Network Working Group E. Kohler
Request for Comments: 4340 UCLA
Category: Standards Track M. Handley
UCL
S. Floyd
ICIR
March 2006
Datagram Congestion Control Protocol (DCCP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Datagram Congestion Control Protocol (DCCP) is a transport
protocol that provides bidirectional unicast connections of
congestion-controlled unreliable datagrams. DCCP is suitable for
applications that transfer fairly large amounts of data and that can
benefit from control over the tradeoff between timeliness and
reliability.
Table of Contents
1. Introduction ....................................................5
2. Design Rationale ................................................6
3. Conventions and Terminology .....................................7
3.1. Numbers and Fields .........................................7
3.2. Parts of a Connection ......................................8
3.3. Features ...................................................9
3.4. Round-Trip Times ...........................................9
3.5. Security Limitation ........................................9
3.6. Robustness Principle ......................................10
4. Overview .......................................................10
4.1. Packet Types ..............................................10
4.2. Packet Sequencing .........................................11
4.3. States ....................................................12
4.4. Congestion Control Mechanisms .............................14
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RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
4.5. Feature Negotiation Options ...............................15
4.6. Differences from TCP ......................................16
4.7. Example Connection ........................................17
5. Packet Formats .................................................18
5.1. Generic Header ............................................19
5.2. DCCP-Request Packets ......................................22
5.3. DCCP-Response Packets .....................................23
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets .............23
5.5. DCCP-CloseReq and DCCP-Close Packets ......................25
5.6. DCCP-Reset Packets ........................................25
5.7. DCCP-Sync and DCCP-SyncAck Packets ........................28
5.8. Options ...................................................29
5.8.1. Padding Option .....................................31
5.8.2. Mandatory Option ...................................31
6. Feature Negotiation ............................................32
6.1. Change Options ............................................32
6.2. Confirm Options ...........................................33
6.3. Reconciliation Rules ......................................33
6.3.1. Server-Priority ....................................34
6.3.2. Non-Negotiable .....................................34
6.4. Feature Numbers ...........................................35
6.5. Feature Negotiation Examples ..............................36
6.6. Option Exchange ...........................................37
6.6.1. Normal Exchange ....................................38
6.6.2. Processing Received Options ........................38
6.6.3. Loss and Retransmission ............................40
6.6.4. Reordering .........................................41
6.6.5. Preference Changes .................................42
6.6.6. Simultaneous Negotiation ...........................42
6.6.7. Unknown Features ...................................43
6.6.8. Invalid Options ....................................43
6.6.9. Mandatory Feature Negotiation ......................44
7. Sequence Numbers ...............................................44
7.1. Variables .................................................45
7.2. Initial Sequence Numbers ..................................45
7.3. Quiet Time ................................................46
7.4. Acknowledgement Numbers ...................................47
7.5. Validity and Synchronization ..............................47
7.5.1. Sequence and Acknowledgement Number Windows ........48
7.5.2. Sequence Window Feature ............................49
7.5.3. Sequence-Validity Rules ............................49
7.5.4. Handling Sequence-Invalid Packets ..................51
7.5.5. Sequence Number Attacks ............................52
7.5.6. Sequence Number Handling Examples ..................54
7.6. Short Sequence Numbers ....................................55
7.6.1. Allow Short Sequence Numbers Feature ...............55
7.6.2. When to Avoid Short Sequence Numbers ...............56
7.7. NDP Count and Detecting Application Loss ..................56
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7.7.1. NDP Count Usage Notes ..............................57
7.7.2. Send NDP Count Feature .............................57
8. Event Processing ...............................................58
8.1. Connection Establishment ..................................58
8.1.1. Client Request .....................................58
8.1.2. Service Codes ......................................59
8.1.3. Server Response ....................................61
8.1.4. Init Cookie Option .................................62
8.1.5. Handshake Completion ...............................63
8.2. Data Transfer .............................................63
8.3. Termination ...............................................64
8.3.1. Abnormal Termination ...............................66
8.4. DCCP State Diagram ........................................66
8.5. Pseudocode ................................................67
9. Checksums ......................................................72
9.1. Header Checksum Field .....................................73
9.2. Header Checksum Coverage Field ............................73
9.2.1. Minimum Checksum Coverage Feature ..................74
9.3. Data Checksum Option ......................................75
9.3.1. Check Data Checksum Feature ........................76
9.3.2. Checksum Usage Notes ...............................76
10. Congestion Control ............................................76
10.1. TCP-like Congestion Control ..............................77
10.2. TFRC Congestion Control ..................................78
10.3. CCID-Specific Options, Features, and Reset Codes .........78
10.4. CCID Profile Requirements ................................80
10.5. Congestion State .........................................81
11. Acknowledgements ..............................................81
11.1. Acks of Acks and Unidirectional Connections ..............82
11.2. Ack Piggybacking .........................................83
11.3. Ack Ratio Feature ........................................84
11.4. Ack Vector Options .......................................85
11.4.1. Ack Vector Consistency ............................88
11.4.2. Ack Vector Coverage ...............................89
11.5. Send Ack Vector Feature ..................................90
11.6. Slow Receiver Option .....................................90
11.7. Data Dropped Option ......................................91
11.7.1. Data Dropped and Normal Congestion Response .......94
11.7.2. Particular Drop Codes .............................95
12. Explicit Congestion Notification ..............................96
12.1. ECN Incapable Feature ....................................96
12.2. ECN Nonces ...............................................97
12.3. Aggression Penalties .....................................98
13. Timing Options ................................................99
13.1. Timestamp Option .........................................99
13.2. Elapsed Time Option ......................................99
13.3. Timestamp Echo Option ...................................100
14. Maximum Packet Size ..........................................101
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14.1. Measuring PMTU ..........................................102
14.2. Sender Behavior .........................................103
15. Forward Compatibility ........................................104
16. Middlebox Considerations .....................................105
17. Relations to Other Specifications ............................106
17.1. RTP .....................................................106
17.2. Congestion Manager and Multiplexing .....................108
18. Security Considerations ......................................108
18.1. Security Considerations for Partial Checksums ...........109
19. IANA Considerations ..........................................110
19.1. Packet Types Registry ...................................110
19.2. Reset Codes Registry ....................................110
19.3. Option Types Registry ...................................110
19.4. Feature Numbers Registry ................................111
19.5. Congestion Control Identifiers Registry .................111
19.6. Ack Vector States Registry ..............................111
19.7. Drop Codes Registry .....................................112
19.8. Service Codes Registry ..................................112
19.9. Port Numbers Registry ...................................112
20. Thanks .......................................................114
A. Appendix: Ack Vector Implementation Notes ....................116
A.1. Packet Arrival ..........................................118
A.1.1. New Packets ......................................118
A.1.2. Old Packets ......................................119
A.2. Sending Acknowledgements ................................120
A.3. Clearing State ..........................................120
A.4. Processing Acknowledgements .............................122
B. Appendix: Partial Checksumming Design Motivation .............123
Normative References .............................................124
Informative References ...........................................125
List of Tables
Table 1: DCCP Packet Types .......................................21
Table 2: DCCP Reset Codes ........................................28
Table 3: DCCP Options ............................................30
Table 4: DCCP Feature Numbers.....................................35
Table 5: DCCP Congestion Control Identifiers .....................77
Table 6: DCCP Ack Vector States ..................................86
Table 7: DCCP Drop Codes .........................................92
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RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
1. Introduction
The Datagram Congestion Control Protocol (DCCP) is a transport
protocol that implements bidirectional, unicast connections of
congestion-controlled, unreliable datagrams. Specifically, DCCP
provides the following:
o Unreliable flows of datagrams.
o Reliable handshakes for connection setup and teardown.
o Reliable negotiation of options, including negotiation of a
suitable congestion control mechanism.
o Mechanisms allowing servers to avoid holding state for
unacknowledged connection attempts and already-finished
connections.
o Congestion control incorporating Explicit Congestion Notification
(ECN) [RFC3168] and the ECN Nonce [RFC3540].
o Acknowledgement mechanisms communicating packet loss and ECN
information. Acks are transmitted as reliably as the relevant
congestion control mechanism requires, possibly completely
reliably.
o Optional mechanisms that tell the sending application, with high
reliability, which data packets reached the receiver, and whether
those packets were ECN marked, corrupted, or dropped in the
receive buffer.
o Path Maximum Transmission Unit (PMTU) discovery [RFC1191].
o A choice of modular congestion control mechanisms. Two mechanisms
are currently specified: TCP-like Congestion Control [RFC4341] and
TCP-Friendly Rate Control (TFRC) [RFC4342]. DCCP is easily
extensible to further forms of unicast congestion control.
DCCP is intended for applications such as streaming media that can
benefit from control over the tradeoffs between delay and reliable
in-order delivery. TCP is not well suited for these applications,
since reliable in-order delivery and congestion control can cause
arbitrarily long delays. UDP avoids long delays, but UDP
applications that implement congestion control must do so on their
own. DCCP provides built-in congestion control, including ECN
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support, for unreliable datagram flows, avoiding the arbitrary delays
associated with TCP. It also implements reliable connection setup,
teardown, and feature negotiation.
2. Design Rationale
One DCCP design goal was to give most streaming UDP applications
little reason not to switch to DCCP, once it is deployed. To
facilitate this, DCCP was designed to have as little overhead as
possible, both in terms of the packet header size and in terms of the
state and CPU overhead required at end hosts. Only the minimal
necessary functionality was included in DCCP, leaving other
functionality, such as forward error correction (FEC), semi-
reliability, and multiple streams, to be layered on top of DCCP as
desired.
Different forms of conformant congestion control are appropriate for
different applications. For example, on-line games might want to
make quick use of any available bandwidth, while streaming media
might trade off this responsiveness for a steadier, less bursty rate.
(Sudden rate changes can cause unacceptable UI glitches such as
audible pauses or clicks in the playout stream.) DCCP thus allows
applications to choose from a set of congestion control mechanisms.
One alternative, TCP-like Congestion Control, halves the congestion
window in response to a packet drop or mark, as in TCP. Applications
using this congestion control mechanism will respond quickly to
changes in available bandwidth, but must tolerate the abrupt changes
in congestion window typical of TCP. A second alternative, TCP-
Friendly Rate Control (TFRC) [RFC3448], a form of equation-based
congestion control, minimizes abrupt changes in the sending rate
while maintaining longer-term fairness with TCP. Other alternatives
can be added as future congestion control mechanisms are
standardized.
DCCP also lets unreliable traffic safely use ECN. A UDP kernel
Application Programming Interface (API) might not allow applications
to set UDP packets as ECN capable, since the API could not guarantee
that the application would properly detect or respond to congestion.
DCCP kernel APIs will have no such issues, since DCCP implements
congestion control itself.
We chose not to require the use of the Congestion Manager [RFC3124],
which allows multiple concurrent streams between the same sender and
receiver to share congestion control. The current Congestion Manager
can only be used by applications that have their own end-to-end
feedback about packet losses, but this is not the case for many of
the applications currently using UDP. In addition, the current
Congestion Manager does not easily support multiple congestion
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control mechanisms or mechanisms where the state about past packet
drops or marks is maintained at the receiver rather than the sender.
DCCP should be able to make use of CM where desired by the
application, but we do not see any benefit in making the deployment
of DCCP contingent on the deployment of CM itself.
We intend for DCCP's protocol mechanisms, which are described in this
document, to suit any application desiring unicast congestion-
controlled streams of unreliable datagrams. However, the congestion
control mechanisms currently approved for use with DCCP, which are
described in separate Congestion Control ID Profiles [RFC4341,
RFC4342], may cause problems for some applications, including high-
bandwidth interactive video. These applications should be able to
use DCCP once suitable Congestion Control ID Profiles are
standardized.
3. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3.1. Numbers and Fields
All multi-byte numerical quantities in DCCP, such as port numbers,
Sequence Numbers, and arguments to options, are transmitted in
network byte order (most significant byte first).
We occasionally refer to the "left" and "right" sides of a bit field.
"Left" means towards the most significant bit, and "right" means
towards the least significant bit.
Random numbers in DCCP are used for their security properties and
SHOULD be chosen according to the guidelines in [RFC4086].
All operations on DCCP sequence numbers use circular arithmetic
modulo 2^48, as do comparisons such as "greater" and "greatest".
This form of arithmetic preserves the relationships between sequence
numbers as they roll over from 2^48 - 1 to 0. Implementation
strategies for DCCP sequence numbers will resemble those for other
circular arithmetic spaces, including TCP's sequence numbers [RFC793]
and DNS's serial numbers [RFC1982]. It may make sense to store DCCP
sequence numbers in the most significant 48 bits of 64-bit integers
and set the least significant 16 bits to zero, since this supports a
common technique that implements circular comparison A < B by testing
whether (A - B) < 0 using conventional two's-complement arithmetic.
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Reserved bitfields in DCCP packet headers MUST be set to zero by
senders and MUST be ignored by receivers, unless otherwise specified.
This allows for future protocol extensions. In particular, DCCP
processors MUST NOT reset a DCCP connection simply because a Reserved
field has non-zero value [RFC3360].
3.2. Parts of a Connection
Each DCCP connection runs between two hosts, which we often name DCCP
A and DCCP B. Each connection is actively initiated by one of the
hosts, which we call the client; the other, initially passive host is
called the server. The term "DCCP endpoint" is used to refer to
either of the two hosts explicitly named by the connection (the
client and the server). The term "DCCP processor" refers more
generally to any host that might need to process a DCCP header; this
includes the endpoints and any middleboxes on the path, such as
firewalls and network address translators.
DCCP connections are bidirectional: data may pass from either
endpoint to the other. This means that data and acknowledgements may
flow in both directions simultaneously. Logically, however, a DCCP
connection consists of two separate unidirectional connections,
called half-connections. Each half-connection consists of the
application data sent by one endpoint and the corresponding
acknowledgements sent by the other endpoint. We can illustrate this
as follows:
+--------+ A-to-B half-connection: +--------+
| | --> application data --> | |
| | <-- acknowledgements <-- | |
| DCCP A | | DCCP B |
| | B-to-A half-connection: | |
| | <-- application data <-- | |
+--------+ --> acknowledgements --> +--------+
Although they are logically distinct, in practice the half-
connections overlap; a DCCP-DataAck packet, for example, contains
application data relevant to one half-connection and acknowledgement
information relevant to the other.
In the context of a single half-connection, the terms "HC-Sender" and
"HC-Receiver" denote the endpoints sending application data and
acknowledgements, respectively. For example, DCCP A is the
HC-Sender and DCCP B is the HC-Receiver in the A-to-B
half-connection.
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3.3. Features
A DCCP feature is a connection attribute on whose value the two
endpoints agree. Many properties of a DCCP connection are controlled
by features, including the congestion control mechanisms in use on
the two half-connections. The endpoints achieve agreement through
the exchange of feature negotiation options in DCCP headers.
DCCP features are identified by a feature number and an endpoint.
The notation "F/X" represents the feature with feature number F
located at DCCP endpoint X. Each valid feature number thus
corresponds to two features, which are negotiated separately and need
not have the same value. The two endpoints know, and agree on, the
value of every valid feature. DCCP A is the "feature location" for
all features F/A, and the "feature remote" for all features F/B.
3.4. Round-Trip Times
DCCP round-trip time measurements are performed by congestion control
mechanisms; different mechanisms may measure round-trip time in
different ways, or not measure it at all. However, the main DCCP
protocol does use round-trip times occasionally, such as in the
initial values for certain timers. Each DCCP implementation thus
defines a default round-trip time for use when no estimate is
available. This parameter should default to not less than 0.2
seconds, a reasonably conservative round-trip time for Internet TCP
connections. Protocol behavior specified in terms of "round-trip
time" values actually refers to "a current round-trip time estimate
taken by some CCID, or, if no estimate is available, the default
round-trip time parameter".
The maximum segment lifetime, or MSL, is the maximum length of time a
packet can survive in the network. The DCCP MSL should equal that of
TCP, which is normally two minutes.
3.5. Security Limitation
DCCP provides no protection against attackers who can snoop on a
connection in progress, or who can guess valid sequence numbers in
other ways. Applications desiring stronger security should use IPsec
[RFC2401]; depending on the level of security required, application-
level cryptography may also suffice. These issues are discussed
further in Sections 7.5.5 and 18.
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3.6. Robustness Principle
DCCP implementations will follow TCP's "general principle of
robustness": "be conservative in what you do, be liberal in what you
accept from others" [RFC793].
4. Overview
DCCP's high-level connection dynamics echo those of TCP. Connections
progress through three phases: initiation, including a three-way
handshake; data transfer; and termination. Data can flow both ways
over the connection. An acknowledgement framework lets senders
discover how much data has been lost and thus avoid unfairly
congesting the network. Of course, DCCP provides unreliable datagram
semantics, not TCP's reliable bytestream semantics. The application
must package its data into explicit frames and must retransmit its
own data as necessary. It may be useful to think of DCCP as TCP
minus bytestream semantics and reliability, or as UDP plus congestion
control, handshakes, and acknowledgements.
4.1. Packet Types
Ten packet types implement DCCP's protocol functions. For example,
every new connection attempt begins with a DCCP-Request packet sent
by the client. In this way a DCCP-Request packet resembles a TCP
SYN, but since DCCP-Request is a packet type there is no way to send
an unexpected flag combination, such as TCP's SYN+FIN+ACK+RST.
Eight packet types occur during the progress of a typical connection,
shown here. Note the three-way handshakes during initiation and
termination.
Client Server
------ ------
(1) Initiation
DCCP-Request -->
<-- DCCP-Response
DCCP-Ack -->
(2) Data transfer
DCCP-Data, DCCP-Ack, DCCP-DataAck -->
<-- DCCP-Data, DCCP-Ack, DCCP-DataAck
(3) Termination
<-- DCCP-CloseReq
DCCP-Close -->
<-- DCCP-Reset
The two remaining packet types are used to resynchronize after bursts
of loss.
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Every DCCP packet starts with a fixed-size generic header.
Particular packet types include additional fixed-size header data;
for example, DCCP-Acks include an Acknowledgement Number. DCCP
options and any application data follow the fixed-size header.
The packet types are as follows:
DCCP-Request
Sent by the client to initiate a connection (the first part of the
three-way initiation handshake).
DCCP-Response
Sent by the server in response to a DCCP-Request (the second part
of the three-way initiation handshake).
DCCP-Data
Used to transmit application data.
DCCP-Ack
Used to transmit pure acknowledgements.
DCCP-DataAck
Used to transmit application data with piggybacked acknowledgement
information.
DCCP-CloseReq
Sent by the server to request that the client close the
connection.
DCCP-Close
Used by the client or the server to close the connection; elicits
a DCCP-Reset in response.
DCCP-Reset
Used to terminate the connection, either normally or abnormally.
DCCP-Sync, DCCP-SyncAck
Used to resynchronize sequence numbers after large bursts of loss.
4.2. Packet Sequencing
Each DCCP packet carries a sequence number so that losses can be
detected and reported. Unlike TCP sequence numbers, which are byte-
based, DCCP sequence numbers increment by one per packet. For
example:
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DCCP A DCCP B
------ ------
DCCP-Data(seqno 1) -->
DCCP-Data(seqno 2) -->
<-- DCCP-Ack(seqno 10, ackno 2)
DCCP-DataAck(seqno 3, ackno 10) -->
<-- DCCP-Data(seqno 11)
Every DCCP packet increments the sequence number, whether or not it
contains application data. DCCP-Ack pure acknowledgements increment
the sequence number; for instance, DCCP B's second packet above uses
sequence number 11, since sequence number 10 was used for an
acknowledgement. This lets endpoints detect all packet loss,
including acknowledgement loss. It also means that endpoints can get
out of sync after long bursts of loss. The DCCP-Sync and DCCP-
SyncAck packet types are used to recover (Section 7.5).
Since DCCP provides unreliable semantics, there are no
retransmissions, and having a TCP-style cumulative acknowledgement
field doesn't make sense. DCCP's Acknowledgement Number field equals
the greatest sequence number received, rather than the smallest
sequence number not received. Separate options indicate any
intermediate sequence numbers that weren't received.
4.3. States
DCCP endpoints progress through different states during the course of
a connection, corresponding roughly to the three phases of
initiation, data transfer, and termination. The figure below shows
the typical progress through these states for a client and server.
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Client Server
------ ------
(0) No connection
CLOSED LISTEN
(1) Initiation
REQUEST DCCP-Request -->
<-- DCCP-Response RESPOND
PARTOPEN DCCP-Ack or DCCP-DataAck -->
(2) Data transfer
OPEN <-- DCCP-Data, Ack, DataAck --> OPEN
(3) Termination
<-- DCCP-CloseReq CLOSEREQ
CLOSING DCCP-Close -->
<-- DCCP-Reset CLOSED
TIMEWAIT
CLOSED
The nine possible states are as follows. They are listed in
increasing order, so that "state >= CLOSEREQ" means the same as
"state = CLOSEREQ or state = CLOSING or state = TIMEWAIT". Section 8
describes the states in more detail.
CLOSED
Represents nonexistent connections.
LISTEN
Represents server sockets in the passive listening state. LISTEN
and CLOSED are not associated with any particular DCCP connection.
REQUEST
A client socket enters this state, from CLOSED, after sending a
DCCP-Request packet to try to initiate a connection.
RESPOND
A server socket enters this state, from LISTEN, after receiving a
DCCP-Request from a client.
PARTOPEN
A client socket enters this state, from REQUEST, after receiving a
DCCP-Response from the server. This state represents the third
phase of the three-way handshake. The client may send application
data in this state, but it MUST include an Acknowledgement Number
on all of its packets.
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OPEN
The central data transfer portion of a DCCP connection. Client
and server sockets enter this state from PARTOPEN and RESPOND,
respectively. Sometimes we speak of SERVER-OPEN and CLIENT-OPEN
states, corresponding to the server's OPEN state and the client's
OPEN state.
CLOSEREQ
A server socket enters this state, from SERVER-OPEN, to order the
client to close the connection and to hold TIMEWAIT state.
CLOSING
Server and client sockets can both enter this state to close the
connection.
TIMEWAIT
A server or client socket remains in this state for 2MSL (4
minutes) after the connection has been torn down, to prevent
mistakes due to the delivery of old packets. Only one of the
endpoints has to enter TIMEWAIT state (the other can enter CLOSED
state immediately), and a server can request its client to hold
TIMEWAIT state using the DCCP-CloseReq packet type.
4.4. Congestion Control Mechanisms
DCCP connections are congestion controlled, but unlike in TCP, DCCP
applications have a choice of congestion control mechanism. In fact,
the two half-connections can be governed by different mechanisms.
Mechanisms are denoted by one-byte congestion control identifiers, or
CCIDs. The endpoints negotiate their CCIDs during connection
initiation. Each CCID describes how the HC-Sender limits data packet
rates, how the HC-Receiver sends congestion feedback via
acknowledgements, and so forth. CCIDs 2 and 3 are currently defined;
CCIDs 0, 1, and 4-255 are reserved. Other CCIDs may be defined in
the future.
CCID 2 provides TCP-like Congestion Control, which is similar to that
of TCP. The sender maintains a congestion window and sends packets
until that window is full. Packets are acknowledged by the receiver.
Dropped packets and ECN [RFC3168] indicate congestion; the response
to congestion is to halve the congestion window. Acknowledgements in
CCID 2 contain the sequence numbers of all received packets within
some window, similar to a selective acknowledgement (SACK) [RFC2018].
CCID 3 provides TCP-Friendly Rate Control (TFRC), an equation-based
form of congestion control intended to respond to congestion more
smoothly than CCID 2. The sender maintains a transmit rate, which it
updates using the receiver's estimate of the packet loss and mark
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rate. CCID 3 behaves somewhat differently than TCP in the short
term, but is designed to operate fairly with TCP over the long term.
Section 10 describes DCCP's CCIDs in more detail. The behaviors of
CCIDs 2 and 3 are fully defined in separate profile documents
[RFC4341, RFC4342].
4.5. Feature Negotiation Options
DCCP endpoints use Change and Confirm options to negotiate and agree
on feature values. Feature negotiation will almost always happen on
the connection initiation handshake, but it can begin at any time.
There are four feature negotiation options in all: Change L, Confirm
L, Change R, and Confirm R. The "L" options are sent by the feature
location and the "R" options are sent by the feature remote. A
Change R option says to the feature location, "change this feature
value as follows". The feature location responds with Confirm L,
meaning, "I've changed it". Some features allow Change R options to
contain multiple values sorted in preference order. For example:
Client Server
------ ------
Change R(CCID, 2) -->
<-- Confirm L(CCID, 2)
* agreement that CCID/Server = 2 *
Change R(CCID, 3 4) -->
<-- Confirm L(CCID, 4, 4 2)
* agreement that CCID/Server = 4 *
Both exchanges negotiate the CCID/Server feature's value, which is
the CCID in use on the server-to-client half-connection. In the
second exchange, the client requests that the server use either CCID
3 or CCID 4, with 3 preferred; the server chooses 4 and supplies its
preference list, "4 2".
The Change L and Confirm R options are used for feature negotiations
initiated by the feature location. In the following example, the
server requests that CCID/Server be set to 3 or 2, with 3 preferred,
and the client agrees.
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Client Server
------ ------
<-- Change L(CCID, 3 2)
Confirm R(CCID, 3, 3 2) -->
* agreement that CCID/Server = 3 *
Section 6 describes the feature negotiation options further,
including the retransmission strategies that make negotiation
reliable.
4.6. Differences from TCP
DCCP's differences from TCP apart from those discussed so far include
the following:
o Copious space for options (up to 1008 bytes or the PMTU).
o Different acknowledgement formats. The CCID for a connection
determines how much acknowledgement information needs to be
transmitted. For example, in CCID 2 (TCP-like), this is about one
ack per 2 packets, and each ack must declare exactly which packets
were received. In CCID 3 (TFRC), it is about one ack per round-
trip time, and acks must declare at minimum just the lengths of
recent loss intervals.
o Denial of Service (DoS) protection. Several mechanisms help limit
the amount of state that possibly-misbehaving clients can force
DCCP servers to maintain. An Init Cookie option analogous to
TCP's SYN Cookies [SYNCOOKIES] avoids SYN-flood-like attacks.
Only one connection endpoint has to hold TIMEWAIT state; the
DCCP-CloseReq packet, which may only be sent by the server, passes
that state to the client. Various rate limits let servers avoid
attacks that might force extensive computation or packet
generation.
o Distinguishing different kinds of loss. A Data Dropped option
(Section 11.7) lets an endpoint declare that a packet was dropped
because of corruption, because of receive buffer overflow, and so
on. This facilitates research into more appropriate rate-control
responses for these non-network-congestion losses (although
currently such losses will cause a congestion response).
o Acknowledgeability. In TCP, a packet may be acknowledged only
once the data is reliably queued for application delivery. This
does not make sense in DCCP, where an application might, for
example, request a drop-from-front receive buffer. A DCCP packet
may be acknowledged as soon as its header has been successfully
processed. Concretely, a packet becomes acknowledgeable at Step 8
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of Section 8.5's packet processing pseudocode. Acknowledgeability
does not guarantee data delivery, however: the Data Dropped option
may later report that the packet's application data was discarded.
o No receive window. DCCP is a congestion control protocol, not a
flow control protocol.
o No simultaneous open. Every connection has one client and one
server.
o No half-closed states. DCCP has no states corresponding to TCP's
FINWAIT and CLOSEWAIT, where one half-connection is explicitly
closed while the other is still active. The Data Dropped option's
Drop Code 1, Application Not Listening (Section 11.7), can achieve
a similar effect, however.
4.7. Example Connection
The progress of a typical DCCP connection is as follows. (This
description is informative, not normative.)
Client Server
------ ------
0. [CLOSED] [LISTEN]
1. DCCP-Request -->
2. <-- DCCP-Response
3. DCCP-Ack -->
4. DCCP-Data, DCCP-Ack, DCCP-DataAck -->
<-- DCCP-Data, DCCP-Ack, DCCP-DataAck
5. <-- DCCP-CloseReq
6. DCCP-Close -->
7. <-- DCCP-Reset
8. [TIMEWAIT]
1. The client sends the server a DCCP-Request packet specifying the
client and server ports, the service being requested, and any
features being negotiated, including the CCID that the client
would like the server to use. The client may optionally piggyback
an application request on the DCCP-Request packet. The server may
ignore this application request.
2. The server sends the client a DCCP-Response packet indicating that
it is willing to communicate with the client. This response
indicates any features and options that the server agrees to,
begins other feature negotiations as desired, and optionally
includes Init Cookies that wrap up all this information and that
must be returned by the client for the connection to complete.
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3. The client sends the server a DCCP-Ack packet that acknowledges
the DCCP-Response packet. This acknowledges the server's initial
sequence number and returns any Init Cookies in the DCCP-Response.
It may also continue feature negotiation. The client may
piggyback an application-level request on this ack, producing a
DCCP-DataAck packet.
4. The server and client then exchange DCCP-Data packets, DCCP-Ack
packets acknowledging that data, and, optionally, DCCP-DataAck
packets containing data with piggybacked acknowledgements. If the
client has no data to send, then the server will send DCCP-Data
and DCCP-DataAck packets, while the client will send DCCP-Acks
exclusively. (However, the client may not send DCCP-Data packets
before receiving at least one non-DCCP-Response packet from the
server.)
5. The server sends a DCCP-CloseReq packet requesting a close.
6. The client sends a DCCP-Close packet acknowledging the close.
7. The server sends a DCCP-Reset packet with Reset Code 1, "Closed",
and clears its connection state. DCCP-Resets are part of normal
connection termination; see Section 5.6.
8. The client receives the DCCP-Reset packet and holds state for two
maximum segment lifetimes, or 2MSL, to allow any remaining packets
to clear the network.
An alternative connection closedown sequence is initiated by the
client:
5b. The client sends a DCCP-Close packet closing the connection.
6b. The server sends a DCCP-Reset packet with Reset Code 1, "Closed",
and clears its connection state.
7b. The client receives the DCCP-Reset packet and holds state for
2MSL to allow any remaining packets to clear the network.
5. Packet Formats
The DCCP header can be from 12 to 1020 bytes long. The initial part
of the header has the same semantics for all currently defined packet
types. Following this comes any additional fixed-length fields
required by the packet type, and then a variable-length list of
options. The application data area follows the header. In some
packet types, this area contains data for the application; in other
packet types, its contents are ignored.
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RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
+---------------------------------------+ -.
| Generic Header | |
+---------------------------------------+ |
| Additional Fields (depending on type) | +- DCCP Header
+---------------------------------------+ |
| Options (optional) | |
+=======================================+ -'
| Application Data Area |
+---------------------------------------+
5.1. Generic Header
The DCCP generic header takes different forms depending on the value
of X, the Extended Sequence Numbers bit. If X is one, the Sequence
Number field is 48 bits long, and the generic header takes 16 bytes,
as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Dest Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Offset | CCVal | CsCov | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |X| | .
| Res | Type |=| Reserved | Sequence Number (high bits) .
| | |1| | .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Sequence Number (low bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If X is zero, only the low 24 bits of the Sequence Number are
transmitted, and the generic header is 12 bytes long.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Dest Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Offset | CCVal | CsCov | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |X| |
| Res | Type |=| Sequence Number (low bits) |
| | |0| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The generic header fields are defined as follows.
Source and Destination Ports: 16 bits each
These fields identify the connection, similar to the corresponding
fields in TCP and UDP. The Source Port represents the relevant
port on the endpoint that sent this packet, and the Destination
Port represents the relevant port on the other endpoint. When
initiating a connection, the client SHOULD choose its Source Port
randomly to reduce the likelihood of attack.
DCCP APIs should treat port numbers similarly to TCP and UDP port
numbers. For example, machines that distinguish between
"privileged" and "unprivileged" ports for TCP and UDP should do
the same for DCCP.
Data Offset: 8 bits
The offset from the start of the packet's DCCP header to the start
of its application data area, in 32-bit words. The receiver MUST
ignore packets whose Data Offset is smaller than the minimum-sized
header for the given Type or larger than the DCCP packet itself.
CCVal: 4 bits
Used by the HC-Sender CCID. For example, the A-to-B CCID's
sender, which is active at DCCP A, MAY send 4 bits of information
per packet to its receiver by encoding that information in CCVal.
The sender MUST set CCVal to zero unless its HC-Sender CCID
specifies otherwise, and the receiver MUST ignore the CCVal field
unless its HC-Receiver CCID specifies otherwise.
Checksum Coverage (CsCov): 4 bits
Checksum Coverage determines the parts of the packet that are
covered by the Checksum field. This always includes the DCCP
header and options, but some or all of the application data may be
excluded. This can improve performance on noisy links for
applications that can tolerate corruption. See Section 9.
Checksum: 16 bits
The Internet checksum of the packet's DCCP header (including
options), a network-layer pseudoheader, and, depending on Checksum
Coverage, all, some, or none of the application data. See Section
9.
Reserved (Res): 3 bits
Senders MUST set this field to all zeroes on generated packets,
and receivers MUST ignore its value.
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RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
Type: 4 bits
The Type field specifies the type of the packet. The following
values are defined:
Type Meaning
---- -------
0 DCCP-Request
1 DCCP-Response
2 DCCP-Data
3 DCCP-Ack
4 DCCP-DataAck
5 DCCP-CloseReq
6 DCCP-Close
7 DCCP-Reset
8 DCCP-Sync
9 DCCP-SyncAck
10-15 Reserved
Table 1: DCCP Packet Types
Receivers MUST ignore any packets with reserved type. That is,
packets with reserved type MUST NOT be processed, and they MUST
NOT be acknowledged as received.
Extended Sequence Numbers (X): 1 bit
Set to one to indicate the use of an extended generic header with
48-bit Sequence and Acknowledgement Numbers. DCCP-Data, DCCP-
DataAck, and DCCP-Ack packets MAY set X to zero or one. All
DCCP-Request, DCCP-Response, DCCP-CloseReq, DCCP-Close, DCCP-
Reset, DCCP-Sync, and DCCP-SyncAck packets MUST set X to one;
endpoints MUST ignore any such packets with X set to zero. High-
rate connections SHOULD set X to one on all packets to gain
increased protection against wrapped sequence numbers and attacks.
See Section 7.6.
Sequence Number: 48 or 24 bits
Identifies the packet uniquely in the sequence of all packets the
source sent on this connection. Sequence Number increases by one
with every packet sent, including packets such as DCCP-Ack that
carry no application data. See Section 7.
All currently defined packet types except DCCP-Request and DCCP-Data
carry an Acknowledgement Number Subheader in the four or eight bytes
immediately following the generic header. When X=1, its format is:
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RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number .
| | (high bits) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. Acknowledgement Number (low bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
When X=0, only the low 24 bits of the Acknowledgement Number are
transmitted, giving the Acknowledgement Number Subheader this format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Acknowledgement Number (low bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved: 16 or 8 bits
Senders MUST set this field to all zeroes on generated packets,
and receivers MUST ignore its value.
Acknowledgement Number: 48 or 24 bits
Generally contains GSR, the Greatest Sequence Number Received on
any acknowledgeable packet so far. A packet is acknowledgeable
if and only if its header was successfully processed by the
receiver; Section 7.4 describes this further. Options such as
Ack Vector (Section 11.4) combine with the Acknowledgement
Number to provide precise information about which packets have
arrived.
Acknowledgement Numbers on DCCP-Sync and DCCP-SyncAck packets
need not equal GSR. See Section 5.7.
5.2. DCCP-Request Packets
A client initiates a DCCP connection by sending a DCCP-Request
packet. These packets MAY contain application data and MUST use
48-bit sequence numbers (X=1).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=0 (DCCP-Request) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Service Code: 32 bits
Describes the application-level service to which the client
application wants to connect. Service Codes are intended to
provide information about which application protocol a connection
intends to use, thus aiding middleboxes and reducing reliance on
globally well-known ports. See Section 8.1.2.
5.3. DCCP-Response Packets
The server responds to valid DCCP-Request packets with DCCP-Response
packets. This is the second phase of the three-way handshake.
DCCP-Response packets MAY contain application data and MUST use
48-bit sequence numbers (X=1).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=1 (DCCP-Response) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Service Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Acknowledgement Number: 48 bits
Contains GSR. Since DCCP-Responses are only sent during
connection initiation, this will always equal the Sequence Number
on a received DCCP-Request.
Service Code: 32 bits
MUST equal the Service Code on the corresponding DCCP-Request.
5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets
The central data transfer portion of every DCCP connection uses
DCCP-Data, DCCP-Ack, and DCCP-DataAck packets. These packets MAY use
24-bit sequence numbers, depending on the value of the Allow Short
Sequence Numbers feature (Section 7.6.1). DCCP-Data packets carry
application data without acknowledgements.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (16 or 12 bytes) /
/ with Type=2 (DCCP-Data) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DCCP-Ack packets dispense with the data but contain an
Acknowledgement Number. They are used for pure acknowledgements.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (16 or 12 bytes) /
/ with Type=3 (DCCP-Ack) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 or 4 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DCCP-DataAck packets carry both application data and an
Acknowledgement Number. This piggybacks acknowledgement information
on a data packet.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header (16 or 12 bytes) /
/ with Type=4 (DCCP-DataAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 or 4 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A DCCP-Data or DCCP-DataAck packet may have a zero-length application
data area, which indicates that the application sent a zero-length
datagram. This differs from DCCP-Request and DCCP-Response packets,
where an empty application data area indicates the absence of
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application data (not the presence of zero-length application data).
The API SHOULD report any received zero-length datagrams to the
receiving application.
A DCCP-Ack packet MAY have a non-zero-length application data area,
which essentially pads the DCCP-Ack to a desired length. Receivers
MUST ignore the content of the application data area in DCCP-Ack
packets.
DCCP-Ack and DCCP-DataAck packets often include additional
acknowledgement options, such as Ack Vector, as required by the
congestion control mechanism in use.
5.5. DCCP-CloseReq and DCCP-Close Packets
DCCP-CloseReq and DCCP-Close packets begin the handshake that
normally terminates a connection. Either client or server may send a
DCCP-Close packet, which will elicit a DCCP-Reset packet. Only the
server can send a DCCP-CloseReq packet, which indicates that the
server wants to close the connection but does not want to hold its
TIMEWAIT state. Both packet types MUST use 48-bit sequence numbers
(X=1).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=5 (DCCP-CloseReq) or 6 (DCCP-Close) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As with DCCP-Ack packets, DCCP-CloseReq and DCCP-Close packets MAY
have non-zero-length application data areas, whose contents receivers
MUST ignore.
5.6. DCCP-Reset Packets
DCCP-Reset packets unconditionally shut down a connection.
Connections normally terminate with a DCCP-Reset, but resets may be
sent for other reasons, including bad port numbers, bad option
behavior, incorrect ECN Nonce Echoes, and so forth. DCCP-Resets MUST
use 48-bit sequence numbers (X=1).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=7 (DCCP-Reset) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reset Code | Data 1 | Data 2 | Data 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Error Text) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reset Code: 8 bits
Represents the reason that the sender reset the DCCP connection.
Data 1, Data 2, and Data 3: 8 bits each
The Data fields provide additional information about why the
sender reset the DCCP connection. The meanings of these fields
depend on the value of Reset Code.
Application Data Area: Error Text
If present, Error Text is a human-readable text string encoded in
Unicode UTF-8, and preferably in English, that describes the error
in more detail. For example, a DCCP-Reset with Reset Code 11,
"Aggression Penalty", might contain Error Text such as "Aggression
Penalty: Received 3 bad ECN Nonce Echoes, assuming misbehavior".
The following Reset Codes are currently defined. Unless otherwise
specified, the Data 1, 2, and 3 fields MUST be set to 0 by the sender
of the DCCP-Reset and ignored by its receiver. Section references
describe concrete situations that will cause each Reset Code to be
generated; they are not meant to be exhaustive.
0, "Unspecified"
Indicates the absence of a meaningful Reset Code. Use of Reset
Code 0 is NOT RECOMMENDED: the sender should choose a Reset Code
that more clearly defines why the connection is being reset.
1, "Closed"
Normal connection close. See Section 8.3.
2, "Aborted"
The sending endpoint gave up on the connection because of lack of
progress. See Sections 8.1.1 and 8.1.5.
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3, "No Connection"
No connection exists. See Section 8.3.1.
4, "Packet Error"
A valid packet arrived with unexpected type. For example, a
DCCP-Data packet with valid header checksum and sequence numbers
arrived at a connection in the REQUEST state. See Section 8.3.1.
The Data 1 field equals the offending packet type as an eight-bit
number; thus, an offending packet with Type 2 will result in a
Data 1 value of 2.
5, "Option Error"
An option was erroneous, and the error was serious enough to
warrant resetting the connection. See Sections 6.6.7, 6.6.8, and
11.4. The Data 1 field equals the offending option type; Data 2
and Data 3 equal the first two bytes of option data (or zero if
the option had less than two bytes of data).
6, "Mandatory Error"
The sending endpoint could not process an option O that was
immediately preceded by Mandatory. The Data fields report the
option type and data of option O, using the format of Reset Code
5, "Option Error". See Section 5.8.2.
7, "Connection Refused"
The Destination Port didn't correspond to a port open for
listening. Sent only in response to DCCP-Requests. See Section
8.1.3.
8, "Bad Service Code"
The Service Code didn't equal the service code attached to the
Destination Port. Sent only in response to DCCP-Requests. See
Section 8.1.3.
9, "Too Busy"
The server is too busy to accept new connections. Sent only in
response to DCCP-Requests. See Section 8.1.3.
10, "Bad Init Cookie"
The Init Cookie echoed by the client was incorrect or missing.
See Section 8.1.4.
11, "Aggression Penalty"
This endpoint has detected congestion control-related misbehavior
on the part of the other endpoint. See Section 12.3.
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12-127, Reserved
Receivers should treat these codes as they do Reset Code 0,
"Unspecified".
128-255, CCID-specific codes
Semantics depend on the connection's CCIDs. See Section 10.3.
Receivers should treat unknown CCID-specific Reset Codes as they
do Reset Code 0, "Unspecified".
The following table summarizes this information.
Reset
Code Name Data 1 Data 2 & 3
----- ---- ------ ----------
0 Unspecified 0 0
1 Closed 0 0
2 Aborted 0 0
3 No Connection 0 0
4 Packet Error pkt type 0
5 Option Error option # option data
6 Mandatory Error option # option data
7 Connection Refused 0 0
8 Bad Service Code 0 0
9 Too Busy 0 0
10 Bad Init Cookie 0 0
11 Aggression Penalty 0 0
12-127 Reserved
128-255 CCID-specific codes
Table 2: DCCP Reset Codes
Options on DCCP-Reset packets are processed before the connection is
shut down. This means that certain combinations of options,
particularly involving Mandatory, may cause an endpoint to respond to
a valid DCCP-Reset with another DCCP-Reset. This cannot lead to a
reset storm; since the first endpoint has already reset the
connection, the second DCCP-Reset will be ignored.
5.7. DCCP-Sync and DCCP-SyncAck Packets
DCCP-Sync packets help DCCP endpoints recover synchronization after
bursts of loss and recover from half-open connections. Each valid
received DCCP-Sync immediately elicits a DCCP-SyncAck. Both packet
types MUST use 48-bit sequence numbers (X=1).
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Generic DCCP Header with X=1 (16 bytes) /
/ with Type=8 (DCCP-Sync) or 9 (DCCP-SyncAck) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Acknowledgement Number Subheader (8 bytes) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Options and Padding /
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
/ Application Data Area (Ignored) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Acknowledgement Number field has special semantics for DCCP-Sync
and DCCP-SyncAck packets. First, the packet corresponding to a
DCCP-Sync's Acknowledgement Number need not have been
acknowledgeable. Thus, receivers MUST NOT assume that a packet was
processed simply because it appears in the Acknowledgement Number
field of a DCCP-Sync packet. This differs from all other packet
types, where the Acknowledgement Number by definition corresponds to
an acknowledgeable packet. Second, the Acknowledgement Number on any
DCCP-SyncAck packet MUST correspond to the Sequence Number on an
acknowledgeable DCCP-Sync packet. In the presence of reordering,
this might not equal GSR.
As with DCCP-Ack packets, DCCP-Sync and DCCP-SyncAck packets MAY have
non-zero-length application data areas, whose contents receivers MUST
ignore. Padded DCCP-Sync packets may be useful when performing Path
MTU discovery; see Section 14.
5.8. Options
Any DCCP packet may contain options, which occupy space at the end of
the DCCP header. Each option is a multiple of 8 bits in length.
Individual options are not padded to multiples of 32 bits, and any
option may begin on any byte boundary. However, the combination of
all options MUST add up to a multiple of 32 bits; Padding options
MUST be added as necessary to fill out option space to a word
boundary. Any options present are included in the header checksum.
The first byte of an option is the option type. Options with types 0
through 31 are single-byte options. Other options are followed by a
byte indicating the option's length. This length value includes the
two bytes of option-type and option-length as well as any option-data
bytes; it must therefore be greater than or equal to two.
Options MUST be processed sequentially, starting with the first
option in the packet header. Options with unknown types MUST be
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ignored. Also, options with nonsensical lengths (length byte less
than two or more than the remaining space in the options portion of
the header) MUST be ignored, and any option space following an option
with nonsensical length MUST likewise be ignored. Unless otherwise
specified, multiple occurrences of the same option MUST be processed
independently; for some options, this will mean in practice that the
last valid occurrence of an option takes precedence.
The following options are currently defined:
Option DCCP- Section
Type Length Meaning Data? Reference
---- ------ ------- ----- ---------
0 1 Padding Y 5.8.1
1 1 Mandatory N 5.8.2
2 1 Slow Receiver Y 11.6
3-31 1 Reserved
32 variable Change L N 6.1
33 variable Confirm L N 6.2
34 variable Change R N 6.1
35 variable Confirm R N 6.2
36 variable Init Cookie N 8.1.4
37 3-8 NDP Count Y 7.7
38 variable Ack Vector [Nonce 0] N 11.4
39 variable Ack Vector [Nonce 1] N 11.4
40 variable Data Dropped N 11.7
41 6 Timestamp Y 13.1
42 6/8/10 Timestamp Echo Y 13.3
43 4/6 Elapsed Time N 13.2
44 6 Data Checksum Y 9.3
45-127 variable Reserved
128-255 variable CCID-specific options - 10.3
Table 3: DCCP Options
Not all options are suitable for all packet types. For example,
since the Ack Vector option is interpreted relative to the
Acknowledgement Number, it isn't suitable on DCCP-Request and DCCP-
Data packets, which have no Acknowledgement Number. If an option
occurs on an unexpected packet type, it MUST generally be ignored;
any such restrictions are mentioned in each option's description.
The table summarizes the most common restriction: when the DCCP-
Data? column value is N, the corresponding option MUST be ignored
when received on a DCCP-Data packet. (Section 7.5.5 describes why
such options are ignored as opposed to, say, causing a reset.)
Options with invalid values MUST be ignored unless otherwise
specified. For example, any Data Checksum option with option length
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4 MUST be ignored, since all valid Data Checksum options have option
length 6.
This section describes two generic options, Padding and Mandatory.
Other options are described later.
5.8.1. Padding Option
+--------+
|00000000|
+--------+
Type=0
Padding is a single-byte "no-operation" option used to pad between or
after options. If the length of a packet's other options is not a
multiple of 32 bits, then Padding options are REQUIRED to pad out the
options area to the length implied by Data Offset. Padding may also
be used between options; for example, to align the beginning of a
subsequent option on a 32-bit boundary. There is no guarantee that
senders will use this option, so receivers must be prepared to
process options even if they do not begin on a word boundary.
5.8.2. Mandatory Option
+--------+
|00000001|
+--------+
Type=1
Mandatory is a single-byte option that marks the immediately
following option as mandatory. Say that the immediately following
option is O. Then the Mandatory option has no effect if the
receiving DCCP endpoint understands and processes O. If the endpoint
does not understand or process O, however, then it MUST reset the
connection using Reset Code 6, "Mandatory Failure". For instance,
the endpoint would reset the connection if it did not understand O's
type; if it understood O's type, but not O's data; if O's data was
invalid for O's type; if O was a feature negotiation option, and the
endpoint did not understand the enclosed feature number; or if the
endpoint understood O, but chose not to perform the action O implies.
This list is not exhaustive and, in particular, individual option
specifications may describe additional situations in which the
endpoint should reset the connection and situations in which it
should not.
Mandatory options MUST NOT be sent on DCCP-Data packets, and any
Mandatory options received on DCCP-Data packets MUST be ignored.
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The connection is in error and should be reset with Reset Code 5,
"Option Error", if option O is absent (Mandatory was the last byte of
the option list), or if option O equals Mandatory. However, the
combination "Mandatory Padding" is valid, and MUST behave like two
bytes of Padding.
Section 6.6.9 describes the behavior of Mandatory feature negotiation
options in more detail.
6. Feature Negotiation
Four DCCP options, Change L, Confirm L, Change R, and Confirm R, are
used to negotiate feature values. Change options initiate a
negotiation; Confirm options complete that negotiation. The "L"
options are sent by the feature location, and the "R" options are
sent by the feature remote. Change options are retransmitted to
ensure reliability.
All these options have the same format. The first byte of option
data is the feature number, and the second and subsequent data bytes
hold one or more feature values. The exact format of the feature
value area depends on the feature type; see Section 6.3.
+--------+--------+--------+--------+--------
| Type | Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Together, the feature number and the option type ("L" or "R")
uniquely identify the feature to which an option applies. The exact
format of the Value(s) area depends on the feature number.
Feature negotiation options MUST NOT be sent on DCCP-Data packets,
and any feature negotiation options received on DCCP-Data packets
MUST be ignored.
6.1. Change Options
Change L and Change R options initiate feature negotiation. The
option to use depends on the relevant feature's location: To start a
negotiation for feature F/A, DCCP A will send a Change L option; to
start a negotiation for F/B, it will send a Change R option. Change
options are retransmitted until some response is received. They
contain at least one Value, and thus have a length of at least 4.
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+--------+--------+--------+--------+--------
Change L: |00100000| Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Type=32
+--------+--------+--------+--------+--------
Change R: |00100010| Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Type=34
6.2. Confirm Options
Confirm L and Confirm R options complete feature negotiation and are
sent in response to Change R and Change L options, respectively.
Confirm options MUST NOT be generated except in response to Change
options. Confirm options need not be retransmitted, since Change
options are retransmitted as necessary. The first byte of the
Confirm option contains the feature number from the corresponding
Change. Following this is the selected Value, and then possibly the
sender's preference list.
+--------+--------+--------+--------+--------
Confirm L: |00100001| Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Type=33
+--------+--------+--------+--------+--------
Confirm R: |00100011| Length |Feature#| Value(s) ...
+--------+--------+--------+--------+--------
Type=35
If an endpoint receives an invalid Change option -- with an unknown
feature number, or an invalid value -- it will respond with an empty
Confirm option containing the problematic feature number, but no
value. Such options have length 3.
6.3. Reconciliation Rules
Reconciliation rules determine how the two sets of preferences for a
given feature are resolved into a unique result. The reconciliation
rule depends only on the feature number. Each reconciliation rule
must have the property that the result is uniquely determined given
the contents of Change options sent by the two endpoints.
All current DCCP features use one of two reconciliation rules:
server-priority ("SP") and non-negotiable ("NN").
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6.3.1. Server-Priority
The feature value is a fixed-length byte string (length determined by
the feature number). Each Change option contains a list of values
ordered by preference, with the most preferred value coming first.
Each Confirm option contains the confirmed value, followed by the
confirmer's preference list. Thus, the feature's current value will
generally appear twice in Confirm options' data, once as the current
value and once in the confirmer's preference list.
To reconcile the preference lists, select the first entry in the
server's list that also occurs in the client's list. If there is no
shared entry, the feature's value MUST NOT change, and the Confirm
option will confirm the feature's previous value (unless the Change
option was Mandatory; see Section 6.6.9).
6.3.2. Non-Negotiable
The feature value is a byte string. Each option contains exactly one
feature value. The feature location signals a new value by sending a
Change L option. The feature remote MUST accept any valid value,
responding with a Confirm R option containing the new value, and it
MUST send empty Confirm R options in response to invalid values
(unless the Change L option was Mandatory; see Section 6.6.9).
Change R and Confirm L options MUST NOT be sent for non-negotiable
features; see Section 6.6.8. Non-negotiable features use the feature
negotiation mechanism to achieve reliability.
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6.4. Feature Numbers
This document defines the following feature numbers.
Rec'n Initial Section
Number Meaning Rule Value Req'd Reference
------ ------- ----- ----- ----- ---------
0 Reserved
1 Congestion Control ID (CCID) SP 2 Y 10
2 Allow Short Seqnos SP 0 Y 7.6.1
3 Sequence Window NN 100 Y 7.5.2
4 ECN Incapable SP 0 N 12.1
5 Ack Ratio NN 2 N 11.3
6 Send Ack Vector SP 0 N 11.5
7 Send NDP Count SP 0 N 7.7.2
8 Minimum Checksum Coverage SP 0 N 9.2.1
9 Check Data Checksum SP 0 N 9.3.1
10-127 Reserved
128-255 CCID-specific features 10.3
Table 4: DCCP Feature Numbers
Rec'n Rule The reconciliation rule used for the feature. SP
means server-priority, NN means non-negotiable.
Initial Value The initial value for the feature. Every feature has
a known initial value.
Req'd This column is "Y" if and only if every DCCP
implementation MUST understand the feature. If it is
"N", then the feature behaves like an extension (see
Section 15), and it is safe to respond to Change
options for the feature with empty Confirm options.
Of course, a CCID might require the feature; a DCCP
that implements CCID 2 MUST support Ack Ratio and
Send Ack Vector, for example.
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6.5. Feature Negotiation Examples
Here are three example feature negotiations for features located at
the server, the first two for the Congestion Control ID feature, the
last for the Ack Ratio.
Client Server
------ ------
1. Change R(CCID, 2 3 1) -->
("2 3 1" is client's preference list)
2. <-- Confirm L(CCID, 3, 3 2 1)
(3 is the negotiated value;
"3 2 1" is server's pref list)
* agreement that CCID/Server = 3 *
1. XXX <-- Change L(CCID, 3 2 1)
2. Retransmission:
<-- Change L(CCID, 3 2 1)
3. Confirm R(CCID, 3, 2 3 1) -->
* agreement that CCID/Server = 3 *
1. <-- Change L(Ack Ratio, 3)
2. Confirm R(Ack Ratio, 3) -->
* agreement that Ack Ratio/Server = 3 *
This example shows a simultaneous negotiation.
Client Server
------ ------
1a. Change R(CCID, 2 3 1) -->
b. <-- Change L(CCID, 3 2 1)
2a. <-- Confirm L(CCID, 3, 3 2 1)
b. Confirm R(CCID, 3, 2 3 1) -->
* agreement that CCID/Server = 3 *
Here are the byte encodings of several Change and Confirm options.
Each option is sent by DCCP A.
Change L(CCID, 2 3) = 32,5,1,2,3
DCCP B should change CCID/A's value (feature number 1, a server-
priority feature); DCCP A's preferred values are 2 and 3, in that
preference order.
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Change L(Sequence Window, 1024) = 32,9,3,0,0,0,0,4,0
DCCP B should change Sequence Window/A's value (feature number 3,
a non-negotiable feature) to the 6-byte string 0,0,0,0,4,0 (the
value 1024).
Confirm L(CCID, 2, 2 3) = 33,6,1,2,2,3
DCCP A has changed CCID/A's value to 2; its preferred values are 2
and 3, in that preference order.
Empty Confirm L(126) = 33,3,126
DCCP A doesn't implement feature number 126, or DCCP B's proposed
value for feature 126/A was invalid.
Change R(CCID, 3 2) = 34,5,1,3,2
DCCP B should change CCID/B's value; DCCP A's preferred values are
3 and 2, in that preference order.
Confirm R(CCID, 2, 3 2) = 35,6,1,2,3,2
DCCP A has changed CCID/B's value to 2; its preferred values were
3 and 2, in that preference order.
Confirm R(Sequence Window, 1024) = 35,9,3,0,0,0,0,4,0
DCCP A has changed Sequence Window/B's value to the 6-byte string
0,0,0,0,4,0 (the value 1024).
Empty Confirm R(126) = 35,3,126
DCCP A doesn't implement feature number 126, or DCCP B's proposed
value for feature 126/B was invalid.
6.6. Option Exchange
A few basic rules govern feature negotiation option exchange.
1. Every non-reordered Change option gets a Confirm option in
response.
2. Change options are retransmitted until a response for the latest
Change is received.
3. Feature negotiation options are processed in strictly-increasing
order by Sequence Number.
The rest of this section describes the consequences of these rules in
more detail.
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6.6.1. Normal Exchange
Change options are generated when a DCCP endpoint wants to change the
value of some feature. Generally, this will happen at the beginning
of a connection, although it may happen at any time. We say the
endpoint "generates" or "sends" a Change L or Change R option, but of
course the option must be attached to a packet. The endpoint may
attach the option to a packet it would have generated anyway (such as
a DCCP-Request), or it may create a "feature negotiation packet",
often a DCCP-Ack or DCCP-Sync, just to carry the option. Feature
negotiation packets are controlled by the relevant congestion control
mechanism. For example, DCCP A may send a DCCP-Ack or DCCP-Sync for
feature negotiation only if the B-to-A CCID would allow sending a
DCCP-Ack. In addition, an endpoint SHOULD generate at most one
feature negotiation packet per round-trip time.
On receiving a Change L or Change R option, a DCCP endpoint examines
the included preference list, reconciles that with its own preference
list, calculates the new value, and sends back a Confirm R or Confirm
L option, respectively, informing its peer of the new value or that
the feature was not understood. Every non-reordered Change option
MUST result in a corresponding Confirm option, and any packet
including a Confirm option MUST carry an Acknowledgement Number.
(Section 6.6.4 describes how Change reordering is detected and
handled.) Generated Confirm options may be attached to packets that
would have been sent anyway (such as DCCP-Response or DCCP-SyncAck)
or to new feature negotiation packets, as described above.
The Change-sending endpoint MUST wait to receive a corresponding
Confirm option before changing its stored feature value. The
Confirm-sending endpoint changes its stored feature value as soon as
it sends the Confirm.
A packet MAY contain more than one feature negotiation option,
possibly including two options that refer to the same feature; as
usual, the options are processed sequentially.
6.6.2. Processing Received Options
DCCP endpoints exist in one of three states relative to each feature.
STABLE is the normal state, where the endpoint knows the feature's
value and thinks the other endpoint agrees. An endpoint enters the
CHANGING state when it first sends a Change for the feature and
returns to STABLE once it receives a corresponding Confirm. The
final state, UNSTABLE, indicates that an endpoint in CHANGING state
changed its preference list but has not yet transmitted a Change
option with the new preference list.
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Feature state transitions at a feature location are implemented
according to this diagram. The diagram ignores sequence number and
option validity issues; these are handled explicitly in the
pseudocode that follows.
timeout/
rcv Confirm R app/protocol evt : snd Change L rcv non-ack
: ignore +---------------------------------------+ : snd Change L
+----+ | | +----+
| v | rcv Change R v | v
+------------+ rcv Confirm R : calc new value, +------------+
| | : accept value snd Confirm L | |
| STABLE |<-----------------------------------| CHANGING |
| | rcv empty Confirm R | |
+------------+ : revert to old value +------------+
| ^ | ^
+----+ pref list | | snd
rcv Change R changes | | Change L
: calc new value, snd Confirm L v |
+------------+
+---| |
rcv Confirm/Change R | | UNSTABLE |
: ignore +-->| |
+------------+
Feature locations SHOULD use the following pseudocode, which
corresponds to the state diagram, to react to each feature
negotiation option on each valid non-Data packet received. The
pseudocode refers to "P.seqno" and "P.ackno", which are properties of
the packet; "O.type" and "O.len", which are properties of the option;
"FGSR" and "FGSS", which are properties of the connection and handle
reordering as described in Section 6.6.4; "F.state", which is the
feature's state (STABLE, CHANGING, or UNSTABLE); and "F.value", which
is the feature's value.
First, check for unknown features (Section 6.6.7);
If F is unknown,
If the option was Mandatory, /* Section 6.6.9 */
Reset connection and return
Otherwise, if O.type == Change R,
Send Empty Confirm L on a future packet
Return
Second, check for reordering (Section 6.6.4);
If F.state == UNSTABLE or P.seqno <= FGSR
or (O.type == Confirm R and P.ackno < FGSS),
Ignore option and return
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Third, process Change R options;
If O.type == Change R,
If the option's value is valid, /* Section 6.6.8 */
Calculate new value
Send Confirm L on a future packet
Set F.state := STABLE
Otherwise, if the option was Mandatory,
Reset connection and return
Otherwise,
Send Empty Confirm L on a future packet
/* Remain in existing state. If that's CHANGING, this
endpoint will retransmit its Change L option later. */
Fourth, process Confirm R options (but only in CHANGING state).
If F.state == CHANGING and O.type == Confirm R,
If O.len > 3, /* nonempty */
If the option's value is valid,
Set F.value := new value
Otherwise,
Reset connection and return
Set F.state := STABLE
Versions of this diagram and pseudocode are also used by feature
remotes; simply switch the "L"s and "R"s, so that the relevant
options are Change R and Confirm L.
6.6.3. Loss and Retransmission
Packets containing Change and Confirm options might be lost or
delayed by the network. Therefore, Change options are repeatedly
transmitted to achieve reliability. We refer to this as
"retransmission", although of course there are no packet-level
retransmissions in DCCP: a Change option that is sent again will be
sent on a new packet with a new sequence number.
A CHANGING endpoint transmits another Change option once it realizes
that it has not heard back from the other endpoint. The new Change
option need not contain the same payload as the original; reordering
protection will ensure that agreement is reached based on the most
recently transmitted option.
A CHANGING endpoint MUST continue retransmitting Change options until
it gets some response or the connection terminates.
Endpoints SHOULD use an exponential-backoff timer to decide when to
retransmit Change options. (Packets generated specifically for
feature negotiation MUST use such a timer.) The timer interval is
initially set to not less than one round-trip time, and should back
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off to not less than 64 seconds. The backoff protects against
delayed agreement due to the reordering protection algorithms
described in the next section. Again, endpoints may piggyback Change
options on packets they would have sent anyway or create new packets
to carry the options. Any new packets are controlled by the relevant
congestion-control mechanism.
Confirm options are never retransmitted, but the Confirm-sending
endpoint MUST generate a Confirm option after every non-reordered
Change.
6.6.4. Reordering
Reordering might cause packets containing Change and Confirm options
to arrive in an unexpected order. Endpoints MUST ignore feature
negotiation options that do not arrive in strictly-increasing order
by Sequence Number. The rest of this section presents two algorithms
that fulfill this requirement.
The first algorithm introduces two sequence number variables that
each endpoint maintains for the connection.
FGSR Feature Greatest Sequence Number Received: The greatest
sequence number received, considering only valid packets
that contained one or more feature negotiation options
(Change and/or Confirm). This value is initialized to
ISR - 1.
FGSS Feature Greatest Sequence Number Sent: The greatest
sequence number sent, considering only packets that
contained one or more new Change options. A Change option
is new if and only if it was generated during a transition
from the STABLE or UNSTABLE state to the CHANGING state;
Change options generated within the CHANGING state are
retransmissions and MUST have exactly the same contents as
previously transmitted options, allowing tolerance for
reordering. FGSS is initialized to ISS.
Each endpoint checks two conditions on sequence numbers to decide
whether to process received feature negotiation options.
1. If a packet's Sequence Number is less than or equal to FGSR, then
its Change options MUST be ignored.
2. If a packet's Sequence Number is less than or equal to FGSR, if it
has no Acknowledgement Number, OR if its Acknowledgement Number is
less than FGSS, then its Confirm options MUST be ignored.
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Alternatively, an endpoint MAY maintain separate FGSR and FGSS values
for every feature. FGSR(F/X) would equal the greatest sequence
number received, considering only packets that contained Change or
Confirm options applying to feature F/X; FGSS(F/X) would be defined
similarly. This algorithm requires more state, but is slightly more
forgiving to multiple overlapped feature negotiations. Either
algorithm MAY be used; the first algorithm, with connection-wide FGSR
and FGSS variables, is RECOMMENDED.
One consequence of these rules is that a CHANGING endpoint will
ignore any Confirm option that does not acknowledge the latest Change
option sent. This ensures that agreement, once achieved, used the
most recent available information about the endpoints' preferences.
6.6.5. Preference Changes
Endpoints are allowed to change their preference lists at any time.
However, an endpoint that changes its preference list while in the
CHANGING state MUST transition to the UNSTABLE state. It will
transition back to CHANGING once it has transmitted a Change option
with the new preference list. This ensures that agreement is based
on active preference lists. Without the UNSTABLE state, simultaneous
negotiation -- where the endpoints began independent negotiations for
the same feature at the same time -- might lead to the negotiation's
terminating with the endpoints thinking the feature had different
values.
6.6.6. Simultaneous Negotiation
The two endpoints might simultaneously open negotiation for the same
feature, after which an endpoint in the CHANGING state will receive a
Change option for the same feature. Such received Change options can
act as responses to the original Change options. The CHANGING
endpoint MUST examine the received Change's preference list,
reconcile that with its own preference list (as expressed in its
generated Change options), and generate the corresponding Confirm
option. It can then transition to the STABLE state.
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6.6.7. Unknown Features
Endpoints may receive Change options referring to feature numbers
they do not understand -- for instance, when an extended DCCP
converses with a non-extended DCCP. Endpoints MUST respond to
unknown Change options with Empty Confirm options (that is, Confirm
options containing no data), which inform the CHANGING endpoint that
the feature was not understood. However, if the Change option was
Mandatory, the connection MUST be reset; see Section 6.6.9.
On receiving an empty Confirm option for some feature, the CHANGING
endpoint MUST transition back to the STABLE state, leaving the
feature's value unchanged. Section 15 suggests that the default
value for any extension feature correspond to "extension not
available".
Some features are required to be understood by all DCCPs (see Section
6.4). The CHANGING endpoint SHOULD reset the connection (with Reset
Code 5, "Option Error") if it receives an empty Confirm option for
such a feature.
Since Confirm options are generated only in response to Change
options, an endpoint should never receive a Confirm option referring
to a feature number it does not understand. Nevertheless, endpoints
MUST ignore any such options they receive.
6.6.8. Invalid Options
A DCCP endpoint might receive a Change or Confirm option for a known
feature that lists one or more values that it does not understand.
Some, but not all, such options are invalid, depending on the
relevant reconciliation rule (Section 6.3). For instance:
o All features have length limitations, and options with invalid
lengths are invalid. For example, the Ack Ratio feature takes
16-bit values, so valid "Confirm R(Ack Ratio)" options have option
length 5.
o Some non-negotiable features have value limitations. The Ack
Ratio feature takes two-byte, non-zero integer values, so a
"Change L(Ack Ratio, 0)" option is never valid. Note that
server-priority features do not have value limitations, since
unknown values are handled as a matter of course.
o Any Confirm option that selects the wrong value, based on the two
preference lists and the relevant reconciliation rule, is invalid.
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However, unexpected Confirm options -- that refer to unknown feature
numbers, or that don't appear to be part of a current negotiation --
are not invalid, although they are ignored by the receiver.
An endpoint receiving an invalid Change option MUST respond with the
corresponding empty Confirm option. An endpoint receiving an invalid
Confirm option MUST reset the connection, with Reset Code 5, "Option
Error".
6.6.9. Mandatory Feature Negotiation
Change options may be preceded by Mandatory options (Section 5.8.2).
Mandatory Change options are processed like normal Change options
except that the following failure cases will cause the receiver to
reset the connection with Reset Code 6, "Mandatory Failure", rather
than send a Confirm option. The connection MUST be reset if:
o the Change option's feature number was not understood;
o the Change option's value was invalid, and the receiver would
normally have sent an empty Confirm option in response; or
o for server-priority features, there was no shared entry in the two
endpoints' preference lists.
Other failure cases do not cause connection reset; in particular,
reordering protection may cause a Mandatory Change option to be
ignored without resetting the connection.
Confirm options behave identically and have the same reset conditions
whether or not they are Mandatory.
7. Sequence Numbers
DCCP uses sequence numbers to arrange packets into sequence, to
detect losses and network duplicates, and to protect against
attackers, half-open connections, and the delivery of very old
packets. Every packet carries a Sequence Number; most packet types
carry an Acknowledgement Number as well.
DCCP sequence numbers are packet based. That is, Sequence Numbers
generated by each endpoint increase by one, modulo 2^48, per packet.
Even DCCP-Ack and DCCP-Sync packets, and other packets that don't
carry user data, increment the Sequence Number. Since DCCP is an
unreliable protocol, there are no true retransmissions, but effective
retransmissions, such as retransmissions of DCCP-Request packets,
also increment the Sequence Number. This lets DCCP implementations
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detect network duplication, retransmissions, and acknowledgement
loss; it is a significant departure from TCP practice.
7.1. Variables
DCCP endpoints maintain a set of sequence number variables for each
connection.
ISS The Initial Sequence Number Sent by this endpoint. This
equals the Sequence Number of the first DCCP-Request or
DCCP-Response sent.
ISR The Initial Sequence Number Received from the other endpoint.
This equals the Sequence Number of the first DCCP-Request or
DCCP-Response received.
GSS The Greatest Sequence Number Sent by this endpoint. Here,
and elsewhere, "greatest" is measured in circular sequence
space.
GSR The Greatest Sequence Number Received from the other endpoint
on an acknowledgeable packet. (Section 7.4 defines this
term.)
GAR The Greatest Acknowledgement Number Received from the other
endpoint on an acknowledgeable packet that was not a DCCP-
Sync.
Some other variables are derived from these primitives.
SWL and SWH
(Sequence Number Window Low and High) The extremes of the
validity window for received packets' Sequence Numbers.
AWL and AWH
(Acknowledgement Number Window Low and High) The extremes of
the validity window for received packets' Acknowledgement
Numbers.
7.2. Initial Sequence Numbers
The endpoints' initial sequence numbers are set by the first DCCP-
Request and DCCP-Response packets sent. Initial sequence numbers
MUST be chosen to avoid two problems:
o delivery of old packets, where packets lingering in the network
from an old connection are delivered to a new connection with the
same addresses and port numbers; and
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o sequence number attacks, where an attacker can guess the sequence
numbers that a future connection would use [M85].
These problems are the same as those faced by TCP, and DCCP
implementations SHOULD use TCP's strategies to avoid them [RFC793,
RFC1948]. The rest of this section explains these strategies in more
detail.
To address the first problem, an implementation MUST ensure that the
initial sequence number for a given 4-tuple doesn't overlap with
recent sequence numbers on previous connections with the same
4-tuple. ("Recent" means sent within 2 maximum segment lifetimes, or
4 minutes.) The implementation MUST additionally ensure that the
lower 24 bits of the initial sequence number don't overlap with the
lower 24 bits of recent sequence numbers (unless the implementation
plans to avoid short sequence numbers; see Section 7.6). An
implementation that has state for a recent connection with the same
4-tuple can pick a good initial sequence number explicitly.
Otherwise, it could tie initial sequence number selection to some
clock, such as the 4-microsecond clock used by TCP [RFC793]. Two
separate clocks may be required, one for the upper 24 bits and one
for the lower 24 bits.
To address the second problem, an implementation MUST provide each
4-tuple with an independent initial sequence number space. Then,
opening a connection doesn't provide any information about initial
sequence numbers on other connections to the same host. [RFC1948]
achieves this by adding a cryptographic hash of the 4-tuple and a
secret to each initial sequence number. For the secret, [RFC1948]
recommends a combination of some truly random data [RFC4086], an
administratively installed passphrase, the endpoint's IP address, and
the endpoint's boot time, but truly random data is sufficient. Care
should be taken when the secret is changed; such a change alters all
initial sequence number spaces, which might make an initial sequence
number for some 4-tuple equal a recently sent sequence number for the
same 4-tuple. To avoid this problem, the endpoint might remember
dead connection state for each 4-tuple or stay quiet for 2 maximum
segment lifetimes around such a change.
7.3. Quiet Time
DCCP endpoints, like TCP endpoints, must take care before initiating
connections when they boot. In particular, they MUST NOT send
packets whose sequence numbers are close to the sequence numbers of
packets lingering in the network from before the boot. The simplest
way to enforce this rule is for DCCP endpoints to avoid sending any
packets until one maximum segment lifetime (2 minutes) after boot.
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Other enforcement mechanisms include remembering recent sequence
numbers across boots and reserving the upper 8 or so bits of initial
sequence numbers for a persistent counter that decrements by two each
boot. (The latter mechanism would require disallowing packets with
short sequence numbers; see Section 7.6.1.)
7.4. Acknowledgement Numbers
Cumulative acknowledgements are meaningless in an unreliable
protocol. Therefore, DCCP's Acknowledgement Number field has a
different meaning from TCP's.
A received packet is classified as acknowledgeable if and only if its
header was successfully processed by the receiving DCCP. In terms of
the pseudocode in Section 8.5, a received packet becomes
acknowledgeable when the receiving endpoint reaches Step 8. This
means, for example, that all acknowledgeable packets have valid
header checksums and sequence numbers. A sent packet's
Acknowledgement Number MUST equal the sending endpoint's GSR, the
Greatest Sequence Number Received on an acknowledgeable packet, for
all packet types except DCCP-Sync and DCCP-SyncAck.
"Acknowledgeable" does not refer to data processing. Even
acknowledgeable packets may have their application data dropped, due
to receive buffer overflow or corruption, for instance. Data Dropped
options report these data losses when necessary, letting congestion
control mechanisms distinguish between network losses and endpoint
losses. This issue is discussed further in Sections 11.4 and 11.7.
DCCP-Sync and DCCP-SyncAck packets' Acknowledgement Numbers differ as
follows: The Acknowledgement Number on a DCCP-Sync packet corresponds
to a received packet, but not necessarily to an acknowledgeable
packet; in particular, it might correspond to an out-of-sync packet
whose options were not processed. The Acknowledgement Number on a
DCCP-SyncAck packet always corresponds to an acknowledgeable DCCP-
Sync packet; it might be less than GSR in the presence of reordering.
7.5. Validity and Synchronization
Any DCCP endpoint might receive packets that are not actually part of
the current connection. For instance, the network might deliver an
old packet, an attacker might attempt to hijack a connection, or the
other endpoint might crash, causing a half-open connection.
DCCP, like TCP, uses sequence number checks to detect these cases.
Packets whose Sequence and/or Acknowledgement Numbers are out of
range are called sequence-invalid and are not processed normally.
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Unlike TCP, DCCP requires a synchronization mechanism to recover from
large bursts of loss. One endpoint might send so many packets during
a burst of loss that when one of its packets finally got through, the
other endpoint would label its Sequence Number as invalid. A
handshake of DCCP-Sync and DCCP-SyncAck packets recovers from this
case.
7.5.1. Sequence and Acknowledgement Number Windows
Each DCCP endpoint defines sequence validity windows that are subsets
of the Sequence and Acknowledgement Number spaces. These windows
correspond to packets the endpoint expects to receive in the next few
round-trip times. The Sequence and Acknowledgement Number windows
always contain GSR and GSS, respectively. The window widths are
controlled by Sequence Window features for the two half-connections.
The Sequence Number validity window for packets from DCCP B is [SWL,
SWH]. This window always contains GSR, the Greatest Sequence Number
Received on a sequence-valid packet from DCCP B. It is W packets
wide, where W is the value of the Sequence Window/B feature. One-
fourth of the sequence window, rounded down, is less than or equal to
GSR, and three-fourths is greater than GSR. (This asymmetric
placement assumes that bursts of loss are more common in the network
than significant reorderings.)
invalid | valid Sequence Numbers | invalid
<---------*|*===========*=======================*|*--------->
GSR -|GSR + 1 - GSR GSR +|GSR + 1 +
floor(W/4)|floor(W/4) ceil(3W/4)|ceil(3W/4)
= SWL = SWH
The Acknowledgement Number validity window for packets from DCCP B is
[AWL, AWH]. The high end of the window, AWH, equals GSS, the
Greatest Sequence Number Sent by DCCP A; the window is W' packets
wide, where W' is the value of the Sequence Window/A feature.
invalid | valid Acknowledgement Numbers | invalid
<---------*|*===================================*|*--------->
GSS - W'|GSS + 1 - W' GSS|GSS + 1
= AWL = AWH
SWL and AWL are initially adjusted so that they are not less than the
initial Sequence Numbers received and sent, respectively:
SWL := max(GSR + 1 - floor(W/4), ISR),
AWL := max(GSS + 1 - W', ISS).
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These adjustments MUST be applied only at the beginning of the
connection. (Long-lived connections may wrap sequence numbers so
that they appear to be less than ISR or ISS; the adjustments MUST NOT
be applied in that case.)
7.5.2. Sequence Window Feature
The Sequence Window/A feature determines the width of the Sequence
Number validity window used by DCCP B and the width of the
Acknowledgement Number validity window used by DCCP A. DCCP A sends
a "Change L(Sequence Window, W)" option to notify DCCP B that the
Sequence Window/A value is W.
Sequence Window has feature number 3 and is non-negotiable. It takes
48-bit (6-byte) integer values, like DCCP sequence numbers. Change
and Confirm options for Sequence Window are therefore 9 bytes long.
New connections start with Sequence Window 100 for both endpoints.
The minimum valid Sequence Window value is Wmin = 32. The maximum
valid Sequence Window value is Wmax = 2^46 - 1 = 70368744177663.
Change options suggesting Sequence Window values out of this range
are invalid and MUST be handled accordingly.
A proper Sequence Window/A value must reflect the number of packets
DCCP A expects to be in flight. Only DCCP A can anticipate this
number. Values that are too small increase the risk of the endpoints
getting out sync after bursts of loss, and values that are much too
small can prevent productive communication whether or not there is
loss. On the other hand, too-large values increase the risk of
connection hijacking; Section 7.5.5 quantifies this risk. One good
guideline is for each endpoint to set Sequence Window to about five
times the maximum number of packets it expects to send in a round-
trip time. Endpoints SHOULD send Change L(Sequence Window) options,
as necessary, as the connection progresses. Also, an endpoint MUST
NOT persistently send more than its Sequence Window number of packets
per round-trip time; that is, DCCP A MUST NOT persistently send more
than Sequence Window/A packets per RTT.
7.5.3. Sequence-Validity Rules
Sequence-validity depends on the received packet's type. This table
shows the sequence and acknowledgement number checks applied to each
packet; a packet is sequence-valid if it passes both tests, and
sequence-invalid if it does not. Many of the checks refer to the
sequence and acknowledgement number validity windows [SWL, SWH] and
[AWL, AWH] defined in Section 7.5.1.
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Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-Request SWL <= seqno <= SWH (*) N/A
DCCP-Response SWL <= seqno <= SWH (*) AWL <= ackno <= AWH
DCCP-Data SWL <= seqno <= SWH N/A
DCCP-Ack SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-DataAck SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-CloseReq GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Close GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Reset GSR < seqno <= SWH GAR <= ackno <= AWH
DCCP-Sync SWL <= seqno AWL <= ackno <= AWH
DCCP-SyncAck SWL <= seqno AWL <= ackno <= AWH
(*) Check not applied if connection is in LISTEN or REQUEST state.
In general, packets are sequence-valid if their Sequence and
Acknowledgement Numbers lie within the corresponding valid windows,
[SWL, SWH] and [AWL, AWH]. The exceptions to this rule are as
follows:
o Since DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets end a
connection, they cannot have Sequence Numbers less than or equal
to GSR, or Acknowledgement Numbers less than GAR.
o DCCP-Sync and DCCP-SyncAck Sequence Numbers are not strongly
checked. These packet types exist specifically to get the
endpoints back into sync; checking their Sequence Numbers would
eliminate their usefulness.
The lenient checks on DCCP-Sync and DCCP-SyncAck packets allow
continued operation after unusual events, such as endpoint crashes
and large bursts of loss, but there's no need for leniency in the
absence of unusual events -- that is, during ongoing successful
communication. Therefore, DCCP implementations SHOULD use the
following, more stringent checks for active connections, where a
connection is considered active if it has received valid packets from
the other endpoint within the last three round-trip times.
Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-Sync SWL <= seqno <= SWH AWL <= ackno <= AWH
DCCP-SyncAck SWL <= seqno <= SWH AWL <= ackno <= AWH
Finally, an endpoint MAY apply the following more stringent checks to
DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets, further lowering
the probability of successful blind attacks using those packet types.
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Since these checks can cause extra synchronization overhead and delay
connection closing when packets are lost, they should be considered
experimental.
Acknowledgement Number
Packet Type Sequence Number Check Check
----------- --------------------- ----------------------
DCCP-CloseReq seqno == GSR + 1 GAR <= ackno <= AWH
DCCP-Close seqno == GSR + 1 GAR <= ackno <= AWH
DCCP-Reset seqno == GSR + 1 GAR <= ackno <= AWH
Note that sequence-validity is only one of the validity checks
applied to received packets.
7.5.4. Handling Sequence-Invalid Packets
Endpoints respond to received sequence-invalid packets as follows.
o Any sequence-invalid DCCP-Sync or DCCP-SyncAck packet MUST be
ignored.
o A sequence-invalid DCCP-Reset packet MUST elicit a DCCP-Sync
packet in response (subject to a possible rate limit). This
response packet MUST use a new Sequence Number, and thus will
increase GSS; GSR will not change, however, since the received
packet was sequence-invalid. The response packet's
Acknowledgement Number MUST equal GSR.
o Any other sequence-invalid packet MUST elicit a similar DCCP-Sync
packet, except that the response packet's Acknowledgement Number
MUST equal the sequence-invalid packet's Sequence Number.
On receiving a sequence-valid DCCP-Sync packet, the peer endpoint
(say, DCCP B) MUST update its GSR variable and reply with a DCCP-
SyncAck packet. The DCCP-SyncAck packet's Acknowledgement Number
will equal the DCCP-Sync's Sequence Number, which is not necessarily
GSR. Upon receiving this DCCP-SyncAck, which will be sequence-valid
since it acknowledges the DCCP-Sync, DCCP A will update its GSR
variable, and the endpoints will be back in sync. As an exception,
if the peer endpoint is in the REQUEST state, it MUST respond with a
DCCP-Reset instead of a DCCP-SyncAck. This serves to clean up DCCP
A's half-open connection.
To protect against denial-of-service attacks, DCCP implementations
SHOULD impose a rate limit on DCCP-Syncs sent in response to
sequence-invalid packets, such as not more than eight DCCP-Syncs per
second.
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DCCP endpoints MUST NOT process sequence-invalid packets except,
perhaps, by generating a DCCP-Sync. For instance, options MUST NOT
be processed. An endpoint MAY temporarily preserve sequence-invalid
packets in case they become valid later, however; this can reduce the
impact of bursts of loss by delivering more packets to the
application. In particular, an endpoint MAY preserve sequence-
invalid packets for up to 2 round-trip times. If, within that time,
the relevant sequence windows change so that the packets become
sequence-valid, the endpoint MAY process them again.
Note that sequence-invalid DCCP-Reset packets cause DCCP-Syncs to be
generated. This is because endpoints in an unsynchronized state
(CLOSED, REQUEST, and LISTEN) might not have enough information to
generate a proper DCCP-Reset on the first try. For example, if a
peer endpoint is in CLOSED state and receives a DCCP-Data packet, it
cannot guess the right Sequence Number to use on the DCCP-Reset it
generates (since the DCCP-Data packet has no Acknowledgement Number).
The DCCP-Sync generated in response to this bad reset serves as a
challenge, and contains enough information for the peer to generate a
proper DCCP-Reset. However, the new DCCP-Reset may carry a different
Reset Code than the original DCCP-Reset; probably the new Reset Code
will be 3, "No Connection". The endpoint SHOULD use information from
the original DCCP-Reset when possible.
7.5.5. Sequence Number Attacks
Sequence and Acknowledgement Numbers form DCCP's main line of defense
against attackers. An attacker that cannot guess sequence numbers
cannot easily manipulate or hijack a DCCP connection, and
requirements like careful initial sequence number choice eliminate
the most serious attacks.
An attacker might still send many packets with randomly chosen
Sequence and Acknowledgement Numbers, however. If one of those
probes ends up sequence-valid, it may shut down the connection or
otherwise cause problems. The easiest such attacks to execute are as
follows:
o Send DCCP-Data packets with random Sequence Numbers. If one of
these packets hits the valid sequence number window, the attack
packet's application data may be inserted into the data stream.
o Send DCCP-Sync packets with random Sequence and Acknowledgement
Numbers. If one of these packets hits the valid acknowledgement
number window, the receiver will shift its sequence number window
accordingly, getting out of sync with the correct endpoint --
perhaps permanently.
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The attacker has to guess both Source and Destination Ports for any
of these attacks to succeed. Additionally, the connection would have
to be inactive for the DCCP-Sync attack to succeed, assuming the
victim implemented the more stringent checks for active connections
recommended in Section 7.5.3.
To quantify the probability of success, let N be the number of attack
packets the attacker is willing to send, W be the relevant sequence
window width, and L be the length of sequence numbers (24 or 48).
The attacker's best strategy is to space the attack packets evenly
over sequence space. Then the probability of hitting one sequence
number window is P = WN/2^L.
The success probability for a DCCP-Data attack using short sequence
numbers thus equals P = WN/2^24. For W = 100, then, the attacker
must send more than 83,000 packets to achieve a 50% chance of
success. For reference, the easiest TCP attack -- sending a SYN with
a random sequence number, which will cause a connection reset if it
falls within the window -- with W = 8760 (a common default) and
L = 32 requires more than 245,000 packets to achieve a 50% chance of
success.
A fast connection's W will generally be high, increasing the attack
success probability for fixed N. If this probability gets
uncomfortably high with L = 24, the endpoint SHOULD prevent the use
of short sequence numbers by manipulating the Allow Short Sequence
Numbers feature (see Section 7.6.1). The probability limit depends
on the application, however. Some applications, such as those
already designed to handle corruption, are quite resilient to data
injection attacks.
The DCCP-Sync attack has L = 48, since DCCP-Sync packets use long
sequence numbers exclusively; in addition, the success probability is
halved, since only half the Sequence Number space is valid. Attacks
have a correspondingly smaller probability of success. For a large W
of 2000 packets, then, the attacker must send more than 10^11 packets
to achieve a 50% chance of success.
Attacks involving DCCP-Ack, DCCP-DataAck, DCCP-CloseReq, DCCP-Close,
and DCCP-Reset packets are more difficult, since Sequence and
Acknowledgement Numbers must both be guessed. The probability of
attack success for these packet types equals P = WXN/2^(2L), where W
is the Sequence Number window, X is the Acknowledgement Number
window, and N and L are as before.
Since DCCP-Data attacks with short sequence numbers are relatively
easy for attackers to execute, DCCP has been engineered to prevent
these attacks from escalating to connection resets or other serious
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consequences. In particular, any options whose processing might
cause the connection to be reset are ignored when they appear on
DCCP-Data packets.
7.5.6. Sequence Number Handling Examples
In the following example, DCCP A and DCCP B recover from a large
burst of loss that runs DCCP A's sequence numbers out of DCCP B's
appropriate sequence number window.
DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
--> DCCP-Data(seq 2) XXX
...
--> DCCP-Data(seq 100) XXX
--> DCCP-Data(seq 101) --> ???
seqno out of range;
send Sync
OK <-- DCCP-Sync(seq 11, ack 101) <--
(GSS=11,GSR=1)
--> DCCP-SyncAck(seq 102, ack 11) --> OK
(GSS=102,GSR=11) (GSS=11,GSR=102)
In the next example, a DCCP connection recovers from a simple blind
attack.
DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
*ATTACKER* --> DCCP-Data(seq 10^6) --> ???
seqno out of range;
send Sync
??? <-- DCCP-Sync(seq 11, ack 10^6) <--
ackno out of range; ignore
(GSS=1,GSR=10) (GSS=11,GSR=1)
The final example demonstrates recovery from a half-open connection.
DCCP A DCCP B
(GSS=1,GSR=10) (GSS=10,GSR=1)
(Crash)
CLOSED OPEN
REQUEST --> DCCP-Request(seq 400) --> ???
!! <-- DCCP-Sync(seq 11, ack 400) <-- OPEN
REQUEST --> DCCP-Reset(seq 401, ack 11) --> (Abort)
REQUEST CLOSED
REQUEST --> DCCP-Request(seq 402) --> ...
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7.6. Short Sequence Numbers
DCCP sequence numbers are 48 bits long. This large sequence space
protects DCCP connections against some blind attacks, such as the
injection of DCCP-Resets into the connection. However, DCCP-Data,
DCCP-Ack, and DCCP-DataAck packets, which make up the body of any
DCCP connection, may reduce header space by transmitting only the
lower 24 bits of the relevant Sequence and Acknowledgement Numbers.
The receiving endpoint will extend these numbers to 48 bits using the
following pseudocode:
procedure Extend_Sequence_Number(S, REF)
/* S is a 24-bit sequence number from the packet header.
REF is the relevant 48-bit reference sequence number:
GSS if S is an Acknowledgement Number, and GSR if S is a
Sequence Number. */
Set REF_low := low 24 bits of REF
Set REF_hi := high 24 bits of REF
If REF_low (<) S /* circular comparison mod 2^24 */
and S |<| REF_low, /* conventional, non-circular
comparison */
Return (((REF_hi + 1) mod 2^24) << 24) | S
Otherwise, if S (<) REF_low and REF_low |<| S,
Return (((REF_hi - 1) mod 2^24) << 24) | S
Otherwise,
Return (REF_hi << 24) | S
The two different kinds of comparison in the if statements detect
when the low-order bits of the sequence space have wrapped. (The
circular comparison "REF_low (<) S" returns true if and only if
(S - REF_low), calculated using two's-complement arithmetic and then
represented as an unsigned number, is less than or equal to 2^23
(mod 2^24).) When this happens, the high-order bits are incremented
or decremented, as appropriate.
7.6.1. Allow Short Sequence Numbers Feature
Endpoints can require that all packets use long sequence numbers by
leaving the Allow Short Sequence Numbers feature value at its default
of zero. This can reduce the risk that data will be inappropriately
injected into the connection. DCCP A sends a "Change L(Allow Short
Seqnos, 1)" option to indicate its desire to send packets with short
sequence numbers.
Allow Short Sequence Numbers has feature number 2 and is server-
priority. It takes one-byte Boolean values. When Allow Short
Seqnos/B is zero, DCCP B MUST NOT send packets with short sequence
numbers and DCCP A MUST ignore any packets with short sequence
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numbers that are received. Values of two or more are reserved. New
connections start with Allow Short Sequence Numbers 0 for both
endpoints.
7.6.2. When to Avoid Short Sequence Numbers
Short sequence numbers reduce the rate DCCP connections can safely
achieve and increase the risks of certain kinds of attacks, including
blind data injection. Very-high-rate DCCP connections, and
connections with large sequence windows (Section 7.5.2), SHOULD NOT
use short sequence numbers on their data packets. The attack risk
issues have been discussed in Section 7.5.5; we discuss the rate
limitation issue here.
The sequence-validity mechanism assumes that the network does not
deliver extremely old data. In particular, it assumes that the
network must have dropped any packet by the time the connection wraps
around and uses its sequence number again. This constraint limits
the maximum connection rate that can be safely achieved. Let MSL
equal the maximum segment lifetime, P equal the average DCCP packet
size in bits, and L equal the length of sequence numbers (24 or 48
bits). Then the maximum safe rate, in bits per second, is
R = P*(2^L)/2MSL.
For the default MSL of 2 minutes, 1500-byte DCCP packets, and short
sequence numbers, the safe rate is therefore approximately 800 Mb/s.
Although 2 minutes is a very large MSL for any networks that could
sustain that rate with such small packets, long sequence numbers
allow much higher rates under the same constraints: up to 14 petabits
a second for 1500-byte packets and the default MSL.
7.7. NDP Count and Detecting Application Loss
DCCP's sequence numbers increment by one on every packet, including
non-data packets (packets that don't carry application data). This
makes DCCP sequence numbers suitable for detecting any network loss,
but not for detecting the loss of application data. The NDP Count
option reports the length of each burst of non-data packets. This
lets the receiving DCCP reliably determine when a burst of loss
included application data.
+--------+--------+-------- ... --------+
|00100101| Length | NDP Count |
+--------+--------+-------- ... --------+
Type=37 Len=3-8 (1-6 bytes)
If a DCCP endpoint's Send NDP Count feature is one (see below), then
that endpoint MUST send an NDP Count option on every packet whose
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immediate predecessor was a non-data packet. Non-data packets
consist of DCCP packet types DCCP-Ack, DCCP-Close, DCCP-CloseReq,
DCCP-Reset, DCCP-Sync, and DCCP-SyncAck. The other packet types,
namely DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck, are
considered data packets, although not all DCCP-Request and DCCP-
Response packets will actually carry application data.
The value stored in NDP Count equals the number of consecutive non-
data packets in the run immediately previous to the current packet.
Packets with no NDP Count option are considered to have NDP Count
zero.
The NDP Count option can carry one to six bytes of data. The
smallest option format that can hold the NDP Count SHOULD be used.
With NDP Count, the receiver can reliably tell only whether a burst
of loss contained at least one data packet. For example, the
receiver cannot always tell whether a burst of loss contained a non-
data packet.
7.7.1. NDP Count Usage Notes
Say that K consecutive sequence numbers are missing in some burst of
loss, and that the Send NDP Count feature is on. Then some
application data was lost within those sequence numbers unless the
packet following the hole contains an NDP Count option whose value is
greater than or equal to K.
For example, say that an endpoint sent the following sequence of
non-data packets (Nx) and data packets (Dx).
N0 N1 D2 N3 D4 D5 N6 D7 D8 D9 D10 N11 N12 D13
Those packets would have NDP Counts as follows.
N0 N1 D2 N3 D4 D5 N6 D7 D8 D9 D10 N11 N12 D13
- 1 2 - 1 - - 1 - - - - 1 2
NDP Count is not useful for applications that include their own
sequence numbers with their packet headers.
7.7.2. Send NDP Count Feature
The Send NDP Count feature lets DCCP endpoints negotiate whether they
should send NDP Count options on their packets. DCCP A sends a
"Change R(Send NDP Count, 1)" option to ask DCCP B to send NDP Count
options.
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Send NDP Count has feature number 7 and is server-priority. It takes
one-byte Boolean values. DCCP B MUST send NDP Count options as
described above when Send NDP Count/B is one, although it MAY send
NDP Count options even when Send NDP Count/B is zero. Values of two
or more are reserved. New connections start with Send NDP Count 0
for both endpoints.
8. Event Processing
This section describes how DCCP connections move between states and
which packets are sent when. Note that feature negotiation takes
place in parallel with the connection-wide state transitions
described here.
8.1. Connection Establishment
DCCP connections' initiation phase consists of a three-way handshake:
an initial DCCP-Request packet sent by the client, a DCCP-Response
sent by the server in reply, and finally an acknowledgement from the
client, usually via a DCCP-Ack or DCCP-DataAck packet. The client
moves from the REQUEST state to PARTOPEN, and finally to OPEN; the
server moves from LISTEN to RESPOND, and finally to OPEN.
Client State Server State
CLOSED LISTEN
1. REQUEST --> Request -->
2. <-- Response <-- RESPOND
3. PARTOPEN --> Ack, DataAck -->
4. <-- Data, Ack, DataAck <-- OPEN
5. OPEN <-> Data, Ack, DataAck <-> OPEN
8.1.1. Client Request
When a client decides to initiate a connection, it enters the REQUEST
state, chooses an initial sequence number (Section 7.2), and sends a
DCCP-Request packet using that sequence number to the intended
server.
DCCP-Request packets will commonly carry feature negotiation options
that open negotiations for various connection parameters, such as
preferred congestion control IDs for each half-connection. They may
also carry application data, but the client should be aware that the
server may not accept such data.
A client in the REQUEST state SHOULD use an exponential-backoff timer
to send new DCCP-Request packets if no response is received. The
first retransmission should occur after approximately one second,
backing off to not less than one packet every 64 seconds; or the
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endpoint can use whatever retransmission strategy is followed for
retransmitting TCP SYNs. Each new DCCP-Request MUST increment the
Sequence Number by one and MUST contain the same Service Code and
application data as the original DCCP-Request.
A client MAY give up on its DCCP-Requests after some time (3 minutes,
for example). When it does, it SHOULD send a DCCP-Reset packet to
the server with Reset Code 2, "Aborted", to clean up state in case
one or more of the Requests actually arrived. A client in REQUEST
state has never received an initial sequence number from its peer, so
the DCCP-Reset's Acknowledgement Number MUST be set to zero.
The client leaves the REQUEST state for PARTOPEN when it receives a
DCCP-Response from the server.
8.1.2. Service Codes
Each DCCP-Request contains a 32-bit Service Code, which identifies
the application-level service to which the client application is
trying to connect. Service Codes should correspond to application
services and protocols. For example, there might be a Service Code
for SIP control connections and one for RTP audio connections.
Middleboxes, such as firewalls, can use the Service Code to identify
the application running on a nonstandard port (assuming the DCCP
header has not been encrypted).
Endpoints MUST associate a Service Code with every DCCP socket, both
actively and passively opened. The application will generally supply
this Service Code. Each active socket MUST have exactly one Service
Code. Passive sockets MAY, at the implementation's discretion, be
associated with more than one Service Code; this might let multiple
applications, or multiple versions of the same application, listen on
the same port, differentiated by Service Code. If the DCCP-Request's
Service Code doesn't equal any of the server's Service Codes for the
given port, the server MUST reject the request by sending a DCCP-
Reset packet with Reset Code 8, "Bad Service Code". A middlebox MAY
also send such a DCCP-Reset in response to packets whose Service Code
is considered unsuitable.
Service Codes are not intended to be DCCP-specific and are allocated
by IANA. Following the policies outlined in [RFC2434], most Service
Codes are allocated First Come First Served, subject to the following
guidelines.
o Service Codes are allocated one at a time, or in small blocks. A
short English description of the intended service is REQUIRED to
obtain a Service Code assignment, but no specification, standards
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track or otherwise, is necessary. IANA maintains an association
of Service Codes to the corresponding phrases.
o Users request specific Service Code values. We suggest that users
request Service Codes that can be represented using the "SC:"
formatting convention described below. Thus, the "Frobodyne Plotz
Protocol" might correspond to Service Code 17178548426 or,
equivalently, "SC:fdpz". The canonical interpretation of a
Service Code field is numeric.
o Service Codes whose bytes each have values in the set {32, 45-57,
65-90} use a Specification Required allocation policy. That is,
these Service Codes are used for international standard or
standards-track specifications, IETF or otherwise. (This set
consists of the ASCII digits, uppercase letters, and characters
space, '-', '.', and '/'.)
o Service Codes whose high-order byte equals 63 (ASCII '?') are
reserved for Private Use.
o Service Code 0 represents the absence of a meaningful Service Code
and MUST NOT be allocated.
o The value 4294967295 is an invalid Service Code. Servers MUST
reject any DCCP-Request with this Service Code value by sending a
DCCP-Reset packet with Reset Code 8, "Bad Service Code".
This design for Service Code allocation is based on the allocation of
4-byte identifiers for Macintosh resources, PNG chunks, and TrueType
and OpenType tables.
In text settings, we recommend that Service Codes be written in one
of three forms, prefixed by the ASCII letters SC and either a colon
":" or equals sign "=". These forms are interpreted as follows.
SC: Indicates a Service Code representable using a subset of the
ASCII characters. The colon is followed by one to four
characters taken from the following set: letters, digits, and
the characters in "-_+.*/?@" (not including quotes).
Numerically, these characters have values in {42-43, 45-57,
63-90, 95, 97-122}. The Service Code is calculated by
padding the string on the right with spaces (value 32) and
intepreting the four-character result as a 32-bit big-endian
number.
SC= Indicates a decimal Service Code. The equals sign is
followed by any number of decimal digits, which specify the
Service Code. Values above 4294967294 are illegal.
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SC=x or SC=X
Indicates a hexadecimal Service Code. The "x" or "X" is
followed by any number of hexadecimal digits (upper or lower
case), which specify the Service Code. Values above
4294967294 are illegal.
Thus, the Service Code 1717858426 might be represented in text as
either SC:fdpz, SC=1717858426, or SC=x6664707A.
8.1.3. Server Response
In the second phase of the three-way handshake, the server moves from
the LISTEN state to RESPOND and sends a DCCP-Response message to the
client. In this phase, a server will often specify the features it
would like to use, either from among those the client requested or in
addition to those. Among these options is the congestion control
mechanism the server expects to use.
The server MAY respond to a DCCP-Request packet with a DCCP-Reset
packet to refuse the connection. Relevant Reset Codes for refusing a
connection include 7, "Connection Refused", when the DCCP-Request's
Destination Port did not correspond to a DCCP port open for
listening; 8, "Bad Service Code", when the DCCP-Request's Service
Code did not correspond to the service code registered with the
Destination Port; and 9, "Too Busy", when the server is currently too
busy to respond to requests. The server SHOULD limit the rate at
which it generates these resets; for example, to not more than 1024
per second.
The server SHOULD NOT retransmit DCCP-Response packets; the client
will retransmit the DCCP-Request if necessary. (Note that the
"retransmitted" DCCP-Request will have, at least, a different
sequence number from the "original" DCCP-Request. The server can
thus distinguish true retransmissions from network duplicates.) The
server will detect that the retransmitted DCCP-Request applies to an
existing connection because of its Source and Destination Ports.
Every valid DCCP-Request received while the server is in the RESPOND
state MUST elicit a new DCCP-Response. Each new DCCP-Response MUST
increment the server's Sequence Number by one and MUST include the
same application data, if any, as the original DCCP-Response.
The server MUST NOT accept more than one piece of DCCP-Request
application data per connection. In particular, the DCCP-Response
sent in reply to a retransmitted DCCP-Request with application data
SHOULD contain a Data Dropped option, in which the retransmitted
DCCP-Request data is reported with Drop Code 0, Protocol Constraints.
The original DCCP-Request SHOULD also be reported in the Data Dropped
option, either in a Normal Block (if the server accepted the data or
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there was no data) or in a Drop Code 0 Drop Block (if the server
refused the data the first time as well).
The Data Dropped and Init Cookie options are particularly useful for
DCCP-Response packets (Sections 11.7 and 8.1.4).
The server leaves the RESPOND state for OPEN when it receives a valid
DCCP-Ack from the client, completing the three-way handshake. It MAY
also leave the RESPOND state for CLOSED after a timeout of not less
than 4MSL (8 minutes); when doing so, it SHOULD send a DCCP-Reset
with Reset Code 2, "Aborted", to clean up state at the client.
8.1.4. Init Cookie Option
+--------+--------+--------+--------+--------+--------
|00100100| Length | Init Cookie Value ...
+--------+--------+--------+--------+--------+--------
Type=36
The Init Cookie option lets a DCCP server avoid having to hold any
state until the three-way connection setup handshake has completed,
in a similar fashion as for TCP SYN cookies [SYNCOOKIES]. The server
wraps up the Service Code, server port, and any options it cares
about from both the DCCP-Request and DCCP-Response in an opaque
cookie. Typically the cookie will be encrypted using a secret known
only to the server and will include a cryptographic checksum or magic
value so that correct decryption can be verified. When the server
receives the cookie back in the response, it can decrypt the cookie
and instantiate all the state it avoided keeping. In the meantime,
it need not move from the LISTEN state.
The Init Cookie option MUST NOT be sent on DCCP-Request or DCCP-Data
packets. Any Init Cookie options received on DCCP-Request or DCCP-
Data packets, or after the connection has been established (when the
connection's state is >= OPEN), MUST be ignored. The server MAY
include Init Cookie options in its DCCP-Response. If so, then the
client MUST echo the same Init Cookie options, in the same order, in
each succeeding DCCP packet until one of those packets is
acknowledged (showing that the three-way handshake has completed) or
the connection is reset. As a result, the client MUST NOT use DCCP-
Data packets until the three-way handshake completes or the
connection is reset. The Init Cookie options on a client packet MUST
equal those received on the DCCP-Request indicated by the client
packet's Acknowledgement Number. The server SHOULD design its Init
Cookie format so that Init Cookies can be checked for tampering; it
SHOULD respond to a tampered Init Cookie option by resetting the
connection with Reset Code 10, "Bad Init Cookie".
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Init Cookie's precise implementation need not be specified here;
since Init Cookies are opaque to the client, there are no
interoperability concerns. An example cookie format might encrypt
(using a secret key) the connection's initial sequence and
acknowledgement numbers, ports, Service Code, any options included on
the DCCP-Request packet and the corresponding DCCP-Response, a random
salt, and a magic number. On receiving a reflected Init Cookie, the
server would decrypt the cookie, validate it by checking its magic
number, sequence numbers, and ports, and, if valid, create a
corresponding socket using the options.
Each individual Init Cookie option can hold at most 253 bytes of
data, but a server can send multiple Init Cookie options to gain more
space.
8.1.5. Handshake Completion
When the client receives a DCCP-Response from the server, it moves
from the REQUEST state to PARTOPEN and completes the three-way
handshake by sending a DCCP-Ack packet to the server. The client
remains in PARTOPEN until it can be sure that the server has received
some packet the client sent from PARTOPEN (either the initial DCCP-
Ack or a later packet). Clients in the PARTOPEN state that want to
send data MUST do so using DCCP-DataAck packets, not DCCP-Data
packets. This is because DCCP-Data packets lack Acknowledgement
Numbers, so the server can't tell from a DCCP-Data packet whether the
client saw its DCCP-Response. Furthermore, if the DCCP-Response
included an Init Cookie, that Init Cookie MUST be included on every
packet sent in PARTOPEN.
The single DCCP-Ack sent when entering the PARTOPEN state might, of
course, be dropped by the network. The client SHOULD ensure that
some packet gets through eventually. The preferred mechanism would
be a roughly 200-millisecond timer, set every time a packet is
transmitted in PARTOPEN. If this timer goes off and the client is
still in PARTOPEN, the client generates another DCCP-Ack and backs
off the timer. If the client remains in PARTOPEN for more than 4MSL
(8 minutes), it SHOULD reset the connection with Reset Code 2,
"Aborted".
The client leaves the PARTOPEN state for OPEN when it receives a
valid packet other than DCCP-Response, DCCP-Reset, or DCCP-Sync from
the server.
8.2. Data Transfer
In the central data transfer phase of the connection, both server and
client are in the OPEN state.
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DCCP A sends DCCP-Data and DCCP-DataAck packets to DCCP B due to
application events on host A. These packets are congestion-
controlled by the CCID for the A-to-B half-connection. In contrast,
DCCP-Ack packets sent by DCCP A are controlled by the CCID for the
B-to-A half-connection. Generally, DCCP A will piggyback
acknowledgement information on DCCP-Data packets when acceptable,
creating DCCP-DataAck packets. DCCP-Ack packets are used when there
is no data to send from DCCP A to DCCP B, or when the congestion
state of the A-to-B CCID will not allow data to be sent.
DCCP-Sync and DCCP-SyncAck packets may also occur in the data
transfer phase. Some cases causing DCCP-Sync generation are
discussed in Section 7.5. One important distinction between DCCP-
Sync packets and other packet types is that DCCP-Sync elicits an
immediate acknowledgement. On receiving a valid DCCP-Sync packet, a
DCCP endpoint MUST immediately generate and send a DCCP-SyncAck
response (subject to any implementation rate limits); the
Acknowledgement Number on that DCCP-SyncAck MUST equal the Sequence
Number of the DCCP-Sync.
A particular DCCP implementation might decide to initiate feature
negotiation only once the OPEN state was reached, in which case it
might not allow data transfer until some time later. Data received
during that time SHOULD be rejected and reported using a Data Dropped
Drop Block with Drop Code 0, Protocol Constraints (see Section 11.7).
8.3. Termination
DCCP connection termination uses a handshake consisting of an
optional DCCP-CloseReq packet, a DCCP-Close packet, and a DCCP-Reset
packet. The server moves from the OPEN state, possibly through the
CLOSEREQ state, to CLOSED; the client moves from OPEN through CLOSING
to TIMEWAIT, and after 2MSL wait time (4 minutes) to CLOSED.
The sequence DCCP-CloseReq, DCCP-Close, DCCP-Reset is used when the
server decides to close the connection but doesn't want to hold
TIMEWAIT state:
Client State Server State
OPEN OPEN
1. <-- CloseReq <-- CLOSEREQ
2. CLOSING --> Close -->
3. <-- Reset <-- CLOSED (LISTEN)
4. TIMEWAIT
5. CLOSED
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A shorter sequence occurs when the client decides to close the
connection.
Client State Server State
OPEN OPEN
1. CLOSING --> Close -->
2. <-- Reset <-- CLOSED (LISTEN)
3. TIMEWAIT
4. CLOSED
Finally, the server can decide to hold TIMEWAIT state:
Client State Server State
OPEN OPEN
1. <-- Close <-- CLOSING
2. CLOSED --> Reset -->
3. TIMEWAIT
4. CLOSED (LISTEN)
In all cases, the receiver of the DCCP-Reset packet holds TIMEWAIT
state for the connection. As in TCP, TIMEWAIT state, where an
endpoint quietly preserves a socket for 2MSL (4 minutes) after its
connection has closed, ensures that no connection duplicating the
current connection's source and destination addresses and ports can
start up while old packets might remain in the network.
The termination handshake proceeds as follows. The receiver of a
valid DCCP-CloseReq packet MUST respond with a DCCP-Close packet.
The receiver of a valid DCCP-Close packet MUST respond with a DCCP-
Reset packet with Reset Code 1, "Closed". The receiver of a valid
DCCP-Reset packet -- which is also the sender of the DCCP-Close
packet (and possibly the receiver of the DCCP-CloseReq packet) --
will hold TIMEWAIT state for the connection.
A DCCP-Reset packet completes every DCCP connection, whether the
termination is clean (due to application close; Reset Code 1,
"Closed") or unclean. Unlike TCP, which has two distinct termination
mechanisms (FIN and RST), DCCP ends all connections in a uniform
manner. This is justified because some aspects of connection
termination are the same independent of whether termination was
clean. For instance, the endpoint that receives a valid DCCP-Reset
SHOULD hold TIMEWAIT state for the connection. Processors that must
distinguish between clean and unclean termination can examine the
Reset Code. DCCP implementations generally transition to the CLOSED
state after sending a DCCP-Reset packet.
Endpoints in the CLOSEREQ and CLOSING states MUST retransmit DCCP-
CloseReq and DCCP-Close packets, respectively, until leaving those
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states. The retransmission timer should initially be set to go off
in two round-trip times and should back off to not less than once
every 64 seconds if no relevant response is received.
Only the server can send a DCCP-CloseReq packet or enter the CLOSEREQ
state. A server receiving a sequence-valid DCCP-CloseReq packet MUST
respond with a DCCP-Sync packet and otherwise ignore the DCCP-
CloseReq.
DCCP-Data, DCCP-DataAck, and DCCP-Ack packets received in CLOSEREQ or
CLOSING states MAY be either processed or ignored.
8.3.1. Abnormal Termination
DCCP endpoints generate DCCP-Reset packets to terminate connections
abnormally; a DCCP-Reset packet may be generated from any state.
Resets sent in the CLOSED, LISTEN, and TIMEWAIT states use Reset Code
3, "No Connection", unless otherwise specified. Resets sent in the
REQUEST or RESPOND states use Reset Code 4, "Packet Error", unless
otherwise specified.
DCCP endpoints in CLOSED, LISTEN, or TIMEWAIT state may need to
generate a DCCP-Reset packet in response to a packet received from a
peer. Since these states have no associated sequence number
variables, the Sequence and Acknowledgement Numbers on the DCCP-Reset
packet R are taken from the received packet P, as follows.
1. If P.ackno exists, then set R.seqno := P.ackno + 1. Otherwise,
set R.seqno := 0.
2. Set R.ackno := P.seqno.
3. If the packet used short sequence numbers (P.X == 0), then set the
upper 24 bits of R.seqno and R.ackno to 0.
8.4. DCCP State Diagram
The most common state transitions discussed above can be summarized
in the following state diagram. The diagram is illustrative; the
text in Section 8.5 and elsewhere should be considered definitive.
For example, there are arcs (not shown) from every state except
CLOSED to TIMEWAIT, contingent on the receipt of a valid DCCP-Reset.
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+---------------------------+ +---------------------------+
| v v |
| +----------+ |
| +-------------+ CLOSED +------------+ |
| | passive +----------+ active | |
| | open open | |
| | snd Request | |
| v v |
| +----------+ +----------+ |
| | LISTEN | | REQUEST | |
| +----+-----+ +----+-----+ |
| | rcv Request rcv Response | |
| | snd Response snd Ack | |
| v v |
| +----------+ +----------+ |
| | RESPOND | | PARTOPEN | |
| +----+-----+ +----+-----+ |
| | rcv Ack/DataAck rcv packet | |
| | | |
| | +----------+ | |
| +------------>| OPEN |<-----------+ |
| +--+-+--+--+ |
| server active close | | | active close |
| snd CloseReq | | | or rcv CloseReq |
| | | | snd Close |
| | | | |
| +----------+ | | | +----------+ |
| | CLOSEREQ |<---------+ | +--------->| CLOSING | |
| +----+-----+ | +----+-----+ |
| | rcv Close | rcv Reset | |
| | snd Reset | | |
|<---------+ | v |
| | +----+-----+ |
| rcv Close | | TIMEWAIT | |
| snd Reset | +----+-----+ |
+-----------------------------+ | |
+-----------+
2MSL timer expires
8.5. Pseudocode
This section presents an algorithm describing the processing steps a
DCCP endpoint must go through when it receives a packet. A DCCP
implementation need not implement the algorithm as it is described
here, but any implementation MUST generate observable effects exactly
as indicated by this pseudocode, except where allowed otherwise by
another part of this document.
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The received packet is written as P, the socket as S. Socket
variables are:
S.SWL - sequence number window low
S.SWH - sequence number window high
S.AWL - acknowledgement number window low
S.AWH - acknowledgement number window high
S.ISS - initial sequence number sent
S.ISR - initial sequence number received
S.OSR - first OPEN sequence number received
S.GSS - greatest sequence number sent
S.GSR - greatest valid sequence number received
S.GAR - greatest valid acknowledgement number received on a
non-Sync; initialized to S.ISS
"Send packet" actions always use, and increment, S.GSS.
Step 1: Check header basics
/* This step checks for malformed packets. Packets that fail
these checks are ignored -- they do not receive Resets in
response */
If the packet is shorter than 12 bytes, drop packet and return
If P.type is not understood, drop packet and return
If P.Data Offset is smaller than the given packet type's
fixed header length or larger than the packet's length,
drop packet and return
If P.type is not Data, Ack, or DataAck and P.X == 0 (the packet
has short sequence numbers), drop packet and return
If the header checksum is incorrect, drop packet and return
If P.CsCov is too large for the packet size, drop packet and
return
Step 2: Check ports and process TIMEWAIT state
/* Flow ID is 4-tuple */
Look up flow ID in table and get corresponding socket
If no socket, or S.state == TIMEWAIT,
/* The following Reset's Sequence and Acknowledgement Numbers
are taken from the input packet; see Section 8.3.1. */
Generate Reset(No Connection) unless P.type == Reset
Drop packet and return
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Step 3: Process LISTEN state
If S.state == LISTEN,
If P.type == Request or P contains a valid Init Cookie option,
/* Must scan the packet's options to check for Init
Cookies. Only Init Cookies are processed here,
however; other options are processed in Step 8. This
scan need only be performed if the endpoint uses Init
Cookies */
/* Generate a new socket and switch to that socket */
Set S := new socket for this port pair
S.state = RESPOND
Choose S.ISS (initial seqno) or set from Init Cookies
Initialize S.GAR := S.ISS
Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init Cookies
Continue with S.state == RESPOND
/* A Response packet will be generated in Step 11 */
Otherwise,
Generate Reset(No Connection) unless P.type == Reset
Drop packet and return
Step 4: Prepare sequence numbers in REQUEST
If S.state == REQUEST,
If (P.type == Response or P.type == Reset)
and S.AWL <= P.ackno <= S.AWH,
/* Set sequence number variables corresponding to the
other endpoint, so P will pass the tests in Step 6 */
Set S.GSR, S.ISR, S.SWL, S.SWH
/* Response processing continues in Step 10; Reset
processing continues in Step 9 */
Otherwise,
/* Only Response and Reset are valid in REQUEST state */
Generate Reset(Packet Error)
Drop packet and return
Step 5: Prepare sequence numbers for Sync
If P.type == Sync or P.type == SyncAck,
If S.AWL <= P.ackno <= S.AWH and P.seqno >= S.SWL,
/* P is valid, so update sequence number variables
accordingly. After this update, P will pass the tests
in Step 6. A SyncAck is generated if necessary in
Step 15 */
Update S.GSR, S.SWL, S.SWH
Otherwise,
Drop packet and return
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Step 6: Check sequence numbers
If P.X == 0 and the relevant Allow Short Seqnos feature is 0,
/* Packet has short seqnos, but short seqnos not allowed */
Drop packet and return
Otherwise, if P.X == 0,
Extend P.seqno and P.ackno to 48 bits using the procedure
in Section 7.6
Let LSWL = S.SWL and LAWL = S.AWL
If P.type == CloseReq or P.type == Close or P.type == Reset,
LSWL := S.GSR + 1, LAWL := S.GAR
If LSWL <= P.seqno <= S.SWH
and (P.ackno does not exist or LAWL <= P.ackno <= S.AWH),
Update S.GSR, S.SWL, S.SWH
If P.type != Sync,
Update S.GAR
Otherwise,
If P.type == Reset,
Send Sync packet acknowledging S.GSR
Otherwise,
Send Sync packet acknowledging P.seqno
Drop packet and return
Step 7: Check for unexpected packet types
If (S.is_server and P.type == CloseReq)
or (S.is_server and P.type == Response)
or (S.is_client and P.type == Request)
or (S.state >= OPEN and P.type == Request
and P.seqno >= S.OSR)
or (S.state >= OPEN and P.type == Response
and P.seqno >= S.OSR)
or (S.state == RESPOND and P.type == Data),
Send Sync packet acknowledging P.seqno
Drop packet and return
Step 8: Process options and mark acknowledgeable
/* Option processing is not specifically described here.
Certain options, such as Mandatory, may cause the connection
to be reset, in which case Steps 9 and on are not executed */
Mark packet as acknowledgeable (in Ack Vector terms, Received
or Received ECN Marked)
Step 9: Process Reset
If P.type == Reset,
Tear down connection
S.state := TIMEWAIT
Set TIMEWAIT timer
Drop packet and return
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RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
Step 10: Process REQUEST state (second part)
If S.state == REQUEST,
/* If we get here, P is a valid Response from the server (see
Step 4), and we should move to PARTOPEN state. PARTOPEN
means send an Ack, don't send Data packets, retransmit
Acks periodically, and always include any Init Cookie from
the Response */
S.state := PARTOPEN
Set PARTOPEN timer
Continue with S.state == PARTOPEN
/* Step 12 will send the Ack completing the three-way
handshake */
Step 11: Process RESPOND state
If S.state == RESPOND,
If P.type == Request,
Send Response, possibly containing Init Cookie
If Init Cookie was sent,
Destroy S and return
/* Step 3 will create another socket when the client
completes the three-way handshake */
Otherwise,
S.OSR := P.seqno
S.state := OPEN
Step 12: Process PARTOPEN state
If S.state == PARTOPEN,
If P.type == Response,
Send Ack
Otherwise, if P.type != Sync,
S.OSR := P.seqno
S.state := OPEN
Step 13: Process CloseReq
If P.type == CloseReq and S.state < CLOSEREQ,
Generate Close
S.state := CLOSING
Set CLOSING timer
Step 14: Process Close
If P.type == Close,
Generate Reset(Closed)
Tear down connection
Drop packet and return
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Step 15: Process Sync
If P.type == Sync,
Generate SyncAck
Step 16: Process data
/* At this point any application data on P can be passed to the
application, except that the application MUST NOT receive
data from more than one Request or Response */
9. Checksums
DCCP uses a header checksum to protect its header against corruption.
Generally, this checksum also covers any application data. DCCP
applications can, however, request that the header checksum cover
only part of the application data, or perhaps no application data at
all. Link layers may then reduce their protection on unprotected
parts of DCCP packets. For some noisy links, and for applications
that can tolerate corruption, this can greatly improve delivery rates
and perceived performance.
Checksum coverage may eventually impact congestion control mechanisms
as well. A packet with corrupt application data and complete
checksum coverage is treated as lost. This incurs a heavy-duty loss
response from the sender's congestion control mechanism, which can
unfairly penalize connections on links with high background
corruption. The combination of reduced checksum coverage and Data
Checksum options may let endpoints report packets as corrupt rather
than dropped, using Data Dropped options and Drop Code 3 (see Section
11.7). This may eventually benefit applications. However, further
research is required to determine an appropriate response to
corruption, which can sometimes correlate with congestion. Corrupt
packets currently incur a loss response.
The Data Checksum option, which contains a strong CRC, lets endpoints
detect application data corruption. An API can then be used to avoid
delivering corrupt data to the application, even if links deliver
corrupt data to the endpoint due to reduced checksum coverage.
However, the use of reduced checksum coverage for applications that
demand correct data is currently considered experimental. This is
because the combined loss-plus-corruption rate for packets with
reduced checksum coverage may be significantly higher than that for
packets with full checksum coverage, although the loss rate will
generally be lower. Actual behavior will depend on link design;
further research and experience is required.
Reduced checksum coverage introduces some security considerations;
see Section 18.1. See Appendix B for further motivation and
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discussion. DCCP's implementation of reduced checksum coverage was
inspired by UDP-Lite [RFC3828].
9.1. Header Checksum Field
DCCP uses the TCP/IP checksum algorithm. The Checksum field in the
DCCP generic header (see Section 5.1) equals the 16-bit one's
complement of the one's complement sum of all 16-bit words in the
DCCP header, DCCP options, a pseudoheader taken from the network-
layer header, and, depending on the value of the Checksum Coverage
field, some or all of the application data. When calculating the
checksum, the Checksum field itself is treated as 0. If a packet
contains an odd number of header and payload bytes to be checksummed,
8 zero bits are added on the right to form a 16-bit word for checksum
purposes. The pad byte is not transmitted as part of the packet.
The pseudoheader is calculated as for TCP. For IPv4, it is 96 bits
long and consists of the IPv4 source and destination addresses, the
IP protocol number for DCCP (padded on the left with 8 zero bits),
and the DCCP length as a 16-bit quantity (the length of the DCCP
header with options, plus the length of any data); see [RFC793],
Section 3.1. For IPv6, it is 320 bits long, and consists of the IPv6
source and destination addresses, the DCCP length as a 32-bit
quantity, and the IP protocol number for DCCP (padded on the left
with 24 zero bits); see [RFC2460], Section 8.1.
Packets with invalid header checksums MUST be ignored. In
particular, their options MUST NOT be processed.
9.2. Header Checksum Coverage Field
The Checksum Coverage field in the DCCP generic header (see Section
5.1) specifies what parts of the packet are covered by the Checksum
field, as follows:
CsCov = 0 The Checksum field covers the DCCP header, DCCP
options, network-layer pseudoheader, and all
application data in the packet, possibly padded on the
right with zeros to an even number of bytes.
CsCov = 1-15 The Checksum field covers the DCCP header, DCCP
options, network-layer pseudoheader, and the initial
(CsCov-1)*4 bytes of the packet's application data.
Thus, if CsCov is 1, none of the application data is protected by the
header checksum. The value (CsCov-1)*4 MUST be less than or equal to
the length of the application data. Packets with invalid CsCov
values MUST be ignored; in particular, their options MUST NOT be
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processed. The meanings of values other than 0 and 1 should be
considered experimental.
Values other than 0 specify that corruption is acceptable in some or
all of the DCCP packet's application data. In fact, DCCP cannot even
detect corruption in areas not covered by the header checksum, unless
the Data Checksum option is used. Applications should not make any
assumptions about the correctness of received data not covered by the
checksum and should, if necessary, introduce their own validity
checks.
A DCCP application interface should let sending applications suggest
a value for CsCov for sent packets, defaulting to 0 (full coverage).
The Minimum Checksum Coverage feature, described below, lets an
endpoint refuse delivery of application data on packets with partial
checksum coverage; by default, only fully covered application data is
accepted. Lower layers that support partial error detection MAY use
the Checksum Coverage field as a hint of where errors do not need to
be detected. Lower layers MUST use a strong error detection
mechanism to detect at least errors that occur in the sensitive part
of the packet, and to discard damaged packets. The sensitive part
consists of the bytes between the first byte of the IP header and the
last byte identified by Checksum Coverage.
For more details on application and lower-layer interface issues
relating to partial checksumming, see [RFC3828].
9.2.1. Minimum Checksum Coverage Feature
The Minimum Checksum Coverage feature lets a DCCP endpoint determine
whether its peer is willing to accept packets with reduced Checksum
Coverage. For example, DCCP A sends a "Change R(Minimum Checksum
Coverage, 1)" option to DCCP B to check whether B is willing to
accept packets with Checksum Coverage set to 1.
Minimum Checksum Coverage has feature number 8 and is server-
priority. It takes one-byte integer values between 0 and 15; values
of 16 or more are reserved. Minimum Checksum Coverage/B reflects
values of Checksum Coverage that DCCP B finds unacceptable. Say that
the value of Minimum Checksum Coverage/B is MinCsCov. Then:
o If MinCsCov = 0, then DCCP B only finds packets with CsCov = 0
acceptable.
o If MinCsCov > 0, then DCCP B additionally finds packets with
CsCov >= MinCsCov acceptable.
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DCCP B MAY refuse to process application data from packets with
unacceptable Checksum Coverage. Such packets SHOULD be reported
using Data Dropped options (Section 11.7) with Drop Code 0, Protocol
Constraints. New connections start with Minimum Checksum Coverage 0
for both endpoints.
9.3. Data Checksum Option
The Data Checksum option holds a 32-bit CRC-32c cyclic redundancy-
check code of a DCCP packet's application data.
+--------+--------+--------+--------+--------+--------+
|00101100|00000110| CRC-32c |
+--------+--------+--------+--------+--------+--------+
Type=44 Length=6
The sending DCCP computes the CRC of the bytes comprising the
application data area and stores it in the option data. The CRC-32c
algorithm used for Data Checksum is the same as that used for SCTP
[RFC3309]; note that the CRC-32c of zero bytes of data equals zero.
The DCCP header checksum will cover the Data Checksum option, so the
data checksum must be computed before the header checksum.
A DCCP endpoint receiving a packet with a Data Checksum option either
MUST or MAY check the Data Checksum; the choice depends on the value
of the Check Data Checksum feature described below. If it checks the
checksum, it computes the received application data's CRC-32c using
the same algorithm as the sender and compares the result with the
Data Checksum value. If the CRCs differ, the endpoint reacts in one
of two ways:
o The receiving application may have requested delivery of known-
corrupt data via some optional API. In this case, the packet's
data MUST be delivered to the application, with a note that it is
known to be corrupt. Furthermore, the receiving endpoint MUST
report the packet as delivered corrupt using a Data Dropped option
(Drop Code 7, Delivered Corrupt).
o Otherwise, the receiving endpoint MUST drop the application data
and report that data as dropped due to corruption using a Data
Dropped option (Drop Code 3, Corrupt).
In either case, the packet is considered acknowledgeable (since its
header was processed) and will therefore be acknowledged using the
equivalent of Ack Vector's Received or Received ECN Marked states.
Although Data Checksum is intended for packets containing application
data, it may be included on other packets, such as DCCP-Ack, DCCP-
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Sync, and DCCP-SyncAck. The receiver SHOULD calculate the
application data area's CRC-32c on such packets, just as it does for
DCCP-Data and similar packets. If the CRCs differ, the packets
similarly MUST be reported using Data Dropped options (Drop Code 3),
although their application data areas would not be delivered to the
application in any case.
9.3.1. Check Data Checksum Feature
The Check Data Checksum feature lets a DCCP endpoint determine
whether its peer will definitely check Data Checksum options. DCCP A
sends a Mandatory "Change R(Check Data Checksum, 1)" option to DCCP B
to require it to check Data Checksum options (the connection will be
reset if it cannot).
Check Data Checksum has feature number 9 and is server-priority. It
takes one-byte Boolean values. DCCP B MUST check any received Data
Checksum options when Check Data Checksum/B is one, although it MAY
check them even when Check Data Checksum/B is zero. Values of two or
more are reserved. New connections start with Check Data Checksum 0
for both endpoints.
9.3.2. Checksum Usage Notes
Internet links must normally apply strong integrity checks to the
packets they transmit [RFC3828, RFC3819]. This is the default case
when the DCCP header's Checksum Coverage value equals zero (full
coverage). However, the DCCP Checksum Coverage value might not be
zero. By setting partial Checksum Coverage, the application
indicates that it can tolerate corruption in the unprotected part of
the application data. Recognizing this, link layers may reduce error
detection and/or correction strength when transmitting this
unprotected part. This, in turn, can significantly increase the
likelihood of the endpoint's receiving corrupt data; Data Checksum
lets the receiver detect that corruption with very high probability.
10. Congestion Control
Each congestion control mechanism supported by DCCP is assigned a
congestion control identifier, or CCID: a number from 0 to 255.
During connection setup, and optionally thereafter, the endpoints
negotiate their congestion control mechanisms by negotiating the
values for their Congestion Control ID features. Congestion Control
ID has feature number 1. The CCID/A value equals the CCID in use for
the A-to-B half-connection. DCCP B sends a "Change R(CCID, K)"
option to ask DCCP A to use CCID K for its data packets.
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CCID is a server-priority feature, so CCID negotiation options can
list multiple acceptable CCIDs, sorted in descending order of
priority. For example, the option "Change R(CCID, 2 3 4)" asks the
receiver to use CCID 2 for its packets, although CCIDs 3 and 4 are
also acceptable. (This corresponds to the bytes "35, 6, 1, 2, 3, 4":
Change R option (35), option length (6), feature ID (1), CCIDs (2, 3,
4).) Similarly, "Confirm L(CCID, 2, 2 3 4)" tells the receiver that
the sender is using CCID 2 for its packets, but that CCIDs 3 and 4
might also be acceptable.
Currently allocated CCIDs are as follows:
CCID Meaning Reference
---- ------- ---------
0-1 Reserved
2 TCP-like Congestion Control [RFC4341]
3 TCP-Friendly Rate Control [RFC4342]
4-255 Reserved
Table 5: DCCP Congestion Control Identifiers
New connections start with CCID 2 for both endpoints. If this is
unacceptable for a DCCP endpoint, that endpoint MUST send Mandatory
Change(CCID) options on its first packets.
All CCIDs standardized for use with DCCP will correspond to
congestion control mechanisms previously standardized by the IETF.
We expect that for quite some time, all such mechanisms will be TCP
friendly, but TCP-friendliness is not an explicit DCCP requirement.
A DCCP implementation intended for general use, such as an
implementation in a general-purpose operating system kernel, SHOULD
implement at least CCID 2. The intent is to make CCID 2 broadly
available for interoperability, although particular applications
might disallow its use.
10.1. TCP-like Congestion Control
CCID 2, TCP-like Congestion Control, denotes Additive Increase,
Multiplicative Decrease (AIMD) congestion control with behavior
modelled directly on TCP, including congestion window, slow start,
timeouts, and so forth [RFC2581]. CCID 2 achieves maximum bandwidth
over the long term, consistent with the use of end-to-end congestion
control, but halves its congestion window in response to each
congestion event. This leads to the abrupt rate changes typical of
TCP. Applications should use CCID 2 if they prefer maximum bandwidth
utilization to steadiness of rate. This is often the case for
applications that are not playing their data directly to the user.
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For example, a hypothetical application that transferred files over
DCCP, using application-level retransmissions for lost packets, would
prefer CCID 2 to CCID 3. On-line games may also prefer CCID 2.
CCID 2 is further described in [RFC4341].
10.2. TFRC Congestion Control
CCID 3 denotes TCP-Friendly Rate Control (TFRC), an equation-based
rate-controlled congestion control mechanism. TFRC is designed to be
reasonably fair when competing for bandwidth with TCP-like flows,
where a flow is "reasonably fair" if its sending rate is generally
within a factor of two of the sending rate of a TCP flow under the
same conditions. However, TFRC has a much lower variation of
throughput over time compared with TCP, which makes CCID 3 more
suitable than CCID 2 for applications such as streaming media where a
relatively smooth sending rate is important.
CCID 3 is further described in [RFC4342]. The TFRC congestion
control algorithms were initially described in [RFC3448].
10.3. CCID-Specific Options, Features, and Reset Codes
Half of the option types, feature numbers, and Reset Codes are
reserved for CCID-specific use. CCIDs may often need new options,
for communicating acknowledgement or rate information, for example;
reserved option spaces let CCIDs create options at will without
polluting the global option space. Option 128 might have different
meanings on a half-connection using CCID 4 and a half-connection
using CCID 8. CCID-specific options and features will never conflict
with global options and features introduced by later versions of this
specification.
Any packet may contain information meant for either half-connection,
so CCID-specific option types, feature numbers, and Reset Codes
explicitly signal the half-connection to which they apply.
o Option numbers 128 through 191 are for options sent from the
HC-Sender to the HC-Receiver; option numbers 192 through 255 are
for options sent from the HC-Receiver to the HC-Sender.
o Reset Codes 128 through 191 indicate that the HC-Sender reset the
connection (most likely because of some problem with
acknowledgements sent by the HC-Receiver). Reset Codes 192
through 255 indicate that the HC-Receiver reset the connection
(most likely because of some problem with data packets sent by the
HC-Sender).
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o Finally, feature numbers 128 through 191 are used for features
located at the HC-Sender; feature numbers 192 through 255 are for
features located at the HC-Receiver. Since Change L and Confirm L
options for a feature are sent by the feature location, we know
that any Change L(128) option was sent by the HC-Sender, while any
Change L(192) option was sent by the HC-Receiver. Similarly,
Change R(128) options are sent by the HC-Receiver, while Change
R(192) options are sent by the HC-Sender.
For example, consider a DCCP connection where the A-to-B half-
connection uses CCID 4 and the B-to-A half-connection uses CCID 5.
Here is how a sampling of CCID-specific options are assigned to
half-connections.
Relevant Relevant
Packet Option Half-conn. CCID
------ ------ ---------- ----
A > B 128 A-to-B 4
A > B 192 B-to-A 5
A > B Change L(128, ...) A-to-B 4
A > B Change R(192, ...) A-to-B 4
A > B Confirm L(128, ...) A-to-B 4
A > B Confirm R(192, ...) A-to-B 4
A > B Change R(128, ...) B-to-A 5
A > B Change L(192, ...) B-to-A 5
A > B Confirm R(128, ...) B-to-A 5
A > B Confirm L(192, ...) B-to-A 5
B > A 128 B-to-A 5
B > A 192 A-to-B 4
B > A Change L(128, ...) B-to-A 5
B > A Change R(192, ...) B-to-A 5
B > A Confirm L(128, ...) B-to-A 5
B > A Confirm R(192, ...) B-to-A 5
B > A Change R(128, ...) A-to-B 4
B > A Change L(192, ...) A-to-B 4
B > A Confirm R(128, ...) A-to-B 4
B > A Confirm L(192, ...) A-to-B 4
Using CCID-specific options and feature options during a negotiation
for the corresponding CCID feature is NOT RECOMMENDED, since it is
difficult to predict which CCID will be in force when the option is
processed. For example, if a DCCP-Request contains the option
sequence "Change L(CCID, 3), 128", the CCID-specific option "128" may
be processed either by CCID 3 (if the server supports CCID 3) or by
the default CCID 2 (if it does not). However, it is safe to include
CCID-specific options following certain Mandatory Change(CCID)
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options. For example, if a DCCP-Request contains the option sequence
"Mandatory, Change L(CCID, 3), 128", then either the "128" option
will be processed by CCID 3 or the connection will be reset.
Servers that do not implement the default CCID 2 might nevertheless
receive CCID 2-specific options on a DCCP-Request packet. (Such a
server MUST send Mandatory Change(CCID) options on its DCCP-Response,
so CCID-specific options on any other packet won't refer to CCID 2.)
The server MUST treat such options as non-understood. Thus, it will
reset the connection on encountering a Mandatory CCID-specific option
or feature negotiation request, send an empty Confirm for a non-
Mandatory Change option for a CCID-specific feature, and ignore other
CCID-specific options.
10.4. CCID Profile Requirements
Each CCID Profile document MUST address at least the following
requirements:
o The profile MUST include the name and number of the CCID being
described.
o The profile MUST describe the conditions in which it is likely to
be useful. Often the best way to do this is by comparison to
existing CCIDs.
o The profile MUST list and describe any CCID-specific options,
features, and Reset Codes and SHOULD list those general options
and features described in this document that are especially
relevant to the CCID.
o Any newly defined acknowledgement mechanism MUST include a way to
transmit ECN Nonce Echoes back to the sender.
o The profile MUST describe the format of data packets, including
any options that should be included and the setting of the CCval
header field.
o The profile MUST describe the format of acknowledgement packets,
including any options that should be included.
o The profile MUST define how data packets are congestion
controlled. This includes responses to congestion events, to idle
and application-limited periods, and to the DCCP Data Dropped and
Slow Receiver options. CCIDs that implement per-packet congestion
control SHOULD discuss how packet size is factored in to
congestion control decisions.
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o The profile MUST specify when acknowledgement packets are
generated and how they are congestion controlled.
o The profile MUST define when a sender using the CCID is considered
quiescent.
o The profile MUST say whether its CCID's acknowledgements ever need
to be acknowledged and, if so, how often.
10.5. Congestion State
Most congestion control algorithms depend on past history to
determine the current allowed sending rate. In CCID 2, this
congestion state includes a congestion window and a measurement of
the number of packets outstanding in the network; in CCID 3, it
includes the lengths of recent loss intervals. Both CCIDs use an
estimate of the round-trip time. Congestion state depends on the
network path and is invalidated by path changes. Therefore, DCCP
senders and receivers SHOULD reset their congestion state --
essentially restarting congestion control from "slow start" or
equivalent -- on significant changes in the end-to-end path. For
example, an endpoint that sends or receives a Mobile IPv6 Binding
Update message [RFC3775] SHOULD reset its congestion state for any
corresponding DCCP connections.
A DCCP implementation MAY also reset its congestion state when a CCID
changes (that is, when a negotiation for the CCID feature completes
successfully and the new feature value differs from the old value).
Thus, a connection in a heavily congested environment might evade
end-to-end congestion control by frequently renegotiating a CCID,
just as it could evade end-to-end congestion control by opening new
connections for the same session. This behavior is prohibited. To
prevent it, DCCP implementations MAY limit the rate at which CCID can
be changed -- for instance, by refusing to change a CCID feature
value more than once per minute.
11. Acknowledgements
Congestion control requires that receivers transmit information about
packet losses and ECN marks to senders. DCCP receivers MUST report
all congestion they see, as defined by the relevant CCID profile.
Each CCID says when acknowledgements should be sent, what options
they must use, and so on. DCCP acknowledgements are congestion
controlled, although it is not required that the acknowledgement
stream be more than very roughly TCP friendly; each CCID defines how
acknowledgements are congestion controlled.
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Most acknowledgements use DCCP options. For example, on a half-
connection with CCID 2 (TCP-like), the receiver reports
acknowledgement information using the Ack Vector option. This
section describes common acknowledgement options and shows how acks
using those options will commonly work. Full descriptions of the ack
mechanisms used for each CCID are laid out in the CCID profile
specifications.
Acknowledgement options, such as Ack Vector, depend on the DCCP
Acknowledgement Number and are thus only allowed on packet types that
carry that number. Acknowledgement options received on other packet
types, namely DCCP-Request and DCCP-Data, MUST be ignored. Detailed
acknowledgement options are not necessarily required on every packet
that carries an Acknowledgement Number, however.
11.1. Acks of Acks and Unidirectional Connections
DCCP was designed to work well for both bidirectional and
unidirectional flows of data, and for connections that transition
between these states. However, acknowledgements required for a
unidirectional connection are very different from those required for
a bidirectional connection. In particular, unidirectional
connections need to worry about acks of acks.
The ack-of-acks problem arises because some acknowledgement
mechanisms are reliable. For example, an HC-Receiver using CCID 2,
TCP-like Congestion Control, sends Ack Vectors containing completely
reliable acknowledgement information. The HC-Sender should
occasionally inform the HC-Receiver that it has received an ack. If
it did not, the HC-Receiver might resend complete Ack Vector
information, going back to the start of the connection, with every
DCCP-Ack packet! However, note that acks-of-acks need not be
reliable themselves: when an ack-of-acks is lost, the HC-Receiver
will simply maintain, and periodically retransmit, old
acknowledgement-related state for a little longer. Therefore, there
is no need for acks-of-acks-of-acks.
When communication is bidirectional, any required acks-of-acks are
automatically contained in normal acknowledgements for data packets.
On a unidirectional connection, however, the receiver DCCP sends no
data, so the sender would not normally send acknowledgements.
Therefore, the CCID in force on that half-connection must explicitly
say whether, when, and how the HC-Sender should generate acks-of-
acks.
For example, consider a bidirectional connection where both half-
connections use the same CCID (either 2 or 3), and where DCCP B goes
"quiescent". This means that the connection becomes unidirectional:
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DCCP B stops sending data and sends only DCCP-Ack packets to DCCP A.
In CCID 2, TCP-like Congestion Control, DCCP B uses Ack Vector to
reliably communicate which packets it has received. As described
above, DCCP A must occasionally acknowledge a pure acknowledgement
from DCCP B so that B can free old Ack Vector state. For instance, A
might send a DCCP-DataAck packet instead of DCCP-Data every now and
then. In CCID 3, however, acknowledgement state is generally
bounded, so A does not need to acknowledge B's acknowledgements.
When communication is unidirectional, a single CCID -- in the
example, the A-to-B CCID -- controls both DCCPs' acknowledgements, in
terms of their content, their frequency, and so forth. For
bidirectional connections, the A-to-B CCID governs DCCP B's
acknowledgements (including its acks of DCCP A's acks) and the B-to-A
CCID governs DCCP A's acknowledgements.
DCCP A switches its ack pattern from bidirectional to unidirectional
when it notices that DCCP B has gone quiescent. It switches from
unidirectional to bidirectional when it must acknowledge even a
single DCCP-Data or DCCP-DataAck packet from DCCP B.
Each CCID defines how to detect quiescence on that CCID, and how that
CCID handles acks-of-acks on unidirectional connections. The B-to-A
CCID defines when DCCP B has gone quiescent. Usually, this happens
when a period has passed without B sending any data packets; in CCID
2, for example, this period is the maximum of 0.2 seconds and two
round-trip times. The A-to-B CCID defines how DCCP A handles
acks-of-acks once DCCP B has gone quiescent.
11.2. Ack Piggybacking
Acknowledgements of A-to-B data MAY be piggybacked on data sent by
DCCP B, as long as that does not delay the acknowledgement longer
than the A-to-B CCID would find acceptable. However, data
acknowledgements often require more than 4 bytes to express. A large
set of acknowledgements prepended to a large data packet might exceed
the allowed maximum packet size. In this case, DCCP B SHOULD send
separate DCCP-Data and DCCP-Ack packets, or wait, but not too long,
for a smaller datagram.
Piggybacking is particularly common at DCCP A when the B-to-A
half-connection is quiescent -- that is, when DCCP A is just
acknowledging DCCP B's acknowledgements. There are three reasons to
acknowledge DCCP B's acknowledgements: to allow DCCP B to free up
information about previously acknowledged data packets from A; to
shrink the size of future acknowledgements; and to manipulate the
rate at which future acknowledgements are sent. Since these are
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secondary concerns, DCCP A can generally afford to wait indefinitely
for a data packet to piggyback its acknowledgement onto; if DCCP B
wants to elicit an acknowledgement, it can send a DCCP-Sync.
Any restrictions on ack piggybacking are described in the relevant
CCID's profile.
11.3. Ack Ratio Feature
The Ack Ratio feature lets HC-Senders influence the rate at which
HC-Receivers generate DCCP-Ack packets, thus controlling reverse-path
congestion. This differs from TCP, which presently has no congestion
control for pure acknowledgement traffic. Ack Ratio reverse-path
congestion control does not try to be TCP friendly. It just tries to
avoid congestion collapse, and to be somewhat better than TCP in the
presence of a high packet loss or mark rate on the reverse path.
Ack Ratio applies to CCIDs whose HC-Receivers clock acknowledgements
off the receipt of data packets. The value of Ack Ratio/A equals the
rough ratio of data packets sent by DCCP A to DCCP-Ack packets sent
by DCCP B. Higher Ack Ratios correspond to lower DCCP-Ack rates; the
sender raises Ack Ratio when the reverse path is congested and lowers
Ack Ratio when it is not. Each CCID profile defines how it controls
congestion on the acknowledgement path, and, particularly, whether
Ack Ratio is used. CCID 2, for example, uses Ack Ratio for
acknowledgement congestion control, but CCID 3 does not. However,
each Ack Ratio feature has a value whether or not that value is used
by the relevant CCID.
Ack Ratio has feature number 5 and is non-negotiable. It takes two-
byte integer values. An Ack Ratio/A value of four means that DCCP B
will send at least one acknowledgement packet for every four data
packets sent by DCCP A. DCCP A sends a "Change L(Ack Ratio)" option
to notify DCCP B of its ack ratio. An Ack Ratio value of zero
indicates that the relevant half-connection does not use an Ack Ratio
to control its acknowledgement rate. New connections start with Ack
Ratio 2 for both endpoints; this Ack Ratio results in acknowledgement
behavior analogous to TCP's delayed acks.
Ack Ratio should be treated as a guideline rather than a strict
requirement. We intend Ack Ratio-controlled acknowledgement behavior
to resemble TCP's acknowledgement behavior when there is no reverse-
path congestion, and to be somewhat more conservative when there is
reverse-path congestion. Following this intent is more important
than implementing Ack Ratio precisely. In particular:
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o Receivers MAY piggyback acknowledgement information on data
packets, creating DCCP-DataAck packets. The Ack Ratio does not
apply to piggybacked acknowledgements. However, if the data
packets are too big to carry acknowledgement information, or if
the data sending rate is lower than Ack Ratio would suggest, then
DCCP B SHOULD send enough pure DCCP-Ack packets to maintain the
rate of one acknowledgement per Ack Ratio received data packets.
o Receivers MAY rate-pace their acknowledgements rather than send
acknowledgements immediately upon the receipt of data packets.
Receivers that rate-pace acknowledgements SHOULD pick a rate that
approximates the effect of Ack Ratio and SHOULD include Elapsed
Time options (Section 13.2) to help the sender calculate round-
trip times.
o Receivers SHOULD implement delayed acknowledgement timers like
TCP's, whereby any packet's acknowledgement is delayed by at most
T seconds. This delay lets the receiver collect additional
packets to acknowledge and thus reduce the per-packet overhead of
acknowledgements; but if T seconds have passed by and the ack is
still around, it is sent out right away. The default value of T
should be 0.2 seconds, as is common in TCP implementations. This
may lead to sending more acknowledgement packets than Ack Ratio
would suggest.
o Receivers SHOULD send acknowledgements immediately on receiving
packets marked ECN Congestion Experienced or packets whose out-
of-order sequence numbers potentially indicate loss. However,
there is no need to send such immediate acknowledgements for
marked packets more than once per round-trip time.
o Receivers MAY ignore Ack Ratio if they perform their own
congestion control on acknowledgements. For example, a receiver
that knows the loss and mark rate for its DCCP-Ack packets might
maintain a TCP-friendly acknowledgement rate on its own. Such a
receiver MUST either ensure that it always obtains sufficient
acknowledgement loss and mark information or fall back to Ack
Ratio when sufficient information is not available, as might
happen during periods when the receiver is quiescent.
11.4. Ack Vector Options
The Ack Vector gives a run-length encoded history of data packets
received at the client. Each byte of the vector gives the state of
that data packet in the loss history, and the number of preceding
packets with the same state. The option's data looks like this:
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+--------+--------+--------+--------+--------+--------
|0010011?| Length |SSLLLLLL|SSLLLLLL|SSLLLLLL| ...
+--------+--------+--------+--------+--------+--------
Type=38/39 \___________ Vector ___________...
The two Ack Vector options (option types 38 and 39) differ only in
the values they imply for ECN Nonce Echo. Section 12.2 describes
this further.
The vector itself consists of a series of bytes, each of whose
encoding is:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Sta| Run Length|
+-+-+-+-+-+-+-+-+
Sta[te] occupies the most significant two bits of each byte and can
have one of four values, as follows:
State Meaning
----- -------
0 Received
1 Received ECN Marked
2 Reserved
3 Not Yet Received
Table 6: DCCP Ack Vector States
The term "ECN marked" refers to packets with ECN code point 11, CE
(Congestion Experienced); packets received with this ECN code point
MUST be reported using State 1, Received ECN Marked. Packets
received with ECN code points 00, 01, or 10 (Non-ECT, ECT(0), or
ECT(1), respectively) MUST be reported using State 0, Received.
Run Length, the least significant six bits of each byte, specifies
how many consecutive packets have the given State. Run Length zero
says the corresponding State applies to one packet only; Run Length
63 says it applies to 64 consecutive packets. Run lengths of 65 or
more must be encoded in multiple bytes.
The first byte in the first Ack Vector option refers to the packet
indicated in the Acknowledgement Number; subsequent bytes refer to
older packets. Ack Vector MUST NOT be sent on DCCP-Data and DCCP-
Request packets, which lack an Acknowledgement Number, and any Ack
Vector options encountered on such packets MUST be ignored.
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An Ack Vector containing the decimal values 0,192,3,64,5 and for
which the Acknowledgement Number is decimal 100 indicates that:
Packet 100 was received (Acknowledgement Number 100, State 0, Run
Length 0);
Packet 99 was lost (State 3, Run Length 0);
Packets 98, 97, 96 and 95 were received (State 0, Run Length 3);
Packet 94 was ECN marked (State 1, Run Length 0); and
Packets 93, 92, 91, 90, 89, and 88 were received (State 0, Run
Length 5).
A single Ack Vector option can acknowledge up to 16192 data packets.
Should more packets need to be acknowledged than can fit in 253 bytes
of Ack Vector, then multiple Ack Vector options can be sent; the
second Ack Vector begins where the first left off, and so forth.
Ack Vector states are subject to two general constraints. (These
principles SHOULD also be followed for other acknowledgement
mechanisms; referring to Ack Vector states simplifies their
explanation.)
1. Packets reported as State 0 or State 1 MUST be acknowledgeable:
their options have been processed by the receiving DCCP stack.
Any data on the packet need not have been delivered to the
receiving application; in fact, the data may have been dropped.
2. Packets reported as State 3 MUST NOT be acknowledgeable. Feature
negotiations and options on such packets MUST NOT have been
processed, and the Acknowledgement Number MUST NOT correspond to
such a packet.
Packets dropped in the application's receive buffer MUST be reported
as Received or Received ECN Marked (States 0 and 1), depending on
their ECN state; such packets' ECN Nonces MUST be included in the
Nonce Echo. The Data Dropped option informs the sender that some
packets reported as received actually had their application data
dropped.
One or more Ack Vector options that, together, report the status of a
packet with a sequence number less than ISN, the initial sequence
number, SHOULD be considered invalid. The receiving DCCP SHOULD
either ignore the options or reset the connection with Reset Code 5,
"Option Error". No Ack Vector option can refer to a packet that has
not yet been sent, as the Acknowledgement Number checks in Section
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7.5.3 ensure, but because of attack, implementation bug, or
misbehavior, an Ack Vector option can claim that a packet was
received before it is actually delivered. Section 12.2 describes how
this is detected and how senders should react. Packets that haven't
been included in any Ack Vector option SHOULD be treated as "not yet
received" (State 3) by the sender.
Appendix A provides a non-normative description of the details of
DCCP acknowledgement handling in the context of an abstract Ack
Vector implementation.
11.4.1. Ack Vector Consistency
A DCCP sender will commonly receive multiple acknowledgements for
some of its data packets. For instance, an HC-Sender might receive
two DCCP-Acks with Ack Vectors, both of which contained information
about sequence number 24. (Information about a sequence number is
generally repeated in every ack until the HC-Sender acknowledges an
ack. In this case, perhaps the HC-Receiver is sending acks faster
than the HC-Sender is acknowledging them.) In a perfect world, the
two Ack Vectors would always be consistent. However, there are many
reasons why they might not be. For example:
o The HC-Receiver received packet 24 between sending its acks, so
the first ack said 24 was not received (State 3) and the second
said it was received or ECN marked (State 0 or 1).
o The HC-Receiver received packet 24 between sending its acks, and
the network reordered the acks. In this case, the packet will
appear to transition from State 0 or 1 to State 3.
o The network duplicated packet 24, and one of the duplicates was
ECN marked. This might show up as a transition between States 0
and 1.
To cope with these situations, HC-Sender DCCP implementations SHOULD
combine multiple received Ack Vector states according to this table:
Received State
0 1 3
+---+---+---+
0 | 0 |0/1| 0 |
Old +---+---+---+
1 | 1 | 1 | 1 |
State +---+---+---+
3 | 0 | 1 | 3 |
+---+---+---+
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To read the table, choose the row corresponding to the packet's old
state and the column corresponding to the packet's state in the newly
received Ack Vector; then read the packet's new state off the table.
For an old state of 0 (received non-marked) and received state of 1
(received ECN marked), the packet's new state may be set to either 0
or 1. The HC-Sender implementation will be indifferent to ack
reordering if it chooses new state 1 for that cell.
The HC-Receiver should collect information about received packets
according to the following table:
Received Packet
0 1 3
+---+---+---+
0 | 0 |0/1| 0 |
Stored +---+---+---+
1 |0/1| 1 | 1 |
State +---+---+---+
3 | 0 | 1 | 3 |
+---+---+---+
This table equals the sender's table except that, when the stored
state is 1 and the received state is 0, the receiver is allowed to
switch its stored state to 0.
An HC-Sender MAY choose to throw away old information gleaned from
the HC-Receiver's Ack Vectors, in which case it MUST ignore newly
received acknowledgements from the HC-Receiver for those old packets.
It is often kinder to save recent Ack Vector information for a while
so that the HC-Sender can undo its reaction to presumed congestion
when a "lost" packet unexpectedly shows up (the transition from State
3 to State 0).
11.4.2. Ack Vector Coverage
We can divide the packets that have been sent from an HC-Sender to an
HC-Receiver into four roughly contiguous groups. From oldest to
youngest, these are:
1. Packets already acknowledged by the HC-Receiver, where the
HC-Receiver knows that the HC-Sender has definitely received the
acknowledgements;
2. Packets already acknowledged by the HC-Receiver, where the
HC-Receiver cannot be sure that the HC-Sender has received the
acknowledgements;
3. Packets not yet acknowledged by the HC-Receiver; and
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4. Packets not yet received by the HC-Receiver.
The union of groups 2 and 3 is called the Acknowledgement Window.
Generally, every Ack Vector generated by the HC-Receiver will cover
the whole Acknowledgement Window: Ack Vector acknowledgements are
cumulative. (This simplifies Ack Vector maintenance at the
HC-Receiver; see Appendix A, below.) As packets are received, this
window both grows on the right and shrinks on the left. It grows
because there are more packets, and shrinks because the HC-Sender's
Acknowledgement Numbers will acknowledge previous acknowledgements,
moving packets from group 2 into group 1.
11.5. Send Ack Vector Feature
The Send Ack Vector feature lets DCCPs negotiate whether they should
use Ack Vector options to report congestion. Ack Vector provides
detailed loss information and lets senders report back to their
applications whether particular packets were dropped. Send Ack
Vector is mandatory for some CCIDs and optional for others.
Send Ack Vector has feature number 6 and is server-priority. It
takes one-byte Boolean values. DCCP A MUST send Ack Vector options
on its acknowledgements when Send Ack Vector/A has value one,
although it MAY send Ack Vector options even when Send Ack Vector/A
is zero. Values of two or more are reserved. New connections start
with Send Ack Vector 0 for both endpoints. DCCP B sends a "Change
R(Send Ack Vector, 1)" option to DCCP A to ask A to send Ack Vector
options as part of its acknowledgement traffic.
11.6. Slow Receiver Option
An HC-Receiver sends the Slow Receiver option to its sender to
indicate that it is having trouble keeping up with the sender's data.
The HC-Sender SHOULD NOT increase its sending rate for approximately
one round-trip time after seeing a packet with a Slow Receiver
option. After one round-trip time, the effect of Slow Receiver
disappears, allowing the HC-Sender to increase its rate. Therefore,
the HC-Receiver SHOULD continue to send Slow Receiver options if it
needs to prevent the HC-Sender from going faster in the long term.
The Slow Receiver option does not indicate congestion, and the HC-
Sender need not reduce its sending rate. (If necessary, the receiver
can force the sender to slow down by dropping packets, with or
without Data Dropped, or by reporting false ECN marks.) APIs should
let receiver applications set Slow Receiver and sending applications
determine whether their receivers are Slow.
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Slow Receiver is a one-byte option.
+--------+
|00000010|
+--------+
Type=2
Slow Receiver does not specify why the receiver is having trouble
keeping up with the sender. Possible reasons include lack of buffer
space, CPU overload, and application quotas. A sending application
might react to Slow Receiver by reducing its application-level
sending rate, for example.
The sending application should not react to Slow Receiver by sending
more data, however. Although the optimal response to a CPU-bound
receiver might be to reduce compression and send more data (a
highly-compressed data format might overwhelm a slow CPU more
seriously than would the higher memory requirements of a less-
compressed data format), this kind of format change should be
requested at the application level, not via the Slow Receiver option.
Slow Receiver implements a portion of TCP's receive window
functionality.
11.7. Data Dropped Option
The Data Dropped option indicates that the application data on one or
more received packets did not actually reach the application. Data
Dropped additionally reports why the data was dropped: perhaps the
data was corrupt, or perhaps the receiver cannot keep up with the
sender's current rate and the data was dropped in some receive
buffer. Using Data Dropped, DCCP endpoints can discriminate between
different kinds of loss; this differs from TCP, in which all loss is
reported the same way.
Unless it is explicitly specified otherwise, DCCP congestion control
mechanisms MUST react as if each Data Dropped packet was marked as
ECN Congestion Experienced by the network. We intend for Data
Dropped to enable research into richer congestion responses to
corrupt and other endpoint-dropped packets, but DCCP CCIDs MUST react
conservatively to Data Dropped until this behavior is standardized.
Section 11.7.2, below, describes congestion responses for all current
Drop Codes.
If a received packet's application data is dropped for one of the
reasons listed below, this SHOULD be reported using a Data Dropped
option. Alternatively, the receiver MAY choose to report as
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"received" only those packets whose data were not dropped, subject to
the constraint that packets not reported as received MUST NOT have
had their options processed.
The option's data looks like this:
+--------+--------+--------+--------+--------+--------
|00101000| Length | Block | Block | Block | ...
+--------+--------+--------+--------+--------+--------
Type=40 \___________ Vector ___________ ...
The Vector consists of a series of bytes, called Blocks, each of
whose encoding corresponds to one of two choices:
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|0| Run Length | or |1|DrpCd|Run Len|
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
Normal Block Drop Block
The first byte in the first Data Dropped option refers to the packet
indicated by the Acknowledgement Number; subsequent bytes refer to
older packets. Data Dropped MUST NOT be sent on DCCP-Data or DCCP-
Request packets, which lack an Acknowledgement Number, and any Data
Dropped options received on such packets MUST be ignored.
Normal Blocks, which have high bit 0, indicate that any received
packets in the Run Length had their data delivered to the
application. Drop Blocks, which have high bit 1, indicate that
received packets in the Run Len[gth] were not delivered as usual.
The 3-bit Drop Code [DrpCd] field says what happened; generally, no
data from that packet reached the application. Packets reported as
"not yet received" MUST be included in Normal Blocks; packets not
covered by any Data Dropped option are treated as if they were in a
Normal Block. Defined Drop Codes for Drop Blocks are as follows.
Drop Code Meaning
--------- -------
0 Protocol Constraints
1 Application Not Listening
2 Receive Buffer
3 Corrupt
4-6 Reserved
7 Delivered Corrupt
Table 7: DCCP Drop Codes
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In more detail:
0 The packet data was dropped due to protocol constraints. For
example, the data was included on a DCCP-Request packet, but
the receiving application does not allow such piggybacking; or
the data was included on a packet with inappropriately low
Checksum Coverage.
1 The packet data was dropped because the application is no
longer listening. See Section 11.7.2.
2 The packet data was dropped in a receive buffer, probably
because of receive buffer overflow. See Section 11.7.2.
3 The packet data was dropped due to corruption. See Section
9.3.
7 The packet data was corrupted but was delivered to the
application anyway. See Section 9.3.
For example, assume that a packet arrives with Acknowledgement Number
100, an Ack Vector reporting all packets as received, and a Data
Dropped option containing the decimal values 0,160,3,162. Then:
Packet 100 was received (Acknowledgement Number 100, Normal Block,
Run Length 0).
Packet 99 was dropped in a receive buffer (Drop Block, Drop Code
2, Run Length 0).
Packets 98, 97, 96, and 95 were received (Normal Block, Run Length
3).
Packets 95, 94, and 93 were dropped in the receive buffer (Drop
Block, Drop Code 2, Run Length 2).
Run lengths of more than 128 (for Normal Blocks) or 16 (for Drop
Blocks) must be encoded in multiple Blocks. A single Data Dropped
option can acknowledge up to 32384 Normal Block data packets,
although the receiver SHOULD NOT send a Data Dropped option when all
relevant packets fit into Normal Blocks. Should more packets need to
be acknowledged than can fit in 253 bytes of Data Dropped, then
multiple Data Dropped options can be sent. The second option will
begin where the first left off, and so forth.
One or more Data Dropped options that, together, report the status of
more packets than have been sent, or that change the status of a
packet, or that disagree with Ack Vector or equivalent options (by
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reporting a "not yet received" packet as "dropped in the receive
buffer", for example) SHOULD be considered invalid. The receiving
DCCP SHOULD either ignore such options, or respond by resetting the
connection with Reset Code 5, "Option Error".
A DCCP application interface should let receiving applications
specify the Drop Codes corresponding to received packets. For
example, this would let applications calculate their own checksums
but still report "dropped due to corruption" packets via the Data
Dropped option. The interface SHOULD NOT let applications reduce the
"seriousness" of a packet's Drop Code; for example, the application
should not be able to upgrade a packet from delivered corrupt (Drop
Code 7) to delivered normally (no Drop Code).
Data Dropped information is transmitted reliably. That is, endpoints
SHOULD continue to transmit Data Dropped options until receiving an
acknowledgement indicating that the relevant options have been
processed. In Ack Vector terms, each acknowledgement should contain
Data Dropped options that cover the whole Acknowledgement Window
(Section 11.4.2), although when every packet in that window would be
placed in a Normal Block, no actual option is required.
11.7.1. Data Dropped and Normal Congestion Response
When deciding on a response to a particular acknowledgement or set of
acknowledgements containing Data Dropped options, a congestion
control mechanism MUST consider dropped packets, ECN Congestion
Experienced marks (including marked packets that are included in Data
Dropped), and packets singled out in Data Dropped. For window-based
mechanisms, the valid response space is defined as follows.
Assume an old window of W. Independently calculate a new window
W_new1 that assumes no packets were Data Dropped (so W_new1 contains
only the normal congestion response), and a new window W_new2 that
assumes no packets were lost or marked (so W_new2 contains only the
Data Dropped response). We are assuming that Data Dropped
recommended a reduction in congestion window, so W_new2 < W.
Then the actual new window W_new MUST NOT be larger than the minimum
of W_new1 and W_new2; and the sender MAY combine the two responses,
by setting
W_new = W + min(W_new1 - W, 0) + min(W_new2 - W, 0).
The details of how this is accomplished are specified in CCID profile
documents. Non-window-based congestion control mechanisms MUST
behave analogously; again, CCID profiles define how.
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11.7.2. Particular Drop Codes
Drop Code 0, Protocol Constraints, does not indicate any kind of
congestion, so the sender's CCID SHOULD react to packets with Drop
Code 0 as if they were received (with or without ECN Congestion
Experienced marks, as appropriate). However, the sending endpoint
SHOULD NOT send data until it believes the protocol constraint no
longer applies.
Drop Code 1, Application Not Listening, means the application running
at the endpoint that sent the option is no longer listening for data.
For example, a server might close its receiving half-connection to
new data after receiving a complete request from the client. This
would limit the amount of state available at the server for incoming
data and thus reduce the potential damage from certain denial-of-
service attacks. A Data Dropped option containing Drop Code 1 SHOULD
be sent whenever received data is ignored due to a non-listening
application. Once an endpoint reports Drop Code 1 for a packet, it
SHOULD report Drop Code 1 for every succeeding data packet on that
half-connection; once an endpoint receives a Drop State 1 report, it
SHOULD expect that no more data will ever be delivered to the other
endpoint's application, so it SHOULD NOT send more data.
Drop Code 2, Receive Buffer, indicates congestion inside the
receiving host. For instance, if a drop-from-tail kernel socket
buffer is too full to accept a packet's application data, that packet
should be reported as Drop Code 2. For a drop-from-head or more
complex socket buffer, the dropped packet should be reported as Drop
Code 2. DCCP implementations may also provide an API by which
applications can mark received packets as Drop Code 2, indicating
that the application ran out of space in its user-level receive
buffer. (However, it is not generally useful to report packets as
dropped due to Drop Code 2 after more than a couple of round-trip
times have passed. The HC-Sender may have forgotten its
acknowledgement state for the packet by that time, so the Data
Dropped report will have no effect.) Every packet newly acknowledged
as Drop Code 2 SHOULD reduce the sender's instantaneous rate by one
packet per round-trip time, unless the sender is already sending one
packet per RTT or less. Each CCID profile defines the CCID-specific
mechanism by which this is accomplished.
Currently, the other Drop Codes (namely Drop Code 3, Corrupt; Drop
Code 7, Delivered Corrupt; and reserved Drop Codes 4-6) MUST cause
the relevant CCID to behave as if the relevant packets were ECN
marked (ECN Congestion Experienced).
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12. Explicit Congestion Notification
The DCCP protocol is fully ECN-aware [RFC3168]. Each CCID specifies
how its endpoints respond to ECN marks. Furthermore, DCCP, unlike
TCP, allows senders to control the rate at which acknowledgements are
generated (with options like Ack Ratio); since acknowledgements are
congestion controlled, they also qualify as ECN-Capable Transport.
Each CCID profile describes how that CCID interacts with ECN, both
for data traffic and pure-acknowledgement traffic. A sender SHOULD
set ECN-Capable Transport on its packets' IP headers unless the
receiver's ECN Incapable feature is on or the relevant CCID disallows
it.
The rest of this section describes the ECN Incapable feature and the
interaction of the ECN Nonce with acknowledgement options such as Ack
Vector.
12.1. ECN Incapable Feature
DCCP endpoints are ECN-aware by default, but the ECN Incapable
feature lets an endpoint reject the use of Explicit Congestion
Notification. The use of this feature is NOT RECOMMENDED. ECN
incapability both avoids ECN's possible benefits and prevents senders
from using the ECN Nonce to check for receiver misbehavior. A DCCP
stack MAY therefore leave the ECN Incapable feature unimplemented,
acting as if all connections were ECN capable. Note that the
inappropriate firewall interactions that dogged TCP's implementation
of ECN [RFC3360] involve TCP header bits, not the IP header's ECN
bits; we know of no middlebox that would block ECN-capable DCCP
packets but allow ECN-incapable DCCP packets.
ECN Incapable has feature number 4 and is server-priority. It takes
one-byte Boolean values. DCCP A MUST be able to read ECN bits from
received frames' IP headers when ECN Incapable/A is zero. (This is
independent of whether it can set ECN bits on sent frames.) DCCP A
thus sends a "Change L(ECN Inapable, 1)" option to DCCP B to inform
it that A cannot read ECN bits. If the ECN Incapable/A feature is
one, then all of DCCP B's packets MUST be sent as ECN incapable. New
connections start with ECN Incapable 0 (that is, ECN capable) for
both endpoints. Values of two or more are reserved.
If a DCCP is not ECN capable, it MUST send Mandatory "Change L(ECN
Incapable, 1)" options to the other endpoint until acknowledged (by
"Confirm R(ECN Incapable, 1)") or the connection closes.
Furthermore, it MUST NOT accept any data until the other endpoint
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sends "Confirm R(ECN Incapable, 1)". It SHOULD send Data Dropped
options on its acknowledgements, with Drop Code 0 ("protocol
constraints"), if the other endpoint does send data inappropriately.
12.2. ECN Nonces
Congestion avoidance will not occur, and the receiver will sometimes
get its data faster, if the sender isn't told about congestion
events. Thus, the receiver has some incentive to falsify
acknowledgement information, reporting that marked or dropped packets
were actually received unmarked. This problem is more serious with
DCCP than with TCP, since TCP provides reliable transport: it is more
difficult with TCP to lie about lost packets without breaking the
application.
ECN Nonces are a general mechanism to prevent ECN cheating (or loss
cheating). Two values for the two-bit ECN header field indicate
ECN-Capable Transport, 01 and 10. The second code point, 10, is the
ECN Nonce. In general, a protocol sender chooses between these code
points randomly on its output packets, remembering the sequence it
chose. On every acknowledgement, the protocol receiver reports the
number of ECN Nonces it has received thus far. This is called the
ECN Nonce Echo. Since ECN marking and packet dropping both destroy
the ECN Nonce, a receiver that lies about an ECN mark or packet drop
has a 50% chance of guessing right and avoiding discipline. The
sender may react punitively to an ECN Nonce mismatch, possibly up to
dropping the connection. The ECN Nonce Echo field need not be an
integer; one bit is enough to catch 50% of infractions, and the
probability of success drops exponentially as more packets are sent
[RFC3540].
In DCCP, the ECN Nonce Echo field is encoded in acknowledgement
options. For example, the Ack Vector option comes in two forms, Ack
Vector [Nonce 0] (option 38) and Ack Vector [Nonce 1] (option 39),
corresponding to the two values for a one-bit ECN Nonce Echo. The
Nonce Echo for a given Ack Vector equals the one-bit sum (exclusive-
or, or parity) of ECN nonces for packets reported by that Ack Vector
as received and not ECN marked. Thus, only packets marked as State 0
matter for this calculation (that is, valid received packets that
were not ECN marked). Every Ack Vector option is detailed enough for
the sender to determine what the Nonce Echo should have been. It can
check this calculation against the actual Nonce Echo and complain if
there is a mismatch. (The Ack Vector could conceivably report every
packet's ECN Nonce state, but this would severely limit its
compressibility without providing much extra protection.)
Each DCCP sender SHOULD set ECN Nonces on its packets and remember
which packets had nonces. When a sender detects an ECN Nonce Echo
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mismatch, it behaves as described in the next section. Each DCCP
receiver MUST calculate and use the correct value for ECN Nonce Echo
when sending acknowledgement options.
ECN incapability, as indicated by the ECN Incapable feature, is
handled as follows: an endpoint sending packets to an ECN-incapable
receiver MUST send its packets as ECN incapable, and an ECN-
incapable receiver MUST use the value zero for all ECN Nonce Echoes.
12.3. Aggression Penalties
DCCP endpoints have several mechanisms for detecting congestion-
related misbehavior. For example:
o A sender can detect an ECN Nonce Echo mismatch, indicating
possible receiver misbehavior.
o A receiver can detect whether the sender is responding to
congestion feedback or Slow Receiver.
o An endpoint may be able to detect that its peer is reporting
inappropriately small Elapsed Time values (Section 13.2).
An endpoint that detects possible congestion-related misbehavior
SHOULD try to verify that its peer is truly misbehaving. For
example, a sending endpoint might send a packet whose ECN header
field is set to Congestion Experienced, 11; a receiver that doesn't
report a corresponding mark is most likely misbehaving.
Upon detecting possible misbehavior, a sender SHOULD respond as if
the receiver had reported one or more recent packets as ECN-marked
(instead of unmarked), while a receiver SHOULD report one or more
recent non-marked packets as ECN-marked. Alternately, a sender might
act as if the receiver had sent a Slow Receiver option, and a
receiver might send Slow Receiver options. Other reactions that
serve to slow the transfer rate are also acceptable. An entity that
detects particularly egregious and ongoing misbehavior MAY also reset
the connection with Reset Code 11, "Aggression Penalty".
However, ECN Nonce mismatches and other warning signs can result from
innocent causes, such as implementation bugs or attack. In
particular, a successful DCCP-Data attack (Section 7.5.5) can cause
the receiver to report an incorrect ECN Nonce Echo. Therefore,
connection reset and other heavyweight mechanisms SHOULD be used only
as last resorts, after multiple round-trip times of verified
aggression.
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13. Timing Options
The Timestamp, Timestamp Echo, and Elapsed Time options help DCCP
endpoints explicitly measure round-trip times.
13.1. Timestamp Option
This option is permitted in any DCCP packet. The length of the
option is 6 bytes.
+--------+--------+--------+--------+--------+--------+
|00101001|00000110| Timestamp Value |
+--------+--------+--------+--------+--------+--------+
Type=41 Length=6
The four bytes of option data carry the timestamp of this packet.
The timestamp is a 32-bit integer that increases monotonically with
time, at a rate of 1 unit per 10 microseconds. At this rate,
Timestamp Value will wrap approximately every 11.9 hours. Endpoints
need not measure time at this fine granularity; for example, an
endpoint that preferred to measure time at millisecond granularity
might send Timestamp Values that were all multiples of 100. The
precise time corresponding to Timestamp Value zero is not specified:
Timestamp Values are only meaningful relative to other Timestamp
Values sent on the same connection. A DCCP receiving a Timestamp
option SHOULD respond with a Timestamp Echo option on the next packet
it sends.
13.2. Elapsed Time Option
This option is permitted in any DCCP packet that contains an
Acknowledgement Number; such options received on other packet types
MUST be ignored. It indicates how much time has elapsed since the
packet being acknowledged -- the packet with the given
Acknowledgement Number -- was received. The option may take 4 or 6
bytes, depending on the size of the Elapsed Time value. Elapsed Time
helps correct round-trip time estimates when the gap between
receiving a packet and acknowledging that packet may be long -- in
CCID 3, for example, where acknowledgements are sent infrequently.
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+--------+--------+--------+--------+
|00101011|00000100| Elapsed Time |
+--------+--------+--------+--------+
Type=43 Len=4
+--------+--------+--------+--------+--------+--------+
|00101011|00000110| Elapsed Time |
+--------+--------+--------+--------+--------+--------+
Type=43 Len=6
The option data, Elapsed Time, represents an estimated lower bound on
the amount of time elapsed since the packet being acknowledged was
received, with units of hundredths of milliseconds. If Elapsed Time
is less than a half-second, the first, smaller form of the option
SHOULD be used. Elapsed Times of more than 0.65535 seconds MUST be
sent using the second form of the option. The special Elapsed Time
value 4294967295, which corresponds to approximately 11.9 hours, is
used to represent any Elapsed Time greater than 42949.67294 seconds.
DCCP endpoints MUST NOT report Elapsed Times that are significantly
larger than the true elapsed times. A connection MAY be reset with
Reset Code 11, "Aggression Penalty", if one endpoint determines that
the other is reporting a much-too-large Elapsed Time.
Elapsed Time is measured in hundredths of milliseconds as a
compromise between two conflicting goals. First, it provides enough
granularity to reduce rounding error when measuring elapsed time over
fast LANs; second, it allows many reasonable elapsed times to fit
into two bytes of data.
13.3. Timestamp Echo Option
This option is permitted in any DCCP packet, as long as at least one
packet carrying the Timestamp option has been received. Generally, a
DCCP endpoint should send one Timestamp Echo option for each
Timestamp option it receives, and it should send that option as soon
as is convenient. The length of the option is between 6 and 10
bytes, depending on whether Elapsed Time is included and how large it
is.
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+--------+--------+--------+--------+--------+--------+
|00101010|00000110| Timestamp Echo |
+--------+--------+--------+--------+--------+--------+
Type=42 Len=6
+--------+--------+------- ... -------+--------+--------+
|00101010|00001000| Timestamp Echo | Elapsed Time |
+--------+--------+------- ... -------+--------+--------+
Type=42 Len=8 (4 bytes)
+--------+--------+------- ... -------+------- ... -------+
|00101010|00001010| Timestamp Echo | Elapsed Time |
+--------+--------+------- ... -------+------- ... -------+
Type=42 Len=10 (4 bytes) (4 bytes)
The first four bytes of option data, Timestamp Echo, carry a
Timestamp Value taken from a preceding received Timestamp option.
Usually, this will be the last packet that was received -- the packet
indicated by the Acknowledgement Number, if any -- but it might be a
preceding packet. Each Timestamp received will generally result in
exactly one Timestamp Echo transmitted. If an endpoint has received
multiple Timestamp options since the last time it sent a packet, then
it MAY ignore all Timestamp options but the one included on the
packet with the greatest sequence number. Alternatively, it MAY
include multiple Timestamp Echo options in its response, each
corresponding to a different Timestamp option.
The Elapsed Time value, similar to that in the Elapsed Time option,
indicates the amount of time elapsed since receiving the packet whose
timestamp is being echoed. This time MUST have units of hundredths
of milliseconds. Elapsed Time is meant to help the Timestamp sender
separate the network round-trip time from the Timestamp receiver's
processing time. This may be particularly important for CCIDs where
acknowledgements are sent infrequently, so that there might be
considerable delay between receiving a Timestamp option and sending
the corresponding Timestamp Echo. A missing Elapsed Time field is
equivalent to an Elapsed Time of zero. The smallest version of the
option SHOULD be used that can hold the relevant Elapsed Time value.
14. Maximum Packet Size
A DCCP implementation MUST maintain the maximum packet size (MPS)
allowed for each active DCCP session. The MPS is influenced by the
maximum packet size allowed by the current congestion control
mechanism (CCMPS), the maximum packet size supported by the path's
links (PMTU, the Path Maximum Transmission Unit) [RFC1191], and the
lengths of the IP and DCCP headers.
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A DCCP application interface SHOULD let the application discover
DCCP's current MPS. Generally, the DCCP implementation will refuse
to send any packet bigger than the MPS, returning an appropriate
error to the application. A DCCP interface MAY allow applications to
request fragmentation for packets larger than PMTU, but not larger
than CCMPS. (Packets larger than CCMPS MUST be rejected in any
case.) Fragmentation SHOULD NOT be the default, since it decreases
robustness: an entire packet is discarded if even one of its
fragments is lost. Applications can usually get better error
tolerance by producing packets smaller than the PMTU.
The MPS reported to the application SHOULD be influenced by the size
expected to be required for DCCP headers and options. If the
application provides data that, when combined with the options the
DCCP implementation would like to include, would exceed the MPS, the
implementation should either send the options on a separate packet
(such as a DCCP-Ack) or lower the MPS, drop the data, and return an
appropriate error to the application.
14.1. Measuring PMTU
Each DCCP endpoint MUST keep track of the current PMTU for each
connection, except that this is not required for IPv4 connections
whose applications have requested fragmentation. The PMTU SHOULD be
initialized from the interface MTU that will be used to send packets.
The MPS will be initialized with the minimum of the PMTU and the
CCMPS, if any.
Classical PMTU discovery uses unfragmentable packets. In IPv4, these
packets have the IP Don't Fragment (DF) bit set; in IPv6, all packets
are unfragmentable once emitted by an end host. As specified in
[RFC1191], when a router receives a packet with DF set that is larger
than the next link's MTU, it sends an ICMP Destination Unreachable
message back to the source whose Code indicates that an
unfragmentable packet was too large to forward (a "Datagram Too Big"
message). When a DCCP implementation receives a Datagram Too Big
message, it decreases its PMTU to the Next-Hop MTU value given in the
ICMP message. If the MTU given in the message is zero, the sender
chooses a value for PMTU using the algorithm described in [RFC1191],
Section 7. If the MTU given in the message is greater than the
current PMTU, the Datagram Too Big message is ignored, as described
in [RFC1191]. (We are aware that this may cause problems for DCCP
endpoints behind certain firewalls.)
A DCCP implementation may allow the application occasionally to
request that PMTU discovery be performed again. This will reset the
PMTU to the outgoing interface's MTU. Such requests SHOULD be rate
limited, to one per two seconds, for example.
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A DCCP sender MAY treat the reception of an ICMP Datagram Too Big
message as an indication that the packet being reported was not lost
due to congestion, and so for the purposes of congestion control it
MAY ignore the DCCP receiver's indication that this packet did not
arrive. However, if this is done, then the DCCP sender MUST check
the ECN bits of the IP header echoed in the ICMP message and only
perform this optimization if these ECN bits indicate that the packet
did not experience congestion prior to reaching the router whose link
MTU it exceeded.
A DCCP implementation SHOULD ensure, as far as possible, that ICMP
Datagram Too Big messages were actually generated by routers, so that
attackers cannot drive the PMTU down to a falsely small value. The
simplest way to do this is to verify that the Sequence Number on the
ICMP error's encapsulated header corresponds to a Sequence Number
that the implementation recently sent. (According to current
specifications, routers should return the full DCCP header and
payload up to a maximum of 576 bytes [RFC1812] or the minimum IPv6
MTU [RFC2463], although they are not required to return more than 64
bits [RFC792]. Any amount greater than 128 bits will include the
Sequence Number.) ICMP Datagram Too Big messages with incorrect or
missing Sequence Numbers may be ignored, or the DCCP implementation
may lower the PMTU only temporarily in response. If more than three
odd Datagram Too Big messages are received and the other DCCP
endpoint reports more than three lost packets, however, the DCCP
implementation SHOULD assume the presence of a confused router and
either obey the ICMP messages' PMTU or (on IPv4 networks) switch to
allowing fragmentation.
DCCP also allows upward probing of the PMTU [PMTUD], where the DCCP
endpoint begins by sending small packets with DF set and then
gradually increases the packet size until a packet is lost. This
mechanism does not require any ICMP error processing. DCCP-Sync
packets are the best choice for upward probing, since DCCP-Sync
probes do not risk application data loss. The DCCP implementation
inserts arbitrary data into the DCCP-Sync application area, padding
the packet to the right length. Since every valid DCCP-Sync
generates an immediate DCCP-SyncAck in response, the endpoint will
have a pretty good idea of when a probe is lost.
14.2. Sender Behavior
A DCCP sender SHOULD send every packet as unfragmentable, as
described above, with the following exceptions.
o On IPv4 connections whose applications have requested
fragmentation, the sender SHOULD send packets with the DF bit not
set.
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o On IPv6 connections whose applications have requested
fragmentation, the sender SHOULD use fragmentation extension
headers to fragment packets larger than PMTU into suitably-sized
chunks. (Those chunks are, of course, unfragmentable.)
o It is undesirable for PMTU discovery to occur on the initial
connection setup handshake, as the connection setup process may
not be representative of packet sizes used during the connection,
and performing MTU discovery on the initial handshake might
unnecessarily delay connection establishment. Thus, DCCP-Request
and DCCP-Response packets SHOULD be sent as fragmentable. In
addition, DCCP-Reset packets SHOULD be sent as fragmentable,
although typically these would be small enough to not be a
problem. For IPv4 connections, these packets SHOULD be sent with
the DF bit not set; for IPv6 connections, they SHOULD be
preemptively fragmented to a size not larger than the relevant
interface MTU.
If the DCCP implementation has decreased the PMTU, the sending
application has not requested fragmentation, and the sending
application attempts to send a packet larger than the new MPS, the
API MUST refuse to send the packet and return an appropriate error to
the application. The application should then use the API to query
the new value of MPS. The kernel might have some packets buffered
for transmission that are smaller than the old MPS but larger than
the new MPS. It MAY send these packets as fragmentable, or it MAY
discard these packets; it MUST NOT send them as unfragmentable.
15. Forward Compatibility
Future versions of DCCP may add new options and features. A few
simple guidelines will let extended DCCPs interoperate with normal
DCCPs.
o DCCP processors MUST NOT act punitively towards options and
features they do not understand. For example, DCCP processors
MUST NOT reset the connection if some field marked Reserved in
this specification is non-zero; if some unknown option is present;
or if some feature negotiation option mentions an unknown feature.
Instead, DCCP processors MUST ignore these events. The Mandatory
option is the single exception: if Mandatory precedes some unknown
option or feature, the connection MUST be reset.
o DCCP processors MUST anticipate the possibility of unknown feature
values, which might occur as part of a negotiation for a known
feature. For server-priority features, unknown values are handled
as a matter of course: since the non-extended DCCP's priority list
will not contain unknown values, the result of the negotiation
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cannot be an unknown value. A DCCP MUST respond with an empty
Confirm option if it is assigned an unacceptable value for some
non-negotiable feature.
o Each DCCP extension SHOULD be controlled by some feature. The
default value of this feature SHOULD correspond to "extension not
available". If an extended DCCP wants to use the extension, it
SHOULD attempt to change the feature's value using a Change L or
Change R option. Any non-extended DCCP will ignore the option,
thus leaving the feature value at its default, "extension not
available".
Section 19 lists DCCP assigned numbers reserved for experimental and
testing purposes.
16. Middlebox Considerations
This section describes properties of DCCP that firewalls, network
address translators, and other middleboxes should consider, including
parts of the packet that middleboxes should not change. The intent
is to draw attention to aspects of DCCP that may be useful, or
dangerous, for middleboxes, or that differ significantly from TCP.
The Service Code field in DCCP-Request packets provides information
that may be useful for stateful middleboxes. With Service Code, a
middlebox can tell what protocol a connection will use without
relying on port numbers. Middleboxes can disallow connections that
attempt to access unexpected services by sending a DCCP-Reset with
Reset Code 8, "Bad Service Code". Middleboxes should not modify the
Service Code unless they are really changing the service a connection
is accessing.
The Source and Destination Port fields are in the same packet
locations as the corresponding fields in TCP and UDP, which may
simplify some middlebox implementations.
The forward compatibility considerations in Section 15 apply to
middleboxes as well. In particular, middleboxes generally shouldn't
act punitively towards options and features they do not understand.
Modifying DCCP Sequence Numbers and Acknowledgement Numbers is more
tedious and dangerous than modifying TCP sequence numbers. A
middlebox that added packets to or removed packets from a DCCP
connection would have to modify acknowledgement options, such as Ack
Vector, and CCID-specific options, such as TFRC's Loss Intervals, at
minimum. On ECN-capable connections, the middlebox would have to
keep track of ECN Nonce information for packets it introduced or
removed, so that the relevant acknowledgement options continued to
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have correct ECN Nonce Echoes, or risk the connection being reset for
"Aggression Penalty". We therefore recommend that middleboxes not
modify packet streams by adding or removing packets.
Note that there is less need to modify DCCP's per-packet sequence
numbers than to modify TCP's per-byte sequence numbers; for example,
a middlebox can change the contents of a packet without changing its
sequence number. (In TCP, sequence number modification is required
to support protocols like FTP that carry variable-length addresses in
the data stream. If such an application were deployed over DCCP,
middleboxes would simply grow or shrink the relevant packets as
necessary without changing their sequence numbers. This might
involve fragmenting the packet.)
Middleboxes may, of course, reset connections in progress. Clearly,
this requires inserting a packet into one or both packet streams, but
the difficult issues do not arise.
DCCP is somewhat unfriendly to "connection splicing" [SHHP00], in
which clients' connection attempts are intercepted, but possibly
later "spliced in" to external server connections via sequence number
manipulations. A connection splicer at minimum would have to ensure
that the spliced connections agreed on all relevant feature values,
which might take some renegotiation.
The contents of this section should not be interpreted as a wholesale
endorsement of stateful middleboxes.
17. Relations to Other Specifications
17.1. RTP
The Real-Time Transport Protocol, RTP [RFC3550], is currently used
over UDP by many of DCCP's target applications (for instance,
streaming media). Therefore, it is important to examine the
relationship between DCCP and RTP and, in particular, the question of
whether any changes in RTP are necessary or desirable when it is
layered over DCCP instead of UDP.
There are two potential sources of overhead in the RTP-over-DCCP
combination: duplicated acknowledgement information and duplicated
sequence numbers. Together, these sources of overhead add slightly
more than 4 bytes per packet relative to RTP-over-UDP, and
eliminating the redundancy would not reduce the overhead.
First, consider acknowledgements. Both RTP and DCCP report feedback
about loss rates to data senders, via RTP Control Protocol Sender and
Receiver Reports (RTCP SR/RR packets) and via DCCP acknowledgement
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options. These feedback mechanisms are potentially redundant.
However, RTCP SR/RR packets contain information not present in DCCP
acknowledgements, such as "interarrival jitter", and DCCP's
acknowledgements contain information not transmitted by RTCP, such as
the ECN Nonce Echo. Neither feedback mechanism makes the other
redundant.
Sending both types of feedback need not be particularly costly
either. RTCP reports may be sent relatively infrequently: once every
5 seconds on average, for low-bandwidth flows. In DCCP, some
feedback mechanisms are expensive -- Ack Vector, for example, is
frequent and verbose -- but others are relatively cheap: CCID 3
(TFRC) acknowledgements take between 16 and 32 bytes of options sent
once per round-trip time. (Reporting less frequently than once per
RTT would make congestion control less responsive to loss.) We
therefore conclude that acknowledgement overhead in RTP-over-DCCP
need not be significantly higher than for RTP-over-UDP, at least for
CCID 3.
One clear redundancy can be addressed at the application level. The
verbose packet-by-packet loss reports sent in RTCP Extended Reports
Loss RLE Blocks [RFC3611] can be derived from DCCP's Ack Vector
options. (The converse is not true, since Loss RLE Blocks contain no
ECN information.) Since DCCP implementations should provide an API
for application access to Ack Vector information, RTP-over-DCCP
applications might request either DCCP Ack Vectors or RTCP Extended
Report Loss RLE Blocks, but not both.
Now consider sequence number redundancy on data packets. The
embedded RTP header contains a 16-bit RTP sequence number. Most data
packets will use the DCCP-Data type; DCCP-DataAck and DCCP-Ack
packets need not usually be sent. The DCCP-Data header is 12 bytes
long without options, including a 24-bit sequence number. This is 4
bytes more than a UDP header. Any options required on data packets
would add further overhead, although many CCIDs (for instance, CCID
3, TFRC) don't require options on most data packets.
The DCCP sequence number cannot be inferred from the RTP sequence
number since it increments on non-data packets as well as data
packets. The RTP sequence number cannot be inferred from the DCCP
sequence number either [RFC3550]. Furthermore, removing RTP's
sequence number would not save any header space because of alignment
issues. We therefore recommend that RTP transmitted over DCCP use
the same headers currently defined. The 4 byte header cost is a
reasonable tradeoff for DCCP's congestion control features and access
to ECN. Truly bandwidth-starved endpoints should use some header
compression scheme.
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17.2. Congestion Manager and Multiplexing
Since DCCP doesn't provide reliable, ordered delivery, multiple
application sub-flows may be multiplexed over a single DCCP
connection with no inherent performance penalty. Thus, there is no
need for DCCP to provide built-in support for multiple sub-flows.
This differs from SCTP [RFC2960].
Some applications might want to share congestion control state among
multiple DCCP flows that share the same source and destination
addresses. This functionality could be provided by the Congestion
Manager [RFC3124], a generic multiplexing facility. However, the CM
would not fully support DCCP without change; it does not gracefully
handle multiple congestion control mechanisms, for example.
18. Security Considerations
DCCP does not provide cryptographic security guarantees.
Applications desiring cryptographic security services (integrity,
authentication, confidentiality, access control, and anti-replay
protection) should use IPsec or end-to-end security of some kind;
Secure RTP is one candidate protocol [RFC3711].
Nevertheless, DCCP is intended to protect against some classes of
attackers: Attackers cannot hijack a DCCP connection (close the
connection unexpectedly, or cause attacker data to be accepted by an
endpoint as if it came from the sender) unless they can guess valid
sequence numbers. Thus, as long as endpoints choose initial sequence
numbers well, a DCCP attacker must snoop on data packets to get any
reasonable probability of success. Sequence number validity checks
provide this guarantee. Section 7.5.5 describes sequence number
security further. This security property only holds assuming that
DCCP's random numbers are chosen according to the guidelines in
[RFC4086].
DCCP also provides mechanisms to limit the potential impact of some
denial-of-service attacks. These mechanisms include Init Cookie
(Section 8.1.4), the DCCP-CloseReq packet (Section 5.5), the
Application Not Listening Drop Code (Section 11.7.2), limitations on
the processing of options that might cause connection reset (Section
7.5.5), limitations on the processing of some ICMP messages (Section
14.1), and various rate limits, which let servers avoid extensive
computation or packet generation (Sections 7.5.3, 8.1.3, and others).
DCCP provides no protection against attackers that can snoop on data
packets.
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18.1. Security Considerations for Partial Checksums
The partial checksum facility has a separate security impact,
particularly in its interaction with authentication and encryption
mechanisms. The impact is the same in DCCP as in the UDP-Lite
protocol, and what follows was adapted from the corresponding text in
the UDP-Lite specification [RFC3828].
When a DCCP packet's Checksum Coverage field is not zero, the
uncovered portion of a packet may change in transit. This is
contrary to the idea behind most authentication mechanisms:
authentication succeeds if the packet has not changed in transit.
Unless authentication mechanisms that operate only on the sensitive
part of packets are developed and used, authentication will always
fail for partially-checksummed DCCP packets whose uncovered part has
been damaged.
The IPsec integrity check (Encapsulation Security Protocol, ESP, or
Authentication Header, AH) is applied (at least) to the entire IP
packet payload. Corruption of any bit within that area will then
result in the IP receiver's discarding a DCCP packet, even if the
corruption happened in an uncovered part of the DCCP application
data.
When IPsec is used with ESP payload encryption, a link can not
determine the specific transport protocol of a packet being forwarded
by inspecting the IP packet payload. In this case, the link MUST
provide a standard integrity check covering the entire IP packet and
payload. DCCP partial checksums provide no benefit in this case.
Encryption (e.g., at the transport or application levels) may be
used. Note that omitting an integrity check can, under certain
circumstances, compromise confidentiality [B98].
If a few bits of an encrypted packet are damaged, the decryption
transform will typically spread errors so that the packet becomes too
damaged to be of use. Many encryption transforms today exhibit this
behavior. There exist encryption transforms, stream ciphers, that do
not cause error propagation. Proper use of stream ciphers can be
quite difficult, especially when authentication checking is omitted
[BB01]. In particular, an attacker can cause predictable changes to
the ultimate plaintext, even without being able to decrypt the
ciphertext.
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19. IANA Considerations
IANA has assigned IP Protocol Number 33 to DCCP.
DCCP introduces eight sets of numbers whose values should be
allocated by IANA. We refer to allocation policies, such as
Standards Action, outlined in [RFC2434], and most registries reserve
some values for experimental and testing use [RFC3692]. In addition,
DCCP requires that the IANA Port Numbers registry be opened for DCCP
port registrations; Section 19.9 describes how. The IANA should feel
free to contact the DCCP Expert Reviewer with questions on any
registry, regardless of the registry policy, for clarification or if
there is a problem with a request.
19.1. Packet Types Registry
Each entry in the DCCP Packet Types registry contains a packet type,
which is a number in the range 0-15; a packet type name, such as
DCCP-Request; and a reference to the RFC defining the packet type.
The registry is initially populated using the values in Table 1
(Section 5.1). This document allocates packet types 0-9, and packet
type 14 is permanently reserved for experimental and testing use.
Packet types 10-13 and 15 are currently reserved and should be
allocated with the Standards Action policy, which requires IESG
review and approval and standards-track IETF RFC publication.
19.2. Reset Codes Registry
Each entry in the DCCP Reset Codes registry contains a Reset Code,
which is a number in the range 0-255; a short description of the
Reset Code, such as "No Connection"; and a reference to the RFC
defining the Reset Code. The registry is initially populated using
the values in Table 2 (Section 5.6). This document allocates Reset
Codes 0-11, and Reset Codes 120-126 are permanently reserved for
experimental and testing use. Reset Codes 12-119 and 127 are
currently reserved and should be allocated with the IETF Consensus
policy, requiring an IETF RFC publication (standards track or not)
with IESG review and approval. Reset Codes 128-255 are permanently
reserved for CCID-specific registries; each CCID Profile document
describes how the corresponding registry is managed.
19.3. Option Types Registry
Each entry in the DCCP option types registry contains an option type,
which is a number in the range 0-255; the name of the option, such as
"Slow Receiver"; and a reference to the RFC defining the option type.
The registry is initially populated using the values in Table 3
(Section 5.8). This document allocates option types 0-2 and 32-44,
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and option types 31 and 120-126 are permanently reserved for
experimental and testing use. Option types 3-30, 45-119, and 127 are
currently reserved and should be allocated with the IETF Consensus
policy, requiring an IETF RFC publication (standards track or not)
with IESG review and approval. Option types 128-255 are permanently
reserved for CCID-specific registries; each CCID Profile document
describes how the corresponding registry is managed.
19.4. Feature Numbers Registry
Each entry in the DCCP feature numbers registry contains a feature
number, which is a number in the range 0-255; the name of the
feature, such as "ECN Incapable"; and a reference to the RFC defining
the feature number. The registry is initially populated using the
values in Table 4 (Section 6). This document allocates feature
numbers 0-9, and feature numbers 120-126 are permanently reserved for
experimental and testing use. Feature numbers 10-119 and 127 are
currently reserved and should be allocated with the IETF Consensus
policy, requiring an IETF RFC publication (standards track or not)
with IESG review and approval. Feature numbers 128-255 are
permanently reserved for CCID-specific registries; each CCID Profile
document describes how the corresponding registry is managed.
19.5. Congestion Control Identifiers Registry
Each entry in the DCCP Congestion Control Identifiers (CCIDs)
registry contains a CCID, which is a number in the range 0-255; the
name of the CCID, such as "TCP-like Congestion Control"; and a
reference to the RFC defining the CCID. The registry is initially
populated using the values in Table 5 (Section 10). CCIDs 2 and 3
are allocated by concurrently published profiles, and CCIDs 248-254
are permanently reserved for experimental and testing use. CCIDs 0,
1, 4-247, and 255 are currently reserved and should be allocated with
the IETF Consensus policy, requiring an IETF RFC publication
(standards track or not) with IESG review and approval.
19.6. Ack Vector States Registry
Each entry in the DCCP Ack Vector States registry contains an Ack
Vector State, which is a number in the range 0-3; the name of the
State, such as "Received ECN Marked"; and a reference to the RFC
defining the State. The registry is initially populated using the
values in Table 6 (Section 11.4). This document allocates States 0,
1, and 3. State 2 is currently reserved and should be allocated with
the Standards Action policy, which requires IESG review and approval
and standards-track IETF RFC publication.
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19.7. Drop Codes Registry
Each entry in the DCCP Drop Codes registry contains a Data Dropped
Drop Code, which is a number in the range 0-7; the name of the Drop
Code, such as "Application Not Listening"; and a reference to the RFC
defining the Drop Code. The registry is initially populated using
the values in Table 7 (Section 11.7). This document allocates Drop
Codes 0-3 and 7. Drop Codes 4-6 are currently reserved, and should
be allocated with the Standards Action policy, which requires IESG
review and approval and standards-track IETF RFC publication.
19.8. Service Codes Registry
Each entry in the Service Codes registry contains a Service Code,
which is a number in the range 0-4294967294; a short English
description of the intended service; and an optional reference to an
RFC or other publicly available specification defining the Service
Code. The registry should list the Service Code's numeric value as a
decimal number. When the Service Code may be represented in "SC:"
format according to the rules in Section 8.1.2, the registry should
also show the corresponding ASCII interpretation of the Service Code
minus the "SC:" prefix. Thus, the number 1717858426 would
additionally appear as "fdpz". Service Codes are not DCCP-specific.
Service Code 0 is permanently reserved (it represents the absence of
a meaningful Service Code), and Service Codes 1056964608-1073741823
(high byte ASCII "?") are reserved for Private Use. Note that
4294967295 is not a valid Service Code. Most of the remaining
Service Codes are allocated First Come First Served, with no RFC
publication required; exceptions are listed in Section 8.1.2. This
document allocates a single Service Code, 1145656131 ("DISC"). This
corresponds to the discard service, which discards all data sent to
the service and sends no data in reply.
19.9. Port Numbers Registry
DCCP services may use contact port numbers to provide service to
unknown callers, as in TCP and UDP. IANA is therefore requested to
open the existing Port Numbers registry for DCCP using the following
rules, which we intend to mesh well with existing Port Numbers
registration procedures.
Port numbers are divided into three ranges. The Well Known Ports are
those from 0 through 1023, the Registered Ports are those from 1024
through 49151, and the Dynamic and/or Private Ports are those from
49152 through 65535. Well Known and Registered Ports are intended
for use by server applications that desire a default contact point on
a system. On most systems, Well Known Ports can only be used by
system (or root) processes or by programs executed by privileged
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users, while Registered Ports can be used by ordinary user processes
or programs executed by ordinary users. Dynamic and/or Private Ports
are intended for temporary use, including client-side ports, out-of-
band negotiated ports, and application testing prior to registration
of a dedicated port; they MUST NOT be registered.
The Port Numbers registry should accept registrations for DCCP ports
in the Well Known Ports and Registered Ports ranges. Well Known and
Registered Ports SHOULD NOT be used without registration. Although
in some cases -- such as porting an application from UDP to DCCP --
it may seem natural to use a DCCP port before registration completes,
we emphasize that IANA will not guarantee registration of particular
Well Known and Registered Ports. Registrations should be requested
as early as possible.
Each port registration SHALL include the following information:
o A short port name, consisting entirely of letters (A-Z and a-z),
digits (0-9), and punctuation characters from "-_+./*" (not
including the quotes).
o The port number that is requested to be registered.
o A short English phrase describing the port's purpose. This MUST
include one or more space-separated textual Service Code
descriptors naming the port's corresponding Service Codes (see
Section 8.1.2).
o Name and contact information for the person or entity performing
the registration, and possibly a reference to a document defining
the port's use. Registrations coming from IETF working groups
need only name the working group, but indicating a contact person
is recommended.
Registrants are encouraged to follow these guidelines when submitting
a registration.
o A port name SHOULD NOT be registered for more than one DCCP port
number.
o A port name registered for UDP MAY be registered for DCCP as well.
Any such registration SHOULD use the same port number as the
existing UDP registration.
o Concrete intent to use a port SHOULD precede port registration.
For example, existing UDP ports SHOULD NOT be registered in
advance of any intent to use those ports for DCCP.
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o A port name generally associated with TCP and/or SCTP SHOULD NOT
be registered for DCCP, since that port name implies reliable
transport. For example, we discourage registration of any "http"
port for DCCP. However, if such a registration makes sense (that
is, if there is concrete intent to use such a port), the DCCP
registration SHOULD use the same port number as the existing
registration.
o Multiple DCCP registrations for the same port number are allowed
as long as the registrations' Service Codes do not overlap.
This document registers the following port. (This should be
considered a model registration.)
discard 9/dccp Discard SC:DISC
# IETF dccp WG, Eddie Kohler , [RFC4340]
The discard service, which accepts DCCP connections on port 9,
discards all incoming application data and sends no data in response.
Thus, DCCP's discard port is analogous to TCP's discard port, and
might be used to check the health of a DCCP stack.
20. Thanks
Thanks to Jitendra Padhye for his help with early versions of this
specification.
Thanks to Junwen Lai and Arun Venkataramani, who, as interns at ICIR,
built a prototype DCCP implementation. In particular, Junwen Lai
recommended that the old feature negotiation mechanism be scrapped
and co-designed the current mechanism. Arun Venkataramani's feedback
improved Appendix A.
We thank the staff and interns of ICIR and, formerly, ACIRI, the
members of the End-to-End Research Group, and the members of the
Transport Area Working Group for their feedback on DCCP. We
especially thank the DCCP expert reviewers Greg Minshall, Eric
Rescorla, and Magnus Westerlund for detailed written comments and
problem spotting, and Rob Austein and Steve Bellovin for verbal
comments and written notes. We also especially thank Aaron Falk, the
working group chair during the development of this specification.
We also thank those who provided comments and suggestions via the
DCCP BOF, Working Group, and mailing lists, including Damon Lanphear,
Patrick McManus, Colin Perkins, Sara Karlberg, Kevin Lai, Bernard
Aboba, Youngsoo Choi, Pengfei Di, Dan Duchamp, Lars Eggert, Gorry
Fairhurst, Derek Fawcus, David Timothy Fleeman, John Loughney,
Ghyslain Pelletier, Hagen Paul Pfeifer, Tom Phelan, Stanislav
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RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
Shalunov, Somsak Vanit-Anunchai, David Vos, Yufei Wang, and Michael
Welzl. In particular, Colin Perkins provided extensive, detailed
feedback, Michael Welzl suggested the Data Checksum option, Gorry
Fairhurst provided extensive feedback on various checksum issues, and
Somsak Vanit-Anunchai, Jonathan Billington, and Tul Kongprakaiwoot's
Colored Petri Net model [VBK05] discovered several problems with
message exchange.
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A. Appendix: Ack Vector Implementation Notes
This appendix discusses particulars of DCCP acknowledgement handling
in the context of an abstract implementation for Ack Vector. It is
informative and not normative.
The first part of our implementation runs at the HC-Receiver, and
therefore acknowledges data packets. It generates Ack Vector
options. The implementation has the following characteristics:
o At most one byte of state per acknowledged packet.
o O(1) time to update that state when a new packet arrives (normal
case).
o Cumulative acknowledgements.
o Quick removal of old state.
The basic data structure is a circular buffer containing information
about acknowledged packets. Each byte in this buffer contains a
state and run length; the state can be 0 (packet received), 1 (packet
ECN marked), or 3 (packet not yet received). The buffer grows from
right to left. The implementation maintains five variables, aside
from the buffer contents:
o "buf_head" and "buf_tail", which mark the live portion of the
buffer.
o "buf_ackno", the Acknowledgement Number of the most recent packet
acknowledged in the buffer. This corresponds to the "head"
pointer.
o "buf_nonce", the one-bit sum (exclusive-or, or parity) of the ECN
Nonces received on all packets acknowledged by the buffer with
State 0.
We draw acknowledgement buffers like this:
+---------------------------------------------------------------+
|S,L|S,L|S,L|S,L| | | | |S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|
+---------------------------------------------------------------+
^ ^
buf_tail buf_head, buf_ackno = A buf_nonce = E
<=== buf_head and buf_tail move this way <===
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Each "S,L" represents a State/Run length byte. We will draw these
buffers showing only their live portion and will add an annotation
showing the Acknowledgement Number for the last live byte in the
buffer. For example:
+-----------------------------------------------+
A |S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L|S,L| T BN[E]
+-----------------------------------------------+
Here, buf_nonce equals E and buf_ackno equals A.
We will use this buffer as a running example.
+---------------------------+
10 |0,0|3,0|3,0|3,0|0,4|1,0|0,0| 0 BN[1] [Example Buffer]
+---------------------------+
In concrete terms, its meaning is as follows:
Packet 10 was received. (The head of the buffer has sequence
number 10, state 0, and run length 0.)
Packets 9, 8, and 7 have not yet been received. (The three bytes
preceding the head each have state 3 and run length 0.)
Packets 6, 5, 4, 3, and 2 were received.
Packet 1 was ECN marked.
Packet 0 was received.
The one-bit sum of the ECN Nonces on packets 10, 6, 5, 4, 3, 2,
and 0 equals 1.
Additionally, the HC-Receiver must keep some information about the
Ack Vectors it has recently sent. For each packet sent carrying an
Ack Vector, it remembers four variables:
o "ack_seqno", the Sequence Number used for the packet. This is an
HC-Receiver sequence number.
o "ack_ptr", the value of buf_head at the time of acknowledgement.
o "ack_runlen", the run length stored in the byte of buffer data at
buf_head at the time of acknowledgement.
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o "ack_ackno", the Acknowledgement Number used for the packet. This
is an HC-Sender sequence number. Since acknowledgements are
cumulative, this single number completely specifies all necessary
information about the packets acknowledged by this Ack Vector.
o "ack_nonce", the one-bit sum of the ECN Nonces for all State 0
packets in the buffer from buf_head to ack_ackno, inclusive.
Initially, this equals the Nonce Echo of the acknowledgement's Ack
Vector (or, if the ack packet contained more than one Ack Vector,
the exclusive-or of all the acknowledgement's Ack Vectors). It
changes as information about old acknowledgements is removed (so
ack_ptr and buf_head diverge) and as old packets arrive (so they
change from State 3 or State 1 to State 0).
A.1. Packet Arrival
This section describes how the HC-Receiver updates its
acknowledgement buffer as packets arrive from the HC-Sender.
A.1.1. New Packets
When a packet with Sequence Number greater than buf_ackno arrives,
the HC-Receiver updates buf_head (by moving it to the left
appropriately), buf_ackno (which is set to the new packet's Sequence
Number), and possibly buf_nonce (if the packet arrived unmarked with
ECN Nonce 1), in addition to the buffer itself. For example, if
HC-Sender packet 11 arrived ECN marked, the Example Buffer above
would enter this new state (changes are marked with stars):
** +***----------------------------+
11 |1,0|0,0|3,0|3,0|3,0|0,4|1,0|0,0| 0 BN[1]
** +***----------------------------+
If the packet's state equals the state at the head of the buffer, the
HC-Receiver may choose to increment its run length (up to the
maximum). For example, if HC-Sender packet 11 arrived without ECN
marking and with ECN Nonce 0, the Example Buffer might enter this
state instead:
** +--*------------------------+
11 |0,1|3,0|3,0|3,0|0,4|1,0|0,0| 0 BN[1]
** +--*------------------------+
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Of course, the new packet's sequence number might not equal the
expected sequence number. In this case, the HC-Receiver will enter
the intervening packets as State 3. If several packets are missing,
the HC-Receiver may prefer to enter multiple bytes with run length 0,
rather than a single byte with a larger run length; this simplifies
table updates if one of the missing packets arrives. For example, if
HC-Sender packet 12 arrived with ECN Nonce 1, the Example Buffer
would enter this state:
** +*******----------------------------+ *
12 |0,0|3,0|0,1|3,0|3,0|3,0|0,4|1,0|0,0| 0 BN[0]
** +*******----------------------------+ *
Of course, the circular buffer may overflow when the HC-Sender is
sending data at a very high rate, when the HC-Receiver's
acknowledgements are not reaching the HC-Sender, or when the
HC-Sender is forgetting to acknowledge those acks (so the HC-Receiver
is unable to clean up old state). In this case, the HC-Receiver
should either compress the buffer (by increasing run lengths when
possible), transfer its state to a larger buffer, or, as a last
resort, drop all received packets, without processing them at all,
until its buffer shrinks again.
A.1.2. Old Packets
When a packet with Sequence Number S <= buf_ackno arrives, the
HC-Receiver will scan the table for the byte corresponding to S.
(Indexing structures could reduce the complexity of this scan.) If S
was previously lost (State 3), and it was stored in a byte with run
length 0, the HC-Receiver can simply change the byte's state. For
example, if HC-Sender packet 8 was received with ECN Nonce 0, the
Example Buffer would enter this state:
+--------*------------------+
10 |0,0|3,0|0,0|3,0|0,4|1,0|0,0| 0 BN[1]
+--------*------------------+
If S was not marked as lost, or if it was not contained in the table,
the packet is probably a duplicate and should be ignored. (The new
packet's ECN marking state might differ from the state in the buffer;
Section 11.4.1 describes what is allowed then.) If S's buffer byte
has a non-zero run length, then the buffer might need to be
reshuffled to make space for one or two new bytes.
The ack_nonce fields may also need manipulation when old packets
arrive. In particular, when S transitions from State 3 or State 1 to
State 0, and S had ECN Nonce 1, then the implementation should flip
the value of ack_nonce for every acknowledgement with ack_ackno >= S.
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It is impossible with this data structure to shift packets from State
0 to State 1, since the buffer doesn't store individual packets' ECN
Nonces.
A.2. Sending Acknowledgements
Whenever the HC-Receiver needs to generate an acknowledgement, the
buffer's contents can simply be copied into one or more Ack Vector
options. Copied Ack Vectors might not be maximally compressed; for
example, the Example Buffer above contains three adjacent 3,0 bytes
that could be combined into a single 3,2 byte. The HC-Receiver
might, therefore, choose to compress the buffer in place before
sending the option, or to compress the buffer while copying it;
either operation is simple.
Every acknowledgement sent by the HC-Receiver SHOULD include the
entire state of the buffer. That is, acknowledgements are
cumulative.
If the acknowledgement fits in one Ack Vector, that Ack Vector's
Nonce Echo simply equals buf_nonce. For multiple Ack Vectors, more
care is required. The Ack Vectors should be split at points
corresponding to previous acknowledgements, since the stored
ack_nonce fields provide enough information to calculate correct
Nonce Echoes. The implementation should therefore acknowledge data
at least once per 253 bytes of buffer state. (Otherwise, there'd be
no way to calculate a Nonce Echo.)
For each acknowledgement it sends, the HC-Receiver will add an
acknowledgement record. ack_seqno will equal the HC-Receiver
sequence number it used for the ack packet; ack_ptr will equal
buf_head; ack_runlen will equal the run length stored in the buffer's
buf_head byte; ack_ackno will equal buf_ackno; and ack_nonce will
equal buf_nonce.
A.3. Clearing State
Some of the HC-Sender's packets will include acknowledgement numbers,
which ack the HC-Receiver's acknowledgements. When such an ack is
received, the HC-Receiver finds the acknowledgement record R with the
appropriate ack_seqno and then does the following:
o If the run length in the buffer's R.ack_ptr byte is greater than
R.ack_runlen, then it decrements that run length by
R.ack_runlen + 1 and sets buf_tail to R.ack_ptr. Otherwise, it
sets buf_tail to R.ack_ptr + 1.
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o If R.ack_nonce is 1, it flips buf_nonce, and the value of
ack_nonce for every later ack record.
o It throws away R and every preceding ack record.
(The HC-Receiver may choose to keep some older information, in case a
lost packet shows up late.) For example, say that the HC-Receiver
storing the Example Buffer had sent two acknowledgements already:
1. ack_seqno = 59, ack_runlen = 1, ack_ackno = 3, ack_nonce = 1.
2. ack_seqno = 60, ack_runlen = 0, ack_ackno = 10, ack_nonce = 0.
Say the HC-Receiver then received a DCCP-DataAck packet with
Acknowledgement Number 59 from the HC-Sender. This informs the
HC-Receiver that the HC-Sender received, and processed, all the
information in HC-Receiver packet 59. This packet acknowledged
HC-Sender packet 3, so the HC-Sender has now received HC-Receiver's
acknowledgements for packets 0, 1, 2, and 3. The Example Buffer
should enter this state:
+------------------*+ * *
10 |0,0|3,0|3,0|3,0|0,2| 4 BN[0]
+------------------*+ * *
The tail byte's run length was adjusted, since packet 3 was in the
middle of that byte. Since R.ack_nonce was 1, the buf_nonce field
was flipped, as were the ack_nonce fields for later acknowledgements
(here, the HC-Receiver Ack 60 record, not shown, has its ack_nonce
flipped to 1). The HC-Receiver can also throw away stored
information about HC-Receiver Ack 59 and any earlier
acknowledgements.
A careful implementation might try to ensure reasonable robustness to
reordering. Suppose that the Example Buffer is as before, but that
packet 9 now arrives, out of sequence. The buffer would enter this
state:
+----*----------------------+
10 |0,0|0,0|3,0|3,0|0,4|1,0|0,0| 0 BN[1]
+----*----------------------+
The danger is that the HC-Sender might acknowledge the HC-Receiver's
previous acknowledgement (with sequence number 60), which says that
Packet 9 was not received, before the HC-Receiver has a chance to
send a new acknowledgement saying that Packet 9 actually was
received. Therefore, when packet 9 arrived, the HC-Receiver might
modify its acknowledgement record as follows:
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1. ack_seqno = 59, ack_ackno = 3, ack_nonce = 1.
2. ack_seqno = 60, ack_ackno = 3, ack_nonce = 1.
That is, Ack 60 is now treated like a duplicate of Ack 59. This
would prevent the Tail pointer from moving past packet 9 until the
HC-Receiver knows that the HC-Sender has seen an Ack Vector
indicating that packet's arrival.
A.4. Processing Acknowledgements
When the HC-Sender receives an acknowledgement, it generally cares
about the number of packets that were dropped and/or ECN marked. It
simply reads this off the Ack Vector. Additionally, it should check
the ECN Nonce for correctness. (As described in Section 11.4.1, it
may want to keep more detailed information about acknowledged packets
in case packets change states between acknowledgements, or in case
the application queries whether a packet arrived.)
The HC-Sender must also acknowledge the HC-Receiver's
acknowledgements so that the HC-Receiver can free old Ack Vector
state. (Since Ack Vector acknowledgements are reliable, the
HC-Receiver must maintain and resend Ack Vector information until it
is sure that the HC-Sender has received that information.) A simple
algorithm suffices: since Ack Vector acknowledgements are cumulative,
a single acknowledgement number tells HC-Receiver how much ack
information has arrived. Assuming that the HC-Receiver sends no
data, the HC-Sender can ensure that at least once a round-trip time,
it sends a DCCP-DataAck packet acknowledging the latest DCCP-Ack
packet it has received. Of course, the HC-Sender only needs to
acknowledge the HC-Receiver's acknowledgements if the HC-Sender is
also sending data. If the HC-Sender is not sending data, then the
HC-Receiver's Ack Vector state is stable, and there is no need to
shrink it. The HC-Sender must watch for drops and ECN marks on
received DCCP-Ack packets so that it can adjust the HC-Receiver's
ack-sending rate in response to congestion, for example, with Ack
Ratio.
If the other half-connection is not quiescent -- that is, the
HC-Receiver is sending data to the HC-Sender, possibly using another
CCID -- then the acknowledgements on that half-connection are
sufficient for the HC-Receiver to free its state.
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B. Appendix: Partial Checksumming Design Motivation
A great deal of discussion has taken place regarding the utility of
allowing a DCCP sender to restrict the checksum so that it does not
cover the complete packet. This section attempts to capture some of
the rationale behind specific details of DCCP design.
Many of the applications that we envisage using DCCP are resilient to
some degree of data loss, or they would typically have chosen a
reliable transport. Some of these applications may also be resilient
to data corruption -- some audio payloads, for example. These
resilient applications might rather receive corrupted data than have
DCCP drop corrupted packets. This is particularly because of
congestion control: DCCP cannot tell the difference between packets
dropped due to corruption and packets dropped due to congestion, and
so it must reduce the transmission rate accordingly. This response
may cause the connection to receive less bandwidth than it is due;
corruption in some networking technologies is independent of, or at
least not always correlated to, congestion. Therefore, corrupted
packets do not need to cause as strong a reduction in transmission
rate as the congestion response would dictate (as long as the DCCP
header and options are not corrupt).
Thus DCCP allows the checksum to cover all of the packet, just the
DCCP header, or both the DCCP header and some number of bytes from
the application data. If the application cannot tolerate any data
corruption, then the checksum must cover the whole packet. If the
application would prefer to tolerate some corruption rather than have
the packet dropped, then it can set the checksum to cover only part
of the packet (but always the DCCP header). In addition, if the
application wishes to decouple checksumming of the DCCP header from
checksumming of the application data, it may do so by including the
Data Checksum option. This would allow DCCP to discard corrupted
application data without mistaking the corruption for network
congestion.
Thus, from the application point of view, partial checksums seem to
be a desirable feature. However, the usefulness of partial checksums
depends on partially corrupted packets being delivered to the
receiver. If the link-layer CRC always discards corrupted packets,
then this will not happen, and so the usefulness of partial checksums
would be restricted to corruption that occurred in routers and other
places not covered by link CRCs. There does not appear to be
consensus on how likely it is that future network links that suffer
significant corruption will not cover the entire packet with a single
strong CRC. DCCP makes it possible to tailor such links to the
application, but it is difficult to predict if this will be
compelling for future link technologies.
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In addition, partial checksums do not co-exist well with IP-level
authentication mechanisms such as IPsec AH, which cover the entire
packet with a cryptographic hash. Thus, if cryptographic
authentication mechanisms are required to co-exist with partial
checksums, the authentication must be carried in the application
data. A possible mode of usage would appear to be similar to that of
Secure RTP. However, such "application-level" authentication does
not protect the DCCP option negotiation and state machine from forged
packets. An alternative would be to use IPsec ESP, and to use
encryption to protect the DCCP headers against attack, while using
the DCCP header validity checks to authenticate that the header is
from someone who possessed the correct key. While this is resistant
to replay (due to the DCCP sequence number), it is not by itself
resistant to some forms of man-in-the-middle attacks because the
application data is not tightly coupled to the packet header. Thus,
an application-level authentication probably needs to be coupled with
IPsec ESP or a similar mechanism to provide a reasonably complete
security solution. The overhead of such a solution might be
unacceptable for some applications that would otherwise wish to use
partial checksums.
On balance, the authors believe that DCCP partial checksums have the
potential to enable some future uses that would otherwise be
difficult. As the cost and complexity of supporting them is small,
it seems worth including them at this time. It remains to be seen
whether they are useful in practice.
Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC
1191, November 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The
Addition of Explicit Congestion Notification (ECN) to
IP", RFC 3168, September 2001.
Kohler, et al. Standards Track [Page 124]
RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
[RFC3309] Stone, J., Stewart, R., and D. Otis, "Stream Control
Transmission Protocol (SCTP) Checksum Change", RFC
3309, September 2002.
[RFC3692] Narten, T., "Assigning Experimental and Testing
Numbers Considered Useful", BCP 82, RFC 3692, January
2004.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility
Support in IPv6", RFC 3775, June 2004.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E.,
and G. Fairhurst, "The Lightweight User Datagram
Protocol (UDP-Lite)", RFC 3828, July 2004.
Informative References
[B98] Bellovin, S.M., "Cryptography and the Internet",
CRYPTO '98 (LNCS 1462), pp 46-55, August 1988.
[BB01] Bellovin, S.M. and M. Blaze, "Cryptographic Modes of
Operation for the Internet", 2nd NIST Workshop on
Modes of Operation, August 2001.
[M85] Morris, R.T., "A Weakness in the 4.2BSD Unix TCP/IP
Software", Computer Science Technical Report 117, AT&T
Bell Laboratories, Murray Hill, NJ, February 1985.
[PMTUD] Mathis, M. and J. Heffner, "Path MTU Discovery", Work
in Progress, March 2006.
[RFC792] Postel, J., "Internet Control Message Protocol", STD
5, RFC 792, September 1981.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC1948] Bellovin, S., "Defending Against Sequence Number
Attacks", RFC 1948, May 1996.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC
1982, August 1996.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow,
"TCP Selective Acknowledgement Options", RFC 2018,
October 1996.
Kohler, et al. Standards Track [Page 125]
RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
[RFC2463] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP
Congestion Control", RFC 2581, April 1999.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L., and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.
[RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion
Manager", RFC 3124, June 2001.
[RFC3360] Floyd, S., "Inappropriate TCP Resets Considered
Harmful", BCP 60, RFC 3360, August 2002.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer,
"TCP Friendly Rate Control (TFRC): Protocol
Specification", RFC 3448, January 2003.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust
Explicit Congestion Notification (ECN) Signaling with
Nonces", RFC 3540, June 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611,
November 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J.,
and L. Wood, "Advice for Internet Subnetwork
Designers", BCP 89, RFC 3819, July 2004.
Kohler, et al. Standards Track [Page 126]
RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
[RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram
Congestion Control Protocol (DCCP) Congestion Control
ID 2: TCP-like Congestion Control", RFC 4341, March
2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC
4342, March 2006.
[SHHP00] Spatscheck, O., Hansen, J.S., Hartman, J.H., and L.L.
Peterson, "Optimizing TCP Forwarder Performance",
IEEE/ACM Transactions on Networking 8(2):146-157,
April 2000.
[SYNCOOKIES] Bernstein, D.J., "SYN Cookies",
http://cr.yp.to/syncookies.html, as of March 2006.
[VBK05] Vanit-Anunchai, S., Billington, J., and T.
Kongprakaiwoot, "Discovering Chatter and
Incompleteness in the Datagram Congestion Control
Protocol", FORTE 2005, pp 143-158, October 2005.
Kohler, et al. Standards Track [Page 127]
RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
Authors' Addresses
Eddie Kohler
4531C Boelter Hall
UCLA Computer Science Department
Los Angeles, CA 90095
USA
EMail: kohler@cs.ucla.edu
Mark Handley
Department of Computer Science
University College London
Gower Street
London WC1E 6BT
UK
EMail: M.Handley@cs.ucl.ac.uk
Sally Floyd
ICSI Center for Internet Research
1947 Center Street, Suite 600
Berkeley, CA 94704
USA
EMail: floyd@icir.org
Kohler, et al. Standards Track [Page 128]
RFC 4340 Datagram Congestion Control Protocol (DCCP) March 2006
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Kohler, et al. Standards Track [Page 129]