Network Working Group D. Marlow
Request for Comments: 1768 NSWC-DD
Category: Experimental March 1995
Host Group Extensions for CLNP Multicasting
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract
This memo documents work performed in the TUBA (TCP/UDP over Bigger
Addresses) working group of IPng area prior to the July 1994 decision
to utilize SIPP-16 as the basis for IPng. The TUBA group worked on
extending the Internet Protocol suite by the use of ISO 8473 (CLNP)
and its related routing protocols. This memo describes multicast
extensions to CLNP and its related routing protocols for Internet
multicast use. Publication of this memo does not imply acceptance by
any IETF Working Group for the ideas expressed within.
This memo provides a specification for multicast extensions to the
CLNP protocol similar to those provided to IP by RFC1112. These
extensions are intended to provide the mechanisms needed by a host
for multicasting in a CLNP based Internet. This memo covers
addressing extensions to the CLNP addressing structure, extensions to
the CLNP protocol and extensions to the ES-IS protocol. An appendix
discusses the differences between IP multicast and the CLNP multicast
approach provided in this memo.
Acknowledgments
The specification provided here was developed by a number of
individuals in the IETF TUBA working group as well as the ANSI X3S3.3
and ISO SC6 WG2 committees. Key contributions were made by Steve
Deering, Joel Halpern, Dave Katz and Dave Oran.
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Table of Contents
1. Introduction .......................................... 2
2. Levels of Conformance.................................. 3
3. Group Network Addresses................................ 4
4. Model of a CLNP End System Multicast Implementation.... 8
5. Extensions to the CLNP Protocol........................ 8
6. Extensions to the ES-IS Routeing Protocol ............. 15
7. Security Considerations ............................... 39
Appendix A. Differences with RFC 1112 .................... 40
Appendix B. Issues Under Study ........................... 43
References ................................................ 44
Author's Address .......................................... 45
1. Introduction
This memo provides a specification for multicast extensions for CLNP
in order to provide a CLNP based Internet the capabilities provided
for IP by RFC 1112 (Host Extensions for IP Multicasting) [RFC1112].
This memo uses an outline similar to that of RFC 1112.
Paraphrasing RFC 1112, "CLNP multicasting is the transmission of a
CLNP datagram to a "host group", a set of zero or more End Systems
identified by a single group Network address (GNA). A multicast
datagram is delivered to all members of its destination host group
with the same "best-efforts" reliability as regular unicast CLNP
datagrams, i.e., the datagram is not guaranteed to arrive intact at
all members of the destination group or in the same order relative to
other datagrams.
"The membership of a host group is dynamic; that is End Systems may
join and leave groups at any time. There is no restrictions on the
location or number of members in a host group. An End System may be a
member of more than one group at a time. An End System need not be a
member of a group to send datagrams to it.
"A host group may be permanent or transient. A permanent group has an
administratively assigned GNA. It is the address, not the membership
of the group, that is permanent; at any time a permanent group may
have any number of members, even zero.
"Internetwork forwarding of CLNP multicast datagrams is handled by
"multicast capable" Intermediate Systems which may be co-resident
with unicast capable Intermediate Systems.
The multicast extensions to the CLNP addressing structure defines
group Network addresses which identify host groups. The multicast
extensions to CLNP provides a means for identifying a CLNP packet and
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provides scope control mechanisms for CLNP multicast packets. The
multicast extensions to the ES-IS protocol provide the mechanisms
needed for a host to exchange control information with multicast
capable routers. These extensions to the ES-IS protocol provide for
a host to "announce" which multicast packets are of interest and for
a multicast capable router to dynamically "map" group Network
addresses to subnetwork addresses.
This memo specifies the extensions required by an End System to make
use of CLNP multicast. In addition the requirements placed upon
multicast capable Intermediate Systems to exchange information with
multicast capable End Systems is specified. No specifications are
provided related to the information exchanges between Intermediate
Systems to support multicast route selection or multicast Protocol
Data Unit (PDU) forwarding. A discussion of multicast route selection
and PDU forwarding has been written by Steve Deering [Deering91].
Note that for these multicast extensions to work there must exist an
uninterrupted path of multicast capable routers between the End
Systems comprising a host group (such paths may utilize tunneling
(i.e., unicast CLNP encapsulated paths between multicast capable CLNP
routers)). In order to support multicast route selection and
forwarding for a CLNP based internet additional specifications are
needed. Specifications of this type could come in the form of new
protocols, extensions to the current CLNP based routing protocols or
use of a technique out of the IETF's Inter-Domain Multicast Routing
(IDMR) group. The IDMR group is currently investigating multicast
protocols for routers which utilize a router's unicast routing
protocols, this approach may extend directly to CLNP routers.
While many of the techniques and assumptions of IP multicasting (as
discussed in RFC 1112) are used in CLNP multicasting, there are
number of differences. Appendix A describes the differences between
CLNP multicasting and IP multicasting. This memo describes techniques
brought in directly from projects within ISO to incorporate multicast
transmission capabilities into CLNP [MULT-AMDS].
2. Levels of Conformance
There are three levels of conformance for End Systems to this
specification:
Level 0: no support for CLNP multicasting.
There is no requirement for a CLNP End System (or Intermediate
System) to support CLNP multicasting. Level 0 hosts should be
unaffected by the presence of multicast activity. The destination
addresses used in support of multicast transfers, the GNA, should not
be enabled by a non-multicast capable End System and the PDUs
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themselves are marked differently than unicast PDUs and thus should
be quietly discarded.
Level 1: support for sending but not receiving CLNP multicast PDUs.
An End System originating multicast PDUs is required to know whether
a multicast capable Intermediate System is attached to the
subnetwork(s) that it originates multicast PDUs (i.e., to determine
the destination SNPA (subnet) address). An End System with Level 1
conformance is required to implement all parts of this specification
except for those supporting only Multicast Announcement. An End
System is not required to know the current Multicast Address Mapping
to start originating multicast PDUs.
Note: It is possible for End System not implementing Multicast
Address Mapping to successfully originate multicast PDUs (but with
the End System knowing of the existence of a multicast capable
Intermediate System). Such operation may lead to inefficient
subnetworks use. Thus when an End System continues (or may continue)
to originate multicast PDUs destined for the same group,
implementations are to provide Multicast Address Mapping support.
Level 2: full support for CLNP multicasting.
Level 2 allows a host to join and leave host groups as well as send
CLNP PDUs to host groups. It requires implementation by the End
System of all parts of this specification.
3. Group Network Addresses
Individual Network addresses used by CLNP for End System addressing
are called Network Service Access Points (NSAPs). RFC 1237 defines
the NSAP address for use in the Internet. In order to provide an
address for a group of End Systems, this specification does not
change the definition of the NSAP address, but adds a new type of
identifier - the group Network address - that supports a multicast
Network service (i.e., between a single source NSAP, identified by an
individual Network address, and a group of destination NSAPs,
identified by a group Network address). Host groups are identified by
group Network addresses.
In the development of multicast address extensions to CLNP,
requirements were identified for: (1)"easily distinguishing" group
addresses at the Network layer from NSAP addresses; (2)leaving the
currently allocated address families unaffected and (3)ensuring that
the approach taken would not require the establishment of new
addressing authorities. In addition, it was agreed that providing
multicast options for all OSI Network layer users was desirable and
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thus the group Network addressing solution should support options for
all address formats covered by ISO/IEC 8348 | CCITT Recommendation
X.213. The only viable means identified for meeting all requirements
was via creating a new set of AFI values with a fixed one-to-one
mapping between each of the existing AFI values and a corresponding
group AFI value.
Group Network addresses are defined by creating a new set of AFI
values, one for each existing AFI value, and a fixed one-to-one
mapping between each of the existing AFI values and a corresponding
group AFI value. The syntax of a group Network address is identical
to the syntax of an individual Network address, except that the value
of the AFI in an individual Network address may be only one of the
values already allocated for individual Network addresses, whereas
the value of the AFI in a group Network address may be only one of
the values allocated here for group Network addresses. The AFI values
allocated for group Network addresses have been chosen in such a way
that they do not overlap, in the preferred encoding defined by
ISO/IEC 8348 | CCITT Recommendation X.213, with any of the AFI values
that have already been allocated for individual Network addresses.
3.1 Definitions
group Network address: an address that identifies a set of zero or
more Network service access points; these may belong to multiple
Network entities, in different End Systems.
individual Network address: an address that identifies a single NSAP.
3.2 CLNP Addresses
A discussion of the CLNP address format is contained in RFC 1237. The
structure of all CLNP addresses is divided into two parts the Initial
Domain Part (IDP) and the Domain Specific Part (DSP). The first two
octets of the IDP are the Authority and Format Identifier (AFI)
field. The AFI has an abstract syntax of two hexadecimal digits with
a value in the range of 00 to FF. In addition to identifying the
address authority responsible for allocating a particular address and
the format of the address, the AFI also identifies whether an address
is an individual Network address or a group Network address. There
are 90 possible AFI values to support individual Network address
allocations. In addition, when the AFI value starts with the value
"0" this identifies that the field contains an incomplete individual
Network address (i.e., identifies an escape code).
Table 1 allocates 90 possible AFI values to support group Network
address allocations. In addition if the first two digits of the IDP
are hexadecimal FF, this indicates the presence of an incomplete
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group Network address. The allocation of group addresses is
restricted to be only from the AFI values allocated for the
assignment of group addresses in Table 1. An addressing authority in
allocating either Network addresses or authorizing one or more
authorities to allocate addresses, allocates both individual and the
corresponding group addresses. Thus each block of addresses allocated
by an addressing authority (or its sub-authority) contains a block of
individual Network addresses and group Network addresses. The
individual and group address block allocated are differentiated by
the AFI values used which are related as shown in Table 1.
Group Network addresses are only used as the destination address
parameter of a CLNP PDU. Source Address parameters are never
permitted to be group Network addresses.
Table 2 lists the AFI values which have not been assigned, at this
time, for the support of neither individual nor group address
allocation. Future assignment of these AFI values is possible.
Additional information concerning individual Network addresses (i.e.,
NSAP and NET (Network Entity Titles)) is contained in RFC 1237.
Note: While the format of the Initial Domain Part of a group Network
address is assigned, the format for the Domain Specific Part of the
group Network address is specified by an addressing authority and is
out of the scope of this memo. While NSAP address assignments are
typically made to support hierarchical unicast routing, a similar
consideration for group Network address assignments may not exist.
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TABLE 1 - Relationship of AFI Individual and Group Values
-----------------------------------------------------------
|Individual Group | Individual Group | Individual Group |
-----------------------------------------------------------
| 0x FF | | |
| 10 A0 | 40 BE | 70 DC |
| 11 A1 | 41 BF | 71 DD |
| 12 A2 | 42 C0 | 72 DE |
| 13 A3 | 43 C1 | 73 DF |
| 14 A4 | 44 C2 | 74 E0 |
| 15 A5 | 45 C3 | 75 E1 |
| 16 A6 | 46 C4 | 76 E2 |
| 17 A7 | 47 C5 | 77 E3 |
| 18 A8 | 48 C6 | 78 E4 |
| 19 A9 | 49 C7 | 79 E5 |
| 20 AA | 50 C8 | 80 E6 |
| 21 AB | 51 C9 | 81 E7 |
| 22 AC | 52 CA | 82 E8 |
| 23 AD | 53 CB | 83 E9 |
| 24 AE | 54 CC | 84 EA |
| 25 AF | 55 CD | 85 EB |
| 26 B0 | 56 CE | 86 EC |
| 27 B1 | 57 CF | 87 ED |
| 28 B2 | 58 D0 | 88 EE |
| 29 B3 | 59 D1 | 89 EF |
| 30 B4 | 60 D2 | 90 F0 |
| 31 B5 | 61 D3 | 91 F1 |
| 32 B6 | 62 D4 | 92 F2 |
| 33 B7 | 63 D5 | 93 F3 |
| 34 B8 | 64 D6 | 94 F4 |
| 35 B9 | 65 D7 | 95 F5 |
| 36 BA | 66 D8 | 96 F6 |
| 37 BB | 67 D9 | 97 F7 |
| 38 BC | 68 DA | 98 F8 |
| 39 BD | 69 DB | 99 F9 |
-----------------------------------------------------------
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TABLE 2 - AFI values reserved for future allocation
--------------
| 1A-1F |
| 2A-2F |
| 3A-3F |
| 4A-4F |
| 5A-5F |
| 6A-6F |
| 7A-7F |
| 8A-8F |
| 9A-9F |
| FA-FE |
--------------
4. Model of a CLNP End System Multicast Implementation
The use of multicast transmission by a CLNP End System involves
extensions to two protocols: CLNP and the ES-IS Routeing Protocol. To
provide level 0 service (no support for CLNP multicast), no
extensions to these two protocols are required. To provide level 1
service (support for sending but not receiving CLNP multicast PDUs)
all extensions contained in the following sections are required
except for those supporting only Multicast Announcement. In order to
support level 2 service (full support for CLNP multicasting), the
extensions contained in the following sections are required.
Extensions identified for Intermediate Systems are not required (or
appropriate) for End Systems. Multicast transmission also requires
the use of a group Network address (as previously described) as the
destination address parameter.
5. Extensions to the CLNP protocol
This section provides extensions to the CLNP Protocol [CLNP] ISO
8473-1, to support multicast transmission. These additions provide
procedures for the connectionless transmission of data and control
information from one network-entity to one or more peer network-
entities.
In developing the multicast extensions for CLNP a decision was needed
on how to "mark" a packet as multicast (versus the current unicast
packets). Such marking is necessary since the forwarding behavior
for multicast packets is different (e.g., multiple copies of a packet
may need to be forwarded). The two alternatives considered were to
mark the packet (via a particular field) or to mark the destination
address, in the end both were done. The destination address for a
multicast PDU identifies a host group which is of a very different
nature than the unicast NSAP address. Rather than changing the
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nature of NSAP addresses, a new set of addresses were created named
group Network addresses which are marked within the first octet
(i.e., the AFI field) with values reserved for group Network
addresses.
Consideration was given to no further marking of the PDU; however, a
problem was identified with only using the group Network address to
identify multicast packets. Currently routers implementing the IS-IS
Intra-Domain protocol as Level 1 routers when receiving a packet with
an unknown destination address are permitted to either discard the
packet or send it to a Level 2 router. Such actions by non-multicast
capable routers to multicast packets can lead to non-deterministic
behavior. Level 1 routers upon receiving a packet containing a group
Network address might pass the packet up to a Level 2 router (which
may or may not be multicast capable) or it might discard it.
Depending upon the circumstances this might lead to whole regions
missing packets or packet duplication (possibly even explosion). The
result was to seek deterministic behavior by non-multicast capable
routers by creating a new PDU type (Multicast Data PDU) and inserting
into the CLNP reasons for discard: receiving a PDU of unknown type.
Note that this reason for discard is mandatory on multicast capable
and non-multicast capable CLNP implementations.
5.1 Definitions
multicast: Data transmission to one or more destinations in a
selected group in a single service invocation.
multicast capable Intermediate System: An Intermediate System which
incorporates the multicast features of the Network layer.
5.2 Addresses
The destination address parameter of a multicast PDU shall contain a
group Network address. The source address parameter shall be an
individual Network address.
5.3 Extensions to the current protocol functions
In order to support multicast transmissions the following optional
CLNP protocol functions will be implemented:
5.3.1 Header Format Analysis function
The header format analysis function optionally provides capabilities
to Network entities which support multicast transfer to supply
applicable PDUs directly to End Systems served by such a Network
entity as well as to forward such PDUs on to other Network entities.
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This optional functionality is realized through a Network entity with
multicast capability identifying a PDU as using multicast transfer
via the PDU type and the PDU's destination address field.
If a Network entity supports multicast transmission, then the header
format analysis function shall provide checking to ensure that a PDU
does not contain a group Network address in the source address field.
Any PDU header analyzed to have a group address in the source address
field shall be discarded.
5.3.2 Route PDU function
The route PDU function optionally provides capabilities to Network
entities which support multicast transfer for determining multiple
Network entities to which a single PDU shall be forwarded to. This
may result in multiple invocations of the forward PDU function and
hence the need to make multiple copies of the PDU. For PDUs that are
received from a different Network entity, the optional functionality
for the route PDU function is realized as a result of the header
format analysis function's recognition of the PDU as being a
multicast PDU. A Network entity attached to more than one subnetwork
when originating a multicast PDU is permitted to originate the PDU on
more than one subnetwork.
Note: The ES-IS function "Extensions to the ISO CLNP Route Function
by End Systems" discussed in section 6.10 identifies on which
subnetworks an End System attached to more than one subnetwork must
originate multicast PDUs on.
Note: The purpose in allowing an originating Network entity to
originate a multicast PDU on multiple subnetworks is to support the
development of multicast IS-IS protocols which will need to determine
on which subnetworks a multicast PDU has visited. This behavior is
predicated on the assumption that the Intermediate Systems in the OSI
environment performing multicast forwarding form a connected set.
5.3.3 Forward PDU function
This function issues an SN-UNITDATA request primitive, supplying the
subnetwork or Subnetwork Dependent Convergence Function (SNDCF)
identified by the route PDU function with the protocol data unit as
user data to be transmitted, the address information required by that
subnetwork or SNDCF to identify the "next" system or systems within
the subnetwork-specific addressing domain (this may be one or more
Intermediate Systems and/or one or more destination End Systems), and
quality of service constraints (if any) to be considered in the
processing of the user data.
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5.3.4 Discard PDU function
Add an additional reason for discard - a PDU is received with an
unknown type code.
5.3.5 Error reporting function
It is important to carefully control the use of the error reporting
capability in the case of multicast transfers. The primary concern
is to avoid the occurrence of broadcast storms and thus a multicast
PDU may not cause the origination of another multicast PDU. This is
the primary reason that the source address is not permitted to be a
group address. In addition, a multicast PDU with error reporting
permitted can result in flooding the source network-entity (as well
as the networks used) with Error Report PDUs.
While error reports are permitted on multicast PDUs, a PDU with a
group Network address in the source address field shall not be
responded to with an Error Report. This is to ensure that a multicast
PDU does not generate another multicast PDU. If the source address is
identified as a group address then an error report PDU shall not be
generated and the original PDU shall be discarded.
5.3.6 Source routing functions
No source routing capability is provided for multicast PDU transfer.
The NS provider shall not accept a multicast PDU with source route
parameters.
5.4 Scope control function
5.4.1 Overview
The scope control function is an option for multicast PDU forwarding
only. The scope control function allows the originator to limit the
forwarding of the multicast PDU. The scope control function provides
the capability to limit the relaying of a particular PDU based on the
individual Network addressing hierarchy and/or limit the amount of
multicast expansion which can take place. In cases where both forms
of scope control are applied to the same PDU, forwarding will cease
once either has reached its scope control limit.
5.4.2 Prefix Based Scope Control
The prefix based scope control function allows the originator to
specify a specific set of address prefixes where the multicast
forwarding of a PDU by an Intermediate System occurs only if one of
the prefixes matches the Network Entity Title (NET) of the
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Intermediate System. Prefix based scope control may be selected only
by the originator of a PDU. Prefix based scope control is
accomplished using one or more address prefixes held in a parameter
within the options part of the PDU header. The length of this
parameter is determined by the originating network entity, and does
not change during the lifetime of a PDU.
When an Intermediate System receives a multicast PDU containing a
prefix based scope control parameter, forwarding is only performed if
every octet of one of the prefixes contained in the prefix based
scope control parameter matches that Intermediate System's NET,
starting from the beginning of its NET. If no such prefix match
exists, the Intermediate System discards the PDU. The error reporting
function shall not be invoked upon PDU discard.
5.4.3 Radius Scope Control
The radius scope control function allows the originator to specify a
maximum logical distance where multicast expansion can occur. It is
closely associated with the header format analysis function. Each IS
receiving a multicast PDU which is capable of expanding and which
contains a Radius Scope Control parameter, decrements the Radius
Scope Control field in the PDU by an administratively set amount
between 0 and the maximum value of the field. An IS, when it
decrements the Radius Scope Control field, shall place a value of 0
into this field if its current value is less than the amount it is to
decrement by. This function determines whether the PDU received may
be forwarded or whether its Radius has been reached, in which case it
shall be discarded. An Intermediate System shall not forward a
multicast PDU containing a Radius Scope Control parameter with a
value of 0. The error reporting function shall not be invoked upon
PDU discard.
5.4.3.1 Radius Scope Control Example
The Radius Scope Control parameter is useful where policies have been
established across the potential forwarding path. One possible
policy for Internet use is for multicast capable routers to treat
this field as a hop count within a domain (decrement by one unit) and
for inter-domain routers to either decrement this field to an even
multiple of 256 when crossing domains where prior agreements have
been made or decrement this field to 0 (i.e., discard the packet) for
other domains.
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5.5 Structure and Encoding of PDUs
Multicast transmission is accomplished via the transfer of Multicast
Data (MD) PDUs. The PDU type code for a MD PDU is "1 1 1 0 1". The
format of the MD PDU is identical to that of the Data (DT) PDU. The
MD and DT PDU may contain the same optional parameters with the
following exceptions: (1)The source routing parameter is permitted
within DT PDUs but not MD PDUs; and (2)The scope control parameter is
permitted within MD PDUs but not DT PDUs.
5.6 Optional parameters for multicast support
5.6.1 Prefix Based Scope Control
The prefix based scope control parameter specifies one or more
address prefixes for which Intermediate System forwarding requires a
match of one of the contained prefixes with the beginning of the
Intermediate System's NET.
Parameter Code: 1100 0100
Parameter Length: variable
Parameter Value: a concatenation of address prefix entries
The parameter value contains an address prefix list. The list
consists of variable length address prefix entries. The first octet
of each entry gives the length of the address prefix denominated in
bits that comprises the remainder of the entry. If the length field
does not specify an integral number of octets then the prefix entry
is followed by enough trailing zeroes to make the end of the entry
fall on an octet boundary. The list must contain at least one entry.
The prefix shall end on a boundary that is legal in the abstract
syntax of the address family from which it is derived. For example,
the encoding of a prefix whose DSP is expressed in decimal syntax
must end on a semi-octet boundary, while the encoding of a prefix
whose DSP is expressed in binary syntax can end on an arbitrary bit
boundary. If the end of the prefix falls within the IDP, then the
prefix must end on a semi-octet boundary and must not contain any
padding characters.
Note: The length of the prefix based scope control parameter is
determined by the originator of the PDU and is not changed during the
lifetime of the PDU.
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5.6.1.1 Prefix matching
A prefix that extends into the DSP shall be compared directly against
the encoded NET address, including any padding characters that may be
present. A prefix which does not extend into the DSP shall be
compared against the derived quantity NET', which is obtained from
the NET address by removing all padding characters (as defined by the
binary encoding process of ISO 8348).
The existence of a match shall be determined as follows:
a) If the encoded NET (or NET') contains fewer bits than the pre-
fix, then there is no match.
b) If the encoded NET (or NET') contains at least as many bits as
the prefix, and all bits of the prefix are identical to the
corresponding leading bits of the encoded NET (or NET'), there
is a match. Otherwise, there is no match.
5.6.2 Radius Scope Control
The radius scope control parameter specifies the logical distance
that a multicast PDU can be forwarded.
Parameter Code: 1100 0110
Parameter Length: two octets
Parameter Value: two octets which represents the remaining
distance, that the PDU can be forwarded,
in administratively set units.
5.7 Provision of the Underlying Service
For a subnetwork that provides an inherent multicast capability, it
is the functionality of the SNDCF to provide the mapping between
group Network addresses and the corresponding addressing capability
of the subnetwork.
5.8 Conformance
All of the extensions provided to the functions to support multicast
capability are optional. For an End System or Intermediate System
which is not multicast capable these extensions are not applicable.
An implementation claiming conformance as a multicast capable End
System shall meet all of the requirements for an End System which is
not multicast capable and also provide all of the multicast
extensions provided here. An implementation claiming conformance as a
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multicast capable Intermediate System shall meet all of the
requirements for an Intermediate System which is not multicast
capable and also provide all of the multicast extensions provided
here.
6. Extensions to the ES-IS Routeing Protocol
This section provides optional extensions to the ES-IS Routeing
Protocol [ES-IS], ISO 9542 to support the transfer of multicast PDUs.
It is an explicit goal of this specification that ESs and ISs, some
of which will have multicast capabilities and some without, will be
able to fully function on the same subnetworks. This specification
does not change any aspect of a currently defined (i.e., non-
multicast) ISO 9542 implementation, it adds new optional
functionality not modifying current functionality. Two basic
functions are provided: multicast announcement and multicast address
mapping.
6.1 Overview of the protocol
6.1.1 Operation of ESs receiving multicast PDUs
ESs, upon initialization and periodically thereafter, will construct
End System Group Hello (ESGH) PDUs identifying, by particular group
Network addresses, the multicast PDUs it wishes to receive. The ES
will periodically originate (announce) these ESGH PDUs on the
subnetwork it wishes to receive these on. Reporting the same group
Network address on multiple subnetworks may result in the reception
of duplicate PDUs. ES-IS operations related to requesting the same
group Network address on multiple subnetworks are handled totally
independently (e.g., using different logical timers,...). It is
permitted for an ES to report a number of group Network addresses in
the same ESGH PDU. The only restrictions placed on providing
multiple group Network addresses within the same ESGH PDU are that
all packets requested are to be received on the same subnet, with the
same holding time and that the ESGH PDU be of length equal to or less
that its maximum packet size constraint. Note that each group
Network address in the ESGH PDU is paired with its own SNPA
(subnetwork point of attachment) address.
An ES will always have an SNPA address associated with each of its
active group Network addresses. An SNPA address is a subnetwork
address, in the case of a subnetwork which uses IEEE 802 addresses
the SNPA address is a 48 bit IEEE 802 MAC (media access control)
address. Of particular interest is the address used to mark the
destination group. For a subnetwork using IEEE 802 addressing a
group SNPA address uses a particular bit position to "mark" group
SNPA addresses.
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Upon initialization the ES may have static SNPA address associations
(Pre-configured SNPA addresses). For any group Network address
without a Pre-configured SNPA address that the ES wishes to receive,
the ES will associate the "All Multicast Capable End Systems" SNPA
address. Upon receiving a Multicast Address Mapping (MAM) PDU
containing a group Network address that the ES is announcing, the ES
will use the SNPA address pairing contained in the MAM PDU for that
group Network address. Upon the expiration of the Mapping Holding
Timer, the ES shall revert back to associating either the Pre-
configured SNPA address if one exists or the "All Multicast Capable
End Systems" SNPA address for the specific group Network address.
While an ES is permitted to listen in on other ESs announcements
(needed for the damping option), an ES is not permitted to change its
group Network address to SNPA address mapping based on the
announcement of other ESs.
Optionally, the ES may perform damping (resetting a Multicast
Announcement Timer corresponding to a particular group Network
address) if the conditions necessary to withhold a particular
announcement are met. In order to perform damping the following
conditions must be met: (1)The ES must be processing other ES's
announcements; (2)An ESGH PDU is received that identifies the exact
same group Network address and SNPA address pairing on a particular
subnetwork that this ES is announcing on; (3) The Multicast Holding
Timer parameter value in the ESGH PDU received is equal to or greater
than the Multicast Holding Timer value, for this subnetwork, that is
being used by the ES processing this ESGH PDU.
ESs will utilize a local default value for their Multicast
Announcement Timer to control the period for sending out their ESGH
PDUs. The Active Multicast IS, if one exists on a particular
subnetwork, may suggest a value for ESs on the subnetwork to use for
their Multicast Announcement Timer for a specific group Network
address. In order to support the optional damping function, ESs are
required to incorporate a 25% jittering to the timer values that they
are using.
6.1.2 Operation of ESs originating multicast PDUs
The ES originating multicast packets identified by a specific group
Network address is not required to be a receiver of such packets (and
thus is not announcing that particular group Network address). The
origination of multicast PDUs involves two differences to the
origination of unicast PDUs. The two differences are: (1)The
mechanism for selecting a destination SNPA address and (2)For End
Systems attached to more than one subnet, the decision on which
subnet(s) to originate the PDUs.
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The destination SNPA address used for originating each multicast
packet depends on whether there is a multicast capable IS attached to
the subnetworks. When a multicast capable IS is attached, the
decision depends on whether there is multicast address mapping
information available for that subnetwork corresponding to the group
Network address used as the destination address parameter of the
multicast packet. When there is a multicast capable IS attached to a
subnetwork and there is multicast address mapping information
available corresponding to the group Network address, then the SNPA
address obtained from the multicast address mapping information is
used. Originating multicast packets using the destination SNPA
address used for receiving such multicast packets ensures that the
multicast packets will not require additional forwarding on the
originating subnetwork(s). When there is a multicast capable IS
attached to a subnetwork but for which there is no multicast address
mapping information available corresponding to the the group Network
address, then the SNPA address used is the "All Multicast Capable
Intermediate Systems" address.
When there is no multicast capable IS attached to a subnetwork then
the ES originating a multicast PDU uses pre-configured information if
it is available or the "All Multicast Capable End Systems" SNPA
address when no pre-configured information is available.
ES's attached to more than one subnetwork forward each multicast
packet that they originate onto every attached subnetwork for which
the NSAP address being used as the source address of the multicast
packet is actively being reported through the unicast ES-IS Report
Configuration function.
6.1.3 Operation of the Active Multicast IS
The Active Multicast IS listens in on all ESGH PDUs originated on the
subnetwork for which it is serving as the Active Multicast IS. All
subnetworks are handled independently (even if multiple subnetworks
have the same ESs attached and the IS is serving as the Active
Multicast IS for these subnetworks).
The Active Multicast IS originates MAM PDUs, for all group Network
addresses for which it has received ESGH PDUs, on the subnetwork due
to the following operational conditions:
1) The IS initializes either as the Active Multicast IS after an
election with other multicast capable ISs or initializes
believing it is the only multicast capable IS;
Note: The determination of such conditions is outside of the scope of
this specification;
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2) The IS receives an ESGH PDU with a group Network address paired
to an incorrect SNPA address;
3) The expiration of the IS's Multicast Address Mapping Timer for
that group Network address; or
Note: This is to prevent the expiration of Mapping Holding Timers in
ESs.
4) The IS receives a multicast PDU originated on the subnetwork
which used an incorrect destination SNPA address.
Note: Of particular concern are those multicast packets using the
"All Multicast Capable Intermediate Systems" SNPA address when
another SNPA address should have been used. In addition the
multicast capable ISs are responsible for listening in on all
multicast packets using destination SNPA addresses that are contained
within the current multicast address mapping information.
As a result of the event driven conditions (i.e., conditions 2 or 4
above), the Active Multicast IS sends a MAM PDU with direct
information (i.e., not needing analysis of the Mask parameters). The
Active Multicast IS limits the number of MAM PDUs that are sent out
per unit of time. Particular MAM PDUs with direct information will
not be sent more than once per second. MAM PDU will be sent in
response to continuing event driven conditions such that events
occurring greater than 10 seconds after the issuance of such a MAM
PDU will result in the issuance of another MAM PDU.
The Active Multicast IS is responsible for forwarding a multicast
packet back on the subnetwork it was originated when a multicast
packet used the "All Multicast Capable Intermediate System" SNPA
address when another SNPA address should have been used. A packet
forwarded back onto the subnetwork the multicast packet was
originated on will be given a CLNP Lifetime of "1" to prevent the
continued relaying of duplicate packets by the multicast ISs.
The further relaying of any multicast packet originated on a
subnetwork is the responsibility of the multicast routing protocol
used and is outside the scope of this specification.
6.2 Definitions
Active Multicast IS: The one multicast capable IS selected (via means
outside of this specification) to originate Multicast Address Mapping
information on a particular subnetwork.
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Paired SNPA Address: The SNPA address associated with a particular
group Network address on a specific subnetwork.
6.3 Routing information supporting multicast transmission
6.3.1 Multicast Announcement Information
An IS should forward a multicast PDU containing a particular
destination group Network address onto a subnetwork to which it is
attached if and only if one or more of the ESs attached to that
subnetwork have declared an interest in receiving multicast PDUs
destined for that group Network address. Multicast announcement
information enables an IS that supports CLNP multicast to dynamically
discover, for each subnetwork to which it is attached, the group
Network addresses for which ESs attached to that subnetwork have
declared an interest.
On a point-to-point subnetwork the multicast announcement information
informs the Network entity, in the case where it is attached to an
End System, of the group Network addresses for which that End System
expects to receive multicast PDUs.
On a broadcast subnetwork the multicast announcement information
informs the multicast capable Intermediate Systems, of the group
Network addresses for which ESs attached to that subnetwork expect to
receive multicast PDUs.
Note: Intermediate Systems with the optional OSI multicast
capabilities do receive information identifying the SNPA address of
ESs on the broadcast network that want PDUs with particular group
Network addresses as their destination address; however, the critical
information is which multicast PDUs are needed, not which ESs need
them.
6.3.2 Multicast Address Mapping Information
In order to receive multicast packets destined for a particular group
Network address, an ES may need to associate with the group Network
address a specific SNPA address. Multicast address mapping
information enables an IS to inform ESs that they can receive
multicast packets destined for a particular group Network address on
a corresponding specific SNPA address. In addition, multicast
address mapping information may provide the specific destination SNPA
addresses needed by an ES for originating multicast packets.
Multicast address mapping information is not employed on point-to-
point subnetworks.
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Multicast address mapping information is employed on broadcast sub-
networks to enable multicast capable Intermediate Systems to inform
the multicast capable End Systems that they can receive, on a
specific broadcast subnetwork, multicast packets destined for a
particular group Network address on a corresponding specific SNPA
address. In addition multicast address mapping information provides
the specific destination SNPA address, that corresponds to a
particular group Network address, for each multicast packet that it
originates on a specific broadcast subnetwork.
6.4 Addresses
All exchanges using this protocol are accomplished over a single
subnetwork. While the control PDU's contain Network addresses (i.e.,
group Network addresses) actual control PDU transfer is accomplished
via Subnetwork based group addresses (i.e., group SNPA addresses).
The following group SNPA addresses are used: (1)All Multicast Capable
End Systems; (2)All Multicast Announcements; (3)All Multicast Capable
Intermediate Systems and (4)a group SNPA address corresponding to a
group Network address
6.5 Timers
Two additional timers are employed: (1)the Multicast Announcement
Timer (MAT) and (2)Multicast Address Mapping Timer (MAMT). Old
multicast announcement or multicast address mapping information shall
be discarded after the Holding Timer expires to ensure the correct
operation of the protocol.
6.5.1 Multicast Announcement Timer
The Multicast Announcement Timer is a local timer (i.e., maintained
independently by each End System, one timer per group Network
address) which assists in performing the Report Multicast
Announcement function. The timer determines how often an End System
reports its desire to receive multicast PDUs with that group Network
address as its destination address parameter. Considerations in
setting this timer are similar to those described for the
Configuration timer in the ES-IS specification.
6.5.2 Multicast Address Mapping Timer
The Multicast Address Mapping Timer is a local timer (i.e.,
maintained independently by an Intermediate System which is actively
participating with End Systems to transfer multicast PDUs) which
assists in performing the Report Multicast Address Mapping function.
The timer determines how often an Intermediate System, actively
participating with End Systems for the transfer of multicast PDUs,
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reports the Multicast Address Mapping for a particular group Network
address. The shorter the Multicast Address Mapping Timer, the more
quickly End Systems on the subnetwork will become aware of the
correct address mapping which may change due to the Intermediate
System becoming available or unavailable. There is a trade off
between increased responsiveness and increased use of resources in
the subnetwork and in the End Systems.
6.6 Extensions to the current protocol functions
In order to support multicast transmissions the following optional
ES-IS protocol functions will be implemented:
6.6.1 Report Configuration by Intermediate Systems
All multicast capable Intermediate Systems on a subnetwork shall use
the Multicast Capable option in all ISH PDUs that they originate.
This will provide multicast capable End Systems with a way to
determine that a multicast capable Intermediate System is operating
on a particular subnetwork.
6.6.2 Query Configuration
Note: The Query Configuration function cannot be performed to find
the corresponding SNPA address of a group Network address since the
addressing information needed is the corresponding group SNPA address
and not the SNPA address of a particular End System responding. On a
large broadcast subnetwork, many different Configuration Responses
could result each incorporating a different End System Address. While
it is possible to design a Query Configuration for use with
multicast, this function does not appear to be required given the use
of the "All Multicast Capable End Systems" address for supplying a
SNPA address when the group SNPA address is not known.
6.7 Multicast Announcement
6.7.1 Report Multicast Announcement Function by End Systems
An End System which needs to receive or continue to receive any
multicast PDUs (i.e., PDUs with group Network addresses as their
destination address), constructs and transmits ESGH PDUs to inform
multicast capable Intermediate Systems of the set of group Network
address destinations for which it wishes to receive PDUs. This may be
done by constructing ESGH PDUs for each group Network address.
Alternatively, ESGH PDUs may be constructed which convey information
about more than one group Network address at a time, up to the limits
imposed by the permitted SNSDU size and the maximum header size of
the ESGH PDU. Each ESGH PDU is transmitted by issuing an SN-
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UNITDATA.Request with the following parameters:
SN_Userdata (SNSDU) <- ESGH PDU
SN_Destination _Address <- multi-destination address that indicates
"All Multicast Announcements"
If an End System is attached to more than one subnetwork, the
information about each group Network address desired for receiving on
a particular subnetwork serving the End System shall be transmitted
via that subnetwork. It is permissible for an End System to report
group Network addresses on multiple subnetworks; however, duplicate
multicast PDUs should be anticipated.
The Group Address Pair parameter carries a list of Group Network
Addresses, each paired with its associated SNPA address. This
information is used by the Active Multicast IS to determine whether a
Multicast Address Mapping PDU should be emitted to update the
association between Group Network Addresses and SNPA addresses.
The Holding Time (HT) field is set to approximately twice the ES's
Multicast Announcement Timer (MAT) parameter. The value shall be
large enough so that even if every other ESGH PDU is discarded (due
to lack of resources), or otherwise lost in the subnetwork, the
multicast announcement information will still be maintained. The
value should be set small enough so that Intermediate Systems
resources are not needlessly consumed when the ES no longer wishes to
receive PDUs destined to a group Network address.
Note: When combining multiple group Network addresses in a single
ESGH PDU, it should be realized that there is a single Holding Time
parameter associated with all of these addresses.
6.7.1.1 Generating Jitter on Multicast Announcement Timers
The ES shall apply a 25% jitter to its Multicast Announcement Timer
(MAT) parameter. When ESGH PDUs are transmitted as a result of timer
expiration, there is a danger that the timers of individual systems
may become synchronised. The result of this is that the traffic
distribution will contain peaks. Where there are a large number of
synchronised systems, this can cause overloading of both the
transmission medium and the systems receiving the PDUs. In order to
prevent this from occurring, all periodic timers, the expiration of
which can cause the transmission of PDUs, shall have "jitter"
introduced as defined in the following algorithm.
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CONSTANT
Jitter = 25;
Resolution = 100;
(* The timer resolution in ms *)
PROCEDURE Random(max: Integer): Integer;
(* This procedure delivers a Uniformly distributed random
integer R such that 0 < R