Network Working Group                               James E. White (JEW)
Request for Comments: 555                                        SRI-ARC
NIC: 17993                                                 July 27, 1973


          Response to Critiques of the Proposed Mail Protocol

   A number of people have responded to my proposal for a Mail Protocol
   (JEW RFC 524 -- 17140,2:y).  In the current RFC, I've attempted to
   collect and respond to the questions, complaints, and suggestions
   that various individuals in the Network community have offered.  I
   intend to critique myself in a forthcoming RFC.

   I hope that dialog on the protocol proposal will continue, and that
   others will join in the discussion.  I will respond via RFC to any
   additional critiques I receive (I hope there'll be many).

I.  QUESTIONS

   HOW DOES THE SERVER VERIFY AN ID?

      References:

         (DHC JBP RFC 539 -- 17644,3g:gy)

      Discussion:

         One postulates the existence of AT LEAST ONE host whose Mail
         server process implements the User Verification Function (JEW
         RFC 524 -- 17140,5f7:gy).  Any process can contact that server,
         give him the name of any Individual in the Net and a test Id,
         and the server will determine whether or not the Individual and
         Id agree.

            The NIC, for one, will without question provide this
            service.

         With such support available to it, ANY FTP server process can
         then require (of any or all user processes that contact it) an
         ID command wherever it wishes within the user-server
         interchange (within the constraints of the Protocol).  The
         server simply prompts for the Id, gets it, opens a connection
         to the User Verification Agent, presents to it the Individual's
         name and purported Id, receives a positive or negative
         response, and deals with the original user process accordingly.






White                                                           [Page 1]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


         Example:

            Suppose a user process opens a connection to UCLA-NMC's
            server process, invokes the Delivery function, and in the
            course of the interchange identifies the Author as Roberts
            at USC-ISI.

            The implementors at UCLA-NMC's server process chose to
            require proof, in all Delivery transactions, that the Author
            is who he claims he is.  It therefore prompts for an Id in
            response to the AUTHOR command from the user process, and
            receives in return the command 'ID arpawheel '.

            UCLA-NMC's server then connects to the NIC's server, invokes
            the User Verification function there, specifying 'REQUESTOR
            roberts @ usc-isi ' and 'ID arpawheel '.  The NIC
            informs UCLA-NMS that the Id is incorrect.

            UCLA-NMC then rejects the original ID command.

         Of course, the Protocol does not require that a server demand
         Ids from users that contact it.  Servers who choose not to
         require proof of identity simply never prompt for ID commands,
         and treat any they receive as NOPs.  For such implementations
         (which represent the current, FTP mail protocol situation), no
         third-part interchanges are ever required.

         Each user in the Net has a single Id that he uses throughout
         the Net for purposes of sending and receiving mail.  That Id
         need not (but may, either coincidentally or by design) have any
         other use.  In particular, a user's Id is independent of the
         passwords by which he gains access to accounts that he might
         possess on hosts around the Net.

            Of course, a user could and might see to it that his
            passwords and Id are the same.  The NIC, for example, might
            require that a user log in to its system with NIC ident and
            Id, rather than with host name and password, as it does
            currently.

         I emphasize again that Ids have nothing whatsoever to do with
         accounting.  UCLA-NMC doesn't force the Author to prove his
         identity so UCLA has someone to whom it can bill the resources
         consumed in processing the Delivery transaction.  It does so to
         prevent Jim White from authoring a piece of mail and claiming
         that Larry Roberts wrote it.





White                                                           [Page 2]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


            UCLA-NMC does have the option of requiring that a user
            process log in before it delivers mail so that it can be
            billed for the resources it uses.  The appropriate commands
            to require of the user process are USER, PASS, and ACCT.
            But, the billing process is separable from that of
            identifying Author, Clerk, etc.

            The NIC, for example, in its role as a Distribution Agent,
            might establish an account at UCLA-NMC to use whenever it
            delivers mail there.  UCLA-NMC will bill ALL of the NIC's
            activity at UCLA to that account.  But when the NIC delivers
            a piece of mail it claims was authored by Larry Roberts,
            UCLA-NMC may still wish to verify that claim.  Hence the ID
            command.

   ACK, PROGRESS REPORT, OR REPLY WITH NO REFERENCE SERIAL NUMBER

      References:

         (DHC JBP RFC 539 -- 17644,3h:gy)

      Discussion:

         A Delivery of type POSITIVE or NEGATIVE ACKNOWLEDGMENT,
         PROGRESS REPORT, or REPLY requires a Reference Serial Number of
         the user process.  Should the server determine that one is
         lacking when the final EXIT command is given, he should reject
         the EXIT command with an appropriate error response.

            The same applies in the Distribution function:  a Reference
            Serial Number MUST be specified if the Delivery Type is
            REPLY.

         The Protocol document is deficient in that it doesn't state the
         above.

II.  COMPLAINTS

   TERMINATING BOTH THE SUBSYSTEM AND FUNCTIONS WITH EXIT

      References:

         (AAM -- 17404,)








White                                                           [Page 3]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


      Discussion:

         I have no objection to defining two terminating commands, one
         to exit a function, the other to exit the subsystem.  I guess
         I'd suggest defining a command 'GO ' to be used to
         terminate a function.

         I don't believe, however, that's it's necessary to distinguish
         the two cases to avoid confusion by human users.

         Even though the command language is ASCII, rather than binary,
         and even though I've adopted Mike Padlipsky's concept of a
         Unified USER Level Protocol', I don't consider that MP is a
         protocol for direct use by humans (although nothing can STOP a
         human user from speaking MP if he has access to a TELNET user
         program and is determined to do so).

         The concept I mean to extract from the UULP and exploit is its
         model of a single process with many subsystems, not its
         philosophy of a Network-standard command language for use by
         human users (the latter may be a good idea, too, but it's not
         the one I'm concerned with at the moment).

         I don't think that designing a protocol to govern an exchange
         between processes is the same task as designing a protocol to
         mediate a conversation between a process and a human user.
         Using ASCII commands suggests (as it did for FTP, RJE, etc.)
         that the latter problem is the one being addressed; it's not.

   USING TELNET GO AHEAD TO TERMINATE CERTAIN COMMANDS

      References:

         (AAM -- 17404,)

         (RCC -- 17822,1a:gy)

         (DHC JBP RFC 539 -- 17644,3b:gy)

      Discussion:

         Agreed.  My mistake.

         I simply have a strong distaste for the current FTP convention
         of terminating commands whose argument may itself contain CR LF
         with 'CR LF . CR LF'.  That seems a little extravagant to me.
         Personally, I'd prefer a single NVT character as a delimiter.




White                                                           [Page 4]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


          only terminates two MP commands (COMMENTS and TEXT).
         Some NVT character (ESC? EXT? ...) can easily be chosen that
         need not appear (and can therefore be prohibited from appearing
         by the Protocol) in the argument to either of those commands.

   SUBSYSTEM OR SEPARATE RJE-LIKE SERVER PROCESS

      References:

         (DHC JBP RFC 539 -- 17644,4a:gy)

         (AAM -- 17404,)

         (ADO RFC 552 -- 17809,3:y)

      Discussion:

         There are two separable issues here:

            (1)  Server Process Proliferation of Not?

               If the consensus of the Network community is that
               Padlipsky's UULP approach to protocol design and
               implementation is in fact superior to the current scheme,
               which calls for the implementation of each new Network
               protocol as a distinct server process with its own
               contact socket, then we should begin to embrace that
               concept and begin reshuffling existing protocol
               implementations accordingly.  Even more surely, NEW
               protocols (like MP), should be designed in accordance
               with the new standards, not the old.

               I think Buz Owen's suggestion (ADO RFC 552 -- 17809,3:y)
               -- that a skeletal UULP be defined, a socket assigned to
               server processes which implement it, and MP defined as a
               subsystem under it -- is excellent.  I retract my
               suggestion (JEW RFC 524 -- 17140,3a2:gy) in favor of
               Owen's.

               I further suggest that the latest revision of FTP (NJN
               RFC 542 -- 17759,) be similarly implemented (i.e., as a
               UULP subsystem), rather then implemented temporarily
               under a new socket and later moved over to socket 3 as
               suggested in RFC 542.







White                                                           [Page 5]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


            (2)  RJE's model for FTP Use or Not?

               If both MP (as currently defined) and RJE were instated
               as UULP subsystems, they would still embrace different
               philosophies regarding their use of FTP.  As the person
               who proposed and fought for the current RJE model (i.e.,
               to its use of FTP),  I (still) believe it to be an
               elegant one, more elegant by far then the one I've
               proposed for MP.

               An alternative I considered and discarded SOLELY for
               reasons of efficiency (neglecting, perhaps, the issue of
               cleanness of implementation), is that the command
               currently defined as 'FILE ' (JEW RFC 524 --
               17140,4q2a:gy), both in specifying Content and in the
               Citation Retrieval function, be 'FILE  '
               instead.

                  The server is then obliged to retrieve the Content of
                  the Mail from the designated server process via a
                  third-party exchange.

               The redefined FILE command would be similar to the
               LOCATION command, except that the former would specify
               JUST Content (and none of the other Static Attributes),
               and that the Server must retrieve the file (which may be
               a temporary file created by the user process) in real
               time, i.e. BEFORE it sends its response to the FILE
               command.

               This alternative eliminates the need to borrow the BYTE,
               SOCK, PASV, TYPE, STRU, MODE, REST, and SITE commands
               from FTP (JEW RFC 524 -- 17140,7c1:gy).  It also allows
               the user process the flexibility of specifying a file at
               a host other than his own.

               After some thought, I think I agree with Crocker and
               Postel that theirs is the better implementation.

                  As they point out, however, this implementation
                  introduces the problem of somehow reconciling the
                  desire to permit (in general) the transfer of mail
                  files without requiring a login, with a server's
                  inability to distinguish that case from the general
                  case of file retrieval (for which many hosts will
                  require a login).





White                                                           [Page 6]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


   USE OF THE DATE FORM 1/2/73 (JAN 2 OR FEB 1?)

      References:
         (RCC -- 17822,1b)
      Discussion:
         Agreed.

   ORDER OF PARAMETER SPECIFICATION

      References:

         (DHC JBP RFC 539 -- 17644,31:gy)

      Discussion:

         The Protocol does not, as Crocker and Postel state, impose an
         order upon command specification within a function (see for
         example, JEW RFC 524 -- 17140,5f1b:gy).

         Having considered their suggestion only briefly, it does seem
         to me appropriate to impose some constraints on the order of
         parameter specification by the user.  Off hand, the order
         suggested -- Dynamic, Optional, Static -- seems good.

III.  SUGGESTED ADDITIONS

   FORWARDING AT DELIVERY TIME

      References:

         (DHC JBP 539 -- 17644,4b:g)

      Discussion:

      Including provision for the forwarding of mail at Delivery Time,
      in contrast to sometime after Delivery in response to a specific
      Forward request (i.e., function), seems to me a useful addition to
      the Protocol.

      As Crocker and Postel note, only one of the three mechanisms for
      such forwarding bears upon the Protocol (although the Protocol
      might mention the other two and either encourage or discourage
      their use).

      I suggest the following reply format, however, rather than the one
      suggested by Crocker and Postel (DHC JBP RFC 539 --
      17644,4b3c2:gy):




White                                                           [Page 7]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


         476  -- is his location.

   DEFAULT SIGNATURE SHOULD BE THE AUTHOR

      References:
         (DHC JBP 539 -- 17644,3c:gy)
      Discussion:
         Agreed.

   LEVELS OF INTERRUPT

      References:

      (DHC JBP 539 -- 17644,3d:gy)

   Discussion:

         I see no value to defining numeric shades of urgency,
         unless the Protocol suggests some particular action the
         server might take in response to each one.

         The whole notion of flagging some pieces of mail as
         urgent seems to me useless unless the MP server process
         (not the human recipient) takes some kind of special
         action for urgent mail, BEFORE the human recipient
         would otherwise be apt to read the mail.  If one
         accepts that argument, there's clearly no point to
         defining shades of urgency if they have meaning only to
         the human recipient.  True, any pair of human users
         could privately agree on meanings, but it seems to me
         preferable to define those meanings formally or not at
         all.

   WARNING THE SERVER OF THE SIZE OF MAIL

      References:
         (DHC JBP RFC 539 -- 17644,3f:gy)
      Discussion:
         Agreed.  Further suggestions as to the implementation?

   DISCOURAGING SERVERS FROM REQUIRING LOGINS

      References:
         (DHC JBP RFC 539 -- 17644,3j:gy)
      Discussion:
         Agreed.  This is not a new issue.





White                                                           [Page 8]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


IV.  META-COMMENTS

   SIZE OF THE PROTOCOL DOCUMENT

      References:

         (RCC -- 17822,1e:gy)

      Discussion:

         I offer an apology for the format of the the Protocol document.
         It differs radically from that of previous Protocol documents
         (e.g., FTP, RJE), and is certainly not tutorial in its
         orientation.  The glossary is a device I found useful in
         designing the Protocol.  If the substance of the Protocol were
         agreed upon, then friendlier documentation would have to be
         written.  The choice of approach was greatly affected by my own
         time constraints.

         As I find time, I would like to define the minimum
         implementation subsets that Clements requests.  For the moment,
         consider the command breakdown below.  It represents the case
         where the server permits only the function by which mail is
         delivered to users in his host.  It has the following
         attributes:

            (1) It supports all of the functions of the current FTP mail
            protocol.  In addition,

            (2) It makes specification of author and title explicit,
            avoiding the current problem of multiple headers (one
            supplied by the server, the other embedded by the user in
            the text of the message),

            (3) It allows the text of the message to reside at a third
            host, and

            (4) It permits multiple recipients.

         The breakdown is the following:

            COMMANDS THAT MUST BE IMPLEMENTED
            (Author and Title could be treated as NOPs)

               To enter the Mail subsystem:
                  MAIL 
               To invoke the Delivery function:
                  DELIVER 



White                                                           [Page 9]

RFC 555     Response to Critiques of Proposed Mail Protocol    July 1973


               To specify the text of the message:
                  FILE 
                  LOCATION  
                  TEXT  
               To identify author(s), recipient(s), and title:
                  AUTHOR  
                  RECIPIENT  
                  TITLE