











Network Working Group                                           N. Freed

Request For Comments: 1854                  Innosoft International, Inc.

Category: Standards Track                          A. Cargille, WG Chair

                                                            October 1995





                         SMTP Service Extension

                         for Command Pipelining



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.



Abstract



   This memo defines an extension to the SMTP service whereby a server

   can indicate the extent of its ability to accept multiple commands in

   a single TCP send operation. Using a single TCP send operation for

   multiple commands can improve SMTP performance significantly.



Introduction



   Although SMTP is widely and robustly deployed, certain extensions may

   nevertheless prove useful. In particular, many parts of the Internet

   make use of high latency network links.



   SMTP's intrinsic one command-one response structure is significantly

   penalized by high latency links, often to the point where the factors

   contributing to overall connection time are dominated by the time

   spent waiting for responses to individual commands (turnaround time).



   In the best of all worlds it would be possible to simply deploy SMTP

   client software that makes use of command pipelining: batching up

   multiple commands into single TCP send operations. Unfortunately, the

   original SMTP specification [1] did not explicitly state that SMTP

   servers must support this.  As a result a non-trivial number of

   Internet SMTP servers cannot adequately handle command pipelining.

   Flaws known to exist in deployed servers include:



 (1)   Connection handoff and buffer flushes in the middle of

       the SMTP dialogue.  Creation of server processes for

       incoming SMTP connections is a useful, obvious, and

       harmless implementation technique. However, some SMTP

       servers defer process forking and connection handoff







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       until some intermediate point in the SMTP dialogue.

       When this is done material read from the TCP connection

       and kept in process buffers can be lost.



 (2)   Flushing the TCP input buffer when an SMTP command

       fails. SMTP commands often fail but there is no reason

       to flush the TCP input buffer when this happens.

       Nevertheless, some SMTP servers do this.



 (3)   Improper processing and promulgation of SMTP command

       failures. For example, some SMTP servers will refuse to

       accept a DATA command if the last RCPT TO command

       fails, paying no attention to the success or failure of

       prior RCPT TO command results. Other servers will

       accept a DATA command even when all previous RCPT TO

       commands have failed. Although it is possible to

       accommodate this sort of behavior in a client that

       employs command pipelining, it does complicate the

       construction of the client unnecessarily.



   This memo uses the mechanism described in [2] to define an extension

   to the SMTP service whereby an SMTP server can declare that it is

   capable of handling pipelined commands. The SMTP client can then

   check for this declaration and use pipelining only when the server

   declares itself capable of handling it.



1.  Framework for the Command Pipelining Extension



   The Command Pipelining extension is defined as follows:



    (1)   the name of the SMTP service extension is Pipelining;



    (2)   the EHLO keyword value associated with the extension is

          PIPELINING;



    (3)   no parameter is used with the PIPELINING EHLO keyword;



    (4)   no additional parameters are added to either the MAIL

          FROM or RCPT TO commands.



    (5)   no additional SMTP verbs are defined by this extension;

          and,



    (6)   the next section specifies how support for the

          extension affects the behavior of a server and client

          SMTP.











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2.  The Pipelining Service Extension



   When a client SMTP wishes to employ command pipelining, it first

   issues the EHLO command to the server SMTP. If the server SMTP

   responds with code 250 to the EHLO command, and the response includes

   the EHLO keyword value PIPELINING, then the server SMTP has indicated

   that it can accommodate SMTP command pipelining.



2.1.  Client use of pipelining



   Once the client SMTP has confirmed that support exists for the

   pipelining extension, the client SMTP may then elect to transmit

   groups of SMTP commands in batches without waiting for a response to

   each individual command. In particular, the commands RSET, MAIL FROM,

   SEND FROM, SOML FROM, SAML FROM, and RCPT TO can all appear anywhere

   in a pipelined command group.  The EHLO, DATA, VRFY, EXPN, TURN,

   QUIT, and NOOP commands can only appear as the last command in a

   group since their success or failure produces a change of state which

   the client SMTP must accommodate. (NOOP is included in this group so

   it can be used as a synchronization point.)



   Additional commands added by other SMTP extensions may only appear as

   the last command in a group unless otherwise specified by the

   extensions that define the commands.



   The actual transfer of message content is explicitly allowed to be

   the first "command" in a group. That is, the RSET/MAIL FROM sequence

   necessary to initiate a new message transaction can be placed in the

   same group as the final transfer of the headers and body of the

   previous message.



   Client SMTP implementations that employ pipelining MUST check ALL

   statuses associated with each command in a group. For example, if

   none of the RCPT TO recipient addresses were accepted the client must

   then check the response to the DATA command -- the client cannot

   assume that the DATA command will be rejected just because none of

   the RCPT TO commands worked.  If the DATA command was properly

   rejected the client SMTP can just issue RSET, but if the DATA command

   was accepted the client SMTP should send a single dot.



   Command statuses MUST be coordinated with responses by counting each

   separate response and correlating that count with the number of

   commands known to have been issued.  Multiline responses MUST be

   supported. Matching on the basis of either the error code value or

   associated text is expressly forbidden.



   Client SMTP implementations MAY elect to operate in a nonblocking

   fashion, processing server responses immediately upon receipt, even







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   if there is still data pending transmission from the client's

   previous TCP send operation. If nonblocking operation is not

   supported, however, client SMTP implementations MUST also check the

   TCP window size and make sure that each group of commands fits

   entirely within the window. The window size is usually, but not

   always, 4K octets.  Failure to perform this check can lead to

   deadlock conditions.



   Clients MUST NOT confuse responses to multiple commands with

   multiline responses. Each command requires one or more lines of

   response, the last line not containing a dash between the response

   code and the response string.



2.2.  Server support of pipelining



   A server SMTP implementation that offers the pipelining extension:



    (1)   MUST NOT flush or otherwise lose the contents of the

          TCP input buffer under any circumstances whatsoever.



    (2)   SHOULD issue a positive response to the DATA command if

          and only if one or more valid RCPT TO addresses have

          been previously received.



    (3)   MUST NOT, after issuing a positive response to a DATA

          command with no valid recipients and subsequently

          receiving an empty message, send any message whatsoever

          to anybody.



    (4)   SHOULD elect to store responses to grouped RSET, MAIL

          FROM, SEND FROM, SOML FROM, SAML FROM, and RCPT TO

          commands in an internal buffer so they can sent as a

          unit.



    (5)   MUST NOT buffer responses to EHLO, DATA, VRFY, EXPN,

          TURN, QUIT, and NOOP.



    (6)   MUST NOT buffer responses to unrecognized commands.



    (7)   MUST send all pending responses immediately whenever

          the local TCP input buffer is emptied.



    (8)   MUST NOT make assumptions about commands that are yet

          to be received.



    (9)   SHOULD issue response text that indicates, either

          implicitly or explicitly, what command the response

          matches.







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   The overriding intent of these server requirements is to make it as

   easy as possible for servers to conform to these pipelining

   extensions.



3.  Examples



   Consider the following SMTP dialogue that does not use pipelining:



   S: <wait for open connection>

   C: <open connection to server>

   S: 220 innosoft.com SMTP service ready

   C: HELO dbc.mtview.ca.us

   S: 250 innosoft.com

   C: MAIL FROM:<mrose@dbc.mtview.ca.us>

   S: 250 sender <mrose@dbc.mtview.ca.us> OK

   C: RCPT TO:<ned@innosoft.com>

   S: 250 recipient <ned@innosoft.com> OK

   C: RCPT TO:<dan@innosoft.com>

   S: 250 recipient <dan@innosoft.com> OK

   C: RCPT TO:<kvc@innosoft.com>

   S: 250 recipient <kvc@innosoft.com> OK

   C: DATA

   S: 354 enter mail, end with line containing only "."

    ...

   C: .

   S: 250 message sent

   C: QUIT

   S: 221 goodbye



   The client waits for a server response a total of 9 times in this

   simple example. But if pipelining is employed the following dialogue

   is possible:



   S: <wait for open connection>

   C: <open connection to server>

   S: 220 innosoft.com SMTP service ready

   C: EHLO dbc.mtview.ca.us

   S: 250-innosoft.com

   S: 250 PIPELINING

   C: MAIL FROM:<mrose@dbc.mtview.ca.us>

   C: RCPT TO:<ned@innosoft.com>

   C: RCPT TO:<dan@innosoft.com>

   C: RCPT TO:<kvc@innosoft.com>

   C: DATA

   S: 250 sender <mrose@dbc.mtview.ca.us> OK

   S: 250 recipient <ned@innosoft.com> OK

   S: 250 recipient <dan@innosoft.com> OK

   S: 250 recipient <kvc@innosoft.com> OK







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   S: 354 enter mail, end with line containing only "."

    ...

   C: .

   C: QUIT

   S: 250 message sent

   S: 221 goodbye



   The total number of turnarounds has been reduced from 9 to 4.



   The next example illustrates one possible form of behavior when

   pipelining is used and all recipients are rejected:



   S: <wait for open connection>

   C: <open connection to server>

   S: 220 innosoft.com SMTP service ready

   C: EHLO dbc.mtview.ca.us

   S: 250-innosoft.com

   S: 250 PIPELINING

   C: MAIL FROM:<mrose@dbc.mtview.ca.us>

   C: RCPT TO:<nsb@thumper.bellcore.com>

   C: RCPT TO:<galvin@tis.com>

   C: DATA

   S: 250 sender <mrose@dbc.mtview.ca.us> OK

   S: 550 remote mail to <nsb@thumper.bellore.com> not allowed

   S: 550 remote mail to <galvin@tis.com> not allowed

   S: 554 no valid recipients given

   C: QUIT

   S: 221 goodbye



   The client SMTP waits for the server 4 times here as well. If the

   server SMTP does not check for at least one valid recipient prior to

   accepting the DATA command, the following dialogue would result:



   S: <wait for open connection>

   C: <open connection to server>

   S: 220 innosoft.com SMTP service ready

   C: EHLO dbc.mtview.ca.us

   S: 250-innosoft.com

   S: 250 PIPELINING

   C: MAIL FROM:<mrose@dbc.mtview.ca.us>

   C: RCPT TO:<nsb@thumper.bellcore.com>

   C: RCPT TO:<galvin@tis.com>

   C: DATA

   S: 250 sender <mrose@dbc.mtview.ca.us> OK

   S: 550 remote mail to <nsb@thumper.bellore.com> not allowed

   S: 550 remote mail to <galvin@tis.com> not allowed

   S: 354 enter mail, end with line containing only "."

   C: .







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RFC 1854                    SMTP Pipelining                 October 1995





   C: QUIT

   S: 554 no valid recipients

   S: 221 goodbye



4.  Security Considerations



   This RFC does not discuss security issues and is not believed to

   raise any security issues not endemic in electronic mail and present

   in fully conforming implementations of [1].



5.  Acknowledgements



   This document is based on the SMTP service extension model presented

   in RFC 1425. Marshall Rose's description of SMTP command pipelining

   in his book "The Internet Message" also served as a source of

   inspiration for this extension.



6.  References



   [1]  Postel, J., "Simple Mail Transfer Protocol", STD 10

        RFC 821, USC/Information Sciences Institute, August

        1982.



   [2]  Klensin, J., Freed, N., Rose, M., Stefferud, E.,

        and D. Crocker, "SMTP Service Extensions", RFC 1651,

        MCI, Innosoft, Dover Beach Consulting, Inc.,

        Network Management Associates, Inc., Silicon Graphics,

        Inc., July 1994.



7.  Author's Address



   Ned Freed

   Innosoft International, Inc.

   1050 East Garvey Avenue South

   West Covina, CA 91790

   USA



   Phone: +1 818 919 3600

   Fax: +1 818 919 3614

   EMail: ned@innosoft.com























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