SMTP Pipelining: SMTP Service Extension for Command Pipelining

Network Working Group                                       N. Freed
Request for Comments: 2197                                  Innosoft
Obsoletes: 1854                                       September 1997
Category: Standards Track


                         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.

1.  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.

   The present document is an updated version of RFC 1854 [3].  Only
   textual and editorial changes have been made; the protocol has not
   changed in any way.

2.  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
          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.

2.1.  Requirements notation

   This document occasionally uses terms that appear in capital letters.
   When the terms "MUST", "SHOULD", "MUST NOT", "SHOULD NOT", and "MAY"
   appear capitalized, they are being used to indicate particular
   requirements of this specification. A discussion of the meanings of
   these terms appears in RFC 2119 [4].

3.  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.

4.  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.

4.1.  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, a RSET/MAIL FROM sequence
   used 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
   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.

4.2.  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.

   The overriding intent of these server requirements is to make it as
   easy as possible for servers to conform to these pipelining
   extensions.

5.  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
   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: .
   C: QUIT
   S: 554 no valid recipients
   S: 221 goodbye

6.  Security Considerations

   This document 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].

7.  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.

8.  References

   [1]  Postel, J., "Simple Mail Transfer Protocol", STD 10,
        RFC 821, August 1982.

   [2]  Klensin, J., Freed, N., Rose, M., Stefferud, E., and
        D. Crocker, "SMTP Service Extensions", RFC 1869,
        November 1995.

   [3]  Freed, N., "SMTP Service Extension for Command Pipelining",
        RFC 1854, October 1995.

   [4]  Bradner, S., "Key words for use in RFCs to Indicate
        Requirement Levels", RFC 2119, March 1997.

9.  Author's Address

   Ned Freed
   Innosoft International, Inc.
   1050 Lakes Drive
   West Covina, CA 91790
   USA

   Phone: +1 626 919 3600
   Fax: +1 626 919 3614
   EMail: ned.freed@innosoft.com

   This document is a product of work done by the Internet Engineering
   Task Force Working Group on Messaging Extensions, Alan Cargille,
   chair.