Network Working Group                                          N. Freed
Request for Comments: 2920                                     Innosoft
STD: 60                                                  September 2000
Obsoletes: 2197
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.

Copyright Notice

  Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

  This memo defines an extension to the Simple Mail Transfer Protocol
  (SMTP) service whereby a server can indicate the extent of its
  ability to accept multiple commands in a single Transmission Control
  Protocol (TCP) send operation. Using a single TCP send operation for
  multiple commands can improve SMTP performance significantly.

1.  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 [RFC-821] 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:





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   (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 [RFC-1869] 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.

1.1.  Requirements Notation

  This document occasionally uses terms that appear in capital letters.
  When the terms "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
  appear capitalized, they are being used to indicate particular
  requirements of this specification. A discussion of the meanings of
  the terms "MUST", "SHOULD", and "MAY" appears in [RFC-1123]; the
  terms "MUST NOT" and "SHOULD NOT" are logical extensions of this
  usage.

2.  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;




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   (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.

3.  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.

3.1.  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



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  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.

3.2.  Server support of pipelining

  A server SMTP implementation that offers the pipelining extension:

   (1)   MUST respond to commands in the order they are received from
         the client.

   (2)   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.

   (3)   SHOULD issue a positive response to the DATA command if and
         only if one or more valid RCPT TO addresses have been
         previously received.

   (4)   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.

   (5)   MUST NOT buffer responses to EHLO, DATA, VRFY, EXPN, TURN,
         QUIT, and NOOP.

   (6)   MUST NOT buffer responses to unrecognized commands.





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   (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)   MUST NOT flush or otherwise lose the contents of the TCP input
         buffer under any circumstances whatsoever.

   (10)  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.

4.  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:<[email protected]>
  S: 250 sender <[email protected]> OK
  C: RCPT TO:<[email protected]>
  S: 250 recipient <[email protected]> OK
  C: RCPT TO:<[email protected]>
  S: 250 recipient <[email protected]> OK
  C: RCPT TO:<[email protected]>
  S: 250 recipient <[email protected]> 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



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  C: EHLO dbc.mtview.ca.us
  S: 250-innosoft.com
  S: 250 PIPELINING
  C: MAIL FROM:<[email protected]>
  C: RCPT TO:<[email protected]>
  C: RCPT TO:<[email protected]>
  C: RCPT TO:<[email protected]>
  C: DATA
  S: 250 sender <[email protected]> OK
  S: 250 recipient <[email protected]> OK
  S: 250 recipient <[email protected]> OK
  S: 250 recipient <[email protected]> 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:<[email protected]>
  C: RCPT TO:<[email protected]>
  C: RCPT TO:<[email protected]>
  C: DATA
  S: 250 sender <[email protected]> OK
  S: 550 remote mail to <[email protected]> not allowed
  S: 550 remote mail to <[email protected]> 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



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  C: EHLO dbc.mtview.ca.us
  S: 250-innosoft.com
  S: 250 PIPELINING
  C: MAIL FROM:<[email protected]>
  C: RCPT TO:<[email protected]>
  C: RCPT TO:<[email protected]>
  C: DATA
  S: 250 sender <[email protected]> OK
  S: 550 remote mail to <[email protected]> not allowed
  S: 550 remote mail to <[email protected]> not allowed
  S: 354 enter mail, end with line containing only "."
  C: .
  C: QUIT
  S: 554 no valid recipients
  S: 221 goodbye

5.  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 [RFC-821].

6.  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.

7.  References

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

  [RFC-1123] Braden, R., "Requirements for Internet Hosts --
             Application and Support", STD 3, RFC 1123, October, 1989.

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

  [RFC-1869] Klensin, J., Freed, N., Rose, M., Stefferud, E. and D.
             Crocker, "SMTP Service Extensions", STD 10, RFC 1869,
             November 1995.

  [RFC-2197] Freed, N., "SMTP Service Extension for Command
             Pipelining", RFC 2197, September 1997.





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8.  Author's Address

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

  Phone: +1 626 919 3600
  Fax:   +1 626 919 361
  EMail: [email protected]

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




































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9.  Full Copyright Statement

  Copyright (C) The Internet Society (2000).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.

















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