GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:std:std60

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:

Freed Standards Track [Page 1] RFC 2920 SMTP for Command Pipelining September 2000

  (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;

Freed Standards Track [Page 2] RFC 2920 SMTP for Command Pipelining September 2000

  (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

Freed Standards Track [Page 3] RFC 2920 SMTP for Command Pipelining September 2000

 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.

Freed Standards Track [Page 4] RFC 2920 SMTP for Command Pipelining September 2000

  (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:<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

Freed Standards Track [Page 5] RFC 2920 SMTP for Command Pipelining September 2000

 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

Freed Standards Track [Page 6] RFC 2920 SMTP for Command Pipelining September 2000

 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

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.

Freed Standards Track [Page 7] RFC 2920 SMTP for Command Pipelining September 2000

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: 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.

Freed Standards Track [Page 8] RFC 2920 SMTP for Command Pipelining September 2000

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.

Freed Standards Track [Page 9]

/data/webs/external/dokuwiki/data/pages/rfc/std/std60.txt · Last modified: 2000/09/13 18:44 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki