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Network Working Group                                          N. FreedRequest for Comments: 2920                                     InnosoftSTD: 60                                                  September 2000Obsoletes:2197Category: Standards TrackSMTP Service Extension for Command PipeliningStatus 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 commandFreed                       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 readyFreed                       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 readyFreed                       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 goodbye5.  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 inRFC 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,RFC821, 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 20008.  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 20009.  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]