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Network Working Group                                       N. FreedRequest for Comments: 2197                                  InnosoftObsoletes:1854                                       September 1997Category: Standards TrackSMTP Service Extensionfor 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.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 ofRFC 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:Freed                       Standards Track                     [Page 1]

RFC 2197                 SMTP Service Extension           September 1997    (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 inRFC 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;Freed                       Standards Track                     [Page 2]

RFC 2197                 SMTP Service Extension           September 1997    (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 mustFreed                       Standards Track                     [Page 3]

RFC 2197                 SMTP Service Extension           September 1997   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.Freed                       Standards Track                     [Page 4]

RFC 2197                 SMTP Service Extension           September 1997    (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 goodbyeFreed                       Standards Track                     [Page 5]

RFC 2197                 SMTP Service Extension           September 1997   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 goodbyeFreed                       Standards Track                     [Page 6]

RFC 2197                 SMTP Service Extension           September 1997   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 goodbye6.  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   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.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.Freed                       Standards Track                     [Page 7]

RFC 2197                 SMTP Service Extension           September 1997   [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.Freed                       Standards Track                     [Page 8]