SMTP Service Extension for Command Pipelining
RFC 2197
Document | Type |
RFC - Draft Standard
(September 1997; No errata)
Obsoleted by RFC 2920
Obsoletes RFC 1854
Was draft-freed-smtp-pipeline (individual)
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Author | Ned Freed | ||
Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 2197 (Draft Standard) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
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: 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 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; 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 theShow full document text