SMTP Service Extension for Command Pipelining (RFC2197)
Original Publication Date: 1997-Sep-01
Included in the Prior Art Database: 2019-Feb-15
Internet Society Requests For Comment (RFCs)
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. [STANDARDS-TRACK]
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.
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 . Only textual and editorial changes have been made; the protocol has not changed in any way.
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  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 tha...