SMTP Service Extension for Command Pipelining (RFC1854)
Original Publication Date: 1995-Oct-01
Included in the Prior Art Database: 2000-Sep-13
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. Using a single TCP send operation for multiple commands can improve SMTP performance significantly.
Network Working Group N. Freed
Request For Comments: 1854 Innosoft International, Inc.
Category: Standards Track A. Cargille, WG Chair
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.
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:
(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...