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Versions: 00 01 RFC 2197

Network Working Group                                Ned Freed
Internet Draft                                        Innosoft
                            <draft-freed-smtp-pipeline-01.txt>

                    SMTP Service Extension
                    for Command Pipelining

                          March 1997


                     Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
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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.


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.











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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:

 (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.











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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.


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;

 (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





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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 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.





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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.

 (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.








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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 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 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>





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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 ready
C: EHLO dbc.mtview.ca.us
S: 250-innosoft.com
S: 250 PIPELINING





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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 goodbye


6.  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 [1].


7.  Acknowledgements

This document is based on the SMTP service extension model
presented in RFC 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.























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8.  References

[1]  J.B. Postel.  Simple Mail Transfer Protocol. Request for
     Comments 821, (August, 1982).

[2]  J.C. Klensin, N. Freed, M.T. Rose, E.A. Stefferud,
     D.H. Crocker.  SMTP Service Extensions. Request for
     Comments 1869, (November, 1995).


9.  Author Address

Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
USA
 tel: +1 818 919 3600           fax: +1 818 919 3614
 email: ned@innosoft.com

This document is a product of work done by the Internet
Engineering Task Force Working Group on Messaging Extensions,
Alan Cargille, chair.



























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