SMTP Service Extension for Checkpoint/Restart
RFC 1845

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• 01

Network Working Group                                         D. Crocker
Request For Comments: 1845                        Brandenburg Consulting
Category: Experimental                                          N. Freed
                                            Innosoft International, Inc.
                                                   A. Cargille, WG Chair
                                                          September 1995

                         SMTP Service Extension
                         for Checkpoint/Restart

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract

   This memo defines an extension to the SMTP service whereby an
   interrupted SMTP transaction can be restarted at a later time without
   having to repeat all of the commands and message content sent prior
   to the interruption.

1.  Introduction

   Although SMTP is widely and robustly deployed, various extensions
   have been requested by parts of the Internet community. In
   particular, when dealing with very large messages over less reliable
   connections it is possible for substantial resources to be consumed
   by repeated unsuccessful attempts to transmit the message in its
   entirety. The original SMTP specification [1] does not provide any
   means to pick up a partially completed transaction after the
   underlying TCP connection has been broken and reestablished.

   This memo provides a facility by which a client can uniquely identify
   a particular SMTP transaction. The server then stores this
   identifying information along with all the information it receives as
   the transaction proceeds. If the transaction is interrupted during
   the data transfer phase the SMTP client may establish a new SMTP
   session at a later time and ask the server to continue the
   transaction from the point where the server lost its connection with
   the client. The server then reestablishes the transaction context and
   tells the client where to resume operations. If this is acceptable
   the client resumes operations at this point.

Crocker, Freed & Cargille     Experimental                      [Page 1]
RFC 1845                SMTP Checkpoint/Restart           September 1995

   This extension may also be used to work around the common timeout
   problem where a client times out waiting for a response from the
   server acknowledging that the message has been accepted. However, use
   of this extension is not an acceptable substitute for proper setting
   of timeout parameters.

2.  Framework for the Checkpointing Extension

   The checkpointing extension is laid out as follows:

 (1)   the name of the SMTP service extension defined here is
       checkpointing;

 (2)   the EHLO keyword value associated with the extension is
       CHECKPOINT;

 (3)   no parameter is used with the CHECKPOINT EHLO keyword;

 (4)   one optional parameter using the keyword TRANSID is
       added to the MAIL FROM command.  The value associated
       with this parameter, coupled with the name of the
       client taken from EHLO command, forms a globally unique
       value that identifies this particular transaction and
       serves to distinguish it from all others. This value is
       case-sensitive. The syntax of the value is as follows,
       using the ABNF notation of [2]:

            transid-value  ::= "<" transid-spec ">"
                               ; transid-value may not be longer than
                               ; 80 characters
            transid-spec   ::= transid-local "@" transid-domain
            transid-domain ::= transid-token
            transid-local  ::= transid-token
            transid-token  ::= transid-atom *("." transid-atom)
            transid-atom   ::= 1*<any (ASCII) CHAR except SPACE,
                                  CTLs, tspecials, or ".">

       NOTE: tspecials is defined in [3]. The TRANSID is
       likely to be different from the RFC822 message id,
       since it must uniquely identify the particular copy of
       the message being sent over this SMTP link. However,
       the syntax of transid-value is designed so that any
       TRANSID is both a legal RFC822 msg-id as well as being
       a legal esmtp-value [4].

 (5)   The maximum length of a MAIL FROM command line is
       increased by 88 characters by the possible addition of
       the TRANSID keyword and value;

Crocker, Freed & Cargille     Experimental                      [Page 2]
RFC 1845                SMTP Checkpoint/Restart           September 1995

 (6)   no additional SMTP verbs are defined by this extension;
       and,

 (7)   the next section specifies how support for the
       extension affects the behavior of a server and client
       SMTP.

3.  The checkpointing service extension