SMTP Checkpoint/Restart: SMTP Service Extension for Checkpoint/Restart

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.

   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;

 (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

   When a client SMTP wishes to use checkpointing to eliminate the need
   to retransmit all message data in its entirety in the event of a
   session interruption, 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 CHECKPOINT, then the
   server SMTP is indicating that it supports SMTP checkpointing and
   will honor requests to restart interrupted SMTP transactions.

   The extended MAIL command is issued by a client SMTP when it wishes
   to enable server checkpointing. The syntax for this command is
   identical to the MAIL command in [1], except that a TRANSID parameter
   must appear after the address.

   The complete syntax of this extended command is defined in [4], with
   the esmtp-keyword TRANSID and transid-value parameter as previously
   defined.

   The value associated with the TRANSID parameter must be an identifier
   that serves to uniquely identify this particular SMTP transaction.
   Only one TRANSID parameter may be used in a single MAIL command. Care
   must be used in constructing TRANSID values to simultaneously insure
   both uniqueness and the ability to reidentify interrupted
   transactions.

   The TRANSID is structured to ensure globally uniqueness without any
   additional registry. The transid-domain part should be a valid domain
   name that uniquely identifies the SMTP client. Note that this is
   usually the same as the domain name given in conjunction with the
   EHLO command, but not always. The EHLO domain name identifies the
   specific host the SMTP connection originated from, whereas the
   transid-domain may refer to a group of hosts that collectively host a
   multi-homed SMTP client. The transid-local part should be an
   identifier that distinguishes this SMTP transaction from any other
   originating from this SMTP client.

   Despite the structured nature of the TRANSID the server should treat
   the value as an opaque, case-sensitive string.

   Note that the contents of the RFC822 message-id header typically are
   NOT appropriate for use as the TRANSID parameter value, since such
   identifiers may be associated with multiple copies of the same
   message -- e.g., as it is split during transmission down different
   network paths -- and hence with multiple distinct SMTP transactions.

   A server which supports the checkpointing extension will then retain
   the transaction identifer as well as the most recent state of the
   transaction in non-volatile storage. This information should deleted
   only when the transaction is known to be complete from the client's
   perspective. Completion is assured only when the client either
   explicitly aborts the transaction, starts a new transaction, or
   requests that the connection be closed with a QUIT command.

   In the event of an interruption prior to completing a transaction
   this preserved state will remain for some period of time defined by
   the operational policies of the server administrator. It is
   recommended that transaction state information be preserved for at
   least 48 hours, although no specific time is required.

   When a client detects that a transaction has been interrupted, it
   then must wait for some period before reconnecting. This period must
   be long enough for server connections to time out and for the
   transaction state associated with such connections to be released for
   use by a new connection. The Internet Host Requirements [5] also
   impose restriction on how quickly reconnection attempts can be made
   (section 5.3.1.1).

   Once the necessary period has elapsed the client first checks the DNS
   as described in [6] and determine the set of acceptable IP addresses
   the message can be transferred to. If the IP address used to connect
   to the original server is still on this list it should be tried
   first, since this server is most likely to be capable of restarting
   the transaction. If this connection attempt fails the client must
   then proceed as described in [6] to try all the remaining IP
   addresses and restart the transaction there. If the attempt to
   restart fails on one of the other servers the client is required to
   retransmit the transaction in its entirety at that point.  Waiting
   for a server with an interrupted transaction state to come back
   online is not acceptable.

   Note: Multi-homed SMTP servers do exist, which means that it is
   entirely possible for a transaction to restart on a different server
   host.

   Once the connection is made the client issues the same MAIL command
   with exactly the same transaction identifier. If the transaction was
   interrupted during or at the end of the transfer of actual message
   data, the server first reestablishes its context to a point close as
   possible to the point of interruption and then responds with the
   status message:

     355 octet-offset is the transaction offset

   The actual status text can vary. However the octet-offset field is
   required to be the first thing on the first line of the reply, it
   must be separated from any following text by linear whitespace, and
   it is structured as follows:

     octet-offset ::= 1*DIGIT

   The octet-offset represents an offset, counting from zero, to the
   particular octet in the actual message data the server expects to see
   next. (This is also a count of how many octets the server has
   received and stored successfully.) This offset does NOT account for
   envelope data, i.e., MAIL FROM and RCPT TO commands. A value of 0
   would indicate that the client needs to start sending the message
   from the beginning, a value of 1 would indicate that the client
   should skip one octet, and so on.

   The SMTP canonical format for messages is used when this offset is
   computed.  Any octets added by any SMTP data-stuffing algorithm do
   not count as part of this offset. In the case of data transferred
   with the DATA command the offset must also correspond to the
   beginning of a line.

   Once this context is reestablished the client issues another data
   transfer command (e.g., DATA) and sends the remaining message data.
   Once this data is terminated the transaction completes in the normal
   fashion and the server deletes the transaction context from non-
   volatile storage.

   Note that the semantics of the octet-offset immediately suggest a
   particularly simple implementation strategy, where the client
   retransmits the message data as it normally would but suppresses
   output of the first octet-offset octets of material. The semantics
   used here are intentionally designed to make such implementation
   possible, but care must be taken to insure that such an
   implementation strategy does not impose a significant performance
   penalty on the client.

5.  Usage Example

   The following dialogue illustrates the use of the checkpointing
   service extension:

S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250-dbc.mtview.ca.us says hello
S: 250 CHECKPOINT
C: MAIL FROM:<ned@ymir.claremont.edu> TRANSID=<12345@claremont.edu>
S: 250 <ned@ymir.claremont.edu>... Sender and TRANSID ok
C: RCPT TO:<mrose@dbc.mtview.ca.us>
S: 250 <mrose@dbc.mtview.ca.us>... Recipient ok
C: DATA
S: 354 Send checkpointed message, ending in CRLF.CRLF
<some amount of message data transmitted>
<session is interrupted and TCP connection is broken>

Some time later a new connection is established:
S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250-dbc.mtview.ca.us says hello
S: 250 CHECKPOINT
C: MAIL FROM:<ned@ymir.claremont.edu> TRANSID=<12345@claremont.edu>
S: 355 6135 is the transaction offset
C: DATA
S: 354 Send previously checkpointed message starting at octet 6135
C: <message data minus first 6135 octets sent>
C: .
S: 250 OK
C: QUIT
S: 221 Goodbye

6.  Security Considerations

   This RFC does not discuss security issues and is not believed to
   raise any security issues not already endemic in electronic mail and
   present in fully conforming implementations of [1].

7.  References

   [1] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
       USC/Information Sciences Institute, August 1982.

   [2] Crocker, D., "Standard for the Format of ARPA Internet Text
       Messages", STD 11, RFC 822, UDEL, August 1982.

   [3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
       Extensions", RFC 1521, Bellcore, Innosoft, September 1993.

   [4] Rose, M., Stefferud, E., Crocker, D., Klensin, J., and N. Freed,
       "SMTP Service Extensions", RFC 1651, Dover Beach Consulting,
       Inc., Network Management Associates, Inc., Silicon Graphics,
       Inc., MCI, Innosoft, July 1994.

   [5] Braden, R., Editor, "Requirements for Internet Hosts -
       Application and Support", STD 3, RFC 1123, USC/Information
       Sciences Institute, October 1989.

   [6] Partridge, C., "Mail Routing and the Domain System", STD 14, RFC
       974, BBN, January 1986.

8.  Authors' Addresses

       Dave Crocker
       Brandenburg Consulting
       675 Spruce Dr.
       Sunnyvale, CA 94086 USA
       USA

       Phone: +1 408 246 8253
       Fax: +1 408 249 6205
       EMail: dcrocker@mordor.stanford.edu


       Ned Freed
       Innosoft International, Inc.
       1050 East Garvey Avenue South
       West Covina, CA 91790
       USA

       Phone: +1 818 919 3600
       Fax: +1 818 919 3614
       EMail: ned@innosoft.com