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+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
+
+   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.
+
+
+
+
+
+Crocker, Freed & Cargille     Experimental                      [Page 3]
+
+RFC 1845                SMTP Checkpoint/Restart           September 1995
+
+
+   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
+
+
+
+Crocker, Freed & Cargille     Experimental                      [Page 4]
+
+RFC 1845                SMTP Checkpoint/Restart           September 1995
+
+
+   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.
+
+
+
+
+
+
+
+
+
+Crocker, Freed & Cargille     Experimental                      [Page 5]
+
+RFC 1845                SMTP Checkpoint/Restart           September 1995
+
+
+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].
+
+
+
+
+
+
+
+
+
+Crocker, Freed & Cargille     Experimental                      [Page 6]
+
+RFC 1845                SMTP Checkpoint/Restart           September 1995
+
+
+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
+
+
+
+
+
+
+Crocker, Freed & Cargille     Experimental                      [Page 7]
+