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Versions: 00 01 RFC 1845

Network Working Group                            David Crocker
Internet Draft                                       Ned Freed
<draft-ietf-mailext-checkp-01.txt>    Allan Cargille, WG Chair

                    SMTP Service Extension
                    for Checkpoint/Restart

                        April 3, 1995

                     Status of this Memo

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1.  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 message content sent prior
to the interruption.

2.  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

Internet Draft     SMTP Checkpoint/Restart          April 1995

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 continue
a transaction once the underlying TCP connection has been

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

3.  Framework for the Checkpointing Extension

The checkpointing extension is laid out as follows:

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

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

 (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

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       case-sensitive. The syntax of the value is as follows,
       using the ABNF notation of [2]:

            transid-value  ::= "<" transid-spec ">"
            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)   no additional SMTP verbs are defined by this extension;

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

4.  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

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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. Completion is assured only when the client
either explicitly aborts the transaction, starts a new
transaction, or closes the connection.

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.

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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

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

     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

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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

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.

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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
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
S: 354 Send checkpointed message, ending in CRLF.CRLF
<some amount of message data transmitted>
<session is interrupted>

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
C: MAIL FROM:<ned@ymir.claremont.edu> TRANSID=<12345@claremont.edu>
S: 355 6135 is the transaction offset
S: 354 Send previously checkpointed message starting at octet 6135
C: <message data minus first 6135 octets sent>
C: .
S: 250 OK
S: 250 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].

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7.  References

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

[2]  D.H. Crocker.  Standard for the Format of ARPA Internet
     Text Messages.  Request for Comments 822, (August, 1982).

[3]  N.S. Borenstein, N. Freed.  Multipurpose Internet Mail
     Extensions.  Request for Comments 1521, (September,

[4]  M.T. Rose, E.A. Stefferud, D.H. Crocker, J.C. Klensin,
     N. Freed.  SMTP Service Extensions.  Request for Comments
     1651, (July, 1994).

[5]  R. Braden, Editor.  Requirements for Internet Hosts --
     Application and Support.  Request for Comments 1123,
     (October, 1989).

[6]  C. Partridge.  Mail Routing and the Domain System.
     Request for Comments 974, (January, 1986).

8.  Author Addresses

Dave Crocker
Brandenburg Consulting
675 Spruce Dr.
Sunnyvale, CA 94086 USA
 tel: +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
 tel: +1 818 919 3600           fax: +1 818 919 3614
 email: ned@innosoft.com

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