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 RFCdoes 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, RFC821,
USC/Information Sciences Institute, August 1982.

[2] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC822, UDEL, August 1982.

[3] Borenstein, N., and N. Freed, "Multipurpose Internet Mail
Extensions", RFC1521, Bellcore, Innosoft, September 1993.

[4] Rose, M., Stefferud, E., Crocker, D., Klensin, J., and N. Freed,
"SMTP Service Extensions", RFC1651, 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, RFC1123, 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