RFC1845 - 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 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