Network Working Group J. Klensin, WG Chair
Request For Comments: 1870 MCI
STD: 10 N. Freed, Editor
Obsoletes: 1653 Innosoft International, Inc.
Category: Standards Track K. Moore
University of Tennessee
November 1995

SMTP Service Extension
for Message Size Declaration

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

1. Abstract

This memo defines an extension to the SMTP service whereby an SMTP
client and server may interact to give the server an opportunity to
decline to accept a message (perhaps temporarily) based on the
client's estimate of the message size.

2. IntrodUCtion

The MIME extensions to the Internet message protocol provide for the
transmission of many kinds of data which were previously unsupported
in Internet mail. One eXPected result of the use of MIME is that
SMTP will be expected to carry a much wider range of message sizes
than was previously the case. This has an impact on the amount of
resources (e.g. disk space) required by a system acting as a server.

This memo uses the mechanism defined in [5] to define extensions to
the SMTP service whereby a client ("sender-SMTP") may declare the
size of a particular message to a server ("receiver-SMTP"), after
which the server may indicate to the client that it is or is not
willing to accept the message based on the declared message size and
whereby a server ("receiver-SMTP") may declare the maximum message
size it is willing to accept to a client ("sender-SMTP").

3. Framework for the Size Declaration Extension

The following service extension is therefore defined:

(1) the name of the SMTP service extension is "Message Size
Declaration";

(2) the EHLO keyWord value associated with this extension is "SIZE";

(3) one optional parameter is allowed with this EHLO keyword value, a
decimal number indicating the fixed maximum message size in bytes
that the server will accept. The syntax of the parameter is as
follows, using the augmented BNF notation of [2]:

size-param ::= [1*DIGIT]

A parameter value of 0 (zero) indicates that no fixed maximum
message size is in force. If the parameter is omitted no
information is conveyed about the server's fixed maximum message
size;

(4) one optional parameter using the keyword "SIZE" is added to the
MAIL FROM command. The value associated with this parameter is a
decimal number indicating the size of the message that is to be
transmitted. The syntax of the value is as follows, using the
augmented BNF notation of [2]:

size-value ::= 1*20DIGIT

(5) the maximum length of a MAIL FROM command line is increased by 26
characters by the possible addition of the SIZE keyword and
value;

(6) no additional SMTP verbs are defined by this extension.

The remainder of this memo specifies how support for the extension
affects the behavior of an SMTP client and server.

4. The Message Size Declaration service extension

An SMTP server may have a fixed upper limit on message size. Any
attempt by a client to transfer a message which is larger than this
fixed upper limit will fail. In addition, a server normally has
limited space with which to store incoming messages. Transfer of a
message may therefore also fail due to a lack of storage space, but
might succeed at a later time.

A client using the unextended SMTP protocol defined in [1], can only
be informed of such failures after transmitting the entire message to
the server (which discards the transferred message). If, however,
both client and server support the Message Size Declaration service
extension, such conditions may be detected before any transfer is
attempted.

An SMTP client wishing to relay a large content may issue the EHLO
command to start an SMTP session, to determine if the server supports
any of several service extensions. If the server responds with code
250 to the EHLO command, and the response includes the EHLO keyword
value SIZE, then the Message Size Declaration extension is supported.

If a numeric parameter follows the SIZE keyword value of the EHLO
response, it indicates the size of the largest message that the
server is willing to accept. Any attempt by a client to transfer a
message which is larger than this limit will be rejected with a
permanent failure (552) reply code.

A server that supports the Message Size Declaration extension will
accept the extended version of the MAIL command described below.
When supported by the server, a client may use the extended MAIL
command (instead of the MAIL command as defined in [1]) to declare an
estimate of the size of a message it wishes to transfer. The server
may then return an appropriate error code if it determines that an
attempt to transfer a message of that size would fail.

5. Definitions

The message size is defined as the number of octets, including CR-LF
pairs, but not the SMTP DATA command's terminating dot or doubled
quoting dots, to be transmitted by the SMTP client after receiving
reply code 354 to the DATA command.

The fixed maximum message size is defined as the message size of the
largest message that a server is ever willing to accept. An attempt
to transfer any message larger than the fixed maximum message size
will always fail. The fixed maximum message size may be an
implementation artifact of the SMTP server, or it may be chosen by
the administrator of the server.

The declared message size is defined as a client's estimate of the
message size for a particular message.

6. The extended MAIL command

The extended MAIL command is issued by a client when it wishes to
inform a server of the size of the message to be sent. The extended
MAIL command is identical to the MAIL command as defined in [1],
except that a SIZE parameter appears after the address.

The complete syntax of this extended command is defined in [5]. The
esmtp-keyword is "SIZE" and the syntax for esmtp-value is given by
the syntax for size-value shown above.

The value associated with the SIZE parameter is a decimal
representation of the declared message size in octets. This number
should include the message header, body, and the CR-LF sequences
between lines, but not the SMTP DATA command's terminating dot or
doubled quoting dots. Only one SIZE parameter may be specified in a
single MAIL command.

Ideally, the declared message size is equal to the true message size.
However, since exact computation of the message size may be
infeasable, the client may use a heuristically-derived estimate.
Such heuristics should be chosen so that the declared message size is
usually larger than the actual message size. (This has the effect of
making the counting or non-counting of SMTP DATA dots largely an
academic point.)

NOTE: Servers MUST NOT use the SIZE parameter to determine end of
content in the DATA command.

6.1 Server action on receipt of the extended MAIL command

Upon receipt of an extended MAIL command containing a SIZE parameter,
a server should determine whether the declared message size exceeds
its fixed maximum message size. If the declared message size is
smaller than the fixed maximum message size, the server may also wish
to determine whether sufficient resources are available to buffer a
message of the declared message size and to maintain it in stable
storage, until the message can be delivered or relayed to each of its
recipients.

A server may respond to the extended MAIL command with any of the
error codes defined in [1] for the MAIL command. In addition, one of
the following error codes may be returned:

(1) If the server currently lacks sufficient resources to accept a
message of the indicated size, but may be able to accept the
message at a later time, it responds with code "452 insufficient
system storage".

(2) If the indicated size is larger than the server's fixed maximum
message size, the server responds with code "552 message size
exceeds fixed maximium message size".

A server is permitted, but not required, to accept a message which
is, in fact, larger than declared in the extended MAIL command, such
as might occur if the client employed a size-estimation heuristic
which was inaccurate.

6.2 Client action on receiving response to extended MAIL command

The client, upon receiving the server's response to the extended MAIL
command, acts as follows:

(1) If the code "452 insufficient system storage" is returned, the
client should next send either a RSET command (if it wishes to
attempt to send other messages) or a QUIT command. The client
should then repeat the attempt to send the message to the server
at a later time.

(2) If the code "552 message exceeds fixed maximum message size" is
received, the client should immediately send either a RSET command
(if it wishes to attempt to send additional messages), or a QUIT
command. The client should then declare the message undeliverable
and return appropriate notification to the sender (if a sender
address was present in the MAIL command).

A successful (250) reply code in response to the extended MAIL
command does not constitute an absolute guarantee that the message
transfer will succeed. SMTP clients using the extended MAIL command
must still be prepared to handle both temporary and permanent error
reply codes (including codes 452 and 552), either immediately after
issuing the DATA command, or after transfer of the message.

6.3 Messages larger than the declared size.

Once a server has agreed (via the extended MAIL command) to accept a
message of a particular size, it should not return a 552 reply code
after the transfer phase of the DATA command, unless the actual size
of the message transferred is greater than the declared message size.
A server may also choose to accept a message which is somewhat larger
than the declared message size.

A client is permitted to declare a message to be smaller than its
actual size. However, in this case, a successful (250) reply code is
no assurance that the server will accept the message or has
sufficient resources to do so. The server may reject such a message
after its DATA transfer.

6.4 Per-recipient rejection based on message size.

A server that implements this extension may return a 452 or 552 reply
code in response to a RCPT command, based on its unwillingness to
accept a message of the declared size for a particular recipient.

(1) If a 452 code is returned, the client may requeue the message for
later delivery to the same recipient.

(2) If a 552 code is returned, the client may not requeue the message
for later delivery to the same recipient.

7. Minimal usage

A "minimal" client may use this extension to simply compare its
(perhaps estimated) size of the message that it wishes to relay, with
the server's fixed maximum message size (from the parameter to the
SIZE keyword in the EHLO response), to determine whether the server
will ever accept the message. Such an implementation need not
declare message sizes via the extended MAIL command. However,
neither will it be able to discover temporary limits on message size
due to server resource limitations, nor per-recipient limitations on
message size.

A minimal server that employs this service extension may simply use
the SIZE keyword value to inform the client of the size of the
largest message it will accept, or to inform the client that there is
no fixed limit on message size. Such a server must accept the
extended MAIL command and return a 552 reply code if the client's
declared size exceeds its fixed size limit (if any), but it need not
detect "temporary" limitations on message size.

The numeric parameter to the EHLO SIZE keyword is optional. If the
parameter is omitted entirely it indicates that the server does not
advertise a fixed maximum message size. A server that returns the
SIZE keyword with no parameter in response to the EHLO command may
not issue a positive (250) response to an extended MAIL command
containing a SIZE specification without first checking to see if
sufficient resources are available to transfer a message of the
declared size, and to retain it in stable storage until it can be
relayed or delivered to its recipients. If possible, the server
should actually reserve sufficient storage space to transfer the
message.

8. Example

The following example illustrates the use of size declaration with
some permanent and temporary failures.

S: <wait for connection on TCP port 25>
C: <open connection to server>
S: 220 sigurd.innosoft.com -- Server SMTP (PMDF V4.2-6 #1992)
C: EHLO ymir.claremont.edu
S: 250-sigurd.innosoft.com
S: 250-EXPN
S: 250-HELP
S: 250 SIZE 1000000
C: MAIL FROM:<ned@thor.innosoft.com> SIZE=500000
S: 250 Address Ok.
C: RCPT TO:<ned@innosoft.com>
S: 250 ned@innosoft.com OK; can accomodate 500000 byte message
C: RCPT TO:<ned@ymir.claremont.edu>
S: 552 Channel size limit exceeded: ned@YMIR.CLAREMONT.EDU
C: RCPT TO:<ned@hmcvax.claremont.edu>
S: 452 Insufficient channel storage: ned@hmcvax.CLAREMONT.EDU
C: DATA
S: 354 Send message, ending in CRLF.CRLF.
...
C: .
S: 250 Some recipients OK
C: QUIT
S: 221 Goodbye

9. Security Considerations

The size declaration extensions described in this memo can
conceivably be used to facilitate crude service denial attacks.
Specifically, both the information contained in the SIZE parameter
and use of the extended MAIL command make it somewhat quicker and
easier to devise an efficacious service denial attack. However,
unless implementations are very weak, these extensions do not create
any vulnerability that has not always existed with SMTP. In addition,
no issues are addressed involving trusted systems and possible
release of information via the mechanisms described in this RFC.

10. Acknowledgements

This document was derived from an earlier Working Group work in
progess contribution. Jim Conklin, Dave Crocker, Neil Katin, Eliot
Lear, Marshall T. Rose, and Einar Stefferud provided extensive
comments in response to earlier works in progress of both this and
the previous memo.

11. 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] Moore, K., "Representation of Non-ASCII Text in Internet Message
Headers", RFC1522, University of Tennessee, September 1993.

[5] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker,
"SMTP Service Extensions", STD 11, RFC1869, MCI, Innosoft
International, Inc., Dover Beach Consulting, Inc., Network
Management Associates, Inc., Brandenburg Consulting, November
1995.

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

12. Chair, Editor, and Author Addresses

John Klensin, WG Chair
MCI
2100 Reston Parkway
Reston, VA 22091

Phone: +1 703 715-7361
Fax: +1 703 715-7436
EMail: klensin@mci.net

Ned Freed, Editor
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

Keith Moore
Computer Science Dept.
University of Tennessee
107 Ayres Hall
Knoxville, TN 37996-1301
USA