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rfc:rfc1653

Network Working Group J. Klensin, WG Chair Request for Comments: 1653 MCI Obsoletes: 1427 N. Freed, Editor Category: Standards Track Innosoft

                                                              K. Moore
                                               University of Tennessee
                                                             July 1994
        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.

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.

1. 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").

Klensin, Freed & Moore [Page 1] RFC 1653 SMTP Size Declaration July 1994

2. 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*DIGIT
 (5) 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.

3. 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

Klensin, Freed & Moore [Page 2] RFC 1653 SMTP Size Declaration July 1994

 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.

4. 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.

4. 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.

Klensin, Freed & Moore [Page 3] RFC 1653 SMTP Size Declaration July 1994

 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.

5.1 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

Klensin, Freed & Moore [Page 4] RFC 1653 SMTP Size Declaration July 1994

 as might occur if the client employed a size-estimation heuristic
 which was inaccurate.

5.2 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.

5.3 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.

5.4 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.

Klensin, Freed & Moore [Page 5] RFC 1653 SMTP Size Declaration July 1994

 (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.

6. 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.

7. 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

Klensin, Freed & Moore [Page 6] RFC 1653 SMTP Size Declaration July 1994

    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: 250 Goodbye

8. 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.

9. Acknowledgements

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

10. 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.

Klensin, Freed & Moore [Page 7] RFC 1653 SMTP Size Declaration July 1994

 [4] Moore, K., "Representation of Non-ASCII Text in Internet Message
     Headers", RFC 1522, University of Tennessee, September 1993.
 [5] Klensin, J., Freed, N., Rose, M., Stefferud, E., and D. Crocker,
     "SMTP Service Extensions", RFC 1651, MCI, Innosoft, Dover Beach
     Consulting, Inc., Network Management Associates, Inc., Silicon
     Graphics, Inc., July 1994.
 [6] Partridge, C., "Mail Routing and the Domain System", STD 14, RFC
     974, BBN, January 1986.

11. Chair, Editor, and Authors' Addresses

 John Klensin, WG Chair
 MCI Data Services Division
 2100 Reston Parkway, 6th floor
 Reston, VA 22091
 USA
 Phone:: 1 703 715 7361
 Fax: +1 703 715 7435
 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
 EMail: moore@cs.utk.edu

Klensin, Freed & Moore [Page 8]

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