rfc1870.txt

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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").








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RFC 1870                 SMTP Size Declaration             November 1995


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.





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RFC 1870                 SMTP Size Declaration             November 1995


   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.








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RFC 1870                 SMTP Size Declaration             November 1995


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".



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RFC 1870                 SMTP Size Declaration             November 1995


   (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