Network Working Group                               J. Klensin, WG Chair
Request for Comments: 1427                     United Nations University
                                                       N. Freed, Editor
                                           Innosoft International, Inc.
                                                               K. Moore
                                                University of Tennessee
                                                          February 1993


                        SMTP Service Extension
                     for Message Size Declaration

Status of this Memo

  This RFC specifies an IAB standards track protocol for the Internet
  community, and requests discussion and suggestions for improvements.
  Please refer to the current edition of the "IAB Official Protocol
  Standards" 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:




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(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.

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



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  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



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  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.

  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:



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(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.





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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:<[email protected]> SIZE=500000
     S: 250 Address Ok.
     C: RCPT TO:<[email protected]>



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     S: 250 [email protected] OK; can accomodate 500000 byte message
     C: RCPT TO:<[email protected]>
     S: 552 channel size limit exceeded: [email protected]
     C: RCPT TO:<[email protected]>
     S: 452 insufficient channel storage: [email protected]
     C: DATA
     S: 354 Send message, ending in CRLF.CRLF.
      ...
     C: .
     S: 250 Some recipients OK
     C: QUIT
     S: 250 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 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.

11.  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 1341, Bellcore, Innosoft, June 1992.

  [4] Moore, K., "Representation of Non-ASCII Text in Internet Message
      Headers", RFC 1342, University of Tennessee, June 1992.






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  [5] Klensin, J., WG Chair, Freed, N., Editor, Rose, M., Stefferud,
      E., and D. Crocker, "SMTP Service Extensions" RFC 1425, United
      Nations University, Innosoft International, Inc., Dover Beach
      Consulting, Inc., Network Management Associates, Inc., The Branch
      Office, February 1993.

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

12.  Chair, Editor, and Author's Addresses

      John Klensin, WG Chair
      United Nations University
      PO Box 500, Charles Street Station
      Boston, MA 02114-0500  USA

      Phone: +1 617 227 8747
      Fax: +1 617 491 6266
      Email: [email protected]


      Ned Freed, Editor
      Innosoft International, Inc.
      250 West First Street, Suite 240
      Claremont, CA 91711  USA

      Phone: +1 909 624 7907
      Fax: +1 909 621 5319
      Email: [email protected]


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

      Email: [email protected]













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