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Network Working Group                               J. Klensin, WG ChairRequest for Comments: 1653                                           MCIObsoletes:1427                                         N. Freed, EditorCategory: Standards Track                                       Innosoft                                                                K. Moore                                                 University of Tennessee                                                               July 1994SMTP Service Extension for Message Size DeclarationStatus 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 19942.  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 isKlensin, 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, suchKlensin, 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.comKlensin, 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 Goodbye8. 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,RFC974, 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.eduKlensin, Freed & Moore                                          [Page 8]