Network Working Group Keith Moore Internet Draft University of Tennessee SMTP Service Extension for Message Size Declaration 24 October 1992 1. Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. (The file 1id-abstracts.txt, available via anonymous ftp from nic.ddn.mil, describes the current status of each Internet Draft.) It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "work in progress". 2. 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. 3. Introduction The Simple Mail Transfer Protocol (SMTP) [1] has provided a stable, effective basis for the relay function of message transfer agents. Although a decade old, SMTP has proven remarkably resilient. 4. Framework for SMTP Extensions For the purpose of service extensions to the SMTP, SMTP relays a mail object containing an envelope and a content. (1) The SMTP envelope is straight-forward, and is sent as a series of SMTP protocol units: it consists of an originator address (to which error reports should be directed); a delivery mode (e.g., deliver to recipient mailboxes); and, one or more recipient addresses. (2) The SMTP content is sent in the SMTP DATA protocol unit and has two parts: the headers and the body. The headers form a collection of field/value pairs structured according to RFC 822 [2], whilst the body, if structured, is defined according to MIME [3]. The content is textual in nature, expressed using the US ASCII repertoire (ISO K. Moore Expires 24 April 1993 [Page 1] Internet Draft 24 October 1992 registered character set number 6). Although extensions (such as MIME) may relax this restriction for the content body, the content headers are always encoded using the US ASCII repertoire. The algorithm defined in [4] is used to represent header values outside the US ASCII repertoire, whilst still encoding them using the US ASCII repertoire. Although SMTP is widely and robustly deployed, some parts of the Internet community might wish to extend the SMTP service. 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 an extension 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: (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 decmial number indicating the fixed maximum message size that the server will accept. The syntax of the parameter is as follows, using the augmented BNF notation of [2]: size-param ::= *DIGIT A parameter value of 0 (zero) indicates that no fixed maximum message size is in force. A null parameter conveys no information about a server's fixed maximum message size. 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. K. Moore Expires 24 April 1993 [Page 2] Internet Draft 24 October 1992 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 attemped. 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, 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, up to, but not including, the final dot, 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. K. Moore Expires 24 April 1993 [Page 3] Internet Draft 24 October 1992 The syntax for this command, using the augmented BNF notation defined in [2], is as follows: extended-mail-cmd = "MAIL FROM:<" reverse-path ">" SPACE "SIZE=" 1*DIGIT CRLF The value associated with the SIZE parameter is a decimal representation of the declared message size in octets. Ideally, the declared message size is equal to the 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. 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, the following error codes are 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: K. Moore Expires 24 April 1993 [Page 4] Internet Draft 24 October 1992 (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. If a particular message is too large to be sent, a client may attempt to relay the message via a different path, (as described in [6]), or to relay the message to the same 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. If a particular message exceeds the server's message size limit, the client may either (a) attempt to relay the message by another path, as described in [6], or (b) 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 to the same server. (2) If a 552 code is returned, the client may not requeue the message for later delivery to the same recipient at the same server. K. Moore Expires 24 April 1993 [Page 5] Internet Draft 24 October 1992 (3) In either case, the client may attempt to deliver the message by another path, as defined in [6]. 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. A null parameter indicates that the server does not advertise a fixed maximum message size. A server that returns the SIZE keyword with a null parameter in response to the EHLO command may not issue a positive (250) response to an extended MAIL command 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. Acknowldegements Dave Crocker, Marshall T. Rose, and Einar Stefferud provided extensive comments in response to earlier drafts of this memo. 9. References [1] J. B. Postel. Simple Mail Transfer Protocol. Request for Comments 821, August 1982. [2] D. H. Crocker. Standard for the Format of ARPA Internet Text Messages. Request for Comments 822, August 1982. [3] N. S. Borenstein, N. Freed. Multipurpose Internet Mail Extensions. Request for Comments 1341, June 1992. K. Moore Expires 24 April 1993 [Page 6] Internet Draft 24 October 1992 [4] K. Moore. Representation of Non-ASCII Text in Internet Message Headers. Request for Comments 1342, June 1992. [5] M. T. Rose, E. A. Stefferud, D. H. Crocker. SMTP Service Extensions. Internet-draft, 21 October 1992. [6] C. Partridge. Mail Routing and the Domain System. Request for Comments 974, January 1986. 10. Author's address Keith Moore Computer Science Dept. University of Tennessee 107 Ayres Hall Knoxville, TN 37996-1301 Internet: moore@cs.utk.edu K. Moore Expires 24 April 1993 [Page 7]