Internet Draft Greg Vaudreuil Expires in six months Lucent Technologies December 1, 1999 SMTP Service Extensions for Transmission of Large and Binary MIME Messages draft-vaudreuil-esmtp-binary2-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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. It is inappropriate to use Internet Drafts as reference material or to cite them other than as a "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. This Internet-Draft is in conformance with Section 10 of RFC 2026. Abstract This memo defines two extensions to the SMTP service. The first service enables a SMTP client and server to negotiate the use of an alternative to the DATA command, called "BDAT" for efficiently sending large MIME messages. The second extension takes advantage of the BDAT command to permit the negotiated sending of binary contents wrapped in MIME but without a transport encoding. This document is intended to update and obsolete RFC1830. Internet Draft Binary ESMTP December 1, 1999 Working Group Summary This protocol is not the product of an IETF working group, however the specification resulted from discussions within the ESMTP working group. The resulting protocol documented in RFC1830 was classified as experimental at that time due to questions about the robustness of the Binary Content-Transport-Encoding deployed in then existent MIME implementations. As MIME has matured and other uses of the Binary Content-Transport-Encoding have been deployed, these concerns have been allayed. With this document, Binary ESMTP is expected to become standards-track. Table of Contents 1. OVERVIEW ..........................................................2 2. FRAMEWORK FOR THE LARGE MESSAGE EXTENSIONS ........................3 3. FRAMEWORK FOR THE BINARY SERVICE EXTENSION ........................5 4. EXAMPLES ..........................................................7 4.1 Simple Chunking .................................................7 4.2 Pipelining Binarymime ...........................................8 5. SECURITY CONSIDERATIONS ...........................................9 6. ACKNOWLEDGMENTS ...................................................9 7. REFERENCES ........................................................9 8. COPYRIGHT NOTICE ..................................................9 9. AUTHOR'S ADDRESS .................................................10 10. APPENDIX A - CHANGES FROM RFC1830 ................................11 1. Overview The MIME extensions to the Internet message protocol provides for the transmission of many kinds of data that was previously unsupported in Internet mail. Anticipating the need to more efficiently transport the new media made possible with MIME, the SMTP protocol has been extended to provide transport for new message types. RFC 1426 defines one such extension for the transmission of unencoded 8-bit MIME messages [8BIT]. This service extension permits the receiver SMTP to declare support for 8-bit body parts and the sender to request 8-bit transmission of a particular message. One expected result of the use of MIME is that the Internet mail system will be expected to carry very large mail messages. In such transactions, there is a performance-based desire to eliminate the requirement that the message be scanned for "CR LF . CR LF" sequences upon sending and receiving to detect the end of message. Independent of the need to send large messages, Internet mail is increasingly multi-media. There is a need to avoid the overhead of base64 and quoted-printable encoding of binary objects sent using the MIME message format over SMTP between hosts that support binary message processing. This memo uses the mechanism defined in [ESMTP] to define two extensions to the SMTP service whereby a client ("sender-SMTP") may Vaudreuil Expires 5/1/00 [Page 2] Internet Draft Binary ESMTP December 1, 1999 declare support for the message chunking transmission mode and support for the receiption of Binary messages. 2. Framework for the Large Message Extensions The following service extension is hereby defined: The name of the data chunking service extension is "CHUNKING". The EHLO keyword value associated with this extension is "CHUNKING". A new SMTP verb is defined "BDAT" as an alternative to the "DATA" command of [RFC821]. The BDAT verb takes two arguments. The first argument indicates the length, in octets, of the binary data chunk. The second optional argument indicates that the data packet is the last. bdat-cmd ::= "BDAT" SP chunk-size [ SP end-marker ] CR LF chunk-size ::= 1*DIGIT end-marker ::= "LAST" The CHUNKING service extension enables the use of the BDAT alternative to the DATA command. This extension can be used for any message, whether 7 bit, 8BITMIME or BINARYMIME. When a client SMTP wishes to submit (using the MAIL command) a large message using the CHUNKING extension, it first issues the EHLO command to the server SMTP. If the server SMTP responds with code 250 to the EHLO command and the response includes the EHLO keyword value CHUNKING, then the server SMTP is indicating that it supports the BDAT command and will accept the sending of messages in chunks. After all MAIL FROM and RCPT TO responses are collected and processed, the message is sent using a series of BDAT commands. The BDAT command takes one required argument, the exact length of the data segment in octets. The message data is sent immediately after the trailing of the BDAT command line. Once the receiver-SMTP receives the specified number of octets, it will return a 250 reply code. The optional LAST parameter on the BDAT command indicates that this is the last chunk of message data to be sent. Any BDAT command sent after the BDAT LAST is illegal and must be replied to with a 503 "Bad sequence of commands" reply code. The state resulting from this error is indeterminate. A RSET command must be sent to clear the transaction before continuing. A 250 response should be sent to each BDAT data block. If a failure ocures after a BDAT command is received, the receiver-SMTP must accept and discard the associated message data before sending the 5XX code. If a 5XX code is received by the sender-SMTP in response to a BDAT chunk, the message should be considered failed and the sender SMTP must not send any additional BDAT segments. If the receiver-SMTP has declared support for streaming, the receiver SMTP must be prepared to Vaudreuil Expires 5/1/00 [Page 3] Internet Draft Binary ESMTP December 1, 1999 accept and discard additional BDAT chunks already in the pipeline after the failed BDAT. Note: An error on the receiver-SMTP such as disk full or imminent shutdown can only be reported after the BDAT segment has been received. It is therefore important to choose a reasonable chunk size given the expected end-to-end bandwidth. Note: Because the receiver-SMTP does not acknowledge the BDAT command before the messafge data is sent, it is important to send the BDAT only to systems that have declared their capability to accept BDAT commands. Illegally sending a BDAT command and associated message data to a non-chunking capable system will result in the receiver-SMTP parsing the associated message data as if it were a potentially very long, binary-data containing E/SMTP command line. The resulting state from a failed BDAT command is indeterminate. A RSET command must be issued to clear the transaction before additional commands may be sent. The RSET command, when issued after the first BDAT and before the BDAT LAST, clears all segments sent during that transaction and resets the session. DATA and BDAT commands cannot be used in the same transaction. If a DATA statement is issued after a BDAT for the current transaction, a 503 "Bad sequence of commands" must be issued. The state resulting from this error is indeterminate. A RSET command must be sent to clear the transaction before continuing. There is no prohibition on using DATA and BDAT in the same session, so long as they are not mixed in the same transaction. The local storage size of a message may not accurately reflect the actual size of the message sent due to local storage conventions. In particular, text messages sent with the BDAT command MUST be sent in the canonical MIME format with lines delimited with a . It may not be possible to convert the entire message to the canonical format at once. Chunking provides a mechanism to convert the message to canonical form, accurately count the bytes, and send the message a single chunk at a time. Note that correct byte counting is essential. If the sender SMTP indicates a chunk-size larger than the actual chunnk-size, the receiver SMTP will continue to wait for the remainder of the data or when using streaming, will read the subsequent command as additional message data. In the case where a portion of the previous command was read as data, the parser will return a syntax error when the incomplete command is read. If the sender SMTP indicates a chunk-size smaller than the actual chunk-size, the receiver SMTP will interpret the remainder of the message data as invalid commands. Note that the remainder of the message data may be binary and as such lexicographical parsers must be prepared to receive, process, and reject lines of arbitrary octets. Vaudreuil Expires 5/1/00 [Page 4] Internet Draft Binary ESMTP December 1, 1999 3. Framework for the Binary Service Extension The following service extension is hereby defined: 1) The name of the binary service extension is "BINARYMIME". 2) The EHLO keyword value associated with this extension is "BINARYMIME". 3) The BINARYMIME service extension can only be used with the "CHUNKING" service extension. 4) No parameter is used with the BINARYMIME keyword. 5) [8BIT] defines the BODY parameter for the MAIL command. This extension defines an additional value for the BODY parameter, "BINARYMIME". The value "BINARYMIME" associated with this parameter indicates that this message is a Binary MIME message (in strict compliance with [MIME]) with arbitrary octet content being sent. The revised syntax of the value is as follows, using the ABNF notation of [RFC822]: body-value ::= "7BIT" / "8BITMIME" / "BINARYMIME" 6) No new verbs are defined for the BINARYMIME extension. A sender SMTP may request that a binary MIME message be sent without transport encoding by sending a BINARYMIME parameter with the MAIL command. When the receiver SMTP accepts a MAIL FROM command with the BINARYMIME body type requested, it agrees to preserve all bits in each octet passed using the BDAT command. BINARYMIME cannot be used with the DATA command. If a DATA command is issued after a MAIL command containing the body-value of "BINARYMIME", a 501 response should be sent. The resulting state from this error condition is indeterminate and the transaction should be reset with the RSET command. It is important to note that when using BINARYMIME, it is especially important to ensure that the MIME message itself is properly formed. In particular, it is essential that text be canonically encoded with each line properly terminated with . Any transformation of text into non-canonical MIME to observe local storage conventions must be reversed before sending as BINARYMIME. Some line-oriented shortcuts will break if used with BINARYMIME. The syntax of the extended MAIL command is identical to the MAIL command in [RFC821], except that a BODY parameter must appear after the address. The complete syntax of this extended command is defined in [ESMTP]. The ESMTP-keyword is BODY and the syntax for ESMTP-value is given by the syntax for body-value in [ESMTP]. Vaudreuil Expires 5/1/00 [Page 5] Internet Draft Binary ESMTP December 1, 1999 If a receiver-SMTP does not indicate support the BINARYMIME message format then the client SMTP must not, under any circumstances, send binary data using the BDAT command. If the receiver-SMTP does not support BINARYMIME and the message content is a MIME object with a binary encoding, a client SMTP has two options with which to forward the message. First, it may implement a gateway transformation to convert the message into valid 7bit-encoded MIME. Second, it may treat this as a permanent error and handle it in the usual manner for delivery failures. The specifics of the transformation from Binary MIME to 7bit MIME are not described by this RFC; the conversion is nevertheless constrained in the following ways: 1. The conversion must cause no loss of information; MIME transport encodings must be employed as needed to insure this is the case. 2. The resulting message must be valid 7bit MIME. In particular, the transformation may not result in nested Base-64 or Quoted-Printable content-transfer-encodings. As of present there are no mechanisms for converting a binary MIME object into an 8-bit MIME object. Such a transformation will require the specification of a new MIME content-transfer-encoding, the standardization of which is discouraged by [MIME]. Vaudreuil Expires 5/1/00 [Page 6] Internet Draft Binary ESMTP December 1, 1999 4. Examples 4.1 Simple Chunking The following simple dialogue illustrates the use of the large message extension to send a short psudo-RFC822 message to one recipient using the CHUNKING extension: R: S: R: 220 cnri.reston.va.us SMTP service ready S: EHLO ymir.claremont.edu R: 250-cnri.reston.va.us says hello R: 250 CHUNKING S: MAIL FROM: R: 250 Sender ok S: RCPT TO: R: 250 Recipient ok S: BDAT 86 LAST S: To: Susan@random.com S: From: Sam@random.com S: Subject: This is a bodyless test message R: 250 Message OK, 86 octets received S: QUIT R: 221 Goodbye Vaudreuil Expires 5/1/00 [Page 7] Internet Draft Binary ESMTP December 1, 1999 4.2 Pipelining Binarymime The following dialogue illustrates the use of the large message extension to send a BINARYMIME object to two recipients using the CHUNKING and PIPELINING extensions: R: R: 220 cnri.reston.va.us SMTP service ready S: EHLO ymir.claremont.edu R: 250-cnri.reston.va.us says hello R: 250-PIPELINING R: 250-BINARYMIME R: 250 CHUNKING S: MAIL FROM: BODY=BINARYMIME S: RCPT TO: S: RCPT TO: R: 250 ... Sender and BINARYMIME ok R: 250 ... Recipient ok R: 250 ... Recipient ok S: BDAT 100000 S: (First 10000 octets of canonical MIME message data) S: BDAT 324 LAST S: (Remaining 324 octets of canonical MIME message data) R: 250 100000 bytes received R: 250 Message OK, 100324 octets received S: QUIT R: 221 Goodbye Vaudreuil Expires 5/1/00 [Page 8] Internet Draft Binary ESMTP December 1, 1999 5. Security Considerations This extension is not known to present any security issues already endemic in electronic mail and present in fully conforming implementations of [RFC821], or otherwise made possible by [MIME]. 6. Acknowledgments This protocol is the result of numerous discussions in the IETF SMTP Extensions Working Group and in particular due to the continued advocacy of "chunking" by Neil Katin. 7. References [BINARY] Vaudreuil, G, " SMTP Service Extensions for Transmission of Large and Binary MIME Messages", RFC 1830, August 1995. [RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821, USC/Information Sciences Institute, August 1982. [RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text Messages", STD 11, RFC 822, UDEL, August 1982. [MIME] Borenstein, N., and N. Freed, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, Bellcore, Innosoft, November 1996. [ESMTP] Klensin, J., WG Chair, Freed, N., Editor, Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extensions" RFC 1869, United Nations University, Innosoft International, Inc., Dover Beach Consulting, Inc., Network Management Associates, Inc., The Branch Office, November 1995. [8BIT] Klensin, J., WG Chair, Freed, N., Editor, Rose, M., Stefferud, E., and D. Crocker, "SMTP Service Extension for 8bit-MIMEtransport" RFC 1652, United Nations University, Innosoft International, Inc., Dover Beach Consulting, Inc., Network Management Associates, Inc., The Branch Office, July 1994. [PIPE] Freed, N., "SMTP Service Extensions for Command Pipelining", RFC 1854, Innosoft International, October 1995. 8. Copyright Notice "Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Vaudreuil Expires 5/1/00 [Page 9] Internet Draft Binary ESMTP December 1, 1999 Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." 9. Author's Address Gregory M. Vaudreuil Lucent Technologies Communications Application Group 17080 Dallas Parkway Dallas, TX 75248-1905 Voice/Fax: +1-972-733-2722 GregV@Lucent.com Vaudreuil Expires 5/1/00 [Page 10] Internet Draft Binary ESMTP December 1, 1999 10. Appendix A - Changes from RFC1830 Numerous editorial changes including required intellectual property boilerplate and revised authors contact information Corrected the simple chunking example to use the correct number of bytes. Vaudreuil Expires 5/1/00 [Page 11]