8BITMIME: SMTP Service Extension for 8bit-MIMEtransport

Network Working Group                               J. Klensin, WG Chair
Request for Comments: 1652                                           MCI
Obsoletes: 1426                                         N. Freed, Editor
Category: Standards Track                                       Innosoft
                                                                 M. Rose
                                            Dover Beach Consulting, Inc.
                                                            E. Stefferud
                                     Network Management Associates, Inc.
                                                              D. Crocker
                                                  Silicon Graphics, Inc.
                                                               July 1994


             SMTP Service Extension for 8bit-MIMEtransport

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.

Abstract

   This memo defines an extension to the SMTP service whereby an SMTP
   content body consisting of text containing octets outside of the US-
   ASCII octet range (hex 00-7F) may be relayed using SMTP.

1.  Introduction

   Although SMTP is widely and robustly deployed, various extensions
   have been requested by parts of the Internet community. In
   particular, a significant portion of the Internet community wishes to
   exchange messages in which the content body consists of a MIME
   message [3] containing arbitrary octet-aligned material. This memo
   uses the mechanism described in [5] to define an extension to the
   SMTP service whereby such contents may be exchanged. Note that this
   extension does NOT eliminate the possibility of an SMTP server
   limiting line length; servers are free to implement this extension
   but nevertheless set a line length limit no lower than 1000 octets.
   Given that this restriction still applies, this extension does NOT
   provide a means for transferring unencoded binary via SMTP.

2.  Framework for the 8bit MIME Transport Extension

   The 8bit MIME transport extension is laid out as follows:

      (1)  the name of the SMTP service extension defined here is
           8bit-MIMEtransport;

      (2)  the EHLO keyword value associated with the extension is
           8BITMIME;

      (3)  no parameter is used with the 8BITMIME EHLO keyword;

      (4)  one optional parameter using the keyword BODY is added to
           the MAIL FROM command.  The value associated with this
           parameter is a keyword indicating whether a 7bit message
           (in strict compliance with [1]) or a MIME message (in
           strict compliance with [3]) with arbitrary octet content
           is being sent. The syntax of the value is as follows,
           using the ABNF notation of [2]:

                body-value ::= "7BIT" / "8BITMIME"

      (5)  no additional SMTP verbs are defined by this extension;
           and,

      (6)  the next section specifies how support for the extension
           affects the behavior of a server and client SMTP.

3.  The 8bit-MIMEtransport service extension

   When a client SMTP wishes to submit (using the MAIL command) a
   content body consisting of a MIME message containing arbitrary lines
   of octet-aligned material, 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 8BITMIME,
   then the server SMTP is indicating that it supports the extended MAIL
   command and will accept MIME messages containing arbitrary octet-
   aligned material.

   The extended MAIL command is issued by a client SMTP when it wishes
   to transmit a content body consisting of a MIME message containing
   arbitrary lines of octet-aligned material. The syntax for this
   command is identical to the MAIL command in [1], except that a BODY
   parameter must appear after the address.  Only one BODY parameter may
   be used in a single MAIL command.

   The complete syntax of this extended command is defined in [5]. The
   esmtp-keyword is BODY and the syntax for esmtp-value is given by the
   syntax for body-value shown above.

   The value associated with the BODY parameter indicates whether the
   content body which will be passed using the DATA command consists of
   a MIME message containing some arbitrary octet-aligned material
   ("8BITMIME") or is encoded entirely in accordance with [1] ("7BIT").

   A server which supports the 8-bit MIME transport service extension
   shall preserve all bits in each octet passed using the DATA command.

   Naturally, the usual SMTP data-stuffing algorithm applies so that a
   content which contains the five-character sequence of

     <CR> <LF> <DOT> <CR> <LF>

   or a content that begins with the three-character sequence of

     <DOT> <CR> <LF>

   does not prematurely terminate the transfer of the content.  Further,
   it should be noted that the CR-LF pair immediately preceeding the
   final dot is considered part of the content.  Finally, although the
   content body contains arbitrary lines of octet-aligned material, the
   length of each line (number of octets between two CR-LF pairs), is
   still subject to SMTP server line length restrictions (which may
   allow as few as 1000 octets on a single line). This restriction means
   that this extension MAY provide the necessary facilities for
   transferring a MIME object with the 8BIT content-transfer-encoding,
   it DOES NOT provide a means of transferring an object with the BINARY
   content-transfer-encoding.

   Once a server SMTP supporting the 8bit-MIMEtransport service
   extension accepts a content body containing octets with the high-
   order (8th) bit set, the server SMTP must deliver or relay the
   content in such a way as to preserve all bits in each octet.

   If a server SMTP does not support the 8-bit MIME transport extension
   (either by not responding with code 250 to the EHLO command, or by
   not including the EHLO keyword value 8BITMIME in its response), then
   the client SMTP must not, under any circumstances, attempt to
   transfer a content which contains characters outside the US-ASCII
   octet range (hex 00-7F).

   A client SMTP has two options in this case: first, it may implement a
   gateway transformation to convert the message into valid 7bit MIME,
   or second, or may treat this as a permanent error and handle it in
   the usual manner for delivery failures.  The specifics of the
   transformation from 8bit MIME to 7bit MIME are not described by this
   RFC; the conversion is nevertheless constrained in the following
   ways:

      (1)  it must cause no loss of information; MIME transport
           encodings must be employed as needed to insure this is
           the case, and

      (2)  the resulting message must be valid 7bit MIME.

4.  Usage Example

   The following dialogue illustrates the use of the 8bit-MIMEtransport
   service extension:

   S: <wait for connection on TCP port 25>
   C: <open connection to server>
   S: 220 dbc.mtview.ca.us SMTP service ready
   C: EHLO ymir.claremont.edu
   S: 250-dbc.mtview.ca.us says hello
   S: 250 8BITMIME
   C: MAIL FROM:<ned@ymir.claremont.edu> BODY=8BITMIME
   S: 250 <ned@ymir.claremont.edu>... Sender and 8BITMIME ok
   C: RCPT TO:<mrose@dbc.mtview.ca.us>
   S: 250 <mrose@dbc.mtview.ca.us>... Recipient ok
   C: DATA
   S: 354 Send 8BITMIME message, ending in CRLF.CRLF.
    ...
   C: .
   S: 250 OK
   C: QUIT
   S: 250 Goodbye

5.  Security Considerations

   This RFC does not discuss security issues and is not believed to
   raise any security issues not already endemic in electronic mail and
   present in fully conforming implementations of [1].

6.  Acknowledgements

   This document represents a synthesis of the ideas of many people and
   reactions to the ideas and proposals of others.  Randall Atkinson,
   Craig Everhart, Risto Kankkunen, and Greg Vaudreuil contributed ideas
   and text sufficient to be considered co-authors.  Other important
   suggestions, text, or encouragement came from Harald Alvestrand, Jim
   Conklin, Mark Crispin, Frank da Cruz, 'Olafur Gudmundsson, Per
   Hedeland, Christian Huitma, Neil Katin, Eliot Lear, Harold A.
   Miller, Keith Moore, Dan Oscarsson, Julian Onions, Neil Rickert, John
   Wagner, Rayan Zachariassen, and the contributions of the entire IETF
   SMTP Working Group. Of course, none of the individuals are
   necessarily responsible for the combination of ideas represented
   here. Indeed, in some cases, the response to a particular criticism
   was to accept the problem identification but to include an entirely
   different solution from the one originally proposed.

7.  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.

   [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, RFC
       974, BBN, January 1986.

8.  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


   Marshall T. Rose
   Dover Beach Consulting, Inc.
   420 Whisman Court
   Moutain View, CA  94043-2186
   USA

   Phone: +1 415 968 1052
   Fax: +1 415 968 2510
   EMail: mrose@dbc.mtview.ca.us


   Einar A. Stefferud
   Network Management Associates, Inc.
   17301 Drey Lane
   Huntington Beach, CA, 92647-5615
   USA

   Phone: +1 714 842 3711
   Fax: +1 714 848 2091
   EMail: stef@nma.com


   Dave Crocker
   Silicon Graphics, Inc.
   2011 N. Shoreline Blvd.
   P.O. Box 7311
   Mountain View, CA 94039
   USA

   Phone: +1 415 390 1804
   Fax: +1 415 962 8404
   EMail: dcrocker@sgi.com