SMTP Service Extension for 8-bit MIME Transport
draft-ietf-yam-rfc1652bis-03

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.

Status of this Memo

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Copyright Notice

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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 [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.)[RFC2046] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types,” November 1996.)[RFC5322] (Resnick, P., Ed., “Internet Message Format,” October 2008.) containing arbitrary octet-aligned material. This memo uses the mechanism described in [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.) 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 [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.)) or a MIME message (in strict compliance with [RFC2046] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types,” November 1996.)[RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.)) with arbitrary octet content is being sent. The syntax of the value is as follows, using the ABNF notation of [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.):

   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 [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.), 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 [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.). 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 [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.) ("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 preceding 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 can allow as few as 1000 octets, inclusive of the CR-LF pair, on a single line). This restriction means that this extension provides 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 [RFC5321] (Klensin, J., “Simple Mail Transfer Protocol,” October 2008.).

6.  IANA Considerations

6.1.  SMTP service extension registration

This document defines an SMTP and Submit service extension, and IANA is asked to update the 8BITMIME entry in the SMTP Service Extensions registry, as follows:

Keyword:

8BITMIME

Description:

SMTP and Submit transport of 8bit MIME content

Reference:

[[IANA: Insert this RFC number.]]

Parameters:

See Section 2 (Framework for the 8bit MIME Transport Extension) in this specification.

6.2.  Acknowledgements

E. Stefferud was an original author. This version of the specification was produced by the YAM working group.

Original 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. Normative References

Authors' Addresses