SMTP and MIME Extensions for Content Conversion
RFC 4141

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Network Working Group                                          K. Toyoda
Request for Comments: 4141                                           PCC
Category: Standards Track                                     D. Crocker
                                                             Brandenburg
                                                           November 2005

            SMTP and MIME Extensions for Content Conversion

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.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   A message originator sometimes sends content in a form the recipient
   cannot process or would prefer not to process a form of lower quality
   than is preferred.  Such content needs to be converted to an
   acceptable form, with the same information or constrained information
   (e.g., changing from color to black and white).  In a store-and-
   forward environment, it may be convenient to have this conversion
   performed by an intermediary.  This specification integrates two
   ESMTP extensions and three MIME content header fields, which defines
   a cooperative service that permits authorized, accountable content
   form conversion by intermediaries.

Toyoda & Crocker            Standards Track                     [Page 1]
RFC 4141     SMTP & MIME Extensions for Content Conversion November 2005

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1. Background. . . . . . . . . . . . . . . . . . . . . . . .  3
       1.2. Overview. . . . . . . . . . . . . . . . . . . . . . . . .  3
       1.3. Notational Conventions. . . . . . . . . . . . . . . . . .  5
   2.  Applicability. . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Service Specification. . . . . . . . . . . . . . . . . . . . .  5
       3.1. Sending Permission. . . . . . . . . . . . . . . . . . . .  9
       3.2. Returning Capabilities. . . . . . . . . . . . . . . . . . 10
       3.3. Next-Hop Non-Support of Service . . . . . . . . . . . . . 12
   4.  Content Conversion Permission SMTP Extension . . . . . . . . . 12
       4.1. Content Conversion Permission Service Extension
            Definition. . . . . . . . . . . . . . . . . . . . . . . . 12
       4.2. CONPERM Parameter to Mail-From. . . . . . . . . . . . . . 13
       4.3. Syntax. . . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Content Negotiation SMTP Extension . . . . . . . . . . . . . . 13
       5.1. Content Negotiation Service Extension Definition. . . . . 13
       5.2. CONNEG Parameter to RCPT-TO . . . . . . . . . . . . . . . 14
       5.3. Syntax. . . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  MIME Content-Features Header Field . . . . . . . . . . . . . . 16
   7.  MIME Content-Convert Header Field. . . . . . . . . . . . . . . 16
   8.  MIME Content-Previous Header Field . . . . . . . . . . . . . . 16
   9.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
       9.1. CONPERM Negotiation . . . . . . . . . . . . . . . . . . . 17
       9.2. Example CONNEG Negotiation. . . . . . . . . . . . . . . . 18
       9.3. Content-Previous. . . . . . . . . . . . . . . . . . . . . 19
   10. Security Considerations. . . . . . . . . . . . . . . . . . . . 19
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   Appendix A. CONNEG with Direct SMTP. . . . . . . . . . . . . . . . 22
   Appendix B. USING Combinations of the Extensions . . . . . . . . . 23
   Appendix C. MIME Content-Type Registrations. . . . . . . . . . . . 24

1.  Introduction

   Internet specifications typically define common capabilities for a
   particular service that are supported by all participants.  This
   permits the sending of basic data without knowing which additional
   capabilities individual recipients support.  However, knowing those
   capabilities permits the sending of additional types of data and data
   of enhanced richness.  Otherwise, a message originator will send
   content in a form the recipient cannot process or will send multiple
   forms of data.  This specification extends the work of [CONMSG],
   which permits a recipient to solicit alternative content forms from
   the originator.  The current specification enables MIME content
   conversion by intermediaries, on behalf of a message originator and a
   message recipient.

Toyoda & Crocker            Standards Track                     [Page 2]
RFC 4141     SMTP & MIME Extensions for Content Conversion November 2005

1.1.  Background

   MIME enables the distinguishing and labeling of different types of