Implementer-Friendly Specification of Message and MIME-Part Header Fields and Field Components
RFC 4249

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Network Working Group                                           B. Lilly
Request for Comments: 4249                                  January 2006
Category: Informational

   Implementer-Friendly Specification of Message and MIME-Part Header
                      Fields and Field Components

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Implementation of generators and parsers of header fields requires
   certain information about those fields.  Interoperability is most
   likely when all such information is explicitly provided by the
   technical specification of the fields.  Lacking such explicit
   information, implementers may guess, and interoperability may suffer.
   This memo identifies information useful to implementers of header
   field generators and parsers.

Table of Contents

   1. Introduction ....................................................2
   2. Scope ...........................................................2
   3. Specification Items .............................................3
      3.1. Established Conventions ....................................3
           3.1.1. Standard Terminology ................................3
           3.1.2. Naming Rules and Conventions ........................3
      3.2. Common Specification Items .................................5
           3.2.1. ABNF ................................................5
           3.2.2. Minimum and Maximum Instances of Fields per Header ..6
           3.2.3. Categorization ......................................7
      3.3. Semantics ..................................................7
           3.3.1. Producers, Modifiers, and Consumers .................7
           3.3.2. What's it all about? ................................7
           3.3.3. Context .............................................7
      3.4. Overall Considerations .....................................7
           3.4.1. Security ............................................8
           3.4.2. Backward Compatibility ..............................8
           3.4.3. Compatibility With Legacy Content ...................8

Lilly                        Informational                      [Page 1]
RFC 4249             Specification of Header Fields         January 2006

           3.4.4. Interaction With Established Mechanisms .............9
   4. Acknowledgements ................................................9
   5. Security Considerations .........................................9
   6. Internationalization Considerations .............................9
   7. IANA Considerations .............................................9
   Appendix A. Disclaimers ...........................................10
   Normative References ..............................................11
   Informative References ............................................11

1.  Introduction

   Internet messages consist of a message header and a body [N1.STD11],
   [N2.RFC2822].  MIME content begins with a MIME-part header
   [N3.RFC2045], [N4.RFC2046].  Message headers and MIME-part headers
   consist of fields.  While the Message Format and MIME specifications
   define their respective overall formats and some specific fields,
   they also have provision for extension fields.  A number of extension
   fields have been specified, some more or less completely than others.
   Incomplete or imprecise specification has led to interoperability
   problems as implementers make assumptions in the absence of
   specifications.  This memo identifies items of potential interest to
   implementers, and section 3 of this memo may serve as an
   informational guide for authors of specifications of extension fields
   and field components.

2.  Scope

   This memo is intended as a non-binding informational supplement to
   various specifications, guidelines, and procedures for specification
   of header fields [N1.STD11], [N2.RFC2822], [N3.RFC2045],
   [N4.RFC2046], [N5.BCP9], [N6.BCP90].  It does not absolve authors of
   header field specifications from compliance with any provisions of
   those or other specifications, guidelines, and procedures.  It offers
   clarification and supplementary suggestions that will promote
   interoperability and may spare specification authors many questions
   regarding incomplete header field specifications.

Lilly                        Informational                      [Page 2]
RFC 4249             Specification of Header Fields         January 2006

3.  Specification Items

3.1.  Established Conventions

   A number of conventions exist for naming and specifying header
   fields.  It would be unwise and confusing to specify a field that
   conflicts with those conventions.

3.1.1.  Standard Terminology

   Terms related to the Internet Message Format are defined in