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Versions: 00 01 02 03 04 RFC 4249

Network Working Group                                           B. Lilly
Internet Draft                                              January 2005
Expires: July 14, 2005

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

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667 [BCP78].  By submitting this Internet-Draft,
   the author represents that any applicable patent or other IPR claims
   of which he is aware have been or will be disclosed, and any of which
   he become aware will be disclosed, in accordance with RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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Copyright Notice

   Copyright(C)The Internet Society (2005).


   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.

1.  Introduction

   Internet messages consist of a message header and a body [STD11],
   [RFC2822].  MIME content begins with a MIME-part header [RFC2045],
   [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

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   implementers make assumptions in the absence of specifications.  This
   memo identifies items of potential interest to implementers and
   section 4 of this memo may serve as a checklist for specifications of
   extension fields and field components.

2.  Requirement Levels

   The key words "SHOULD", and "SHOULD NOT", in this document are to be
   interpreted as described in [BCP14].

3.  Scope

   This memo is intended to supplement various specifications,
   guidelines, and procedures for specification of header fields
   [STD11], [BCP9], [RFC2045], [RFC2046], [RFC2822], [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

4.  Specification Items

4.1.  Established Conventions

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

4.1.1.  Naming Conventions

   Several conventions have been established for naming of header
   fields.  Resent- prefix

   Field names beginning with "Resent-" have particular semantics as
   given in [STD11] and [RFC2822].  If a Resent- version of a field is
   applicable, an author SHOULD say so explicitly, and SHOULD provide a
   comprehensive specification of any differences between the plain
   field and the Resent- version.  Content- prefix

   Field names beginning with "Content-" are MIME extension fields
   [RFC2045].  This prefix SHOULD be used for all MIME extension fields
   and SHOULD NOT be used for fields which are not MIME extension

4.2.  Common Specification Items

   Several items are specified for standard header fields; these items
   should also be specified for extension fields.

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4.2.1.  ABNF

   [STD11] is vague about where whitespace is permitted or required in
   header field syntax.  [RFC2822] addresses that issue by defining
   grammar productions such as FWS and CFWS.  Extension field ABNF
   SHOULD clearly specify where comments, line folding, and whitespace
   are prohibited and permitted, and SHOULD use the RFC 2822 grammar
   productions in ABNF for that purpose.

   All ABNF should be carefully checked for ambiguities and to ensure
   that all productions resolve to some combination of terminal
   productions provided by a normative reference.  [RFC2234] provides
   several productions that may be useful.  While use of suitable
   productions defined and in use is encouraged, specification authors
   are cautioned that some such productions have been amended by
   subsequently issued RFCs and/or by formal errata [Errata].

   It is sometimes necessary or desirable to define keywords as protocol
   elements in structured fields.  Protocol elements SHOULD be
   case-insensitive per the Internet Architecture [RFC1958].  Keywords
   are typically registered by IANA; a specification using registered
   keywords SHOULD include an IANA considerations section, and SHOULD
   indicate to readers of the specification precisely where IANA has set
   up the registry (authors will need to coordinate this with IANA prior
   to publication as an RFC).  In many cases, it will be desirable to
   make provision for extending the set of keywords; that may be done by
   specifying that the set may be extended by publication of an RFC, or
   a formal review and registration procedure may be specified
   (typically as a BCP RFC).

   Provision may be made for experimental or private-use keywords.
   These typically begin with a case-insensitive "x-" prefix.  Note that
   [BCP82] has specific considerations for use of experimental keywords.

   If some field content is to be considered human-readable text, there
   should be provision for specifying language in accordance with
   [BCP18].  Header fields typically use the mechanism specified in
   [RFC2047] as amended by [RFC2231] and [Errata] for that purpose.
   Note, however, that that mechanism applies only to three specific
   cases; unstructured fields, an RFC 822 "word" in an RFC 822 "phrase",
   and comments in structured fields.  Any internationalization
   considerations should be detailed in an Internationalization
   Considerations section of the specification as specified in [BCP18].

4.2.2.  Minimum and Maximum Instances per Header

   Some fields are mandatory, others are optional.  It may make sense to
   permit multiple instances of a field in a given header; in other
   cases at most a single instance is sensible.  RFC 2822 specifies a
   minimum and maximum count per header for each standard field in a
   message; specification authors SHOULD likewise specify minimum and
   maximum counts for extension fields.

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4.2.3.  Categorization

   RFC 2822 defines categories of header fields (e.g. trace fields,
   address fields).  Such categories have implications for processing
   and handling of fields.  A specification author SHOULD indicate any
   applicable categories.

4.3.  Semantics

   In addition to specifying syntax of a field, a specification document
   should indicate the semantics of each field.  Such semantics are
   comprised of several aspects:

4.3.1.  Producers, Modifiers, and Consumers

   Some fields are intended for end-to-end communication between author
   or sender and recipient; such fields should not be generated or
   altered by intermediaries in the transmission chain [Crocker04].
   Other fields comprise trace information which is added during
   transport.  Authors SHOULD clearly specify who may generate a field,
   who may modify it in transit, who should interpret such a field, and
   who is prohibited from interpreting or modifying the field.

4.3.2.  What's it All About?

   When introducing a new field or modifying an existing field, an
   author should present a clear description of what problem or
   situation is being addressed by the extension or change.

4.3.3.  Context

   The permitted types of headers in which the field may appear should
   be specified.  Some fields might only be appropriate in a message
   header, some might appear in MIME-part headers as well as message
   headers, still others might appear in specialized MIME media types.

4.4.  Overall Considerations

   Several factors should be considered regarding how a field interacts
   with the Internet at large, with the applications for which it is
   intended, and in interacting with other applications.

4.4.1.  Security

   Every specification is supposed to include a carefully-considered
   Security Considerations section [RFC2223].

4.4.2.  Backwards Compatibility

   There is a large deployed base of applications which use header
   fields.  Implementations that comprise that deployed base may change
   very slowly.  It is therefore critically important to consider the
   impact of a new or revised field or field component on that deployed
   base.  A new field, or extensions to the syntax of an existing field

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   or field component, might not be recognizable to deployed
   implementations.  Depending on the care with which the authors of an
   extension have considered such backwards compatibility, such an
   extension might, for example:

   a. Cause a deployed implementation to simply ignore the field in its
      entirety.  That is not a problem provided that it is a new field
      and that there is no assumption that such deployed implementations
      will do otherwise.

   b. Cause a deployed implementation to behave differently from how it
      would behave in the absence of the proposed change, in ways that
      are not intended by the proposal.  That is a failure of the
      proposal to remain backwards compatible with the deployed base of

   There are many subtleties and variations that may come into play.
   Authors SHOULD very carefully consider backwards compatibility when
   devising extensions, and SHOULD clearly describe all known
   compatibility issues.

4.4.3.  Compatibility With Legacy Content

   Content is sometimes archived for various reasons.  It is sometimes
   necessary or desirable to access archived content, with the semantics
   of that archived content unchanged.  It is therefore important that
   lack of presence of an extension field or field component should not
   be construed (by an extension specification) as conferring new
   semantics on a message or piece of MIME content which lacks that
   field or field component.

4.4.4.  Interaction With Established Mechanisms

   Header fields are handled specially by gateways under various
   circumstances.  Message fragmentation and reassembly [RFC2046] is one
   example.  If special treatment is required for a header field under
   such circumstances, it SHOULD be clearly specified by the author of
   the specification.  [RFC3798] is an example of how this might be
   handled (however, because that specification requires deployed RFC
   2046-conforming implementations to be modified, it is not strictly
   backwards compatible).

5.  Acknowledgments

   The author would like to acknowledge the helpful comments provided by
   members of the ietf-822 mailing list.  In particular, Peter Koch and
   Keith Moore have made useful comments.

6.  Security Considerations

   No new security considerations are addressed by this memo.  The memo
   reinforces the need for careful consideration of security issues.

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7.  Internationalization Considerations

   This memo does not directly have internationalization considerations,
   however it reminds specification authors of the need to consider
   internationalization of textual field components.

8.  IANA Considerations

   While no specific action is required of IANA in regard to this memo,
   it does note that some coordination between IANA and specification
   authors who do require IANA to set up registries is at least
   desirable, if not a necessity.  IANA should also closely coordinate
   with the RFC Editor so that registries are set up and properly
   referenced at the time of publication of an RFC which refers to such
   a registry.  IANA is also encouraged to work closely with authors and
   the RFC Editor to ensure that descriptions of registries maintained
   by IANA are accurate and meaningful.

Normative References

   [BCP9]      Bradner, S., "The Internet Standards Process -- Revision
               3", BCP 9, RFC 2026, October 1996.

   [BCP14]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [BCP18]     Alvestrand, H., "IETF Policy on Character Sets and
               Languages", BCP 18, RFC 2277, January 1998.

   [BCP82]     Narten, T., "Assigning Experimental and Testing Numbers
               Considered Useful", BCP 82, RFC 3692, January 2004.

   [BCP90]     Klyne, G., Nottingham, M., and J. Mogul, "Registration
               Procedures for Message Header Fields", BCP 90, RFC 3864,
               September 2004.

   [Errata]    RFC-Editor errata page,

   [RFC2045]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part One: Format of Internet Message
               Bodies", RFC 2045, November 1996.

   [RFC2046]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
               Extensions (MIME) Part Two: Media Types", RFC 2046,
               November 1996.

   [RFC2223]   Postel, J. and J. Reynolds, "Instructions to RFC
               Authors", RFC 2223, October 1997.

   [RFC2234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 2234, November 1997.

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   [RFC2822]   Resnick, P., "Internet Message Format", RFC 2822, April

   [STD11]     Crocker, D., "Standard for the format of ARPA Internet
               text messages", STD 11, RFC 822, August 1982.

Informative References

   [BCP78]     Bradner, S., "IETF Rights in Contributions", BCP 78,
               RFC 3667, February 2004.

   [BCP79]     Bradner, S., "Intellectual Property Rights in IETF
               Technology", BCP 79, RFC 3668, February 2004.

   [Crocker04] Crocker, D., "Internet Mail Architecture", Work in

   [RFC1958]   Carpenter, B., "Architectural Principles of the
               Internet", RFC 1958, June 1996.

   [RFC3798]   Hansen, T. and G. Vaudreuil, "Message Disposition
               Notification", RFC 3798, May 2004.

Author's Address

   Bruce Lilly

   Email: blilly@erols.com

Full Copyright Statement

   Copyright(C)The Internet Society (2005).  This document is subject to
   the rights, licenses and restrictions contained in BCP 78 [BCP78],
   and except as set forth therein, the author retains all his rights.

   This document and the information contained herein are provided on an

Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
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   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 [BCP78] and BCP 79 [BCP79].

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