1. RFC 4249
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).


   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

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           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.

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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
   [N2.RFC2822].  Authors specifying extension header fields should use
   the same terms in the same manner in order to provide clarity and
   avoid confusion.  For example, a "header" [I1.FYI18], [N2.RFC2822] is
   comprised of "header fields", each of which has a "field name" and
   usually has a "field body".  Each message may have multiple
   "headers", viz. a message header and MIME-part [N4.RFC2046] headers.

   A message header has a Date header field (i.e., a field with field
   name "Date").  However, there is no "Date header"; use of such non-
   standard terms is likely to lead to confusion, possibly resulting in
   interoperability failures of implementations.

3.1.2.  Naming Rules and Conventions

   Several rules and conventions have been established for naming of
   header fields.  Rules are codified in published RFCs; conventions
   reflect common use.  Naming Rules

   Some RFCs define a particular prefix, reserving use of that prefix
   for specific purposes.  Content- prefix rule

   This prefix must be used for all MIME extension fields and must not
   be used for fields that are not MIME extension fields [N3.RFC2045]
   (section 9).  Resent- prefix rule

   Specified for certain standard fields as given in [N1.STD11] (also
   used by [N2.RFC2822], although not specified as a prefix therein).
   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.

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RFC 4249             Specification of Header Fields         January 2006  Naming Conventions

   Some prefixes have developed as conventions.  Although not formally
   specified as reserved prefixes, these conventions are or have been in
   use in multiple fields with common semantics for each prefix.  Accept- prefix convention

   This prefix should be used for all extension fields intended for use
   in content negotiation [I2.RFC2616] and should not be used for fields
   that are not intended for such use.  An example may be found in
   [I3.RFC3282].  List- prefix convention

   Used to indicate information about mailing lists when a list
   expansion takes place.  Examples of defined fields can be found in
   [I4.RFC2369] and [I5.RFC2919].  Illegal- prefix convention

   This prefix provides a record of illegal content in a field when
   fields are transformed at a gateway [I6.RFC886].  Disposition-Notification- prefix convention

   Specification of information used in conjunction with Message
   Disposition Notifications (MDNs) [I7.RFC3798].  Original- prefix convention

   Used to reference some content from a related message.  Examples
   include Original-Message-ID as used by [I8.RFC3297] and [I7.RFC3798],
   Original-Encoded-Information-Types [I9.RFC2156], Original-Envelope-ID
   [I10.RFC3464], and Original-Recipient [I7.RFC3798].  Reporting- prefix

   Specifies a host that generated a type of report, such as those
   defined in [I7.RFC3798], [I10.RFC3464].  X400- prefix convention

   Used in conversion from X.400 environments by gateways [I9.RFC2156].  Discarded-X400- prefix convention

   Also used by gateways from X.400 [I9.RFC2156].

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RFC 4249             Specification of Header Fields         January 2006  P1- prefix convention

   Was used by X.400 gateways [I11.RFC987].  Delivery-Report-Content- prefix convention

   Also used by legacy X.400 gateways [I11.RFC987].

3.2.  Common Specification Items

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

3.2.1.  ABNF

   [N1.STD11] is vague about where whitespace is permitted or required
   in header field syntax.  [N2.RFC2822] addresses that issue by
   defining grammar productions such as FWS and CFWS, in conjunction
   with formal ABNF [N7.RFC4234] and in accordance with the necessity
   for specificity of such issues as noted in section 3.1 of
   [N7.RFC4234].  Extension field ABNF should clearly specify where
   comments, line folding, and whitespace are prohibited and permitted,
   and should use the [N2.RFC2822] grammar productions in ABNF for that

   All ABNF must be carefully checked for ambiguities and to ensure that
   all productions resolve to some combination of terminal productions
   provided by a normative reference [N8.CKLIST] ("All ABNF must be
   checked").  [N7.RFC4234] 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 [I12.Errata].

   Authors and designers should be careful not to mix syntax with
   disparate semantics within a single field.  Examples of disparate
   semantics are [N2.RFC2822] comments (which use parentheses as
   delimiters), [I13.RFC2533] feature sets (which also use parentheses
   as delimiters, but not for comments), and [I14.RFC3986] Uniform
   Resource Identifiers (URIs), which permit parentheses in URI text.

   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 [I15.RFC1958] (section
   4.3).  Keywords are typically registered by IANA; a specification
   using registered keywords must include an IANA Considerations section
   [N9.BCP26], [I16.RFC3692], and should indicate to readers of the
   specification precisely where IANA has set up the registry (authors

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   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).

   If keywords are defined, and if there is any chance that the set of
   keywords might be expanded, a registry should be established via
   IANA.  If a registry is not established initially, there is a good
   chance that initially-defined keywords will not be registered or will
   subsequently be registered with different semantics (this has

   Provision may be made for experimental or private-use keywords.
   These typically begin with a case-insensitive "x-" prefix.  Note that
   [N10.BCP82] has specific considerations for use of experimental

   If some field content is to be considered human-readable text, there
   must be provision for specifying language in accordance with
   [N11.BCP18] (section 4.2).  Header fields typically use the mechanism
   specified in [I17.RFC2047] as amended by [I18.RFC2231] and
   [I12.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 [N11.BCP18] (section 6).

   Some field bodies may include ABNF representing numerical values.
   Such ABNF, its comments, and supporting normative text should clearly
   indicate whether such a numerical value is decimal, octal,
   hexadecimal, etc.; whether or not leading and/or trailing zeroes are
   significant and/or permitted; and how any combinations of numeric
   fields are intended to be interpreted.  For example, two numeric
   fields separated by a dot, exemplified by "001.100", "1.1", "1.075",
   and "1.75", might be interpreted in several ways, depending on
   factors such as those enumerated above.

   While ABNF [N7.RFC4234] is used by [N2.RFC2822] and is mentioned
   above, alternate formal syntax formats may be used in specifications

3.2.2.  Minimum and Maximum Instances of Fields 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.  [N2.RFC2822] specifies

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   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.

3.2.3.  Categorization

   [N2.RFC2822] 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.

3.3.  Semantics

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

3.3.1.  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 [I20.Arch].
   Other fields comprise trace information that 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.

3.3.2.  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.

3.3.3.  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 [N4.RFC2046] as well
   as message headers, still others might appear in specialized MIME
   media types.

3.4.  Overall Considerations

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

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3.4.1.  Security

   Every specification is supposed to include a carefully-considered
   Security Considerations section [N12.RFC2223] (section 9),

3.4.2.  Backward Compatibility

   There is a large deployed base of applications that use header
   fields.  Implementations that comprise that deployed base may change
   very slowly.  It is therefore critically important to consider and
   specify 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 or field component, might not be recognizable to
   deployed implementations.  Depending on the care with which the
   authors of an extension have considered such backward 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 backward compatible with the deployed base of

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

3.4.3.  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 that lacks that field
   or field component.  Any such semantics should be explicitly

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3.4.4.  Interaction With Established Mechanisms

   Header fields are handled specially by gateways under various
   circumstances, e.g., message fragmentation and reassembly
   [N4.RFC2046].  If special treatment is required for a header field
   under such circumstances, it should be clearly specified by the
   author of the specification.  [I7.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 backward compatible).

4.  Acknowledgements

   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.

5.  Security Considerations

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

6.  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.

7. 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 that 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.

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Appendix A.  Disclaimers

   This document has exactly one (1) author.

   In spite of the fact that the author's given name may also be the
   surname of other individuals, and the fact that the author's surname
   may also be a given name for some females, the author is, and has
   always been, male.

   The presence of "/SHE", "their", and "authors" (plural) in the
   boilerplate sections of this document is irrelevant.  The author of
   this document is not responsible for the boilerplate text.

   Comments regarding the silliness, lack of accuracy, and lack of
   precision of the boilerplate text should be directed to the IESG, not
   to the author.

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Normative References

   [N1.STD11]    Crocker, D., "Standard for the format of ARPA Internet
                 text messages", STD 11, RFC 822, August 1982.

   [N2.RFC2822]  Resnick, P., "Internet Message Format", RFC 2822, April

   [N3.RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of Internet
                 Message Bodies", RFC 2045, November 1996.

   [N4.RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part Two: Media Types", RFC
                 2046, November 1996.

   [N5.BCP9]     Bradner, S., "The Internet Standards Process --
                 Revision 3", BCP 9, RFC 2026, October 1996.

   [N6.BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
                 Procedures for Message Header Fields", BCP 90, RFC
                 3864, September 2004.

   [N7.RFC4234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
                 Specifications: ABNF", RFC 4234, October 2005.

   [N8.CKLIST]   "Checklist for Internet-Drafts (IDs) submitted for RFC
                 publication", http://www.ietf.org/ID-Checklist.html.

   [N9.BCP26]    Narten, T. and H. Alvestrand, "Guidelines for Writing
                 an IANA Considerations Section in RFCs", BCP 26, RFC
                 2434, October 1998.

   [N10.BCP82]   Narten, T., "Assigning Experimental and Testing Numbers
                 Considered Useful", BCP 82, RFC 3692, January 2004.

   [N11.BCP18]   Alvestrand, H., "IETF Policy on Character Sets and
                 Languages", BCP 18, RFC 2277, January 1998.

   [N12.RFC2223] Postel, J. and J. Reynolds, "Instructions to RFC
                 Authors", RFC 2223, October 1997.

Informative References

   [I1.FYI18]    Malkin, G., "Internet Users' Glossary", FYI 18, RFC
                 1983, August 1996.

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   [I2.RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [I3.RFC3282]  Alvestrand, H., "Content Language Headers", RFC 3282,
                 May 2002.

   [I4.RFC2369]  Neufeld, G. and J. Baer, "The Use of URLs as Meta-
                 Syntax for Core Mail List Commands and their Transport
                 through Message Header Fields", RFC 2369, July 1998.

   [I5.RFC2919]  Chandhok, R. and G. Wenger, "List-Id: A Structured
                 Field and Namespace for the Identification of Mailing
                 Lists", RFC 2919, March 2001.

   [I6.RFC886]   Rose, M., "Proposed standard for message header
                 munging", RFC 886, December 1983.

   [I7.RFC3798]  Hansen, T. and G. Vaudreuil, "Message Disposition
                 Notification", RFC 3798, May 2004.

   [I8.RFC3297]  Klyne, G., Iwazaki, R., and D. Crocker, "Content
                 Negotiation for Messaging Services based on Email", RFC
                 3297, July 2002.

   [I9.RFC2156]  Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):
                 Mapping between X.400 and RFC 822/MIME", RFC 2156,
                 January 1998.

   [I10.RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message
                 Format for Delivery Status Notifications", RFC 3464,
                 January 2003.

   [I11.RFC987]  Kille, S., "Mapping between X.400 and RFC 822", RFC
                 987, June 1986.

   [I12.Errata]  RFC-Editor errata page,

   [I13.RFC2533] Klyne, G., "A Syntax for Describing Media Feature
                 Sets", RFC 2533, March 1999.

   [I14.RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
                 "Uniform Resource Identifier (URI): Generic Syntax",
                 STD 66, RFC 3986, January 2005.

   [I15.RFC1958] Carpenter, B., "Architectural Principles of the
                 Internet", RFC 1958, June 1996.

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   [I16.RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
                 Considered Useful", BCP 82, RFC 3692, January 2004.

   [I17.RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
                 Extensions) Part Three: Message Header Extensions for
                 Non-ASCII Text", RFC 2047, November 1996.

   [I18.RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and
                 Encoded Word Extensions: Character Sets, Languages, and
                 Continuations", RFC 2231, November 1997.

   [I19.Syntax]  Carpenter, B., "Syntax for format definitions",

   [I20.Arch]    Crocker, D., "Internet Mail Architecture", Work in
                 Progress, March 2005.

   [I21.BCP72]   Rescorla, E. and B. Korver, "Guidelines for Writing RFC
                 Text on Security Considerations", BCP 72, RFC 3552,
                 July 2003.

Author's Address

   Bruce Lilly

   EMail: blilly@erols.com

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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

Intellectual Property

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   Copies of IPR disclosures made to the IETF Secretariat and any
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