GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc4249

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
 [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.

3.1.2.1. Naming Rules

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

3.1.2.1.1. 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).

3.1.2.1.2. 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.

Lilly Informational [Page 3] RFC 4249 Specification of Header Fields January 2006

3.1.2.2. 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.

3.1.2.2.1. 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].

3.1.2.2.2. 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].

3.1.2.2.3. Illegal- prefix convention

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

3.1.2.2.4. Disposition-Notification- prefix convention

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

3.1.2.2.5. 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].

3.1.2.2.6. Reporting- prefix

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

3.1.2.2.7. X400- prefix convention

 Used in conversion from X.400 environments by gateways [I9.RFC2156].

3.1.2.2.8. Discarded-X400- prefix convention

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

Lilly Informational [Page 4] RFC 4249 Specification of Header Fields January 2006

3.1.2.2.9. P1- prefix convention

 Was used by X.400 gateways [I11.RFC987].

3.1.2.2.10. 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
 purpose.
 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

Lilly Informational [Page 5] RFC 4249 Specification of Header Fields January 2006

 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
 happened!).
 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
 keywords.
 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
 [I19.Syntax].

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

Lilly Informational [Page 6] RFC 4249 Specification of Header Fields January 2006

 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.

Lilly Informational [Page 7] RFC 4249 Specification of Header Fields January 2006

3.4.1. Security

 Every specification is supposed to include a carefully-considered
 Security Considerations section [N12.RFC2223] (section 9),
 [I21.BCP72].

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
    implementations.
 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
 specified.

Lilly Informational [Page 8] RFC 4249 Specification of Header Fields January 2006

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.

Lilly Informational [Page 9] RFC 4249 Specification of Header Fields January 2006

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.

Lilly Informational [Page 10] RFC 4249 Specification of Header Fields January 2006

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
               2001.
 [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.

Lilly Informational [Page 11] RFC 4249 Specification of Header Fields January 2006

 [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,
               http://www.rfc-editor.org/errata.html
 [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.

Lilly Informational [Page 12] RFC 4249 Specification of Header Fields January 2006

 [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",
               http://ietf.org/IESG/STATEMENTS/syntax-format-def.txt
 [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

Lilly Informational [Page 13] RFC 4249 Specification of Header Fields January 2006

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
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

 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
 might or might not be available; nor does it represent that it has
 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 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
 assurances of licenses to be made available, or the result of an
 attempt made to obtain a general license or permission for the use of
 such proprietary rights by implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 http://www.ietf.org/ipr.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at
 ietf-ipr@ietf.org.

Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Lilly Informational [Page 14]

/data/webs/external/dokuwiki/data/pages/rfc/rfc4249.txt · Last modified: 2006/01/16 18:31 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki