Nine by Nine

GK-IETF@ninebynine.org http://www.ninebynine.net/

BEA Systems

235 Montgomery St. Level 15 San Francisco CA 94104 USA mnot@pobox.com

Western Research Laboratory, Compaq Computer Corporation

JeffMogul@acm.org 250 University Avenue Palo Alto CA 94305 US 1 650 617 3304 (email preferred)

Application message headers header fields email headers HTTP headers RFC822 MIME registration IANA This specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications. Please send comments to <ietf-822@imc.org>. To subscribe, send a message with body 'subscribe' to <ietf-822-request@imc.org>.

This specification defines registration procedures for the message header field names used by Internet mail, HTTP, newsgroup feeds and other Internet applications. It is not intended to be a replacement for protocol-specific registries, such as the SIP registry . Benefits of a central registry for message header field names include: providing a single point of reference for standardized and widely-used header field names; providing a central point of discovery for established header fields, and easy location of their defining documents; discouraging multiple definitions of a header field name for different purposes; helping those proposing new header fields discern established trends and conventions, and avoid names that might be confused with existing ones; encouraging convergence of header field name usage across multiple applications and protocols. The primary specification for Internet message header fields in email is the Internet mail message format specification, RFC 2822 . HTTP/1.0 and HTTP/1.1 define message header fields (respectively, the HTTP-header and message-header protocol elements) for use with HTTP. RFC 1036 defines message header elements for use with Netnews feeds. These specifications also define a number of header fields, and provide for extension through the use of new field-names. There are many other Internet standards track documents that define additional header fields for use within the same namespaces, notably MIME and related specifications. Other Internet applications that use MIME, such as SIP (RFC 3261 ) may also use many of the same header fields (but note that IANA maintains a separate registry of header fields used with SIP). Although in principle each application defines its own set of valid header fields, exchange of messages between applications (e.g. mail to Netnews gateways), common use of MIME encapsulation, and the possibility of common processing for various message types (e.g. a common message archive and retrieval facility) makes it desirable to have a common point of reference for standardized and proposed header fields. Listing header fields together reduces the chance of an accidental collision, and helps implementers find relevant information. The message header field registries defined here serve that purpose.

discusses the purpose of this specification, and indicates some sources of information about defined message header fields. defines the message header field name repositories, and sets out requirements and procedures for creating entries in them.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 . NOTE: indented comments like this provide additional nonessential information about the rationale behind this document. [[[Editorial comments and questions about outstanding issues are provided in triple brackets like this. These working comments should be resolved and removed prior to final publication.]]]

Many message header fields are defined in standards-track documents, which means they have been subjected to a process of community review and achieved consensus that they provide a useful and well-founded capability, or represent a widespread use of which developers should be aware. Some are defined for experimental use, typically indicating consensus regarding their purpose but not necessarily concerning their technical details. Many others have been defined and adopted ad-hoc to address a locally occurring requirement; some of these have found widespread use. The catalogues defined here are intended to cater for all of these header fields, while maintaining a clear distinction and status for those which have community consensus. To this end, two repositories are defined: A Permanent Message Header Field Registry, intended for headers defined in IETF standards-track documents, those that have achieved a comparable level of community review, or are generally recognized to be in widespread use. The assignment policy for such registration is "Specification Required", as defined by RFC 2434 , where the specification must be published in an RFC (standards-track, experimental, informational or historic), or as an "Open Standard" in the sense of RFC 2026, section 7 . A Provisional Message Header Field Repository, intended for any header field proposed by any developer, without making any claim about its usefulness or the quality of its definition. The policy for recording these is "Private Use", per RFC 2434 . Neither repository tracks the syntax, semantics or type of field-values. Only the field-names, applicable protocols and status are registered; all other details are specified in the defining documents referenced by repository entries. Significant updates to such references (e.g., the replacement of a Proposed Standard RFC by a Draft Standard RFC, but not necessarily the revision of an Internet-draft) SHOULD be accompanied by updates to the corresponding repository entries.

RFC 2822 defines a general syntax for message headers, and also defines a number of fields for use with Internet mail. HTTP/1.0 and HTTP/1.1 do likewise for HTTP. Additional field names are defined in a variety of standards-track RFC documents, including: RFC 1036 , RFC 1496 , RFC 1505 , RFC 1864 , RFC 2156 , RFC 2183 , RFC 2045 , RFC 2046 , RFC 2557 , RFC 2227 , RFC 2231 , RFC 2298 , RFC 2369 , RFC 2421 , RFC 2518 , RFC 2617 , RFC 2821 , RFC 2912 , RFC 2919 , RFC 2965 , and RFC 3282 .

Internet applications that use similar message headers include Internet mail , NNTP newsgroup feeds , HTTP web access and any other that uses MIME encapsulation of message content. In some cases (notably HTTP ), the header syntax and usage is redefined for the specific application. This registration is concerned only with the allocation and specification of field names, and not with the details of header implementation in specific protocols. In some cases, the same field name may be specified differently (by different documents) for use with different application protocols; e.g. The Date: header field used with HTTP has a different syntax than the Date: used with Internet mail. In other cases, a field name may have a common specification across multiple protocols (ignoring protocol-specific lexical and character set conventions); e.g. this is generally the case for MIME header fields with names of the form 'Content-*'. Thus, we need to accommodate application-specific fields, while wishing to recognize and promote (where appropriate) commonality of other fields across multiple applications. Common repositories are used for all applications, and each registered header field specifies the application protocol for which the corresponding definition applies. A given field name may have multiple registry entries for different protocols; in the Permanent Message Header Field registry, a given header field name may be registered only once for any given protocol. (In some cases, the registration may reference several defining documents.)

Some header fields with names of the form Content-* are associated with the MIME data object encapsulation and labelling framework. These header fields can meaningfully be applied to a data object separately from the protocol used to carry it. MIME is used with email messages and other protocols that specify a MIME-based data object format. MIME header fields used with such protocols are defined in the registry with the protocol "mime", and as such are presumed to be usable in conjunction with any protocol that conveys MIME objects. Other protocols do not convey MIME objects, but define a number of header fields with similar names and functions to MIME. Notably, HTTP defines a number of entity header fields that serve a purpose in HTTP similar to MIME header fields in email. Some of these header fields have the same names and similar functions to their MIME counterparts (though there are some variations). Such header fields must be registered separately for any non-MIME-carrying protocol with which they may be used. It is poor practice to reuse a header field name from another protocol simply because the fields have similar (even "very similar") meanings. Protocols should share header field names only when their meanings are identical in all foreseeable circumstances. In particular, new header field names of the form Content-* should not be defined for non-MIME-carrying protocols unless their specification is exactly the same as in MIME.

RFCs defining new header fields for Internet mail, HTTP, or MIME MUST include appropriate header registration templates(s) (as given in Section 3.2) for all headers defined in the document in their IANA considerations section. Use of the header registry MAY be mandated by other protocol specifications, however, in the absence of such a mandate use of the registry is not required.

The procedure for registering a message header field is: Construct a header field specification Prepare a registration template Submit the registration template

Registration of a new message header field starts with construction of a proposal that describes the syntax, semantics and intended use of the field. For entries in the Permanent Message Header Field Registry, this proposal MUST be published as an RFC, or as an Open Standard in the sense described by RFC 2026, section 7 . A registered field name SHOULD conform at least to the syntax defined by RFC 2822 , section 3.6.8. Further, the "." character is reserved to indicate a naming sub-structure and MUST NOT be included in any registered field name. Currently, no specific sub-structure is defined; if used, any such structure MUST be defined by a standards track RFC document. Header field names may sometimes be used in URIs, URNs and/or XML. To comply with the syntactic constraints of these forms, it is recommended that characters in a registered field name are restricted to those that can be used without escaping in a URI or URN , and that are also legal in XML element names. Thus, for maximum flexibility, header field names SHOULD further be restricted to just letters, digits, hyphen ('-') and underscore ('_') characters, with the first character being a letter or underscore.

The registration template for a message header field may be contained in the defining document, or prepared separately.

A header registered in the Permanent Message Header Field Registry MUST be published as an RFC or as an "Open Standard" in the sense described by RFC 2026, section 7 , and MUST have a name which is unique among all the registered permanent field names that may be used with the same application protocol. The registration template has the following form. The name requested for the new header field. This MUST conform to the header field specification details noted in . Specify "mail" (RFC 2822), "mime" (RFC2045), "http" (RFC 2616), "netnews" (RFC 1036), or cite any other standards-track RFC defining the protocol with which the header is intended to be used. Specify "standard", "experimental", "informational", "historic", "obsoleted", or some other appropriate value according to the type and status of the primary document in which it is defined. For non-IETF specifications, those formally approved by other standards bodies should be labelled as "standard"; others may be "informational" or "deprecated" depending on the reason for registration. For Internet standards-track, state "IETF". For other open standards, give the name of the publishing body (e.g. ANSI, ISO, ITU, W3C, etc.). For other specifications, give the name, email address, and organization name of the primary specification author. A postal address, home page URI, telephone and fax numbers may also be included. Reference to document that specifies the header for use with the indicated protocol, preferably including a URI that can be used to retrieve a copy of the document. An indication of the relevant sections MAY also be included, but is not required. Optionally, citations to additional documents containing further relevant information. (This part of the registry may also be used for IESG comments.) Where a primary specification refers to another document for substantial technical detail, the referenced document is usefully mentioned here.

Registration as a Provisional Message Header Field does not imply any kind of endorsement by the IETF, IANA or any other body. The main requirements for a header field to be included in the provisional repository are that it MUST have a citable specification, and there MUST NOT be a corresponding entry (with same field name and protocol) in the permanent header field registry. The specification SHOULD indicate an email address for sending technical comments and discussion of the proposed message header. The submission template has the following form. The name proposed for the new header field. This SHOULD conform to the field name specification details noted in . Specify "mail" (RFC 2822), "mime" (RFC 2045), "http" (RFC 2616), "netnews" (RFC 1036), or cite any other standards-track RFC defining the protocol with which the header is intended to be used. Specify: "provisional". This will be updated if and when the header registration is subsequently moved to the permanent registry. The name, email address, and organization name of the submission author, who may authorize changes to or retraction of the repository entry. A postal address, home page URI, telephone and fax numbers may also be included. If the proposal comes from a standards body working group, give the name and home page URI of the working group, and an email address for discussion of or comments on the specification. Reference to document that specifies the header for use with the indicated protocol. The document MUST be an RFC, a current Internet-draft or the URL of a publicly accessible document (so IANA can verify availability of the specification). An indication of the relevant sections MAY also be included, but is not required. NOTE: if the specification is available in printed form only, then an Internet draft containing full reference to the paper document should be published and cited in the registration template. The paper specification MAY be cited under related information. Optionally, citations to additional documents containing further relevant information.

The registration template is submitted for incorporation in one of the IANA message header field repositories by one of the following methods: An IANA considerations section in a defining RFC, calling for registration of the message header and referencing information as required by the registration template within the same document. Registration of the header is then processed as part of the RFC publication process. Send a copy of the template to the designated email discussion list . Allow a reasonable period - at least 2 weeks - for discussion and comments, then send the template to IANA at the designated email address . IANA will publish the template information if the requested name and the specification document meet the criteria noted in and , unless the IESG or their designated expert have requested that it not be published (see ). IESG's designated expert should confirm to IANA that the registration criteria have been satisfied. When a new entry is recorded in the permanent message header field registry, IANA will remove any corresponding entries (with the same field name and protocol) from the provisional registry.

Listing of an entry in the provisional repository should not be lightly refused. An entry MAY be refused if there is some credible reason to believe that such registration will be harmful. In the absence of such objection, IANA SHOULD allow any registration that meets the criteria set out in and . Some reasonable grounds for refusal might be: There is IETF consensus that publication is considered likely to harm the Internet technical infrastructure in some way. Disreputable or frivolous use of the registration facilities. The proposal is sufficiently lacking in purpose, or misleading about its purpose, that it can be held to be a waste of time and effort. Conflict with some current IETF activity. Note that objections or disagreements about technical detail are not, of themselves, considered grounds to refuse listing in the provisional repository. After all, one of its purposes is to allow developers to communicate with a view to combining their ideas, expertise and energy to the maximum benefit of the Internet community. Publication in an RFC or other form of Open Standard document (per RFC 2026 , section 7) is sufficient grounds for publication in the permanent registry. To assist IANA in determining whether or not there is a sustainable objection to any registration, IESG nominates a designated expert to liaise with IANA about new registrations. For the most part, the designated expert's role is to confirm to IANA that the registration criteria have been satisfied. The IESG or their designated expert MAY require any change or commentary to be attached to any registry entry. The IESG is the final arbiter of any objection.

Change control of a header field registration is subject to the same condition as the initial registration; i.e. publication (or reclassification) of an Open Standards specification for a Permanent Message Header Field, or on request of the indicated author/change controller for a Provisional Message Header (like the original submission, subject to review on the designated email discussion list .) A change to a permanent message header field registration MAY be requested by the IESG. A change to or retraction of any Provisional Message Header Field Repository entry MAY be requested by the IESG or designated expert. IANA MAY remove any Provisional Message Header Field Repository entry whose corresponding specification document is no longer available (e.g. expired Internet-draft, or URL not resolvable). Anyone may notify IANA of any such cases by sending an email to the designated email address . removing an entry for this reason, IANA SHOULD contact the registered Author/Change controller to determine whether a replacement for the specification document (consistent with the requirements of section ) is available. It is intended that entries in the Permanent Message Header Field Registry may be used in the construction of URNs (per RFC 2141 ) which have particular requirements for uniqueness and persistence (per RFC 1737 ). Therefore, once an entry is made in the Permanent Message Header Registry, the combination of the header name and applicable protocol MUST NOT subsequently be registered for any other purpose. (This is not to preclude revision of the applicable specification(s) within the appropriate IETF Consensus rules, and corresponding updates to the specification citation in the header registration.)

Comments on proposed registrations should be sent to the designated email discussion list .

The message header field registry is accessible from IANA's web site http://[[[IANA registry URL here]]].

This specification calls for: A new IANA registry for permanent message header fields per of this document. The policy for inclusion in this registry is described in and . A new IANA repository listing provisional message header fields per of this document. The policy for inclusion in this registry is described in and . IESG appoints a designated expert to advise IANA whether registration criteria for proposed registrations have been satisfied. [[[During the publication process for this document, there are IANA-controlled email addresses and URLs to be allocated. All such references are indicated in this memo by '[[[(some description)]]]'. All such text (including this) should be replaced or removed in the publication process.]]] No initial registry entries are provided.

No security considerations are introduced by this specification beyond those already inherent in the use of message headers.

The shape of the registries described here owes much to energetic discussion of previous versions by many denizens of the IETF-822 mailing list. The authors also gratefully acknowledge the contribution of those who provided valuable feedback on earlier versions of this memo: Charles Lindsey, Dave Crocker, Pete Resnick, Jacob Palme, Ned Freed, Michelle Cotton.

Stockholm University/KTH

[[[Please remove this section on final publication]]]

Fix up references to avoid private bibliography includes.

Changes in response to feedback from IESG: Added notes clarifying that this header field registry does not supplant the SIP header field registry. Updated references in cases where cited RFCs have been obsoleted (these were all informational references, being part of the survey of protocols and specifications that define header fields). Final changes requested by IESG, for RFC publication.

Fixed typo. Response to last-call comments: Updated author details. Added section on MIME header fields, noting that MIME is treated as a separate protocol for registration purposes. Separated normative references from informational references. (Many protocols mentioned as motivation for an aspect of this specification are cited as informational, on the basis that their specification is not needed for validation of a proposed registry entry.) Added "mime" as an option under applicable protocols. Moved URL of IANA registry from references section to body of document. Added requirement that IESG's designated expert should confirm to IANA that registration criteria have been satisfied. Noted appointment of IESG-designated expert in IANA considerations section. Added temporary note to IANA considerations section that there are IANA-controlled email addresses and URLs yet to be allocated. Widened options for status of a header field. Revised some references.

Updated contact details. Updated reference to mail header field registration document.

Added note about referencing paper-only specification documents. Change 'news' -> 'netnews'. Experimental standards don't cite IETF as change controller. Added note that provisional registry entries SHOULD indicate a discussion list. Clarify that a single registry may reference several specification documents. Add "deprecated" to list of possible status values for permanent registry. Conflict with IETF work noted as possible objection to listing in the provisional registry. Several small editorial changes. Simplify the text describing recommended character limitations for header field names. Re-arrange the text dealing with status for permanent registry entries. More small editorial changes.

Re-organized repositories into Permanent and Provisional, names chosen with a view to reducing any spurious impression of legitimacy. All entries have a status indicator. For provisional entries, this clearly indicates the non-settled nature of the entry. Make provision for prior email notification to interested parties of submissions not published in an RFC. Make limited provision for refusal or removal of harmful submissions. IESG may appoint a designated expert to assist IANA with any assessments needed. Make provision for IANA to remove provisional entries whose specification is no longer available. Proposed mailing list for announcement/discussion of new registrations, and another for notifying IANA. Removed any reference to header names beginning with 'X-', on the basis that this is a protocol-specific convention. Updated terminology to use "header field" more consistently, rather than just "header". Revised permanent and provisional submission templates to have the same form and structure. Editorial fixes. Extend range of specifications acceptable for permanent registry entries to include "Open standards" (per RFC 2026). Trim references to other informational lists of header information. Focused the requirement for permanent registration on persistent publication by an open standards body. Noted in provisional submission template that a proposal may come from a working group. Allowed for fuller contact information under change controller. Improved wording of submission process. Indicate that there should be IETF consensus that publication would harm the technical infrastructure, for publication of a provisional entry to be blocked. Adjust registration criteria and procedures so that corresponding entries cannot appear simultaneously in the permanent and provisional listings.

Merged with HTTP header registry proposal. Move initial registrations to separate documents. Editorial revisions.

In response to feedback from interested parties, expanded the registry to cover Normative and Provisional message header registrations. Defined a formal role for the applicable protocol(s) in the registry: the combination of header name and any applicable protocol must be unique for a Normative Message Header. Noted further constraints to the header name format for XML name compatibility. Fixed registration policy for a Normative Message Header to be "IETF Consensus".

Document initially created.

[[[Please remove this section on final publication]]] Finalize references to initial registrations. Finalize email address for submission of registration templates. Finalize web address for registry.