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RFC 2048 - Multipurpose Internet Mail Extensions (MIME) Part Fou


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Network Working Group                                           N. Freed
Request for Comments: 2048                                      Innosoft
BCP: 13                                                       J. Klensin
Obsoletes: 1521, 1522, 1590                                          MCI
Category: Best Current Practice                                J. Postel
                                                                     ISI
                                                           November 1996

                 Multipurpose Internet Mail Extensions
                           (MIME) Part Four:
                        Registration Procedures

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Abstract

   STD 11, RFC 822, defines a message representation protocol specifying
   considerable detail about US-ASCII message headers, and leaves the
   message content, or message body, as flat US-ASCII text.  This set of
   documents, collectively called the Multipurpose Internet Mail
   Extensions, or MIME, redefines the format of messages to allow for

    (1)   textual message bodies in character sets other than
          US-ASCII,

    (2)   an extensible set of different formats for non-textual
          message bodies,

    (3)   multi-part message bodies, and

    (4)   textual header information in character sets other than
          US-ASCII.

   These documents are based on earlier work documented in RFC 934, STD
   11, and RFC 1049, but extends and revises them.  Because RFC 822 said
   so little about message bodies, these documents are largely
   orthogonal to (rather than a revision of) RFC 822.

   This fourth document, RFC 2048, specifies various IANA registration
   procedures for the following MIME facilities:

    (1)   media types,

    (2)   external body access types,

    (3)   content-transfer-encodings.

   Registration of character sets for use in MIME is covered elsewhere
   and is no longer addressed by this document.

   These documents are revisions of RFCs 1521 and 1522, which themselves
   were revisions of RFCs 1341 and 1342.  An appendix in RFC 2049
   describes differences and changes from previous versions.

Table of Contents

   1. Introduction .........................................    3
   2. Media Type Registration ..............................    4
   2.1 Registration Trees and Subtype Names ................    4
   2.1.1 IETF Tree .........................................    4
   2.1.2 Vendor Tree .......................................    4
   2.1.3 Personal or Vanity Tree ...........................    5
   2.1.4 Special `x.' Tree .................................    5
   2.1.5 Additional Registration Trees .....................    6
   2.2 Registration Requirements ...........................    6
   2.2.1 Functionality Requirement .........................    6
   2.2.2 Naming Requirements ...............................    6
   2.2.3 Parameter Requirements ............................    7
   2.2.4 Canonicalization and Format Requirements ..........    7
   2.2.5 Interchange Recommendations .......................    8
   2.2.6 Security Requirements .............................    8
   2.2.7 Usage and Implementation Non-requirements .........    9
   2.2.8 Publication Requirements ..........................   10
   2.2.9 Additional Information ............................   10
   2.3 Registration Procedure ..............................   11
   2.3.1 Present the Media Type to the Community for  Review   11
   2.3.2 IESG Approval .....................................   12
   2.3.3 IANA Registration .................................   12
   2.4 Comments on Media Type Registrations ................   12
   2.5 Location of Registered Media Type List ..............   12
   2.6 IANA Procedures for Registering Media Types .........   12
   2.7 Change Control ......................................   13
   2.8 Registration Template ...............................   14
   3. External Body Access Types ...........................   14
   3.1 Registration Requirements ...........................   15
   3.1.1 Naming Requirements ...............................   15

   3.1.2 Mechanism Specification Requirements ..............   15
   3.1.3 Publication Requirements ..........................   15
   3.1.4 Security Requirements .............................   15
   3.2 Registration Procedure ..............................   15
   3.2.1 Present the Access Type to the Community ..........   16
   3.2.2 Access Type Reviewer ..............................   16
   3.2.3 IANA Registration .................................   16
   3.3 Location of Registered Access Type List .............   16
   3.4 IANA Procedures for Registering Access Types ........   16
   4. Transfer Encodings ...................................   17
   4.1 Transfer Encoding Requirements ......................   17
   4.1.1 Naming Requirements ...............................   17
   4.1.2 Algorithm Specification Requirements ..............   18
   4.1.3 Input Domain Requirements .........................   18
   4.1.4 Output Range Requirements .........................   18
   4.1.5 Data Integrity and Generality Requirements ........   18
   4.1.6 New Functionality Requirements ....................   18
   4.2 Transfer Encoding Definition Procedure ..............   19
   4.3 IANA Procedures for Transfer Encoding Registration...   19
   4.4 Location of Registered Transfer Encodings List ......   19
   5. Authors' Addresses ...................................   20
   A. Grandfathered Media Types ............................   21

1.  Introduction

   Recent Internet protocols have been carefully designed to be easily
   extensible in certain areas.  In particular, MIME [RFC 2045] is an
   open-ended framework and can accommodate additional object types,
   character sets, and access methods without any changes to the basic
   protocol.  A registration process is needed, however, to ensure that
   the set of such values is developed in an orderly, well-specified,
   and public manner.

   This document defines registration procedures which use the Internet
   Assigned Numbers Authority (IANA) as a central registry for such
   values.

   Historical Note: The registration process for media types was
   initially defined in the context of the asynchronous Internet mail
   environment.  In this mail environment there is a need to limit the
   number of possible media types to increase the likelihood of
   interoperability when the capabilities of the remote mail system are
   not known.  As media types are used in new environments, where the
   proliferation of media types is not a hindrance to interoperability,
   the original procedure was excessively restrictive and had to be
   generalized.

2.  Media Type Registration

   Registration of a new media type or types starts with the
   construction of a registration proposal.  Registration may occur in
   several different registration trees, which have different
   requirements as discussed below.  In general, the new registration
   proposal is circulated and reviewed in a fashion appropriate to the
   tree involved.  The media type is then registered if the proposal is
   acceptable.  The following sections describe the requirements and
   procedures used for each of the different registration trees.

2.1.  Registration Trees and Subtype Names

   In order to increase the efficiency and flexibility of the
   registration process, different structures of subtype names may be
   registered to accomodate the different natural requirements for,
   e.g., a subtype that will be recommended for wide support and
   implementation by the Internet Community or a subtype that is used to
   move files associated with proprietary software.  The following
   subsections define registration "trees", distinguished by the use of
   faceted names (e.g., names of the form "tree.subtree...type").  Note
   that some media types defined prior to this document do not conform
   to the naming conventions described below.  See Appendix A for a
   discussion of them.

2.1.1.  IETF Tree

   The IETF tree is intended for types of general interest to the
   Internet Community. Registration in the IETF tree requires approval
   by the IESG and publication of the media type registration as some
   form of RFC.

   Media types in the IETF tree are normally denoted by names that are
   not explicitly faceted, i.e., do not contain period (".", full stop)
   characters.

   The "owner" of a media type registration in the IETF tree is assumed
   to be the IETF itself.  Modification or alteration of the
   specification requires the same level of processing (e.g.  standards
   track) required for the initial registration.

2.1.2.  Vendor Tree

   The vendor tree is used for media types associated with commercially
   available products.  "Vendor" or "producer" are construed as
   equivalent and very broadly in this context.

   A registration may be placed in the vendor tree by anyone who has
   need to interchange files associated with the particular product.
   However, the registration formally belongs to the vendor or
   organization producing the software or file format.  Changes to the
   specification will be made at their request, as discussed in
   subsequent sections.

   Registrations in the vendor tree will be distinguished by the leading
   facet "vnd.".  That may be followed, at the discretion of the
   registration, by either a media type name from a well-known producer
   (e.g., "vnd.mudpie") or by an IANA-approved designation of the
   producer's name which is then followed by a media type or product
   designation (e.g., vnd.bigcompany.funnypictures).

   While public exposure and review of media types to be registered in
   the vendor tree is not required, using the ietf-types list for review
   is strongly encouraged to improve the quality of those
   specifications. Registrations in the vendor tree may be submitted
   directly to the IANA.

2.1.3.  Personal or Vanity Tree

   Registrations for media types created experimentally or as part of
   products that are not distributed commercially may be registered in
   the personal or vanity tree.  The registrations are distinguished by
   the leading facet "prs.".

   The owner of "personal" registrations and associated specifications
   is the person or entity making the registration, or one to whom
   responsibility has been transferred as described below.

   While public exposure and review of media types to be registered in
   the personal tree is not required, using the ietf-types list for
   review is strongly encouraged to improve the quality of those
   specifications.  Registrations in the personl tree may be submitted
   directly to the IANA.

2.1.4.  Special `x.' Tree

   For convenience and symmetry with this registration scheme, media
   type names with "x." as the first facet may be used for the same
   purposes for which names starting in "x-" are normally used.  These
   types are unregistered, experimental, and should be used only with
   the active agreement of the parties exchanging them.

   However, with the simplified registration procedures described above
   for vendor and personal trees, it should rarely, if ever, be
   necessary to use unregistered experimental types, and as such use of
   both "x-" and "x." forms is discouraged.

2.1.5.  Additional Registration Trees

   From time to time and as required by the community, the IANA may,
   with the advice and consent of the IESG, create new top-level
   registration trees.  It is explicitly assumed that these trees may be
   created for external registration and management by well-known
   permanent bodies, such as scientific societies for media types
   specific to the sciences they cover.  In general, the quality of
   review of specifications for one of these additional registration
   trees is expected to be equivalent to that which IETF would give to
   registrations in its own tree. Establishment of these new trees will
   be announced through RFC publication approved by the IESG.

2.2.  Registration Requirements

   Media type registration proposals are all expected to conform to
   various requirements laid out in the following sections.  Note that
   requirement specifics sometimes vary depending on the registration
   tree, again as detailed in the following sections.

2.2.1.  Functionality Requirement

   Media types must function as an actual media format: Registration of
   things that are better thought of as a transfer encoding, as a
   character set, or as a collection of separate entities of another
   type, is not allowed.  For example, although applications exist to
   decode the base64 transfer encoding [RFC 2045], base64 cannot be
   registered as a media type.

   This requirement applies regardless of the registration tree
   involved.

2.2.2.  Naming Requirements

   All registered media types must be assigned MIME type and subtype
   names. The combination of these names then serves to uniquely
   identify the media type and the format of the subtype name identifies
   the registration tree.

   The choice of top-level type name must take the nature of media type
   involved into account. For example, media normally used for
   representing still images should be a subtype of the image content
   type, whereas media capable of representing audio information belongs

   under the audio content type. See RFC 2046 for additional information
   on the basic set of top-level types and their characteristics.

   New subtypes of top-level types must conform to the restrictions of
   the top-level type, if any. For example, all subtypes of the
   multipart content type must use the same encapsulation syntax.

   In some cases a new media type may not "fit" under any currently
   defined top-level content type. Such cases are expected to be quite
   rare. However, if such a case arises a new top-level type can be
   defined to accommodate it. Such a definition must be done via
   standards-track RFC; no other mechanism can be used to define
   additional top-level content types.

   These requirements apply regardless of the registration tree
   involved.

2.2.3.  Parameter Requirements

   Media types may elect to use one or more MIME content type
   parameters, or some parameters may be automatically made available to
   the media type by virtue of being a subtype of a content type that
   defines a set of parameters applicable to any of its subtypes.  In
   either case, the names, values, and meanings of any parameters must
   be fully specified when a media type is registered in the IETF tree,
   and should be specified as completely as possible when media types
   are registered in the vendor or personal trees.

   New parameters must not be defined as a way to introduce new
   functionality in types registered in the IETF tree, although new
   parameters may be added to convey additional information that does
   not otherwise change existing functionality.  An example of this
   would be a "revision" parameter to indicate a revision level of an
   external specification such as JPEG.  Similar behavior is encouraged
   for media types registered in the vendor or personal trees but is not
   required.

2.2.4.  Canonicalization and Format Requirements

   All registered media types must employ a single, canonical data
   format, regardless of registration tree.

   A precise and openly available specification of the format of each
   media type is required for all types registered in the IETF tree and
   must at a minimum be referenced by, if it isn't actually included in,
   the media type registration proposal itself.

   The specifications of format and processing particulars may or may
   not be publically available for media types registered in the vendor
   tree, and such registration proposals are explicitly permitted to
   include only a specification of which software and version produce or
   process such media types.  References to or inclusion of format
   specifications in registration proposals is encouraged but not
   required.

   Format specifications are still required for registration in the
   personal tree, but may be either published as RFCs or otherwise
   deposited with IANA. The deposited specifications will meet the same
   criteria as those required to register a well-known TCP port and, in
   particular, need not be made public.

   Some media types involve the use of patented technology.  The
   registration of media types involving patented technology is
   specifically permitted.  However, the restrictions set forth in RFC
   1602 on the use of patented technology in standards-track protocols
   must be respected when the specification of a media type is part of a
   standards-track protocol.

2.2.5.  Interchange Recommendations

   Media types should, whenever possible, interoperate across as many
   systems and applications as possible. However, some media types will
   inevitably have problems interoperating across different platforms.
   Problems with different versions, byte ordering, and specifics of
   gateway handling can and will arise.

   Universal interoperability of media types is not required, but known
   interoperability issues should be identified whenever possible.
   Publication of a media type does not require an exhaustive review of
   interoperability, and the interoperability considerations section is
   subject to continuing evaluation.

   These recommendations apply regardless of the registration tree
   involved.

2.2.6.  Security Requirements

   An analysis of security issues is required for for all types
   registered in the IETF Tree.  (This is in accordance with the basic
   requirements for all IETF protocols.) A similar analysis for media
   types registered in the vendor or personal trees is encouraged but
   not required.  However, regardless of what security analysis has or
   has not been done, all descriptions of security issues must be as
   accurate as possible regardless of registration tree.  In particular,
   a statement that there are "no security issues associated with this

   type" must not be confused with "the security issues associates with
   this type have not been assessed".

   There is absolutely no requirement that media types registered in any
   tree be secure or completely free from risks.  Nevertheless, all
   known security risks must be identified in the registration of a
   media type, again regardless of registration tree.

   The security considerations section of all registrations is subject
   to continuing evaluation and modification, and in particular may be
   extended by use of the "comments on media types" mechanism described
   in subsequent sections.

   Some of the issues that should be looked at in a security analysis of
   a media type are:

    (1)   Complex media types may include provisions for
          directives that institute actions on a recipient's
          files or other resources.  In many cases provision is
          made for originators to specify arbitrary actions in an
          unrestricted fashion which may then have devastating
          effects.  See the registration of the
          application/postscript media type in RFC 2046 for
          an example of such directives and how to handle them.

    (2)   Complex media types may include provisions for
          directives that institute actions which, while not
          directly harmful to the recipient, may result in
          disclosure of information that either facilitates a
          subsequent attack or else violates a recipient's
          privacy in some way.  Again, the registration of the
          application/postscript media type illustrates how such
          directives can be handled.

    (3)   A media type might be targeted for applications that
          require some sort of security assurance but not provide
          the necessary security mechanisms themselves. For
          example, a media type could be defined for storage of
          confidential medical information which in turn requires
          an external confidentiality service.

2.2.7.  Usage and Implementation Non-requirements

   In the asynchronous mail environment, where information on the
   capabilities of the remote mail agent is frequently not available to
   the sender, maximum interoperability is attained by restricting the
   number of media types used to those "common" formats expected to be
   widely implemented.  This was asserted in the past as a reason to

   limit the number of possible media types and resulted in a
   registration process with a significant hurdle and delay for those
   registering media types.

   However, the need for "common" media types does not require limiting
   the registration of new media types. If a limited set of media types
   is recommended for a particular application, that should be asserted
   by a separate applicability statement specific for the application
   and/or environment.

   As such, universal support and implementation of a media type is NOT
   a requirement for registration.  If, however, a media type is
   explicitly intended for limited use, this should be noted in its
   registration.

2.2.8.  Publication Requirements

   Proposals for media types registered in the IETF tree must be
   published as RFCs. RFC publication of vendor and personal media type
   proposals is encouraged but not required. In all cases IANA will
   retain copies of all media type proposals and "publish" them as part
   of the media types registration tree itself.

   Other than in the IETF tree, the registration of a data type does not
   imply endorsement, approval, or recommendation by IANA or IETF or
   even certification that the specification is adequate.  To become
   Internet Standards, protocol, data objects, or whatever must go
   through the IETF standards process.  This is too difficult and too
   lengthy a process for the convenient registration of media types.

   The IETF tree exists for media types that do require require a
   substantive review and approval process with the vendor and personal
   trees exist for those that do not. It is expected that applicability
   statements for particular applications will be published from time to
   time that recommend implementation of, and support for, media types
   that have proven particularly useful in those contexts.

   As discussed above, registration of a top-level type requires
   standards-track processing and, hence, RFC publication.

2.2.9.  Additional Information

   Various sorts of optional information may be included in the
   specification of a media type if it is available:

    (1)   Magic number(s) (length, octet values). Magic numbers
          are byte sequences that are always present and thus can
          be used to identify entities as being of a given media

          type.

    (2)   File extension(s) commonly used on one or more
          platforms to indicate that some file containing a given
          type of media.

    (3)   Macintosh File Type code(s) (4 octets) used to label
          files containing a given type of media.

   Such information is often quite useful to implementors and if
   available should be provided.

2.3.  Registration Procedure

   The following procedure has been implemented by the IANA for review
   and approval of new media types.  This is not a formal standards
   process, but rather an administrative procedure intended to allow
   community comment and sanity checking without excessive time delay.
   For registration in the IETF tree, the normal IETF processes should
   be followed, treating posting of an internet-draft and announcement
   on the ietf-types list (as described in the next subsection) as a
   first step.  For registrations in the vendor or personal tree, the
   initial review step described below may be omitted and the type
   registered directly by submitting the template and an explanation
   directly to IANA (at iana@iana.org).  However, authors of vendor or
   personal media type specifications are encouraged to seek community
   review and comment whenever that is feasible.

2.3.1.  Present the Media Type to the Community for Review

   Send a proposed media type registration to the "ietf-types@iana.org"
   mailing list for a two week review period.  This mailing list has
   been established for the purpose of reviewing proposed media and
   access types. Proposed media types are not formally registered and
   must not be used; the "x-" prefix specified in RFC 2045 can be used
   until registration is complete.

   The intent of the public posting is to solicit comments and feedback
   on the choice of type/subtype name, the unambiguity of the references
   with respect to versions and external profiling information, and a
   review of any interoperability or security considerations. The
   submitter may submit a revised registration, or withdraw the
   registration completely, at any time.

2.3.2.  IESG Approval

   Media types registered in the IETF tree must be submitted to the IESG
   for approval.

2.3.3.  IANA Registration

   Provided that the media type meets the requirements for media types
   and has obtained approval that is necessary, the author may submit
   the registration request to the IANA, which will register the media
   type and make the media type registration available to the community.

2.4.  Comments on Media Type Registrations

   Comments on registered media types may be submitted by members of the
   community to IANA.  These comments will be passed on to the "owner"
   of the media type if possible.  Submitters of comments may request
   that their comment be attached to the media type registration itself,
   and if IANA approves of this the comment will be made accessible in
   conjunction with the type registration itself.

2.5.  Location of Registered Media Type List

   Media type registrations will be posted in the anonymous FTP
   directory "ftp://ftp.isi.edu/in-notes/iana/assignments/media-types/"
   and all registered media types will be listed in the periodically
   issued "Assigned Numbers" RFC [currently STD 2, RFC 1700].  The media
   type description and other supporting material may also be published
   as an Informational RFC by sending it to "rfc-editor@isi.edu" (please
   follow the instructions to RFC authors [RFC-1543]).

2.6.  IANA Procedures for Registering Media Types

   The IANA will only register media types in the IETF tree in response
   to a communication from the IESG stating that a given registration
   has been approved. Vendor and personal types will be registered by
   the IANA automatically and without any formal review as long as the
   following minimal conditions are met:

    (1)   Media types must function as an actual media format.
          In particular, character sets and transfer encodings
          may not be registered as media types.

    (2)   All media types must have properly formed type and
          subtype names. All type names must be defined by a
          standards-track RFC. All subtype names must be unique,
          must conform to the MIME grammar for such names, and
          must contain the proper tree prefix.

    (3)   Types registered in the personal tree must either
          provide a format specification or a pointer to one.

    (4)   Any security considerations given must not be obviously
          bogus. (It is neither possible nor necessary for the
          IANA to conduct a comprehensive security review of
          media type registrations.  Nevertheless, IANA has the
          authority to identify obviously incompetent material
          and exclude it.)

2.7.  Change Control

   Once a media type has been published by IANA, the author may request
   a change to its definition. The descriptions of the different
   registration trees above designate the "owners" of each type of
   registration. The change request follows the same procedure as the
   registration request:

    (1)   Publish the revised template on the ietf-types list.

    (2)   Leave at least two weeks for comments.

    (3)   Publish using IANA after formal review if required.

   Changes should be requested only when there are serious omission or
   errors in the published specification. When review is required, a
   change request may be denied if it renders entities that were valid
   under the previous definition invalid under the new definition.

   The owner of a content type may pass responsibility for the content
   type to another person or agency by informing IANA and the ietf-types
   list; this can be done without discussion or review.

   The IESG may reassign responsibility for a media type. The most
   common case of this will be to enable changes to be made to types
   where the author of the registration has died, moved out of contact
   or is otherwise unable to make changes that are important to the
   community.

   Media type registrations may not be deleted; media types which are no
   longer believed appropriate for use can be declared OBSOLETE by a
   change to their "intended use" field; such media types will be
   clearly marked in the lists published by IANA.

2.8.  Registration Template

     To: ietf-types@iana.org
     Subject: Registration of MIME media type XXX/YYY

     MIME media type name:

     MIME subtype name:

     Required parameters:

     Optional parameters:

     Encoding considerations:

     Security considerations:

     Interoperability considerations:

     Published specification:

     Applications which use this media type:

     Additional information:

       Magic number(s):
       File extension(s):
       Macintosh File Type Code(s):

     Person & email address to contact for further information:

     Intended usage:

     (One of COMMON, LIMITED USE or OBSOLETE)

     Author/Change controller:

     (Any other information that the author deems interesting may be
     added below this line.)

3.  External Body Access Types

   RFC 2046 defines the message/external-body media type, whereby a MIME
   entity can act as pointer to the actual body data in lieu of
   including the data directly in the entity body. Each
   message/external-body reference specifies an access type, which
   determines the mechanism used to retrieve the actual body data. RFC
   2046 defines an initial set of access types, but allows for the

   registration of additional access types to accommodate new retrieval
   mechanisms.

3.1.  Registration Requirements

   New access type specifications must conform to a number of
   requirements as described below.

3.1.1.  Naming Requirements

   Each access type must have a unique name.  This name appears in the
   access-type parameter in the message/external-body content-type
   header field, and must conform to MIME content type parameter syntax.

3.1.2.  Mechanism Specification Requirements

   All of the protocols, transports, and procedures used by a given
   access type must be described, either in the specification of the
   access type itself or in some other publicly available specification,
   in sufficient detail for the access type to be implemented by any
   competent implementor.  Use of secret and/or proprietary methods in
   access types are expressly prohibited. The restrictions imposed by
   RFC 1602 on the standardization of patented algorithms must be
   respected as well.

3.1.3.  Publication Requirements

   All access types must be described by an RFC. The RFC may be
   informational rather than standards-track, although standard-track
   review and approval are encouraged for all access types.

3.1.4.  Security Requirements

   Any known security issues that arise from the use of the access type
   must be completely and fully described. It is not required that the
   access type be secure or that it be free from risks, but that the
   known risks be identified.  Publication of a new access type does not
   require an exhaustive security review, and the security
   considerations section is subject to continuing evaluation.
   Additional security considerations should be addressed by publishing
   revised versions of the access type specification.

3.2.  Registration Procedure

   Registration of a new access type starts with the construction of a
   draft of an RFC.

3.2.1.  Present the Access Type to the Community

   Send a proposed access type specification to the "ietf-
   types@iana.org" mailing list for a two week review period.  This
   mailing list has been established for the purpose of reviewing
   proposed access and media types.  Proposed access types are not
   formally registered and must not be used.

   The intent of the public posting is to solicit comments and feedback
   on the access type specification and a review of any security
   considerations.

3.2.2.  Access Type Reviewer

   When the two week period has passed, the access type reviewer, who is
   appointed by the IETF Applications Area Director, either forwards the
   request to iana@isi.edu, or rejects it because of significant
   objections raised on the list.

   Decisions made by the reviewer must be posted to the ietf-types
   mailing list within 14 days. Decisions made by the reviewer may be
   appealed to the IESG.

3.2.3.  IANA Registration

   Provided that the access type has either passed review or has been
   successfully appealed to the IESG, the IANA will register the access
   type and make the registration available to the community. The
   specification of the access type must also be published as an RFC.
   Informational RFCs are published by sending them to "rfc-
   editor@isi.edu" (please follow the instructions to RFC authors [RFC-
   1543]).

3.3.  Location of Registered Access Type List

   Access type registrations will be posted in the anonymous FTP
   directory "ftp://ftp.isi.edu/in-notes/iana/assignments/access-types/"
   and all registered access types will be listed in the periodically
   issued "Assigned Numbers" RFC [currently RFC-1700].

3.4.  IANA Procedures for Registering Access Types

   The identity of the access type reviewer is communicated to the IANA
   by the IESG.  The IANA then only acts in response to access type
   definitions that either are approved by the access type reviewer and
   forwarded by the reviewer to the IANA for registration, or in
   response to a communication from the IESG that an access type
   definition appeal has overturned the access type reviewer's ruling.

4.  Transfer Encodings

   Transfer encodings are tranformations applied to MIME media types
   after conversion to the media type's canonical form.  Transfer
   encodings are used for several purposes:

    (1)   Many transports, especially message transports, can
          only handle data consisting of relatively short lines
          of text. There can also be severe restrictions on what
          characters can be used in these lines of text -- some
          transports are restricted to a small subset of US-ASCII
          and others cannot handle certain character sequences.
          Transfer encodings are used to transform binary data
          into textual form that can survive such transports.
          Examples of this sort of transfer encoding include the
          base64 and quoted-printable transfer encodings defined
          in RFC 2045.

    (2)   Image, audio, video, and even application entities are
          sometimes quite large. Compression algorithms are often
          quite effective in reducing the size of large entities.
          Transfer encodings can be used to apply general-purpose
          non-lossy compression algorithms to MIME entities.

    (3)   Transport encodings can be defined as a means of
          representing existing encoding formats in a MIME
          context.

   IMPORTANT:  The standardization of a large numbers of different
   transfer encodings is seen as a significant barrier to widespread
   interoperability and is expressely discouraged.  Nevertheless, the
   following procedure has been defined to provide a means of defining
   additional transfer encodings, should standardization actually be
   justified.

4.1.  Transfer Encoding Requirements

   Transfer encoding specifications must conform to a number of
   requirements as described below.

4.1.1.  Naming Requirements

   Each transfer encoding must have a unique name.  This name appears in
   the Content-Transfer-Encoding header field and must conform to the
   syntax of that field.

4.1.2.  Algorithm Specification Requirements

   All of the algorithms used in a transfer encoding (e.g.  conversion
   to printable form, compression) must be described in their entirety
   in the transfer encoding specification.  Use of secret and/or
   proprietary algorithms in standardized transfer encodings are
   expressly prohibited. The restrictions imposed by RFC 1602 on the
   standardization of patented algorithms must be respected as well.

4.1.3.  Input Domain Requirements

   All transfer encodings must be applicable to an arbitrary sequence of
   octets of any length.  Dependence on particular input forms is not
   allowed.

   It should be noted that the 7bit and 8bit encodings do not conform to
   this requirement. Aside from the undesireability of having
   specialized encodings, the intent here is to forbid the addition of
   additional encodings along the lines of 7bit and 8bit.

4.1.4.  Output Range Requirements

   There is no requirement that a particular tranfer encoding produce a
   particular form of encoded output.  However, the output format for
   each transfer encoding must be fully and completely documented.  In
   particular, each specification must clearly state whether the output
   format always lies within the confines of 7bit data, 8bit data, or is
   simply pure binary data.

4.1.5.  Data Integrity and Generality Requirements

   All transfer encodings must be fully invertible on any platform; it
   must be possible for anyone to recover the original data by
   performing the corresponding decoding operation.  Note that this
   requirement effectively excludes all forms of lossy compression as
   well as all forms of encryption from use as a transfer encoding.

4.1.6.  New Functionality Requirements

   All transfer encodings must provide some sort of new functionality.
   Some degree of functionality overlap with previously defined transfer
   encodings is acceptable, but any new transfer encoding must also
   offer something no other transfer encoding provides.

4.2.  Transfer Encoding Definition Procedure

   Definition of a new transfer encoding starts with the construction of
   a draft of a standards-track RFC.  The RFC must define the transfer
   encoding precisely and completely, and must also provide substantial
   justification for defining and standardizing a new transfer encoding.
   This specification must then be presented to the IESG for
   consideration.  The IESG can

    (1)   reject the specification outright as being
          inappropriate for standardization,

    (2)   approve the formation of an IETF working group to work
          on the specification in accordance with IETF
          procedures, or,

    (3)   accept the specification as-is and put it directly on
          the standards track.

   Transfer encoding specifications on the standards track follow normal
   IETF rules for standards track documents.  A transfer encoding is
   considered to be defined and available for use once it is on the
   standards track.

4.3.  IANA Procedures for Transfer Encoding Registration

   There is no need for a special procedure for registering Transfer
   Encodings with the IANA. All legitimate transfer encoding
   registrations must appear as a standards-track RFC, so it is the
   IESG's responsibility to notify the IANA when a new transfer encoding
   has been approved.

4.4.  Location of Registered Transfer Encodings List

   Transfer encoding registrations will be posted in the anonymous FTP
   directory "ftp://ftp.isi.edu/in-notes/iana/assignments/transfer-
   encodings/" and all registered transfer encodings will be listed in
   the periodically issued "Assigned Numbers" RFC [currently RFC-1700].

5.  Authors' Addresses

   For more information, the authors of this document are best
   contacted via Internet mail:

   Ned Freed
   Innosoft International, Inc.
   1050 East Garvey Avenue South
   West Covina, CA 91790
   USA

   Phone: +1 818 919 3600
   Fax:   +1 818 919 3614
   EMail: ned@innosoft.com

   John Klensin
   MCI
   2100 Reston Parkway
   Reston, VA 22091

   Phone: +1 703 715-7361
   Fax:   +1 703 715-7436
   EMail: klensin@mci.net

   Jon Postel
   USC/Information Sciences Institute
   4676 Admiralty Way
   Marina del Rey, CA  90292
   USA

   Phone: +1 310 822 1511
   Fax:   +1 310 823 6714
   EMail: Postel@ISI.EDU

Appendix A -- Grandfathered Media Types

   A number of media types, registered prior to 1996, would, if
   registered under the guidelines in this document, be placed into
   either the vendor or personal trees.  Reregistration of those types
   to reflect the appropriate trees is encouraged, but not required.
   Ownership and change control principles outlined in this document
   apply to those types as if they had been registered in the trees
   described above.

 

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