











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.



















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RFC 2048              MIME Registration Procedures         November 1996





   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







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   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.











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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.











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   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.















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   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







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   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.











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   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







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   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







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   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







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          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.

















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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.







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    (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.

















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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







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   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.











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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.







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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.













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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.

















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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].



























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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































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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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