





Network Working Group                                           N. Freed
Request for Comments: 4289                              Sun Microsystems
BCP: 13                                                       J. Klensin
Obsoletes: 2048                                            December 2005
Category: Best Current Practice


        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.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies IANA registration procedures for MIME
   external body access types and content-transfer-encodings.



























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Table of Contents

   1. Introduction ....................................................2
   2. External Body Access Types ......................................3
      2.1. Registration Requirements ..................................3
         2.1.1. Naming Requirements ...................................3
         2.1.2. Mechanism Specification Requirements ..................3
         2.1.3. Publication Requirements ..............................4
         2.1.4. Security Requirements .................................4
      2.2. Registration Procedure .....................................4
         2.2.1. Present the Access Type to the Community ..............4
         2.2.2. Access Type Reviewer ..................................4
         2.2.3. IANA Registration .....................................5
      2.3. Location of Registered Access Type List ....................5
      2.4. IANA Procedures for Registering Access Types ...............5
   3. Transfer Encodings ..............................................5
      3.1. Transfer Encoding Requirements .............................6
         3.1.1. Naming Requirements ...................................6
         3.1.2. Algorithm Specification Requirements ..................6
         3.1.3. Input Domain Requirements .............................6
         3.1.4. Output Range Requirements .............................6
         3.1.5. Data Integrity and Generality Requirements ............7
         3.1.6. New Functionality Requirements ........................7
         3.1.7. Security Requirements .................................7
      3.2. Transfer Encoding Definition Procedure .....................7
      3.3. IANA Procedures for Transfer Encoding Registration .........8
      3.4. Location of Registered Transfer Encodings List .............8
   4. Security Considerations .........................................8
   5. IANA Considerations .............................................8
   6. Acknowledgements ................................................8
   7. References ......................................................9
   A.  Changes Since RFC 2048 .........................................9

1.  Introduction

   Recent Internet protocols have been carefully designed to be easily
   extensible in certain areas.  In particular, MIME [RFC2045] is an
   open-ended framework and can accommodate additional object types,
   charsets, 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 that use the Internet
   Assigned Numbers Authority (IANA) as a central registry for these
   values.





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   Note:

      Registration of media types and charsets for use in MIME are
      specified in separate documents [RFC4288] [RFC2978] and are not
      addressed here.

1.1.  Conventions Used in This Document

   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 [RFC2119].

2.  External Body Access Types

   [RFC2046] 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.

2.1.  Registration Requirements

   New access type specifications MUST conform to the requirements
   described below.

2.1.1.  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.

2.1.2.  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 is expressly prohibited.  The restrictions imposed by
   [RFC2026] on the standardization of patented algorithms must be
   respected as well.







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2.1.3.  Publication Requirements

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

2.1.4.  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 it is
   required 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.

2.2.  Registration Procedure

   Registration of a new access type starts with the publication of the
   specification as an Internet Draft.

2.2.1.  Present the Access Type to the Community

   A proposed access type specification is sent 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.

2.2.2.  Access Type Reviewer

   When the two-week period has passed, the access type reviewer, who is
   appointed by the IETF Applications Area Director(s), either forwards
   the request to iana@iana.org 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 as specified in [RFC2026].







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2.2.3.  IANA Registration

   Provided that the access type either has 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.

2.3.  Location of Registered Access Type List

   Access type registrations are listed by the IANA on the following web
   page:

     http://www.iana.org/assignments/access-types

2.4.  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 either in response to access
   type definitions that are approved by the access type reviewer and
   forwarded 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.

3.  Transfer Encodings

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

   o  Many transports, especially message transports, can only handle
      data consisting of relatively short lines of text.  There can 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 a
      textual form that can survive such transports.  Examples of this
      sort of transfer encoding include the base64 and quoted-printable
      transfer encodings defined in [RFC2045].

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

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




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

3.1.  Transfer Encoding Requirements

   Transfer encoding specifications MUST conform to the requirements
   described below.

3.1.1.  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.

3.1.2.  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 is
   expressly prohibited.  The restrictions imposed by [RFC2026] on the
   standardization of patented algorithms MUST be respected as well.

3.1.3.  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 undesirability of having
   specialized encodings, the intent here is to forbid the addition of
   additional encodings similar to, or redundant with, 7bit and 8bit.

3.1.4.  Output Range Requirements

   There is no requirement that a particular transfer 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 or 8bit or is simply
   pure binary data.




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3.1.5.  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.

3.1.6.  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.

3.1.7.  Security Requirements

   To the greatest extent possible, transfer encodings SHOULD NOT
   contain known security issues.  Regardless, any known security issues
   that arise from the use of the transfer encoding MUST be completely
   and fully described.  If additional security issues come to light
   after initial publication and registration, they SHOULD be addressed
   by publishing revised versions of the transfer encoding
   specification.

3.2.  Transfer Encoding Definition Procedure

   Definition of a new transfer encoding starts with the publication of
   the specification as an Internet Draft.  The draft MUST define the
   transfer encoding precisely and completely, and it 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:

   o  reject the specification outright as being inappropriate for
      standardization,

   o  assign the specification to an existing IETF working group for
      further work,

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

   o  accept the specification as-is for processing as an individual
      standards-track submission.

   Transfer encoding specifications on the standards track follow normal
   IETF rules for standards-track documents.  A transfer encoding is



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   considered to be defined and available for use once it is on the
   standards track.

3.3.  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.

3.4.  Location of Registered Transfer Encodings List

   The list of transfer encoding registrations can be found at:

     http://www.iana.org/assignments/transfer-encodings

4.  Security Considerations

   Security requirements for access types are discussed in Section
   2.1.4.  Security requirements for transfer encodings are discussed in
   Section 3.1.7.

5.  IANA Considerations

   The sole purpose of this document is to define IANA registries for
   access types and transfer encodings.  The IANA procedures for these
   registries are specified in Section 2.4 and Section 3.3 respectively.

6.  Acknowledgements

   The current authors would like to acknowledge their debt to the late
   Dr. Jon Postel, whose general model of IANA registration procedures
   and specific contributions shaped the predecessors of this document
   [RFC2048].  We hope that the current version is one with which he
   would have agreed but, as it is impossible to verify that agreement,
   we have regretfully removed his name as a co-author.














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7.  References

7.1.  Normative References

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

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

7.2.  Informative References

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2048]  Freed, N., Klensin, J., and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, October 2000.






















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Appendix A.  Changes Since RFC 2048

   o  Media type registration procedures are now described in a separate
      document [RFC4288].

   o  The various URLs and addresses in this document have been changed
      so they all refer to iana.org rather than isi.edu.  Additionally,
      many of the URLs have been changed to use HTTP; formerly they used
      FTP.

   o  Much of the document has been clarified in the light of
      operational experience with these procedures.

   o  Several of the references in this document have been updated to
      refer to current versions of the relevant specifications.

   o  The option of assigning the task of working on a new transfer
      encoding to an existing working group has been added to the list
      of possible actions the IESG can take.

   o  Security considerations and IANA considerations sections have been
      added.

   o  Registration of charsets for use in MIME is specified in [RFC2978]
      and is no longer addressed by this document.

Authors' Addresses

   Ned Freed
   Sun Microsystems
   3401 Centrelake Drive, Suite 410
   Ontario, CA  92761-1205
   USA

   Phone: +1 909 457 4293
   EMail: ned.freed@mrochek.com


   John C. Klensin
   1770 Massachusetts Ave, #322
   Cambridge, MA  02140

   EMail: klensin+ietf@jck.com








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Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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