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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.
Freed & Klensin Best Current Practice [Page 1]
RFC 4289 MIME Registration December 2005
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.
Freed & Klensin Best Current Practice [Page 2]
RFC 4289 MIME Registration December 2005
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.
Freed & Klensin Best Current Practice [Page 3]
RFC 4289 MIME Registration December 2005
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].
Freed & Klensin Best Current Practice [Page 4]
RFC 4289 MIME Registration December 2005
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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RFC 4289 MIME Registration December 2005
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.
Freed & Klensin Best Current Practice [Page 6]
RFC 4289 MIME Registration December 2005
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
Freed & Klensin Best Current Practice [Page 7]
RFC 4289 MIME Registration December 2005
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.
Freed & Klensin Best Current Practice [Page 8]
RFC 4289 MIME Registration December 2005
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.
Freed & Klensin Best Current Practice [Page 9]
RFC 4289 MIME Registration December 2005
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
Freed & Klensin Best Current Practice [Page 10]
RFC 4289 MIME Registration December 2005
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
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The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Freed & Klensin Best Current Practice [Page 11]
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