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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Registration of media types and charsets for use in MIME are specified in separate documents [RFC4288] [RFC2978] and are not addressed here.
[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.
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.
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.
A proposed access type specification is sent to the "firstname.lastname@example.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.
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 email@example.com 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].
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.
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.
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.
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.
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.
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.
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.
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.
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:
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.
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.
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.
Ned Freed Sun Microsystems 3401 Centrelake Drive, Suite 410 Ontario, CA 92761-1205 USA
John C. Klensin 1770 Massachusetts Ave, #322 Cambridge, MA 02140
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