Updated by:
Obsoletes:
Obsoleted by:
Keywords: media, types, external, body, access, content-transfer-encodings
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.
Freed, et. al. Best Current Practice [Page 1]
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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
Freed, et. al. Best Current Practice [Page 8]
RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
(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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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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RFC 2048 MIME Registration Procedures November 1996
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
Freed, et. al. Best Current Practice [Page 20]
RFC 2048 MIME Registration Procedures November 1996
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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