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[Note that this file is a concatenation of more than one RFC.] Network Working Group N. Freed Request for Comments: 4288 Sun Microsystems BCP: 13 J. Klensin Obsoletes: 2048 December 2005 Category: Best Current Practice Media Type Specifications and 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 defines procedures for the specification and registration of media types for use in MIME and other Internet protocols. Table of Contents 1. Introduction ....................................................3 2. Media Type Registration Preliminaries ...........................4 3. Registration Trees and Subtype Names ............................4 3.1. Standards Tree .............................................4 3.2. Vendor Tree ................................................5 3.3. Personal or Vanity Tree ....................................5 3.4. Special x. Tree ............................................5 3.5. Additional Registration Trees ..............................6 4. Registration Requirements .......................................6 4.1. Functionality Requirement ..................................6 4.2. Naming Requirements ........................................6 4.2.1. Text Media Types ......................................7 4.2.2. Image Media Types .....................................8 4.2.3. Audio Media Types .....................................8 4.2.4. Video Media Types .....................................8 4.2.5. Application Media Types ...............................9 4.2.6. Multipart and Message Media Types .....................9 4.2.7. Additional Top-level Types ............................9 4.3. Parameter Requirements ....................................10 4.4. Canonicalization and Format Requirements ..................10 4.5. Interchange Recommendations ...............................11 4.6. Security Requirements .....................................11 4.7. Requirements specific to XML media types ..................13 4.8. Encoding Requirements .....................................13 4.9. Usage and Implementation Non-requirements .................13 4.10. Publication Requirements .................................14 4.11. Additional Information ...................................15 5. Registration Procedure .........................................15 5.1. Preliminary Community Review ..............................16 5.2. IESG Approval .............................................16 5.3. IANA Registration .........................................16 5.4. Media Types Reviewer ......................................16 6. Comments on Media Type Registrations ...........................17 7. Location of Registered Media Type List .........................17 8. IANA Procedures for Registering Media Types ....................17 9. Change Procedures ..............................................18 10. Registration Template .........................................19 11. Security Considerations .......................................20 12. IANA Considerations ...........................................20 13. Acknowledgements ..............................................20 14. References ....................................................20 Appendix A. Grandfathered Media Types ............................22 Appendix B. Changes Since RFC 2048 ...............................22 1. Introduction Recent Internet protocols have been carefully designed to be easily extensible in certain areas. In particular, many protocols, including but not limited to MIME [RFC2045], are capable of carrying arbitrary labeled content. A mechanism is needed to label such content and a registration process is needed for these labels, to ensure that the set of such values is developed in an orderly, well- specified, and public manner. This document defines media type specification and registration procedures that use the Internet Assigned Numbers Authority (IANA) as a central registry. Historical Note The media type registration process was initially defined for registering media types for use 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 in which the proliferation of media types is not a hindrance to interoperability, the original procedure proved excessively restrictive and had to be generalized. This was initially done in [RFC2048], but the procedure defined there was still part of the MIME document set. The media type specification and registration procedure has now been moved to this separate document, to make it clear that it is independent of MIME. It may be desirable to restrict the use of media types to specific environments or to prohibit their use in other environments. This revision attempts for the first time to incorporate such restrictions into media type registrations in a systematic way. See Section 4.9 for additional discussion. 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]. This specification makes use of the Augmented Backus-Naur Form (ABNF) [RFC4234] notation, including the core rules defined in Appendix A of that document. 2. Media Type Registration Preliminaries Registration of a new media type or types starts with the construction of a registration proposal. Registration may occur within several different registration trees that have different requirements, as discussed below. In general, a 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. 3. 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 accommodate 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" that are distinguished by the use of faceted names, e.g., names of the form "tree.subtree...subtype". 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. 3.1. Standards Tree The standards tree is intended for types of general interest to the Internet community. Registrations in the standards tree MUST be approved by the IESG and MUST correspond to a formal publication by a recognized standards body. In the case of registration for the IETF itself, the registration proposal MUST be published as an RFC. Standards-tree registration RFCs can either be standalone "registration only" RFCs, or they can be incorporated into a more general specification of some sort. Media types in the standards 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 standards tree is assumed to be the standards body itself. Modification or alteration of the specification requires the same level of processing (e.g., standards track) required for the initial registration. 3.2. 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. A registration may be placed in the vendor tree by anyone who needs to interchange files associated with the particular product. However, the registration formally belongs to the vendor or organization producing the software or file format being registered. 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 registrant, by either a media subtype name from a well-known producer (e.g., "vnd.mudpie") or by an IANA-approved designation of the producer's name that is 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 email@example.com mailing 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. 3.3. 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 personal tree may be submitted directly to the IANA. 3.4. Special x. Tree For convenience and symmetry with this registration scheme, subtype names with "x." as the first facet may be used for the same purposes for which names starting in "x-" are used. These types are unregistered, experimental, and for use only with the active agreement of the parties exchanging them. 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. Therefore, use of both "x-" and "x." forms is discouraged. Types in this tree MUST NOT be registered. 3.5. Additional Registration Trees From time to time and as required by the community, the IANA may, by and 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; for example, scientific societies may register 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 registrations in the standards tree. Establishment of these new trees will be announced through RFC publication approved by the IESG. 4. 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. 4.1. 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 charset, or as a collection of separate entities of another type, is not allowed. For example, although applications exist to decode the base64 transfer encoding [RFC2045], base64 cannot be registered as a media type. This requirement applies regardless of the registration tree involved. 4.2. Naming Requirements All registered media types MUST be assigned type and subtype names. The combination of these names serves to uniquely identify the media type, and the format of the subtype name identifies the registration tree. Both type and subtype names are case-insensitive. Type and subtype names beginning with "X-" are reserved for experimental use and MUST NOT be registered. This parallels the restriction on the x. tree, as discussed in Section 3.4. Type and subtype names MUST conform to the following ABNF: type-name = reg-name subtype-name = reg-name reg-name = 1*127reg-name-chars reg-name-chars = ALPHA / DIGIT / "!" / "#" / "$" / "&" / "." / "+" / "-" / "^" / "_" Note that this syntax is somewhat more restrictive than what is allowed by the ABNF in [RFC2045]. In accordance with the rules specified in [RFC3023], media subtypes that do not represent XML entities MUST NOT be given a name that ends with the "+xml" suffix. More generally, "+suffix" constructs should be used with care, given the possibility of conflicts with future suffix definitions. While it is possible for a given media type to be assigned additional names, the use of different names to identify the same media type is discouraged. These requirements apply regardless of the registration tree involved. The choice of top-level type name MUST take into account the nature of media type involved. New subtypes of top-level types MUST conform to the restrictions of the top-level type, if any. The following sections describe each of the initial set of top-level types and their associated restrictions. Additionally, various protocols, including but not limited to MIME, MAY impose additional restrictions on the media types they can transport. (See [RFC2046] for additional information on the restrictions MIME imposes.) 4.2.1. Text Media Types The "text" media type is intended for sending material that is principally textual in form. A "charset" parameter MAY be used to indicate the charset of the body text for "text" subtypes, notably including the subtype "text/plain", which is a generic subtype for plain text defined in [RFC2046]. If defined, a text "charset" parameter MUST be used to specify a charset name defined in accordance to the procedures laid out in [RFC2978]. Plain text does not provide for or allow formatting commands, font attribute specifications, processing instructions, interpretation directives, or content markup. Plain text is seen simply as a linear sequence of characters, possibly interrupted by line breaks or page breaks. Plain text MAY allow the stacking of several characters in the same position in the text. Plain text in scripts like Arabic and Hebrew may also include facilities that allow the arbitrary mixing of text segments with opposite writing directions. Beyond plain text, there are many formats for representing what might be known as "rich text". An interesting characteristic of many such representations is that they are to some extent readable even without the software that interprets them. It is useful to distinguish them, at the highest level, from such unreadable data as images, audio, or text represented in an unreadable form. In the absence of appropriate interpretation software, it is reasonable to present subtypes of "text" to the user, while it is not reasonable to do so with most non-textual data. Such formatted textual data should be represented using subtypes of "text". 4.2.2. Image Media Types A media type of "image" indicates that the content specifies or more separate images that require appropriate hardware to display. The subtype names the specific image format. 4.2.3. Audio Media Types A media type of "audio" indicates that the content contains audio data. 4.2.4. Video Media Types A media type of "video" indicates that the content specifies a time- varying-picture image, possibly with color and coordinated sound. The term 'video' is used in its most generic sense, rather than with reference to any particular technology or format, and is not meant to preclude subtypes such as animated drawings encoded compactly. Note that although in general this document strongly discourages the mixing of multiple media in a single body, it is recognized that many so-called video formats include a representation for synchronized audio and/or text, and this is explicitly permitted for subtypes of "video". 4.2.5. Application Media Types The "application" media type is to be used for discrete data that do not fit in any of the media types, and particularly for data to be processed by some type of application program. This is information that must be processed by an application before it is viewable or usable by a user. Expected uses for the "application" media type include but are not limited to file transfer, spreadsheets, presentations, scheduling data, and languages for "active" (computational) material. (The latter, in particular, can pose security problems that must be understood by implementors, and are considered in detail in the discussion of the "application/ PostScript" media type in [RFC2046].) For example, a meeting scheduler might define a standard representation for information about proposed meeting dates. An intelligent user agent would use this information to conduct a dialog with the user, and might then send additional material based on that dialog. More generally, there have been several "active" languages developed in which programs in a suitably specialized language are transported to a remote location and automatically run in the recipient's environment. Such applications may be defined as subtypes of the "application" media type. The subtype of "application" will often be either the name or include part of the name of the application for which the data are intended. This does not mean, however, that any application program name may be used freely as a subtype of "application". 4.2.6. Multipart and Message Media Types Multipart and message are composite types, that is, they provide a means of encapsulating zero or more objects, each labeled with its own media type. All subtypes of multipart and message MUST conform to the syntax rules and other requirements specified in [RFC2046]. 4.2.7. Additional Top-level Types 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 does arise 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. 4.3. Parameter Requirements Media types MAY elect to use one or more media 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 standards tree, and SHOULD be specified as completely as possible when media types are registered in the vendor or personal trees. Parameter names have the syntax as media type names and values: parameter-name = reg-name Note that this syntax is somewhat more restrictive than what is allowed by the ABNF in [RFC2045] and amended by [RFC2231]. There is no defined syntax for parameter values. Therefore registrations MUST specify parameter value syntax. Additionally, some transports impose restrictions on parameter value syntax, so care should be taken to limit the use of potentially problematic syntaxes; e.g., pure binary valued parameters, while permitted in some protocols, probably should be avoided. New parameters SHOULD NOT be defined as a way to introduce new functionality in types registered in the standards 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. 4.4. 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 MUST exist for all types registered in the standards tree and MUST at a minimum be referenced by, if it isn't actually included in, the media type registration proposal itself. The specifications of format and processing particulars may or may not be publicly available for media types registered in the vendor tree, and such registration proposals are explicitly permitted to limit specification to 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 the 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 [RFC2026] on the use of patented technology in IETF standards-track protocols must be respected when the specification of a media type is part of a standards-track protocol. In addition, other standards bodies making use of the standards tree may have their own rules regarding intellectual property that must be observed in their registrations. 4.5. Interchange Recommendations Media types SHOULD 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. 4.6. Security Requirements An analysis of security issues MUST be done for all types registered in the standards Tree. 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 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 Section 6 below. Some of the issues that should be looked at in a security analysis of a media type are: o 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 that may then have devastating effects. See the registration of the application/postscript media type in [RFC2046] for an example of such directives and how they should be described in a media type registration. o All registrations MUST state whether or not they employ such "active content", and if they do, they MUST state what steps have been taken to protect users of the media type from harm. o Complex media types may include provisions for directives that institute actions that, 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. o A media type that employs compression may provide an opportunity for sending a small amount of data that, when received and evaluated, expands enormously to consume all of the recipient's resources. All media types SHOULD state whether or not they employ compression, and if they do they should discuss what steps need to be taken to avoid such attacks. o 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 that in turn requires an external confidentiality service, or which is designed for use only within a secure environment. 4.7. Requirements specific to XML media types There are a number of additional requirements specific to the registration of XML media types. These requirements are specified in [RFC3023]. 4.8. Encoding Requirements Some transports impose restrictions on the type of data they can carry. For example, Internet mail traditionally was limited to 7bit US-ASCII text. Encoding schemes are often used to work around such transport limitations. It is therefore useful to note what sort of data a media type can consist of as part of its registration. An "encoding considerations" field is provided for this purpose. Possible values of this field are: 7bit: The content of the media type consists solely of CRLF-delimited 7bit US-ASCII text. 8bit: The content of the media type consists solely of CRLF-delimited 8bit text. binary: The content consists of unrestricted sequence of octets. framed: The content consists of a series of frames or packets without internal framing or alignment indicators. Additional out-of-band information is needed to interpret the data properly, including but not necessarily limited to, knowledge of the boundaries between successive frames and knowledge of the transport mechanism. Note that media types of this sort cannot simply be stored in a file or transported as a simple stream of octets; therefore, such media types are unsuitable for use in many traditional protocols. A commonly used transport with framed encoding is the Real-time Transport Protocol, RTP. Additional rules for framed encodings defined for transport using RTP are given in [RFC3555]. Additional restrictions on 7bit and 8bit text are given in [RFC2046]. 4.9. 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 media types used to those "common" formats expected to be widely implemented. This was asserted in the past as a reason to limit the number of possible media types, and it 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. Therefore, universal support and implementation of a media type is NOT a requirement for registration. However, if a media type is explicitly intended for limited use, this MUST be noted in its registration. The "Restrictions on Usage" field is provided for this purpose. 4.10. Publication Requirements Proposals for media types registered in the standards tree by the IETF itself MUST be published as RFCs. RFC publication of vendor and personal media type proposals is encouraged but not required. In all cases the IANA will retain copies of all media type proposals and "publish" them as part of the media types registration tree itself. As stated previously, standards tree registrations for media types defined in documents produced by other standards bodies MUST be described by a formal standards specification produced by that body. Such specifications MUST contain an appropriate media type registration template taken from Section 10. Additionally, the copyright on the registration template MUST allow the IANA to copy it into the IANA registry. Other than IETF registrations in the standards tree, the registration of a data type does not imply endorsement, approval, or recommendation by the IANA or the IETF or even certification that the specification is adequate. To become Internet Standards, a protocol or data object must go through the IETF standards process. This is too difficult and too lengthy a process for the convenient registration of media types. The standards tree exists for media types that do require a substantive review and approval process in a recognized standards body. The vendor and personal trees exist for those media types that do not require such a process. It is expected that applicability statements for particular applications will be published from time to time in the IETF, recommending 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 in the IETF and, hence, RFC publication. 4.11. Additional Information Various sorts of optional information SHOULD be included in the specification of a media type if it is available: o Magic number(s) (length, octet values). Magic numbers are byte sequences that are always present at a given place in the file and thus can be used to identify entities as being of a given media type. o File name extension(s) commonly used on one or more platforms to indicate that some file contains a given media type. o Mac OS File Type code(s) (4 octets) used to label files containing a given media type. o Information about how fragment/anchor identifiers [RFC3986] are constructed for use in conjunction with this media type. In the case of a registration in the standards tree, this additional information MAY be provided in the formal specification of the media type. It is suggested that this be done by incorporating the IANA media type registration form into the specification itself. 5. Registration Procedure The media type registration procedure is not a formal standards process, but rather an administrative procedure intended to allow community comment and sanity checking without excessive time delay. The normal IETF processes should be followed for all IETF registrations in the standards tree. The posting of an Internet Draft is a necessary first step, followed by posting to the firstname.lastname@example.org list as discussed below. Registrations in the vendor and personal tree should be submitted directly to the IANA, ideally after first posting to the email@example.com list for review. Proposed registrations in the standards tree by other standards bodies should be communicated to the IESG (at firstname.lastname@example.org) and to the ietf-types list (at email@example.com). Prior posting as an Internet Draft is not required for these registrations, but may be helpful to the IESG and is encouraged. 5.1. Preliminary Community Review Notice of a potential media type registration in the standards tree MUST be sent to the "firstname.lastname@example.org" mailing list for review. This mailing list has been established for the purpose of reviewing proposed media and access types. Registrations in other trees MAY be sent to the list for review as well. The intent of the public posting to this list 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 abandon the registration completely and at any time. 5.2. IESG Approval Media types registered in the standards tree MUST be approved by the IESG prior to registration. 5.3. IANA Registration Provided that the media type meets all of the relevant requirements and has obtained whatever approval is necessary, the author may submit the registration request to the IANA. Registration requests can be sent to email@example.com. A web form for registration requests is also available: http://www.iana.org/cgi-bin/mediatypes.pl Sending to firstname.lastname@example.org does not constitute submitting the registration to the IANA. When the registration is either part of an RFC publication request or a registration in the standards tree submitted to the IESG, close coordination between the IANA and the IESG means IESG approval in effect submits the registration to the IANA. There is no need for an additional registration request in such cases. 5.4. Media Types Reviewer Registrations submitted to the IANA will be passed on to the media types reviewer. The media types reviewer, who is appointed by the IETF Applications Area Director(s), will review the registration to make sure it meets the requirements set forth in this document. Registrations that do not meet these requirements will be returned to the submitter for revision. Decisions made by the media types reviewer may be appealed to the IESG using the procedure specified in [RFC2026] section 6.5.4. Once a media type registration has passed review, the IANA will register the media type and make the media type registration available to the community. 6. Comments on Media Type Registrations Comments on registered media types may be submitted by members of the community to the IANA. These comments will be reviewed by the media types reviewer and then 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 the IANA approves of this, the comment will be made accessible in conjunction with the type registration. 7. Location of Registered Media Type List Media type registrations are listed by the IANA at: http://www.iana.org/assignments/media-types/ 8. IANA Procedures for Registering Media Types The IANA will only register media types in the standards 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 approval process as long as the following minimal conditions are met: o Media types MUST function as an actual media format. In particular, charsets and transfer encodings MUST NOT be registered as media types. o All media types MUST have properly formed type and subtype names. All type names MUST be defined by a standards-track RFC. All type/subtype name pairs MUST be unique and MUST contain the proper tree prefix. o Types registered in the personal tree MUST either provide a format specification or a pointer to one. o All media types MUST have a reasonable security considerations section. (It is neither possible nor necessary for the IANA to conduct a comprehensive security review of media type registrations. Nevertheless, the IANA has the authority to identify obviously incompetent material and return it to the submitter for revision.) Registrations in the standards tree MUST satisfy the additional requirement that they originate from the IETF itself or from another standards body recognized as such by the IETF. 9. Change Procedures Once a media type has been published by the IANA, the owner may request a change to its definition. The descriptions of the different registration trees above designate the "owners" of each type of registration. The same procedure that would be appropriate for the original registration request is used to process a change request. Changes should be requested only when there are serious omissions 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 media type may pass responsibility to another person or agency by informing the 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 that 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 the IANA. 10. Registration Template To: email@example.com Subject: Registration of media type XXX/YYY Type name: Subtype name: Required parameters: Optional parameters: Encoding considerations: Security considerations: Interoperability considerations: Published specification: Applications that 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.) Restrictions on usage: (Any restrictions on where the media type can be used go here.) Author: Change controller: (Any other information that the author deems interesting may be added below this line.) Some discussion of Macintosh file type codes and their purpose can be found in [MacOSFileTypes]. Additionally, please refrain from writing "none" or anything similar when no file extension or Macintosh file type is specified, lest "none" be confused with an actual code value. 11. Security Considerations Security requirements for media type registrations are discussed in Section 4.6. 12. IANA Considerations The purpose of this document is to define IANA registries for media types. 13. 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. 14. References 14.1. 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. [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration Procedures", BCP 19, RFC 2978, October 2000. [RFC3023] Murata, M., St. Laurent, S., and D. Kohn, "XML Media Types", RFC 3023, January 2001. [RFC3555] Casner, S. and P. Hoschka, "MIME Type Registration of RTP Payload Formats", RFC 3555, July 2003. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC4234] Crocker, D. Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. 14.2. Informative References [MacOSFileTypes] Apple Computer, Inc., "Mac OS: File Type and Creator Codes, and File Formats", Apple Knowledge Base Article 55381, June 1993, <http://www.info.apple.com/kbnum/n55381>. [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. [RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and Continuations", RFC 2231, November 1997. 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. Appendix B. Changes Since RFC 2048 o Media type specification and registration procedures have been moved out of the MIME document set to this separate specification. 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 The unfaceted IETF tree is now called the standards tree, and the registration rules for this tree have been relaxed to allow use by other standards bodies. o The text describing the media type registration procedure has clarified. o The rules and requirements for constructing security considerations sections have been extended and clarified. o RFC 3023 is now referenced as the source of additional information concerning the registration of XML media types. o Several of the references in this document have been updated to refer to current versions of the relevant specifications. o A note has been added discouraging the assignment of multiple names to a single media type. o Security considerations and IANA considerations sections have been added. o Concerns regarding copyrights on media type registration templates produced by other standards bodies have been dealt with by requiring that the IANA be allowed to copy the registration template into the registry. o The basic registration requirements for the various top-level types have been moved from RFC 2046 to this document. o A syntax is now specified for media type, subtype, and parameter names. o Imposed a maximum length of 127 on all media type and subtype names. o A note has been added to caution against excessive use of "+suffix" constructs in subtype names. o The encoding considerations field has been extended to allow the value "framed". o A reference describing Macintosh Type codes has been added. o Ietf-types list review of registrations in the standards tree is now required rather than just recommended. Authors' Addresses Ned Freed Sun Microsystems 3401 Centrelake Drive, Suite 410 Ontario, CA 92761-1205 USA Phone: +1 909 457 4293 EMail: firstname.lastname@example.org John C. Klensin 1770 Massachusetts Ave, #322 Cambridge, MA 02140 EMail: email@example.com 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 http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- firstname.lastname@example.org. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Freed & Klensin Best Current Practice [Page 24] ======================================================================== 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. 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. Note: Registration of media types and charsets for use in MIME are specified in separate documents [RFC4288] [RFC2278] 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. 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 "email@example.com" 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 firstname.lastname@example.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]. 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. 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. 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 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. 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. [RFC2278] Freed, N. and J. Postel, "IANA Charset Registration Procedures", BCP 19, RFC 2278, January 1998. 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 [RFC2278] 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: email@example.com John C. Klensin 1770 Massachusetts Ave, #322 Cambridge, MA 02140 EMail: firstname.lastname@example.org 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 http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- email@example.com. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.