Internet Engineering Task Force Audio/Video Transport Working Group Internet-Draft S. Casner draft-ietf-avt-rfc3555bis-04.txt Packet Design Obsoletes: RFC 3555 (if approved) April 17, 2006 Expires: October 17, 2006 Media Type Registration of RTP Payload Formats Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 26, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document specifies the procedure to register RTP payload formats as audio, video or other media subtype names. This is useful in a text-based format description or control protocol to identify the type of an RTP transmission. Expires October 2006 [Page 1] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 Table of Contents 1. Introduction .................................................. 3 1.2. Terminology .............................................. 3 2. Procedure For Registering Media Types for RTP Payload Types ... 3 2.1. Example Media Type Registration ........................... 5 2.2. Restrictions on Sharing a Subtype Name ................... 6 3. Mapping to SDP Parameters ..................................... 7 4. Changes from RFC 3555 ......................................... 8 5. Security Considerations ....................................... 8 6. IANA Considerations ........................................... 9 7. References .................................................... 9 8. Author's Address .............................................. 10 9. Intellectual Property Statement ............................... 10 10. Disclaimer of Validity ........................................ 10 11. Copyright Statement ........................................... 11 Expires October 2006 [Page 2] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 1. Introduction RFC 4288 [1] defines media type specification and registration procedures that use the Internet Assigned Numbers Authority (IANA) as a central registry. That document covers general requirements independent of particular application environments and transport modes. This document defines the specific requirements for registration of media types for use with the Real-time Transport Protocol (RTP), RFC 3550 [2], to identify RTP payload formats. 1.1. Terminology 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 RFC 2119 [3] and indicate requirement levels for implementations compliant with this specification. 2. Procedure For Registering Media Types for RTP Payload Types Registering an RTP payload type as a media type follows the same procedures as described in RFC 4288 [1] and uses the registration template shown in Section 10 of that RFC. To specify how the particular payload format is transported over RTP, some additional information is required in the following sections of that template: Required parameters If the payload format does not have a fixed RTP timestamp clock rate, then a "rate" parameter is required to specify the RTP timestamp clock rate. A particular payload format may have additional required parameters. Optional parameters Most audio payload formats can have an optional "channels" parameter to specify the number of audio channels included in the transmission. The default channel order is as specified in RFC 3551 [4]. Any payload format, but most likely audio formats, may also include the optional parameters "ptime", to specify the recommended length of time in milliseconds represented by the media in a packet, and/or "maxptime" to specify the maximum amount of media that can be encapsulated in each packet, expressed as time in milliseconds. The "ptime" and "maxptime" parameters are defined in the Session Description Protocol (SDP) [5]. A particular payload format may have additional optional parameters. As allowed in Section 4.3 of [1], new parameters MAY be added to RTP media types that have been previously Expires October 2006 [Page 3] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 defined, but the new parameters MUST NOT change existing functionality and it MUST be possible for existing implementations to ignore the additional parameters without impairing operation. Encoding considerations Most RTP payload formats include binary or framed data as described in Section 4.8 of [1]. The appropriate encoding considerations MUST be noted. Published specification A description of the media encoding and a specification of the payload format must be provided, usually by reference to an RTP payload format specification RFC. That RFC may be separate, or the media type registration may be incorporated into the payload format specification RFC. The payload format specification MUST include the RTP timestamp clock rate (or multiple rates for audio encodings with multiple sampling rates). A reference to a further description of the data compression format itself should be provided, if available. Restrictions on usage: The fact that the media type is defined for transfer via RTP MUST be noted, in particular if the transfer depends on RTP framing and hence the media type is only defined for transfer via RTP. Depending on whether the type has already been registered for transfer with a non-RTP protocol (e.g. MIME mail or http) or not, several different cases can occur: a) Not yet registered as a media type A new registration should be constructed using the media type registration template. The registration may specify transfer via other means in addition to RTP if that is feasible and desired. The appropriate encoding considerations must be specified, and the restrictions on usage must specify whether the type is only defined for transfer via RTP or via other modes as well. Optional parameters may be defined as needed, and it must be clearly stated to which mode(s) of transfer the parameters apply. Expires October 2006 [Page 4] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 b) Media type exists for a non-RTP protocol The restrictions on usage of the existing type should be changed, if present, or added, if not, to indicate that the type can also be transferred via RTP. RTP-specific parameters may be added, and it must be clearly stated that these are only to be used when the media type is transmitted via RTP transport. c) Update an existing media type for RTP to be used for a non-RTP protocol The restrictions on usage of the existing type should be changed to indicate that the type can also be transferred via a non-RTP protocol (e.g. SMTP, HTTP). Non-RTP-specific parameters can be added, and it must be clearly stated that these are only to be used when the media type is transmitted via a non-RTP transport. 2.1. Example Media Type Registration The following sample registration of a fake media type audio/example provides examples for some of the required text. References to RFC nnnn would be replaced by references to the RFC that contains the payload format specification and the media type registration. Type name: audio Subtype name: example Required parameters: rate: RTP timestamp clock rate, which is equal to the sampling rate. The typical rate is 8000; other rates may be specified. Optional parameters: channels: number of interleaved audio streams, either 1 for mono or 2 for stereo, and defaults to 1 if omitted. Interleaving takes place between on a frame-by-frame basis, with the left channel followed by the right channel. ptime: recommended length of time in milliseconds represented by the media in a packet (see RFC YYYY). maxptime: maximum amount of media that can be encapsulated in each packet, expressed as time in milliseconds (see RFC YYYY). Expires October 2006 [Page 5] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 Encoding considerations: This media type is framed binary data (see RFC 4288, Section 4.8). Security considerations: See Section n of RFC nnnn Interoperability considerations: Some receivers may only be capable of receiving single-channel audio. Published specification: RFC nnnn Applications that use this media type: Audio and video streaming and conferencing tools. Additional information: none Person & email address to contact for further information: Fred Audio Intended usage: COMMON Restrictions on usage: This media type depends on RTP framing, and hence is only defined for transfer via RTP (RFC 3550). Transfer within other framing protocols is not defined at this time. Author: Fred Audio Change controller: IETF Audio/Video Transport working group delegated from the IESG. 2.2. Restrictions on Sharing a Subtype Name The same media subtype name MUST NOT be shared for RTP and non-RTP (file-based) transfer methods unless the data format is the same for both methods. The data format is considered to be same if the file format is equivalent to a concatenated sequence of payloads from RTP packets not including the RTP header or any RTP payload-format header. The file format MAY include a magic number or other header at the start of the file that is not included when the data is transferred via RTP. A second requirement for sharing a media subtype name is that the sets of required parameters must be the same for both methods. Expires October 2006 [Page 6] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 For cases where the data format or required parameters cannot be the same for RTP and non-RTP transfer methods, then the data formats MUST be registered as separate types. It is RECOMMENDED that the subtype names be related, such as by using a common root plus a suffix. For those cases where a suffix is applied in the subtype name for the RTP transfer method, the suffix "+rtp" is suggested. 3. Mapping to SDP Parameters The representation of a media type is specified in the syntax of the Content-Type header field in RFC 2045 [6] as follows: type "/" subtype *(";" parameter) Parameters may be required for a particular type or subtype or they may be optional. For media types which represent RTP payload formats, the parameters "rate", "channels", "ptime", and "maxptime" have general definitions (given above) that may apply across types and subtypes. The format for a parameter is specified in RFC 2045 as attribute "=" value where attribute is the parameter name and the permissible values are specified for each parameter. RFC 2045 specifies that a value MUST be present and that the value MUST be a quoted string if it contains any of the special characters listed in that RFC. The information carried in the media type string has a specific mapping to fields in the Session Description Protocol (SDP) [5], which is commonly used to describe RTP sessions. The mapping is as follows: o The media type (e.g., audio) goes in SDP "m=" as the media name. o The media subtype (payload format) goes in SDP "a=rtpmap" as the encoding name. o The general (possibly optional) parameters "rate" and "channels" also go in "a=rtpmap" as clock rate and encoding parameters, respectively. o The general (and optional) parameters "ptime" and "maxptime" go in the SDP "a=ptime" and "a=maxptime" attributes, respectively. o Any payload-format-specific parameters go in the SDP "a=fmtp" attribute. The set of allowed parameters is defined by the RFC that specifies the payload format and MUST NOT be extended by the media type registration without a corresponding revision of the payload format specification. The format and syntax of these Expires October 2006 [Page 7] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 parameters may also be defined by the payload format specification, but it is suggested that the parameters be copied directly from the media type string as a semicolon separated list of parameter=value pairs. For payload formats that specify some other syntax for the fmtp parameters, the registration of that payload format as a media type must specify what the parameters are in MIME format and how to map them to the "a=fmtp" attribute. An example mapping is as follows: audio/L16; rate=48000; channels=2; ptime=5; emphasis=50-15 m=audio 49170 RTP/AVP 97 a=rtpmap:97 L16/48000/2 a=fmtp:97 emphasis=50-15 a=ptime:5 Note that the payload format (encoding) names defined in the RTP Profile [4] are commonly shown in upper case. Media subtype names are commonly shown in lower case. These names are case-insensitive in both places. Similarly, parameter names are case-insensitive both in media type strings and in the default mapping to the SDP a=fmtp attribute. 4. Changes from RFC 3555 This document updates RFC 3555 to conform to the revised media type registration procedures in RFC 4288 [1]. Whereas RFC 3555 required the encoding considerations to specify transfer via RTP, that is now specified under restrictions on usage. This document also specifies the conditions under which new optional parameters may be added to a previously defined RTP media type and adds a new Section 2.2 to clarify the requirements for sharing a media type among RTP and non-RTP transfer methods. RFC 3555 included media type registrations for the RTP payload formats defined in the RTP Profile for Audio and Video Conferences, RFC 3551 [4]. Those media type registrations have been removed from this document. Some of them have been assembled into a separate companion RFC XXXX [7], leaving out those that have been, or are intended to be, registered in revisions of their own payload format specification RFCs. Philipp Hoschka is a co-author of RFC 3555; his contributions to the foundation of this document are appreciated. 5. Security Considerations The media type registration procedure specified in this memo does not impose any security considerations on its own. Registrations conforming Expires October 2006 [Page 8] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 to this procedure also do not themselves impose security risks, but each is required to state any security considerations specific to use of the media type being registered. Several audio and video encodings are perfect for hiding data using steganography. The RTP specification, RFC 3550, provides security considerations for the transport of audio and video data over RTP, including the use of encryption where confidentiality is required. 6. IANA Considerations The purpose of this document is to specify the requirements and procedures for registering RTP payload formats in the IANA media type registry. No registrations are defined here, so no IANA actions are required for this document. 7. References 7.1. Normative References [1] Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December, 2005. [2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [4] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3551, July 2003. [5] Handley, M., V. Jacobson and C. Perkins, "SDP: Session Description Protocol", draft-ietf-mmusic-sdp-new-26.txt, January 2006. (Approved for publication as Proposed Standard to obsolete RFC 2327, so this reference should be to the RFC when published and that number inserted where RFC YYYY appears in this document) [6] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. Expires October 2006 [Page 9] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 7.2. Informative References [7] Casner, S., "Media Type Registration of Payload Formats in the RTP Profile for Audio and Video Conferences", draft-ietf-avt-rfc3555bis-part2-00.txt, February, 2006. (Companion to this document, referenced herein as RFC XXXX). 8. Author's Address Stephen L. Casner Packet Design 3400 Hillview Avenue, Building 3 Palo Alto, CA 94304 United States Phone: +1 650 739-1843 EMail: casner@acm.org 9. Intellectual Property Statement 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- ipr@ietf.org. 10. Disclaimer of Validity 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, Expires October 2006 [Page 10] Internet Draft draft-ietf-avt-rfc3555bis-04.txt April 17, 2006 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. 11. Copyright Statement Copyright (C) The Internet Society (2006). 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. Expires October 2006 [Page 11]