AVT D. Singer Internet-Draft Apple Computer Inc. Expires: December 17, 2006 June 15, 2006 A general mechanism for RTP Header Extensions draft-ietf-avt-rtp-hdrext-03.txt 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 December 17, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document provides a general mechanism to use the header- extension feature of RTP (the Real Time Protocol). It provides the option to use a small number of small extensions in each RTP packet, where the universe of possible extensions is large and unregistered. The actual extensions in use in a session are signaled in the setup information for that session. Singer Expires December 17, 2006 [Page 1] Internet-Draft RTP Header Extensions June 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements notation . . . . . . . . . . . . . . . . . . . . 5 3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Packet Design . . . . . . . . . . . . . . . . . . . . . . . . 7 5. SDP Signalling Design . . . . . . . . . . . . . . . . . . . . 8 6. Transmission offset . . . . . . . . . . . . . . . . . . . . . 9 7. BNF Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 9.1. New spaces for IANA to manage . . . . . . . . . . . . . . 12 9.2. Registration of the transmit-time offset . . . . . . . . . 12 9.3. Registration of the SDP extmap attribute . . . . . . . . . 12 10. RFC Editor Considerations . . . . . . . . . . . . . . . . . . 13 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 14 Appendix B. Change History . . . . . . . . . . . . . . . . . . . 15 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16 Intellectual Property and Copyright Statements . . . . . . . . . . 17 Singer Expires December 17, 2006 [Page 2] Internet-Draft RTP Header Extensions June 2006 1. Introduction In the RTP specification [RFC3550] there is provision for header extensions. The following text is drawn from that specification for the convenience of readers. * * * * * * * * * * * * * * 5.3.1 RTP Header Extension An extension mechanism is provided to allow individual implementations to experiment with new payload-format-independent functions that require additional information to be carried in the RTP data packet header. This mechanism is designed so that the header extension may be ignored by other interoperating implementations that have not been extended. Note that this header extension is intended only for limited use. Most potential uses of this mechanism would be better done another way, using the methods described in the previous section. For example, a profile-specific extension to the fixed header is less expensive to process because it is not conditional nor in a variable location. Additional information required for a particular payload format SHOULD NOT use this header extension, but SHOULD be carried in the payload section of the packet. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | defined by profile | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | header extension | | .... | If the X bit in the RTP header is one, a variable-length header extension MUST be appended to the RTP header, following the CSRC list if present. The header extension contains a 16-bit length field that counts the number of 32-bit words in the extension, excluding the four-octet extension header (therefore zero is a valid length). Only a single extension can be appended to the RTP data header. To allow multiple interoperating implementations to each experiment independently with different header extensions, or to allow a particular implementation to experiment with more than one type of header extension, the first 16 bits of the header extension are left open for distinguishing identifiers or parameters. The format of these 16 bits is to be defined by the profile specification under which the implementations are operating. This RTP specification does Singer Expires December 17, 2006 [Page 3] Internet-Draft RTP Header Extensions June 2006 not define any header extensions itself. * * * * * * * * * * * * * * This mechanism has two conspicuous drawbacks: only one extension is possible, and there is no documentation of how the 16-bit identifiers are allocated. Singer Expires December 17, 2006 [Page 4] Internet-Draft RTP Header Extensions June 2006 2. Requirements notation 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]. Singer Expires December 17, 2006 [Page 5] Internet-Draft RTP Header Extensions June 2006 3. Design Goals The goal of this design is to provide a simple mechanism whereby multiple identified extensions can be used in RTP packets, without the need for formal registration of those extensions but nonetheless avoiding collision. This mechanism provides an alternative to the practice of burying associated metadata into the media format bit stream. This has often been done in media data sent over fixed-bandwidth channels. Once this is done, a decoder for the specific media format is required to extract the metadata. Also, depending on the media format, the metadata may need to be added at the time of encoding the media so that the bit-rate required for the metadata is taken into account. But the metadata may not be known at that time. Inserting metadata at a later time can require a decode and re-encode to meet bit-rate requirements. In some cases a more appropriate, higher level mechanism may be available, and if so, it should be used. For cases where a higher level mechanism is not available, it is better to provide a mechanism at the RTP level than have the meta-data be tied to a specific form of media data. Singer Expires December 17, 2006 [Page 6] Internet-Draft RTP Header Extensions June 2006 4. Packet Design The following design is fit into the "header extension" of the RTP extension, as shown above. The presence and format of this header extension is negotiated out-of-band, such as through signaling (see below for SDP signaling), and therefore the "identifier" used above is only defined here for diagnostic and cross-check purposes (e.g. by network analyzers); it is the negotiation which is the definitive indication that this header extension is present. The 16-bit identifier takes the fixed bit pattern 0xBEDE (the first draft of this specification was written on the feast day of the Venerable Bede). The header extension is formed of a set of extension elements. Each extension element has a local identifier and a length. Since it is expected that (a) the number of extensions in any given RTP session is small and (b) the extensions themselves are small, only 4 bits are allocated to each of these. The local identifiers may be mapped to a larger namespace in the negotation (e.g. session signaling). The form of the header extension block is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID | len | extension element bytes... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | .... | The 4-bit ID is the local identifier of this element; the values 0 and 15 are reserved and MUST NOT be used. The remaining values present in the stream MUST have been negotiated out-of-band. There are no static allocations of identifiers. The 4-bit length is the length minus one of the data bytes of this header extension element (excluding this one-byte header). Therefore the value zero in this field indicates that one byte of data follows, and a value of 15 (the maximum) indicates element data of 16 bytes. There are as many extension elements as fit into the length as indicated in the RTP header-extension length. Since the extension length is signaled in full 32-bit words, padding bytes are placed after the last extension element to pad to a 32-bit boundary. These padding bytes MUST have the value zero (0). Singer Expires December 17, 2006 [Page 7] Internet-Draft RTP Header Extensions June 2006 5. SDP Signalling Design The map of the local identifiers used in the packets to a larger un- allocated namespace may be performed in the session signaling. This section defines such signaling in SDP. The universe of possible names is identified by reversed domain-names (as used in Java, for example), combined with a month-date in the form mmyyyy. That date MUST be near the time of the definition of the extension, and it MUST be true that the extension was defined in a way authorized by the owner of the domain at that date. (This avoids problems when domain names change ownership). An example name (this is only an example) might be "org.example.avt.rtp.transmittime/082005". The mapping may be provided per media-stream (in the media-specific section of SDP, i.e. after an "m=" line) or globally for all streams (i.e. before the first "m=" line). The definitions MUST be either all global or all local; it is not permitted to mix the two styles. Each local identifier potentially used in the stream is mapped to a string using an attribute of the form: a=extmap: / where is a reversed domain name, is an integer in the range 1-14 inclusive (0 and 15 are reserved, as noted above), and takes the form of a 6-digit number mmyyyy. Example: a=extmap:1 org.example.avt.rtp.toffset/082005 a=extmap:2 org.ietf.avt.smpte12M/022006 When SDP signaling is used for the RTP session, it is the presence of the 'extmap' attribute(s) which is diagnostic that this style of header extensions is used, not the magic number indicated above. Rationale: the use of reversed domain names provides for a large, unallocated space, while still giving some clue as to what organization or body is responsible for the definition of the extension. Extension names starting with "org.ietf" and "arpa." are reserved to the IETF and MUST be documented in RFCs, and registered with IANA. Singer Expires December 17, 2006 [Page 8] Internet-Draft RTP Header Extensions June 2006 6. Transmission offset This specification defines only one extension element. The name is "org.ietf.avt.rtp.toffset/082005". The value is a fixed 24-bit signed integer. In the case when the sender knows that it is not transmitting the RTP packets strictly according to their relative RTP timestamps, this extension element may be used to indicate the "effective" RTP transmission time of the packet, on the RTP timescale. The value is added to the RTP timestamp of the packet to give the RTP time of transmission. There are no extension attributes defined for this name. Singer Expires December 17, 2006 [Page 9] Internet-Draft RTP Header Extensions June 2006 7. BNF Syntax The syntax element 'hostname' is as defined in [RFC2396], except that (a) numeric hostnames are not permitted and (b) the name is in reverse order (e.g. "com.example.multimedia.tag"). The syntax element 'extmap' is an attribute as defined in [RFC2327]. Extensionattributes are defined by the specification that defines a specific extension name; there may be several, separated by spaces. digit = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9" month = "01"|"02"|"03"|"04"|"05|"06"|"07"|"08"|"09"|"10|"11"|"12" year = digit digit digit digit date = month year integer = digit *(digit) space = " " extensionname = hostname "/" date mapentry = "extmap" ":" integer mapattrs = (space extensionattributes) extmap = mapentry space extensionname mapattrs Singer Expires December 17, 2006 [Page 10] Internet-Draft RTP Header Extensions June 2006 8. Security Considerations This defines only a place to transmit information; the security implications of the extensions must be discussed with those extensions. Singer Expires December 17, 2006 [Page 11] Internet-Draft RTP Header Extensions June 2006 9. IANA Considerations 9.1. New spaces for IANA to manage IANA needs to manage the extension names that start with "org.ietf" or "arpa". Any use of a name with these prefixes MUST be documented in a standards-track RFC which defines both the name and the format of the data. IANA manages the registration by documenting the association of name to RFC number. Names drawn from other spaces are managed outside both the IETF and IANA, and the handling of registration and documentation is the responsibility of the owner of the internet domain name as of the date specified in the registration. 9.2. Registration of the transmit-time offset The first registration within the IETF/IANA space is as documented above. The registered name is "org.ietf.avt.rtp.toffset/082005", associated with this specification. 9.3. Registration of the SDP extmap attribute This section contains the information required by [RFC2327] (which is identical to that required by draft-ietf-mmusic-sdp-new-26.txt) for an SDP attribute. o contact name, email address and telephone number: are as indicated for this document o attribute-name (as it will appear in SDP): is 'extmap' (without the quotation marks) o long-form attribute name in English: is "header extension map definition" o type of attribute (session level, media level, or both): both o whether the attribute value is subject to the charset attribute: no, it is not o a one paragraph explanation of the purpose of the attribute: This attribute defines the mapping from the extension numbers used in packet headers into extension names as documented in specifications and appropriately registered. o a specification of appropriate attribute values for this attribute: is as defined in this document Singer Expires December 17, 2006 [Page 12] Internet-Draft RTP Header Extensions June 2006 10. RFC Editor Considerations None. Singer Expires December 17, 2006 [Page 13] Internet-Draft RTP Header Extensions June 2006 Appendix A. Acknowledgments Both Brian Link and John Lazzaro provided helpful comments on an initial draft. Colin Perkins was helpful in reviewing and dealing with the details. Singer Expires December 17, 2006 [Page 14] Internet-Draft RTP Header Extensions June 2006 Appendix B. Change History o August 2005: 00 Draft-avt-rtp-hdrext made from draft-singer-rtp-hdrext; added requirement for dates in DNS names o January 2006: 01 Updated to XML2RFC, inserted BNF, other cleanup; IANA considerations cleaned o June 2006: 02 Made the signaling optional, and indicate only that the presence of the extensions must be negotiated somehow; more rationale o June 2006: 03 Fixed small error in the BNF 11. References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [RFC2396] Berners-Lee, MT., Fielding, R., and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, STD 0064, July 2003. Singer Expires December 17, 2006 [Page 15] Internet-Draft RTP Header Extensions June 2006 Author's Address David Singer Apple Computer Inc. 1 Infinite Loop Cupertino, CA 95014 US Phone: +1 408 996 1010 Email: singer@apple.com Singer Expires December 17, 2006 [Page 16] Internet-Draft RTP Header Extensions June 2006 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. 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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, 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. 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Singer Expires December 17, 2006 [Page 17]