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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 5285

AVT                                                            D. Singer
Internet-Draft                                       Apple Computer Inc.
Expires: June 18, 2007                                       H. Desineni
                                                       December 15, 2006

             A general mechanism for RTP Header Extensions

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document provides a general mechanism to use the header-
   extension feature of RTP (the Real Time Transport 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Design Goals . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Packet Design  . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  SDP Signalling Design  . . . . . . . . . . . . . . . . . . . .  8
   6.  Offer/Answer . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  BNF Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  New space for IANA to manage . . . . . . . . . . . . . . . 15
     9.2.  Registration of the SDP extmap attribute . . . . . . . . . 15
   10. RFC Editor Considerations  . . . . . . . . . . . . . . . . . . 16
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 17
   12. Normative References . . . . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19

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1.  Introduction

   In the RTP specification [RFC3550] there is provision for header
   extensions in section 5.3.1.

   It permits at most one extension in a given packet; the extension has
   a length in 32-bit words, and there is a 16-bit identifier 'defined
   by profile' to identify the extension in use.

   This mechanism has two conspicuous drawbacks: only one extension is
   possible, and there is no documentation of how the 16-bit identifiers
   are allocated.

   This header extension value applies to the RTP/AVP profile and its

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2.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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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

   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.

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4.  Packet Design

   The following design is fit into the "header extension" of the RTP
   extension, as described above.  The presence and format of this
   header extension is negotiated or defined 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/definition which is the definitive indication that this
   header extension is present.  For this specification the 16-bit value
   required by the RTP specification for a header extension, labelled in
   the RTP specification as "defined by profile", takes the fixed bit
   pattern 0xBEDE (the first draft of this specification was written on
   the feast day of the Venerable Bede).

   The RTP specification states that the header extension "is designed
   so that the header extension may be ignored".  This specification
   therefore inherits this requirement.  To be specific, header
   extensions using this specification MUST only be used for data that
   can safely be ignored by the recipient without affecting
   interoperability.  Examples might include meta-data that is
   additional to the usual RTP information.

   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 negotiation (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 in the range
   1-14 inclusive.  The values present in the stream MUST have been
   negotiated or defined out-of-band.  There are no static allocations
   of identifiers.  Each distinct extension MUST have a unique ID.

   The value 0 is reserved for padding and MUST NOT be used as an
   identifier.  The value 15 is reserved for future extension and MUST
   NOT be used as an identifier.  If the value 15 is encountered,

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   processing of the extension should terminate at that point, and only
   the extension elements present prior to the ID=15 considered.

   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 RTP header
   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.

   Padding bytes have the value of 0 (zero).  They may be placed between
   extension elements, if desired for alignment, or after the last
   extension element, as needed to pad to full 32-bit words.  A padding
   byte is not interpreted as the ID of an ID/length pair, and no length
   byte follows it.  When a padding byte is found it is ignored and the
   parser moves on to interpreting the next byte.

   As is good network practice, data should only be transmitted when
   needed.  The RTP header extension should only be present in a packet
   if that packet also contains one or more extension elements, as
   defined here.  An extension element should only be present in a
   packet when needed; the signaling setup of extension elements
   indicates only that those elements may be present in some packets,
   not that they are in fact present in all (or indeed, any) packets.

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5.  SDP Signalling Design

   The mapping of local identifiers used in the header extension to a
   larger namespace MUST be performed out of band, for example as part
   of a SIP offer/answer exchange using SDP.  This section defines such
   signaling in SDP.

   A usable mapping MUST use IDs in the range 1-14, and each ID in this
   range MUST be used only once for each media (or only once if the
   mappings are session level).  Mappings which do not conform to these
   rules MAY be presented, for instance during offer/answer negotiation
   as described in the next section, but remapping to conformant values
   will be necessary before they can be applied.

   Each extension is named by a URI.  That URI MUST be absolute, and
   should precisely identify the format and meaning of the extension.
   In general, the URI SHOULD also be de-referencable by any system that
   sees or receives the SDP containing it.  URIs that contain a domain
   name SHOULD also contain 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 name at that date.  (This avoids problems when
   domain names change ownership).  If the resource or document defines
   several extensions, then the URI MUST identify the actual extension
   in use, e.g. using a fragment or query identifier (characters after a
   '#' or '?' in the URI).

   An extension URI MUST NOT appear more than once applying to the same
   stream, i.e. at session level or in the declarations for a single
   stream at media level.  (The same extension may, of course, be used
   for several streams.)

   For extensions defined in RFCs, the URI used SHOULD be a URN starting
   "urn:ietf:params:rtp-hdrext:" and followed by a registered,
   descriptive name.  These URNs are managed by IANA.  An example (this
   is only an example), where 'avt-example-metadata' is the hypothetical
   name of a header extension, might be:


   An example name not from the IETF (this is only an example) might be


   The mapping may be provided per media-stream (in the media level
   section(s) of SDP, i.e. after an "m=" line) or globally for all
   streams (i.e. before the first "m=" line, at session level).  The
   definitions MUST be either all session level or all media level; it

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   is not permitted to mix the two styles.  In addition, as noted above,
   the IDs used MUST be unique for each stream type for a given media,
   or for the session for session level declarations.

   Each local identifier potentially used in the stream is mapped to a
   string using an attribute of the form:

   a=extmap:<value>["/"<direction>] <URI> <extensionattributes>

   where <URI> is a URI, as above, <value> is the local identifier (ID)
   of this extension, and is an integer in the range 1-14 inclusive (0
   and 15 are reserved, as noted above), and <direction> is one of
   "sendonly", "recvonly", "sendrecv", "inactive" (without the quotes).

   The formal BNF syntax is presented in a later section of this


   a=extmap:1 http://example.com/082005/ext.htm#ttime

   a=extmap:2/sendrecv http://example.com/082005/ext.htm#xmeta short

   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 URIs provides for a large, unallocated space,
   gives documentation on the extension.  The URIs are not required to
   be de-referencable in order to permit confidential or experimental
   use, and to cover the case when extensions continue to be used after
   the organization that defined them ceases to exist.

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6.  Offer/Answer

   The simple signaling described above may be enhanced in an offer/
   answer context, to permit:

   o  asymmetric behavior (extensions sent in only one direction);

   o  the offer of mutually-exclusive alternatives;

   o  the offer of more extensions than can be sent in a single session.

   A direction attribute may be included in an extmap; without it, the
   direction implicitly inherits, of course, from the stream direction,
   or is "sendrecv" for session level attributes or extensions of
   "inactive" streams.  The direction may be one of "sendonly",
   "recvonly", "sendrecv", "inactive".  A "sendonly" direction indicates
   an ability to send; a "recvonly" direction indicates a desire to
   receive; a "sendrecv" direction indicates both.  An "inactive"
   direction indicates neither, but later re-negotiation may make an
   extension active.

   Extensions, with their directions, may be signaled for an "inactive"
   stream.  It is an error to use an extension direction incompatible
   with the stream direction (e.g. a "sendonly" attribute for a
   "recvonly" stream).

   If an offer or answer contains session level mappings (and hence no
   media level mappings), and different behavior is desired for each
   stream, then the entire set of extension map declarations may be
   moved into the media level section(s) of the SDP.  (Note that this
   specification does not permit mixing global and local declarations,
   to make identifier management easier).

   If an extension map is offered as "sendrecv", explicitly or
   implicitly, and asymmetric behavior is desired, the SDP may be
   modified to modify or add direction qualifiers for that extension.

   If an extension is marked as "sendonly" and the answerer desires to
   receive it, the extension MUST be marked as "recvonly" in the SDP
   answer.  An answerer which has no desire to receive the extension or
   does not understand the extension SHOULD remove it from the SDP

   If an extension is marked as "recvonly" and the answerer desires to
   send it, the extension MUST be marked as "sendonly" in the SDP
   answer.  An answerer which has no desire to, or is unable to, send
   the extension SHOULD remove it from the SDP answer.

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   Identifiers in the range 1-14 inclusive in an offer or answer must
   not be used more than once per media section (including the session
   level section).  A session update MAY change the direction qualifiers
   of extensions under use.  A session update MAY add or remove
   extension(s).  Identifiers values in the range 1-14 MUST NOT be
   altered (remapped).

   Note that, under this rule, the same identifier cannot be used for
   two extensions for the same media, even when one is "sendonly" and
   the other "recvonly", as it would then be impossible to make either
   of then sendrecv (since re-numbering is not permitted either).

   If a party wishes to offer mutually exclusive alternatives, then
   multiple extensions with the same identifier in the (unusable) range
   4096-4351 may be offered; the answerer should select at most one of
   the offered extensions with the same identifier, and remap it to a
   free identifier in the range 1-14, for that extension to be usable.

   Similarly, if more than 14 extensions are offered, identifiers in the
   range 4096-4351 may be offered; the answerer should choose those that
   are desired, and remap them to a free identifier in the range 1-14.

   It is always allowed to place the offered identifier value "as is" in
   the SDP answer (for example, due to lack of a free identifier value
   in the range 1-14).  Extensions with an identifier outside the range
   1-14 cannot, of course, be used.  If required, the offerer or
   answerer can update the session to make space for such an extension.

   Rationale: the range 4096-4351 for these negotiation identifiers is
   deliberately restricted to allow expansion of the range of valid
   identifiers in future (e.g. by using a full byte for an ID).

   Note that either party may include extensions in the stream other
   than those negotiated, or those negotiated as "inactive", for example
   for the benefit of intermediate nodes.  Only extensions with an
   identifier in the range 1-14, and with a declaration, can be sent.

   Example (port numbers, RTP profiles, payload IDs and rtpmaps etc. all
   omitted for brevity):

   The offer:

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   a=extmap:1 URI-toffset
   a=extmap:14 URI-obscure
   a=extmap:4096 URI-gps-string
   a=extmap:4096 URI-gps-binary
   a=extmap:4097 URI-frametype

   The answerer is interested in receiving GPS in string format only on
   video, but cannot send GPS at all.  They are not interested in
   transmission offsets on audio, and do not understand the URI-obscure
   extension.  They therefore move the extensions from session level to
   media level, and adjust the declarations:

   a=extmap:1 URI-toffset
   a=extmap:2/recvonly URI-gps-string
   a=extmap:3 URI-frametype
   a=extmap:1/sendonly URI-toffset

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7.  BNF Syntax

   The syntax element 'URI-reference' is as defined in [RFC3986], except
   that only absolute URIs are permitted here.  The syntax element
   'extmap' is an attribute as defined in [RFC4566].
   Extensionattributes are not defined here, but by the specification
   that defines a specific extension name; there may be several,
   separated by spaces.

     space = " "

     extensionname = URI-reference

     direction = "sendonly" | "recvonly" | "sendrecv" | "inactive"

     mapentry = "extmap" ":" integer ("/" direction)

     mapattrs = (space extensionattributes)

     extmap = mapentry space extensionname mapattrs

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8.  Security Considerations

   This defines only a place to transmit information; the security
   implications of the extensions must be discussed with those

   Care should be taken when defining extensions.  Clearly, they should
   be solely informative, but even when the information is extracted,
   should not cause security concerns.

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9.  IANA Considerations

9.1.  New space for IANA to manage

   The rtp-hdrext namespace under urn:ietf:params: needs to be created
   for management, referenced to RFCxxxx.

   Note: Names drawn from other spaces than the IETF 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; no IANA
   action is required for these names.

9.2.  Registration of the SDP extmap attribute

   This section contains the information required by [RFC4566] for an
   SDP attribute.

   o  contact name, email address and telephone number: D. Singer,
      singer@apple.com, +1 408-974-3162

   o  attribute-name (as it will appear in SDP): extmap

   o  long-form attribute name in English: generic header extension map

   o  type of attribute (session level, media level, or both): both

   o  whether the attribute value is subject to the charset attribute:
      not subject to the charset attribute

   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: see RFCxxxx.

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10.  RFC Editor Considerations

   RFCxxxx in the IANA considerations needs to be replaced with the RFC
   number (two places).

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11.  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.  The use of URNs for IETF-defined extensions was
   suggested by Jonathan Lennox, and Pete Cordell was instrumental in
   improving the padding wording.

12.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", RFC 3550, STD 0064, July 2003.

   [RFC3986]  Berners-Lee, MT., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", RFC 3986,
              January 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

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Authors' Addresses

   David Singer
   Apple Computer Inc.
   1 Infinite Loop
   Cupertino, CA  95014

   Phone: +1 408 996 1010
   Email: singer@apple.com
   URI:   http://www.apple.com/quicktime

   Harikishan Desineni
   5775 Morehouse Drive
   San Diego, CA  92126

   Phone: +1 858 845 8996
   Email: hd@qualcomm.com
   URI:   http://www.qualcomm.com

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Intellectual Property Statement

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   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
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Disclaimer of Validity

   This document and the information contained herein are provided on an

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.


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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