AVT                                                            D. Singer
Internet-Draft                                       Apple Computer Inc.
Intended status: Standards Track                             H. Desineni
Expires: August 30, 2007                                        Qualcomm
                                                       February 26, 2007

             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 IETF Trust (2007).

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   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
   registration is de-centralized.  The actual extensions in use in a
   session are signaled in the setup information for that session.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Design Goals . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Packet Design  . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  SDP Signalling Design  . . . . . . . . . . . . . . . . . . . .  9
   6.  Offer/Answer . . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  BNF Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  New space for IANA to manage . . . . . . . . . . . . . . . 16
     9.2.  Registration of the SDP extmap attribute . . . . . . . . . 16
   10. RFC Editor Considerations  . . . . . . . . . . . . . . . . . . 18
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   12. Normative References . . . . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
   Intellectual Property and Copyright Statements . . . . . . . . . . 22

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1.  Introduction

   The RTP Specification [RFC3550] provides a capability to extend the
   RTP header.  It defines the header extension format and rules for its
   use in section 5.3.1.  The existing header extension method permits
   at most one extension per RTP packet, identified by a 16-bit
   identifier and a 16-bit length field specifying the length of the
   header extension in 32-bit words.

   This mechanism has two conspicuous drawbacks.  First, it permits only
   one header extension in a single RTP packet.  Second, the
   specification gives no guidance as to how the 16-bit header extension
   identifiers are allocated to avoid collisions.

   This specification removes the first drawback by defining a backward-
   compatible and extensible means to carry multiple header extension
   elements in a single RTP packet.  It removes the second drawback by
   defining that these extension elements are labelled by URNs, defines
   an IANA registry for extension elements defined in IETF
   specifications, and an SDP method for mapping between the URNs and
   the identifier values carried in the RTP packets.

   This header extension 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 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 presence and format of this header extension and its contents is
   negotiated or defined out-of-band, such as through signaling (see
   below for SDP signaling).  The fixed value defined above is only an
   architectural constant (e.g. for use by by network analyzers); it is
   the negotiation/definition (e.g. in SDP) which is the definitive
   indication that this header extension is present.

   This specification inherits the requirement from the RTP
   specification that the header extension "is designed so that the
   header extension may be ignored".  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

   The RTP header extension is formed as a sequence of extension
   elements, with possible padding.  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).

   Each extension element starts with a byte containing an ID and a

       0 1 2 3 4 5 6 7
      |  ID   |  len  |

   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

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   The value 15 is reserved for future extension and MUST NOT be used as
   an identifier.  If the ID value 15 is encountered, its length field
   should be ignored, processing of the entire extension should
   terminate at that point, and only the extension elements present
   prior to the element with ID 15 considered.

   The 4-bit length is the number minus one of data bytes of this header
   extension element following the 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.  (This
   permits carriage of 16-byte values, which is a common length of
   labels and identifiers, while losing the possibility of zero-length
   values - which would often be padded anyway.)

   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, if needed for padding.  The first four bits of a
   padding byte are not interpreted as the ID of an ID/length pair, nor
   are the second four bits interpreted as a length field.  When a
   padding byte is found it is ignored and the parser moves on to
   interpreting the next byte.

   A sequence of extension elements, possibly with padding, forms the
   header extension defined in the RTP specification.  There are as many
   extension elements as fit into the length as indicated in the RTP
   header-extension length.  Since this length is signaled in full 32-
   bit words, padding bytes are used to pad to a 32-bit boundary.  The
   entire extension is parsed byte-by-byte to find each extension
   element (no alignment is required), and parsing stops at the earlier
   of the end of the entire header extension, or on encountering an
   identifier with the reserved value of 15.

   An example header extension, with three extension elements, some
   padding, and including the required RTP fields, 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
      |       0xBE    |    0xDE       |           length=3            |
      |  ID   | L=0   |     data      |  ID   |  L=1  |   data...
            ...data   |    0 (pad)    |    0 (pad)    |  ID   | L=3   |
      |                          data                                 |

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   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 indication of the presence of this extension, and 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

   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
   is necessary before they can be applied.

   Each extension is named by a URI.  That URI MUST be absolute, and
   precisely identifies 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.  The
   definition of the element and assignment of the URI MUST have been
   authorized by the owner of the domain name on or very close to 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).

   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.

   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

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   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
   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.

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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 MUST 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 them 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).

   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 that appeared with an
   identifier in the range 1-14 in SDP originated by the sender can be

   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.

     digit = "0"/"1"/"2"/"3"/"4"/"5"/"6"/"7"/"8"/"9"

     integer = 1*digit

     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.

   Header extensions have the same security coverage as the RTP header
   itself.  When SRTP [RFC3711] is used to protect RTP sessions, the RTP
   payload may be both encrypted and integrity protected, while the RTP
   header is either unprotected or integrity protected.  Therefore, it
   is inappropriate to place information in header extensions which
   cause security problems if disclosed, unless the entire RTP packet is
   protected by a lower-layer security protocol providing both
   confidentiality and integrity capability.

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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.  Additions in this namespace
   shall be made on the basis of "Specification Required".

   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.

   The IANA will also maintain a server available via at least HTTP and
   FTP that contains all of the registered elements in some publicly
   accessible space in the same way that all of the IANA's registered
   elements are available via http://www.iana.org/assignments/.  The
   suggested path is

   Here is the formal declaration required by the IETF URN Sub-namespace
   specification [RFC3553].

   o  Registry name: urn:ietf:params:rtp-hdrext: (RTP header extensions
      defined by the IETF)

   o  Specification: RFCxxxx and RFCs updating RFCxxxx.

   o  Repository: see above.

   o  Index value: -- The index value is an absolute URI, chosen for
      uniqueness within the parameter space.

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

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   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.  Dave Oran provided feedback and text
   in the final review.

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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.

   [RFC3553]  Mealling, T., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", RFC 3553, June 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [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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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   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

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