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Internet Engineering Task Force
INTERNET-DRAFT                                                 D. Singer
draft-ietf-avt-rtp-hdrext-00.doc                          Apple Computer

                                                             Aug 11 2005
                                                    Expires: Feb 11 2006

             A general mechanism for RTP Header Extensions

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   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.

D. Singer                                                       [Page 1]

Internet Draft         draft-avt-rtp-hdrext-00.doc         Aug 11 2005


   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.

1 Introduction

   In the RTP specification [1] 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

D. Singer                                                       [Page 2]

Internet Draft         draft-avt-rtp-hdrext-00.doc         Aug 11 2005

   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
   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.

2 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.

3 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 signaled out-of-band (see below for a suggested 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 signaling 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 are mapped to a
   larger namespace in the session signaling.

D. Singer                                                       [Page 3]

Internet Draft         draft-avt-rtp-hdrext-00.doc         Aug 11 2005

   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 defined out-of-band (see below)
   in some kind of signaling.  There are no static allocations of

   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).

4 Signalling Design

   The map of the local identifiers used in the packets to a larger un-
   allocated namespace is performed in the session signaling.  The
   universe of possible names is identified by reversed domain-names (as
   used in Java, for example).  These names MUST include 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

   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

D. Singer                                                       [Page 4]

Internet Draft         draft-avt-rtp-hdrext-00.doc         Aug 11 2005

   string using an attribute of the form:

   a=EXTMAP <name>:<value>

   where <name> is a reversed domain name, and value is an integer in
   the range 1-14 inclusive (0 and 15 are reserved, as noted above).


   a=EXTMAP org.example.082005.avt.rtp.toffset:1
   a=EXTMAP org.smpte.082005.12M:2

   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 names starting with "org.ietf" and "arpa." are reserved to
   the IETF and MUST be documented in RFCs.

5 Transmission Offset

   This specification defines only one extension element.  The name is
   "org.ietf.082005.avt.rtp.toffset".  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.

6 Security Considerations

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

6 IANA Considerations

   Potentially management of the "ietf.org." and "arpa." namespaces.

7 RFC Editor Considerations

D. Singer                                                       [Page 5]

Internet Draft         draft-avt-rtp-hdrext-00.doc         Aug 11 2005


8 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

9 Intellectual Property Notice

   The IETF takes no position regarding the validity or scope of any
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   The IETF invites any interested party to bring to its attention any
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   this standard. Please address the information to the IETF at ietf-

10 References
   RFC3550, STD0064, RTP: A Transport Protocol for Real-Time

D. Singer                                                       [Page 6]

Internet Draft         draft-avt-rtp-hdrext-00.doc         Aug 11 2005

   Applications, H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
   July 2003


   Both Brian Link and John Lazzaro provided helpful comments on an
   initial draft.

D. Singer                                                       [Page 7]

Internet Draft         draft-avt-rtp-hdrext-00.doc         Aug 11 2005

Authors' Contact Information
   David Singer
   Apple Computer, Inc.
   One Infinite Loop, MS:302-3MT
   Cupertino  CA 95014
   Email: singer@apple.com
   Tel: +1 408 974 3162

Change History

   August 2005: Draft-avt-rtp-hdrext made from draft-singer-rtp-hdrext;
   added requirement for dates in DNS names

                             Written: Aug 11 2005
                             Expires: Feb 11 2006

D. Singer                                                       [Page 8]

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