Network Working Group                           Jack Moffitt
Internet-Draft                                  Xiph.org Foundation
Expire in six months                            February 2001


            RTP Payload Format for Vorbis Encoded Audio

                   <draft-moffitt-vorbis-rtp-00.txt>

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   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.

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Abstract

   This document describes a RTP payload format for transporting Vorbis
   encoded audio.

 Introduction

   This document describes how Vorbis encoded audio may be formatted for
   use as an RTP payload type.

   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 [1].

 Background

   The Xiph.org Foundation creates and defines codecs for use in
   multimedia that are not encumbered by patents and thus may be freely
   implemented by any individual or organization.

   Vorbis is the general purpose multi-channel audio codec created by
   the Xiph.org Foundation.

   Vorbis encoded audio is generally found within an Ogg format
   bitstream, which provides framing and synchronization.  For the
   purposes of RTP transport, this layer is unnecessary, and so raw
   Vorbis packets are used in the payload.

   Vorbis packets are unbounded in length currently.  At some future
   point there will likely be a practical limit placed on packet
   length.

   Typical Vorbis packet sizes are from very small (2-3 bytes) to
   quite large (8-12 kilobytes).  The reference implementation seems to
   make every packet less than ~800 bytes, except for the codebooks
   packet which is ~8-12 kilobytes.

 Payload Format

   The standard RTP header is followed by an 8 bit payload header, and
   the payload data.

3.1 RTP Header

   The following fields of the RTP header are used for Vorbis payloads:

   Payload Type (PT): 7 bits
      An RTP profile for a class of applications is expected to assign a
      payload type for this format, or a dynamically allocated payload
      type should be chosen which designates the payload as Vorbis.

   Timestamp: 32 bits
      A timestamp representing the sampling time of the first sample of
      the first Vorbis packet in the RTP packet.  The clock frequency
      MUST be set to the sample rate of the encoded audio data and is
      conveyed out-of-band.

   Marker (M): 1 bit
      Set to one if the payload contains complete packets or if it
      contains the last fragment of a fragmented packet.

3.2 Payload Header

   The first byte of the payload data is the payload header:

           +---+---+---+---+---+---+---+---+
           | C | R | R |  # of packets     |
           +---+---+---+---+---+---+---+---+

   C: 1 bit
      Set to one if this is a continuation of a fragmented packet.

   R: 1 bit x 2
      Reserved, must be set to zero by senders, and ignored by
      receivers.

   The last 5 bits are the number of complete packets in this payload.
   If C is set to one, this number should be 0.

3.3 Payload Data

   If the payload contains a single Vorbis packet or a Vorbis packet
   fragment, the Vorbis packet data follows the payload header.

   For payloads which consist of multiple Vorbis packets, payload data
   consists of one byte representing the packet length followed by the
   packet data for each of the Vorbis packets in the payload.

   The Vorbis packet length byte is the length minus one.  A value of
   0 means a length of 1.

3.4 Example RTP Packet

   Here is an example RTP packet containing two Vorbis packets.

   RTP Packet Header:

    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 8 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|X|  CC   |M|      PT     |      sequence number          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 timestamp (in sample rate units)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         sychronization source (SSRC) identifier               |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |         contributing source (CSRC) identifiers                |
   |                      ...                                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Payload Data:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|0|0| # pks: 2|      len      |  vorbis data ...              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  ...vorbis data...                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     ...       |      len      | next vorbis packet data...    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 Frame Packetizing

   Each RTP packet contains either one complete Vorbis packet, one
   Vorbis packet fragment, or an integer number of complete Vorbis
   packets (a max of 32 packets, since the number of packets is
   defined by a 5 bit value).

   Any Vorbis packet that is larger than 256 bytes and less than the
   path-MTU should be placed in a RTP packet by itself.

   Any Vorbis packet that is 256 bytes or less should be bundled in the
   RTP packet with as many Vorbis packets as will fit, up to a maximum
   of 32.

   If a packet will not fit into the RTP packet, it must be fragmented.
   A fragmented packet has a zero in the last five bits of the payload
   header.  Each fragment after the first will also set the Continued
   (C) bit to one in the payload header.  The RTP packet containing the
   last fragment of the Vorbis packet will have the Marker (M) bit set
   to one.

 Open Issues

   To decode a Vorbis stream, a set of codebooks is required.  These
   codebooks are allowed to change for each logical bitstream (for
   example, for each song encoded in a radio stream).

   The codebooks must be completely intact and a client can not decode
   a stream with an incomplete or corrupted set.

   A client connecting to a multicast RTP Vorbis session needs to get the
   first set of codebooks in some manner.  These codebooks are typically
   between 4 kilobytes and 8 kilobytes in size.

   A final solution to how best to deliver the codebooks has not yet been
   realized.  Here are the current proposals:

   - Including the first set of codebooks in the SDP description

   - Broadcasting a codebook only stream as a second multicast Vorbis
     stream

   - Create some method of requesting the codebooks via RTCP

   - Periodically retransmit the headers inline

 Security Considerations

   RTP packets using this payload format are subject to the security
   considerations discussed in the RTP specification [1].  This implies
   that the confidentiality of the media stream is achieved by using
   encryption.  Becase the data compression used with this payload
   format is applied end-to-end, encryption may be performed on the
   compressed data.

 Acknowledgements

   Thanks to the rest of the Xiph.org team, especially Monty
   <monty@xiph.org>. Thanks also to Rob Lanphier <robla@real.com> for
   his guidance.

 References

   1. RTP: A Transport Protocol for Real-Time Applications (RFC 1889)

   2. Xiph.org's Ogg Vorbis pages http://www.xiph.org/ogg/vorbis/
      Vorbis documentation only currently exists as API documenation,
      or as source code.  The source can be obtained at
      http://www.xiph.org/ogg/vorbis/download.html

 Author's Address

   Jack Moffitt
   Executive Director
   Xiph.org Foundation
   email: jack@xiph.org
   WWW: http://www.xiph.org/