



AVT Working Group                                             L. Barbato
Internet-Draft                                                  Xiph.Org
Expires: April 24, 2006                                 October 21, 2005


                      draft-kerr-avt-vorbis-rtp-05
              RTP Payload Format for Vorbis Encoded Audio

Status of this Memo

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   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on April 24, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document describes an RTP payload format for transporting Vorbis
   encoded audio.  It details the RTP encapsulation mechanism for raw
   Vorbis data and details the delivery mechanisms for the decoder
   probability model, referred to as a codebook and other setup
   information.

   Also included within the document are the necessary details for the
   use of Vorbis with MIME and Session Description Protocol (SDP).




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Editors Note

   All references to RFC XXXX are to be replaced by references to the
   RFC number of this memo, when published.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Payload Format . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  RTP Header . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Payload Header . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Payload Data . . . . . . . . . . . . . . . . . . . . . . .  6
     2.4.  Example RTP Packet . . . . . . . . . . . . . . . . . . . .  7
   3.  Configuration Headers  . . . . . . . . . . . . . . . . . . . .  8
     3.1.  In-band Header Transmission  . . . . . . . . . . . . . . .  9
       3.1.1.  Packed Configuration . . . . . . . . . . . . . . . . .  9
     3.2.  Out of Band Transmission . . . . . . . . . . . . . . . . . 10
       3.2.1.  Packed Headers . . . . . . . . . . . . . . . . . . . . 10
     3.3.  Loss of Configuration Headers  . . . . . . . . . . . . . . 12
   4.  Comment Headers  . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Frame Packetizing  . . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Example Fragmented Vorbis Packet . . . . . . . . . . . . . 14
     5.2.  Packet Loss  . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
     6.1.  Mapping MIME Parameters into SDP . . . . . . . . . . . . . 18
   7.  Congestion Control . . . . . . . . . . . . . . . . . . . . . . 19
   8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  Stream Radio . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     11.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Intellectual Property and Copyright Statements . . . . . . . . . . 23














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1.  Introduction

   Vorbis is a general purpose perceptual audio codec intended to allow
   maximum encoder flexibility, thus allowing it to scale competitively
   over an exceptionally wide range of bitrates.  At the high quality/
   bitrate end of the scale (CD or DAT rate stereo, 16/24 bits), it is
   in the same league as MPEG-2 and MPC.  Similarly, the version 1.1
   reference encoder can encode high-quality CD and DAT rate stereo at
   below 48k bits/sec without resampling to a lower rate.  Vorbis is
   also intended for lower and higher sample rates (from 8kHz telephony
   to 192kHz digital masters) and a range of channel representations
   (monaural, polyphonic, stereo, quadraphonic, 5.1, ambisonic, or up to
   255 discrete channels).

   Vorbis encoded audio is generally encapsulated within an Ogg format
   bitstream [1], 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.

1.1.  Terminology

   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 [2].


2.  Payload Format

   For RTP based transportation of Vorbis encoded audio the standard RTP
   header is followed by a 4 octet payload header, then the payload
   data.  The payload headers are used to associate the Vorbis data with
   its associated decoding codebooks as well as indicating if the
   following packet contains fragmented Vorbis data and/or the the
   number of whole Vorbis data frames.  The payload data contains the
   raw Vorbis bitstream information.

2.1.  RTP Header

   The format of the RTP header is specified in [3] and shown in Figure
   Figure 1.  This payload format uses the fields of the header in a
   manner consistent with that specification.










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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |       sequence number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           timestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: RTP Header

   The RTP header begins with an octet of fields (V, P, X, and CC) to
   support specialized RTP uses (see [3] and [4] for details).  For
   Vorbis RTP, the following values are used.

   Version (V): 2 bits

   This field identifies the version of RTP.  The version used by this
   specification is two (2).

   Padding (P): 1 bit

   Padding MAY be used with this payload format according to section 5.1
   of [3].

   Extension (X): 1 bit

   The Extension bit is used in accordance with [3].

   CSRC count (CC): 4 bits

   The CSRC count is used in accordance with [3].

   Marker (M): 1 bit

   Set to zero.  Audio silence suppression not used.  This conforms to
   section 4.1 of [12].

   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.




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   Sequence number: 16 bits

   The sequence number increments by one for each RTP data packet sent,
   and may be used by the receiver to detect packet loss and to restore
   packet sequence.  This field is detailed further in [3].

   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 as a SDP attribute.

   SSRC/CSRC identifiers:

   These two fields, 32 bits each with one SSRC field and a maximum of
   16 CSRC fields, are as defined in [3].

2.2.  Payload Header

   After the RTP Header section the following 4 octets are the Payload
   Header.  This header is split into a number of bitfields detailing
   the format of the following payload data packets.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Ident                     | F |VDT|# pkts.|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 2: Payload Header

   Ident: 24 bits

   This 24 bit field is used to associate the Vorbis data to a decoding
   Configuration.

   Fragment type (F): 2 bits

   This field is set accordingly the following list

      0 = Not Fragmented
      1 = Start Fragment
      2 = Continuation Fragment
      3 = End Fragment

   Vorbis Data Type (VDT): 2 bits




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   This field sets the packet payload type for the Vorbis data.  There
   are currently three type of Vorbis payloads.

      0 = Raw Vorbis payload
      1 = Vorbis Packed Configuration payload
      2 = Legacy Vorbis Comment payload
      3 = Reserved

   The last 4 bits are the number of complete packets in this payload.
   This provides for a maximum number of 15 Vorbis packets in the
   payload.  If the packet contains fragmented data the number of
   packets MUST be set to 0.

2.3.  Payload Data

   Raw Vorbis packets are unbounded in length currently, although at
   some future point there will likely be a practical limit placed on
   them.  Typical Vorbis packet sizes are from very small (2-3 bytes) to
   quite large (8-12 kilobytes).  The reference implementation [11]
   typically produces packets less than ~800 bytes, except for the setup
   header packets which are ~4-12 kilobytes.  Within an RTP context the
   maximum packet size, including the RTP and payload headers, SHOULD be
   kept below the path MTU to avoid packet fragmentation.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            length             |       vorbis packet data     ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3: Payload Data Header

   Each Vorbis payload packet starts with a two octet length header,
   which is used to represent the size of the following data payload,
   followed by the raw Vorbis data padded to the nearest byte boundary.

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

   The Vorbis packet length header is the length of the Vorbis data
   block only and does not count the length field.

   The payload packing of the Vorbis data packets MUST follow the
   guidelines set-out in [4] where the oldest packet occurs immediately
   after the RTP packet header.

   Channel mapping of the audio is in accordance with the Vorbis I



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   Specification [12].

2.4.  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 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | 2 |0|0|  0    |0|      PT     |       sequence number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               timestamp (in sample rate units)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronisation source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: Example Packet (RTP Headers)

   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Ident                     | 0 | 0 | 2 pks |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             length            |          vorbis data         ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        vorbis data                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            length             |   next vorbis packet data    ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        vorbis data                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5: Example Packet (Payload Data)

   The payload data section of the RTP packet starts with the 24 bit
   Ident field followed by the one octet bitfield header, which has the
   number of Vorbis frames set to 2.  Each of the Vorbis data frames is
   prefixed by the two octet length field.  The Packet Type and Fragment
   Type are set to 0.  The decode Configuration that will be used to
   decode the packets is the one indexed by the ident value.




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3.  Configuration Headers

   Unlike other mainstream audio codecs Vorbis has no statically
   configured probability model.  Instead, it packs all entropy decoding
   configuration, VQ and Huffman models into a data block that must be
   transmitted to the decoder along with the compressed data.  A decoder
   also requires identification information detailing the number of
   audio channels, bitrates and other information to configure itself
   for a particular compressed data stream.  These two blocks of
   information are often referred to collectively as the "codebooks" for
   a Vorbis stream, and are nominally included as special "header"
   packets at the start of the compressed data.

   Thus these two codebook header packets must be received by the
   decoder before any audio data can be interpreted.  In addition, the
   Vorbis I specification [12] requires the presense of a comment header
   packet which gives simple metadata about the stream.  This
   requirement poses problems in RTP, which is often used over
   unreliable transports.

   Since this information must be transmitted reliably and, as the RTP
   stream may change certain configuration data mid-session, there are
   different methods for delivering this configuration data to a client,
   both in-band and out-of-band which is detailed below.  SDP delivery
   is used to setup an initial state for the client application.  The
   changes may be due to different codebooks as well as different
   bitrates of the stream.

   The delivery vectors in use are specified by an SDP attribute to
   indicate the method and the optional URI where the Vorbis Packed
   Configuration (Section 3.1.1) Packets could be fetched.  Different
   delivery methods MAY be advertised for the same session.  The in-band
   Configuration delivery SHOULD be considered as baseline, out-of-band
   delivery methods that don't use RTP will not be described in this
   document.  For non chained streams, the Configuration delivery method
   RECOMMENDED is inline the Packed Configuration (Section 3.1.1) in the
   SDP as explained in the IANA considerations (Section 6.1) section.

   The 24 bit Ident field is used to map which Configuration will be
   used to decodea packet.  When the Ident field changes, it indicates
   that a change in the stream has taken place.  The client application
   MUST have in advance the correct configuration and if the client
   detects a change in the Ident value and does not have this
   information it MUST NOT decode the raw Vorbis data associated until
   it fetches the correct Configuration.






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3.1.  In-band Header Transmission

   The Packed Configuration (Section 3.1.1) Payload is sent in-band with
   the packet type bits set to match the payload type.  Clients MUST be
   capable of dealing with fragmentation and periodic re-transmission of
   the configuration headers.

3.1.1.  Packed Configuration

   A Vorbis Packed Configuration is indicated with the payload type
   field set to 1.  Of the three headers, defined in the Vorbis I
   specification [12], the identification and the setup will be packed
   together, the comment header is completely suppressed.  Is up to the
   client provide a minimal size comment header to the decoder if
   required by the implementation.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |             xxxx              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Ident                    | 0 | 1 |      1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           length              |        Identification       ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Identification                       ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Identification                       ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Identification                       ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..              |                       Setup                  ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                            Setup                            ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                            Setup                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6: Packed Configuration Figure




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   The Ident field is set with the value that will be used by the Raw
   Payload Packets to address this Configuration.  The Fragment type is
   set to 0 since the packet bears the full Packed configuration, the
   number of packet is set to 1.

3.2.  Out of Band Transmission

   This section, as stated before, won't cover all the possible out-of-
   band delivery methods since they rely to different protocols and be
   linked to a specific application.  The following packet definition
   SHOULD be used in out-of-band delivery and MUST be used when
   Configuration is inlined in the SDP.

3.2.1.  Packed Headers

   As mentioned above the RECOMMENDED delivery vector for Vorbis
   configuration data is via a retrieval method that can be performed
   using a reliable transport protocol.  As the RTP headers are not
   required for this method of delivery the structure of the
   configuration data is slightly different.  The packed header starts
   with a 32 bit count field which details the number of packed headers
   that are contained in the bundle.  Next is the Packed header payload
   for each chained Vorbis stream.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Number of packed headers                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Packed header                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Packed header                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7: Packed Headers Overview

   Since the Configuration Ident and the Identification Header are fixed
   length there is only a 2 byte length tag to define the length of the
   packed headers.












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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Ident                       |              ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..   length     |              Identification Header           ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                    Identification Header                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Setup Header                        ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                         Setup Header                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 8: Packed Headers Detail

   The key difference between the in-band format and this one, is there
   is no need for the payload header octet.

3.2.1.1.  Packed Headers IANA Considerations

   The following IANA considerations MUST only be applied to the packed
   headers.

   MIME media type name: audio

   MIME subtype: vorbis-config

   Required Parameters:

   None.

   Optional Parameters:

   None.

   Encoding considerations:

   This type is only defined for transfer via non RTP protocols as
   specified in RFC XXXX.

   Security Considerations:

   See Section 6 of RFC 3047.

   Interoperability considerations: none

   Published specification:



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   See RFC XXXX for details.

   Applications which use this media type:

   Vorbis encoded audio, configuration data.

   Additional information: none

   Person & email address to contact for further information:

   Luca Barbato: <lu_zero@gentoo.org>

   Intended usage: COMMON

   Author/Change controller:

   Author: Luca Barbato

   Change controller: IETF AVT Working Group

3.3.  Loss of Configuration Headers

   Unlike the loss of raw Vorbis payload data, loss of a configuration
   header can lead to a situation where it will not be possible to
   successfully decode the stream.

   Loss of Configuration Packet results in the halting of stream
   decoding and SHOULD be reported to the client as well as a loss
   report sent via RTCP.


4.  Comment Headers

   With the payload type flag set to 2, this indicates that the packet
   contain the comment metadata, such as artist name, track title and so
   on.  These metadata messages are not intended to be fully descriptive
   but to offer basic track/song information.  Clients MAY ignore it
   completely.  The details on the format of the comments can be found
   in the Vorbis documentation [12].












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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |             xxxx              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Ident                    | 0 | 2 |      1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            length             |            Comment           ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                           Comment                           ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                           Comment                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 9: Comment Packet

   The 2 bytes length field is necessary since this packet could be
   fragmented.


5.  Frame Packetizing

   Each RTP packet contains either one Vorbis packet fragment, or an
   integer number of complete Vorbis packets (up to a max of 15 packets,
   since the number of packets is defined by a 4 bit value).

   Any Vorbis data packet that is less than path MTU SHOULD be bundled
   in the RTP packet with as many Vorbis packets as will fit, up to a
   maximum of 15.  Path MTU is detailed in [6] and [7].

   If a Vorbis packet, not only data but also Configuration and Comment,
   is larger than 65535 octets it MUST be fragmented.  A fragmented
   packet has a zero in the last four bits of the payload header.  The
   first fragment will set the Fragment type to 1.  Each fragment after
   the first will set the Fragment type to 2 in the payload header.  The
   RTP packet containing the last fragment of the Vorbis packet will
   have the Fragment type set to 3.  To maintain the correct sequence
   for fragmented packet reception the timestamp field of fragmented
   packets MUST be the same as the first packet sent, with the sequence
   number incremented as normal for the subsequent RTP packets.  The



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   length field shows the fragment length.

5.1.  Example Fragmented Vorbis Packet

   Here is an example fragmented Vorbis packet split over three RTP
   packets.  Each packet contains the standard RTP headers as well as
   the 4 octet Vorbis headers.

      Packet 1:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |           1000                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Ident                   | 1 | 0 |      0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             length            |            vorbis data       ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        vorbis data                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 10: Example Fragmented Packet (Packet 1)

   In this packet the initial sequence number is 1000 and the timestamp
   is xxxxx.  The Fragment type is set to 1, the number of packets field
   is set to 0, and as the payload is raw Vorbis data the VDT field is
   set to 0.















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      Packet 2:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |           1001                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Ident                   | 2 | 0 |      0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             length            |          vorbis data         ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        vorbis data                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 11: Example Fragmented Packet (Packet 2)

   The Fragment type field is set to 2 and the number of packets field
   is set to 0.  For large Vorbis fragments there can be several of
   these type of payload packets.  The maximum packet size SHOULD be no
   greater than the path MTU, including all RTP and payload headers.
   The sequence number has been incremented by one but the timestamp
   field remains the same as the initial packet.





















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      Packet 3:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |           1002                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Ident                    | 3 | 0 |      0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             length            |          vorbis data         ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        vorbis data                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 12: Example Fragmented Packet (Packet 3)

   This is the last Vorbis fragment packet.  The Fragment type is set to
   3 and the packet count remains set to 0.  As in the previous packets
   the timestamp remains set to the first packet in the sequence and the
   sequence number has been incremented.

5.2.  Packet Loss

   As there is no error correction within the Vorbis stream, packet loss
   will result in a loss of signal.  Packet loss is more of an issue for
   fragmented Vorbis packets as the client will have to cope with the
   handling of the Fragment Type.  In case of loss of fragments the
   client MUST discard all the remaining fragments and decode the
   incomplete packet.  If we use the fragmented Vorbis packet example
   above and the first packet is lost the client MUST detect that the
   next packet has the packet count field set to 0 and the Fragment type
   2 and MUST drop it.  The next packet, which is the final fragmented
   packet, MUST be dropped in the same manner.  If the missing packet is
   the last, the received two fragments will be kept and the incomplete
   vorbis packet decoded.  Feedback reports on lost and dropped packets
   MUST be sent back via RTCP.

   If a particular multicast session has a large number of participants
   care must be taken to prevent an RTCP feedback implosion, [10], in
   the event of packet loss from a large number of participants.



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   Loss of any of the Configuration fragment will result in the loss of
   the full Configuration packet with the result detailed in the Loss of
   Configuration Headers (Section 3.3) section.


6.  IANA Considerations

   MIME media type name: audio

   MIME subtype: vorbis

   Required Parameters:

   delivery-method: indicates the delivery methods in use, the possible
   values are:inline, in_band, out_band

   configuration: the base16 [9] (hexadecimal) representation of the
   Packed Headers (Section 3.2.1).

   Optional Parameters:

   configuration-uri: the URI of the configuration headers in case of
   out of band transmission.  In the form of
   "protocol://path/to/resource/".  Depending on the specific method the
   single ident packet could be retrived by their number, or aggregated
   in a single stream.

   Encoding considerations:

   This type is only defined for transfer via RTP as specified in RFC
   XXXX.

   Security Considerations:

   See Section 6 of RFC 3047.

   Interoperability considerations: none

   Published specification:

   See the Vorbis documentation [12] for details.

   Applications which use this media type:

   Audio streaming and conferencing tools

   Additional information: none




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   Person & email address to contact for further information:

   Luca Barbato: <lu_zero@gentoo.org>

   Intended usage: COMMON

   Author/Change controller:

   Author: Luca Barbato

   Change controller: IETF AVT Working Group

6.1.  Mapping MIME Parameters into SDP

   The information carried in the MIME media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [5], which is commonly used to describe RTP sessions.  When SDP is
   used to specify sessions the mapping are as follows:

   o  The MIME type ("audio") goes in SDP "m=" as the media name.

   o  The MIME subtype ("vorbis") goes in SDP "a=rtpmap" as the encoding
      name.

   o  The parameter "rate" also goes in "a=rtpmap" as clock rate.

   o  The parameter "channels" also goes in "a=rtpmap" as channel count.

   o  The mandated parameters "delivery-method" and "configuration" MUST
      be included in the SDP "a=fmpt" attribute.

   o  The optional parameter "configuration-uri", when present, MUST be
      included in the SDP "a=fmpt" attribute.

   If the stream comprises chained Vorbis files and all of them are
   known in advance, the Configuration Packet for each file SHOULD be
   passed to the client using the configuration attribute.

   The URI specified in the configuration-uri attribute MUST point to a
   location where all of the Configuration Packets needed for the life
   of the session reside.

   The port value is specified by the server application bound to the
   address specified in the c attribute.  The bitrate value and channels
   specified in the rtpmap attribute MUST match the Vorbis sample rate
   value.  An example is found below.

   The answer to any offer, [8], MUST NOT change the URI specified in



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   the configuration-uri attribute.  The Configuration inlined in the
   configuration parameter MAY change.

      c=IN IP4/6
      m=audio RTP/AVP 98
      a=rtpmap:98 VORBIS/44100/2
      a=delivery:out_band/http
      a=fmtp:98 delivery-method:in_band,out_band/http;
      configuration=base16string1;
      configuration-uri=http://path/to/the/resource

   Note that the payload format (encoding) names are commonly shown in
   upper case.  MIME subtypes are commonly shown in lower case.  These
   names are case-insensitive in both places.  Similarly, parameter
   names are case-insensitive both in MIME types and in the default
   mapping to the SDP a=fmtp attribute.  The exception regarding case
   sensitivity is the configuration-uri URI which MUST be regarded as
   being case sensitive.


7.  Congestion Control

   Vorbis clients SHOULD send regular receiver reports detailing
   congestion.  A mechanism for dynamically downgrading the stream,
   known as bitrate peeling, will allow for a graceful backing off of
   the stream bitrate.  This feature is not available at present so an
   alternative would be to redirect the client to a lower bitrate stream
   if one is available.

   If a particular multicast session has a large number of participants
   care must be taken to prevent an RTCP feedback implosion, [10], in
   the event of congestion.


8.  Examples

   The following examples are common usage patterns that MAY be applied
   in such situations, the main scope of this section is to explain
   better usage of the transmission vectors.

8.1.  Stream Radio

   That is one of the most common situation: one single server streaming
   content in multicast, the clients may start a session at random time.
   The content itself could be a mix of live stream as the dj's speech
   and stored streams as the music she plays.

   In this situation we don't know in advance how many codebooks we will



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   use and.  The clients can join anytime and users expect to start
   listening to the content in a short time

   On join the client will receive the current Configuration necessary
   to decode the current stream inlined in the SDP.  And can start
   decoding the current stream.

   When the streamed content changes the new Configuration is sent in-
   band befoe the actual stream, and the Configuration that has to be
   sent inline in the SDP updated.

   A serverside optimization would be keep an hash list of the
   Configurations per session to avoid packing them and send the same
   Configuration with different Ident tags

   A clientside optimization would be keep a tag list of the
   Configurations per session and don't process configuration packets
   already known.

   Let's assume that the client playout buffer can store at least 7
   packets and that is the maximum latency.


9.  Security Considerations

   RTP packets using this payload format are subject to the security
   considerations discussed in the RTP specification [3].  This implies
   that the confidentiality of the media stream is achieved by using
   encryption.  Because the data compression used with this payload
   format is applied end-to-end, encryption may be performed on the
   compressed data.  Where the size of a data block is set care MUST be
   taken to prevent buffer overflows in the client applications.


10.  Acknowledgments

   This document is a continuation of draft-moffitt-vorbis-rtp-00.txt
   and draft-kerr-avt-vorbis-rtp-04.txt.  The MIME type section is a
   continuation of draft-short-avt-rtp-vorbis-mime-00.txt.

   Thanks to the AVT, Ogg Vorbis Communities / Xiph.org including Steve
   Casner, Aaron Colwell, Ross Finlayson, Fluendo, Ramon Garcia, Pascal
   Hennequin, Ralph Giles, Tor-Einar Jarnbjo, Colin Law, John Lazzaro,
   Jack Moffitt, Christopher Montgomery, Colin Perkins, Barry Short,
   Mike Smith, Phil Kerr, Michael Sparks, Magnus Westerlund, David
   Barrett, Silvia Pfeiffer, Politecnico di Torino (LS)^3/IMG Group in
   particular Federico Ridolfo, Francesco Varano, Giampaolo Mancini,
   Juan Carlos De Martin.



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11.  References

11.1.  Normative References

   [1]   Pfeiffer, S., "The Ogg Encapsulation Format Version 0",
         RFC 3533.

   [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", RFC 2119.

   [3]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for real-time applications",
         RFC 3550.

   [4]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
         Conferences with Minimal Control.", RFC 3551.

   [5]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327.

   [6]   Mogul et al., J., "Path MTU Discovery", RFC 1063.

   [7]   McCann et al., J., "Path MTU Discovery for IP version 6",
         RFC 1981.

   [8]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264.

   [9]   Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
         RFC 3548.

   [10]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
         "Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)",
         Internet Draft (draft-ietf-avt-rtcp-feedback-11: Work in
         progress).

11.2.  Informative References

   [11]  "libvorbis: Available from the Xiph website,
         http://www.xiph.org".

   [12]  "Ogg Vorbis I specification:  Codec setup and packet decode.
         Available from the Xiph website, http://www.xiph.org".

   [13]  "Ogg Vorbis I specification:  Comment field and header
         specification.  Available from the Xiph website,
         http://www.xiph.org".




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Author's Address

   Luca Barbato
   Xiph.Org

   Email: lu_zero@gentoo.org
   URI:   http://www.xiph.org/












































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