

Network Working Group                                          Phil Kerr
Internet-Draft                                      Ogg Vorbis Community
October 27, 2003                                               OpenDrama
Expires: April 27, 2003      


              RTP Payload Format for Vorbis Encoded Audio

                   <draft-kerr-avt-vorbis-rtp-03.txt>

Status of this Memo

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

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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract
        
   This document describes a 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, metadata 
   and other setup information.

   [Note to RFC Editor: All references to RFC XXXX are to be replaced 
   by references to the RFC number of this memo, when published.]








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Table of Contents

   1.         Introduction ........................................    2
   1.1        Terminology .........................................    2
   2.         Payload Format ......................................    3
   2.1        RTP Header ..........................................    3
   2.2        Payload Header ......................................    4
   2.3        Payload Data ........................................    5
   2.4        Example RTP Packet ..................................    5
   3.         Frame Packetizing ...................................    6
   3.1        Example Fragmented Vorbis Packet ....................    6
   3.2        Packet Loss .........................................    8
   4.         Configuration Headers ...............................    8
   4.1        RTCP Based Config Header Transmission ...............    9
   4.2        Codebook Caching ....................................   11
   5.         Session Description .................................   11
   5.1        SDP Based Config Header Transmission ................   12
   6.         IANA Considerations .................................   13
   7.         Congestion Control ..................................   13
   8.         Security Considerations .............................   14
   9.         Acknowledgements ....................................   14
   10.        Normative References ................................   14
   10.1       Informative References ................................ 15
   11.        Full Copyright Statement ............................   15
   11.1       IPR Statement .......................................   15
   12.        Authors Address .....................................   15


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 1.0 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].


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2 Payload Format

   For RTP based transportation of Vorbis encoded audio the standard 
   RTP header is followed by an 8 bit payload header, then the payload 
   data.  The payload header is used to signify 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

     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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The RTP header begins with an octet of fields (V, P, X, and CC) to   
   support specialized RTP uses (see [4] and [5] 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
      Always set to 0, as audio silence suppression is not used by 
      the Vorbis codec. 

   CSRC count (CC): 4 bits
      The CSRC count is used in accordance with [3].






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   Marker (M): 1 bit
      Set to zero.  Audio silence suppression not used.  This conforms
      to section 4.1 of [6].

   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.

   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 next octet is the Payload Header.  
   This octet is split into a number of bitfields detailing the format
   of the following Payload Data packets.

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | C | F | R |  # of packets     |
   +---+---+---+---+---+---+---+---+

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

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

   Reserved (R): 1 bit
      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.  
   This provides for a maximum number of 32 Vorbis packets in the 
   payload.  If C is set to one, this number MUST be 0.

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2.3 Payload Data

   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 [11] 
   typically produces packets less than ~800 bytes, except for the
   header packets which are ~4-12 kilobytes.

   Within a RTP context the maximum Vorbis packet SHOULD be kept below
   the MTU size, typically 1500 octets, including the RTP and payload 
   headers, to avoid fragmentation.  For the delivery of Vorbis audio 
   using RTP the maximum size of the header block is limited to 64K.

   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 octet representing the packet length followed by 
   the packet data for each of the Vorbis packets in the payload.

   The Vorbis packet length field is the length of the Vorbis data 
   block minus one octet.   

   The payload packing of the Vorbis data packets SHOULD follow the
   guidelines set-out in section 4.4 of [5] where the oldest packet
   occurs immediately after the RTP packet header.

   Channel mapping of the audio is in accordance with BS. 775-1 
   ITU-R.


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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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   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|1|0| # pks: 2|      len      |         vorbis data ...       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      ...vorbis data...                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ...      |      len      |   next vorbis packet data...  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


3 Frame Packetizing

   Each RTP packet contains either one complete Vorbis packet, one 
   Vorbis packet fragment, or an integer number of complete Vorbis 
   packets (up to 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 octets and less than the
   path-MTU MUST 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 Vorbis packet will not fit within the network MTU, it SHOULD 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 
   Fragmented (F) bit set to one.  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.  Path 
   MTU is detailed in [9] and [10].




3.1 Example Fragmented Vorbis Packet

   Here is an example fragmented Vorbis packet split over three RTP
   packets.  









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   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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|0|0|        0|      len      |         vorbis data ..        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ..vorbis data..                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In this packet the initial sequence number is 1000 and the 
   timestamp is xxxxx.  The number of packets field is set to 0.


   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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0|0|        0|      len      |         vorbis data ...       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ..vorbis data..                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The C bit is set to 1 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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|1|0|        0|      len      |         vorbis data ..        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ..vorbis data..                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This is the last Vorbis fragment packet.  The C and F bits are 
   set 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.


3.2 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 C and F flags.  If we use the 
   fragmented Vorbis packet example above and the first packet is
   lost the client SHOULD detect that the next packet has the packet
   count field set to 0 and the C bit is set and MUST drop it.  The
   next packet, which is the final fragmented packet, SHOULD be dropped
   in the same manner, or buffered.  Feedback reports on lost and 
   dropped packets MUST be sent back via RTCP.


4 Configuration Headers

   To decode a Vorbis stream three configuration header blocks are
   needed.  The first header indicates the sample and bitrates, the 
   number of channels and the version of the Vorbis encoder used.  
   The second header contains the decoders probability model, or 
   codebooks and the third header details stream metadata.






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   As the RTP stream may change certain configuration data mid-session
   there are two different methods for delivering this configuration
   data to a client, RTCP which is detailed below and SDP which is 
   detailed in section 5.  SDP delivery is used to set-up an initial
   state for the client application and RTCP is used to change state
   during the session.  The changes may be due to different metadata
   or codebooks as well as different bitrates of the stream.

   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 self-contained codebook.
   This codebook block also requires additional identification 
   information detailing the number of audio channels, bit rates and
   other information used to initialise the Vorbis stream.


4.1 RTCP Based Header Transmission

   The three header data blocks are sent out-of-band as an APP defined 
   RTCP message with the 4 octet name field set to VORB. 

   Synchronizing the configuration headers to the RTP stream is 
   critical.  A 32 bit timestamp field is used to indicate the
   timepoint when a VORB header MUST be applied to the RTP stream. 
   VORB RTCP packets SHOULD be sent just ahead of the change in the
   RTP stream.  As the reception loss of the RTCP header will mean 
   the RTP stream will fail to decode properly the freqency of their 
   periodic retransmission SHOULD be high enough to minimize the    
   stream disturbance whilst remaining under the RTCP bandwidth
   allocation.

    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| subtype |   PT=APP=204  |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           SSRC/CSRC                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             VORB                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Timestamp (in sample rate units)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Vorbis Version                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Audio Sample Rate                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Bitrate Maximum                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Bitrate Nominal                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Bitrate Minimum                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | bsz 0 | bsz 1 |       Num Audio Channels      |c|m|o|x|x|x|x|x|
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |     Codebook length           |      Codebook checksum        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ..                          Codebook                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ..                          URI string                          |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                      Vendor string length                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Vendor string                        ..
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    User comments list length                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ..               User comment length / User comment             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The first Vorbis config header defines the Vorbis stream 
   attributes.  The Vorbis version MUST be set to zero to comply with
   this document.  The fields Sample Rate, Bitrate Maximum/Nominal/
   Minimum and Num Audio Channels are set in accordance with [6] with 
   the bsz fields above referring to the blocksize parameters.  The 
   framing bit is not used for RTP transportation and so applications 
   constructing Vorbis files MUST take care to set this if required.

   The next 8 bits are used to indicate the presence of the two 
   other Vorbis stream config headers and the size overflow header.

   The c flag indicates the presence of a codebook header block, the
   m flag indicates the presence of a comment metadata block.  The o
   flag indicates if the size of either of the c and m headers would
   make the VORB packet greater than that allowed for a RTCP message.

   The remaining five bits, indicated with an x, are reserved/unused
   and MUST be set to 0 for this version of the document.

   If the c flag is set then the next header block will contain the 
   codebook configuration data.  

   The configuration information detailed above MUST be completely 
   intact, as a client can not decode a stream with an incomplete 
   or corrupted codebook set.

   A 16 bit codebook length field and a 16 bit 1's complement checksum
   of the codebook precedes the codebook datablock.  The length field 
   allows for codebooks to be up to 64K in size. The checksum is used 
   to detect a corrupted codebook.  

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   If a checksum failure is detected then a new config header file
   SHOULD be obtained from SDP, if the codebook has not changed since
   the session has started.  If no SDP value is set and no other method
   for obtaining the config headers exists then this is considered to 
   be a failure and SHOULD be reported to the client application.

   If the m flag is set then the next header block will 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.  This message MUST be sent at 
   the start of the stream, together with the setup and codebook 
   headers, even if it contains no information.  During a session the
   metadata associated with the stream may change from that specified 
   at the start, e.g. a live concert broadcast changing acts/scenes, so
   clients MUST have the ability to receive m header blocks.  Details
   on the format of the comments can be found in the Vorbis 
   documentation [7].

   The format for the data takes the form of a 32 bit codec vendors
   name length field followed by the name encoded in UTF-8.  The next
   field denotes the number of user comments and then the user comments
   length and text field pairs, up to the number indicated by the user 
   comment list length.   

   If the o, overflow, bit is set then the URI of a whole header block
   is specified in an overflow URI field, which is a null terminated 
   UTF-8 string.  The header file specified at the URI MUST NOT have 
   the overflow flag set, otherwise a loop condition will occur. 


4.2 Codebook Caching

   Codebook caching allows clients that have previously connected to a 
   stream to re-use the codebooks and thus begin the playback of the 
   session faster.  When a client receives a codebook it may store
   it, together with the MD5 key, locally and can compare the MD5 key
   of locally cached codebooks with the key it receives via SDP, which
   is detailed in section 5.   
   

5 Session Description for Vorbis RTP Streams

   Session description information concerning the Vorbis stream 
   SHOULD be provided if possible and MUST be in accordance with [8].  
   The SDP information is split into two sections, a mandatory 
   section detailing the RTP stream and an optional section used to 
   convey information needed for codebook caching.






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   Below is an outline of the mandatory SDP attributes.

   c=IN IP4/6 <Vorbis stream>
   m=audio <port> RTP/AVP 98
   a=rtpmap:98 vorbis/<sample rate>
   a=fmtp:98 header=<URI of Vorbis codebooks>
   a=fmtp:98 md5key=<MD5 key of codebook>

   The port value is specified by the server application bound to 
   the address specified in the c attribute.  The bitrate value 
   specified in the a attribute MUST match the Vorbis sample rate 
   value.

   The Vorbis codebook specified in the header attribute MUST contain
   all of the configuration data.  If the codebook MD5 attribute, 
   md5key, is set the key is compared to a locally held cache and 
   if found the associated local codebook is used, if not the 
   client MUST use the configuration headers specified with the 
   header attribute.  

5.1 SDP Based Config Header Transmission

   The optional SDP attributes are used to convey details of the 
   Vorbis stream which are required for codebook caching.  If the 
   following attributes are set they take precedent over values 
   specified in the u attribute detailed above.  The maximum size
   of the mandatory and optional SDP attributes MUST be less than
   1K in size to conform to section 4.1 of [8].

   a=fmtp:98 bitrate_min=<Bitrate Minimum>
   a=fmtp:98 bitrate_norm=<Bitrate Normal>
   a=fmtp:98 bitrate_max=<Bitrate Maximum>
   a=fmtp:98 bsz0=<Block Size 0>
   a=fmtp:98 bsz1=<Block Size 1>
   a=fmtp:98 channels=<Num Audio Channels>
   a=fmtp:98 meta_vendor=<Vendor Name>


   The metadata attribute, meta_vendor, provides the bare minimum 
   information required for decoding but does not convey any 
   meaningful stream metadata information.  As outlined in the Vorbis 
   comment field and header specification documentation, [7], a number 
   of predefined field names are available which SHOULD be used.  An 
   example would be:




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   a=fmtp:98 meta_vendor=Xiph.Org libVorbis I 20020717
   a=fmtp:98 meta_artist=Honest Bob and the Factory-to-Dealer-Incentives
   a=fmtp:98 meta_title=I'm Still Around
   a=fmtp:98 meta_tracknumber=5


6 IANA Considerations

   MIME media type name: audio

   MIME subtype: vorbis

   Required Parameters:
         header indicates the URI of the decoding codebook.
         md5key indicates the MD5 key of the codebooks.

   Optional Parameters: 
         bitrate_min, bitrate_norm and bitrate_max indicate the 
         minimum, nominal and maximum bitrates.  bsz0 and bsz1
         indicate the blocksize values.  channels indicates the 
         number of audio channels in the stream.  meta_vendor 
         indicates the encoding codec vendor.

   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 [2] for details.

   Applications which use this media type:
         Audio streaming and conferencing tools

   Additional information: none

   Person & email address to contact for further information:
         Phil Kerr
         philkerr@elec.gla.ac.uk/phil@plus24.com

   Intended usage: COMMON

   Author/Change controller:
         Author: Phil Kerr
         Change controller: IETF AVT Working Group

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.  
 
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8 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.


9 Acknowledgments

   This document is a continuation of draft-moffitt-vorbis-rtp-00.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, Ramon Garcia, Pascal Hennequin, Ralph Jiles, 
   Tor-Einar Jarnbjo, Colin Law, John Lazzaro, Jack Moffitt, 
   Colin Perkins, Barry Short, Mike Smith, Magnus Westerlund.


10 Normative References

   1.  The Ogg Encapsulation Format Version 0 (RFC 3533), S. Pfeiffer.

   2.  Key words for use in RFCs to Indicate Requirement Levels 
       (RFC 2119), S. Bradner.

   3.  RTP: A Transport Protocol for Real-Time Applications (RFC 1889),
       Schulzrinne, et al.
  
   4.  RTP: A transport protocol for real-time applications. Work   
       in progress, draft-ietf-avt-rtp-new-11.txt.

   5.  RTP Profile for Audio and Video Conferences with Minimal Control. 
       Work in progress, draft-ietf-avt-profile-new-12.txt.

   6.  Ogg Vorbis I spec:  Codec setup and packet decode.
       http://www.xiph.org/ogg/vorbis/doc/vorbis-spec-ref.html

   7.  Ogg Vorbis I spec:  Comment field and header specification. 
       http://www.xiph.org/ogg/vorbis/doc/v-comment.html

   8.  SDP: Session Description Protocol (RFC 2327), Handley, M. and 
       V. Jacobson.
  
   9.  Path MTU Discovery (RFC 1063), Mogul & Deering 

   10. Path MTU Discovery for IP version 6 (RFC 1981), McCann, J. et al.





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10.1 Informative References

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


11 Full Copyright Statement

   Copyright (C) The Internet Society (2003). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works. However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

10.1 IPR Statement

   "The IETF takes no position regarding the validity or scope of   any intellectual property or other rights that might be claimed   to  pertain to the implementation or use of the technology   described in this document or the extent to which any license   under such rights might or might not be available; neither does   it represent that it has made any effort to identify any such   rights.  Information on the IETF's procedures with respect to   rights in standards-track and standards-related documentation   can be found in BCP-11.  Copies of claims of rights made   available for publication and any assurances of licenses to   be made available, or the result of an attempt made   to obtain a general license or permission for the use of such   proprietary rights by implementors or users of this   specification can be obtained from the IETF Secretariat."

   Further IPR details on the Vorbis bitstream may be found on the
   Xiph website: http://www.xiph.org



12 Authors Address

   Phil Kerr
   Centre for Music Technology
   University of Glasgow
   Glasgow, Scotland
   UK, G12 8LT
   Phone: +44 141 330 5740
   Email: philkerr@elec.gla.ac.uk
          phil@plus24.com

   WWW: http://www.xiph.org/


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