Network Working Group                                         L. Barbato

Request for Comments: 5215                                          Xiph

Category: Standards Track                                    August 2008

              RTP Payload Format for Vorbis Encoded Audio

Status of This Memo

   This document specifies an Internet standards track protocol for the

   Internet community, and requests discussion and suggestions for

   improvements.  Please refer to the current edition of the "Internet

   Official Protocol Standards" (STD 1) for the standardization state

   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This document describes an RTP payload format for transporting Vorbis

   encoded audio.  It details the RTP encapsulation mechanism for raw

   Vorbis data and the delivery mechanisms for the decoder probability

   model (referred to as a codebook), as well as other setup

   information.

   Also included within this memo are media type registrations and the

   details necessary for the use of Vorbis with the Session Description

   Protocol (SDP).

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RFC 5215               Vorbis RTP Payload Format             August 2008

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

     1.1.  Conformance and Document Conventions . . . . . . . . . . .  3

   2.  Payload Format . . . . . . . . . . . . . . . . . . . . . . . .  3

     2.1.  RTP Header . . . . . . . . . . . . . . . . . . . . . . . .  4

     2.2.  Payload Header . . . . . . . . . . . . . . . . . . . . . .  5

     2.3.  Payload Data . . . . . . . . . . . . . . . . . . . . . . .  6

     2.4.  Example RTP Packet . . . . . . . . . . . . . . . . . . . .  8

   3.  Configuration Headers  . . . . . . . . . . . . . . . . . . . .  8

     3.1.  In-band Header Transmission  . . . . . . . . . . . . . . .  9

       3.1.1.  Packed Configuration . . . . . . . . . . . . . . . . . 10

     3.2.  Out of Band Transmission . . . . . . . . . . . . . . . . . 12

       3.2.1.  Packed Headers . . . . . . . . . . . . . . . . . . . . 12

     3.3.  Loss of Configuration Headers  . . . . . . . . . . . . . . 13

   4.  Comment Headers  . . . . . . . . . . . . . . . . . . . . . . . 13

   5.  Frame Packetization  . . . . . . . . . . . . . . . . . . . . . 14

     5.1.  Example Fragmented Vorbis Packet . . . . . . . . . . . . . 15

     5.2.  Packet Loss  . . . . . . . . . . . . . . . . . . . . . . . 17

   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18

     6.1.  Packed Headers IANA Considerations . . . . . . . . . . . . 19

   7.  SDP Related Considerations . . . . . . . . . . . . . . . . . . 20

     7.1.  Mapping Media Type Parameters into SDP . . . . . . . . . . 20

       7.1.1.  SDP Example  . . . . . . . . . . . . . . . . . . . . . 21

     7.2.  Usage with the SDP Offer/Answer Model  . . . . . . . . . . 22

   8.  Congestion Control . . . . . . . . . . . . . . . . . . . . . . 22

   9.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

     9.1.  Stream Radio . . . . . . . . . . . . . . . . . . . . . . . 22

   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 23

   11. Copying Conditions . . . . . . . . . . . . . . . . . . . . . . 23

   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23

   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24

     13.1. Normative References . . . . . . . . . . . . . . . . . . . 24

     13.2. Informative References . . . . . . . . . . . . . . . . . . 25

Barbato                     Standards Track                     [Page 2]

RFC 5215               Vorbis RTP Payload Format             August 2008

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 bit rates.  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-4 AAC.  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 [RFC3533], 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.  Conformance and Document Conventions

   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 BCP 14, [RFC2119] and

   indicate requirement levels for compliant implementations.

   Requirements apply to all implementations unless otherwise stated.

   An implementation is a software module that supports one of the media

   types defined in this document.  Software modules may support

   multiple media types, but conformance is considered individually for

   each type.

   Implementations that fail to satisfy one or more "MUST" requirements

   are considered non-compliant.  Implementations that satisfy all

   "MUST" requirements, but fail to satisfy one or more "SHOULD"

   requirements, are said to be "conditionally compliant".  All other

   implementations are "unconditionally compliant".

2.  Payload Format

   For RTP-based transport of Vorbis-encoded audio, the standard RTP

   header is followed by a 4-octet payload header, and then the payload

   data.  The payload headers are used to associate the Vorbis data with

   its associated decoding codebooks as well as indicate if the

   following packet contains fragmented Vorbis data and/or the number of

   whole Vorbis data frames.  The payload data contains the raw Vorbis

   bitstream information.  There are 3 types of Vorbis data; an RTP

   payload MUST contain just one of them at a time.

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RFC 5215               Vorbis RTP Payload Format             August 2008

2.1.  RTP Header

   The format of the RTP header is specified in [RFC3550] and shown in

   Figure 1.  This payload format uses the fields of the header in a

   manner consistent with that specification.

       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 [RFC3550] and [RFC3551] 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 [RFC3550].

   Extension (X): 1 bit

   The Extension bit is used in accordance with [RFC3550].

   CSRC count (CC): 4 bits

   The CSRC count is used in accordance with [RFC3550].

   Marker (M): 1 bit

   Set to zero.  Audio silence suppression is not used.  This conforms

   to Section 4.1 of [VORBIS-SPEC-REF].

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RFC 5215               Vorbis RTP Payload Format             August 2008

   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 that 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

   the packet sequence.  This field is detailed further in [RFC3550].

   Timestamp: 32 bits

   A timestamp representing the sampling time of the first sample of the

   first Vorbis packet in the RTP payload.  The clock frequency MUST be

   set to the sample rate of the encoded audio data and is conveyed out-

   of-band (e.g., as an SDP parameter).

   SSRC/CSRC identifiers:

   These two fields, 32 bits each with one SSRC field and a maximum of

   16 CSRC fields, are as defined in [RFC3550].

2.2.  Payload Header

   The 4 octets following the RTP Header section are the Payload Header.

   This header is split into a number of bit fields 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.  It is stored as a network byte order integer.

   Fragment type (F): 2 bits

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RFC 5215               Vorbis RTP Payload Format             August 2008

   This field is set according to the following list:

      0 = Not Fragmented

      1 = Start Fragment

      2 = Continuation Fragment

      3 = End Fragment

   Vorbis Data Type (VDT): 2 bits

   This field specifies the kind of Vorbis data stored in this RTP

   packet.  There are currently three different types of Vorbis

   payloads.  Each packet MUST contain only a single type of Vorbis

   packet (e.g., you must not aggregate configuration and comment

   packets in the same RTP payload).

      0 = Raw Vorbis payload

      1 = Vorbis Packed Configuration payload

      2 = Legacy Vorbis Comment payload

      3 = Reserved

   The packets with a VDT of value 3 MUST be ignored.

   The last 4 bits represent the number of complete packets in this

   payload.  This provides for a maximum number of 15 Vorbis packets in

   the payload.  If the payload contains fragmented data, the number of

   packets MUST be set to 0.

2.3.  Payload Data

   Raw Vorbis packets are currently unbounded in length; application

   profiles will likely define a practical limit.  Typical Vorbis packet

   sizes range from very small (2-3 bytes) to quite large (8-12

   kilobytes).  The reference implementation [LIBVORBIS] typically

   produces packets less than ~800 bytes, except for the setup header

   packets, which are ~4-12 kilobytes.  Within an RTP context, to avoid

   fragmentation, the Vorbis data packet size SHOULD be kept

   sufficiently small so that after adding the RTP and payload headers,

   the complete RTP packet is smaller than the path MTU.

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RFC 5215               Vorbis RTP Payload Format             August 2008

       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 in bytes of the following data

   payload, and is followed by the raw Vorbis data padded to the nearest

   byte boundary, as explained by the Vorbis I Specification

   [VORBIS-SPEC-REF].  The length value is stored as a network byte

   order integer.

   For payloads that 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 include the length field.

   The payload packing of the Vorbis data packets MUST follow the

   guidelines set out in [RFC3551], where the oldest Vorbis packet

   occurs immediately after the RTP packet header.  Subsequent Vorbis

   packets, if any, MUST follow in temporal order.

   Audio channel mapping is in accordance with the Vorbis I

   Specification [VORBIS-SPEC-REF].

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RFC 5215               Vorbis RTP Payload Format             August 2008

2.4.  Example RTP Packet

   Here is an example RTP payload containing two Vorbis 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

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      | 2 |0|0|  0    |0|      PT     |       sequence number         |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |               timestamp (in sample rate units)                |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |           synchronisation source (SSRC) identifier            |

      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

      |            contributing source (CSRC) identifiers             |

      |                              ...                              |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                     Ident                     | 0 | 0 | 2 pks |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |            length             |          vorbis data         ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                        vorbis data                           |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |            length             |   next vorbis packet data    ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                        vorbis data                          ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..               vorbis data                    |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: Example Raw Vorbis Packet

   The payload data section of the RTP packet begins with the 24-bit

   Ident field followed by the one octet bit field header, which has the

   number of Vorbis frames set to 2.  Each of the Vorbis data frames is

   prefixed by the two octets length field.  The Packet Type and

   Fragment Type are set to 0.  The Configuration that will be used to

   decode the packets is the one indexed by the ident value.

3.  Configuration Headers

   Unlike other mainstream audio codecs, Vorbis has no statically

   configured probability model.  Instead, it packs all entropy decoding

   configuration, Vector Quantization and Huffman models into a data

   block that must be transmitted to the decoder with the compressed

   data.  A decoder also requires information detailing the number of

   audio channels, bitrates, and similar information to configure itself

   for a particular compressed data stream.  These two blocks of

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RFC 5215               Vorbis RTP Payload Format             August 2008

   information are often referred to collectively as the "codebooks" for

   a Vorbis stream, and are included as special "header" packets at the

   start of the compressed data.  In addition, the Vorbis I

   specification [VORBIS-SPEC-REF] requires the presence of a comment

   header packet that gives simple metadata about the stream, but this

   information is not required for decoding the frame sequence.

   Thus, these two codebook header packets must be received by the

   decoder before any audio data can be interpreted.  These requirements

   pose 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 are detailed below.  In order to

   set up an initial state for the client application, the configuration

   MUST be conveyed via the signalling channel used to set up the

   session.  One example of such signalling is SDP [RFC4566] with the

   Offer/Answer Model [RFC3264].  Changes to the configuration MAY be

   communicated via a re-invite, conveying a new SDP, or sent in-band in

   the RTP channel.  Implementations MUST support an in-band delivery of

   updated codebooks, and SHOULD support out-of-band codebook update

   using a new SDP file.  The changes may be due to different codebooks

   as well as different bitrates of the RTP stream.

   For non-chained streams, the recommended Configuration delivery

   method is inside the Packed Configuration (Section 3.1.1) in the SDP

   as explained the Mapping Media Type Parameters into SDP

   (Section 7.1).

   The 24-bit Ident field is used to map which Configuration will be

   used to decode a 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.  If the client

   detects a change in the Ident value and does not have this

   information, it MUST NOT decode the raw associated Vorbis data until

   it fetches the correct Configuration.

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 Vorbis Data Type.  Clients MUST

   be capable of dealing with fragmentation and periodic re-transmission

   of [RFC4588] the configuration headers.  The RTP timestamp value MUST

   reflect the transmission time of the first data packet for which this

   configuration applies.

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RFC 5215               Vorbis RTP Payload Format             August 2008

3.1.1.  Packed Configuration

   A Vorbis Packed Configuration is indicated with the Vorbis Data Type

   field set to 1.  Of the three headers defined in the Vorbis I

   specification [VORBIS-SPEC-REF], the Identification and the Setup

   MUST be packed as they are, while the Comment header MAY be replaced

   with a dummy one.

   The packed configuration stores Xiph codec configurations in a

   generic way: the first field stores the number of the following

   packets minus one (count field), the next ones represent the size of

   the headers (length fields), and the headers immediately follow the

   list of length fields.  The size of the last header is implicit.

   The count and the length fields are encoded using the following

   logic: the data is in network byte order; every byte has the most

   significant bit used as a flag, and the following 7 bits are used to

   store the value.  The first 7 most significant bits are stored in the

   first byte.  If there are remaining bits, the flag bit is set to 1

   and the subsequent 7 bits are stored in the following byte.  If there

   are remaining bits, set the flag to 1 and the same procedure is

   repeated.  The ending byte has the flag bit set to 0.  To decode,

   simply iterate over the bytes until the flag bit is set to 0.  For

   every byte, the data is added to the accumulated value multiplied by

   128.

   The headers are packed in the same order as they are present in Ogg

   [VORBIS-SPEC-REF]: Identification, Comment, Setup.

   The 2 byte length tag defines the length of the packed headers as the

   sum of the Configuration, Comment, and Setup lengths.

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RFC 5215               Vorbis RTP Payload Format             August 2008

       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              | n. of headers |    length1    |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |    length2    |                  Identification              ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                        Identification                       ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                        Identification                       ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                        Identification                       ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..               Identification                 |    Comment   ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                            Comment                          ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                            Comment                          ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                            Comment                          ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..           Comment            |             Setup            ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                            Setup                            ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                            Setup                            ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: Packed Configuration Figure

   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 because the packet bears the full Packed configuration.  The

   number of the packet is set to 1.

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RFC 5215               Vorbis RTP Payload Format             August 2008

3.2.  Out of Band Transmission

   The following packet definition MUST be used when Configuration is

   inside 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 (network-byte ordered) count field, which details the

   number of packed headers that are contained in the bundle.  The

   following shows the Packed header payload for each chained Vorbis

   stream.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                     Number of packed headers                  |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                          Packed header                        |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                          Packed header                        |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 6: Packed Headers Overview

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RFC 5215               Vorbis RTP Payload Format             August 2008

       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    ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..              | n. of headers |    length1    |    length2   ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..              |             Identification Header            ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      .................................................................

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..              |         Comment Header                       ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      .................................................................

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                        Comment Header                        |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                          Setup Header                        ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      .................................................................

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                         Setup Header                         |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 7: Packed Headers Detail

   The key difference between the in-band format and this one is that

   there is no need for the payload header octet.  In this figure, the

   comment has a size bigger than 127 bytes.

3.3.  Loss of Configuration Headers

   Unlike the loss of raw Vorbis payload data, loss of a configuration

   header leads to a situation where it will not be possible to

   successfully decode the stream.  Implementations MAY try to recover

   from an error by requesting again the missing Configuration or, if

   the delivery method is in-band, by buffering the payloads waiting for

   the Configuration needed to decode them.  The baseline reaction

   SHOULD either be reset or end the RTP session.

4.  Comment Headers

   Vorbis Data Type flag set to 2 indicates that the packet contains the

   comment metadata, such as artist name, track title, and so on.  These

   metadata messages are not intended to be fully descriptive but rather

   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 I Specification [VORBIS-SPEC-REF].

Barbato                     Standards Track                    [Page 13]

RFC 5215               Vorbis RTP Payload Format             August 2008

       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 8: Comment Packet

   The 2-byte length field is necessary since this packet could be

   fragmented.

5.  Frame Packetization

   Each RTP payload contains either one Vorbis packet fragment or an

   integer number of complete Vorbis packets (up to a maximum 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 payload with as many Vorbis packets as will fit, up to a

   maximum of 15, except when such bundling would exceed an

   application's desired transmission latency.  Path MTU is detailed in

   [RFC1191] and [RFC1981].

   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 consecutive fragments MUST be sent without any

   other payload being sent between the first and the last fragment.

   The RTP payload 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

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RFC 5215               Vorbis RTP Payload Format             August 2008

   the sequence number incremented as normal for the subsequent RTP

   payloads; this will affect the RTCP jitter measurement.  The length

   field shows the fragment length.

5.1.  Example Fragmented Vorbis Packet

   Here is an example of a fragmented Vorbis packet split over three RTP

   payloads.  Each of them 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                |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                            12345                              |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |           synchronization source (SSRC) identifier            |

      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

      |            contributing source (CSRC) identifiers             |

      |                              ...                              |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                       Ident                   | 1 | 0 |      0|

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |             length            |            vorbis data       ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                        vorbis data                           |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 9: Example Fragmented Packet (Packet 1)

   In this payload, the initial sequence number is 1000 and the

   timestamp is 12345.  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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RFC 5215               Vorbis RTP Payload Format             August 2008

      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                |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                             12345                             |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |           synchronization source (SSRC) identifier            |

      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

      |            contributing source (CSRC) identifiers             |

      |                              ...                              |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                       Ident                   | 2 | 0 |      0|

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |             length            |          vorbis data         ..

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      ..                        vorbis data                           |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 10: 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 types of payloads.  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 payload.