1177 lines
47 KiB
XML
Executable File
1177 lines
47 KiB
XML
Executable File
<?xml version="1.0" encoding="US-ASCII" ?>
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<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
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<?rfc rfcedstyle="yes"?>
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<?rfc subcompact="no"?>
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<?rfc toc="yes"?>
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<?rfc symrefs="yes" ?>
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<?rfc sortrefs="yes" ?>
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<rfc number="5215" category="std">
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<front>
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<title abbrev="Vorbis RTP Payload Format">RTP Payload Format for Vorbis Encoded Audio</title>
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<author initials="L" surname="Barbato" fullname="Luca Barbato">
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<organization abbrev="Xiph">Xiph.Org Foundation</organization>
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<address>
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<email>lu_zero@gentoo.org</email>
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<uri>http://xiph.org/</uri>
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</address>
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</author>
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<date month="August" year="2008" />
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<area>General</area>
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<workgroup>AVT Working Group</workgroup>
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<keyword>I-D</keyword>
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<keyword>Internet-Draft</keyword>
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<keyword>Vorbis</keyword>
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<keyword>RTP</keyword>
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<keyword>example</keyword>
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<abstract>
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<t>
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This document describes an RTP payload format for transporting Vorbis encoded
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audio. It details the RTP encapsulation mechanism for raw Vorbis data and
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the delivery mechanisms for the decoder probability model (referred to
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as a codebook), as well as other setup information.
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</t>
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<t>
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Also included within this memo are media type registrations and the details
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necessary for the use of Vorbis with the Session Description Protocol (SDP).
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</t>
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</abstract>
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</front>
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<middle>
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<section anchor="Introduction" title="Introduction">
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<t>
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Vorbis is a general purpose perceptual audio codec intended to allow
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maximum encoder flexibility, thus allowing it to scale competitively
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over an exceptionally wide range of bit rates. At the high
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quality/bitrate end of the scale (CD or DAT rate stereo, 16/24 bits), it
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is in the same league as MPEG-4 AAC.
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Vorbis is also intended for lower and higher sample rates (from
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8kHz telephony to 192kHz digital masters) and a range of channel
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representations (monaural, polyphonic, stereo, quadraphonic, 5.1,
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ambisonic, or up to 255 discrete channels).
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</t>
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<t>
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Vorbis encoded audio is generally encapsulated within an Ogg format bitstream
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<xref target="RFC3533"></xref>, which provides framing and synchronization.
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For the purposes of RTP transport, this layer is unnecessary, and so raw Vorbis
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packets are used in the payload.
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</t>
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<section anchor="Terminology" title="Conformance and Document Conventions">
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<t>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, <xref target="RFC2119"/> and indicate requirement levels for compliant implementations. Requirements apply to all implementations unless otherwise stated.</t>
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<t>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.</t>
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<t>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".</t>
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</section>
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</section>
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<section anchor="Payload Format" title="Payload Format">
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<t>
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For RTP-based transport of Vorbis-encoded audio, the standard RTP header is
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followed by a 4-octet payload header, and then the payload data. The payload
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headers are used to associate the Vorbis data with its associated decoding
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codebooks as well as indicate if the following packet contains fragmented
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Vorbis data and/or the number of whole Vorbis data frames. The payload data
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contains the raw Vorbis bitstream information. There are 3 types of Vorbis
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data; an RTP payload MUST contain just one of them at a time.
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</t>
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<section anchor="RTP Header" title="RTP Header">
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<t>
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The format of the RTP header is specified in <xref target="RFC3550"></xref>
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and shown in <xref target="RTP Header Figure"/>. This payload format
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uses the fields of the header in a manner consistent with that specification.
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</t>
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<t>
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<figure anchor="RTP Header Figure" title="RTP Header">
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<artwork><![CDATA[
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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|V=2|P|X| CC |M| PT | sequence number |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| timestamp |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| synchronization source (SSRC) identifier |
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+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
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| contributing source (CSRC) identifiers |
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| ... |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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]]></artwork>
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</figure>
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</t>
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<t>
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The RTP header begins with an octet of fields (V, P, X, and CC) to support
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specialized RTP uses (see <xref target="RFC3550"></xref> and
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<xref target="RFC3551"></xref> for details). For Vorbis RTP, the following
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values are used.
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</t>
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<t>
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Version (V): 2 bits</t>
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<t>
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This field identifies the version of RTP. The version used by this
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specification is two (2).
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</t>
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<t>
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Padding (P): 1 bit</t>
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<t>
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Padding MAY be used with this payload format according to Section 5.1 of
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<xref target="RFC3550"></xref>.
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</t>
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<t>
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Extension (X): 1 bit</t>
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<t>
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The Extension bit is used in accordance with <xref target="RFC3550"></xref>.
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</t>
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<t>
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CSRC count (CC): 4 bits</t>
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<t>
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The CSRC count is used in accordance with <xref target="RFC3550"></xref>.
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</t>
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<t>
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Marker (M): 1 bit</t>
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<t>
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Set to zero. Audio silence suppression is not used. This conforms to Section 4.1
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of <xref target="VORBIS-SPEC-REF"></xref>.
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</t>
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<t>
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Payload Type (PT): 7 bits</t>
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<t>
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An RTP profile for a class of applications is expected to assign a payload type
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for this format, or a dynamically allocated payload type SHOULD be chosen that
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designates the payload as Vorbis.
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</t>
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<t>
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Sequence number: 16 bits</t>
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<t>
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The sequence number increments by one for each RTP data packet sent, and may be
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used by the receiver to detect packet loss and to restore the packet sequence. This
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field is detailed further in <xref target="RFC3550"></xref>.
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</t>
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<t>
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Timestamp: 32 bits</t>
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<t>
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A timestamp representing the sampling time of the first sample of the first
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Vorbis packet in the RTP payload. The clock frequency MUST be set to the sample
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rate of the encoded audio data and is conveyed out-of-band (e.g., as an SDP parameter).
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</t>
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<t>
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SSRC/CSRC identifiers: </t>
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<t>
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These two fields, 32 bits each with one SSRC field and a maximum of 16 CSRC
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fields, are as defined in <xref target="RFC3550">
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</xref>.
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</t>
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</section>
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<section anchor="Payload Header" title="Payload Header">
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<t>
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The 4 octets following the RTP Header section are the Payload Header. This
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header is split into a number of bit fields detailing the format of the
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following payload data packets.
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</t>
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<figure anchor="Payload Header Figure" title="Payload Header">
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<artwork><![CDATA[
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Ident | F |VDT|# pkts.|
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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]]></artwork>
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</figure>
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<t>
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Ident: 24 bits</t>
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<t>
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This 24-bit field is used to associate the Vorbis data to a decoding
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Configuration. It is stored as a network byte order integer.
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</t>
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<t>
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Fragment type (F): 2 bits</t>
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<t>
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This field is set according to the following list:
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</t>
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<vspace blankLines="1" />
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<list style="empty">
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<t> 0 = Not Fragmented</t>
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<t> 1 = Start Fragment</t>
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<t> 2 = Continuation Fragment</t>
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<t> 3 = End Fragment</t>
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</list>
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<t>
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Vorbis Data Type (VDT): 2 bits</t>
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<t>
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This field specifies the kind of Vorbis data stored in this RTP packet. There
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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).
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</t>
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<vspace blankLines="1" />
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<list style="empty">
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<t> 0 = Raw Vorbis payload</t>
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<t> 1 = Vorbis Packed Configuration payload</t>
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<t> 2 = Legacy Vorbis Comment payload</t>
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<t> 3 = Reserved</t>
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</list>
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<t> The packets with a VDT of value 3 MUST be ignored.</t>
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<t>
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The last 4 bits represent the number of complete packets in this payload. This
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provides for a maximum number of 15 Vorbis packets in the payload. If the
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payload contains fragmented data, the number of packets MUST be set to 0.
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</t>
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</section>
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<section anchor="Payload Data" title="Payload Data">
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<t>
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Raw Vorbis packets are currently unbounded in length; application profiles will
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likely define a practical limit. Typical Vorbis packet sizes range from very
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small (2-3 bytes) to quite large (8-12 kilobytes). The reference implementation
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<xref target="LIBVORBIS"></xref> typically produces packets less than ~800
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bytes, except for the setup header packets, which are ~4-12 kilobytes. Within an
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RTP context, to avoid fragmentation, the Vorbis data packet size SHOULD be kept
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sufficiently small so that after adding the RTP and payload headers, the
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complete RTP packet is smaller than the path MTU.
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</t>
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<figure anchor="Payload Data Figure" title="Payload Data Header">
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<artwork><![CDATA[
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| length | vorbis packet data ..
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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]]></artwork>
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</figure>
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<t>
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Each Vorbis payload packet starts with a two octet length header, which is used
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to represent the size in bytes of the following data payload, and is followed by the
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raw Vorbis data padded to the nearest byte boundary, as explained by the <xref target="VORBIS-SPEC-REF">Vorbis I Specification</xref>. The length value is stored
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as a network byte order integer.
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</t>
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<t>
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For payloads that consist of multiple Vorbis packets, the payload data consists
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of the packet length followed by the packet data for each of the Vorbis packets
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in the payload.
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</t>
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<t>
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The Vorbis packet length header is the length of the Vorbis data block only and
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does not include the length field.
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</t>
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<t>
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The payload packing of the Vorbis data packets MUST follow the guidelines
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set out in <xref target="RFC3551"></xref>, where the oldest Vorbis packet occurs
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immediately after the RTP packet header. Subsequent Vorbis packets, if any, MUST
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follow in temporal order.
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</t>
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<t>
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Audio channel mapping is in accordance with the
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<xref target="VORBIS-SPEC-REF">Vorbis I Specification</xref>.
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</t>
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</section>
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<section anchor="Example RTP Packet" title="Example RTP Packet">
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<t>
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Here is an example RTP payload containing two Vorbis packets.
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</t>
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<figure anchor="Example Raw Vorbis Packet" title="Example Raw Vorbis Packet">
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<artwork><![CDATA[
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0 1 2 3
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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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 2 |0|0| 0 |0| PT | sequence number |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| timestamp (in sample rate units) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| synchronisation source (SSRC) identifier |
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+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
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| contributing source (CSRC) identifiers |
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| ... |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Ident | 0 | 0 | 2 pks |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| length | vorbis data ..
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.. vorbis data |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| length | next vorbis packet data ..
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.. vorbis data ..
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.. vorbis data |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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]]></artwork>
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</figure>
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<t>
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The payload data section of the RTP packet begins with the 24-bit Ident field
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followed by the one octet bit field header, which has the number of Vorbis
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frames set to 2. Each of the Vorbis data frames is prefixed by the two octets
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length field. The Packet Type and Fragment Type are set to 0. The Configuration
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that will be used to decode the packets is the one indexed by the ident value.
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</t>
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</section>
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</section>
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<section anchor="Configuration Headers" title="Configuration Headers">
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<t>
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Unlike other mainstream audio codecs, Vorbis has no statically
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configured probability model. Instead, it packs all entropy decoding
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configuration, Vector Quantization and Huffman models into a data block
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that must be transmitted to the decoder with the compressed data.
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A decoder also requires information detailing the number of audio
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channels, bitrates, and similar information to configure itself for a
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particular compressed data stream. These two blocks of information are
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often referred to collectively as the "codebooks" for a Vorbis stream,
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and are included as special "header" packets at the start
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of the compressed data. In addition,
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the <xref target="VORBIS-SPEC-REF">Vorbis I specification</xref>
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requires the presence of a comment header packet that gives simple
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metadata about the stream, but this information is not required for
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decoding the frame sequence.
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</t>
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<t>
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Thus, these two codebook header packets must be received by the decoder before
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any audio data can be interpreted. These requirements pose problems in RTP,
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which is often used over unreliable transports.
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</t>
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<t>
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Since this information must be transmitted reliably and, as the RTP
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stream may change certain configuration data mid-session, there are
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different methods for delivering this configuration data to a
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client, both in-band and out-of-band, which are detailed below.
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In order to set up an initial state for the client application, the
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configuration MUST be conveyed via the signalling channel used to set up
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the session. One example of such signalling is
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<xref target="RFC4566">SDP</xref> with the
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<xref target="RFC3264">Offer/Answer Model</xref>.
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Changes to the configuration MAY be communicated via a re-invite,
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conveying a new SDP, or sent in-band in the RTP channel.
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Implementations MUST support an in-band delivery of updated codebooks,
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and SHOULD support out-of-band codebook update using a new SDP file.
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The changes may be due to different codebooks as well as
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different bitrates of the RTP stream.
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</t>
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<t>For non-chained streams, the recommended Configuration delivery
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method is inside the <xref target="Packed Configuration">Packed
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Configuration</xref> in the SDP as explained the <xref
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target="Mapping Media Type Parameters into SDP"> Mapping Media Type
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Parameters into SDP</xref>.
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</t>
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<t>
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The 24-bit Ident field is used to map which Configuration will be used to
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decode a packet. When the Ident field changes, it indicates that a change in
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the stream has taken place. The client application MUST have in advance the
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correct configuration. If the client detects a change in the Ident value and
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does not have this information, it MUST NOT decode the raw associated Vorbis
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data until it fetches the correct Configuration.
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</t>
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<section anchor="In-band Header Transmission" title="In-band Header Transmission">
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<t>
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The <xref target="Packed Configuration">Packed Configuration</xref> Payload is
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sent in-band with the packet type bits set to match the Vorbis Data Type.
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Clients MUST be capable of dealing with fragmentation and periodic
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<xref target="RFC4588">re-transmission of</xref> the configuration headers.
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The RTP timestamp value MUST reflect the transmission time of the first data packet for which this configuration applies.
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</t>
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<section anchor="Packed Configuration" title="Packed Configuration">
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<t>
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A Vorbis Packed Configuration is indicated with the Vorbis Data Type field set
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to 1. Of the three headers defined in the
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<xref target="VORBIS-SPEC-REF">Vorbis I specification</xref>, the
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Identification and the Setup MUST be packed as they are, while the Comment
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header MAY be replaced with a dummy one.</t>
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<t>
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The packed configuration stores Xiph codec
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configurations in a generic way: the first field stores the number of the following packets
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minus one (count field), the next ones represent the size of the headers
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(length fields), and the headers immediately follow the list of length fields.
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The size of the last header is implicit.</t>
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<t>
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The count and the length fields are encoded using the following logic: the data
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is in network byte order; every byte has the most significant bit used
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as a flag, and the following 7 bits are used to store the value.
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The first 7 most significant bits are stored in the first byte.
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If there are remaining bits, the flag bit is set to 1 and the subsequent
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7 bits are stored in the following byte.
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If there are remaining bits, set the flag to 1 and the same procedure is
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repeated.
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The ending byte has the flag bit set to 0. To decode, simply iterate
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over the bytes until the flag bit is set to 0. For every byte, the data
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is added to the accumulated value multiplied by 128.</t>
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<t>
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The headers are packed in the same order as they are present in Ogg <xref target="VORBIS-SPEC-REF" />:
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Identification, Comment, Setup.</t>
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<t>
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The 2 byte length tag defines the length of the packed headers as the sum of
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the Configuration, Comment, and Setup lengths.</t>
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<figure anchor="Packed Configuration Figure" title="Packed Configuration Figure">
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<artwork><![CDATA[
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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 | 1 | 1|
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| length | n. of headers | length1 |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| length2 | Identification ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Identification ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Identification ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Identification ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Identification | Comment ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Comment ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Comment ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Comment ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Comment | Setup ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Setup ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. Setup ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
]]></artwork>
|
|
</figure>
|
|
|
|
<t>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.</t>
|
|
</section>
|
|
</section>
|
|
|
|
<section anchor="Out of Band Transmission" title="Out of Band Transmission">
|
|
|
|
<t>
|
|
The following packet definition MUST be used when Configuration is inside
|
|
in the SDP.
|
|
</t>
|
|
|
|
<section anchor="Packed Headers" title="Packed Headers">
|
|
|
|
<t>
|
|
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.
|
|
</t>
|
|
|
|
<figure anchor="Packed Headers Overview Figure" title="Packed Headers Overview">
|
|
<artwork><![CDATA[
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Number of packed headers |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Packed header |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Packed header |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
]]></artwork>
|
|
</figure>
|
|
|
|
<figure anchor="Packed Headers Detail Figure" title="Packed Headers Detail">
|
|
<artwork><![CDATA[
|
|
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 |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
]]></artwork>
|
|
</figure>
|
|
<t>
|
|
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.
|
|
</t>
|
|
</section>
|
|
|
|
</section>
|
|
|
|
<section anchor="Loss of Configuration Headers" title="Loss of Configuration Headers">
|
|
|
|
<t>
|
|
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.
|
|
</t>
|
|
|
|
</section>
|
|
|
|
</section>
|
|
|
|
<section anchor="Comment Headers" title="Comment Headers">
|
|
|
|
<t>
|
|
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 <xref target="VORBIS-SPEC-REF">Vorbis I Specification</xref>.
|
|
</t>
|
|
<figure anchor="Comment Packet Figure" title="Comment Packet">
|
|
<artwork><![CDATA[
|
|
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 |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
]]></artwork>
|
|
</figure>
|
|
|
|
<t>
|
|
The 2-byte length field is necessary since this packet could be fragmented.
|
|
</t>
|
|
|
|
</section>
|
|
<section anchor="Frame Packetization" title="Frame Packetization">
|
|
|
|
<t>
|
|
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).
|
|
</t>
|
|
|
|
<t>
|
|
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 <xref target="RFC1191"></xref> and <xref target="RFC1981"></xref>.
|
|
</t>
|
|
|
|
<t>
|
|
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 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.
|
|
</t>
|
|
|
|
<section anchor="Example Fragmented Vorbis Packet" title="Example Fragmented Vorbis Packet">
|
|
|
|
<t>
|
|
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.
|
|
</t>
|
|
|
|
<figure anchor="Example Fragmented Packet (Packet 1)" title="Example Fragmented Packet (Packet 1)">
|
|
<artwork><![CDATA[
|
|
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 |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
]]></artwork>
|
|
</figure>
|
|
|
|
<t>
|
|
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.
|
|
</t>
|
|
|
|
<figure anchor="Example Fragmented Packet (Packet 2)" title="Example Fragmented Packet (Packet 2)">
|
|
<artwork><![CDATA[
|
|
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 |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
]]></artwork>
|
|
</figure>
|
|
|
|
<t>
|
|
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.
|
|
</t>
|
|
|
|
<figure anchor="Example Fragmented Packet (Packet 3)" title="Example Fragmented Packet (Packet 3)">
|
|
<artwork><![CDATA[
|
|
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 |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| 12345 |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| synchronization source (SSRC) identifier |
|
|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|
|
| contributing source (CSRC) identifiers |
|
|
| ... |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| Ident | 3 | 0 | 0|
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| length | vorbis data ..
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
.. vorbis data |
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
]]></artwork>
|
|
</figure>
|
|
|
|
<t>
|
|
This is the last Vorbis fragment payload. The Fragment type is set to 3 and the
|
|
packet count remains set to 0. As in the previous payloads, the timestamp remains
|
|
set to the first payload timestamp in the sequence and the sequence number has
|
|
been incremented.
|
|
</t>
|
|
</section>
|
|
|
|
<section anchor="Packet Loss" title="Packet Loss">
|
|
|
|
<t>
|
|
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 Vorbis fragments and decode the incomplete packet. If we use the
|
|
fragmented Vorbis packet example above and the first RTP payload is lost, the
|
|
client MUST detect that the next RTP payload has the packet count field set
|
|
to 0 and the Fragment type 2 and MUST drop it.
|
|
The next RTP payload, which is the final fragmented packet, MUST be dropped
|
|
in the same manner.
|
|
If the missing RTP payload is the last, the two fragments received will be
|
|
kept and the incomplete Vorbis packet decoded.
|
|
</t>
|
|
|
|
<t>
|
|
Loss of any of the Configuration fragment will result in the loss of the full
|
|
Configuration packet with the result detailed in the <xref target="Loss of Configuration Headers">Loss of Configuration Headers</xref> section.
|
|
</t>
|
|
|
|
</section>
|
|
</section>
|
|
<section anchor="IANA Considerations" title="IANA Considerations">
|
|
|
|
<list style="hanging">
|
|
<t hangText="Type name:"> audio </t>
|
|
|
|
<t hangText="Subtype name:"> vorbis </t>
|
|
|
|
<t hangText="Required parameters:">
|
|
|
|
<list style="hanging">
|
|
<t hangText="rate:"> indicates the RTP timestamp clock rate as described in <xref target="RFC3551">RTP Profile for Audio and Video Conferences with Minimal Control</xref>.
|
|
</t>
|
|
|
|
<t hangText="channels:"> indicates the number of audio channels as described in <xref target="RFC3551">RTP Profile for Audio and Video Conferences with Minimal Control</xref>.
|
|
</t>
|
|
|
|
|
|
<t hangText="configuration:"> the <xref target="RFC4648">base64</xref> representation of the <xref target="Packed Headers">Packed Headers</xref>.
|
|
</t>
|
|
</list>
|
|
</t>
|
|
|
|
<t hangText="Encoding considerations:">
|
|
<vspace blankLines="1" />
|
|
This media type is framed and contains binary data.
|
|
</t>
|
|
|
|
<t hangText="Security considerations:">
|
|
<vspace blankLines="1" />
|
|
See Section 10 of RFC 5215.</t>
|
|
|
|
<t hangText="Interoperability considerations:">
|
|
<vspace blankLines="1" />
|
|
None</t>
|
|
|
|
<t hangText="Published specification:">
|
|
<vspace blankLines="1" />
|
|
RFC 5215
|
|
<vspace blankLines="1" />
|
|
Ogg Vorbis I specification: Codec setup and packet decode. Available from the Xiph website, http://xiph.org/
|
|
</t>
|
|
|
|
|
|
<t hangText="Applications which use this media type:">
|
|
<vspace blankLines="1"/>
|
|
Audio streaming and conferencing tools </t>
|
|
|
|
<t hangText="Additional information:">
|
|
<vspace blankLines="1" />
|
|
None </t>
|
|
|
|
<t hangText="Person & email address to contact for further information:">
|
|
<vspace blankLines="1" />
|
|
Luca Barbato: <lu_zero@gentoo.org><br/>
|
|
<vspace blankLines="0" />
|
|
IETF Audio/Video Transport Working Group
|
|
|
|
</t>
|
|
|
|
<t hangText="Intended usage:">
|
|
<vspace blankLines="1" />
|
|
COMMON</t>
|
|
|
|
<t hangText="Restriction on usage:">
|
|
<vspace blankLines="1" />
|
|
This media type depends on RTP framing, hence is only defined for transfer via <xref target="RFC3550">RTP</xref>.</t>
|
|
|
|
<t hangText="Author:">
|
|
<vspace blankLines="1"/>Luca Barbato</t>
|
|
|
|
<t hangText="Change controller:">
|
|
<vspace blankLines="1"/>IETF AVT Working Group delegated from the IESG</t>
|
|
</list>
|
|
|
|
<section anchor="Packed Headers IANA Considerations" title="Packed Headers IANA Considerations">
|
|
|
|
<t>
|
|
The following IANA considerations refers to the split configuration <xref target="Packed Headers">Packed Headers</xref> used within RFC 5215.
|
|
</t>
|
|
|
|
<list style="hanging">
|
|
<t hangText="Type name:"> audio </t>
|
|
|
|
<t hangText="Subtype name:"> vorbis-config </t>
|
|
|
|
<t hangText="Required parameters:">
|
|
<vspace blankLines="1" />
|
|
None
|
|
</t>
|
|
|
|
<t hangText="Optional parameters:">
|
|
<vspace blankLines="1" />
|
|
None
|
|
</t>
|
|
|
|
<t hangText="Encoding considerations:">
|
|
<vspace blankLines="1" />
|
|
This media type contains binary data.
|
|
</t>
|
|
|
|
<t hangText="Security considerations:">
|
|
<vspace blankLines="1" />
|
|
See Section 10 of RFC 5215.
|
|
</t>
|
|
|
|
<t hangText="Interoperability considerations:">
|
|
<vspace blankLines="1" />
|
|
None
|
|
</t>
|
|
|
|
<t hangText="Published specification:">
|
|
<vspace blankLines="1" />
|
|
RFC 5215
|
|
</t>
|
|
|
|
<t hangText="Applications which use this media type:">
|
|
<vspace blankLines="1" />
|
|
Vorbis encoded audio, configuration data
|
|
</t>
|
|
|
|
<t hangText="Additional information:">
|
|
<vspace blankLines="1" />
|
|
None
|
|
</t>
|
|
|
|
<t hangText="Person & email address to contact for further information:">
|
|
<vspace blankLines="1" />
|
|
Luca Barbato: <lu_zero@gentoo.org>
|
|
<vspace blankLines="0" />
|
|
IETF Audio/Video Transport Working Group
|
|
</t>
|
|
|
|
<t hangText="Intended usage:">
|
|
COMMON
|
|
</t>
|
|
|
|
<t hangText="Restriction on usage:">
|
|
<vspace blankLines="1" />
|
|
This media type doesn't depend on the transport.
|
|
</t>
|
|
|
|
<t hangText="Author:">
|
|
<vspace blankLines="1" />
|
|
Luca Barbato</t>
|
|
|
|
<t hangText="Change controller:">
|
|
<vspace blankLines="1" />
|
|
IETF AVT Working Group delegated from the IESG</t>
|
|
</list>
|
|
|
|
</section>
|
|
|
|
</section>
|
|
|
|
<section anchor="SDP related considerations" title="SDP Related Considerations">
|
|
<t>
|
|
The following paragraphs define the mapping of the parameters described in the IANA considerations section and their usage in the <xref target="RFC3264">Offer/Answer Model</xref>. In order to be forward compatible, the implementation MUST ignore unknown parameters.
|
|
</t>
|
|
|
|
<section anchor="Mapping Media Type Parameters into SDP" title="Mapping Media Type Parameters into SDP">
|
|
|
|
<t>
|
|
The information carried in the Media Type specification has a
|
|
specific mapping to fields in the <xref target="RFC4566">Session Description
|
|
Protocol (SDP)</xref>, which is commonly used to describe RTP sessions.
|
|
When SDP is used to specify sessions, the mapping are as follows:
|
|
</t>
|
|
|
|
<list style="symbols">
|
|
|
|
<t>The type name ("audio") goes in SDP "m=" as the media name.</t>
|
|
|
|
<t>The subtype name ("vorbis") goes in SDP "a=rtpmap" as the encoding name.</t>
|
|
|
|
<t>The parameter "rate" also goes in "a=rtpmap" as the clock rate.</t>
|
|
|
|
<t>The parameter "channels" also goes in "a=rtpmap" as the channel count.</t>
|
|
|
|
<t>The mandated parameters "configuration" MUST be included in the SDP
|
|
"a=fmtp" attribute.</t>
|
|
|
|
</list>
|
|
|
|
<t>
|
|
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.
|
|
</t>
|
|
|
|
<t>
|
|
The port value is specified by the server application bound to the address
|
|
specified in the c= line. The channel count value specified in the rtpmap
|
|
attribute SHOULD match the current Vorbis stream or should be considered the maximum
|
|
number of channels to be expected. The timestamp clock rate MUST be a multiple
|
|
of the sample rate; a different payload number MUST be used if the clock rate
|
|
changes. The Configuration payload delivers the exact information, thus the
|
|
SDP information SHOULD be considered a hint.
|
|
An example is found below.
|
|
</t>
|
|
|
|
<section anchor="SDP Example" title="SDP Example">
|
|
<t>The following example shows a basic SDP single stream. The first
|
|
configuration packet is inside the SDP; other configurations could be
|
|
fetched at any time from the URIs provided. The following
|
|
<xref target="RFC4648">base64</xref> configuration string is folded in this
|
|
example due to RFC line length limitations.</t>
|
|
|
|
|
|
<list style="empty">
|
|
<t>c=IN IP4 192.0.2.1</t>
|
|
<t>m=audio RTP/AVP 98</t>
|
|
<t>a=rtpmap:98 vorbis/44100/2</t>
|
|
<t>a=fmtp:98 configuration=AAAAAZ2f4g9NAh4aAXZvcmJpcwA...;</t>
|
|
</list>
|
|
</section>
|
|
|
|
<t>
|
|
Note that the payload format (encoding) names are commonly shown in uppercase.
|
|
Media Type subtypes are commonly shown in lowercase. These names are
|
|
case-insensitive in both places. Similarly, parameter names are
|
|
case-insensitive both in Media Type types and in the default mapping to the SDP
|
|
a=fmtp attribute. The a=fmtp line is a single line, even if it is shown as multiple lines in this document for clarity.
|
|
</t>
|
|
|
|
</section>
|
|
|
|
<section anchor="Usage with the SDP Offer/Answer Mode" title="Usage with the SDP Offer/Answer Model">
|
|
|
|
<t>
|
|
There are no negotiable parameters. All of them are declarative.
|
|
</t>
|
|
|
|
</section>
|
|
|
|
</section>
|
|
<section anchor="Congestion Control" title="Congestion Control">
|
|
<t>
|
|
The general congestion control considerations for transporting RTP
|
|
data apply to Vorbis audio over RTP as well. See the RTP specification
|
|
<xref target="RFC3550" /> and any applicable RTP profile (e.g., <xref target="RFC3551" />).
|
|
Audio data can be encoded using a range of different bit rates, so
|
|
it is possible to adapt network bandwidth by adjusting the encoder
|
|
bit rate in real time or by having multiple copies of content encoded
|
|
at different bit rates.
|
|
</t>
|
|
</section>
|
|
<section anchor="Example" title="Example">
|
|
|
|
<t>
|
|
The following example shows a common usage pattern that MAY be applied in
|
|
such a situation. The main scope of this section is to explain better usage
|
|
of the transmission vectors.
|
|
</t>
|
|
|
|
<section anchor="Stream Radio" title="Stream Radio">
|
|
|
|
<t>This is one of the most common situations: there is one single server streaming
|
|
content in multicast, and the clients may start a session at a random time. The
|
|
content itself could be a mix of a live stream (as the webjockey's voice)
|
|
and stored streams (as the music she plays).</t>
|
|
|
|
<t>In this situation, we don't know in advance how many codebooks we will use.
|
|
The clients can join anytime and users expect to start listening to the content
|
|
in a short time.</t>
|
|
|
|
<t>Upon joining, the client will receive the current Configuration necessary to
|
|
decode the current stream inside the SDP so that the decoding will start
|
|
immediately after.</t>
|
|
|
|
<t>When the streamed content changes, the new Configuration is sent in-band
|
|
before the actual stream, and the Configuration that has to be sent inside
|
|
the SDP is updated. Since the in-band method is unreliable, an out-of-band
|
|
fallback is provided.</t>
|
|
|
|
<t>The client may choose to fetch the Configuration from the alternate source
|
|
as soon as it discovers a Configuration packet got lost in-band, or use
|
|
<xref target="RFC3611">selective retransmission</xref> if the server supports
|
|
this feature.</t>
|
|
|
|
<t>A server-side optimization would be to keep a hash list of the
|
|
Configurations per session, which avoids packing all of them and sending the same
|
|
Configuration with different Ident tags.</t>
|
|
|
|
<t>A client-side optimization would be to keep a tag list of the Configurations
|
|
per session and not process configuration packets that are already known.</t>
|
|
|
|
</section>
|
|
</section>
|
|
|
|
<section anchor="Security Considerations" title="Security Considerations">
|
|
<t>
|
|
RTP packets using this payload format are subject to the security
|
|
considerations discussed in the
|
|
<xref target="RFC3550">RTP specification</xref>, the
|
|
<xref target="RFC4648">base64 specification</xref>, and the
|
|
<xref target="RFC3986">URI Generic syntax specification</xref>.
|
|
Among other considerations, 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.
|
|
</t>
|
|
|
|
</section>
|
|
<section title="Copying Conditions">
|
|
<t>The authors agree to grant third parties the irrevocable right to copy,
|
|
use, and distribute the work, with or without modification, in any medium,
|
|
without royalty, provided that, unless separate permission is granted,
|
|
redistributed modified works do not contain misleading author, version,
|
|
name of work, or endorsement information.</t>
|
|
</section>
|
|
<section anchor="Acknowledgments" title="Acknowledgments">
|
|
|
|
<t>
|
|
This document is a continuation of the following documents:
|
|
</t><t>
|
|
Moffitt, J., "RTP Payload Format for Vorbis Encoded Audio", February 2001.
|
|
</t><t>
|
|
Kerr, R., "RTP Payload Format for Vorbis Encoded Audio", December 2004.
|
|
</t><t>
|
|
The Media Type declaration is a continuation of the following
|
|
document:</t><t>
|
|
Short, B., "The audio/rtp-vorbis MIME Type", January 2008.
|
|
</t>
|
|
|
|
<t>
|
|
Thanks to the AVT, Vorbis Communities / Xiph.Org Foundation 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, Stefan Ehmann, Gianni Ceccarelli and Alessandro Salvatori. Thanks to the LScube Group, in particular Federico
|
|
Ridolfo, Francesco Varano, Giampaolo Mancini, Dario Gallucci, and Juan Carlos De Martin.
|
|
</t>
|
|
|
|
</section>
|
|
|
|
</middle>
|
|
|
|
<back>
|
|
|
|
<references title="Normative References">
|
|
|
|
<?rfc include="reference.RFC.2119" ?>
|
|
<?rfc include="reference.RFC.3550" ?>
|
|
<?rfc include="reference.RFC.3551" ?>
|
|
<?rfc include="reference.RFC.3986" ?>
|
|
<?rfc include="reference.RFC.4566" ?>
|
|
<?rfc include="reference.RFC.1191" ?>
|
|
<?rfc include="reference.RFC.1981" ?>
|
|
<?rfc include="reference.RFC.3264" ?>
|
|
<?rfc include="reference.RFC.4648" ?>
|
|
|
|
<reference anchor="VORBIS-SPEC-REF">
|
|
<front>
|
|
<title>Ogg Vorbis I specification: Codec setup and packet decode. Available from the Xiph website, http://xiph.org/vorbis/doc/Vorbis_I_spec.html</title>
|
|
</front>
|
|
</reference>
|
|
|
|
</references>
|
|
|
|
<references title="Informative References">
|
|
|
|
<?rfc include="reference.RFC.3533" ?>
|
|
|
|
<reference anchor="LIBVORBIS">
|
|
<front>
|
|
<title>libvorbis: Available from the dedicated website, http://vorbis.com/</title>
|
|
</front>
|
|
</reference>
|
|
|
|
<?rfc include="reference.RFC.3611" ?>
|
|
<?rfc include="reference.RFC.4588" ?>
|
|
|
|
</references>
|
|
</back>
|
|
</rfc>
|