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1<?xml version="1.0" encoding="US-ASCII" ?>
2<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
3
4<?rfc rfcedstyle="yes"?>
5<?rfc subcompact="no"?>
6<?rfc toc="yes"?>
7<?rfc symrefs="yes" ?>
8<?rfc sortrefs="yes" ?>
9
10<rfc number="5215" category="std">
11
12<front>
13<title abbrev="Vorbis RTP Payload Format">RTP Payload Format for Vorbis Encoded Audio</title>
14
15<author initials="L" surname="Barbato" fullname="Luca Barbato">
16<organization abbrev="Xiph">Xiph.Org Foundation</organization>
17<address>
18<email>lu_zero@gentoo.org</email>
19<uri>http://xiph.org/</uri>
20</address>
21</author>
22
23<date month="August" year="2008" />
24
25<area>General</area>
26<workgroup>AVT Working Group</workgroup>
27<keyword>I-D</keyword>
28
29<keyword>Internet-Draft</keyword>
30<keyword>Vorbis</keyword>
31<keyword>RTP</keyword>
32
33<keyword>example</keyword>
34
35<abstract>
36
37<t>
38This document describes an RTP payload format for transporting Vorbis encoded
39audio. It details the RTP encapsulation mechanism for raw Vorbis data and
40the delivery mechanisms for the decoder probability model (referred to
41as a codebook), as well as other setup information.
42</t>
43
44<t>
45Also included within this memo are media type registrations and the details
46necessary for the use of Vorbis with the Session Description Protocol (SDP).
47</t>
48
49</abstract>
50
51</front>
52
53<middle>
54
55<section anchor="Introduction" title="Introduction">
56
57<t>
58Vorbis is a general purpose perceptual audio codec intended to allow
59maximum encoder flexibility, thus allowing it to scale competitively
60over an exceptionally wide range of bit rates. At the high
61quality/bitrate end of the scale (CD or DAT rate stereo, 16/24 bits), it
62is in the same league as MPEG-4 AAC.
63Vorbis is also intended for lower and higher sample rates (from
648kHz telephony to 192kHz digital masters) and a range of channel
65representations (monaural, polyphonic, stereo, quadraphonic, 5.1,
66ambisonic, or up to 255 discrete channels).
67</t>
68
69<t>
70Vorbis encoded audio is generally encapsulated within an Ogg format bitstream
71<xref target="RFC3533"></xref>, which provides framing and synchronization.
72For the purposes of RTP transport, this layer is unnecessary, and so raw Vorbis
73packets are used in the payload.
74</t>
75
76<section anchor="Terminology" title="Conformance and Document Conventions">
77
78<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>
79<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>
80<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>
81
82</section>
83</section>
84
85<section anchor="Payload Format" title="Payload Format">
86
87<t>
88For RTP-based transport of Vorbis-encoded audio, the standard RTP header is
89followed by a 4-octet payload header, and then the payload data. The payload
90headers are used to associate the Vorbis data with its associated decoding
91codebooks as well as indicate if the following packet contains fragmented
92Vorbis data and/or the number of whole Vorbis data frames. The payload data
93contains the raw Vorbis bitstream information. There are 3 types of Vorbis
94data; an RTP payload MUST contain just one of them at a time.
95</t>
96
97<section anchor="RTP Header" title="RTP Header">
98
99<t>
100The format of the RTP header is specified in <xref target="RFC3550"></xref>
101and shown in <xref target="RTP Header Figure"/>. This payload format
102uses the fields of the header in a manner consistent with that specification.
103</t>
104
105<t>
106<figure anchor="RTP Header Figure" title="RTP Header">
107<artwork><![CDATA[
108 0 1 2 3
109 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
110 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
111 |V=2|P|X| CC |M| PT | sequence number |
112 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
113 | timestamp |
114 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
115 | synchronization source (SSRC) identifier |
116 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
117 | contributing source (CSRC) identifiers |
118 | ... |
119 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
120]]></artwork>
121</figure>
122</t>
123
124<t>
125The RTP header begins with an octet of fields (V, P, X, and CC) to support
126specialized RTP uses (see <xref target="RFC3550"></xref> and
127<xref target="RFC3551"></xref> for details). For Vorbis RTP, the following
128values are used.
129</t>
130
131<t>
132Version (V): 2 bits</t>
133<t>
134This field identifies the version of RTP. The version used by this
135specification is two (2).
136</t>
137
138<t>
139Padding (P): 1 bit</t>
140<t>
141Padding MAY be used with this payload format according to Section 5.1 of
142<xref target="RFC3550"></xref>.
143</t>
144
145<t>
146Extension (X): 1 bit</t>
147<t>
148The Extension bit is used in accordance with <xref target="RFC3550"></xref>.
149</t>
150
151<t>
152CSRC count (CC): 4 bits</t>
153<t>
154The CSRC count is used in accordance with <xref target="RFC3550"></xref>.
155</t>
156
157<t>
158Marker (M): 1 bit</t>
159<t>
160Set to zero. Audio silence suppression is not used. This conforms to Section 4.1
161of <xref target="VORBIS-SPEC-REF"></xref>.
162</t>
163
164<t>
165Payload Type (PT): 7 bits</t>
166<t>
167An RTP profile for a class of applications is expected to assign a payload type
168for this format, or a dynamically allocated payload type SHOULD be chosen that
169designates the payload as Vorbis.
170</t>
171
172<t>
173Sequence number: 16 bits</t>
174<t>
175The sequence number increments by one for each RTP data packet sent, and may be
176used by the receiver to detect packet loss and to restore the packet sequence. This
177field is detailed further in <xref target="RFC3550"></xref>.
178</t>
179
180<t>
181Timestamp: 32 bits</t>
182<t>
183A timestamp representing the sampling time of the first sample of the first
184Vorbis packet in the RTP payload. The clock frequency MUST be set to the sample
185rate of the encoded audio data and is conveyed out-of-band (e.g., as an SDP parameter).
186</t>
187
188<t>
189SSRC/CSRC identifiers: </t>
190<t>
191These two fields, 32 bits each with one SSRC field and a maximum of 16 CSRC
192fields, are as defined in <xref target="RFC3550">
193</xref>.
194</t>
195
196</section>
197
198<section anchor="Payload Header" title="Payload Header">
199
200<t>
201The 4 octets following the RTP Header section are the Payload Header. This
202header is split into a number of bit fields detailing the format of the
203following payload data packets.
204</t>
205
206<figure anchor="Payload Header Figure" title="Payload Header">
207<artwork><![CDATA[
208 0 1 2 3
209 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
210 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
211 | Ident | F |VDT|# pkts.|
212 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
213]]></artwork>
214</figure>
215
216<t>
217Ident: 24 bits</t>
218<t>
219This 24-bit field is used to associate the Vorbis data to a decoding
220Configuration. It is stored as a network byte order integer.
221</t>
222
223<t>
224Fragment type (F): 2 bits</t>
225<t>
226This field is set according to the following list:
227</t>
228<vspace blankLines="1" />
229<list style="empty">
230<t> 0 = Not Fragmented</t>
231<t> 1 = Start Fragment</t>
232<t> 2 = Continuation Fragment</t>
233<t> 3 = End Fragment</t>
234</list>
235
236<t>
237Vorbis Data Type (VDT): 2 bits</t>
238<t>
239This field specifies the kind of Vorbis data stored in this RTP packet. There
240are 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).
241</t>
242
243<vspace blankLines="1" />
244<list style="empty">
245<t> 0 = Raw Vorbis payload</t>
246<t> 1 = Vorbis Packed Configuration payload</t>
247<t> 2 = Legacy Vorbis Comment payload</t>
248<t> 3 = Reserved</t>
249</list>
250
251<t> The packets with a VDT of value 3 MUST be ignored.</t>
252
253<t>
254The last 4 bits represent the number of complete packets in this payload. This
255provides for a maximum number of 15 Vorbis packets in the payload. If the
256payload contains fragmented data, the number of packets MUST be set to 0.
257</t>
258
259</section>
260
261<section anchor="Payload Data" title="Payload Data">
262
263<t>
264Raw Vorbis packets are currently unbounded in length; application profiles will
265likely define a practical limit. Typical Vorbis packet sizes range from very
266small (2-3 bytes) to quite large (8-12 kilobytes). The reference implementation
267<xref target="LIBVORBIS"></xref> typically produces packets less than ~800
268bytes, except for the setup header packets, which are ~4-12 kilobytes. Within an
269RTP context, to avoid fragmentation, the Vorbis data packet size SHOULD be kept
270sufficiently small so that after adding the RTP and payload headers, the
271complete RTP packet is smaller than the path MTU.
272</t>
273
274<figure anchor="Payload Data Figure" title="Payload Data Header">
275<artwork><![CDATA[
276 0 1 2 3
277 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
278 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
279 | length | vorbis packet data ..
280 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
281]]></artwork>
282</figure>
283
284<t>
285Each Vorbis payload packet starts with a two octet length header, which is used
286to represent the size in bytes of the following data payload, and is followed by the
287raw 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
288as a network byte order integer.
289</t>
290
291<t>
292For payloads that consist of multiple Vorbis packets, the payload data consists
293of the packet length followed by the packet data for each of the Vorbis packets
294in the payload.
295</t>
296
297<t>
298The Vorbis packet length header is the length of the Vorbis data block only and
299does not include the length field.
300</t>
301
302<t>
303The payload packing of the Vorbis data packets MUST follow the guidelines
304set out in <xref target="RFC3551"></xref>, where the oldest Vorbis packet occurs
305immediately after the RTP packet header. Subsequent Vorbis packets, if any, MUST
306follow in temporal order.
307</t>
308
309<t>
310Audio channel mapping is in accordance with the
311<xref target="VORBIS-SPEC-REF">Vorbis I Specification</xref>.
312</t>
313
314</section>
315
316<section anchor="Example RTP Packet" title="Example RTP Packet">
317
318<t>
319Here is an example RTP payload containing two Vorbis packets.
320</t>
321
322<figure anchor="Example Raw Vorbis Packet" title="Example Raw Vorbis Packet">
323<artwork><![CDATA[
324 0 1 2 3
325 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
326 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
327 | 2 |0|0| 0 |0| PT | sequence number |
328 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
329 | timestamp (in sample rate units) |
330 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
331 | synchronisation source (SSRC) identifier |
332 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
333 | contributing source (CSRC) identifiers |
334 | ... |
335 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
336 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
337 | Ident | 0 | 0 | 2 pks |
338 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
339 | length | vorbis data ..
340 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
341 .. vorbis data |
342 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
343 | length | next vorbis packet data ..
344 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
345 .. vorbis data ..
346 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
347 .. vorbis data |
348 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
349]]></artwork>
350</figure>
351
352<t>
353The payload data section of the RTP packet begins with the 24-bit Ident field
354followed by the one octet bit field header, which has the number of Vorbis
355frames set to 2. Each of the Vorbis data frames is prefixed by the two octets
356length field. The Packet Type and Fragment Type are set to 0. The Configuration
357that will be used to decode the packets is the one indexed by the ident value.
358</t>
359
360</section>
361</section>
362
363
364
365<section anchor="Configuration Headers" title="Configuration Headers">
366
367<t>
368Unlike other mainstream audio codecs, Vorbis has no statically
369configured probability model. Instead, it packs all entropy decoding
370configuration, Vector Quantization and Huffman models into a data block
371that must be transmitted to the decoder with the compressed data.
372A decoder also requires information detailing the number of audio
373channels, bitrates, and similar information to configure itself for a
374particular compressed data stream. These two blocks of information are
375often referred to collectively as the "codebooks" for a Vorbis stream,
376and are included as special "header" packets at the start
377of the compressed data. In addition,
378the <xref target="VORBIS-SPEC-REF">Vorbis I specification</xref>
379requires the presence of a comment header packet that gives simple
380metadata about the stream, but this information is not required for
381decoding the frame sequence.
382</t>
383
384<t>
385Thus, these two codebook header packets must be received by the decoder before
386any audio data can be interpreted. These requirements pose problems in RTP,
387which is often used over unreliable transports.
388</t>
389
390<t>
391Since this information must be transmitted reliably and, as the RTP
392stream may change certain configuration data mid-session, there are
393different methods for delivering this configuration data to a
394client, both in-band and out-of-band, which are detailed below.
395In order to set up an initial state for the client application, the
396configuration MUST be conveyed via the signalling channel used to set up
397the session. One example of such signalling is
398<xref target="RFC4566">SDP</xref> with the
399<xref target="RFC3264">Offer/Answer Model</xref>.
400Changes to the configuration MAY be communicated via a re-invite,
401conveying a new SDP, or sent in-band in the RTP channel.
402Implementations MUST support an in-band delivery of updated codebooks,
403and SHOULD support out-of-band codebook update using a new SDP file.
404The changes may be due to different codebooks as well as
405different bitrates of the RTP stream.
406</t>
407
408<t>For non-chained streams, the recommended Configuration delivery
409method is inside the <xref target="Packed Configuration">Packed
410Configuration</xref> in the SDP as explained the <xref
411target="Mapping Media Type Parameters into SDP"> Mapping Media Type
412Parameters into SDP</xref>.
413</t>
414
415<t>
416The 24-bit Ident field is used to map which Configuration will be used to
417decode a packet. When the Ident field changes, it indicates that a change in
418the stream has taken place. The client application MUST have in advance the
419correct configuration. If the client detects a change in the Ident value and
420does not have this information, it MUST NOT decode the raw associated Vorbis
421data until it fetches the correct Configuration.
422</t>
423
424<section anchor="In-band Header Transmission" title="In-band Header Transmission">
425
426<t>
427The <xref target="Packed Configuration">Packed Configuration</xref> Payload is
428sent in-band with the packet type bits set to match the Vorbis Data Type.
429Clients MUST be capable of dealing with fragmentation and periodic
430<xref target="RFC4588">re-transmission of</xref> the configuration headers.
431The RTP timestamp value MUST reflect the transmission time of the first data packet for which this configuration applies.
432</t>
433
434<section anchor="Packed Configuration" title="Packed Configuration">
435
436<t>
437A Vorbis Packed Configuration is indicated with the Vorbis Data Type field set
438to 1. Of the three headers defined in the
439<xref target="VORBIS-SPEC-REF">Vorbis I specification</xref>, the
440Identification and the Setup MUST be packed as they are, while the Comment
441header MAY be replaced with a dummy one.</t>
442<t>
443The packed configuration stores Xiph codec
444configurations in a generic way: the first field stores the number of the following packets
445minus one (count field), the next ones represent the size of the headers
446(length fields), and the headers immediately follow the list of length fields.
447The size of the last header is implicit.</t>
448<t>
449The count and the length fields are encoded using the following logic: the data
450is in network byte order; every byte has the most significant bit used
451as a flag, and the following 7 bits are used to store the value.
452The first 7 most significant bits are stored in the first byte.
453If there are remaining bits, the flag bit is set to 1 and the subsequent
4547 bits are stored in the following byte.
455If there are remaining bits, set the flag to 1 and the same procedure is
456repeated.
457The ending byte has the flag bit set to 0. To decode, simply iterate
458over the bytes until the flag bit is set to 0. For every byte, the data
459is added to the accumulated value multiplied by 128.</t>
460<t>
461The headers are packed in the same order as they are present in Ogg <xref target="VORBIS-SPEC-REF" />:
462Identification, Comment, Setup.</t>
463
464<t>
465The 2 byte length tag defines the length of the packed headers as the sum of
466the Configuration, Comment, and Setup lengths.</t>
467
468<figure anchor="Packed Configuration Figure" title="Packed Configuration Figure">
469<artwork><![CDATA[
470 0 1 2 3
471 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
472 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
473 |V=2|P|X| CC |M| PT | xxxx |
474 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
475 | xxxxx |
476 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
477 | synchronization source (SSRC) identifier |
478 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
479 | contributing source (CSRC) identifiers |
480 | ... |
481 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
482 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
483 | Ident | 0 | 1 | 1|
484 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
485 | length | n. of headers | length1 |
486 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
487 | length2 | Identification ..
488 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
489 .. Identification ..
490 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
491 .. Identification ..
492 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
493 .. Identification ..
494 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
495 .. Identification | Comment ..
496 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
497 .. Comment ..
498 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
499 .. Comment ..
500 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
501 .. Comment ..
502 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
503 .. Comment | Setup ..
504 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
505 .. Setup ..
506 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
507 .. Setup ..
508 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
509]]></artwork>
510</figure>
511
512<t>The Ident field is set with the value that will be used by the Raw Payload
513Packets to address this Configuration. The Fragment type is set to 0 because the
514packet bears the full Packed configuration. The number of the packet is set to 1.</t>
515</section>
516</section>
517
518<section anchor="Out of Band Transmission" title="Out of Band Transmission">
519
520<t>
521The following packet definition MUST be used when Configuration is inside
522in the SDP.
523</t>
524
525<section anchor="Packed Headers" title="Packed Headers">
526
527<t>
528As mentioned above, the RECOMMENDED delivery vector for Vorbis configuration
529data is via a retrieval method that can be performed using a reliable transport
530protocol. As the RTP headers are not required for this method of delivery, the
531structure of the configuration data is slightly different. The packed header
532starts with a 32-bit (network-byte ordered) count field, which details
533the number of packed headers that are contained in the bundle. The
534following shows the Packed header
535payload for each chained Vorbis stream.
536</t>
537
538<figure anchor="Packed Headers Overview Figure" title="Packed Headers Overview">
539<artwork><![CDATA[
540 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
541 | Number of packed headers |
542 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
543 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
544 | Packed header |
545 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
546 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
547 | Packed header |
548 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
549]]></artwork>
550</figure>
551
552<figure anchor="Packed Headers Detail Figure" title="Packed Headers Detail">
553<artwork><![CDATA[
554 0 1 2 3
555 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
556 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
557 | Ident | length ..
558 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
559 .. | n. of headers | length1 | length2 ..
560 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
561 .. | Identification Header ..
562 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
563 .................................................................
564 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
565 .. | Comment Header ..
566 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
567 .................................................................
568 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
569 .. Comment Header |
570 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
571 | Setup Header ..
572 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
573 .................................................................
574 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
575 .. Setup Header |
576 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
577]]></artwork>
578</figure>
579<t>
580The key difference between the in-band format and this one is that there is no
581need for the payload header octet. In this figure, the comment has a size bigger
582than 127 bytes.
583</t>
584</section>
585
586</section>
587
588<section anchor="Loss of Configuration Headers" title="Loss of Configuration Headers">
589
590<t>
591Unlike the loss of raw Vorbis payload data, loss of a configuration header
592leads to a situation where it will not be possible to successfully decode the
593stream. Implementations MAY try to recover from an error by requesting again the
594missing Configuration or, if the delivery method is in-band, by buffering the
595payloads waiting for the Configuration needed to decode them.
596The baseline reaction SHOULD either be reset or end the RTP session.
597</t>
598
599</section>
600
601</section>
602
603<section anchor="Comment Headers" title="Comment Headers">
604
605<t>
606Vorbis Data Type flag set to 2 indicates that the packet contains
607the comment metadata, such as artist name, track title, and so on. These
608metadata messages are not intended to be fully descriptive but rather to offer basic
609track/song information. Clients MAY ignore it completely. The details on the
610format of the comments can be found in the <xref target="VORBIS-SPEC-REF">Vorbis I Specification</xref>.
611</t>
612<figure anchor="Comment Packet Figure" title="Comment Packet">
613<artwork><![CDATA[
614 0 1 2 3
615 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
616 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
617 |V=2|P|X| CC |M| PT | xxxx |
618 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
619 | xxxxx |
620 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
621 | synchronization source (SSRC) identifier |
622 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
623 | contributing source (CSRC) identifiers |
624 | ... |
625 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
626 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
627 | Ident | 0 | 2 | 1|
628 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
629 | length | Comment ..
630 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
631 .. Comment ..
632 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
633 .. Comment |
634 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
635]]></artwork>
636</figure>
637
638<t>
639The 2-byte length field is necessary since this packet could be fragmented.
640</t>
641
642</section>
643<section anchor="Frame Packetization" title="Frame Packetization">
644
645<t>
646Each RTP payload contains either one Vorbis packet fragment or an integer
647number of complete Vorbis packets (up to a maximum of 15 packets, since the
648number of packets is defined by a 4-bit value).
649</t>
650
651<t>
652Any Vorbis data packet that is less than path MTU SHOULD be bundled in the RTP
653payload with as many Vorbis packets as will fit, up to a maximum of 15, except
654when such bundling would exceed an application's desired transmission latency.
655Path MTU is detailed in <xref target="RFC1191"></xref> and <xref target="RFC1981"></xref>.
656</t>
657
658<t>
659A fragmented packet has a zero in the last four bits of the payload header.
660The first fragment will set the Fragment type to 1. Each fragment after the
661first will set the Fragment type to 2 in the payload header. The consecutive
662fragments MUST be sent without any other payload being sent between the first
663and the last fragment. The RTP payload containing the last fragment of the
664Vorbis packet will have the Fragment type set to 3. To maintain the correct
665sequence for fragmented packet reception, the timestamp field of fragmented
666packets MUST be the same as the first packet sent, with the sequence number
667incremented as normal for the subsequent RTP payloads; this will affect the
668RTCP jitter measurement. The length field shows the fragment length.
669</t>
670
671<section anchor="Example Fragmented Vorbis Packet" title="Example Fragmented Vorbis Packet">
672
673<t>
674Here is an example of a fragmented Vorbis packet split over three RTP payloads.
675Each of them contains the standard RTP headers as well as the 4-octet Vorbis
676headers.
677</t>
678
679<figure anchor="Example Fragmented Packet (Packet 1)" title="Example Fragmented Packet (Packet 1)">
680<artwork><![CDATA[
681 Packet 1:
682
683 0 1 2 3
684 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
685 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
686 |V=2|P|X| CC |M| PT | 1000 |
687 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
688 | 12345 |
689 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
690 | synchronization source (SSRC) identifier |
691 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
692 | contributing source (CSRC) identifiers |
693 | ... |
694 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
695 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
696 | Ident | 1 | 0 | 0|
697 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
698 | length | vorbis data ..
699 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
700 .. vorbis data |
701 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
702]]></artwork>
703</figure>
704
705<t>
706In 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
707the payload is raw Vorbis data, the VDT field is set to 0.
708</t>
709
710<figure anchor="Example Fragmented Packet (Packet 2)" title="Example Fragmented Packet (Packet 2)">
711<artwork><![CDATA[
712 Packet 2:
713
714 0 1 2 3
715 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
716 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
717 |V=2|P|X| CC |M| PT | 1001 |
718 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
719 | 12345 |
720 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
721 | synchronization source (SSRC) identifier |
722 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
723 | contributing source (CSRC) identifiers |
724 | ... |
725 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
726 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
727 | Ident | 2 | 0 | 0|
728 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
729 | length | vorbis data ..
730 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
731 .. vorbis data |
732 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
733]]></artwork>
734</figure>
735
736<t>
737The Fragment type field is set to 2, and the number of packets field is set to 0.
738For large Vorbis fragments, there can be several of these types of payloads.
739The maximum packet size SHOULD be no greater than the path MTU,
740including all RTP and payload headers. The sequence number has been incremented
741by one, but the timestamp field remains the same as the initial payload.
742</t>
743
744<figure anchor="Example Fragmented Packet (Packet 3)" title="Example Fragmented Packet (Packet 3)">
745<artwork><![CDATA[
746 Packet 3:
747
748 0 1 2 3
749 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
750 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
751 |V=2|P|X| CC |M| PT | 1002 |
752 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
753 | 12345 |
754 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
755 | synchronization source (SSRC) identifier |
756 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
757 | contributing source (CSRC) identifiers |
758 | ... |
759 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
760 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
761 | Ident | 3 | 0 | 0|
762 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
763 | length | vorbis data ..
764 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
765 .. vorbis data |
766 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
767]]></artwork>
768</figure>
769
770<t>
771This is the last Vorbis fragment payload. The Fragment type is set to 3 and the
772packet count remains set to 0. As in the previous payloads, the timestamp remains
773set to the first payload timestamp in the sequence and the sequence number has
774been incremented.
775</t>
776</section>
777
778<section anchor="Packet Loss" title="Packet Loss">
779
780<t>
781As there is no error correction within the Vorbis stream, packet loss will
782result in a loss of signal. Packet loss is more of an issue for fragmented
783Vorbis packets as the client will have to cope with the handling of the
784Fragment Type. In case of loss of fragments, the client MUST discard all the
785remaining Vorbis fragments and decode the incomplete packet. If we use the
786fragmented Vorbis packet example above and the first RTP payload is lost, the
787client MUST detect that the next RTP payload has the packet count field set
788to 0 and the Fragment type 2 and MUST drop it.
789The next RTP payload, which is the final fragmented packet, MUST be dropped
790in the same manner.
791If the missing RTP payload is the last, the two fragments received will be
792kept and the incomplete Vorbis packet decoded.
793</t>
794
795<t>
796Loss of any of the Configuration fragment will result in the loss of the full
797Configuration packet with the result detailed in the <xref target="Loss of Configuration Headers">Loss of Configuration Headers</xref> section.
798</t>
799
800</section>
801</section>
802<section anchor="IANA Considerations" title="IANA Considerations">
803
804<list style="hanging">
805<t hangText="Type name:"> audio </t>
806
807<t hangText="Subtype name:"> vorbis </t>
808
809<t hangText="Required parameters:">
810
811<list style="hanging">
812<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>.
813</t>
814
815<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>.
816</t>
817
818
819<t hangText="configuration:"> the <xref target="RFC4648">base64</xref> representation of the <xref target="Packed Headers">Packed Headers</xref>.
820</t>
821</list>
822</t>
823
824<t hangText="Encoding considerations:">
825<vspace blankLines="1" />
826This media type is framed and contains binary data.
827</t>
828
829<t hangText="Security considerations:">
830<vspace blankLines="1" />
831See Section 10 of RFC 5215.</t>
832
833<t hangText="Interoperability considerations:">
834<vspace blankLines="1" />
835None</t>
836
837<t hangText="Published specification:">
838<vspace blankLines="1" />
839RFC 5215
840<vspace blankLines="1" />
841Ogg Vorbis I specification: Codec setup and packet decode. Available from the Xiph website, http://xiph.org/
842</t>
843
844
845<t hangText="Applications which use this media type:">
846<vspace blankLines="1"/>
847Audio streaming and conferencing tools </t>
848
849<t hangText="Additional information:">
850<vspace blankLines="1" />
851None </t>
852
853<t hangText="Person &amp; email address to contact for further information:">
854<vspace blankLines="1" />
855Luca Barbato: &lt;lu_zero@gentoo.org&gt;<br/>
856<vspace blankLines="0" />
857IETF Audio/Video Transport Working Group
858
859</t>
860
861<t hangText="Intended usage:">
862<vspace blankLines="1" />
863COMMON</t>
864
865<t hangText="Restriction on usage:">
866<vspace blankLines="1" />
867This media type depends on RTP framing, hence is only defined for transfer via <xref target="RFC3550">RTP</xref>.</t>
868
869<t hangText="Author:">
870<vspace blankLines="1"/>Luca Barbato</t>
871
872<t hangText="Change controller:">
873<vspace blankLines="1"/>IETF AVT Working Group delegated from the IESG</t>
874</list>
875
876<section anchor="Packed Headers IANA Considerations" title="Packed Headers IANA Considerations">
877
878<t>
879The following IANA considerations refers to the split configuration <xref target="Packed Headers">Packed Headers</xref> used within RFC 5215.
880</t>
881
882<list style="hanging">
883<t hangText="Type name:"> audio </t>
884
885<t hangText="Subtype name:"> vorbis-config </t>
886
887<t hangText="Required parameters:">
888<vspace blankLines="1" />
889None
890</t>
891
892<t hangText="Optional parameters:">
893<vspace blankLines="1" />
894None
895</t>
896
897<t hangText="Encoding considerations:">
898<vspace blankLines="1" />
899This media type contains binary data.
900</t>
901
902<t hangText="Security considerations:">
903<vspace blankLines="1" />
904See Section 10 of RFC 5215.
905</t>
906
907<t hangText="Interoperability considerations:">
908<vspace blankLines="1" />
909None
910</t>
911
912<t hangText="Published specification:">
913<vspace blankLines="1" />
914RFC 5215
915</t>
916
917<t hangText="Applications which use this media type:">
918<vspace blankLines="1" />
919Vorbis encoded audio, configuration data
920</t>
921
922<t hangText="Additional information:">
923<vspace blankLines="1" />
924None
925</t>
926
927<t hangText="Person &amp; email address to contact for further information:">
928<vspace blankLines="1" />
929Luca Barbato: &lt;lu_zero@gentoo.org&gt;
930<vspace blankLines="0" />
931IETF Audio/Video Transport Working Group
932</t>
933
934<t hangText="Intended usage:">
935COMMON
936</t>
937
938<t hangText="Restriction on usage:">
939<vspace blankLines="1" />
940This media type doesn't depend on the transport.
941</t>
942
943<t hangText="Author:">
944<vspace blankLines="1" />
945Luca Barbato</t>
946
947<t hangText="Change controller:">
948<vspace blankLines="1" />
949IETF AVT Working Group delegated from the IESG</t>
950</list>
951
952</section>
953
954</section>
955
956<section anchor="SDP related considerations" title="SDP Related Considerations">
957<t>
958The 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.
959</t>
960
961<section anchor="Mapping Media Type Parameters into SDP" title="Mapping Media Type Parameters into SDP">
962
963<t>
964The information carried in the Media Type specification has a
965specific mapping to fields in the <xref target="RFC4566">Session Description
966Protocol (SDP)</xref>, which is commonly used to describe RTP sessions.
967When SDP is used to specify sessions, the mapping are as follows:
968</t>
969
970<list style="symbols">
971
972<t>The type name ("audio") goes in SDP "m=" as the media name.</t>
973
974<t>The subtype name ("vorbis") goes in SDP "a=rtpmap" as the encoding name.</t>
975
976<t>The parameter "rate" also goes in "a=rtpmap" as the clock rate.</t>
977
978<t>The parameter "channels" also goes in "a=rtpmap" as the channel count.</t>
979
980<t>The mandated parameters "configuration" MUST be included in the SDP
981"a=fmtp" attribute.</t>
982
983</list>
984
985<t>
986If the stream comprises chained Vorbis files and all of them are known in
987advance, the Configuration Packet for each file SHOULD be passed to the client
988using the configuration attribute.
989</t>
990
991<t>
992The port value is specified by the server application bound to the address
993specified in the c= line. The channel count value specified in the rtpmap
994attribute SHOULD match the current Vorbis stream or should be considered the maximum
995number of channels to be expected. The timestamp clock rate MUST be a multiple
996of the sample rate; a different payload number MUST be used if the clock rate
997changes. The Configuration payload delivers the exact information, thus the
998SDP information SHOULD be considered a hint.
999An example is found below.
1000</t>
1001
1002<section anchor="SDP Example" title="SDP Example">
1003<t>The following example shows a basic SDP single stream. The first
1004configuration packet is inside the SDP; other configurations could be
1005fetched at any time from the URIs provided. The following
1006<xref target="RFC4648">base64</xref> configuration string is folded in this
1007example due to RFC line length limitations.</t>
1008
1009
1010<list style="empty">
1011<t>c=IN IP4 192.0.2.1</t>
1012<t>m=audio RTP/AVP 98</t>
1013<t>a=rtpmap:98 vorbis/44100/2</t>
1014<t>a=fmtp:98 configuration=AAAAAZ2f4g9NAh4aAXZvcmJpcwA...;</t>
1015</list>
1016</section>
1017
1018<t>
1019Note that the payload format (encoding) names are commonly shown in uppercase.
1020Media Type subtypes are commonly shown in lowercase. These names are
1021case-insensitive in both places. Similarly, parameter names are
1022case-insensitive both in Media Type types and in the default mapping to the SDP
1023a=fmtp attribute. The a=fmtp line is a single line, even if it is shown as multiple lines in this document for clarity.
1024</t>
1025
1026</section>
1027
1028<section anchor="Usage with the SDP Offer/Answer Mode" title="Usage with the SDP Offer/Answer Model">
1029
1030<t>
1031There are no negotiable parameters. All of them are declarative.
1032</t>
1033
1034</section>
1035
1036</section>
1037<section anchor="Congestion Control" title="Congestion Control">
1038<t>
1039The general congestion control considerations for transporting RTP
1040data apply to Vorbis audio over RTP as well. See the RTP specification
1041<xref target="RFC3550" /> and any applicable RTP profile (e.g., <xref target="RFC3551" />).
1042Audio data can be encoded using a range of different bit rates, so
1043it is possible to adapt network bandwidth by adjusting the encoder
1044bit rate in real time or by having multiple copies of content encoded
1045 at different bit rates.
1046</t>
1047</section>
1048<section anchor="Example" title="Example">
1049
1050<t>
1051The following example shows a common usage pattern that MAY be applied in
1052such a situation. The main scope of this section is to explain better usage
1053of the transmission vectors.
1054</t>
1055
1056<section anchor="Stream Radio" title="Stream Radio">
1057
1058<t>This is one of the most common situations: there is one single server streaming
1059content in multicast, and the clients may start a session at a random time. The
1060content itself could be a mix of a live stream (as the webjockey's voice)
1061and stored streams (as the music she plays).</t>
1062
1063<t>In this situation, we don't know in advance how many codebooks we will use.
1064The clients can join anytime and users expect to start listening to the content
1065in a short time.</t>
1066
1067<t>Upon joining, the client will receive the current Configuration necessary to
1068decode the current stream inside the SDP so that the decoding will start
1069immediately after.</t>
1070
1071<t>When the streamed content changes, the new Configuration is sent in-band
1072before the actual stream, and the Configuration that has to be sent inside
1073the SDP is updated. Since the in-band method is unreliable, an out-of-band
1074fallback is provided.</t>
1075
1076<t>The client may choose to fetch the Configuration from the alternate source
1077as soon as it discovers a Configuration packet got lost in-band, or use
1078<xref target="RFC3611">selective retransmission</xref> if the server supports
1079this feature.</t>
1080
1081<t>A server-side optimization would be to keep a hash list of the
1082Configurations per session, which avoids packing all of them and sending the same
1083Configuration with different Ident tags.</t>
1084
1085<t>A client-side optimization would be to keep a tag list of the Configurations
1086per session and not process configuration packets that are already known.</t>
1087
1088</section>
1089</section>
1090
1091<section anchor="Security Considerations" title="Security Considerations">
1092<t>
1093RTP packets using this payload format are subject to the security
1094considerations discussed in the
1095<xref target="RFC3550">RTP specification</xref>, the
1096<xref target="RFC4648">base64 specification</xref>, and the
1097<xref target="RFC3986">URI Generic syntax specification</xref>.
1098Among other considerations, this implies that the confidentiality of the
1099media stream is achieved by using encryption. Because the data compression used
1100with this payload format is applied end-to-end, encryption may be performed on
1101the compressed data.
1102</t>
1103
1104</section>
1105<section title="Copying Conditions">
1106 <t>The authors agree to grant third parties the irrevocable right to copy,
1107 use, and distribute the work, with or without modification, in any medium,
1108 without royalty, provided that, unless separate permission is granted,
1109 redistributed modified works do not contain misleading author, version,
1110 name of work, or endorsement information.</t>
1111</section>
1112<section anchor="Acknowledgments" title="Acknowledgments">
1113
1114<t>
1115This document is a continuation of the following documents:
1116</t><t>
1117Moffitt, J., "RTP Payload Format for Vorbis Encoded Audio", February 2001.
1118</t><t>
1119Kerr, R., "RTP Payload Format for Vorbis Encoded Audio", December 2004.
1120</t><t>
1121The Media Type declaration is a continuation of the following
1122document:</t><t>
1123Short, B., "The audio/rtp-vorbis MIME Type", January 2008.
1124</t>
1125
1126<t>
1127Thanks to the AVT, Vorbis Communities / Xiph.Org Foundation including Steve Casner,
1128Aaron Colwell, Ross Finlayson, Fluendo, Ramon Garcia, Pascal Hennequin, Ralph
1129Giles, Tor-Einar Jarnbjo, Colin Law, John Lazzaro, Jack Moffitt, Christopher
1130Montgomery, Colin Perkins, Barry Short, Mike Smith, Phil Kerr, Michael Sparks,
1131Magnus Westerlund, David Barrett, Silvia Pfeiffer, Stefan Ehmann, Gianni Ceccarelli and Alessandro Salvatori. Thanks to the LScube Group, in particular Federico
1132Ridolfo, Francesco Varano, Giampaolo Mancini, Dario Gallucci, and Juan Carlos De Martin.
1133</t>
1134
1135</section>
1136
1137</middle>
1138
1139<back>
1140
1141<references title="Normative References">
1142
1143<?rfc include="reference.RFC.2119" ?>
1144<?rfc include="reference.RFC.3550" ?>
1145<?rfc include="reference.RFC.3551" ?>
1146<?rfc include="reference.RFC.3986" ?>
1147<?rfc include="reference.RFC.4566" ?>
1148<?rfc include="reference.RFC.1191" ?>
1149<?rfc include="reference.RFC.1981" ?>
1150<?rfc include="reference.RFC.3264" ?>
1151<?rfc include="reference.RFC.4648" ?>
1152
1153<reference anchor="VORBIS-SPEC-REF">
1154<front>
1155<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>
1156</front>
1157</reference>
1158
1159</references>
1160
1161<references title="Informative References">
1162
1163<?rfc include="reference.RFC.3533" ?>
1164
1165<reference anchor="LIBVORBIS">
1166<front>
1167<title>libvorbis: Available from the dedicated website, http://vorbis.com/</title>
1168</front>
1169</reference>
1170
1171<?rfc include="reference.RFC.3611" ?>
1172<?rfc include="reference.RFC.4588" ?>
1173
1174</references>
1175</back>
1176</rfc>
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