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 & email address to contact for further information:"> 854<vspace blankLines="1" /> 855Luca Barbato: <lu_zero@gentoo.org><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 & email address to contact for further information:"> 928<vspace blankLines="1" /> 929Luca Barbato: <lu_zero@gentoo.org> 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> 1177