idnits 2.17.1 draft-ietf-avt-rtp-speex-07.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** The document seems to lack a License Notice according IETF Trust Provisions of 28 Dec 2009, Section 6.b.i or Provisions of 12 Sep 2009 Section 6.b -- however, there's a paragraph with a matching beginning. Boilerplate error? (You're using the IETF Trust Provisions' Section 6.b License Notice from 12 Feb 2009 rather than one of the newer Notices. See https://trustee.ietf.org/license-info/.) Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- No issues found here. Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (May 7, 2009) is 5468 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) -- Obsolete informational reference (is this intentional?): RFC 4288 (Obsoleted by RFC 6838) Summary: 2 errors (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 AVT G. Herlein 3 Internet-Draft 4 Intended status: Standards Track J. Valin 5 Expires: November 8, 2009 Xiph.Org Foundation 6 A. Heggestad 7 Creytiv.com 8 A. Moizard 9 Antisip 10 May 7, 2009 12 RTP Payload Format for the Speex Codec 13 draft-ietf-avt-rtp-speex-07 15 Status of this Memo 17 This Internet-Draft is submitted to IETF in full conformance with the 18 provisions of BCP 78 and BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on November 8, 2009. 38 Copyright Notice 40 Copyright (c) 2009 IETF Trust and the persons identified as the 41 document authors. All rights reserved. 43 This document is subject to BCP 78 and the IETF Trust's Legal 44 Provisions Relating to IETF Documents in effect on the date of 45 publication of this document (http://trustee.ietf.org/license-info). 46 Please review these documents carefully, as they describe your rights 47 and restrictions with respect to this document. 49 Abstract 51 Speex is an open-source voice codec suitable for use in Voice over IP 52 (VoIP) type applications. This document describes the payload format 53 for Speex generated bit streams within an RTP packet. Also included 54 here are the necessary details for the use of Speex with the Session 55 Description Protocol (SDP). 57 Editors Note 59 All references to RFC XXXX are to be replaced by references to the 60 RFC number of this memo, when published. 62 Table of Contents 64 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 65 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 66 3. RTP usage for Speex . . . . . . . . . . . . . . . . . . . . . 6 67 3.1. RTP Speex Header Fields . . . . . . . . . . . . . . . . . 6 68 3.2. RTP payload format for Speex . . . . . . . . . . . . . . . 6 69 3.3. Speex payload . . . . . . . . . . . . . . . . . . . . . . 6 70 3.4. Example Speex packet . . . . . . . . . . . . . . . . . . . 7 71 3.5. Multiple Speex frames in a RTP packet . . . . . . . . . . 8 72 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 73 4.1. Media Type Registration . . . . . . . . . . . . . . . . . 9 74 4.1.1. Registration of media type audio/speex . . . . . . . . 9 75 5. SDP usage of Speex . . . . . . . . . . . . . . . . . . . . . . 12 76 5.1. Example supporting all modes, prefer mode 4 . . . . . . . 15 77 5.2. Example supporting only mode 3 and 5 . . . . . . . . . . . 15 78 5.3. Example with Variable Bit Rate and Comfort Noise . . . . . 15 79 5.4. Example with Voice Activity Detection . . . . . . . . . . 15 80 5.5. Example with Multiple sampling rates . . . . . . . . . . . 15 81 5.6. Example with ptime and Multiple Speex frames . . . . . . . 16 82 5.7. Example with Complete Offer/Answer exchange . . . . . . . 16 83 6. Implementation Guidelines . . . . . . . . . . . . . . . . . . 18 84 7. Security Considerations . . . . . . . . . . . . . . . . . . . 19 85 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20 86 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 87 9.1. Normative References . . . . . . . . . . . . . . . . . . . 21 88 9.2. Informative References . . . . . . . . . . . . . . . . . . 21 89 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 91 1. Introduction 93 Speex is based on the CELP [CELP] encoding technique with support for 94 either narrowband (nominal 8kHz), wideband (nominal 16kHz) or ultra- 95 wideband (nominal 32kHz). The main characteristics can be summarized 96 as follows: 98 o Free software/open-source 100 o Integration of wideband and narrowband in the same bit-stream 102 o Wide range of bit-rates available 104 o Dynamic bit-rate switching and variable bit-rate (VBR) 106 o Voice Activity Detection (VAD, integrated with VBR) 108 o Variable complexity 110 The Speex codec supports a wide range of bit-rates from 2.15 kbit/s 111 to 44 kbit/s. In some cases however, it may not be possible for an 112 implementation to include support for all rates (e.g. because of 113 bandwidth, RAM or CPU constraints). In those cases, to be compliant 114 with this specification, implementations MUST support at least 115 narrowband (8 kHz) encoding and decoding at 8 kbit/s bit-rate 116 (narrowband mode 3). Support for narrowband at 15 kbit/s (narrowband 117 mode 5) is RECOMMENDED and support for wideband at 27.8 kbit/s 118 (wideband mode 8) is also RECOMMENDED. The sampling rate MUST be 8, 119 16 or 32 kHz. This specification defines only single channel audio 120 (mono). 122 2. Terminology 124 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 125 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 126 document are to be interpreted as described in RFC2119 [RFC2119] and 127 indicate requirement levels for compliant RTP implementations. 129 3. RTP usage for Speex 131 3.1. RTP Speex Header Fields 133 The RTP header is defined in the RTP specification [RFC3550]. This 134 section defines how fields in the RTP header are used. 136 Payload Type (PT): The assignment of an RTP payload type for this 137 packet format is outside the scope of this document; it is 138 specified by the RTP profile under which this payload format is 139 used, or signaled dynamically out-of-band (e.g., using SDP). 141 Marker (M) bit: The M bit is set to one on the first packet sent 142 after a silence period, during which packets have not been 143 transmitted contiguously. 145 Extension (X) bit: Defined by the RTP profile used. 147 Timestamp: A 32-bit word that corresponds to the sampling instant 148 for the first frame in the RTP packet. 150 3.2. RTP payload format for Speex 152 The RTP payload for Speex has the format shown in Figure 1. No 153 additional header fields specific to this payload format are 154 required. For RTP based transportation of Speex encoded audio the 155 standard RTP header [RFC3550] is followed by one or more payload data 156 blocks. An optional padding terminator may also be used. 158 0 1 2 3 159 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 1 160 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 161 | RTP Header | 162 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 163 | one or more frames of Speex .... | 164 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 165 | one or more frames of Speex .... | padding | 166 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 168 Figure 1: RTP payload for Speex 170 3.3. Speex payload 172 For the purposes of packetizing the bit stream in RTP, it is only 173 necessary to consider the sequence of bits as output by the Speex 174 encoder [speex_manual], and present the same sequence to the decoder. 175 The payload format described here maintains this sequence. 177 A typical Speex frame, encoded at the maximum bitrate, is approx. 110 178 octets and the total number of Speex frames SHOULD be kept less than 179 the path MTU to prevent fragmentation. Speex frames MUST NOT be 180 fragmented across multiple RTP packets, 182 The Speex frames must be placed starting with the oldest frame and 183 then continue consecutive in time. 185 An RTP packet MAY contain Speex frames of the same bit rate or of 186 varying bit rates, since the bit-rate for a frame is conveyed in band 187 with the signal. 189 The encoding and decoding algorithm can change the bit rate at any 20 190 msec frame boundary, with the bit rate change notification provided 191 in-band with the bit stream. Each frame contains both sampling rate 192 (narrowband, wideband or ultra-wideband) and "mode" (bit-rate) 193 information in the bit stream. No out-of-band notification is 194 required for the decoder to process changes in the bit rate sent by 195 the encoder. 197 The sampling rate MUST be either 8000 Hz, 16000 Hz, or 32000 Hz. 199 The RTP payload MUST be padded to provide an integer number of octets 200 as the payload length. These padding bits are LSB aligned in network 201 octet order and consist of a 0 followed by all ones (until the end of 202 the octet). This padding is only required for the last frame in the 203 packet, and only to ensure the packet contents ends on an octet 204 boundary. 206 3.4. Example Speex packet 208 In the example below we have a single Speex frame with 5 bits of 209 padding to ensure the packet size falls on an octet boundary. 211 0 1 2 3 212 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 1 213 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 214 | RTP Header | 215 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 216 | ..speex data.. | 217 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 218 | ..speex data.. |0 1 1 1 1| 219 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 221 3.5. Multiple Speex frames in a RTP packet 223 Below is an example of two Speex frames contained within one RTP 224 packet. The Speex frame length in this example fall on an octet 225 boundary so there is no padding. 227 The Speex decoder [speex_manual] can detect the bitrate from the 228 payload and is responsible for detecting the 20 msec boundaries 229 between each frame. 231 0 1 2 3 232 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 1 233 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 234 | RTP Header | 235 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ 236 | ..speex frame 1.. | 237 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 238 | ..speex frame 1.. | ..speex frame 2.. | 239 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 240 | ..speex frame 2.. | 241 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 243 4. IANA Considerations 245 This document defines the Speex media type. 247 4.1. Media Type Registration 249 This section describes the media types and names associated with this 250 payload format. The section registers the media types, as per 251 RFC4288 [RFC4288] 253 4.1.1. Registration of media type audio/speex 255 Media type name: audio 257 Media subtype name: speex 259 Required parameters: 261 rate: RTP timestamp clock rate, which is equal to the sampling 262 rate in Hz. The sampling rate MUST be either 8000, 16000, or 263 32000. 265 Optional parameters: 267 ptime: SHOULD be a multiple of 20 msec [RFC4566] 269 maxptime: SHOULD be a multiple of 20 msec [RFC4566] 271 vbr: variable bit rate - either 'on' 'off' or 'vad' (defaults to 272 off). If on, variable bit rate is enabled. If off, disabled. If 273 set to 'vad' then constant bit rate is used but silence will be 274 encoded with special short frames to indicate a lack of voice for 275 that period. This parameter is a preference to the encoder. 277 cng: comfort noise generation - either 'on' or 'off' (defaults to 278 off). If off then silence frames will be silent; if 'on' then 279 those frames will be filled with comfort noise. This parameter is 280 a preference to the encoder. 282 mode: Comma-separated list of supported Speex decoding modes, in 283 order of preference. The first is the most preferred and the 284 remaining is in decreasing order of preference. The valid modes 285 are different for narrowband and wideband, and are defined as 286 follows: 288 * {1,2,3,4,5,6,7,8,any} for narrowband 290 * {0,1,2,3,4,5,6,7,8,9,10,any} for wideband and ultra-wideband 292 The 'mode' parameters may contain multiple values. In this case, 293 the remote party SHOULD configure its encoder using the first 294 supported mode provided. When 'any' is used, the offerer 295 indicates that it supports all decoding modes. The 'mode' 296 parameter value MUST always be quoted. If the 'mode' parameter is 297 not provided, the mode value is considered to be equivalent to 298 'mode="3,any"' in narrowband and 'mode="8,any"' in wideband and 299 ultra-wideband. Note that each Speex frame does contains the mode 300 (or bit-rate) that should be used to decode it. Thus application 301 MUST be able to decode any Speex frame unless the SDP clearly 302 specify that some modes are not supported. (e.g., by not including 303 'mode="any"') Indicating support for a given set of decoding 304 modes, also implies that the implementation support the same 305 encoding modes. 307 Encoding considerations: 309 This media type is framed and binary, see section 4.8 in 310 [RFC4288]. 312 Security considerations: See Section 6 314 Interoperability considerations: 316 None. 318 Published specification: 320 RFC XXXX [RFC Editor: please replace by the RFC number of this 321 memo, when published] 323 Applications which use this media type: 325 Audio streaming and conferencing applications. 327 Additional information: none 329 Person and email address to contact for further information : 331 Alfred E. Heggestad: aeh@db.org 333 Intended usage: COMMON 335 Restrictions on usage: 337 This media type depends on RTP framing, and hence is only defined 338 for transfer via RTP [RFC3550]. Transport within other framing 339 protocols is not defined at this time. 341 Author: Alfred E. Heggestad 343 Change controller: 345 IETF Audio/Video Transport working group delegated from the IESG. 347 5. SDP usage of Speex 349 The information carried in the media type specification has a 350 specific mapping to fields in the Session Description Protocol (SDP) 351 [RFC4566], which is commonly used to describe RTP sessions. When SDP 352 is used to specify sessions employing the Speex codec, the mapping is 353 as follows: 355 o The media type ("audio") goes in SDP "m=" as the media name. 357 o The media subtype ("speex") goes in SDP "a=rtpmap" as the encoding 358 name. The required parameter "rate" also goes in "a=rtpmap" as 359 the clock rate. 361 o The parameters "ptime" and "maxptime" go in the SDP "a=ptime" and 362 "a=maxptime" attributes, respectively. 364 o Any remaining parameters go in the SDP "a=fmtp" attribute by 365 copying them directly from the media type string as a semicolon 366 separated list of parameter=value pairs. 368 The tables below include the equivalence between modes and bitrates 369 for narrowband, wideband and ultra-wideband. Also, the corresponding 370 "Speex quality" setting (see SPEEX_SET_QUALITY in The Speex Codec 371 Manual [speex_manual]) is included as an indication. 373 +------+---------------+-------------+ 374 | mode | Speex quality | bitrate | 375 +------+---------------+-------------+ 376 | 1 | 0 | 2.15 kbit/s | 377 | | | | 378 | 2 | 2 | 5.95 kbit/s | 379 | | | | 380 | 3 | 3 or 4 | 8.00 kbit/s | 381 | | | | 382 | 4 | 5 or 6 | 11.0 kbit/s | 383 | | | | 384 | 5 | 7 or 8 | 15.0 kbit/s | 385 | | | | 386 | 6 | 9 | 18.2 kbit/s | 387 | | | | 388 | 7 | 10 | 24.6 kbit/s | 389 | | | | 390 | 8 | 1 | 3.95 kbit/s | 391 +------+---------------+-------------+ 393 Mode vs Bitrate table for narrowband 395 Table 1 397 +------+---------------+------------------+------------------------+ 398 | mode | Speex quality | wideband bitrate | ultra wideband bitrate | 399 +------+---------------+------------------+------------------------+ 400 | 0 | 0 | 3.95 kbit/s | 5.75 kbit/s | 401 | | | | | 402 | 1 | 1 | 5.75 kbit/s | 7.55 kbit/s | 403 | | | | | 404 | 2 | 2 | 7.75 kbit/s | 9.55 kbit/s | 405 | | | | | 406 | 3 | 3 | 9.80 kbit/s | 11.6 kbit/s | 407 | | | | | 408 | 4 | 4 | 12.8 kbit/s | 14.6 kbit/s | 409 | | | | | 410 | 5 | 5 | 16.8 kbit/s | 18.6 kbit/s | 411 | | | | | 412 | 6 | 6 | 20.6 kbit/s | 22.4 kbit/s | 413 | | | | | 414 | 7 | 7 | 23.8 kbit/s | 25.6 kbit/s | 415 | | | | | 416 | 8 | 8 | 27.8 kbit/s | 29.6 kbit/s | 417 | | | | | 418 | 9 | 9 | 34.2 kbit/s | 36.0 kbit/s | 419 | | | | | 420 | 10 | 10 | 42.2 kbit/s | 44.0 kbit/s | 421 +------+---------------+------------------+------------------------+ 423 Mode vs Bitrate table for wideband and ultra-wideband 425 Table 2 427 The Speex parameters indicate the decoding capabilities of the agent, 428 and what the agent prefers to receive. 430 The Speex parameters in an SDP Offer/Answer exchange are completely 431 orthogonal, and there is no relationship between the SDP Offer and 432 the Answer. 434 Several Speex specific parameters can be given in a single a=fmtp 435 line provided that they are separated by a semi-colon: 437 a=fmtp:97 mode="1,any";vbr=on 439 Some example SDP session descriptions utilizing Speex encodings 440 follow. 442 5.1. Example supporting all modes, prefer mode 4 444 The offerer indicates that it wishes to receive a Speex stream at 445 8000Hz, and wishes to receive Speex 'mode 4'. It is important to 446 understand that any other mode might still be sent by remote party: 447 the device might have bandwidth limitation or might only be able to 448 send 'mode="3"'. Thus, application that support all decoding modes 449 SHOULD include 'mode="any"' as shown in the example below: 451 m=audio 8088 RTP/AVP 97 452 a=rtpmap:97 speex/8000 453 a=fmtp:97 mode="4,any" 455 5.2. Example supporting only mode 3 and 5 457 The offerer indicates the mode he wishes to receive (Speex 'mode 3'). 458 This offer indicates mode 3 and mode 5 are supported and that no 459 other modes are supported. The remote party MUST NOT configure its 460 encoder using another Speex mode. 462 m=audio 8088 RTP/AVP 97 463 a=rtmap:97 speex/8000 464 a=fmtp:97 mode="3,5" 466 5.3. Example with Variable Bit Rate and Comfort Noise 468 The offerer indicates that it wishes to receive variable bit rate 469 frames with comfort noise: 471 m=audio 8088 RTP/AVP 97 472 a=rtmap:97 speex/8000 473 a=fmtp:97 vbr=on;cng=on 475 5.4. Example with Voice Activity Detection 477 The offerer indicates that it wishes to use silence suppression. In 478 this case vbr=vad parameter will be used: 480 m=audio 8088 RTP/AVP 97 481 a=rtmap:97 speex/8000 482 a=fmtp:97 vbr=vad 484 5.5. Example with Multiple sampling rates 486 The offerer indicates that it wishes to receive Speex audio at 16000 487 Hz with mode 10 (42.2 kbit/s), alternatively Speex audio at 8000 Hz 488 with mode 7 (24.6 kbit/s). The offerer supports decoding all modes. 490 m=audio 8088 RTP/AVP 97 98 491 a=rtmap:97 speex/16000 492 a=fmtp:97 mode="10,any" 493 a=rtmap:98 speex/8000 494 a=fmtp:98 mode="7,any" 496 5.6. Example with ptime and Multiple Speex frames 498 The "ptime" SDP attribute is used to denote the packetization 499 interval (ie, how many milliseconds of audio is encoded in a single 500 RTP packet). Since Speex uses 20 msec frames, ptime values of 501 multiples of 20 denote multiple Speex frames per packet. It is 502 recommended to use ptime values which are a multiple of 20. 504 If ptime contains a value which is not multiple of 20, the internal 505 interpretation of it should be rounded up to the nearest multiple of 506 20 before the number of Speex frames is calculated. For example if 507 the "ptime" attribute is set to 30, the internal interpretation 508 should be rounded up to 40 and then used to calculate 2 Speex frames 509 per packet. 511 In the example below the ptime value is set to 40, indicating that 512 there are 2 frames in each packet. 514 m=audio 8088 RTP/AVP 97 515 a=rtpmap:97 speex/8000 516 a=ptime:40 518 Note that the ptime parameter applies to all payloads listed in the 519 media line and is not used as part of an a=fmtp directive. 521 Care must be taken when setting the value of ptime so that the RTP 522 packet size does not exceed the path MTU. 524 5.7. Example with Complete Offer/Answer exchange 526 The offerer indicates that it wishes to receive Speex audio at 16000 527 Hz, alternatively Speex audio at 8000 Hz. The offerer does support 528 ALL modes because no mode is specified. 530 m=audio 8088 RTP/AVP 97 98 531 a=rtmap:97 speex/16000 532 a=rtmap:98 speex/8000 534 The answerer indicates that it wishes to receive Speex audio at 8000 535 Hz, which is the only sampling rate it supports. The answerer does 536 support ALL modes because no mode is specified. 538 m=audio 8088 RTP/AVP 99 539 a=rtmap:99 speex/8000 541 6. Implementation Guidelines 543 Implementations that supports Speex are responsible for correctly 544 decoding incoming Speex frames. 546 Each Speex frame does contains all needed informations to decode 547 itself. In particular, the 'mode' and 'ptime' values proposed in the 548 SDP contents MUST NOT be used for decoding: those values are not 549 needed to properly decode a RTP Speex stream. 551 7. Security Considerations 553 RTP packets using the payload format defined in this specification 554 are subject to the security considerations discussed in the RTP 555 specification [RFC3550], and any appropriate RTP profile. This 556 implies that confidentiality of the media streams is achieved by 557 encryption. Because the data compression used with this payload 558 format is applied end-to-end, encryption may be performed after 559 compression so there is no conflict between the two operations. 561 A potential denial-of-service threat exists for data encodings using 562 compression techniques that have non-uniform receiver-end 563 computational load. The attacker can inject pathological datagrams 564 into the stream which are complex to decode and cause the receiver to 565 be overloaded. However, this encoding does not exhibit any 566 significant non-uniformity. 568 As with any IP-based protocol, in some circumstances a receiver may 569 be overloaded simply by the receipt of too many packets, either 570 desired or undesired. Network-layer authentication may be used to 571 discard packets from undesired sources, but the processing cost of 572 the authentication itself may be too high. 574 8. Acknowledgements 576 The authors would like to thank Equivalence Pty Ltd of Australia for 577 their assistance in attempting to standardize the use of Speex in 578 H.323 applications, and for implementing Speex in their open source 579 OpenH323 stack. The authors would also like to thank Brian C. Wiles 580 of StreamComm for his assistance in developing 581 the proposed standard for Speex use in H.323 applications. 583 The authors would also like to thank the following members of the 584 Speex and AVT communities for their input: Ross Finlayson, Federico 585 Montesino Pouzols, Henning Schulzrinne, Magnus Westerlund, Colin 586 Perkins, Ivo Emanuel Goncalves. 588 Thanks to former authors of this document; Simon Morlat, Roger 589 Hardiman, Phil Kerr. 591 9. References 593 9.1. Normative References 595 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 596 Requirement Levels", BCP 14, RFC 2119, March 1997. 598 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 599 Jacobson, "RTP: A Transport Protocol for Real-Time 600 Applications", STD 64, RFC 3550, July 2003. 602 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 603 Description Protocol", RFC 4566, July 2006. 605 9.2. Informative References 607 [CELP] "CELP, U.S. Federal Standard 1016.", National Technical 608 Information Service (NTIS) website http://www.ntis.gov/. 610 [RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and 611 Registration Procedures", BCP 13, RFC 4288, December 2005. 613 [speex_manual] 614 Valin, J., "The Speex Codec Manual", Speex 615 website http://www.speex.org/docs/. 617 Authors' Addresses 619 Greg Herlein 620 2034 Filbert Street 621 San Francisco, California 94123 622 United States 624 Email: gherlein@herlein.com 626 Jean-Marc Valin 627 Xiph.Org Foundation 629 Email: jean-marc.valin@usherbrooke.ca 631 Alfred E. Heggestad 632 Creytiv.com 633 Biskop J. Nilssonsgt. 20a 634 Oslo 0659 635 Norway 637 Email: aeh@db.org 639 Aymeric Moizard 640 Antisip 641 5 Place Benoit Crepu 642 Lyon, 69005 643 France