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Even, Ed. 3 Internet-Draft Huawei Technologies 4 Obsoletes: RFC5285 (if approved) D. Singer 5 Intended status: Standards Track Apple, Inc. 6 Expires: November 12, 2016 H. Desineni 7 May 11, 2016 9 A General Mechanism for RTP Header Extensions 10 draft-ietf-avtcore-rfc5285-bis-02.txt 12 Abstract 14 This document provides a general mechanism to use the header 15 extension feature of RTP (the Real-Time Transport Protocol). It 16 provides the option to use a small number of small extensions in each 17 RTP packet, where the universe of possible extensions is large and 18 registration is de-centralized. The actual extensions in use in a 19 session are signaled in the setup information for that session. 21 Status of This Memo 23 This Internet-Draft is submitted in full conformance with the 24 provisions of BCP 78 and BCP 79. 26 Internet-Drafts are working documents of the Internet Engineering 27 Task Force (IETF). Note that other groups may also distribute 28 working documents as Internet-Drafts. The list of current Internet- 29 Drafts is at http://datatracker.ietf.org/drafts/current/. 31 Internet-Drafts are draft documents valid for a maximum of six months 32 and may be updated, replaced, or obsoleted by other documents at any 33 time. It is inappropriate to use Internet-Drafts as reference 34 material or to cite them other than as "work in progress." 36 This Internet-Draft will expire on November 12, 2016. 38 Copyright Notice 40 Copyright (c) 2016 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 45 (http://trustee.ietf.org/license-info) in effect on the date of 46 publication of this document. Please review these documents 47 carefully, as they describe your rights and restrictions with respect 48 to this document. Code Components extracted from this document must 49 include Simplified BSD License text as described in Section 4.e of 50 the Trust Legal Provisions and are provided without warranty as 51 described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 56 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 3 57 3. Design Goals . . . . . . . . . . . . . . . . . . . . . . . . 3 58 4. Packet Design . . . . . . . . . . . . . . . . . . . . . . . . 3 59 4.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 3 60 4.2. One-Byte Header . . . . . . . . . . . . . . . . . . . . . 5 61 4.3. Two-Byte Header . . . . . . . . . . . . . . . . . . . . . 7 62 5. SDP Signaling Design . . . . . . . . . . . . . . . . . . . . 8 63 6. SDP Signaling for support of mixed one byte and two bytes 64 header extensions. . . . . . . . . . . . . . . . . . . . . . 10 65 7. Offer/Answer . . . . . . . . . . . . . . . . . . . . . . . . 11 66 8. BNF Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 13 67 9. Security Considerations . . . . . . . . . . . . . . . . . . . 14 68 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 69 10.1. Identifier Space for IANA to Manage . . . . . . . . . . 14 70 10.2. Registration of the SDP extmap Attribute . . . . . . . . 16 71 10.3. Registration of the SDP Attribute . . . . . . . . . . . 16 72 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 73 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 74 12.1. Normative References . . . . . . . . . . . . . . . . . . 17 75 12.2. Informative References . . . . . . . . . . . . . . . . . 18 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 78 1. Introduction 80 The RTP specification [RFC3550] provides a capability to extend the 81 RTP header. It defines the header extension format and rules for its 82 use in Section 5.3.1. The existing header extension method permits 83 at most one extension per RTP packet, identified by a 16-bit 84 identifier and a 16-bit length field specifying the length of the 85 header extension in 32-bit words. 87 This mechanism has two conspicuous drawbacks. First, it permits only 88 one header extension in a single RTP packet. Second, the 89 specification gives no guidance as to how the 16-bit header extension 90 identifiers are allocated to avoid collisions. 92 This specification removes the first drawback by defining a backward- 93 compatible and extensible means to carry multiple header extension 94 elements in a single RTP packet. It removes the second drawback by 95 defining that these extension elements are named by URIs, defining an 96 IANA registry for extension elements defined in IETF specifications, 97 and a Session Description Protocol (SDP) method for mapping between 98 the naming URIs and the identifier values carried in the RTP packets. 100 This header extension applies to RTP/AVP (the Audio/Visual Profile) 101 and its extensions. 103 This document removes a limitation from RFC5285 that did not allow 104 sending both one byte and two bytes header extensions in the same RTP 105 stream 107 2. Requirements Notation 109 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 110 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 111 document are to be interpreted as described in [RFC2119]. 113 3. Design Goals 115 The goal of this design is to provide a simple mechanism whereby 116 multiple identified extensions can be used in RTP packets, without 117 the need for formal registration of those extensions but nonetheless 118 avoiding collision. 120 This mechanism provides an alternative to the practice of burying 121 associated metadata into the media format bit stream. This has often 122 been done in media data sent over fixed-bandwidth channels. Once 123 this is done, a decoder for the specific media format needs to 124 extract the metadata. Also, depending on the media format, the 125 metadata can be added at the time of encoding the media so that the 126 bit-rate used for the metadata is taken into account. But the 127 metadata can be unknown at that time. Inserting metadata at a later 128 time can cause a decode and re-encode to meet bit-rate requirements. 130 In some cases, a more appropriate, higher-level mechanism can be 131 available, and if so, it can be used. For cases where a higher-level 132 mechanism is not available, it is better to provide a mechanism at 133 the RTP level than have the metadata be tied to a specific form of 134 media data. 136 4. Packet Design 138 4.1. General 140 The following design is fit into the "header extension" of the RTP 141 extension, as described above. 143 The presence and format of this header extension and its contents are 144 negotiated or defined out-of-band, such as through signaling (see 145 below for SDP signaling). The value defined for an RTP extension 146 (defined below for the one-byte and two-byte header forms) is only an 147 architectural constant (e.g., for use by network analyzers); it is 148 the negotiation/definition (e.g., in SDP) that is the definitive 149 indication that this header extension is present. 151 This specification updates the requirement from the RTP specification 152 that the header extension "is designed so that the header extension 153 MAY be ignored". To be specific, header extensions using this 154 specification SHOULD be used for data that can safely be ignored by 155 the recipient without affecting interoperability, there can be 156 essential header extensions for interoperability and intermediaries 157 SHOULD NOT remove such header extensions. Note that the support of 158 header extension as specified in this recommendation is negotiated. 159 RTP Header extensions MUST NOT be used when the presence of the 160 extension has changed the form or nature of the rest of the packet in 161 a way that is not compatible with the way the stream is signaled 162 (e.g., as defined by the payload type). Valid examples might include 163 metadata that is additional to the usual RTP information, e.g. Audio 164 level from Client to mixer [RFC6464]. 166 The RTP header extension is formed as a sequence of extension 167 elements, with possible padding. Each extension element has a local 168 identifier and a length. The local identifiers MAY be mapped to a 169 larger namespace in the negotiation (e.g., session signaling). 171 As is good network practice, data SHOULD only be transmitted when 172 needed. The RTP header extension SHOULD only be present in a packet 173 if that packet also contains one or more extension elements, as 174 defined here. An extension element SHOULD only be present in a 175 packet when needed; the signaling setup of extension elements 176 indicates only that those elements can be present in some packets, 177 not that they are in fact present in all (or indeed, any) packets. 179 Each extension element in a packet has a local identifier (ID) and a 180 length. The local identifiers present in the stream MUST have been 181 negotiated or defined out-of-band. There are no static allocations 182 of local identifiers. Each distinct extension MUST have a unique ID. 183 The value 0 is reserved for padding and MUST NOT be used as a local 184 identifier. 186 There are two variants of the extension: one-byte and two-byte 187 headers. Since it is expected that (a) the number of extensions in 188 any given RTP session is small and (b) the extensions themselves are 189 small, the one-byte header form is preferred and MUST be supported by 190 all receivers. A stream MUST contain only one-byte or two-byte 191 headers unless it is known that all recipients support mixing, either 192 by offer/answer negotiation (see section 6) or by out-of-band 193 knowledge. One-byte and two-byte headers MUST NOT be mixed in a 194 single RTP packet. Transmitters SHOULD NOT use the two-byte form 195 when all extensions are small enough for the one-byte header form. A 196 transmitter MAY be aware that an intermediary may add RTP header 197 extensions in this case, the transmitter SHOULD use two-byte form. 199 A sequence of extension elements, possibly with padding, forms the 200 header extension defined in the RTP specification. There are as many 201 extension elements as fit into the length as indicated in the RTP 202 header extension length. Since this length is signaled in full 203 32-bit words, padding bytes are used to pad to a 32-bit boundary. 204 The entire extension is parsed byte-by-byte to find each extension 205 element (no alignment is needed), and parsing stops at the earlier of 206 the end of the entire header extension, or in one-byte headers only 207 case, on encountering an identifier with the reserved value of 15. 209 In both forms, padding bytes have the value of 0 (zero). They MAY be 210 placed between extension elements, if desired for alignment, or after 211 the last extension element, if needed for padding. A padding byte 212 does not supply the ID of an element, nor the length field. When a 213 padding byte is found, it is ignored and the parser moves on to 214 interpreting the next byte. 216 Note carefully that the one-byte header form allows for data lengths 217 between 1 and 16 bytes, by adding 1 to the signaled length value 218 (thus, 0 in the length field indicates 1 byte of data follows). This 219 allows for the important case of 16-byte payloads. This addition is 220 not performed for the two-byte headers, where the length field 221 signals data lengths between 0 and 255 bytes. 223 Use of RTP header extensions will reduce the efficiency of RTP header 224 compression, since the header extension will be sent uncompressed 225 unless the RTP header compression module is updated to recognize the 226 extension header. If header extensions are present in some packets, 227 but not in others, this can also reduce compression efficiency by 228 requiring an update to the fixed header to be conveyed when header 229 extensions start or stop being sent. The interactions of the RTP 230 header extension and header compression is explored further in 231 [RFC2508] and [RFC3095]. 233 4.2. One-Byte Header 235 In the one-byte header form of extensions, the 16-bit value REQUIRED 236 by the RTP specification for a header extension, labeled in the RTP 237 specification as "defined by profile", MUST have the fixed bit 238 pattern 0xBEDE (the first version of this specification was written 239 on the feast day of the Venerable Bede). 241 Each extension element MUST starts with a byte containing an ID and a 242 length: 244 0 245 0 1 2 3 4 5 6 7 246 +-+-+-+-+-+-+-+-+ 247 | ID | len | 248 +-+-+-+-+-+-+-+-+ 250 The 4-bit ID is the local identifier of this element in the range 251 1-14 inclusive. In the signaling section, this is referred to as the 252 valid range. 254 The local identifier value 15 is reserved for future extension and 255 MUST NOT be used as an identifier. If the ID value 15 is 256 encountered, its length field MUST be ignored, processing of the 257 entire extension MUST terminate at that point, and only the extension 258 elements present prior to the element with ID 15 SHOULD be 259 considered. 261 The 4-bit length is the number minus one of data bytes of this header 262 extension element following the one-byte header. Therefore, the 263 value zero in this field indicates that one byte of data follows, and 264 a value of 15 (the maximum) indicates element data of 16 bytes. 265 (This permits carriage of 16-byte values, which is a common length of 266 labels and identifiers, while losing the possibility of zero-length 267 values -- which would often be padded anyway.) 269 An example header extension, with three extension elements, and some 270 padding follows: 272 0 1 2 3 273 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 1 274 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 275 | 0xBE | 0xDE | length=3 | 276 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 277 | ID | L=0 | data | ID | L=1 | data... 278 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 279 ...data | 0 (pad) | 0 (pad) | ID | L=3 | 280 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 281 | data | 282 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 284 4.3. Two-Byte Header 286 In the two-byte header form, the 16-bit value defined by the RTP 287 specification for a header extension, labeled in the RTP 288 specification as "defined by profile", is defined as shown below. 290 0 1 291 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 292 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 293 | 0x100 |appbits| 294 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 296 The appbits field is 4 bits that are application-dependent and MAY be 297 defined to be any value or meaning, and are outside the scope of this 298 specification. For the purposes of signaling, this field is treated 299 as a special extension value assigned to the local identifier 256. 300 If no extension has been specified through configuration or signaling 301 for this local identifier value 256, the appbits field SHOULD be set 302 to all 0s by the sender and MUST be ignored by the receiver. 304 Each extension element starts with a byte containing an ID and a byte 305 containing a length: 307 0 1 308 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 309 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 310 | ID | length | 311 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 313 The 8-bit ID is the local identifier of this element in the range 314 1-255 inclusive. In the signaling section, the range 1-256 is 315 referred to as the valid range, with the values 1-255 referring to 316 extension elements, and the value 256 referring to the 4-bit field 317 'appbits' (above). 319 The 8-bit length field is the length of extension data in bytes not 320 including the ID and length fields. The value zero indicates there 321 is no data following. 323 An example header extension, with three extension elements, and some 324 padding follows: 326 0 1 2 3 327 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 1 328 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 329 | 0x10 | 0x00 | length=3 | 330 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 331 | ID | L=0 | ID | L=1 | 332 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 333 | data | 0 (pad) | ID | L=4 | 334 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 335 | data | 336 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 338 5. SDP Signaling Design 340 The indication of the presence of this extension, and the mapping of 341 local identifiers used in the header extension to a larger namespace, 342 MUST be performed out-of-band, for example, as part of a SIP offer/ 343 answer exchange using SDP. This section defines such signaling in 344 SDP. 346 A usable mapping MUST use IDs in the valid range, and each ID in this 347 range MUST be used only once for each media (or only once if the 348 mappings are session level). Mappings that do not conform to these 349 rules MAY be presented, for instance, during offer/answer negotiation 350 as described in the next section, but remapping to conformant values 351 is necessary before they can be applied. 353 Each extension is named by a URI. That URI MUST be absolute, and 354 precisely identifies the format and meaning of the extension. URIs 355 that contain a domain name SHOULD also contain a month-date in the 356 form mmyyyy. The definition of the element and assignment of the URI 357 MUST have been authorized by the owner of the domain name on or very 358 close to that date. (This avoids problems when domain names change 359 ownership.) If the resource or document defines several extensions, 360 then the URI MUST identify the actual extension in use, e.g., using a 361 fragment or query identifier (characters after a '#' or '?' in the 362 URI). 364 Rationale: the use of URIs provides for a large, unallocated space, 365 and gives documentation on the extension. The URIs do not have to be 366 de-referencable, in order to permit confidential or experimental use, 367 and to cover the case when extensions continue to be used after the 368 organization that defined them ceases to exist. 370 An extension URI with the same attributes MUST NOT appear more than 371 once applying to the same stream, i.e., at session level or in the 372 declarations for a single stream at media level. (The same extension 373 can, of course, be used for several streams, and can appear 374 differently parameterized for the same stream.) 376 For extensions defined in RFCs, the URI used SHOULD be a URN starting 377 "urn:ietf:params:rtp-hdrext:" and followed by a registered, 378 descriptive name. 380 The registration requirements are detailed in the IANA Considerations 381 section, below. 383 An example (this is only an example), where 'avt-example-metadata' is 384 the hypothetical name of a header extension, might be: 386 urn:ietf:params:rtp-hdrext:avt-example-metadata 388 An example name not from the IETF (this is only an example) might be: 390 http://example.com/082005/ext.htm#example-metadata 392 The mapping MAY be provided per media stream (in the media-level 393 section(s) of SDP, i.e., after an "m=" line) or globally for all 394 streams (i.e., before the first "m=" line, at session level). The 395 definitions MUST be either all session level or all media level; it 396 is not permitted to mix the two styles. In addition, as noted above, 397 the IDs used MUST be unique for each stream type for a given media, 398 or for the session for session-level declarations. 400 Each local identifier potentially used in the stream is mapped to a 401 string using an attribute of the form: 403 a=extmap:["/"] 405 where is a URI, as above, is the local identifier (ID) 406 of this extension and is an integer in the valid range inclusive (0 407 is reserved for padding in both forms, and 15 is reserved in the one- 408 byte header form, as noted above), and is one of 409 "sendonly", "recvonly", "sendrecv", or "inactive" (without the 410 quotes). 412 The formal BNF syntax is presented in a later section of this 413 specification. 415 Example: 417 a=extmap:1 http://example.com/082005/ext.htm#ttime 418 a=extmap:2/sendrecv http://example.com/082005/ext.htm#xmeta short 420 When SDP signaling is used for the RTP session, it is the presence of 421 the 'extmap' attribute(s) that is diagnostic that this style of 422 header extensions is used, not the magic number indicated above. 424 6. SDP Signaling for support of mixed one byte and two bytes header 425 extensions. 427 In order to allow for backward interoperability with systems that do 428 not support mixing of one byte and two bytes header extensions this 429 document defines the "a=extmap-allow-mixed" Session Description 430 Protocol (SDP) [RFC4566] attribute to indicate if the participant is 431 capable of supporting this new mode. The attribute takes no value. 432 This attribute can be used at the session or media levels. A 433 participant that proposes the use of this mode SHALL itself support 434 the reception of mixed one byte and two bytes header extensions. 436 The negotiation for mixed one byte and two bytes extension MUST be 437 negotiated in offer/answer [RFC3264]. In the absence of negotiation 438 using offer/answer, mixed headers MUST NOT occur unless the 439 transmitter has some (out of band) knowledge that all potential 440 recipients support this mode. 442 The formal definition of this attribute is: 444 Name: extmap-allow-mixed 446 Value: 448 Usage Level: session, media 450 Charset Dependent: no 452 Example: 454 a=extmap-allow-mixed 456 When doing SDP Offer/Answer [RFC3264] an offering client that wishes 457 to use both one and two bytes extensions MUST include the attribute 458 "a= extmap-allow-mixed " in the SDP offer. If "a= extmap-allow-mixed 459 " is present in the offer SDP, the answerer that supports this mode 460 and wishes to use it SHALL include the "a=extmap-allow-mixed " 461 attribute in the answer. In cases the answer has been excluded, 462 neither clients SHALL use mixed one bytes and two bytes extensions in 463 the same RTP stream. 465 7. Offer/Answer 467 The simple signaling described above MAY be enhanced in an offer/ 468 answer context, to permit: 470 o asymmetric behavior (extensions sent in only one direction), 472 o the offer of mutually exclusive alternatives, or 474 o the offer of more extensions than can be sent in a single session. 476 A direction attribute MAY be included in an extmap; without it, the 477 direction implicitly inherits, of course, from the stream direction, 478 or is "sendrecv" for session-level attributes or extensions of 479 "inactive" streams. The direction MUST be one of "sendonly", 480 "recvonly", "sendrecv", or "inactive". A "sendonly" direction 481 indicates an ability to send; a "recvonly" direction indicates a 482 desire to receive; a "sendrecv" direction indicates both. An 483 "inactive" direction indicates neither, but later re-negotiation MAY 484 make an extension active. 486 Extensions, with their directions, MAY be signaled for an "inactive" 487 stream. It is an error to use an extension direction incompatible 488 with the stream direction (e.g., a "sendonly" attribute for a 489 "recvonly" stream). 491 If an offer or answer contains session-level mappings (and hence no 492 media-level mappings), and different behavior is desired for each 493 stream, then the entire set of extension map declarations MAY be 494 moved into the media-level section(s) of the SDP. (Note that this 495 specification does not permit mixing global and local declarations, 496 to make identifier management easier.) 498 If an extension map is offered as "sendrecv", explicitly or 499 implicitly, and asymmetric behavior is desired, the SDP MAY be 500 modified to modify or add direction qualifiers for that extension. 502 If an extension is marked as "sendonly" and the answerer desires to 503 receive it, the extension MUST be marked as "recvonly" in the SDP 504 answer. An answerer that has no desire to receive the extension or 505 does not understand the extension SHOULD remove it from the SDP 506 answer. 508 If an extension is marked as "recvonly" and the answerer desires to 509 send it, the extension MUST be marked as "sendonly" in the SDP 510 answer. An answerer that has no desire to, or is unable to, send the 511 extension SHOULD remove it from the SDP answer. 513 Local identifiers in the valid range inclusive in an offer or answer 514 MUST NOT be used more than once per media section (including the 515 session-level section). A session update MAY change the direction 516 qualifiers of extensions under use. A session update MAY add or 517 remove extension(s). Identifiers values in the valid range MUST NOT 518 be altered (remapped). 520 Note that, under this rule, the same local identifier cannot be used 521 for two extensions for the same media, even when one is "sendonly" 522 and the other "recvonly", as it would then be impossible to make 523 either of them sendrecv (since re-numbering is not permitted either). 525 If a party wishes to offer mutually exclusive alternatives, then 526 multiple extensions with the same identifier in the (unusable) range 527 4096-4351 MAY be offered; the answerer SHOULD select at most one of 528 the offered extensions with the same identifier, and remap it to a 529 free identifier in the valid range, for that extension to be usable. 531 Similarly, if more extensions are offered than can be fit in the 532 valid range, identifiers in the range 4096-4351 MAY be offered; the 533 answerer SHOULD choose those that are desired, and remap them to a 534 free identifier in the valid range. 536 It is always allowed to place the offered identifier value "as is" in 537 the SDP answer (for example, due to lack of a free identifier value 538 in the valid range). Extensions with an identifier outside the valid 539 range MUST NOT, of course, be used. If needed, the offerer or 540 answerer can update the session to make space for such an extension. 542 Rationale: the range 4096-4351 for these negotiation identifiers is 543 deliberately restricted to allow expansion of the range of valid 544 identifiers in future. 546 Either party MAY include extensions in the stream other than those 547 negotiated, or those negotiated as "inactive", for example, for the 548 benefit of intermediate nodes. Only extensions that appeared with an 549 identifier in the valid range in SDP originated by the sender can be 550 sent. 552 Example (port numbers, RTP profiles, payload IDs and rtpmaps, etc. 553 all omitted for brevity): 555 The offer: 557 a=extmap:1 URI-toffset 558 a=extmap:14 URI-obscure 559 a=extmap:4096 URI-gps-string 560 a=extmap:4096 URI-gps-binary 561 a=extmap:4097 URI-frametype 562 m=video 563 a=sendrecv 564 m=audio 565 a=sendrecv 567 The answerer is interested in receiving GPS in string format only on 568 video, but cannot send GPS at all. It is not interested in 569 transmission offsets on audio, and does not understand the URI- 570 obscure extension. It therefore moves the extensions from session 571 level to media level, and adjusts the declarations: 573 m=video 574 a=sendrecv 575 a=extmap:1 URI-toffset 576 a=extmap:2/recvonly URI-gps-string 577 a=extmap:3 URI-frametype 578 m=audio 579 a=sendrecv 580 a=extmap:1/sendonly URI-toffset 582 8. BNF Syntax 584 The syntax definition below uses ABNF according to [RFC5234]. The 585 syntax element 'URI' is defined in [RFC3986] (only absolute URIs are 586 permitted here). The syntax element 'extmap' is an attribute as 587 defined in [RFC4566], i.e., "a=" precedes the extmap definition. 588 Specific extensionattributes are defined by the specification that 589 defines a specific extension name; there can be several. 591 extmap = mapentry SP extensionname [SP extensionattributes] 593 extensionname = URI 595 direction = "sendonly" / "recvonly" / "sendrecv" / "inactive" 597 mapentry = "extmap:" 1*5DIGIT ["/" direction] 599 extensionattributes = byte-string 601 URI = 603 byte-string = 605 SP = 607 DIGIT = 609 9. Security Considerations 611 This document defines only a place to transmit information; the 612 security implications of each of the extensions MUST be discussed 613 with those extensions. 615 Header extensions have the same security coverage as the RTP header 616 itself. When Secure Real-time Transport Protocol (SRTP) [RFC3711] is 617 used to protect RTP sessions, the RTP payload can be both encrypted 618 and integrity protected, while the RTP header is either unprotected 619 or integrity protected. RTP header extensions can carry sensitive 620 information for which participants in multimedia sessions want 621 confidentiality. RFC6904 [RFC6904] provides a mechanism, extending 622 the mechanisms of SRTP, to selectively encrypt RTP header extensions 623 in SRTP. 625 10. IANA Considerations 627 This document updates the IANA consideration to reference this 628 document and adds a new SDP attribute in section 10.3 630 Note to IANA : change RFCxxxx to this RFC number and remove the note. 632 10.1. Identifier Space for IANA to Manage 634 The mapping from the naming URI form to a reference to a 635 specification is managed by IANA. Insertion into this registry is 636 under the requirements of "Expert Review" as defined in [RFC5226]. 638 The IANA will also maintain a server that contains all of the 639 registered elements in a publicly accessible space. 641 Here is the formal declaration to comply with the IETF URN Sub- 642 namespace specification [RFC3553]. 644 o Registry name: RTP Compact Header Extensions 646 o Specification: RFC 5285 and RFCs updating RFC 5285. 648 o Information required: 650 A. The desired extension naming URI 652 B. A formal reference to the publicly available specification 654 C. A short phrase describing the function of the extension 656 D. Contact information for the organization or person making the 657 registration 659 For extensions defined in RFCs, the URI SHOULD be of the form 660 urn:ietf:params:rtp-hdrext:, and the formal reference is the RFC 661 number of the RFC documenting the extension. 663 o Review process: Expert review is REQUIRED. The expert review 664 SHOULD check the following requirements: 666 1. that the specification is publicly available; 668 2. that the extension complies with the requirements of RTP and 669 this specification, for extensions (notably, that the stream 670 is still decodable if the extension is ignored or not 671 recognized); 673 3. that the extension specification is technically consistent (in 674 itself and with RTP), complete, and comprehensible; 676 4. that the extension does not duplicate functionality in 677 existing IETF specifications (including RTP itself), or other 678 extensions already registered; 680 5. that the specification contains a security analysis regarding 681 the content of the header extension; 683 6. that the extension is generally applicable, for example point- 684 to-multipoint safe, and the specification correctly describes 685 limitations if they exist; and 687 7. that the suggested naming URI form is appropriately chosen and 688 unique. 690 o Size and format of entries: a mapping from a naming URI string to 691 a formal reference to a publicly available specification, with a 692 descriptive phrase and contact information. 694 o Initial assignments: none. 696 10.2. Registration of the SDP extmap Attribute 698 This section contains the information requested by [RFC4566] for an 699 SDP attribute. 701 o contact name, email address, and telephone number: 703 D. Singer 704 singer@apple.com 705 +1 408-974-3162 707 o attribute name (as it will appear in SDP): extmap 709 o long-form attribute name in English: generic header extension map 710 definition 712 o type of attribute (session level, media level, or both): both 714 o whether the attribute value is subject to the charset attribute: 715 not subject to the charset attribute 717 o a one-paragraph explanation of the purpose of the attribute: This 718 attribute defines the mapping from the extension numbers used in 719 packet headers into extension names as documented in 720 specifications and appropriately registered. 722 o a specification of appropriate attribute values for this 723 attribute: see RFC 5285. 725 10.3. Registration of the SDP Attribute 727 The IANA is requested to register one new SDP attribute: 729 SDP Attribute ("att-field"): 730 Attribute name: extmap-allow-mixed 731 Long form: One and Two bytes mixed mode 732 Type of name: att-field 733 Type of attribute: Media or session level 734 Subject to charset: No 735 Purpose: Negotiate the use of One and Two bytes 736 in the same RTP stream. 737 Reference: [RFCXXXX] 738 Values: None 740 11. Acknowledgments 742 Both Brian Link and John Lazzaro provided helpful comments on an 743 initial draft of this document. Colin Perkins was helpful in 744 reviewing and dealing with the details. The use of URNs for IETF- 745 defined extensions was suggested by Jonathan Lennox, and Pete Cordell 746 was instrumental in improving the padding wording. Dave Oran 747 provided feedback and text in the review. Mike Dolan contributed the 748 two-byte header form. Magnus Westerlund and Tom Taylor were 749 instrumental in managing the registration text. 751 12. References 753 12.1. Normative References 755 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 756 Requirement Levels", BCP 14, RFC 2119, 757 DOI 10.17487/RFC2119, March 1997, 758 . 760 [RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP 761 Headers for Low-Speed Serial Links", RFC 2508, February 762 1999. 764 [RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., 765 Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, 766 K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., 767 Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header 768 Compression (ROHC): Framework and four profiles: RTP, UDP, 769 ESP, and uncompressed", RFC 3095, July 2001. 771 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 772 Jacobson, "RTP: A Transport Protocol for Real-Time 773 Applications", STD 64, RFC 3550, July 2003. 775 [RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An 776 IETF URN Sub-namespace for Registered Protocol 777 Parameters", BCP 73, RFC 3553, June 2003. 779 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 780 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 781 RFC 3711, March 2004. 783 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 784 Resource Identifier (URI): Generic Syntax", STD 66, 785 RFC 3986, January 2005. 787 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 788 Description Protocol", RFC 4566, July 2006. 790 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 791 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 792 May 2008. 794 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 795 Specifications: ABNF", STD 68, RFC 5234, January 2008. 797 [RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure 798 Real-time Transport Protocol (SRTP)", RFC 6904, 799 DOI 10.17487/RFC6904, April 2013, 800 . 802 12.2. Informative References 804 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 805 with Session Description Protocol (SDP)", RFC 3264, June 806 2002. 808 [RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time 809 Transport Protocol (RTP) Header Extension for Client-to- 810 Mixer Audio Level Indication", RFC 6464, 811 DOI 10.17487/RFC6464, December 2011, 812 . 814 Authors' Addresses 816 Roni Even (editor) 817 Huawei Technologies 818 Shabazi 12A 819 Tel Aviv 820 Israel 822 Email: Roni.even@mail01.huawei.com 823 David Singer 824 Apple, Inc. 825 1 Infinite Loop 826 Cupertino, CA 95014 827 USA 829 Phone: +1 408 996 1010 830 Email: singer@apple.com 831 URI: http://www.apple.com/quicktime 833 Harikishan Desineni 834 10001 Pacific Heights Blvd 835 San Diego, CA 92121 836 USA 838 Phone: +1 858 845 8996 839 Email: hdesinen@quicinc.com