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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: June 2, 2016 H. Desineni 7 November 30, 2015 9 A General Mechanism for RTP Header Extensions 10 draft-ietf-avtcore-rfc5285-bis-00.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 June 2, 2016. 38 Copyright Notice 40 Copyright (c) 2015 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 . . . . . . . . . . 15 70 10.2. Registration of the SDP extmap Attribute . . . . . . . . 16 71 10.3. Registration of the SDP Attribute . . . . . . . . . . . 17 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 is required to 124 extract the metadata. Also, depending on the media format, the 125 metadata may need to be added at the time of encoding the media so 126 that the bit-rate required for the metadata is taken into account. 127 But the metadata may not be known at that time. Inserting metadata 128 at a later time can require a decode and re-encode to meet bit-rate 129 requirements. 131 In some cases, a more appropriate, higher-level mechanism may be 132 available, and if so, it should be used. For cases where a higher- 133 level mechanism is not available, it is better to provide a mechanism 134 at the RTP level than have the metadata be tied to a specific form of 135 media data. 137 4. Packet Design 139 4.1. General 141 The following design is fit into the "header extension" of the RTP 142 extension, as described above. 144 The presence and format of this header extension and its contents are 145 negotiated or defined out-of-band, such as through signaling (see 146 below for SDP signaling). The value defined for an RTP extension 147 (defined below for the one-byte and two-byte header forms) is only an 148 architectural constant (e.g., for use by network analyzers); it is 149 the negotiation/definition (e.g., in SDP) that is the definitive 150 indication that this header extension is present. 152 This specification inherits the requirement from the RTP 153 specification that the header extension "is designed so that the 154 header extension may be ignored". To be specific, header extensions 155 using this specification MUST only be used for data that can safely 156 be ignored by the recipient without affecting interoperability, and 157 MUST NOT be used when the presence of the extension has changed the 158 form or nature of the rest of the packet in a way that is not 159 compatible with the way the stream is signaled (e.g., as defined by 160 the payload type). Valid examples might include metadata that is 161 additional to the usual RTP information. 163 The RTP header extension is formed as a sequence of extension 164 elements, with possible padding. Each extension element has a local 165 identifier and a length. The local identifiers may be mapped to a 166 larger namespace in the negotiation (e.g., session signaling). 168 As is good network practice, data should only be transmitted when 169 needed. The RTP header extension should only be present in a packet 170 if that packet also contains one or more extension elements, as 171 defined here. An extension element should only be present in a 172 packet when needed; the signaling setup of extension elements 173 indicates only that those elements may be present in some packets, 174 not that they are in fact present in all (or indeed, any) packets. 176 Each extension element in a packet has a local identifier (ID) and a 177 length. The local identifiers present in the stream MUST have been 178 negotiated or defined out-of-band. There are no static allocations 179 of local identifiers. Each distinct extension MUST have a unique ID. 180 The value 0 is reserved for padding and MUST NOT be used as a local 181 identifier. 183 There are two variants of the extension: one-byte and two-byte 184 headers. Since it is expected that (a) the number of extensions in 185 any given RTP session is small and (b) the extensions themselves are 186 small, the one-byte header form is preferred and MUST be supported by 187 all receivers.A stream MUST contain only one-byte or two-byte headers 188 unless it is known that all recipients support mixing, either by 189 offer/answer negotiation (see section 6) or by out-of-band knowledge. 190 One-byte and two-byte headers MUST NOT be mixed in a single RTP 191 packet. Transmitters SHOULD NOT use the two-byte form when all 192 extensions are small enough for the one-byte header form. 194 A sequence of extension elements, possibly with padding, forms the 195 header extension defined in the RTP specification. There are as many 196 extension elements as fit into the length as indicated in the RTP 197 header extension length. Since this length is signaled in full 198 32-bit words, padding bytes are used to pad to a 32-bit boundary. 199 The entire extension is parsed byte-by-byte to find each extension 200 element (no alignment is required), and parsing stops at the earlier 201 of the end of the entire header extension, or in one-byte headers 202 only case, on encountering an identifier with the reserved value of 203 15. 205 In both forms, padding bytes have the value of 0 (zero). They may be 206 placed between extension elements, if desired for alignment, or after 207 the last extension element, if needed for padding. A padding byte 208 does not supply the ID of an element, nor the length field. When a 209 padding byte is found, it is ignored and the parser moves on to 210 interpreting the next byte. 212 Note carefully that the one-byte header form allows for data lengths 213 between 1 and 16 bytes, by adding 1 to the signaled length value 214 (thus, 0 in the length field indicates 1 byte of data follows). This 215 allows for the important case of 16-byte payloads. This addition is 216 not performed for the two-byte headers, where the length field 217 signals data lengths between 0 and 255 bytes. 219 Use of RTP header extensions will reduce the efficiency of RTP header 220 compression, since the header extension will be sent uncompressed 221 unless the RTP header compression module is updated to recognize the 222 extension header. If header extensions are present in some packets, 223 but not in others, this can also reduce compression efficiency by 224 requiring an update to the fixed header to be conveyed when header 225 extensions start or stop being sent. The interactions of the RTP 226 header extension and header compression is explored further in 227 [RFC2508] and [RFC3095]. 229 4.2. One-Byte Header 231 In the one-byte header form of extensions, the 16-bit value required 232 by the RTP specification for a header extension, labeled in the RTP 233 specification as "defined by profile", takes the fixed bit pattern 234 0xBEDE (the first version of this specification was written on the 235 feast day of the Venerable Bede). 237 Each extension element starts with a byte containing an ID and a 238 length: 240 0 241 0 1 2 3 4 5 6 7 242 +-+-+-+-+-+-+-+-+ 243 | ID | len | 244 +-+-+-+-+-+-+-+-+ 246 The 4-bit ID is the local identifier of this element in the range 247 1-14 inclusive. In the signaling section, this is referred to as the 248 valid range. 250 The local identifier value 15 is reserved for future extension and 251 MUST NOT be used as an identifier. If the ID value 15 is 252 encountered, its length field should be ignored, processing of the 253 entire extension should terminate at that point, and only the 254 extension elements present prior to the element with ID 15 255 considered. 257 The 4-bit length is the number minus one of data bytes of this header 258 extension element following the one-byte header. Therefore, the 259 value zero in this field indicates that one byte of data follows, and 260 a value of 15 (the maximum) indicates element data of 16 bytes. 261 (This permits carriage of 16-byte values, which is a common length of 262 labels and identifiers, while losing the possibility of zero-length 263 values -- which would often be padded anyway.) 265 An example header extension, with three extension elements, some 266 padding, and including the required RTP fields, follows: 268 0 1 2 3 269 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 1 270 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 271 | 0xBE | 0xDE | length=3 | 272 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 273 | ID | L=0 | data | ID | L=1 | data... 274 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 275 ...data | 0 (pad) | 0 (pad) | ID | L=3 | 276 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 277 | data | 278 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 280 4.3. Two-Byte Header 282 In the two-byte header form, the 16-bit value required by the RTP 283 specification for a header extension, labeled in the RTP 284 specification as "defined by profile", is defined as shown below. 286 0 1 287 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 288 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 289 | 0x100 |appbits| 290 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 292 The appbits field is 4 bits that are application-dependent and may be 293 defined to be any value or meaning, and are outside the scope of this 294 specification. For the purposes of signaling, this field is treated 295 as a special extension value assigned to the local identifier 256. 296 If no extension has been specified through configuration or signaling 297 for this local identifier value 256, the appbits field SHOULD be set 298 to all 0s by the sender and MUST be ignored by the receiver. 300 Each extension element starts with a byte containing an ID and a byte 301 containing a length: 303 0 1 304 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 305 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 306 | ID | length | 307 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 309 The 8-bit ID is the local identifier of this element in the range 310 1-255 inclusive. In the signaling section, the range 1-256 is 311 referred to as the valid range, with the values 1-255 referring to 312 extension elements, and the value 256 referring to the 4-bit field 313 'appbits' (above). 315 The 8-bit length field is the length of extension data in bytes not 316 including the ID and length fields. The value zero indicates there 317 is no data following. 319 An example header extension, with three extension elements, some 320 padding, and including the required RTP fields, follows: 322 0 1 2 3 323 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 1 324 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 325 | 0x10 | 0x00 | length=3 | 326 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 327 | ID | L=0 | ID | L=1 | 328 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 329 | data | 0 (pad) | ID | L=4 | 330 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 331 | data | 332 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 334 5. SDP Signaling Design 336 The indication of the presence of this extension, and the mapping of 337 local identifiers used in the header extension to a larger namespace, 338 MUST be performed out-of-band, for example, as part of a SIP offer/ 339 answer exchange using SDP. This section defines such signaling in 340 SDP. 342 A usable mapping MUST use IDs in the valid range, and each ID in this 343 range MUST be used only once for each media (or only once if the 344 mappings are session level). Mappings that do not conform to these 345 rules MAY be presented, for instance, during offer/answer negotiation 346 as described in the next section, but remapping to conformant values 347 is necessary before they can be applied. 349 Each extension is named by a URI. That URI MUST be absolute, and 350 precisely identifies the format and meaning of the extension. URIs 351 that contain a domain name SHOULD also contain a month-date in the 352 form mmyyyy. The definition of the element and assignment of the URI 353 MUST have been authorized by the owner of the domain name on or very 354 close to that date. (This avoids problems when domain names change 355 ownership.) If the resource or document defines several extensions, 356 then the URI MUST identify the actual extension in use, e.g., using a 357 fragment or query identifier (characters after a '#' or '?' in the 358 URI). 360 Rationale: the use of URIs provides for a large, unallocated space, 361 and gives documentation on the extension. The URIs are not required 362 to be de-referencable, in order to permit confidential or 363 experimental use, and to cover the case when extensions continue to 364 be used after the organization that defined them ceases to exist. 366 An extension URI with the same attributes MUST NOT appear more than 367 once applying to the same stream, i.e., at session level or in the 368 declarations for a single stream at media level. (The same extension 369 may, of course, be used for several streams, and may appear 370 differently parameterized for the same stream.) 372 For extensions defined in RFCs, the URI used SHOULD be a URN starting 373 "urn:ietf:params:rtp-hdrext:" and followed by a registered, 374 descriptive name. 376 The registration requirements are detailed in the IANA Considerations 377 section, below. 379 An example (this is only an example), where 'avt-example-metadata' is 380 the hypothetical name of a header extension, might be: 382 urn:ietf:params:rtp-hdrext:avt-example-metadata 384 An example name not from the IETF (this is only an example) might be: 386 http://example.com/082005/ext.htm#example-metadata 388 The mapping may be provided per media stream (in the media-level 389 section(s) of SDP, i.e., after an "m=" line) or globally for all 390 streams (i.e., before the first "m=" line, at session level). The 391 definitions MUST be either all session level or all media level; it 392 is not permitted to mix the two styles. In addition, as noted above, 393 the IDs used MUST be unique for each stream type for a given media, 394 or for the session for session-level declarations. 396 Each local identifier potentially used in the stream is mapped to a 397 string using an attribute of the form: 399 a=extmap:["/"] 401 where is a URI, as above, is the local identifier (ID) 402 of this extension and is an integer in the valid range inclusive (0 403 is reserved for padding in both forms, and 15 is reserved in the one- 404 byte header form, as noted above), and is one of 405 "sendonly", "recvonly", "sendrecv", or "inactive" (without the 406 quotes). 408 The formal BNF syntax is presented in a later section of this 409 specification. 411 Example: 413 a=extmap:1 http://example.com/082005/ext.htm#ttime 414 a=extmap:2/sendrecv http://example.com/082005/ext.htm#xmeta short 416 When SDP signaling is used for the RTP session, it is the presence of 417 the 'extmap' attribute(s) that is diagnostic that this style of 418 header extensions is used, not the magic number indicated above. 420 6. SDP Signaling for support of mixed one byte and two bytes header 421 extensions. 423 In order to allow for backward interoperability with systems that do 424 not support mixing of one byte and two bytes header extensions this 425 document defines the "a=extmap-allow-mixed" Session Description 426 Protocol (SDP) [RFC4566] attribute to indicate if the participant is 427 capable of supporting this new mode. The attribute takes no value. 428 This attribute can be used at the session or media levels. A 429 participant that proposes the use of this mode SHALL itself support 430 the reception of mixed one byte and two bytes header extensions. 432 The negotiation for mixed one byte and two bytes extension MUST be 433 negotiated in offer/answer [RFC3264]. In the absence of negotiation 434 using offer/answer, mixed headers MUST NOT occur unless the 435 transmitter has some (out of band) knowledge that all potential 436 recipients support this mode. 438 The formal definition of this attribute is: 440 Name: extmap-allow-mixed 442 Value: 444 Usage Level: session, media 446 Charset Dependent: no 448 Example: 450 a=extmap-allow-mixed 452 When doing SDP Offer/Answer [RFC3264] an offering client that wishes 453 to use both one and two bytes extensions MUST include the attribute 454 "a= extmap-allow-mixed " in the SDP offer. If "a= extmap-allow-mixed 455 " is present in the offer SDP, the answerer that supports this mode 456 and wishes to use it SHALL include the "a=extmap-allow-mixed " 457 attribute in the answer. In cases the answer has been excluded, 458 neither clients SHALL use mixed one bytes and two bytes extensions in 459 the same RTP stream. 461 7. Offer/Answer 463 The simple signaling described above may be enhanced in an offer/ 464 answer context, to permit: 466 o asymmetric behavior (extensions sent in only one direction), 468 o the offer of mutually exclusive alternatives, or 470 o the offer of more extensions than can be sent in a single session. 472 A direction attribute MAY be included in an extmap; without it, the 473 direction implicitly inherits, of course, from the stream direction, 474 or is "sendrecv" for session-level attributes or extensions of 475 "inactive" streams. The direction MUST be one of "sendonly", 476 "recvonly", "sendrecv", or "inactive". A "sendonly" direction 477 indicates an ability to send; a "recvonly" direction indicates a 478 desire to receive; a "sendrecv" direction indicates both. An 479 "inactive" direction indicates neither, but later re-negotiation may 480 make an extension active. 482 Extensions, with their directions, may be signaled for an "inactive" 483 stream. It is an error to use an extension direction incompatible 484 with the stream direction (e.g., a "sendonly" attribute for a 485 "recvonly" stream). 487 If an offer or answer contains session-level mappings (and hence no 488 media-level mappings), and different behavior is desired for each 489 stream, then the entire set of extension map declarations may be 490 moved into the media-level section(s) of the SDP. (Note that this 491 specification does not permit mixing global and local declarations, 492 to make identifier management easier.) 494 If an extension map is offered as "sendrecv", explicitly or 495 implicitly, and asymmetric behavior is desired, the SDP may be 496 modified to modify or add direction qualifiers for that extension. 498 If an extension is marked as "sendonly" and the answerer desires to 499 receive it, the extension MUST be marked as "recvonly" in the SDP 500 answer. An answerer that has no desire to receive the extension or 501 does not understand the extension SHOULD remove it from the SDP 502 answer. 504 If an extension is marked as "recvonly" and the answerer desires to 505 send it, the extension MUST be marked as "sendonly" in the SDP 506 answer. An answerer that has no desire to, or is unable to, send the 507 extension SHOULD remove it from the SDP answer. 509 Local identifiers in the valid range inclusive in an offer or answer 510 must not be used more than once per media section (including the 511 session-level section). A session update MAY change the direction 512 qualifiers of extensions under use. A session update MAY add or 513 remove extension(s). Identifiers values in the valid range MUST NOT 514 be altered (remapped). 516 Note that, under this rule, the same local identifier cannot be used 517 for two extensions for the same media, even when one is "sendonly" 518 and the other "recvonly", as it would then be impossible to make 519 either of them sendrecv (since re-numbering is not permitted either). 521 If a party wishes to offer mutually exclusive alternatives, then 522 multiple extensions with the same identifier in the (unusable) range 523 4096-4351 may be offered; the answerer should select at most one of 524 the offered extensions with the same identifier, and remap it to a 525 free identifier in the valid range, for that extension to be usable. 527 Similarly, if more extensions are offered than can be fit in the 528 valid range, identifiers in the range 4096-4351 may be offered; the 529 answerer should choose those that are desired, and remap them to a 530 free identifier in the valid range. 532 It is always allowed to place the offered identifier value "as is" in 533 the SDP answer (for example, due to lack of a free identifier value 534 in the valid range). Extensions with an identifier outside the valid 535 range cannot, of course, be used. If required, the offerer or 536 answerer can update the session to make space for such an extension. 538 Rationale: the range 4096-4351 for these negotiation identifiers is 539 deliberately restricted to allow expansion of the range of valid 540 identifiers in future. 542 Either party MAY include extensions in the stream other than those 543 negotiated, or those negotiated as "inactive", for example, for the 544 benefit of intermediate nodes. Only extensions that appeared with an 545 identifier in the valid range in SDP originated by the sender can be 546 sent. 548 Example (port numbers, RTP profiles, payload IDs and rtpmaps, etc. 549 all omitted for brevity): 551 The offer: 553 a=extmap:1 URI-toffset 554 a=extmap:14 URI-obscure 555 a=extmap:4096 URI-gps-string 556 a=extmap:4096 URI-gps-binary 557 a=extmap:4097 URI-frametype 558 m=video 559 a=sendrecv 560 m=audio 561 a=sendrecv 563 The answerer is interested in receiving GPS in string format only on 564 video, but cannot send GPS at all. It is not interested in 565 transmission offsets on audio, and does not understand the URI- 566 obscure extension. It therefore moves the extensions from session 567 level to media level, and adjusts the declarations: 569 m=video 570 a=sendrecv 571 a=extmap:1 URI-toffset 572 a=extmap:2/recvonly URI-gps-string 573 a=extmap:3 URI-frametype 574 m=audio 575 a=sendrecv 576 a=extmap:1/sendonly URI-toffset 578 8. BNF Syntax 580 The syntax definition below uses ABNF according to [RFC5234]. The 581 syntax element 'URI' is defined in [RFC3986] (only absolute URIs are 582 permitted here). The syntax element 'extmap' is an attribute as 583 defined in [RFC4566], i.e., "a=" precedes the extmap definition. 584 Specific extensionattributes are defined by the specification that 585 defines a specific extension name; there may be several. 587 extmap = mapentry SP extensionname [SP extensionattributes] 589 extensionname = URI 591 direction = "sendonly" / "recvonly" / "sendrecv" / "inactive" 593 mapentry = "extmap:" 1*5DIGIT ["/" direction] 595 extensionattributes = byte-string 597 URI = 599 byte-string = 601 SP = 603 DIGIT = 605 9. Security Considerations 607 This defines only a place to transmit information; the security 608 implications of the extensions must be discussed with those 609 extensions. 611 Care should be taken when defining extensions. Clearly, they should 612 be solely informative, but even when the information is extracted, 613 should not cause security concerns. 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 may be both encrypted 618 and integrity protected, while the RTP header is either unprotected 619 or integrity protected. Therefore, it is inappropriate to place 620 information in header extensions that cause security problems if 621 disclosed, unless the entire RTP packet is protected by a lower-layer 622 security protocol providing both confidentiality and integrity 623 capability. 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 required by the IETF URN Sub-namespace 642 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 is recommended to be of 660 the form urn:ietf:params:rtp-hdrext:, and the formal reference is 661 the RFC 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 required 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 12.2. Informative References 799 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 800 with Session Description Protocol (SDP)", RFC 3264, June 801 2002. 803 Authors' Addresses 805 Roni Even (editor) 806 Huawei Technologies 807 Shabazi 12A 808 Tel Aviv 809 Israel 811 EMail: Roni.even@mail01.huawei.com 812 David Singer 813 Apple, Inc. 814 1 Infinite Loop 815 Cupertino, CA 95014 816 USA 818 Phone: +1 408 996 1010 819 EMail: singer@apple.com 820 URI: http://www.apple.com/quicktime 822 Harikishan Desineni 823 10001 Pacific Heights Blvd 824 San Diego, CA 92121 825 USA 827 Phone: +1 858 845 8996 828 EMail: hdesinen@quicinc.com