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'E164' == Outdated reference: A later version (-28) exists of draft-ietf-mmusic-data-channel-sdpneg-17 == Outdated reference: A later version (-19) exists of draft-ietf-mmusic-sdp-mux-attributes-17 ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) == Outdated reference: A later version (-54) exists of draft-ietf-mmusic-sdp-bundle-negotiation-50 -- Obsolete informational reference (is this intentional?): RFC 2327 (Obsoleted by RFC 4566) -- Obsolete informational reference (is this intentional?): RFC 5245 (Obsoleted by RFC 8445, RFC 8839) Summary: 1 error (**), 0 flaws (~~), 12 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Begen 3 Internet-Draft Networked Media 4 Obsoletes: 4566 (if approved) P. Kyzivat 5 Intended status: Standards Track 6 Expires: November 1, 2018 C. Perkins 7 University of Glasgow 8 M. Handley 9 UCL 10 April 30, 2018 12 SDP: Session Description Protocol 13 draft-ietf-mmusic-rfc4566bis-26 15 Abstract 17 This memo defines the Session Description Protocol (SDP). SDP is 18 intended for describing multimedia sessions for the purposes of 19 session announcement, session invitation, and other forms of 20 multimedia session initiation. This document obsoletes RFC 4566. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on November 1, 2018. 39 Copyright Notice 41 Copyright (c) 2018 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 This document may contain material from IETF Documents or IETF 55 Contributions published or made publicly available before November 56 10, 2008. The person(s) controlling the copyright in some of this 57 material may not have granted the IETF Trust the right to allow 58 modifications of such material outside the IETF Standards Process. 59 Without obtaining an adequate license from the person(s) controlling 60 the copyright in such materials, this document may not be modified 61 outside the IETF Standards Process, and derivative works of it may 62 not be created outside the IETF Standards Process, except to format 63 it for publication as an RFC or to translate it into languages other 64 than English. 66 Table of Contents 68 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 69 2. Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . 4 70 3. Examples of SDP Usage . . . . . . . . . . . . . . . . . . . . 5 71 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 5 72 3.2. Streaming Media . . . . . . . . . . . . . . . . . . . . . 5 73 3.3. Email and the World Wide Web . . . . . . . . . . . . . . 5 74 3.4. Multicast Session Announcement . . . . . . . . . . . . . 5 75 4. Requirements and Recommendations . . . . . . . . . . . . . . 6 76 4.1. Media and Transport Information . . . . . . . . . . . . . 7 77 4.2. Timing Information . . . . . . . . . . . . . . . . . . . 7 78 4.3. Obtaining Further Information about a Session . . . . . . 8 79 4.4. Categorization . . . . . . . . . . . . . . . . . . . . . 8 80 4.5. Internationalization . . . . . . . . . . . . . . . . . . 8 81 5. SDP Specification . . . . . . . . . . . . . . . . . . . . . . 8 82 5.1. Protocol Version ("v=") . . . . . . . . . . . . . . . . . 11 83 5.2. Origin ("o=") . . . . . . . . . . . . . . . . . . . . . . 12 84 5.3. Session Name ("s=") . . . . . . . . . . . . . . . . . . . 13 85 5.4. Session Information ("i=") . . . . . . . . . . . . . . . 13 86 5.5. URI ("u=") . . . . . . . . . . . . . . . . . . . . . . . 14 87 5.6. Email Address and Phone Number ("e=" and "p=") . . . . . 14 88 5.7. Connection Information ("c=") . . . . . . . . . . . . . . 15 89 5.8. Bandwidth Information ("b=") . . . . . . . . . . . . . . 17 90 5.9. Time Active ("t=") . . . . . . . . . . . . . . . . . . . 18 91 5.10. Repeat Times ("r=") . . . . . . . . . . . . . . . . . . . 19 92 5.11. Time Zones ("z=") . . . . . . . . . . . . . . . . . . . . 19 93 5.12. Encryption Keys ("k=") . . . . . . . . . . . . . . . . . 20 94 5.13. Attributes ("a=") . . . . . . . . . . . . . . . . . . . . 20 95 5.14. Media Descriptions ("m=") . . . . . . . . . . . . . . . . 21 96 6. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . . 24 97 6.1. cat (category) . . . . . . . . . . . . . . . . . . . . . 24 98 6.2. keywds (keywords) . . . . . . . . . . . . . . . . . . . . 25 99 6.3. tool . . . . . . . . . . . . . . . . . . . . . . . . . . 25 100 6.4. ptime (packet time) . . . . . . . . . . . . . . . . . . . 26 101 6.5. maxptime (maximum packet time) . . . . . . . . . . . . . 26 102 6.6. rtpmap . . . . . . . . . . . . . . . . . . . . . . . . . 27 103 6.7. Media Direction Attributes . . . . . . . . . . . . . . . 29 104 6.7.1. recvonly (receive-only) . . . . . . . . . . . . . . . 29 105 6.7.2. sendrecv (send-receive) . . . . . . . . . . . . . . . 30 106 6.7.3. sendonly (send-only) . . . . . . . . . . . . . . . . 30 107 6.7.4. inactive . . . . . . . . . . . . . . . . . . . . . . 30 108 6.8. orient (orientation) . . . . . . . . . . . . . . . . . . 31 109 6.9. type (conference type) . . . . . . . . . . . . . . . . . 31 110 6.10. charset (character set) . . . . . . . . . . . . . . . . . 32 111 6.11. sdplang (SDP language) . . . . . . . . . . . . . . . . . 33 112 6.12. lang (language) . . . . . . . . . . . . . . . . . . . . . 34 113 6.13. framerate (frame rate) . . . . . . . . . . . . . . . . . 35 114 6.14. quality . . . . . . . . . . . . . . . . . . . . . . . . . 36 115 6.15. fmtp (format parameters) . . . . . . . . . . . . . . . . 36 116 7. Security Considerations . . . . . . . . . . . . . . . . . . . 37 117 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39 118 8.1. The "application/sdp" Media Type . . . . . . . . . . . . 39 119 8.2. Registration of Parameters . . . . . . . . . . . . . . . 40 120 8.2.1. Media Types ("media") . . . . . . . . . . . . . . . . 40 121 8.2.2. Transport Protocols ("proto") . . . . . . . . . . . . 41 122 8.2.3. Media Formats ("fmt") . . . . . . . . . . . . . . . . 41 123 8.2.4. Attribute Names ("att-field") . . . . . . . . . . . . 42 124 8.2.5. Bandwidth Specifiers ("bwtype") . . . . . . . . . . . 44 125 8.2.6. Network Types ("nettype") . . . . . . . . . . . . . . 45 126 8.2.7. Address Types ("addrtype") . . . . . . . . . . . . . 45 127 8.2.8. Registration Procedure . . . . . . . . . . . . . . . 45 128 8.3. Encryption Key Access Methods . . . . . . . . . . . . . . 46 129 8.4. Reorganization of the nettype Registry . . . . . . . . . 46 130 8.5. Reorganization of the att-field Registries . . . . . . . 47 131 9. SDP Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 47 132 10. Summary of Changes from RFC 4566 . . . . . . . . . . . . . . 53 133 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 53 134 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 53 135 12.1. Normative References . . . . . . . . . . . . . . . . . . 53 136 12.2. Informative References . . . . . . . . . . . . . . . . . 55 137 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57 139 1. Introduction 141 When initiating multimedia teleconferences, voice-over-IP calls, 142 streaming video, or other sessions, there is a requirement to convey 143 media details, transport addresses, and other session description 144 metadata to the participants. 146 SDP provides a standard representation for such information, 147 irrespective of how that information is transported. SDP is purely a 148 format for session description -- it does not incorporate a transport 149 protocol, and it is intended to use different transport protocols as 150 appropriate, including the Session Announcement Protocol (SAP) 151 [RFC2974], Session Initiation Protocol (SIP) [RFC3261], Real Time 152 Streaming Protocol (RTSP) [RFC7826], electronic mail using the MIME 153 extensions, and the Hypertext Transport Protocol (HTTP). 155 SDP is intended to be general purpose so that it can be used in a 156 wide range of network environments and applications. However, it is 157 not intended to support negotiation of session content or media 158 encodings: this is viewed as outside the scope of session 159 description. 161 This memo obsoletes [RFC4566]. The changes relative to [RFC4566] are 162 limited to essential corrections, and are outlined in Section 10 of 163 this memo. 165 2. Glossary of Terms 167 The following terms are used in this document and have specific 168 meaning within the context of this document. 170 Session Description: A well-defined format for conveying sufficient 171 information to discover and participate in a multimedia session. 173 Media Description: A media description starts with an "m=" line and 174 is terminated by either the next "m=" line or by the end of the 175 session description. 177 Session-level Section: This refers to the parts that are not media 178 descriptions, whereas the session description refers to the whole 179 body that includes the session-level section and the media 180 description(s). 182 The terms "multimedia conference" and "multimedia session" are used 183 in this document as defined in [RFC7656]. The terms "session" and 184 "multimedia session" are used interchangeably in this document. 186 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 187 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 188 "OPTIONAL" in this document are to be interpreted as described in BCP 189 14 [RFC2119] [RFC8174] when, and only when, they appear in all 190 capitals, as shown here. 192 3. Examples of SDP Usage 194 3.1. Session Initiation 196 The Session Initiation Protocol (SIP) [RFC3261] is an application- 197 layer control protocol for creating, modifying, and terminating 198 sessions such as Internet multimedia conferences, Internet telephone 199 calls, and multimedia distribution. The SIP messages used to create 200 sessions carry session descriptions that allow participants to agree 201 on a set of compatible media types. These session descriptions are 202 commonly formatted using SDP. When used with SIP, the offer/answer 203 model [RFC3264] provides a limited framework for negotiation using 204 SDP. 206 3.2. Streaming Media 208 The Real Time Streaming Protocol (RTSP) [RFC7826], is an application- 209 level protocol for control over the delivery of data with real-time 210 properties. RTSP provides an extensible framework to enable 211 controlled, on-demand delivery of real-time data, such as audio and 212 video. An RTSP client and server negotiate an appropriate set of 213 parameters for media delivery, partially using SDP syntax to describe 214 those parameters. 216 3.3. Email and the World Wide Web 218 Alternative means of conveying session descriptions include 219 electronic mail and the World Wide Web (WWW). For both email and WWW 220 distribution, the media type "application/sdp" is used. This enables 221 the automatic launching of applications for participation in the 222 session from the WWW client or mail reader in a standard manner. 224 Note that announcements of multicast sessions made only via email or 225 the WWW do not have the property that the receiver of a session 226 announcement can necessarily receive the session because the 227 multicast sessions may be restricted in scope, and access to the WWW 228 server or reception of email is possible outside this scope. 230 3.4. Multicast Session Announcement 232 In order to assist the advertisement of multicast multimedia 233 conferences and other multicast sessions, and to communicate the 234 relevant session setup information to prospective participants, a 235 distributed session directory may be used. An instance of such a 236 session directory periodically sends packets containing a description 237 of the session to a well-known multicast group. These advertisements 238 are received by other session directories such that potential remote 239 participants can use the session description to start the tools 240 required to participate in the session. 242 One protocol used to implement such a distributed directory is the 243 SAP [RFC2974]. SDP provides the recommended session description 244 format for such session announcements. 246 4. Requirements and Recommendations 248 The purpose of SDP is to convey information about media streams in 249 multimedia sessions to allow the recipients of a session description 250 to participate in the session. SDP is primarily intended for use in 251 an internetwork, although it is sufficiently general that it can 252 describe multimedia conferences in other network environments. Media 253 streams can be many-to-many. Sessions need not be continually 254 active. 256 Thus far, multicast-based sessions on the Internet have differed from 257 many other forms of conferencing in that anyone receiving the traffic 258 can join the session (unless the session traffic is encrypted). In 259 such an environment, SDP serves two primary purposes. It is a means 260 to communicate the existence of a session, and it is a means to 261 convey sufficient information to enable joining and participating in 262 the session. In a unicast environment, only the latter purpose is 263 likely to be relevant. 265 An SDP description includes the following: 267 o Session name and purpose 269 o Time(s) the session is active 271 o The media comprising the session 273 o Information needed to receive those media (addresses, ports, 274 formats, etc.) 276 As resources necessary to participate in a session may be limited, 277 some additional information may also be desirable: 279 o Information about the bandwidth to be used by the session 281 o Contact information for the person responsible for the session 283 In general, SDP must convey sufficient information to enable 284 applications to join a session (with the possible exception of 285 encryption keys) and to announce the resources to be used to any non- 286 participants that may need to know. (This latter feature is 287 primarily useful when SDP is used with a multicast session 288 announcement protocol.) 290 4.1. Media and Transport Information 292 An SDP description includes the following media information: 294 o The type of media (video, audio, etc.) 296 o The media transport protocol (RTP/UDP/IP, H.320, etc.) 298 o The format of the media (H.261 video, MPEG video, etc.) 300 In addition to media format and transport protocol, SDP conveys 301 address and port details. For an IP multicast session, these 302 comprise: 304 o The multicast group address for media 306 o The transport port for media 308 This address and port is the destination address and destination port 309 of the multicast stream, whether being sent, received, or both. 311 For unicast IP sessions, the following are conveyed: 313 o The remote address for media 315 o The remote transport port for media 317 The semantics of this address and port depend on the media and 318 transport protocol defined. By default, this SHOULD be the remote 319 address and remote port to which data is sent. Some media types may 320 redefine this behaviour, but this is NOT RECOMMENDED since it 321 complicates implementations (including middleboxes that must parse 322 the addresses to open Network Address Translation (NAT) or firewall 323 pinholes). 325 4.2. Timing Information 327 Sessions may be either bounded or unbounded in time. Whether or not 328 they are bounded, they may be only active at specific times. SDP can 329 convey: 331 o An arbitrary list of start and stop times bounding the session 333 o For each bound, repeat times such as "every Wednesday at 10am for 334 one hour" 336 This timing information is globally consistent, irrespective of local 337 time zone or daylight saving time (see Section 5.9). 339 4.3. Obtaining Further Information about a Session 341 A session description could convey enough information to decide 342 whether or not to participate in a session. SDP may include 343 additional pointers in the form of Uniform Resource Identifiers 344 (URIs) for more information about the session. 346 4.4. Categorization 348 When many session descriptions are being distributed by an 349 advertisement mechanism, it may be desirable to filter session 350 announcements that are of interest from those that are not. SDP 351 supports a categorization mechanism for sessions that is capable of 352 being automated (the "a=cat:" attribute; see Section 6). 354 4.5. Internationalization 356 The SDP specification recommends the use of the ISO 10646 character 357 set in the UTF-8 encoding [RFC3629] to allow many different languages 358 to be represented. However, to assist in compact representations, 359 SDP also allows other character sets such as ISO 8859-1 to be used 360 when desired. Internationalization only applies to free-text fields 361 (session name and background information), and not to SDP as a whole. 363 5. SDP Specification 365 An SDP description is denoted by the media type "application/sdp" 366 (See Section 8). 368 An SDP description is entirely textual. SDP field names and 369 attribute names use only the US-ASCII subset of UTF-8, but textual 370 fields and attribute values MAY use the full ISO 10646 character set 371 in UTF-8 encoding, or some other character set defined by the 372 "a=charset:" attribute. Field and attribute values that use the full 373 UTF-8 character set are never directly compared, hence there is no 374 requirement for UTF-8 normalization. The textual form, as opposed to 375 a binary encoding such as ASN.1 or XDR, was chosen to enhance 376 portability, to enable a variety of transports to be used, and to 377 allow flexible, text-based toolkits to be used to generate and 378 process session descriptions. However, since SDP may be used in 379 environments where the maximum permissible size of a session 380 description is limited, the encoding is deliberately compact. Also, 381 since announcements may be transported via very unreliable means or 382 damaged by an intermediate caching server, the encoding was designed 383 with strict order and formatting rules so that most errors would 384 result in malformed session announcements that could be detected 385 easily and discarded. This also allows rapid discarding of encrypted 386 session announcements for which a receiver does not have the correct 387 key. 389 An SDP description consists of a number of lines of text of the form: 391 = 393 where MUST be exactly one case-significant character and 394 is structured text whose format depends on . In 395 general, is either a number of fields delimited by a single 396 space character or a free format string, and is case-significant 397 unless a specific field defines otherwise. Whitespace separators 398 MUST NOT be used on either side of the "=" sign, however, the value 399 can contain a leading whitespace as part of its syntax, i.e., that 400 whitespace is part of the value. 402 An SDP description consists of a session-level section followed by 403 zero or more media descriptions. The session-level section starts 404 with a "v=" line and continues to the first media description (or the 405 end of the whole description, whichever comes first). Each media 406 description starts with an "m=" line and continues to the next media 407 description or the end of the whole session description - whichever 408 comes first. In general, session-level values are the default for 409 all media unless overridden by an equivalent media-level value. 411 Some lines in each description are REQUIRED and some are OPTIONAL, 412 but all MUST appear in exactly the order given here (the fixed order 413 greatly enhances error detection and allows for a simple parser). 414 OPTIONAL items are marked with a "*". 416 Session description 417 v= (protocol version) 418 o= (originator and session identifier) 419 s= (session name) 420 i=* (session information) 421 u=* (URI of description) 422 e=* (email address) 423 p=* (phone number) 424 c=* (connection information -- not required if included in 425 all media descriptions) 426 b=* (zero or more bandwidth information lines) 427 One or more time descriptions ("t=" and "r=" lines; see below) 428 z=* (time zone adjustments) 429 k=* (encryption key) 430 a=* (zero or more session attribute lines) 431 Zero or more media descriptions 433 Time description 434 t= (time the session is active) 435 r=* (zero or more repeat times) 437 Media description, if present 438 m= (media name and transport address) 439 i=* (media title) 440 c=* (connection information -- optional if included at 441 session level) 442 b=* (zero or more bandwidth information lines) 443 k=* (encryption key) 444 a=* (zero or more media attribute lines) 446 The set of type letters is deliberately small and not intended to be 447 extensible -- an SDP parser MUST completely ignore any session 448 description that contains a type letter that it does not understand. 449 The attribute mechanism ("a=" described below) is the primary means 450 for extending SDP and tailoring it to particular applications or 451 media. Some attributes (the ones listed in Section 6 of this memo) 452 have a defined meaning, but others may be added on an application-, 453 media-, or session-specific basis. An SDP parser MUST ignore any 454 attribute it doesn't understand. 456 An SDP description may contain URIs that reference external content 457 in the "u=", "k=", and "a=" lines. These URIs may be dereferenced in 458 some cases, making the session description non-self-contained. 460 The connection ("c=") information in the session-level section 461 applies to all the media descriptions of that session unless 462 overridden by connection information in the media description. For 463 instance, in the example below, each audio media description behaves 464 as if it were given a "c=IN IP4 233.252.0.2". 466 An example SDP description is: 468 v=0 469 o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1 470 s=SDP Seminar 471 i=A Seminar on the session description protocol 472 u=http://www.example.com/seminars/sdp.pdf 473 e=j.doe@example.com (Jane Doe) 474 c=IN IP4 233.252.0.2 475 t=2873397496 2873404696 476 a=recvonly 477 m=audio 49170 RTP/AVP 0 478 m=audio 49180 RTP/AVP 0 479 m=video 51372 RTP/AVP 99 480 c=IN IP4 233.252.0.1/127 481 a=rtpmap:99 h263-1998/90000 483 Text fields such as the session name and information are octet 484 strings that may contain any octet with the exceptions of 0x00 (Nul), 485 0x0a (ASCII newline), and 0x0d (ASCII carriage return). The sequence 486 CRLF (0x0d0a) is used to end a record, although parsers SHOULD be 487 tolerant and also accept records terminated with a single newline 488 character. If the "a=charset" attribute is not present, these octet 489 strings MUST be interpreted as containing ISO-10646 characters in 490 UTF-8 encoding (the presence of the "a=charset" attribute may force 491 some fields to be interpreted differently). 493 A session description can contain domain names in the "o=", "u=", 494 "e=", "c=", and "a=" lines. Any domain name used in SDP MUST comply 495 with [RFC1034], [RFC1035]. Internationalized domain names (IDNs) 496 MUST be represented using the ASCII Compatible Encoding (ACE) form 497 defined in [RFC5890] and MUST NOT be directly represented in UTF-8 or 498 any other encoding (this requirement is for compatibility with 499 [RFC2327] and other early SDP-related standards, which predate the 500 development of internationalized domain names). 502 5.1. Protocol Version ("v=") 504 v=0 506 The "v=" line gives the version of the Session Description Protocol. 507 This memo defines version 0. There is no minor version number. 509 5.2. Origin ("o=") 511 o= 512 514 The "o=" line gives the originator of the session (her username and 515 the address of the user's host) plus a session identifier and version 516 number: 518 is the user's login on the originating host, or it is "-" 519 if the originating host does not support the concept of user IDs. 520 The MUST NOT contain spaces. 522 is a numeric string such that the tuple of , 523 , , , and forms a 524 globally unique identifier for the session. The method of allocation is up to the creating tool, but it has been 526 suggested that a Network Time Protocol (NTP) format timestamp be 527 used to ensure uniqueness [RFC5905]. 529 is a version number for this session description. 530 Its usage is up to the creating tool, so long as is 531 increased when a modification is made to the session data. Again, 532 it is RECOMMENDED that an NTP format timestamp is used. 534 is a text string giving the type of network. Initially 535 "IN" is defined to have the meaning "Internet", but other values 536 MAY be registered in the future (see Section 8). 538 is a text string giving the type of the address that 539 follows. Initially "IP4" and "IP6" are defined, but other values 540 MAY be registered in the future (see Section 8). 542 is an address of the machine from which the 543 session was created. For an address type of IP4, this is either a 544 fully qualified domain name of the machine or the dotted-decimal 545 representation of an IP version 4 address of the machine. For an 546 address type of IP6, this is either a fully qualified domain name 547 of the machine or the compressed textual representation of an IP 548 version 6 address of the machine. For both IP4 and IP6, the fully 549 qualified domain name is the form that SHOULD be given unless this 550 is unavailable, in which case a globally unique address MAY be 551 substituted. Unless an SDP extension for NAT traversal is used 552 (e.g., ICE [RFC5245], ICE TCP [RFC6544]), a local IP address MUST 553 NOT be used in any context where the SDP description might leave 554 the scope in which the address is meaningful (for example, a local 555 address MUST NOT be included in an application-level referral that 556 might leave the scope). 558 In general, the "o=" line serves as a globally unique identifier for 559 this version of the session description, and the sub-fields excepting 560 the version, taken together identify the session irrespective of any 561 modifications. 563 For privacy reasons, it is sometimes desirable to obfuscate the 564 username and IP address of the session originator. If this is a 565 concern, an arbitrary and private MAY be 566 chosen to populate the "o=" line, provided that these are selected in 567 a manner that does not affect the global uniqueness of the field. 569 5.3. Session Name ("s=") 571 s= 573 The "s=" line is the textual session name. There MUST be one and 574 only one "s=" line per session description. The "s=" line MUST NOT 575 be empty and SHOULD contain ISO 10646 characters (but see also the 576 "a=charset" attribute). If a session has no meaningful name, the "s= 577 " line SHOULD be used (i.e., a single space as the session name). 579 5.4. Session Information ("i=") 581 i= 583 The "i=" line provides textual information about the session. There 584 MUST be at most one session-level "i=" line per session description, 585 and at most one "i=" line in each media description. Unless a media 586 level "i= line is provided, the session-level "i= line applies to 587 that media description. If the "a=charset" attribute is present, it 588 specifies the character set used in the "i=" line. If the 589 "a=charset" attribute is not present, the "i=" line MUST contain ISO 590 10646 characters in UTF-8 encoding. 592 At most one "i=" line can be used for each media description. In 593 media definitions, "i=" lines are primarily intended for labelling 594 media streams. As such, they are most likely to be useful when a 595 single session has more than one distinct media stream of the same 596 media type. An example would be two different whiteboards, one for 597 slides and one for feedback and questions. 599 The "i=" line is intended to provide a free-form human-readable 600 description of the session or the purpose of a media stream. It is 601 not suitable for parsing by automata. 603 5.5. URI ("u=") 605 u= 607 A URI is a Uniform Resource Identifier as used by WWW clients 608 [RFC3986]. The URI should be a pointer to additional information 609 about the session. This line is OPTIONAL. No more than one URI line 610 is allowed per session description. 612 5.6. Email Address and Phone Number ("e=" and "p=") 614 e= 615 p= 617 The "e=" and "p=" lines specify contact information for the person 618 responsible for the session. This is not necessarily the same person 619 that created the session description. 621 Inclusion of an email address or phone number is OPTIONAL. 623 If an email address or phone number is present, it MUST be specified 624 before the first media description. More than one email or phone 625 line can be given for a session description. 627 Phone numbers SHOULD be given in the form of an international public 628 telecommunication number (see ITU-T Recommendation E.164 [E164]) 629 preceded by a "+". Spaces and hyphens may be used to split up a 630 phone field to aid readability if desired. For example: 632 p=+1 617 555-6011 634 Both email addresses and phone numbers can have an OPTIONAL free text 635 string associated with them, normally giving the name of the person 636 who may be contacted. This MUST be enclosed in parentheses if it is 637 present. For example: 639 e=j.doe@example.com (Jane Doe) 641 The alternative [RFC5322] name quoting convention is also allowed for 642 both email addresses and phone numbers. For example: 644 e=Jane Doe 646 The free text string SHOULD be in the ISO-10646 character set with 647 UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if 648 the appropriate session-level "a=charset" attribute is set. 650 5.7. Connection Information ("c=") 652 c= 654 The "c=" line contains connection data. 656 A session description MUST contain either at least one "c=" line in 657 each media description or a single "c=" line at the session level. 658 It MAY contain a single session-level "c=" line and additional "c=" 659 line(s) per media description, in which case the per-media values 660 override the session-level settings for the respective media. 662 The first sub-field ("") is the network type, which is a 663 text string giving the type of network. Initially, "IN" is defined 664 to have the meaning "Internet", but other values MAY be registered in 665 the future (see Section 8). 667 The second sub-field ("") is the address type. This allows 668 SDP to be used for sessions that are not IP based. This memo only 669 defines IP4 and IP6, but other values MAY be registered in the future 670 (see Section 8). 672 The third sub-field ("") is the connection 673 address. Additional sub-fields MAY be added after the connection 674 address depending on the value of the field. 676 When the is IP4 and IP6, the connection address is defined 677 as follows: 679 o If the session is multicast, the connection address will be an IP 680 multicast group address. If the session is not multicast, then 681 the connection address contains the unicast IP address of the 682 expected data source, data relay or data sink as determined by 683 additional attribute fields. It is not expected that unicast 684 addresses will be given in a session description that is 685 communicated by a multicast announcement, though this is not 686 prohibited. 688 o Sessions using an IP4 multicast connection address MUST also have 689 a time to live (TTL) value present in addition to the multicast 690 address. The TTL and the address together define the scope with 691 which multicast packets sent in this session will be sent. TTL 692 values MUST be in the range 0-255. Although the TTL MUST be 693 specified, its use to scope multicast traffic is deprecated; 694 applications SHOULD use an administratively scoped address 695 instead. 697 The TTL for the session is appended to the address using a slash as a 698 separator. An example is: 700 c=IN IP4 233.252.0.1/127 702 IP6 multicast does not use TTL scoping, and hence the TTL value MUST 703 NOT be present for IP6 multicast. It is expected that IP6 scoped 704 addresses will be used to limit the scope of multimedia conferences. 706 Hierarchical or layered encoding schemes are data streams where the 707 encoding from a single media source is split into a number of layers. 708 The receiver can choose the desired quality (and hence bandwidth) by 709 only subscribing to a subset of these layers. Such layered encodings 710 are normally transmitted in multiple multicast groups to allow 711 multicast pruning. This technique keeps unwanted traffic from sites 712 only requiring certain levels of the hierarchy. For applications 713 requiring multiple multicast groups, we allow the following notation 714 to be used for the connection address: 716 [/]/ 718 If the number of addresses is not given, it is assumed to be one. 719 Multicast addresses so assigned are contiguously allocated above the 720 base address, so that, for example: 722 c=IN IP4 233.252.0.1/127/3 724 would state that addresses 233.252.0.1, 233.252.0.2, and 233.252.0.3 725 are to be used with a TTL of 127. This is semantically identical to 726 including multiple "c=" lines in a media description: 728 c=IN IP4 233.252.0.1/127 729 c=IN IP4 233.252.0.2/127 730 c=IN IP4 233.252.0.3/127 732 Similarly, an IP6 example would be: 734 c=IN IP6 FF15::101/3 736 which is semantically equivalent to: 738 c=IN IP6 FF15::101 739 c=IN IP6 FF15::102 740 c=IN IP6 FF15::103 742 (remembering that the TTL field is not present in IP6 multicast). 744 Multiple addresses or "c=" lines MAY be specified on a per media 745 description basis only if they provide multicast addresses for 746 different layers in a hierarchical or layered encoding scheme. They 747 MUST NOT be specified for a session-level "c=" line. 749 The slash notation for multiple addresses described above MUST NOT be 750 used for IP unicast addresses. 752 5.8. Bandwidth Information ("b=") 754 b=: 756 This OPTIONAL line denotes the proposed bandwidth to be used by the 757 session or media description. The is an alphanumeric 758 modifier giving the meaning of the figure. Two values 759 are defined in this specification, but other values MAY be registered 760 in the future (see Section 8 and [RFC3556], [RFC3890]): 762 CT If the bandwidth of a session is different from the bandwidth 763 implicit from the scope, a "b=CT:..." line SHOULD be supplied for 764 the session giving the proposed upper limit to the bandwidth used 765 (the "conference total" bandwidth). Similarly, if the bandwidth 766 of bundled media streams in an m line is different from the 767 implicit value from the scope, a "b=CT:..." line SHOULD be 768 supplied in the media level. The primary purpose of this is to 769 give an approximate idea as to whether two or more sessions (or 770 bundled media streams) can coexist simultaneously. Note that CT 771 gives a total bandwidth figure for all the media at all endpoints. 773 AS The bandwidth is interpreted to be application specific (it will 774 be the application's concept of maximum bandwidth). Normally, 775 this will coincide with what is set on the application's "maximum 776 bandwidth" control if applicable. For RTP-based applications, AS 777 gives the RTP "session bandwidth" as defined in Section 6.2 of 778 [RFC3550]. Note that AS gives a bandwidth figure for a single 779 media at a single endpoint, although there may be many endpoints 780 sending simultaneously. 782 A prefix "X-" is defined for names. This is intended for 783 experimental purposes only. For example: 785 b=X-YZ:128 787 Use of the "X-" prefix is NOT RECOMMENDED: instead new names 788 SHOULD be registered with IANA in the standard namespace. SDP 789 parsers MUST ignore bandwidth fields with unknown names. 790 The names MUST be alphanumeric and, although no length limit 791 is given, it is recommended that they be short. 793 The is interpreted as kilobits per second by default 794 (including the transport and network-layer but not the link-layer 795 overhead). The definition of a new modifier MAY specify 796 that the bandwidth is to be interpreted in some alternative unit (the 797 "CT" and "AS" modifiers defined in this memo use the default units). 799 5.9. Time Active ("t=") 801 t= 803 The "t=" lines specify the start and stop times for a session. 804 Multiple "t=" lines MAY be used if a session is active at multiple 805 irregularly spaced times; each additional "t=" line specifies an 806 additional period of time for which the session will be active. If 807 the session is active at regular repeat times, an "r=" line (see 808 below) should be used in addition to, and following, a "t=" line -- 809 in which case the "t=" line specifies the start and stop times of the 810 entire repeat sequence. 812 The first and second sub-fields give the start and stop times, 813 respectively, for the session. These values are the decimal 814 representation of Network Time Protocol (NTP) time values in seconds 815 since 1900 [RFC5905]. To convert these values to UNIX time (UTC), 816 subtract decimal 2208988800. 818 NTP timestamps are elsewhere represented by 64-bit values, which wrap 819 sometime in the year 2036. Since SDP uses an arbitrary length 820 decimal representation, this should not cause an issue (SDP 821 timestamps MUST continue counting seconds since 1900 - NTP will use 822 the value modulo the 64-bit limit). 824 If the is set to zero, then the session is not bounded, 825 though it will not become active until after the . If 826 the is also zero, the session is regarded as permanent. 828 User interfaces SHOULD strongly discourage the creation of unbounded 829 and permanent sessions as they give no information about when the 830 session is actually going to terminate, and so make scheduling 831 difficult. 833 The general assumption may be made, when displaying unbounded 834 sessions that have not timed out to the user, that an unbounded 835 session will only be active until half an hour from the current time 836 or the session start time, whichever is the later. If behaviour 837 other than this is required, an end-time SHOULD be given and modified 838 as appropriate when new information becomes available about when the 839 session should really end. 841 Permanent sessions may be shown to the user as never being active 842 unless there are associated repeat times that state precisely when 843 the session will be active. 845 5.10. Repeat Times ("r=") 847 r= 849 An "r=" line specifies repeat times for a session. For example, if a 850 session is active at 10am on Monday and 11am on Tuesday for one hour 851 each week for three months, then the in the 852 corresponding "t=" line would be the NTP representation of 10am on 853 the first Monday, the would be 1 week, the would be 1 hour, and the offsets would be zero and 25 855 hours. The corresponding "t=" line stop time would be the NTP 856 representation of the end of the last session three months later. By 857 default, all fields are in seconds, so the "r=" and "t=" lines might 858 be the following: 860 t=3034423619 3042462419 861 r=604800 3600 0 90000 863 To make the description more compact, times may also be given in 864 units of days, hours, or minutes. The syntax for these is a number 865 immediately followed by a single case-sensitive character. 866 Fractional units are not allowed -- a smaller unit should be used 867 instead. The following unit specification characters are allowed: 869 d - days (86400 seconds) 870 h - hours (3600 seconds) 871 m - minutes (60 seconds) 872 s - seconds (allowed for completeness) 874 Thus, the above session announcement could also have been written: 876 r=7d 1h 0 25h 878 Monthly and yearly repeats cannot be directly specified with a single 879 SDP repeat time; instead, separate "t=" lines should be used to 880 explicitly list the session times. 882 5.11. Time Zones ("z=") 884 z= .... 886 This field is an optional modifier to the Repeat Times ("r=") field. 887 It does not apply to any other fields. 889 To schedule a repeated session that spans a change from daylight 890 saving time to standard time or vice versa, it is necessary to 891 specify offsets from the base time. This is required because 892 different time zones change time at different times of day, different 893 countries change to or from daylight saving time on different dates, 894 and some countries do not have daylight saving time at all. 896 Thus, in order to schedule a session that is at the same time winter 897 and summer, it must be possible to specify unambiguously by whose 898 time zone a session is scheduled. To simplify this task for 899 receivers, we allow the sender to specify the NTP time that a time 900 zone adjustment happens and the offset from the time when the session 901 was first scheduled. The "z=" line allows the sender to specify a 902 list of these adjustment times and offsets from the base time. 904 An example might be the following: 906 z=2882844526 -1h 2898848070 0 908 This specifies that at time 2882844526, the time base by which the 909 session's repeat times are calculated is shifted back by 1 hour, and 910 that at time 2898848070, the session's original time base is 911 restored. Adjustments are always relative to the specified start 912 time -- they are not cumulative. Adjustments apply to all "t=" and 913 "r=" lines in a session description. 915 If a session is likely to last several years, it is expected that the 916 session description will be modified periodically rather than 917 transmit several years' worth of adjustments in one session 918 description. 920 5.12. Encryption Keys ("k=") 922 k= 923 k=: 925 The "k=" line is obsolete and MUST NOT be used. It is included in 926 this document for legacy reasons. One MUST NOT include a "k=" line 927 in an SDP, and MUST discard it if it is received in an SDP. 929 5.13. Attributes ("a=") 931 a= 932 a=: 934 Attributes are the primary means for extending SDP. Attributes may 935 be defined to be used as "session-level" attributes, "media-level" 936 attributes, or both. 938 A media description may have any number of attributes ("a=" lines) 939 that are media description specific. These are referred to as 940 "media-level" attributes and add information about the media 941 description. Attribute lines can also be added before the first 942 media description; these "session-level" attributes convey additional 943 information that applies to the session as a whole rather than to 944 individual media descriptions. 946 Attribute lines may be of two forms: 948 o A property attribute is simply of the form "a=". These 949 are binary attributes, and the presence of the attribute conveys 950 that the attribute is a property of the session. An example might 951 be "a=recvonly". 953 o A value attribute is of the form "a=:". For 954 example, a whiteboard could have the value attribute 955 "a=orient:landscape" 957 Attribute interpretation depends on the media tool being invoked. 958 Thus receivers of session descriptions should be configurable in 959 their interpretation of session descriptions in general and of 960 attributes in particular. 962 Attribute names MUST use the US-ASCII subset of ISO-10646/UTF-8. 964 Attribute values are octet strings, and MAY use any octet value 965 except 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute 966 values are to be interpreted as in ISO-10646 character set with UTF-8 967 encoding. Unlike other text fields, attribute values are NOT 968 normally affected by the "charset" attribute as this would make 969 comparisons against known values problematic. However, when an 970 attribute is defined, it can be defined to be charset dependent, in 971 which case its value should be interpreted in the session charset 972 rather than in ISO-10646. 974 Attributes MUST be registered with IANA (see Section 8). If an 975 attribute is received that is not understood, it MUST be ignored by 976 the receiver. 978 5.14. Media Descriptions ("m=") 980 m= ... 982 A session description may contain a number of media descriptions. 983 Each media description starts with an "m=" line (media field) and is 984 terminated by either the next "m=" line or by the end of the session 985 description. A media field has several sub-fields: 987 is the media type. This document defines the values 988 "audio", "video", "text", "application", and "message". This list 989 is extended by other memos and may be further extended by 990 additional memos registering media types in the future (see 991 Section 8). For example, [RFC6466] defined the "image" media 992 type. 994 is the transport port to which the media stream is sent. The 995 meaning of the transport port depends on the network being used as 996 specified in the relevant "c=" line, and on the transport protocol 997 defined in the sub-field of the media field. Other ports 998 used by the media application (such as the RTP Control Protocol 999 (RTCP) port [RFC3550]) MAY be derived algorithmically from the 1000 base media port or MAY be specified in a separate attribute (for 1001 example, "a=rtcp:" as defined in [RFC3605]). 1003 If non-contiguous ports are used or if they don't follow the 1004 parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:" 1005 attribute MUST be used. Applications that are requested to send 1006 media to a that is odd and where the "a=rtcp:" is present 1007 MUST NOT subtract 1 from the RTP port: that is, they MUST send the 1008 RTP to the port indicated in and send the RTCP to the port 1009 indicated in the "a=rtcp" attribute. 1011 For applications where hierarchically encoded streams are being 1012 sent to a unicast address, it may be necessary to specify multiple 1013 transport ports. This is done using a similar notation to that 1014 used for IP multicast addresses in the "c=" line: 1016 m= / ... 1018 In such a case, the ports used depend on the transport protocol. 1019 For RTP, the default is that only the even-numbered ports are used 1020 for data with the corresponding one-higher odd ports used for the 1021 RTCP belonging to the RTP session, and the 1022 denoting the number of RTP sessions. For example: 1024 m=video 49170/2 RTP/AVP 31 1026 would specify that ports 49170 and 49171 form one RTP/RTCP pair 1027 and 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the 1028 transport protocol and 31 is the format (see below). If non- 1029 contiguous ports are required, they must be signalled using a 1030 separate attribute (for example, "a=rtcp:" as defined in 1031 [RFC3605]). 1033 If multiple addresses are specified in the "c=" line and multiple 1034 ports are specified in the "m=" line, a one-to-one mapping from 1035 port to the corresponding address is implied. For example: 1037 c=IN IP4 233.252.0.1/127/2 1038 m=video 49170/2 RTP/AVP 31 1040 would imply that address 233.252.0.1 is used with ports 49170 and 1041 49171, and address 233.252.0.2 is used with ports 49172 and 49173. 1043 This document provides no semantics for using multiple "m=" lines 1044 using the same transport address. This implies that, unlike 1045 limited past practice, there is no implicit grouping defined by 1046 such means and an explicit grouping framework (for example, 1047 [RFC5888]) should instead be used to express the intended 1048 semantics. Such semantics may alo be added as extensions. For 1049 instance, see [I-D.ietf-mmusic-sdp-bundle-negotiation]. 1051 is the transport protocol. The meaning of the transport 1052 protocol is dependent on the address type field in the relevant 1053 "c=" line. Thus a "c=" field of IP4 indicates that the transport 1054 protocol runs over IP4. The following transport protocols are 1055 defined, but may be extended through registration of new protocols 1056 with IANA (see Section 8): 1058 * udp: denotes that the data is transported directly in UDP with 1059 no additional framing. 1061 * RTP/AVP: denotes RTP [RFC3550] used under the RTP Profile for 1062 Audio and Video Conferences with Minimal Control [RFC3551] 1063 running over UDP. 1065 * RTP/SAVP: denotes the Secure Real-time Transport Protocol 1066 [RFC3711] running over UDP. 1068 The main reason to specify the transport protocol in addition to 1069 the media format is that the same standard media formats may be 1070 carried over different transport protocols even when the network 1071 protocol is the same -- a historical example is VAT (MBone's 1072 popular multimedia audio tool) Pulse Code Modulation (PCM) audio 1073 and RTP PCM audio; another might be TCP/RTP PCM audio. In 1074 addition, relays and monitoring tools that are transport-protocol- 1075 specific but format-independent are possible. 1077 is a media format description. The fourth and any subsequent 1078 sub-fields describe the format of the media. The interpretation 1079 of the media format depends on the value of the sub-field. 1081 If the sub-field is "RTP/AVP" or "RTP/SAVP" the sub- 1082 fields contain RTP payload type numbers. When a list of payload 1083 type numbers is given, this implies that all of these payload 1084 formats MAY be used in the session, but the first of these formats 1085 SHOULD be used as the default format for the session. For dynamic 1086 payload type assignments the "a=rtpmap:" attribute (see Section 6) 1087 SHOULD be used to map from an RTP payload type number to a media 1088 encoding name that identifies the payload format. The "a=fmtp:" 1089 attribute MAY be used to specify format parameters (see 1090 Section 6). 1092 If the sub-field is "udp" the sub-fields MUST 1093 reference a media type describing the format under the "audio", 1094 "video", "text", "application", or "message" top-level media 1095 types. The media type registration SHOULD define the packet 1096 format for use with UDP transport. 1098 For media using other transport protocols, the field is 1099 protocol specific. Rules for interpretation of the sub- 1100 field MUST be defined when registering new protocols (see 1101 Section 8.2.2). 1103 Section 3 of [RFC4855] states that the payload format (encoding) 1104 names defined in the RTP Profile are commonly shown in upper case, 1105 while media subtype names are commonly shown in lower case. It 1106 also states that both of these names are case-insensitive in both 1107 places, similar to parameter names which are case-insensitive both 1108 in media type strings and in the default mapping to the SDP a=fmtp 1109 attribute. 1111 6. SDP Attributes 1113 The following attributes are defined. Since application writers may 1114 add new attributes as they are required, this list is not exhaustive. 1115 Registration procedures for new attributes are defined in 1116 Section 8.2.4. Syntax is provided using ABNF [RFC7405] with some of 1117 the rules defined further in Section 9. 1119 6.1. cat (category) 1121 Name: cat 1123 Value: cat-value 1125 Usage Level: session 1127 Charset Dependent: no 1128 Syntax: 1130 cat-value = category 1131 category = non-ws-string 1133 Example: 1135 a=cat:foo.bar 1137 This attribute gives the dot-separated hierarchical category of the 1138 session. This is to enable a receiver to filter unwanted sessions by 1139 category. There is no central registry of categories. This 1140 attribute is obsoleted. 1142 6.2. keywds (keywords) 1144 Name: keywds 1146 Value: keywds-value 1148 Usage Level: session 1150 Charset Dependent: yes 1152 Syntax: 1154 keywds-value = keywords 1155 keywords = text 1157 Example: 1159 a=keywds:SDP session description protocol 1161 Like the cat attribute, this is to assist identifying wanted sessions 1162 at the receiver. This allows a receiver to select interesting 1163 sessions based on keywords describing the purpose of the session; 1164 there is no central registry of keywords. Its value should be 1165 interpreted in the charset specified for the session description if 1166 one is specified, or by default in ISO 10646/UTF-8. This attribute 1167 is obsoleted. 1169 6.3. tool 1171 Name: tool 1173 Value: tool-value 1175 Usage Level: session 1176 Charset Dependent: no 1178 Syntax: 1180 tool-value = tool-name-and-version 1181 tool-name-and-version = text 1183 Example: 1185 a=tool:foobar V3.2 1187 This gives the name and version number of the tool used to create the 1188 session description. 1190 6.4. ptime (packet time) 1192 Name: ptime 1194 Value: ptime-value 1196 Usage Level: media 1198 Charset Dependent: no 1200 Syntax: 1202 ptime-value = non-zero-int-or-real 1204 Example: 1206 a=ptime:20 1208 This gives the length of time in milliseconds represented by the 1209 media in a packet. This is probably only meaningful for audio data, 1210 but may be used with other media types if it makes sense. It should 1211 not be necessary to know ptime to decode RTP or vat audio, and it is 1212 intended as a recommendation for the encoding/packetization of audio. 1214 6.5. maxptime (maximum packet time) 1216 Name: maxptime 1218 Value: maxptime-value 1220 Usage Level: media 1222 Charset Dependent: no 1223 Syntax: 1225 maxptime-value = non-zero-int-or-real 1227 Example: 1229 a=maxptime:20 1231 This gives the maximum amount of media that can be encapsulated in 1232 each packet, expressed as time in milliseconds. The time SHALL be 1233 calculated as the sum of the time the media present in the packet 1234 represents. For frame-based codecs, the time SHOULD be an integer 1235 multiple of the frame size. This attribute is probably only 1236 meaningful for audio data, but may be used with other media types if 1237 it makes sense. Note that this attribute was introduced after 1238 [RFC2327], and non-updated implementations will ignore this 1239 attribute. 1241 6.6. rtpmap 1243 Name: rtpmap 1245 Value: rtpmap-value 1247 Usage Level: media 1249 Charset Dependent: no 1251 Syntax: 1253 rtpmap-value = payload-type SP encoding-name 1254 "/" clock-rate [ "/" encoding-params ] 1255 payload-type = zero-based-integer 1256 encoding-name = token 1257 clock-rate = integer 1258 encoding-params = channels 1259 channels = integer 1261 This attribute maps from an RTP payload type number (as used in an 1262 "m=" line) to an encoding name denoting the payload format to be 1263 used. It also provides information on the clock rate and encoding 1264 parameters. Note that the payload type number is indicated in a 1265 7-bit field, limiting the values to incusively between 0 and 127. 1267 Although an RTP profile can make static assignments of payload type 1268 numbers to payload formats, it is more common for that assignment to 1269 be done dynamically using "a=rtpmap:" attributes. As an example of a 1270 static payload type, consider u-law PCM coded single-channel audio 1271 sampled at 8 kHz. This is completely defined in the RTP Audio/Video 1272 profile as payload type 0, so there is no need for an "a=rtpmap:" 1273 attribute, and the media for such a stream sent to UDP port 49232 can 1274 be specified as: 1276 m=audio 49232 RTP/AVP 0 1278 An example of a dynamic payload type is 16-bit linear encoded stereo 1279 audio sampled at 16 kHz. If we wish to use the dynamic RTP/AVP 1280 payload type 98 for this stream, additional information is required 1281 to decode it: 1283 m=audio 49232 RTP/AVP 98 1284 a=rtpmap:98 L16/16000/2 1286 Up to one rtpmap attribute can be defined for each media format 1287 specified. Thus, we might have the following: 1289 m=audio 49230 RTP/AVP 96 97 98 1290 a=rtpmap:96 L8/8000 1291 a=rtpmap:97 L16/8000 1292 a=rtpmap:98 L16/11025/2 1294 RTP profiles that specify the use of dynamic payload types MUST 1295 define the set of valid encoding names and/or a means to register 1296 encoding names if that profile is to be used with SDP. The "RTP/AVP" 1297 and "RTP/SAVP" profiles use media subtypes for encoding names, under 1298 the top-level media type denoted in the "m=" line. In the example 1299 above, the media types are "audio/L8" and "audio/L16". 1301 For audio streams, encoding-params indicates the number of audio 1302 channels. This parameter is OPTIONAL and may be omitted if the 1303 number of channels is one, provided that no additional parameters are 1304 needed. 1306 For video streams, no encoding parameters are currently specified. 1308 Additional encoding parameters MAY be defined in the future, but 1309 codec-specific parameters SHOULD NOT be added. Parameters added to 1310 an "a=rtpmap:" attribute SHOULD only be those required for a session 1311 directory to make the choice of appropriate media to participate in a 1312 session. Codec-specific parameters should be added in other 1313 attributes (for example, "a=fmtp:"). 1315 Note: RTP audio formats typically do not include information about 1316 the number of samples per packet. If a non-default (as defined in 1317 the RTP Audio/Video Profile [RFC3551]) packetization is required, the 1318 "ptime" attribute is used as given above. 1320 6.7. Media Direction Attributes 1322 At most one of recvonly/sendrecv/sendonly/inactive MAY appear at 1323 session level, and at most one MAY appear in each media description. 1325 If any one of these appears in a media description then it applies 1326 for that media description. If none appear in a media description 1327 then the one from session level, if any, applies to that media 1328 description. 1330 If none of the media direction attributes is present at either 1331 session level or media level, "sendrecv" SHOULD be assumed as the 1332 default for sessions that are not of the multimedia conference type 1333 "broadcast" or "H332" (see below). 1335 Within the following SDP example, the "inactive" attribute applies to 1336 audio media and the "recvonly" attribute applies to video media. 1338 v=0 1339 o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1 1340 s=SDP Seminar 1341 i=A Seminar on the session description protocol 1342 u=http://www.example.com/seminars/sdp.pdf 1343 e=j.doe@example.com (Jane Doe) 1344 c=IN IP4 233.252.0.1/127 1345 t=2873397496 2873404696 1346 a=inactive 1347 m=audio 49170 RTP/AVP 0 1348 m=video 51372 RTP/AVP 99 1349 a=rtpmap:99 h263-1998/90000 1350 a=recvonly 1352 6.7.1. recvonly (receive-only) 1354 Name: recvonly 1356 Value: 1358 Usage Level: session, media 1360 Charset Dependent: no 1362 Example: 1364 a=recvonly 1366 This specifies that the tools should be started in receive-only mode 1367 where applicable. Note that recvonly applies to the media only, not 1368 to any associated control protocol (e.g., an RTP-based system in 1369 recvonly mode SHOULD still send RTCP packets). 1371 6.7.2. sendrecv (send-receive) 1373 Name: sendrecv 1375 Value: 1377 Usage Level: session, media 1379 Charset Dependent: no 1381 Example: 1383 a=sendrecv 1385 This specifies that the tools should be started in send and receive 1386 mode. This is necessary for interactive multimedia conferences with 1387 tools that default to receive-only mode. 1389 6.7.3. sendonly (send-only) 1391 Name: sendonly 1393 Value: 1395 Usage Level: session, media 1397 Charset Dependent: no 1399 Example: 1401 a=sendonly 1403 This specifies that the tools should be started in send-only mode. 1404 An example may be where a different unicast address is to be used for 1405 a traffic destination than for a traffic source. In such a case, two 1406 media descriptions may be used, one sendonly and one recvonly. Note 1407 that sendonly applies only to the media, and any associated control 1408 protocol (e.g., RTCP) SHOULD still be received and processed as 1409 normal. 1411 6.7.4. inactive 1413 Name: inactive 1415 Value: 1417 Usage Level: session, media 1419 Charset Dependent: no 1421 Example: 1423 a=inactive 1425 This specifies that the tools should be started in inactive mode. 1426 This is necessary for interactive multimedia conferences where users 1427 can put other users on hold. No media is sent over an inactive media 1428 stream. Note that an RTP-based system MUST still send RTCP (if RTCP 1429 is used), even if started inactive. 1431 6.8. orient (orientation) 1433 Name: orient 1435 Value: orient-value 1437 Usage Level: media 1439 Charset Dependent: no 1441 Syntax: 1443 orient-value = portrait / landscape / seascape 1444 portrait = %s"portrait" 1445 landscape = %s"landscape" 1446 seascape = %s"seascape" 1447 ; NOTE: These names are case-sensitive. 1449 Example: 1451 a=orient:portrait 1453 Normally this is only used for a whiteboard or presentation tool. It 1454 specifies the orientation of the workspace on the screen. Permitted 1455 values are "portrait", "landscape", and "seascape" (upside-down 1456 landscape). 1458 6.9. type (conference type) 1460 Name: type 1462 Value: type-value 1464 Usage Level: session 1465 Charset Dependent: no 1467 Syntax: 1469 type-value = conference-type 1470 conference-type = broadcast / meeting / moderated / test / 1471 H332 1472 broadcast = %s"broadcast" 1473 meeting = %s"meeting" 1474 moderated = %s"moderated" 1475 test = %s"test" 1476 H332 = %s"H332" 1477 ; NOTE: These names are case-sensitive. 1479 Example: 1481 a=type:moderated 1483 This specifies the type of the multimedia conference. Suggested 1484 values are "broadcast", "meeting", "moderated", "test", and "H332". 1485 "recvonly" should be the default for "type:broadcast" sessions, 1486 "type:meeting" should imply "sendrecv", and "type:moderated" should 1487 indicate the use of a floor control tool and that the media tools are 1488 started so as to mute new sites joining the multimedia conference. 1490 Specifying the attribute "type:H332" indicates that this loosely 1491 coupled session is part of an H.332 session as defined in the ITU 1492 H.332 specification [ITU.H332.1998]. Media tools should be started 1493 "recvonly". 1495 Specifying the attribute "type:test" is suggested as a hint that, 1496 unless explicitly requested otherwise, receivers can safely avoid 1497 displaying this session description to users. 1499 6.10. charset (character set) 1501 Name: charset 1503 Value: charset-value 1505 Usage Level: session 1507 Charset Dependent: no 1509 Syntax: 1511 charset-value = mime-charset 1512 (as defined in [RFC 2978]) 1514 This specifies the character set to be used to display the session 1515 name and information data. By default, the ISO-10646 character set 1516 in UTF-8 encoding is used. If a more compact representation is 1517 required, other character sets may be used. For example, the ISO 1518 8859-1 is specified with the following SDP attribute: 1520 a=charset:ISO-8859-1 1522 The charset specified MUST be one of those registered in the IANA 1523 Character Sets registry (http://www.iana.org/assignments/character- 1524 sets), such as ISO-8859-1. The character set identifier is a US- 1525 ASCII string and MUST be compared against identifiers from the "Name" 1526 or "Preferred MIME Name" field of the registry using a case- 1527 insensitive comparison. If the identifier is not recognised or not 1528 supported, all strings that are affected by it SHOULD be regarded as 1529 octet strings. 1531 Note that a character set specified MUST still prohibit the use of 1532 bytes 0x00 (Nul), 0x0A (LF), and 0x0d (CR). Character sets requiring 1533 the use of these characters MUST define a quoting mechanism that 1534 prevents these bytes from appearing within text fields. 1536 6.11. sdplang (SDP language) 1538 Name: sdplang 1540 Value: sdplang-value 1542 Usage Level: session, media 1544 Charset Dependent: no 1546 Syntax: 1548 sdplang-value = Language-Tag 1549 ; Language-Tag defined in RFC5646 1551 Example: 1553 a=sdplang:fr 1555 Multiple sdplang attributes can be provided either at session or 1556 media level if the session description or media use multiple 1557 languages. 1559 As a session-level attribute, it specifies the language for the 1560 session description (not the language of the media). As a media- 1561 level attribute, it specifies the language for any media-level SDP 1562 information field associated with that media (again not the language 1563 of the media), overriding any sdplang attributes specified at session 1564 level. 1566 In general, sending session descriptions consisting of multiple 1567 languages is discouraged. Instead, multiple sesssion descriptions 1568 SHOULD be sent describing the session, one in each language. 1569 However, this is not possible with all transport mechanisms, and so 1570 multiple sdplang attributes are allowed although NOT RECOMMENDED. 1572 The "sdplang" attribute value must be a single [RFC5646] language tag 1573 in US-ASCII. An "sdplang" attribute SHOULD be specified when a 1574 session is distributed with sufficient scope to cross geographic 1575 boundaries, where the language of recipients cannot be assumed, or 1576 where the session is in a different language from the locally assumed 1577 norm. 1579 6.12. lang (language) 1581 Name: lang 1583 Value: lang-value 1585 Usage Level: session, media 1587 Charset Dependent: no 1589 Syntax: 1591 lang-value = Language-Tag 1592 ; Language-Tag defined in RFC5646 1594 Example: 1596 a=lang:de 1598 Multiple lang attributes can be provided either at session or media 1599 level if the session or media has capabilities in more than one 1600 language, in which case the order of the attributes indicates the 1601 order of preference of the various languages in the session or media, 1602 from most preferred to least preferred. 1604 As a session-level attribute, lang specifies a language capability 1605 for the session being described. As a media-level attribute, it 1606 specifies a language capability for that media, overriding any 1607 session-level language(s) specified. 1609 The "lang" attribute value must be a single [RFC5646] language tag in 1610 US-ASCII. A "lang" attribute SHOULD be specified when a session is 1611 of sufficient scope to cross geographic boundaries where the language 1612 of participants cannot be assumed, or where the session has 1613 capabilities in languages different from the locally assumed norm. 1615 The "lang" attribute is supposed to be used for setting the initial 1616 language(s) used in the session. Events during the session may 1617 influence which language(s) are used, and the participants are not 1618 strictly bound to only use the declared languages. 1620 Most real-time use cases start with just one language used, while 1621 other cases involve a range of languages, e.g. an interpreted or 1622 subtitled session. When more than one 'lang' attribute is specified, 1623 the "lang" attribute itself does not provide any information about 1624 multiple languages being intended to be used during the session, or 1625 if the intention is to only select one of the languages. If needed, 1626 a new attribute can be defined and used to indicate such intentions. 1627 Without such semantics, it is assumed that for a negotiated session 1628 one of the declared languages will be selected and used. 1630 6.13. framerate (frame rate) 1632 Name: framerate 1634 Value: framerate-value 1636 Usage Level: media 1638 Charset Dependent: no 1640 Syntax: 1642 framerate-value = non-zero-int-or-real 1644 Example: 1646 a=framerate:60 1648 This gives the maximum video frame rate in frames/sec. It is 1649 intended as a recommendation for the encoding of video data. Decimal 1650 representations of fractional values are allowed. It is defined only 1651 for video media. 1653 6.14. quality 1655 Name: quality 1657 Value: quality-value 1659 Usage Level: media 1661 Charset Dependent: no 1663 Syntax: 1665 quality-value = zero-based-integer 1667 Example: 1669 a=quality:10 1671 This gives a suggestion for the quality of the encoding as an integer 1672 value. The intention of the quality attribute for video is to 1673 specify a non-default trade-off between frame-rate and still-image 1674 quality. For video, the value is in the range 0 to 10, with the 1675 following suggested meaning: 1677 10 - the best still-image quality the compression scheme 1678 can give. 1679 5 - the default behaviour given no quality suggestion. 1680 0 - the worst still-image quality the codec designer 1681 thinks is still usable. 1683 6.15. fmtp (format parameters) 1685 Name: fmtp 1687 Value: fmtp-value 1689 Usage Level: media 1691 Charset Dependent: no 1693 Syntax: 1695 fmtp-value = fmt SP format-specific-params 1696 format-specific-params = byte-string 1697 ; Notes: 1698 ; - The format parameters are media type parameters and 1699 need to reflect their syntax. 1701 Example: 1703 a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600 1705 This attribute allows parameters that are specific to a particular 1706 format to be conveyed in a way that SDP does not have to understand 1707 them. The format must be one of the formats specified for the media. 1708 Format-specific parameters, semicolon separated, may be any set of 1709 parameters required to be conveyed by SDP and given unchanged to the 1710 media tool that will use this format. At most one instance of this 1711 attribute is allowed for each format. 1713 The fmtp attribute may be used to specify parameters for any protocol 1714 and format that defines use of such parameters. 1716 7. Security Considerations 1718 SDP is frequently used with the Session Initiation Protocol [RFC3261] 1719 using the offer/answer model [RFC3264] to agree on parameters for 1720 unicast sessions. When used in this manner, the security 1721 considerations of those protocols apply. 1723 SDP is a session description format that describes multimedia 1724 sessions. Entities receiving and acting upon an SDP message SHOULD 1725 be aware that a session description cannot be trusted unless it has 1726 been obtained by an authenticated and integrity-protected transport 1727 protocol from a known and trusted source. Many different transport 1728 protocols may be used to distribute session descriptions, and the 1729 nature of the authentication and integrity-protection will differ 1730 from transport to transport. For some transports, security features 1731 are often not deployed. In case a session description has not been 1732 obtained in a trusted manner, the endpoint SHOULD exercise care 1733 because, among other attacks, the media sessions received may not be 1734 the intended ones, the destination where media is sent to may not be 1735 the expected one, any of the parameters of the session may be 1736 incorrect, or the media security may be compromised. It is up to the 1737 endpoint to make a sensible decision taking into account the security 1738 risks of the application and the user preferences - the endpoint may 1739 decide to ask the user whether or not to accept the session. 1741 On receiving a session description over an unauthenticated transport 1742 mechanism or from an untrusted party, software parsing the session 1743 should take a few precautions. Similar concerns apply if integrity 1744 protection is not in place. Session descriptions contain information 1745 required to start software on the receiver's system. Software that 1746 parses a session description MUST NOT be able to start other software 1747 except that which is specifically configured as appropriate software 1748 to participate in multimedia sessions. It is normally considered 1749 inappropriate for software parsing a session description to start, on 1750 a user's system, software that is appropriate to participate in 1751 multimedia sessions, without the user first being informed that such 1752 software will be started and giving the user's consent. Thus, a 1753 session description arriving by session announcement, email, session 1754 invitation, or WWW page MUST NOT deliver the user into an interactive 1755 multimedia session unless the user has explicitly pre-authorised such 1756 action. As it is not always simple to tell whether or not a session 1757 is interactive, applications that are unsure should assume sessions 1758 are interactive. 1760 In this specification, there are no attributes that would allow the 1761 recipient of a session description to be informed to start multimedia 1762 tools in a mode where they default to transmitting. Under some 1763 circumstances it might be appropriate to define such attributes. If 1764 this is done, an application parsing a session description containing 1765 such attributes SHOULD either ignore them or inform the user that 1766 joining this session will result in the automatic transmission of 1767 multimedia data. The default behaviour for an unknown attribute is 1768 to ignore it. 1770 In certain environments, it has become common for intermediary 1771 systems to intercept and analyse session descriptions contained 1772 within other signalling protocols. This is done for a range of 1773 purposes, including but not limited to opening holes in firewalls to 1774 allow media streams to pass, or to mark, prioritize, or block traffic 1775 selectively. In some cases, such intermediary systems may modify the 1776 session description, for example, to have the contents of the session 1777 description match NAT bindings dynamically created. These behaviours 1778 are NOT RECOMMENDED unless the session description is conveyed in 1779 such a manner that allows the intermediary system to conduct proper 1780 checks to establish the authenticity of the session description, and 1781 the authority of its source to establish such communication sessions. 1782 SDP by itself does not include sufficient information to enable these 1783 checks: they depend on the encapsulating protocol (e.g., SIP or 1784 RTSP). 1786 Use of the "k=" line poses a significant security risk, since it 1787 conveys session encryption keys in the clear. SDP MUST NOT be used 1788 to convey keying material, unless it can be guaranteed that the 1789 channel over which the SDP is delivered is both private and 1790 authenticated. Moreover, the "k=" line provides no way to indicate 1791 or negotiate cryptographic key algorithms. As it provides for only a 1792 single symmetric key, rather than separate keys for confidentiality 1793 and integrity, its utility is severely limited. The "k=" line MUST 1794 NOT be used, as discussed in Section 5.12. 1796 8. IANA Considerations 1798 8.1. The "application/sdp" Media Type 1800 One media type registration from [RFC4566] is to be updated, as 1801 defined below. 1803 To: ietf-types@iana.org 1804 Subject: Registration of media type "application/sdp" 1806 Type name: application 1808 Subtype name: sdp 1810 Required parameters: None. 1812 Optional parameters: None. 1814 Encoding considerations: 1815 SDP files are primarily UTF-8 format text. The "a=charset:" 1816 attribute may be used to signal the presence of other character 1817 sets in certain parts of an SDP file (see Section 6 of RFC 1818 XXXX). Arbitrary binary content cannot be directly 1819 represented in SDP. 1821 Security considerations: 1822 See Section 7 of RFC XXXX. 1824 Interoperability considerations: 1825 See RFC XXXX. 1827 Published specification: 1828 See RFC XXXX. 1830 Applications which use this media type: 1831 Voice over IP, video teleconferencing, streaming media, instant 1832 messaging, among others. See also Section 3 of RFC XXXX. 1834 Fragment identifier considerations: None 1836 Additional information: 1838 Deprecated alias names for this type: N/A 1839 Magic number(s): None. 1840 File extension(s): The extension ".sdp" is commonly used. 1841 Macintosh File Type Code(s): "sdp " 1843 Person & email address to contact for further information: 1845 IETF MMUSIC working group 1847 Intended usage: COMMON 1849 Restrictions on usage: None 1851 Author/Change controller: 1852 Authors of RFC XXXX 1853 IETF MMUSIC working group delegated from the IESG 1855 8.2. Registration of Parameters 1857 There are seven field names that are registered with IANA. Using the 1858 terminology in the SDP specification Augmented Backus-Naur Form 1859 (ABNF), they are "media", "proto", "fmt", "att-field", "bwtype", 1860 "nettype", and "addrtype". 1862 The contact address for all parameters registered below is: 1864 IETF MMUSIC working group 1866 8.2.1. Media Types ("media") 1868 The set of media types is intended to be small and SHOULD NOT be 1869 extended except under rare circumstances. The same rules should 1870 apply for media names as for top-level media types, and where 1871 possible the same name should be registered for SDP as for MIME. For 1872 media other than existing top-level media types, a Standards Track 1873 RFC MUST be produced for a new top-level media type to be registered, 1874 and the registration MUST provide good justification why no existing 1875 media name is appropriate (the "Standards Action" policy of 1876 [RFC8126]). 1878 This memo registers the media types "audio", "video", "text", 1879 "application", and "message". 1881 Note: The media types "control" and "data" were listed as valid in an 1882 early version of this specification (RFC 2327); however, their 1883 semantics were never fully specified and they are not widely used. 1884 These media types have been removed in this specification, although 1885 they still remain valid media type capabilities for a SIP user agent 1886 as defined in [RFC3840]. If these media types are considered useful 1887 in the future, a Standards Track RFC MUST be produced to document 1888 their use. Until that is done, applications SHOULD NOT use these 1889 types and SHOULD NOT declare support for them in SIP capabilities 1890 declarations (even though they exist in the registry created by 1891 [RFC3840]). Also note that [RFC6466] defined the "image" media type. 1893 8.2.2. Transport Protocols ("proto") 1895 The "proto" field describes the transport protocol used. The 1896 registration procedure for this field is "RFC Required". 1898 This document registers two values: "RTP/AVP" is a reference to 1899 [RFC3550] used under the RTP Profile for Audio and Video Conferences 1900 with Minimal Control [RFC3551] running over UDP/IP, and "udp" 1901 indicates direct use of the UDP protocol. 1903 New transport protocols MAY be defined, and SHOULD be registered with 1904 IANA. Registrations MUST reference an RFC describing the protocol. 1905 Such an RFC MAY be Experimental or Informational, although it is 1906 preferable that it be Standards Track. The RFC defining a new 1907 protocol MUST define the rules by which the "fmt" (see below) 1908 namespace is managed. 1910 "proto" names starting with "RTP/" MUST only be used for defining 1911 transport protocols that are profiles of the RTP protocol. For 1912 example, a profile whose short name is "XYZ" would be denoted by a 1913 "proto" field of "RTP/XYZ". 1915 8.2.3. Media Formats ("fmt") 1917 Each transport protocol, defined by the "proto" field, has an 1918 associated "fmt" namespace that describes the media formats that may 1919 be conveyed by that protocol. Formats cover all the possible 1920 encodings that could be transported in a multimedia session. 1922 RTP payload formats under the "RTP/AVP" and other "RTP/*" profiles 1923 MUST use the payload type number as their "fmt" value. If the 1924 payload type number is dynamically assigned by this session 1925 description, an additional "rtpmap" attribute MUST be included to 1926 specify the format name and parameters as defined by the media type 1927 registration for the payload format. It is RECOMMENDED that other 1928 RTP profiles that are registered (in combination with RTP) as SDP 1929 transport protocols specify the same rules for the "fmt" namespace. 1931 For the "udp" protocol, allowed "fmt" values are media subtypes from 1932 the IANA Media Types registry. The media type and subtype 1933 combination / specifies the format of the body of UDP 1934 packets. Use of an existing media subtype for the format is 1935 encouraged. If no suitable media subtype exists, it is RECOMMENDED 1936 that a new one be registered through the IETF process [RFC6838] by 1937 production of, or reference to, a standards-track RFC that defines 1938 the format. 1940 For other protocols, formats MAY be registered according to the rules 1941 of the associated "proto" specification. 1943 Registrations of new formats MUST specify which transport protocols 1944 they apply to. 1946 8.2.4. Attribute Names ("att-field") 1948 8.2.4.1. New Attributes 1950 Attribute field names ("att-field") MUST be registered with IANA and 1951 documented, to avoid any issues due to conflicting attribute 1952 definitions under the same name. Unknown attributes in SDP are 1953 simply ignored, but conflicting ones that fragment the protocol are a 1954 serious problem. 1956 New attribute registrations are accepted according to the 1957 "Specification Required" policy of [RFC8126], provided that the 1958 specification includes the following information: 1960 o Contact Name. 1962 o Contact Email Address. 1964 o Attribute Name: The name of the attribute that will appear in 1965 SDP). This MUST conform to the definition of . 1967 o Attribute Syntax: For a value attribute (see clause 5.13), an ABNF 1968 definition of the attribute value syntax (See 1969 Section 9) MUST be provided. The syntax MUST follow the rule form 1970 as per Section 2.2 of [RFC5234] and [RFC7405]. This SHALL define 1971 the allowable values that the attribute might take. It MAY also 1972 define an extension method for the addition of future values. For 1973 a property attribute, the ABNF definition is omitted as the 1974 property attribute takes no values. 1976 o Attribute Semantics: For a value attribute, a semantic description 1977 of the values that the attribute might take MUST be provided. The 1978 usage of a property attribute is described under purpose below. 1980 o Attribute Value: The name of an ABNF syntax rule defining the 1981 syntax of the value. Absence of a rule name indicates that the 1982 attribute takes no values. Enclosing the rule name in "[" and "]" 1983 indicates that a value is optional. 1985 o Usage Level: Usage level(s) of the attribute. One or more of: 1986 session, media, source, dcsa, dcsa(subprotocol). For a definition 1987 of source level attributes, see [RFC5576]. For a definition of 1988 dcsa attributes see: [I-D.ietf-mmusic-data-channel-sdpneg]. 1990 o Charset Dependent: Whether the attribute value is subject to the 1991 charset attribute or not (Yes/No). 1993 o Purpose: An explanation of the purpose and usage of the attribute. 1995 o O/A Procedures: Offer/Answer procedures as explained in [RFC3264]. 1997 o Mux Category: Indication of which multiplexing "category" 1998 [I-D.ietf-mmusic-sdp-mux-attributes] an attribute is associated 1999 with. 2001 o Reference: A reference to the specification defining the 2002 attribute. 2004 The above is the minimum that IANA will accept. Attributes that are 2005 expected to see widespread use and interoperability SHOULD be 2006 documented with a standards-track RFC that specifies the attribute 2007 more precisely. 2009 Submitters of registrations should ensure that the specification is 2010 in the spirit of SDP attributes, most notably that the attribute is 2011 platform independent in the sense that it makes no implicit 2012 assumptions about operating systems and does not name specific pieces 2013 of software in a manner that might inhibit interoperability. 2015 Submitters of registrations should also carefully choose the 2016 attribute usage level. They should not choose only "session" when 2017 the attribute can have different values when media is disaggregated, 2018 i.e., when each m= section has its own IP address on a different 2019 endpoint. In that case the attribute type chosen should be "session, 2020 media" or "media" (depending on desired semantics). The default rule 2021 is that for all new SDP attributes that can occur both in session and 2022 media level, the media level overrides the session level. When this 2023 is not the case for a new SDP attribute, it MUST be explicitly 2024 stated. 2026 IANA has registered the initial set of attribute names ("att-field" 2027 values) with definitions as in Section 6 of this memo (these 2028 definitions replace those in [RFC4566]). 2030 8.2.4.2. Updates to Existing Attributes 2032 Updated attribute registrations are accepted according to the 2033 "Specification Required" policy of [RFC8126], provided that the 2034 specification updating the attribute (for example, by adding a new 2035 value) considers the registration information items from 2036 Section 8.2.4.1 according to the following bullets: 2038 o Contact Name: A name MUST be provided. 2040 o Contact Email Address: An email address MUST be provided. 2042 o Attribute Name: MUST be provided and MUST NOT be changed. 2043 Otherwise it is a new attribute. 2045 o Attribute Syntax: The existing rule syntax with the syntax 2046 extensions MUST be provided if there is a change to the syntax. A 2047 revision to an existing attribute usage MAY extend the syntax of 2048 an attribute, but MUST be backward compatible. 2050 o Attribute Semantics: A semantic description of new additional 2051 attributes values or a semantic extension of existing values. 2052 Existing attribute values semantics MUST only be extended in a 2053 backward compatible manner. 2055 o Usage Level: Updates MAY only add additional levels. 2057 o Charset Dependent: MUST NOT be changed. 2059 o Purpose: MAY be extended according to the updated usage. 2061 o O/A Procedures: MAY be updated in a backward compatible manner 2062 and/or it applies to a new usage level only. 2064 o Mux Category: No change unless from "TBD" to another value (see 2065 [I-D.ietf-mmusic-sdp-mux-attributes]. It MAY also change if 2066 'media' level is being added to the definition of an attribute 2067 that previously did not include it. 2069 o Reference: A new reference MUST be provided. 2071 Items SHOULD be omitted if there is no impact to them as a result of 2072 the attribute update. 2074 8.2.5. Bandwidth Specifiers ("bwtype") 2076 A proliferation of bandwidth specifiers is strongly discouraged. 2078 New bandwidth specifiers ("bwtype" fields) MUST be registered with 2079 IANA. The submission MUST reference a standards-track RFC specifying 2080 the semantics of the bandwidth specifier precisely, and indicating 2081 when it should be used, and why the existing registered bandwidth 2082 specifiers do not suffice. 2084 IANA has registered the bandwidth specifiers "CT" and "AS" with 2085 definitions as in Section 5.8 of this memo (these definitions update 2086 those in [RFC4566]). 2088 8.2.6. Network Types ("nettype") 2090 New network types (the "nettype" field) MUST be registered with IANA 2091 if SDP needs to be used in the context of non-Internet environments. 2092 The registration is subject to the "RFC Required" policy of 2093 [RFC8126]. Although these are not normally the preserve of IANA, 2094 there may be circumstances when an Internet application needs to 2095 interoperate with a non-Internet application, such as when gatewaying 2096 an Internet telephone call into the Public Switched Telephone Network 2097 (PSTN). The number of network types should be small and should be 2098 rarely extended. A new network type cannot be registered without 2099 registering at least one address type to be used with that network 2100 type. A new network type registration MUST reference an RFC that 2101 gives details of the network type and address type(s) and specifies 2102 how and when they would be used. 2104 IANA has registered the network type "IN" to represent the Internet, 2105 with definition as in Sections 5.2 and 5.7 of this memo (these 2106 definitions update those in [RFC4566]). 2108 8.2.7. Address Types ("addrtype") 2110 New address types ("addrtype") MUST be registered with IANA. The 2111 registration is subject to the "RFC Required" policy of [RFC8126]. 2112 An address type is only meaningful in the context of a network type, 2113 and any registration of an address type MUST specify a registered 2114 network type or be submitted along with a network type registration. 2115 A new address type registration MUST reference an RFC giving details 2116 of the syntax of the address type. Address types are not expected to 2117 be registered frequently. 2119 IANA has registered the address types "IP4" and "IP6" with 2120 definitions as in Sections 5.2 and 5.7 of this memo (these 2121 definitions update those in [RFC4566]). 2123 8.2.8. Registration Procedure 2125 In the RFC documentation that registers SDP "media", "proto", "fmt", 2126 "bwtype", "nettype", and "addrtype" fields, the authors MUST include 2127 the following information for IANA to place in the appropriate 2128 registry: 2130 o contact name, email address, and telephone number 2131 o name being registered (as it will appear in SDP) 2133 o long-form name in English 2135 o type of name ("media", "proto", "fmt", "bwtype", "nettype", or 2136 "addrtype") 2138 o a one-paragraph explanation of the purpose of the registered name 2140 o a reference to the specification for the registered name (this 2141 will typically be an RFC number) 2143 In the case of a new addrtype registration, the author has to check 2144 whether the new address type is usable with the existing network 2145 types. If yes, the "nettype" registry MUST be updated accordingly. 2146 In the case of a new nettype registration, the author MUST specify 2147 the usable address type(s). 2149 IANA may refer any registration to the IESG for review, and may 2150 request revisions to be made before a registration will be made. 2152 8.3. Encryption Key Access Methods 2154 The IANA previously maintained a table of SDP encryption key access 2155 method ("enckey") names. This table is obsolete, since the "k=" line 2156 is not extensible. New registrations MUST NOT be accepted. 2158 8.4. Reorganization of the nettype Registry 2160 This document adds a new column in the "nettype" registry with the 2161 title "Usable addrtype Values" and updates the "nettype" registry as 2162 follows: 2164 -------------------------------------------------------------------- 2165 |Type | SDP Name | Usable addrtype Values | Reference | 2166 -------------------------------------------------------------------- 2167 |nettype | IN | IP4, IP6 | [RFCXXXX] | 2168 |nettype | TN | RFC2543 | [RFC2848] | 2169 |nettype | ATM | NSAP, GWID, E164 | [RFC3108] | 2170 |nettype | PSTN | E164 | [RFC7195] | 2171 -------------------------------------------------------------------- 2173 Note that both [RFC7195] and [RFC3108] registered "E164" as an 2174 address type, although [RFC7195] mentions that the "E164" address 2175 type has a different context for ATM and PSTN networks. 2177 8.5. Reorganization of the att-field Registries 2179 This document combines all of the (currently) five "att-field" 2180 registries into one registry called "att-field" registry, and updates 2181 the columns to reflect the name, usage level(s), charset dependency 2182 and reference. As such IANA is requested to create a new combined 2183 registry using the following columns: 2185 Name | Usage Level | Dependent on Charset? | Mux Category | Reference 2187 The "Name" column reflects the attribute name (as it will appear in 2188 the SDP). The "Usage Level" column MUST indicate one or more of the 2189 following: session, media, source, dcsa and dcsa(subprotocol). The 2190 "Dependent on Charset?" column MUST indicate "Yes" or "No" depending 2191 on whether the attribute value is subject to the charset attribute. 2192 The "Mux Category" column MUST indicate one of the following 2193 categories: NORMAL, NOT RECOMMENDED, IDENTICAL, SUM, TRANSPORT, 2194 INHERIT, IDENTICAL-PER-PT, SPECIAL or TBD as defined by 2195 [I-D.ietf-mmusic-sdp-mux-attributes]. Finally, the "Reference" 2196 column indicates the specification(s) where the attribute is defined. 2198 For example, the attribute "setup" which is defined for both session 2199 and media level, will be listed in the new registry as follows: 2201 Name | Usage Level | Dependent on Charset?|Mux Category| Reference | 2202 setup | session,media, | No |IDENTICAL | [RFC4145] | 2203 | dcsa,dcsa(msrp)| | | [RFC6135] | 2204 | | | | [I-D.mmusic 2205 | | | |-msrp-usage- 2206 | | | |data-channel 2207 | | | |] | 2209 9. SDP Grammar 2211 This section provides an Augmented BNF grammar for SDP. ABNF is 2212 defined in [RFC5234] and [RFC7405]. 2214 ; SDP Syntax 2215 session-description = proto-version 2216 origin-field 2217 session-name-field 2218 [information-field] 2219 [uri-field] 2220 *email-field 2221 *phone-field 2222 [connection-field] 2223 *bandwidth-field 2224 1*time-field 2226 [key-field] 2227 *attribute-field 2228 *media-description 2230 proto-version = %s"v" "=" 1*DIGIT CRLF 2231 ;this memo describes version 0 2233 origin-field = %s"o" "=" username SP sess-id SP sess-version SP 2234 nettype SP addrtype SP unicast-address CRLF 2236 session-name-field = %s"s" "=" text CRLF 2238 information-field = %s"i" "=" text CRLF 2240 uri-field = %s"u" "=" uri CRLF 2242 email-field = %s"e" "=" email-address CRLF 2244 phone-field = %s"p" "=" phone-number CRLF 2246 connection-field = %s"c" "=" nettype SP addrtype SP 2247 connection-address CRLF 2248 ;a connection field must be present 2249 ;in every media description or at the 2250 ;session-level 2252 bandwidth-field = %s"b" "=" bwtype ":" bandwidth CRLF 2254 time-field = %s"t" "=" start-time SP stop-time 2255 *(CRLF repeat-fields) CRLF 2256 [zone-adjustments CRLF] 2258 repeat-fields = %s"r" "=" repeat-interval SP typed-time 2259 1*(SP typed-time) 2261 zone-adjustments = %s"z" "=" time SP ["-"] typed-time 2262 *(SP time SP ["-"] typed-time) 2264 key-field = %s"k" "=" key-type CRLF 2266 attribute-field = %s"a" "=" attribute CRLF 2268 media-description = media-field 2269 [information-field] 2270 *connection-field 2271 *bandwidth-field 2272 [key-field] 2273 *attribute-field 2275 media-field = %s"m" "=" media SP port ["/" integer] 2276 SP proto 1*(SP fmt) CRLF 2278 ; sub-rules of 'o=' 2279 username = non-ws-string 2280 ;pretty wide definition, but doesn't 2281 ;include space 2283 sess-id = 1*DIGIT 2284 ;should be unique for this username/host 2286 sess-version = 1*DIGIT 2288 nettype = token 2289 ;typically "IN" 2291 addrtype = token 2292 ;typically "IP4" or "IP6" 2294 ; sub-rules of 'u=' 2295 uri = URI-reference 2296 ; see RFC 3986 2298 ; sub-rules of 'e=', see RFC 5322 for definitions 2299 email-address = address-and-comment / dispname-and-address 2300 / addr-spec 2301 address-and-comment = addr-spec 1*SP "(" 1*email-safe ")" 2302 dispname-and-address = 1*email-safe 1*SP "<" addr-spec ">" 2304 ; sub-rules of 'p=' 2305 phone-number = phone *SP "(" 1*email-safe ")" / 2306 1*email-safe "<" phone ">" / 2307 phone 2309 phone = ["+"] DIGIT 1*(SP / "-" / DIGIT) 2311 ; sub-rules of 'c=' 2312 connection-address = multicast-address / unicast-address 2314 ; sub-rules of 'b=' 2315 bwtype = token 2317 bandwidth = 1*DIGIT 2319 ; sub-rules of 't=' 2320 start-time = time / "0" 2322 stop-time = time / "0" 2323 time = POS-DIGIT 9*DIGIT 2324 ; Decimal representation of NTP time in 2325 ; seconds since 1900. The representation 2326 ; of NTP time is an unbounded length field 2327 ; containing at least 10 digits. Unlike the 2328 ; 64-bit representation used elsewhere, time 2329 ; in SDP does not wrap in the year 2036. 2331 ; sub-rules of 'r=' and 'z=' 2332 repeat-interval = POS-DIGIT *DIGIT [fixed-len-time-unit] 2334 typed-time = 1*DIGIT [fixed-len-time-unit] 2336 fixed-len-time-unit = %s"d" / %s"h" / %s"m" / %s"s" 2337 ; NOTE: These units are case-sensitive. 2339 ; sub-rules of 'k=' 2340 key-type = %s"prompt" / 2341 %s"clear:" text / 2342 %s"base64:" base64 / 2343 %s"uri:" uri 2344 ; NOTE: These names are case-sensitive. 2346 base64 = *base64-unit [base64-pad] 2347 base64-unit = 4base64-char 2348 base64-pad = 2base64-char "==" / 3base64-char "=" 2349 base64-char = ALPHA / DIGIT / "+" / "/" 2351 ; sub-rules of 'a=' 2352 attribute = (att-field ":" att-value) / att-field 2354 att-field = token 2356 att-value = byte-string 2358 ; sub-rules of 'm=' 2359 media = token 2360 ;typically "audio", "video", "text", "image" 2361 ;or "application" 2363 fmt = token 2364 ;typically an RTP payload type for audio 2365 ;and video media 2367 proto = token *("/" token) 2368 ;typically "RTP/AVP" or "udp" 2370 port = 1*DIGIT 2371 ; generic sub-rules: addressing 2372 unicast-address = IP4-address / IP6-address / FQDN / extn-addr 2374 multicast-address = IP4-multicast / IP6-multicast / FQDN 2375 / extn-addr 2377 IP4-multicast = m1 3( "." decimal-uchar ) 2378 "/" ttl [ "/" numaddr ] 2379 ; IP4 multicast addresses may be in the 2380 ; range 224.0.0.0 to 239.255.255.255 2382 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / 2383 ("23" DIGIT ) 2385 IP6-multicast = IP6-address [ "/" numaddr ] 2386 ; IP6 address starting with FF 2388 numaddr = integer 2390 ttl = (POS-DIGIT *2DIGIT) / "0" 2392 FQDN = 4*(alpha-numeric / "-" / ".") 2393 ; fully qualified domain name as specified 2394 ; in RFC 1035 (and updates) 2396 IP4-address = b1 3("." decimal-uchar) 2398 b1 = decimal-uchar 2399 ; less than "224" 2401 IP6-address = 6( h16 ":" ) ls32 2402 / "::" 5( h16 ":" ) ls32 2403 / [ h16 ] "::" 4( h16 ":" ) ls32 2404 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2405 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2406 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2407 / [ *4( h16 ":" ) h16 ] "::" ls32 2408 / [ *5( h16 ":" ) h16 ] "::" h16 2409 / [ *6( h16 ":" ) h16 ] "::" 2411 h16 = 1*4HEXDIG 2413 ls32 = ( h16 ":" h16 ) / IP4-address 2415 ; Generic for other address families 2416 extn-addr = non-ws-string 2418 ; generic sub-rules: datatypes 2419 text = byte-string 2420 ;default is to interpret this as UTF8 text. 2421 ;ISO 8859-1 requires "a=charset:ISO-8859-1" 2422 ;session-level attribute to be used 2424 byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF) 2425 ;any byte except NUL, CR, or LF 2427 non-ws-string = 1*(VCHAR/%x80-FF) 2428 ;string of visible characters 2430 token-char = ALPHA / DIGIT 2431 / "!" / "#" / "$" / "%" / "&" 2432 / "'" ; (single quote) 2433 / "*" / "+" / "-" / "." / "^" / "_" 2434 / "`" ; (Grave accent) 2435 / "{" / "|" / "}" / "~" 2437 token = 1*(token-char) 2439 email-safe = %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF 2440 ;any byte except NUL, CR, LF, or the quoting 2441 ;characters ()<> 2443 integer = POS-DIGIT *DIGIT 2445 zero-based-integer = "0" / integer 2447 non-zero-int-or-real = integer / non-zero-real 2449 non-zero-real = zero-based-integer "." *DIGIT POS-DIGIT 2451 ; generic sub-rules: primitives 2452 alpha-numeric = ALPHA / DIGIT 2454 POS-DIGIT = %x31-39 ; 1 - 9 2456 decimal-uchar = DIGIT 2457 / POS-DIGIT DIGIT 2458 / ("1" 2*(DIGIT)) 2459 / ("2" ("0"/"1"/"2"/"3"/"4") DIGIT) 2460 / ("2" "5" ("0"/"1"/"2"/"3"/"4"/"5")) 2462 ; external references: 2463 ; ALPHA, DIGIT, CRLF, HEXDIG, SP, VCHAR: from RFC 5234 2464 ; URI-reference: from RFC 3986 2465 ; addr-spec: from RFC 5322 2467 10. Summary of Changes from RFC 4566 2469 The ABNF rule for IP6-address has been corrected. As a result, the 2470 ABNF rule for IP6-multicast has changed, and the (now unused) rules 2471 for hexpart, hexseq, and hex4 have been removed. 2473 IP4 unicast and multicast addresses in the example SDP descriptions 2474 have been revised per RFCs 5735 and 5771. 2476 Text in Section 5.2 has been revised to clarify the use of local 2477 addresses in case of ICE-like SDP extensions. 2479 Normative and informative references have been updated. 2481 The text regarding the session vs. media-level attribute usage has 2482 been clarified. 2484 The case-insensitivity rules from RFC 4855 have been included in this 2485 document. 2487 11. Acknowledgements 2489 Many people in the IETF Multiparty Multimedia Session Control 2490 (MMUSIC) working group have made comments and suggestions 2491 contributing to this document. 2493 12. References 2495 12.1. Normative References 2497 [E164] International Telecommunication Union, "E.164 : The 2498 international public telecommunication numbering plan", 2499 ITU Recommendation E.164, November 2010. 2501 [I-D.ietf-mmusic-data-channel-sdpneg] 2502 Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak, R., 2503 Marcon, J., and R. Even, "SDP-based Data Channel 2504 Negotiation", draft-ietf-mmusic-data-channel-sdpneg-17 2505 (work in progress), April 2018. 2507 [I-D.ietf-mmusic-sdp-mux-attributes] 2508 Nandakumar, S., "A Framework for SDP Attributes when 2509 Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-17 2510 (work in progress), February 2018. 2512 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 2513 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 2514 . 2516 [RFC1035] Mockapetris, P., "Domain names - implementation and 2517 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 2518 November 1987, . 2520 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2521 Requirement Levels", BCP 14, RFC 2119, 2522 DOI 10.17487/RFC2119, March 1997, 2523 . 2525 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration 2526 Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978, 2527 October 2000, . 2529 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 2530 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2531 2003, . 2533 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2534 Resource Identifier (URI): Generic Syntax", STD 66, 2535 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2536 . 2538 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 2539 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 2540 July 2006, . 2542 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 2543 Specifications: ABNF", STD 68, RFC 5234, 2544 DOI 10.17487/RFC5234, January 2008, 2545 . 2547 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 2548 Media Attributes in the Session Description Protocol 2549 (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009, 2550 . 2552 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying 2553 Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, 2554 September 2009, . 2556 [RFC5890] Klensin, J., "Internationalized Domain Names for 2557 Applications (IDNA): Definitions and Document Framework", 2558 RFC 5890, DOI 10.17487/RFC5890, August 2010, 2559 . 2561 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2562 Writing an IANA Considerations Section in RFCs", BCP 26, 2563 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2564 . 2566 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2567 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2568 May 2017, . 2570 12.2. Informative References 2572 [I-D.ietf-mmusic-sdp-bundle-negotiation] 2573 Holmberg, C., Alvestrand, H., and C. Jennings, 2574 "Negotiating Media Multiplexing Using the Session 2575 Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- 2576 negotiation-50 (work in progress), April 2018. 2578 [ITU.H332.1998] 2579 International Telecommunication Union, "H.323 extended for 2580 loosely coupled conferences", ITU Recommendation H.332, 2581 September 1998. 2583 [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description 2584 Protocol", RFC 2327, DOI 10.17487/RFC2327, April 1998, 2585 . 2587 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 2588 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 2589 October 2000, . 2591 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 2592 A., Peterson, J., Sparks, R., Handley, M., and E. 2593 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 2594 DOI 10.17487/RFC3261, June 2002, 2595 . 2597 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2598 with Session Description Protocol (SDP)", RFC 3264, 2599 DOI 10.17487/RFC3264, June 2002, 2600 . 2602 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 2603 Jacobson, "RTP: A Transport Protocol for Real-Time 2604 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 2605 July 2003, . 2607 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 2608 Video Conferences with Minimal Control", STD 65, RFC 3551, 2609 DOI 10.17487/RFC3551, July 2003, 2610 . 2612 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth 2613 Modifiers for RTP Control Protocol (RTCP) Bandwidth", 2614 RFC 3556, DOI 10.17487/RFC3556, July 2003, 2615 . 2617 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 2618 in Session Description Protocol (SDP)", RFC 3605, 2619 DOI 10.17487/RFC3605, October 2003, 2620 . 2622 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 2623 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 2624 RFC 3711, DOI 10.17487/RFC3711, March 2004, 2625 . 2627 [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, 2628 "Indicating User Agent Capabilities in the Session 2629 Initiation Protocol (SIP)", RFC 3840, 2630 DOI 10.17487/RFC3840, August 2004, 2631 . 2633 [RFC3890] Westerlund, M., "A Transport Independent Bandwidth 2634 Modifier for the Session Description Protocol (SDP)", 2635 RFC 3890, DOI 10.17487/RFC3890, September 2004, 2636 . 2638 [RFC4855] Casner, S., "Media Type Registration of RTP Payload 2639 Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007, 2640 . 2642 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 2643 (ICE): A Protocol for Network Address Translator (NAT) 2644 Traversal for Offer/Answer Protocols", RFC 5245, 2645 DOI 10.17487/RFC5245, April 2010, 2646 . 2648 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 2649 DOI 10.17487/RFC5322, October 2008, 2650 . 2652 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 2653 Protocol (SDP) Grouping Framework", RFC 5888, 2654 DOI 10.17487/RFC5888, June 2010, 2655 . 2657 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 2658 "Network Time Protocol Version 4: Protocol and Algorithms 2659 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 2660 . 2662 [RFC6466] Salgueiro, G., "IANA Registration of the 'image' Media 2663 Type for the Session Description Protocol (SDP)", 2664 RFC 6466, DOI 10.17487/RFC6466, December 2011, 2665 . 2667 [RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach, 2668 "TCP Candidates with Interactive Connectivity 2669 Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544, 2670 March 2012, . 2672 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 2673 Specifications and Registration Procedures", BCP 13, 2674 RFC 6838, DOI 10.17487/RFC6838, January 2013, 2675 . 2677 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 2678 RFC 7405, DOI 10.17487/RFC7405, December 2014, 2679 . 2681 [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and 2682 B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms 2683 for Real-Time Transport Protocol (RTP) Sources", RFC 7656, 2684 DOI 10.17487/RFC7656, November 2015, 2685 . 2687 [RFC7826] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., 2688 and M. Stiemerling, Ed., "Real-Time Streaming Protocol 2689 Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December 2690 2016, . 2692 Authors' Addresses 2694 Ali Begen 2695 Networked Media 2696 Konya 2697 Turkey 2699 EMail: ali.begen@networked.media 2700 Paul Kyzivat 2701 USA 2703 EMail: pkyzivat@alum.mit.edu 2705 Colin Perkins 2706 University of Glasgow 2707 School of Computing Science 2708 University of Glasgow 2709 Glasgow G12 8QQ 2710 UK 2712 EMail: csp@csperkins.org 2714 Mark Handley 2715 University College London 2716 Department of Computer Science 2717 London WC1E 6BT 2718 UK 2720 EMail: M.Handley@cs.ucl.ac.uk