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'E164' == Outdated reference: A later version (-28) exists of draft-ietf-mmusic-data-channel-sdpneg-18 == Outdated reference: A later version (-19) exists of draft-ietf-mmusic-sdp-mux-attributes-17 ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) == Outdated reference: A later version (-54) exists of draft-ietf-mmusic-sdp-bundle-negotiation-51 -- Obsolete informational reference (is this intentional?): RFC 2327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 22, 2018 C. Perkins 7 University of Glasgow 8 M. Handley 9 UCL 10 May 21, 2018 12 SDP: Session Description Protocol 13 draft-ietf-mmusic-rfc4566bis-28 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 22, 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 Zone Adjustment ("z=") . . . . . . . . . . . . . . . 20 93 5.12. Encryption Keys ("k=") . . . . . . . . . . . . . . . . . 20 94 5.13. Attributes ("a=") . . . . . . . . . . . . . . . . . . . . 21 95 5.14. Media Descriptions ("m=") . . . . . . . . . . . . . . . . 22 96 6. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . . 24 97 6.1. cat (category) . . . . . . . . . . . . . . . . . . . . . 25 98 6.2. keywds (keywords) . . . . . . . . . . . . . . . . . . . . 25 99 6.3. tool . . . . . . . . . . . . . . . . . . . . . . . . . . 26 100 6.4. ptime (packet time) . . . . . . . . . . . . . . . . . . . 26 101 6.5. maxptime (maximum packet time) . . . . . . . . . . . . . 27 102 6.6. rtpmap . . . . . . . . . . . . . . . . . . . . . . . . . 27 103 6.7. Media Direction Attributes . . . . . . . . . . . . . . . 29 104 6.7.1. recvonly (receive-only) . . . . . . . . . . . . . . . 30 105 6.7.2. sendrecv (send-receive) . . . . . . . . . . . . . . . 30 106 6.7.3. sendonly (send-only) . . . . . . . . . . . . . . . . 30 107 6.7.4. inactive . . . . . . . . . . . . . . . . . . . . . . 31 108 6.8. orient (orientation) . . . . . . . . . . . . . . . . . . 31 109 6.9. type (conference type) . . . . . . . . . . . . . . . . . 32 110 6.10. charset (character set) . . . . . . . . . . . . . . . . . 33 111 6.11. sdplang (SDP language) . . . . . . . . . . . . . . . . . 34 112 6.12. lang (language) . . . . . . . . . . . . . . . . . . . . . 34 113 6.13. framerate (frame rate) . . . . . . . . . . . . . . . . . 36 114 6.14. quality . . . . . . . . . . . . . . . . . . . . . . . . . 36 115 6.15. fmtp (format parameters) . . . . . . . . . . . . . . . . 37 116 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") . . . . . . . . . . . . . . . . 41 121 8.2.2. Transport Protocols ("proto") . . . . . . . . . . . . 41 122 8.2.3. Media Formats ("fmt") . . . . . . . . . . . . . . . . 42 123 8.2.4. Attribute Names ("att-field") . . . . . . . . . . . . 42 124 8.2.5. Bandwidth Specifiers ("bwtype") . . . . . . . . . . . 45 125 8.2.6. Network Types ("nettype") . . . . . . . . . . . . . . 45 126 8.2.7. Address Types ("addrtype") . . . . . . . . . . . . . 46 127 8.2.8. Registration Procedure . . . . . . . . . . . . . . . 46 128 8.3. Encryption Key Access Methods . . . . . . . . . . . . . . 47 129 8.4. Reorganization of the nettype Registry . . . . . . . . . 47 130 8.5. Reorganization of the att-field Registries . . . . . . . 47 131 9. SDP Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 48 132 10. Summary of Changes from RFC 4566 . . . . . . . . . . . . . . 53 133 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 54 134 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 54 135 12.1. Normative References . . . . . . . . . . . . . . . . . . 54 136 12.2. Informative References . . . . . . . . . . . . . . . . . 55 137 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 58 139 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 sub- 361 fields (session name and background information), and not to SDP as a 362 whole. 364 5. SDP Specification 366 An SDP description is denoted by the media type "application/sdp" 367 (See Section 8). 369 An SDP description is entirely textual. SDP field names and 370 attribute names use only the US-ASCII subset of UTF-8, but textual 371 fields and attribute values MAY use the full ISO 10646 character set 372 in UTF-8 encoding, or some other character set defined by the 373 "a=charset:" attribute. Field and attribute values that use the full 374 UTF-8 character set are never directly compared, hence there is no 375 requirement for UTF-8 normalization. The textual form, as opposed to 376 a binary encoding such as ASN.1 or XDR, was chosen to enhance 377 portability, to enable a variety of transports to be used, and to 378 allow flexible, text-based toolkits to be used to generate and 379 process session descriptions. However, since SDP may be used in 380 environments where the maximum permissible size of a session 381 description is limited, the encoding is deliberately compact. Also, 382 since announcements may be transported via very unreliable means or 383 damaged by an intermediate caching server, the encoding was designed 384 with strict order and formatting rules so that most errors would 385 result in malformed session announcements that could be detected 386 easily and discarded. This also allows rapid discarding of encrypted 387 session announcements for which a receiver does not have the correct 388 key. 390 An SDP description consists of a number of lines of text of the form: 392 = 394 where MUST be exactly one case-significant character and 395 is structured text whose format depends on . In 396 general, is either a number of sub-fields delimited by a 397 single space character or a free format string, and is case- 398 significant unless a specific field defines otherwise. Whitespace 399 separators MUST NOT be used on either side of the "=" sign, however, 400 the value can contain a leading whitespace as part of its syntax, 401 i.e., that whitespace is part of the value. 403 An SDP description consists of a session-level section followed by 404 zero or more media descriptions. The session-level section starts 405 with a "v=" line and continues to the first media description (or the 406 end of the whole description, whichever comes first). Each media 407 description starts with an "m=" line and continues to the next media 408 description or the end of the whole session description - whichever 409 comes first. In general, session-level values are the default for 410 all media unless overridden by an equivalent media-level value. 412 Some lines in each description are REQUIRED and some are OPTIONAL, 413 but all MUST appear in exactly the order given here (the fixed order 414 greatly enhances error detection and allows for a simple parser). 415 OPTIONAL items are marked with a "*". 417 Session description 418 v= (protocol version) 419 o= (originator and session identifier) 420 s= (session name) 421 i=* (session information) 422 u=* (URI of description) 423 e=* (email address) 424 p=* (phone number) 425 c=* (connection information -- not required if included in 426 all media descriptions) 427 b=* (zero or more bandwidth information lines) 428 One or more time descriptions: 429 ("t=", "r=" and "z=" lines; see below) 430 k=* (encryption key) 431 a=* (zero or more session attribute lines) 432 Zero or more media descriptions 434 Time description 435 t= (time the session is active) 436 r=* (zero or more repeat times) 437 z= (optional time zone offset line) 439 Media description, if present 440 m= (media name and transport address) 441 i=* (media title) 442 c=* (connection information -- optional if included at 443 session level) 444 b=* (zero or more bandwidth information lines) 445 k=* (encryption key) 446 a=* (zero or more media attribute lines) 448 The set of type letters is deliberately small and not intended to be 449 extensible -- an SDP parser MUST completely ignore any session 450 description that contains a type letter that it does not understand. 451 The attribute mechanism ("a=" described below) is the primary means 452 for extending SDP and tailoring it to particular applications or 453 media. Some attributes (the ones listed in Section 6 of this memo) 454 have a defined meaning, but others may be added on an application-, 455 media-, or session-specific basis. An SDP parser MUST ignore any 456 attribute it doesn't understand. 458 An SDP description may contain URIs that reference external content 459 in the "u=", "k=", and "a=" lines. These URIs may be dereferenced in 460 some cases, making the session description non-self-contained. 462 The connection ("c=") information in the session-level section 463 applies to all the media descriptions of that session unless 464 overridden by connection information in the media description. For 465 instance, in the example below, each audio media description behaves 466 as if it were given a "c=IN IP4 233.252.0.2". 468 An example SDP description is: 470 v=0 471 o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1 472 s=SDP Seminar 473 i=A Seminar on the session description protocol 474 u=http://www.example.com/seminars/sdp.pdf 475 e=j.doe@example.com (Jane Doe) 476 c=IN IP4 233.252.0.2 477 t=2873397496 2873404696 478 a=recvonly 479 m=audio 49170 RTP/AVP 0 480 m=audio 49180 RTP/AVP 0 481 m=video 51372 RTP/AVP 99 482 c=IN IP4 233.252.0.1/127 483 a=rtpmap:99 h263-1998/90000 485 Text containing fields such as the session name and information are 486 octet strings that may contain any octet with the exceptions of 0x00 487 (Nul), 0x0a (ASCII newline), and 0x0d (ASCII carriage return). The 488 sequence CRLF (0x0d0a) is used to end a record, although parsers 489 SHOULD be tolerant and also accept records terminated with a single 490 newline character. If the "a=charset" attribute is not present, 491 these octet strings MUST be interpreted as containing ISO-10646 492 characters in UTF-8 encoding (the presence of the "a=charset" 493 attribute may force some fields to be interpreted differently). 495 A session description can contain domain names in the "o=", "u=", 496 "e=", "c=", and "a=" lines. Any domain name used in SDP MUST comply 497 with [RFC1034], [RFC1035]. Internationalized domain names (IDNs) 498 MUST be represented using the ASCII Compatible Encoding (ACE) form 499 defined in [RFC5890] and MUST NOT be directly represented in UTF-8 or 500 any other encoding (this requirement is for compatibility with 501 [RFC2327] and other early SDP-related standards, which predate the 502 development of internationalized domain names). 504 5.1. Protocol Version ("v=") 506 v=0 508 The "v=" line (version-field) gives the version of the Session 509 Description Protocol. This memo defines version 0. There is no 510 minor version number. 512 5.2. Origin ("o=") 514 o= 515 517 The "o=" line (origin-field) gives the originator of the session (her 518 username and the address of the user's host) plus a session 519 identifier and version number: 521 is the user's login on the originating host, or it is "-" 522 if the originating host does not support the concept of user IDs. 523 The MUST NOT contain spaces. 525 is a numeric string such that the tuple of , 526 , , , and forms a 527 globally unique identifier for the session. The method of allocation is up to the creating tool, but it has been 529 suggested that a Network Time Protocol (NTP) format timestamp be 530 used to ensure uniqueness [RFC5905]. 532 is a version number for this session description. 533 Its usage is up to the creating tool, so long as is 534 increased when a modification is made to the session data. Again, 535 it is RECOMMENDED that an NTP format timestamp is used. 537 is a text string giving the type of network. Initially 538 "IN" is defined to have the meaning "Internet", but other values 539 MAY be registered in the future (see Section 8). 541 is a text string giving the type of the address that 542 follows. Initially "IP4" and "IP6" are defined, but other values 543 MAY be registered in the future (see Section 8). 545 is an address of the machine from which the 546 session was created. For an address type of IP4, this is either a 547 fully qualified domain name of the machine or the dotted-decimal 548 representation of an IP version 4 address of the machine. For an 549 address type of IP6, this is either a fully qualified domain name 550 of the machine or the compressed textual representation of an IP 551 version 6 address of the machine. For both IP4 and IP6, the fully 552 qualified domain name is the form that SHOULD be given unless this 553 is unavailable, in which case a globally unique address MAY be 554 substituted. Unless an SDP extension for NAT traversal is used 555 (e.g., ICE [RFC5245], ICE TCP [RFC6544]), a local IP address MUST 556 NOT be used in any context where the SDP description might leave 557 the scope in which the address is meaningful (for example, a local 558 address MUST NOT be included in an application-level referral that 559 might leave the scope). 561 In general, the "o=" line serves as a globally unique identifier for 562 this version of the session description, and the sub-fields excepting 563 the version, taken together identify the session irrespective of any 564 modifications. 566 For privacy reasons, it is sometimes desirable to obfuscate the 567 username and IP address of the session originator. If this is a 568 concern, an arbitrary and private MAY be 569 chosen to populate the "o=" line, provided that these are selected in 570 a manner that does not affect the global uniqueness of the field. 572 5.3. Session Name ("s=") 574 s= 576 The "s=" line (session-name-field) is the textual session name. 577 There MUST be one and only one "s=" line per session description. 578 The "s=" line MUST NOT be empty and SHOULD contain ISO 10646 579 characters (but see also the "a=charset" attribute). If a session 580 has no meaningful name, the "s= " line SHOULD be used (i.e., a single 581 space as the session name). 583 5.4. Session Information ("i=") 585 i= 587 The "i=" line (information-field) provides textual information about 588 the session. There MUST be at most one session-level "i=" line per 589 session description, and at most one "i=" line in each media 590 description. Unless a media level "i=" line is provided, the 591 session-level "i=" line applies to that media description. If the 592 "a=charset" attribute is present, it specifies the character set used 593 in the "i=" line. If the "a=charset" attribute is not present, the 594 "i=" line MUST contain ISO 10646 characters in UTF-8 encoding. 596 At most one "i=" line can be used for each media description. In 597 media definitions, "i=" lines are primarily intended for labelling 598 media streams. As such, they are most likely to be useful when a 599 single session has more than one distinct media stream of the same 600 media type. An example would be two different whiteboards, one for 601 slides and one for feedback and questions. 603 The "i=" line is intended to provide a free-form human-readable 604 description of the session or the purpose of a media stream. It is 605 not suitable for parsing by automata. 607 5.5. URI ("u=") 609 u= 611 The "u=" line (uri-field) provides URI (Uniform Resource Identifier) 612 as used by WWW clients [RFC3986]. The URI should be a pointer to 613 additional information about the session. This line is OPTIONAL. No 614 more than one "u=" line is allowed per session description. 616 5.6. Email Address and Phone Number ("e=" and "p=") 618 e= 619 p= 621 The "e=" line (email-field) and "p=" line (phone-field) specify 622 contact information for the person responsible for the session. This 623 is not necessarily the same person that created the session 624 description. 626 Inclusion of an email address or phone number is OPTIONAL. 628 If an email address or phone number is present, it MUST be specified 629 before the first media description. More than one email or phone 630 line can be given for a session description. 632 Phone numbers SHOULD be given in the form of an international public 633 telecommunication number (see ITU-T Recommendation E.164 [E164]) 634 preceded by a "+". Spaces and hyphens may be used to split up a 635 phone field to aid readability if desired. For example: 637 p=+1 617 555-6011 639 Both email addresses and phone numbers can have an OPTIONAL free text 640 string associated with them, normally giving the name of the person 641 who may be contacted. This MUST be enclosed in parentheses if it is 642 present. For example: 644 e=j.doe@example.com (Jane Doe) 646 The alternative [RFC5322] name quoting convention is also allowed for 647 both email addresses and phone numbers. For example: 649 e=Jane Doe 651 The free text string SHOULD be in the ISO-10646 character set with 652 UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if 653 the appropriate session-level "a=charset" attribute is set. 655 5.7. Connection Information ("c=") 657 c= 659 The "c=" line (connection-field) contains connection data. 661 A session description MUST contain either at least one "c=" line in 662 each media description or a single "c=" line at the session level. 663 It MAY contain a single session-level "c=" line and additional "c=" 664 line(s) per media description, in which case the per-media values 665 override the session-level settings for the respective media. 667 The first sub-field ("") is the network type, which is a 668 text string giving the type of network. Initially, "IN" is defined 669 to have the meaning "Internet", but other values MAY be registered in 670 the future (see Section 8). 672 The second sub-field ("") is the address type. This allows 673 SDP to be used for sessions that are not IP based. This memo only 674 defines IP4 and IP6, but other values MAY be registered in the future 675 (see Section 8). 677 The third sub-field ("") is the connection 678 address. Additional sub-fields MAY be added after the connection 679 address depending on the value of the sub-field. 681 When the is IP4 and IP6, the connection address is defined 682 as follows: 684 o If the session is multicast, the connection address will be an IP 685 multicast group address. If the session is not multicast, then 686 the connection address contains the unicast IP address of the 687 expected data source, data relay or data sink as determined by 688 additional attribute fields. It is not expected that unicast 689 addresses will be given in a session description that is 690 communicated by a multicast announcement, though this is not 691 prohibited. 693 o Sessions using an IP4 multicast connection address MUST also have 694 a time to live (TTL) value present in addition to the multicast 695 address. The TTL and the address together define the scope with 696 which multicast packets sent in this session will be sent. TTL 697 values MUST be in the range 0-255. Although the TTL MUST be 698 specified, its use to scope multicast traffic is deprecated; 699 applications SHOULD use an administratively scoped address 700 instead. 702 The TTL for the session is appended to the address using a slash as a 703 separator. An example is: 705 c=IN IP4 233.252.0.1/127 707 IP6 multicast does not use TTL scoping, and hence the TTL value MUST 708 NOT be present for IP6 multicast. It is expected that IP6 scoped 709 addresses will be used to limit the scope of multimedia conferences. 711 Hierarchical or layered encoding schemes are data streams where the 712 encoding from a single media source is split into a number of layers. 713 The receiver can choose the desired quality (and hence bandwidth) by 714 only subscribing to a subset of these layers. Such layered encodings 715 are normally transmitted in multiple multicast groups to allow 716 multicast pruning. This technique keeps unwanted traffic from sites 717 only requiring certain levels of the hierarchy. For applications 718 requiring multiple multicast groups, we allow the following notation 719 to be used for the connection address: 721 [/]/ 723 If the number of addresses is not given, it is assumed to be one. 724 Multicast addresses so assigned are contiguously allocated above the 725 base address, so that, for example: 727 c=IN IP4 233.252.0.1/127/3 729 would state that addresses 233.252.0.1, 233.252.0.2, and 233.252.0.3 730 are to be used with a TTL of 127. This is semantically identical to 731 including multiple "c=" lines in a media description: 733 c=IN IP4 233.252.0.1/127 734 c=IN IP4 233.252.0.2/127 735 c=IN IP4 233.252.0.3/127 737 Similarly, an IP6 example would be: 739 c=IN IP6 FF15::101/3 741 which is semantically equivalent to: 743 c=IN IP6 FF15::101 744 c=IN IP6 FF15::102 745 c=IN IP6 FF15::103 747 (remembering that the TTL sub-field is not present in IP6 multicast). 749 Multiple addresses or "c=" lines MAY be specified on a per media 750 description basis only if they provide multicast addresses for 751 different layers in a hierarchical or layered encoding scheme. They 752 MUST NOT be specified for a session-level "c=" line. 754 The slash notation for multiple addresses described above MUST NOT be 755 used for IP unicast addresses. 757 5.8. Bandwidth Information ("b=") 759 b=: 761 The OPTIONAL "b=" line (bandwidth-field) denotes the proposed 762 bandwidth to be used by the session or media description. The 763 is an alphanumeric modifier giving the meaning of the 764 figure. Two values are defined in this specification, 765 but other values MAY be registered in the future (see Section 8 and 766 [RFC3556], [RFC3890]): 768 CT If the bandwidth of a session is different from the bandwidth 769 implicit from the scope, a "b=CT:..." line SHOULD be supplied for 770 the session giving the proposed upper limit to the bandwidth used 771 (the "conference total" bandwidth). Similarly, if the bandwidth 772 of bundled media streams in an m line is different from the 773 implicit value from the scope, a "b=CT:..." line SHOULD be 774 supplied in the media level. The primary purpose of this is to 775 give an approximate idea as to whether two or more sessions (or 776 bundled media streams) can coexist simultaneously. Note that CT 777 gives a total bandwidth figure for all the media at all endpoints. 779 AS The bandwidth is interpreted to be application specific (it will 780 be the application's concept of maximum bandwidth). Normally, 781 this will coincide with what is set on the application's "maximum 782 bandwidth" control if applicable. For RTP-based applications, AS 783 gives the RTP "session bandwidth" as defined in Section 6.2 of 784 [RFC3550]. Note that AS gives a bandwidth figure for a single 785 media at a single endpoint, although there may be many endpoints 786 sending simultaneously. 788 A prefix "X-" is defined for names. This is intended for 789 experimental purposes only. For example: 791 b=X-YZ:128 793 Use of the "X-" prefix is NOT RECOMMENDED: instead new (non "X-" 794 prefix) names SHOULD be defined, and then MUST be registered 795 with IANA in the standard namespace. SDP parsers MUST ignore 796 bandwidth fields with unknown names. The names 797 MUST be alphanumeric and, although no length limit is given, it is 798 recommended that they be short. 800 The is interpreted as kilobits per second by default 801 (including the transport and network-layer but not the link-layer 802 overhead). The definition of a new modifier MAY specify 803 that the bandwidth is to be interpreted in some alternative unit (the 804 "CT" and "AS" modifiers defined in this memo use the default units). 806 5.9. Time Active ("t=") 808 t= 810 A "t=" line (time-field) initiates a time description that specifies 811 the start and stop times for a session. Multiple time descriptions 812 MAY be used if a session is active at multiple irregularly spaced 813 times; each additional time description specifies additional periods 814 of time for which the session will be active. If the session is 815 active at regular repeat times, a repeat description, initiated by an 816 "r=" line (see below) can be included following the time-field -- in 817 which case the time-field specifies the start and stop times of the 818 entire repeat sequence. 820 The first and second sub-fields of the time-field give the start and 821 stop times, respectively, for the session. These values are the 822 decimal representation of Network Time Protocol (NTP) time values in 823 seconds since 1900 [RFC5905]. To convert these values to UNIX time 824 (UTC), subtract decimal 2208988800. 826 NTP timestamps are elsewhere represented by 64-bit values, which wrap 827 sometime in the year 2036. Since SDP uses an arbitrary length 828 decimal representation, this should not cause an issue (SDP 829 timestamps MUST continue counting seconds since 1900 - NTP will use 830 the value modulo the 64-bit limit). 832 If the is set to zero, then the session is not bounded, 833 though it will not become active until after the . If 834 the is also zero, the session is regarded as permanent. 836 User interfaces SHOULD strongly discourage the creation of unbounded 837 and permanent sessions as they give no information about when the 838 session is actually going to terminate, and so make scheduling 839 difficult. 841 The general assumption may be made, when displaying unbounded 842 sessions that have not timed out to the user, that an unbounded 843 session will only be active until half an hour from the current time 844 or the session start time, whichever is the later. If behaviour 845 other than this is required, an end-time SHOULD be given and modified 846 as appropriate when new information becomes available about when the 847 session should really end. 849 Permanent sessions may be shown to the user as never being active 850 unless there are associated repeat times that state precisely when 851 the session will be active. 853 5.10. Repeat Times ("r=") 855 r= 857 An"r=" line (repeat-field) specifies repeat times for a session. If 858 needed to express complex schedules, multiple repeat-fields may be 859 included. For example, if a session is active at 10am on Monday and 860 11am on Tuesday for one hour each week for three months, then the 861 in the corresponding "t=" line would be the NTP 862 representation of 10am on the first Monday, the 863 would be 1 week, the would be 1 hour, and the 864 offsets would be zero and 25 hours. The corresponding "t=" line stop 865 time would be the NTP representation of the end of the last session 866 three months later. By default, all sub-fields are in seconds, so 867 the "r=" and "t=" lines might be the following: 869 t=3034423619 3042462419 870 r=604800 3600 0 90000 872 To make the description more compact, times may also be given in 873 units of days, hours, or minutes. The syntax for these is a number 874 immediately followed by a single case-sensitive character. 875 Fractional units are not allowed -- a smaller unit should be used 876 instead. The following unit specification characters are allowed: 878 d - days (86400 seconds) 879 h - hours (3600 seconds) 880 m - minutes (60 seconds) 881 s - seconds (allowed for completeness) 883 Thus, the above session announcement could also have been written: 885 r=7d 1h 0 25h 887 Monthly and yearly repeats cannot be directly specified with a single 888 SDP repeat time; instead, separate time-descriptions should be used 889 to explicitly list the session times. 891 5.11. Time Zone Adjustment ("z=") 893 z= .... 895 A "z=" line (zone-field) is an optional modifier to the repeat-fields 896 it immediately follows. It does not apply to any other fields. 898 To schedule a repeated session that spans a change from daylight 899 saving time to standard time or vice versa, it is necessary to 900 specify offsets from the base time. This is required because 901 different time zones change time at different times of day, different 902 countries change to or from daylight saving time on different dates, 903 and some countries do not have daylight saving time at all. 905 Thus, in order to schedule a session that is at the same time winter 906 and summer, it must be possible to specify unambiguously by whose 907 time zone a session is scheduled. To simplify this task for 908 receivers, we allow the sender to specify the NTP time that a time 909 zone adjustment happens and the offset from the time when the session 910 was first scheduled. The "z=" line allows the sender to specify a 911 list of these adjustment times and offsets from the base time. 913 An example might be the following: 915 z=2882844526 -1h 2898848070 0 917 This specifies that at time 2882844526, the time base by which the 918 session's repeat times are calculated is shifted back by 1 hour, and 919 that at time 2898848070, the session's original time base is 920 restored. Adjustments are always relative to the specified start 921 time -- they are not cumulative. Adjustments apply to all "t=" and 922 "r=" lines in a session description. 924 If a session is likely to last several years, it is expected that the 925 session description will be modified periodically rather than 926 transmit several years' worth of adjustments in one session 927 description. 929 5.12. Encryption Keys ("k=") 931 k= 932 k=: 934 The "k=" line (key-field) is obsolete and MUST NOT be used. It is 935 included in this document for legacy reasons. One MUST NOT include a 936 "k=" line in an SDP, and MUST discard it if it is received in an SDP. 938 5.13. Attributes ("a=") 940 a= 941 a=: 943 Attributes are the primary means for extending SDP. Attributes may 944 be defined to be used as "session-level" attributes, "media-level" 945 attributes, or both. 947 A media description may have any number of "a=" lines (attribute- 948 fields) that are media description specific. These are referred to 949 as "media-level" attributes and add information about the media 950 description. Attribute fields can also be added before the first 951 media description; these "session-level" attributes convey additional 952 information that applies to the session as a whole rather than to 953 individual media descriptions. 955 Attribute fields may be of two forms: 957 o A property attribute is simply of the form "a=". These 958 are binary attributes, and the presence of the attribute conveys 959 that the attribute is a property of the session. An example might 960 be "a=recvonly". 962 o A value attribute is of the form "a=:". For 963 example, a whiteboard could have the value attribute 964 "a=orient:landscape" 966 Attribute interpretation depends on the media tool being invoked. 967 Thus receivers of session descriptions should be configurable in 968 their interpretation of session descriptions in general and of 969 attributes in particular. 971 Attribute names MUST use the US-ASCII subset of ISO-10646/UTF-8. 973 Attribute values are octet strings, and MAY use any octet value 974 except 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute 975 values are to be interpreted as in ISO-10646 character set with UTF-8 976 encoding. Unlike other text fields, attribute values are NOT 977 normally affected by the "charset" attribute as this would make 978 comparisons against known values problematic. However, when an 979 attribute is defined, it can be defined to be charset dependent, in 980 which case its value should be interpreted in the session charset 981 rather than in ISO-10646. 983 Attributes MUST be registered with IANA (see Section 8). If an 984 attribute is received that is not understood, it MUST be ignored by 985 the receiver. 987 5.14. Media Descriptions ("m=") 989 m= ... 991 A session description may contain a number of media descriptions. 992 Each media description starts with an "m=" line (media-field) and is 993 terminated by either the next "m=" line or by the end of the session 994 description. A media field has several sub-fields: 996 is the media type. This document defines the values 997 "audio", "video", "text", "application", and "message". This list 998 is extended by other memos and may be further extended by 999 additional memos registering media types in the future (see 1000 Section 8). For example, [RFC6466] defined the "image" media 1001 type. 1003 is the transport port to which the media stream is sent. The 1004 meaning of the transport port depends on the network being used as 1005 specified in the relevant "c=" line, and on the transport protocol 1006 defined in the sub-field of the media field. Other ports 1007 used by the media application (such as the RTP Control Protocol 1008 (RTCP) port [RFC3550]) MAY be derived algorithmically from the 1009 base media port or MAY be specified in a separate attribute (for 1010 example, "a=rtcp:" as defined in [RFC3605]). 1012 If non-contiguous ports are used or if they don't follow the 1013 parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:" 1014 attribute MUST be used. Applications that are requested to send 1015 media to a that is odd and where the "a=rtcp:" is present 1016 MUST NOT subtract 1 from the RTP port: that is, they MUST send the 1017 RTP to the port indicated in and send the RTCP to the port 1018 indicated in the "a=rtcp" attribute. 1020 For applications where hierarchically encoded streams are being 1021 sent to a unicast address, it may be necessary to specify multiple 1022 transport ports. This is done using a similar notation to that 1023 used for IP multicast addresses in the "c=" line: 1025 m= / ... 1027 In such a case, the ports used depend on the transport protocol. 1028 For RTP, the default is that only the even-numbered ports are used 1029 for data with the corresponding one-higher odd ports used for the 1030 RTCP belonging to the RTP session, and the 1031 denoting the number of RTP sessions. For example: 1033 m=video 49170/2 RTP/AVP 31 1035 would specify that ports 49170 and 49171 form one RTP/RTCP pair 1036 and 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the 1037 transport protocol and 31 is the format (see below). If non- 1038 contiguous ports are required, they must be signalled using a 1039 separate attribute (for example, "a=rtcp:" as defined in 1040 [RFC3605]). 1042 If multiple addresses are specified in the "c=" line and multiple 1043 ports are specified in the "m=" line, a one-to-one mapping from 1044 port to the corresponding address is implied. For example: 1046 c=IN IP4 233.252.0.1/127/2 1047 m=video 49170/2 RTP/AVP 31 1049 would imply that address 233.252.0.1 is used with ports 49170 and 1050 49171, and address 233.252.0.2 is used with ports 49172 and 49173. 1052 This document provides no semantics for using multiple "m=" lines 1053 using the same transport address. This implies that, unlike 1054 limited past practice, there is no implicit grouping defined by 1055 such means and an explicit grouping framework (for example, 1056 [RFC5888]) should instead be used to express the intended 1057 semantics. Such semantics may alo be added as extensions. For 1058 instance, see [I-D.ietf-mmusic-sdp-bundle-negotiation]. 1060 is the transport protocol. The meaning of the transport 1061 protocol is dependent on the address type sub-field in the 1062 relevant "c=" line. Thus a "c=" field of IP4 indicates that the 1063 transport protocol runs over IP4. The following transport 1064 protocols are defined, but may be extended through registration of 1065 new protocols with IANA (see Section 8): 1067 * udp: denotes that the data is transported directly in UDP with 1068 no additional framing. 1070 * RTP/AVP: denotes RTP [RFC3550] used under the RTP Profile for 1071 Audio and Video Conferences with Minimal Control [RFC3551] 1072 running over UDP. 1074 * RTP/SAVP: denotes the Secure Real-time Transport Protocol 1075 [RFC3711] running over UDP. 1077 The main reason to specify the transport protocol in addition to 1078 the media format is that the same standard media formats may be 1079 carried over different transport protocols even when the network 1080 protocol is the same -- a historical example is VAT (MBone's 1081 popular multimedia audio tool) Pulse Code Modulation (PCM) audio 1082 and RTP PCM audio; another might be TCP/RTP PCM audio. In 1083 addition, relays and monitoring tools that are transport-protocol- 1084 specific but format-independent are possible. 1086 is a media format description. The fourth and any subsequent 1087 sub-fields describe the format of the media. The interpretation 1088 of the media format depends on the value of the sub-field. 1090 If the sub-field is "RTP/AVP" or "RTP/SAVP" the sub- 1091 fields contain RTP payload type numbers. When a list of payload 1092 type numbers is given, this implies that all of these payload 1093 formats MAY be used in the session, but the first of these formats 1094 SHOULD be used as the default format for the session. For dynamic 1095 payload type assignments the "a=rtpmap:" attribute (see Section 6) 1096 SHOULD be used to map from an RTP payload type number to a media 1097 encoding name that identifies the payload format. The "a=fmtp:" 1098 attribute MAY be used to specify format parameters (see 1099 Section 6). 1101 If the sub-field is "udp" the sub-fields MUST 1102 reference a media type describing the format under the "audio", 1103 "video", "text", "application", or "message" top-level media 1104 types. The media type registration SHOULD define the packet 1105 format for use with UDP transport. 1107 For media using other transport protocols, the sub-field is 1108 protocol specific. Rules for interpretation of the sub- 1109 field MUST be defined when registering new protocols (see 1110 Section 8.2.2). 1112 Section 3 of [RFC4855] states that the payload format (encoding) 1113 names defined in the RTP Profile are commonly shown in upper case, 1114 while media subtype names are commonly shown in lower case. It 1115 also states that both of these names are case-insensitive in both 1116 places, similar to parameter names which are case-insensitive both 1117 in media type strings and in the default mapping to the SDP a=fmtp 1118 attribute. 1120 6. SDP Attributes 1122 The following attributes are defined. Since application writers may 1123 add new attributes as they are required, this list is not exhaustive. 1124 Registration procedures for new attributes are defined in 1125 Section 8.2.4. Syntax is provided using ABNF [RFC7405] with some of 1126 the rules defined further in Section 9. 1128 6.1. cat (category) 1130 Name: cat 1132 Value: cat-value 1134 Usage Level: session 1136 Charset Dependent: no 1138 Syntax: 1140 cat-value = category 1141 category = non-ws-string 1143 Example: 1145 a=cat:foo.bar 1147 This attribute gives the dot-separated hierarchical category of the 1148 session. This is to enable a receiver to filter unwanted sessions by 1149 category. There is no central registry of categories. This 1150 attribute is obsoleted. 1152 6.2. keywds (keywords) 1154 Name: keywds 1156 Value: keywds-value 1158 Usage Level: session 1160 Charset Dependent: yes 1162 Syntax: 1164 keywds-value = keywords 1165 keywords = text 1167 Example: 1169 a=keywds:SDP session description protocol 1171 Like the cat attribute, this is to assist identifying wanted sessions 1172 at the receiver. This allows a receiver to select interesting 1173 sessions based on keywords describing the purpose of the session; 1174 there is no central registry of keywords. Its value should be 1175 interpreted in the charset specified for the session description if 1176 one is specified, or by default in ISO 10646/UTF-8. This attribute 1177 is obsoleted. 1179 6.3. tool 1181 Name: tool 1183 Value: tool-value 1185 Usage Level: session 1187 Charset Dependent: no 1189 Syntax: 1191 tool-value = tool-name-and-version 1192 tool-name-and-version = text 1194 Example: 1196 a=tool:foobar V3.2 1198 This gives the name and version number of the tool used to create the 1199 session description. 1201 6.4. ptime (packet time) 1203 Name: ptime 1205 Value: ptime-value 1207 Usage Level: media 1209 Charset Dependent: no 1211 Syntax: 1213 ptime-value = non-zero-int-or-real 1215 Example: 1217 a=ptime:20 1219 This gives the length of time in milliseconds represented by the 1220 media in a packet. This is probably only meaningful for audio data, 1221 but may be used with other media types if it makes sense. It should 1222 not be necessary to know ptime to decode RTP or vat audio, and it is 1223 intended as a recommendation for the encoding/packetization of audio. 1225 6.5. maxptime (maximum packet time) 1227 Name: maxptime 1229 Value: maxptime-value 1231 Usage Level: media 1233 Charset Dependent: no 1235 Syntax: 1237 maxptime-value = non-zero-int-or-real 1239 Example: 1241 a=maxptime:20 1243 This gives the maximum amount of media that can be encapsulated in 1244 each packet, expressed as time in milliseconds. The time SHALL be 1245 calculated as the sum of the time the media present in the packet 1246 represents. For frame-based codecs, the time SHOULD be an integer 1247 multiple of the frame size. This attribute is probably only 1248 meaningful for audio data, but may be used with other media types if 1249 it makes sense. Note that this attribute was introduced after 1250 [RFC2327], and non-updated implementations will ignore this 1251 attribute. 1253 6.6. rtpmap 1255 Name: rtpmap 1257 Value: rtpmap-value 1259 Usage Level: media 1261 Charset Dependent: no 1263 Syntax: 1265 rtpmap-value = payload-type SP encoding-name 1266 "/" clock-rate [ "/" encoding-params ] 1267 payload-type = zero-based-integer 1268 encoding-name = token 1269 clock-rate = integer 1270 encoding-params = channels 1271 channels = integer 1273 This attribute maps from an RTP payload type number (as used in an 1274 "m=" line) to an encoding name denoting the payload format to be 1275 used. It also provides information on the clock rate and encoding 1276 parameters. Note that the payload type number is indicated in a 1277 7-bit field, limiting the values to incusively between 0 and 127. 1279 Although an RTP profile can make static assignments of payload type 1280 numbers to payload formats, it is more common for that assignment to 1281 be done dynamically using "a=rtpmap:" attributes. As an example of a 1282 static payload type, consider u-law PCM coded single-channel audio 1283 sampled at 8 kHz. This is completely defined in the RTP Audio/Video 1284 profile as payload type 0, so there is no need for an "a=rtpmap:" 1285 attribute, and the media for such a stream sent to UDP port 49232 can 1286 be specified as: 1288 m=audio 49232 RTP/AVP 0 1290 An example of a dynamic payload type is 16-bit linear encoded stereo 1291 audio sampled at 16 kHz. If we wish to use the dynamic RTP/AVP 1292 payload type 98 for this stream, additional information is required 1293 to decode it: 1295 m=audio 49232 RTP/AVP 98 1296 a=rtpmap:98 L16/16000/2 1298 Up to one rtpmap attribute can be defined for each media format 1299 specified. Thus, we might have the following: 1301 m=audio 49230 RTP/AVP 96 97 98 1302 a=rtpmap:96 L8/8000 1303 a=rtpmap:97 L16/8000 1304 a=rtpmap:98 L16/11025/2 1306 RTP profiles that specify the use of dynamic payload types MUST 1307 define the set of valid encoding names and/or a means to register 1308 encoding names if that profile is to be used with SDP. The "RTP/AVP" 1309 and "RTP/SAVP" profiles use media subtypes for encoding names, under 1310 the top-level media type denoted in the "m=" line. In the example 1311 above, the media types are "audio/L8" and "audio/L16". 1313 For audio streams, encoding-params indicates the number of audio 1314 channels. This parameter is OPTIONAL and may be omitted if the 1315 number of channels is one, provided that no additional parameters are 1316 needed. 1318 For video streams, no encoding parameters are currently specified. 1320 Additional encoding parameters MAY be defined in the future, but 1321 codec-specific parameters SHOULD NOT be added. Parameters added to 1322 an "a=rtpmap:" attribute SHOULD only be those required for a session 1323 directory to make the choice of appropriate media to participate in a 1324 session. Codec-specific parameters should be added in other 1325 attributes (for example, "a=fmtp:"). 1327 Note: RTP audio formats typically do not include information about 1328 the number of samples per packet. If a non-default (as defined in 1329 the RTP Audio/Video Profile [RFC3551]) packetization is required, the 1330 "ptime" attribute is used as given above. 1332 6.7. Media Direction Attributes 1334 At most one of recvonly/sendrecv/sendonly/inactive MAY appear at 1335 session level, and at most one MAY appear in each media description. 1337 If any one of these appears in a media description then it applies 1338 for that media description. If none appear in a media description 1339 then the one from session level, if any, applies to that media 1340 description. 1342 If none of the media direction attributes is present at either 1343 session level or media level, "sendrecv" SHOULD be assumed as the 1344 default for sessions that are not of the multimedia conference type 1345 "broadcast" or "H332" (see below). 1347 Within the following SDP example, the "inactive" attribute applies to 1348 audio media and the "recvonly" attribute applies to video media. 1350 v=0 1351 o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1 1352 s=SDP Seminar 1353 i=A Seminar on the session description protocol 1354 u=http://www.example.com/seminars/sdp.pdf 1355 e=j.doe@example.com (Jane Doe) 1356 c=IN IP4 233.252.0.1/127 1357 t=2873397496 2873404696 1358 a=inactive 1359 m=audio 49170 RTP/AVP 0 1360 m=video 51372 RTP/AVP 99 1361 a=rtpmap:99 h263-1998/90000 1362 a=recvonly 1364 6.7.1. recvonly (receive-only) 1366 Name: recvonly 1368 Value: 1370 Usage Level: session, media 1372 Charset Dependent: no 1374 Example: 1376 a=recvonly 1378 This specifies that the tools should be started in receive-only mode 1379 where applicable. Note that recvonly applies to the media only, not 1380 to any associated control protocol (e.g., an RTP-based system in 1381 recvonly mode SHOULD still send RTCP packets). 1383 6.7.2. sendrecv (send-receive) 1385 Name: sendrecv 1387 Value: 1389 Usage Level: session, media 1391 Charset Dependent: no 1393 Example: 1395 a=sendrecv 1397 This specifies that the tools should be started in send and receive 1398 mode. This is necessary for interactive multimedia conferences with 1399 tools that default to receive-only mode. 1401 6.7.3. sendonly (send-only) 1403 Name: sendonly 1405 Value: 1407 Usage Level: session, media 1409 Charset Dependent: no 1410 Example: 1412 a=sendonly 1414 This specifies that the tools should be started in send-only mode. 1415 An example may be where a different unicast address is to be used for 1416 a traffic destination than for a traffic source. In such a case, two 1417 media descriptions may be used, one sendonly and one recvonly. Note 1418 that sendonly applies only to the media, and any associated control 1419 protocol (e.g., RTCP) SHOULD still be received and processed as 1420 normal. 1422 6.7.4. inactive 1424 Name: inactive 1426 Value: 1428 Usage Level: session, media 1430 Charset Dependent: no 1432 Example: 1434 a=inactive 1436 This specifies that the tools should be started in inactive mode. 1437 This is necessary for interactive multimedia conferences where users 1438 can put other users on hold. No media is sent over an inactive media 1439 stream. Note that an RTP-based system MUST still send RTCP (if RTCP 1440 is used), even if started inactive. 1442 6.8. orient (orientation) 1444 Name: orient 1446 Value: orient-value 1448 Usage Level: media 1450 Charset Dependent: no 1451 Syntax: 1453 orient-value = portrait / landscape / seascape 1454 portrait = %s"portrait" 1455 landscape = %s"landscape" 1456 seascape = %s"seascape" 1457 ; NOTE: These names are case-sensitive. 1459 Example: 1461 a=orient:portrait 1463 Normally this is only used for a whiteboard or presentation tool. It 1464 specifies the orientation of the workspace on the screen. Permitted 1465 values are "portrait", "landscape", and "seascape" (upside-down 1466 landscape). 1468 6.9. type (conference type) 1470 Name: type 1472 Value: type-value 1474 Usage Level: session 1476 Charset Dependent: no 1478 Syntax: 1480 type-value = conference-type 1481 conference-type = broadcast / meeting / moderated / test / 1482 H332 1483 broadcast = %s"broadcast" 1484 meeting = %s"meeting" 1485 moderated = %s"moderated" 1486 test = %s"test" 1487 H332 = %s"H332" 1488 ; NOTE: These names are case-sensitive. 1490 Example: 1492 a=type:moderated 1494 This specifies the type of the multimedia conference. Suggested 1495 values are "broadcast", "meeting", "moderated", "test", and "H332". 1496 "recvonly" should be the default for "type:broadcast" sessions, 1497 "type:meeting" should imply "sendrecv", and "type:moderated" should 1498 indicate the use of a floor control tool and that the media tools are 1499 started so as to mute new sites joining the multimedia conference. 1501 Specifying the attribute "type:H332" indicates that this loosely 1502 coupled session is part of an H.332 session as defined in the ITU 1503 H.332 specification [ITU.H332.1998]. Media tools should be started 1504 "recvonly". 1506 Specifying the attribute "type:test" is suggested as a hint that, 1507 unless explicitly requested otherwise, receivers can safely avoid 1508 displaying this session description to users. 1510 6.10. charset (character set) 1512 Name: charset 1514 Value: charset-value 1516 Usage Level: session 1518 Charset Dependent: no 1520 Syntax: 1522 charset-value = mime-charset 1523 (as defined in [RFC 2978]) 1525 This specifies the character set to be used to display the session 1526 name and information data. By default, the ISO-10646 character set 1527 in UTF-8 encoding is used. If a more compact representation is 1528 required, other character sets may be used. For example, the ISO 1529 8859-1 is specified with the following SDP attribute: 1531 a=charset:ISO-8859-1 1533 The charset specified MUST be one of those registered in the IANA 1534 Character Sets registry (http://www.iana.org/assignments/character- 1535 sets), such as ISO-8859-1. The character set identifier is a US- 1536 ASCII string and MUST be compared against identifiers from the "Name" 1537 or "Preferred MIME Name" field of the registry using a case- 1538 insensitive comparison. If the identifier is not recognised or not 1539 supported, all strings that are affected by it SHOULD be regarded as 1540 octet strings. 1542 Note that a character set specified MUST still prohibit the use of 1543 bytes 0x00 (Nul), 0x0A (LF), and 0x0d (CR). Character sets requiring 1544 the use of these characters MUST define a quoting mechanism that 1545 prevents these bytes from appearing within text fields. 1547 6.11. sdplang (SDP language) 1549 Name: sdplang 1551 Value: sdplang-value 1553 Usage Level: session, media 1555 Charset Dependent: no 1557 Syntax: 1559 sdplang-value = Language-Tag 1560 ; Language-Tag defined in RFC5646 1562 Example: 1564 a=sdplang:fr 1566 Multiple sdplang attributes can be provided either at session or 1567 media level if the session description or media use multiple 1568 languages. 1570 As a session-level attribute, it specifies the language for the 1571 session description (not the language of the media). As a media- 1572 level attribute, it specifies the language for any media-level SDP 1573 information field associated with that media (again not the language 1574 of the media), overriding any sdplang attributes specified at session 1575 level. 1577 In general, sending session descriptions consisting of multiple 1578 languages is discouraged. Instead, multiple sesssion descriptions 1579 SHOULD be sent describing the session, one in each language. 1580 However, this is not possible with all transport mechanisms, and so 1581 multiple sdplang attributes are allowed although NOT RECOMMENDED. 1583 The "sdplang" attribute value must be a single [RFC5646] language tag 1584 in US-ASCII. An "sdplang" attribute SHOULD be specified when a 1585 session is distributed with sufficient scope to cross geographic 1586 boundaries, where the language of recipients cannot be assumed, or 1587 where the session is in a different language from the locally assumed 1588 norm. 1590 6.12. lang (language) 1592 Name: lang 1594 Value: lang-value 1595 Usage Level: session, media 1597 Charset Dependent: no 1599 Syntax: 1601 lang-value = Language-Tag 1602 ; Language-Tag defined in RFC5646 1604 Example: 1606 a=lang:de 1608 Multiple lang attributes can be provided either at session or media 1609 level if the session or media has capabilities in more than one 1610 language, in which case the order of the attributes indicates the 1611 order of preference of the various languages in the session or media, 1612 from most preferred to least preferred. 1614 As a session-level attribute, lang specifies a language capability 1615 for the session being described. As a media-level attribute, it 1616 specifies a language capability for that media, overriding any 1617 session-level language(s) specified. 1619 The "lang" attribute value must be a single [RFC5646] language tag in 1620 US-ASCII. A "lang" attribute SHOULD be specified when a session is 1621 of sufficient scope to cross geographic boundaries where the language 1622 of participants cannot be assumed, or where the session has 1623 capabilities in languages different from the locally assumed norm. 1625 The "lang" attribute is supposed to be used for setting the initial 1626 language(s) used in the session. Events during the session may 1627 influence which language(s) are used, and the participants are not 1628 strictly bound to only use the declared languages. 1630 Most real-time use cases start with just one language used, while 1631 other cases involve a range of languages, e.g. an interpreted or 1632 subtitled session. When more than one 'lang' attribute is specified, 1633 the "lang" attribute itself does not provide any information about 1634 multiple languages being intended to be used during the session, or 1635 if the intention is to only select one of the languages. If needed, 1636 a new attribute can be defined and used to indicate such intentions. 1637 Without such semantics, it is assumed that for a negotiated session 1638 one of the declared languages will be selected and used. 1640 6.13. framerate (frame rate) 1642 Name: framerate 1644 Value: framerate-value 1646 Usage Level: media 1648 Charset Dependent: no 1650 Syntax: 1652 framerate-value = non-zero-int-or-real 1654 Example: 1656 a=framerate:60 1658 This gives the maximum video frame rate in frames/sec. It is 1659 intended as a recommendation for the encoding of video data. Decimal 1660 representations of fractional values are allowed. It is defined only 1661 for video media. 1663 6.14. quality 1665 Name: quality 1667 Value: quality-value 1669 Usage Level: media 1671 Charset Dependent: no 1673 Syntax: 1675 quality-value = zero-based-integer 1677 Example: 1679 a=quality:10 1681 This gives a suggestion for the quality of the encoding as an integer 1682 value. The intention of the quality attribute for video is to 1683 specify a non-default trade-off between frame-rate and still-image 1684 quality. For video, the value is in the range 0 to 10, with the 1685 following suggested meaning: 1687 10 - the best still-image quality the compression scheme 1688 can give. 1689 5 - the default behaviour given no quality suggestion. 1690 0 - the worst still-image quality the codec designer 1691 thinks is still usable. 1693 6.15. fmtp (format parameters) 1695 Name: fmtp 1697 Value: fmtp-value 1699 Usage Level: media 1701 Charset Dependent: no 1703 Syntax: 1705 fmtp-value = fmt SP format-specific-params 1706 format-specific-params = byte-string 1707 ; Notes: 1708 ; - The format parameters are media type parameters and 1709 need to reflect their syntax. 1711 Example: 1713 a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600 1715 This attribute allows parameters that are specific to a particular 1716 format to be conveyed in a way that SDP does not have to understand 1717 them. The format must be one of the formats specified for the media. 1718 Format-specific parameters, semicolon separated, may be any set of 1719 parameters required to be conveyed by SDP and given unchanged to the 1720 media tool that will use this format. At most one instance of this 1721 attribute is allowed for each format. 1723 The fmtp attribute may be used to specify parameters for any protocol 1724 and format that defines use of such parameters. 1726 7. Security Considerations 1728 SDP is frequently used with the Session Initiation Protocol [RFC3261] 1729 using the offer/answer model [RFC3264] to agree on parameters for 1730 unicast sessions. When used in this manner, the security 1731 considerations of those protocols apply. 1733 SDP is a session description format that describes multimedia 1734 sessions. Entities receiving and acting upon an SDP message SHOULD 1735 be aware that a session description cannot be trusted unless it has 1736 been obtained by an authenticated and integrity-protected transport 1737 protocol from a known and trusted source. Many different transport 1738 protocols may be used to distribute session descriptions, and the 1739 nature of the authentication and integrity-protection will differ 1740 from transport to transport. For some transports, security features 1741 are often not deployed. In case a session description has not been 1742 obtained in a trusted manner, the endpoint SHOULD exercise care 1743 because, among other attacks, the media sessions received may not be 1744 the intended ones, the destination where media is sent to may not be 1745 the expected one, any of the parameters of the session may be 1746 incorrect, or the media security may be compromised. It is up to the 1747 endpoint to make a sensible decision taking into account the security 1748 risks of the application and the user preferences - the endpoint may 1749 decide to ask the user whether or not to accept the session. 1751 On receiving a session description over an unauthenticated transport 1752 mechanism or from an untrusted party, software parsing the session 1753 should take a few precautions. Similar concerns apply if integrity 1754 protection is not in place. Session descriptions contain information 1755 required to start software on the receiver's system. Software that 1756 parses a session description MUST NOT be able to start other software 1757 except that which is specifically configured as appropriate software 1758 to participate in multimedia sessions. It is normally considered 1759 inappropriate for software parsing a session description to start, on 1760 a user's system, software that is appropriate to participate in 1761 multimedia sessions, without the user first being informed that such 1762 software will be started and giving the user's consent. Thus, a 1763 session description arriving by session announcement, email, session 1764 invitation, or WWW page MUST NOT deliver the user into an interactive 1765 multimedia session unless the user has explicitly pre-authorised such 1766 action. As it is not always simple to tell whether or not a session 1767 is interactive, applications that are unsure should assume sessions 1768 are interactive. 1770 In this specification, there are no attributes that would allow the 1771 recipient of a session description to be informed to start multimedia 1772 tools in a mode where they default to transmitting. Under some 1773 circumstances it might be appropriate to define such attributes. If 1774 this is done, an application parsing a session description containing 1775 such attributes SHOULD either ignore them or inform the user that 1776 joining this session will result in the automatic transmission of 1777 multimedia data. The default behaviour for an unknown attribute is 1778 to ignore it. 1780 In certain environments, it has become common for intermediary 1781 systems to intercept and analyse session descriptions contained 1782 within other signalling protocols. This is done for a range of 1783 purposes, including but not limited to opening holes in firewalls to 1784 allow media streams to pass, or to mark, prioritize, or block traffic 1785 selectively. In some cases, such intermediary systems may modify the 1786 session description, for example, to have the contents of the session 1787 description match NAT bindings dynamically created. These behaviours 1788 are NOT RECOMMENDED unless the session description is conveyed in 1789 such a manner that allows the intermediary system to conduct proper 1790 checks to establish the authenticity of the session description, and 1791 the authority of its source to establish such communication sessions. 1792 SDP by itself does not include sufficient information to enable these 1793 checks: they depend on the encapsulating protocol (e.g., SIP or 1794 RTSP). 1796 Use of the "k=" line poses a significant security risk, since it 1797 conveys session encryption keys in the clear. SDP MUST NOT be used 1798 to convey keying material, unless it can be guaranteed that the 1799 channel over which the SDP is delivered is both private and 1800 authenticated. Moreover, the "k=" line provides no way to indicate 1801 or negotiate cryptographic key algorithms. As it provides for only a 1802 single symmetric key, rather than separate keys for confidentiality 1803 and integrity, its utility is severely limited. The "k=" line MUST 1804 NOT be used, as discussed in Section 5.12. 1806 8. IANA Considerations 1808 8.1. The "application/sdp" Media Type 1810 One media type registration from [RFC4566] is to be updated, as 1811 defined below. 1813 To: ietf-types@iana.org 1814 Subject: Registration of media type "application/sdp" 1816 Type name: application 1818 Subtype name: sdp 1820 Required parameters: None. 1822 Optional parameters: None. 1824 Encoding considerations: 1825 SDP files are primarily UTF-8 format text. The "a=charset:" 1826 attribute may be used to signal the presence of other character 1827 sets in certain parts of an SDP file (see Section 6 of RFC 1828 XXXX). Arbitrary binary content cannot be directly 1829 represented in SDP. 1831 Security considerations: 1832 See Section 7 of RFC XXXX. 1834 Interoperability considerations: 1835 See RFC XXXX. 1837 Published specification: 1838 See RFC XXXX. 1840 Applications which use this media type: 1841 Voice over IP, video teleconferencing, streaming media, instant 1842 messaging, among others. See also Section 3 of RFC XXXX. 1844 Fragment identifier considerations: None 1846 Additional information: 1848 Deprecated alias names for this type: N/A 1849 Magic number(s): None. 1850 File extension(s): The extension ".sdp" is commonly used. 1851 Macintosh File Type Code(s): "sdp " 1853 Person & email address to contact for further information: 1854 IETF MMUSIC working group 1856 Intended usage: COMMON 1858 Restrictions on usage: None 1860 Author/Change controller: 1861 Authors of RFC XXXX 1862 IETF MMUSIC working group delegated from the IESG 1864 8.2. Registration of Parameters 1866 This specification establishes and initializes IANA parameter 1867 registries for seven named SDP sub-fields. Using the terminology in 1868 the SDP specification Augmented Backus-Naur Form (ABNF), they are 1869 "media", "proto", "fmt", "att-field", "bwtype", "nettype", and 1870 "addrtype". 1872 The contact address for all parameters registered below is: 1874 IETF MMUSIC working group 1876 8.2.1. Media Types ("media") 1878 The set of media types is intended to be small and SHOULD NOT be 1879 extended except under rare circumstances. The same rules should 1880 apply for media names as for top-level media types, and where 1881 possible the same name should be registered for SDP as for MIME. For 1882 media other than existing top-level media types, a Standards Track 1883 RFC MUST be produced for a new top-level media type to be registered, 1884 and the registration MUST provide good justification why no existing 1885 media name is appropriate (the "Standards Action" policy of 1886 [RFC8126]). 1888 This memo registers the media types "audio", "video", "text", 1889 "application", and "message". 1891 Note: The media types "control" and "data" were listed as valid in an 1892 early version of this specification (RFC 2327); however, their 1893 semantics were never fully specified and they are not widely used. 1894 These media types have been removed in this specification, although 1895 they still remain valid media type capabilities for a SIP user agent 1896 as defined in [RFC3840]. If these media types are considered useful 1897 in the future, a Standards Track RFC MUST be produced to document 1898 their use. Until that is done, applications SHOULD NOT use these 1899 types and SHOULD NOT declare support for them in SIP capabilities 1900 declarations (even though they exist in the registry created by 1901 [RFC3840]). Also note that [RFC6466] defined the "image" media type. 1903 8.2.2. Transport Protocols ("proto") 1905 The "proto" sub-field describes the transport protocol used. The 1906 registration procedure for this registry is "RFC Required". 1908 This document registers two values: "RTP/AVP" is a reference to 1909 [RFC3550] used under the RTP Profile for Audio and Video Conferences 1910 with Minimal Control [RFC3551] running over UDP/IP, and "udp" 1911 indicates direct use of the UDP protocol. 1913 New transport protocols MAY be defined, and MUST be registered with 1914 IANA. Registrations MUST reference an RFC describing the protocol. 1915 Such an RFC MAY be Experimental or Informational, although it is 1916 preferable that it be Standards Track. The RFC defining a new 1917 protocol MUST define the rules by which the "fmt" (see below) 1918 namespace is managed. 1920 "proto" names starting with "RTP/" MUST only be used for defining 1921 transport protocols that are profiles of the RTP protocol. For 1922 example, a profile whose short name is "XYZ" would be denoted by a 1923 "proto" sub-field of "RTP/XYZ". 1925 8.2.3. Media Formats ("fmt") 1927 Each transport protocol, defined by the "proto" sub-field, has an 1928 associated "fmt" namespace that describes the media formats that may 1929 be conveyed by that protocol. Formats cover all the possible 1930 encodings that could be transported in a multimedia session. 1932 RTP payload formats under the "RTP/AVP" and other "RTP/*" profiles 1933 MUST use the payload type number as their "fmt" value. If the 1934 payload type number is dynamically assigned by this session 1935 description, an additional "rtpmap" attribute MUST be included to 1936 specify the format name and parameters as defined by the media type 1937 registration for the payload format. It is RECOMMENDED that other 1938 RTP profiles that are registered (in combination with RTP) as SDP 1939 transport protocols specify the same rules for the "fmt" namespace. 1941 For the "udp" protocol, allowed "fmt" values are media subtypes from 1942 the IANA Media Types registry. The media type and subtype 1943 combination / specifies the format of the body of UDP 1944 packets. Use of an existing media subtype for the format is 1945 encouraged. If no suitable media subtype exists, it is RECOMMENDED 1946 that a new one be registered through the IETF process [RFC6838] by 1947 production of, or reference to, a standards-track RFC that defines 1948 the format. 1950 For other protocols, formats MAY be registered according to the rules 1951 of the associated "proto" specification. 1953 Registrations of new formats MUST specify which transport protocols 1954 they apply to. 1956 8.2.4. Attribute Names ("att-field") 1958 8.2.4.1. New Attributes 1960 Attribute field names ("att-field") MUST be registered with IANA and 1961 documented, to avoid any issues due to conflicting attribute 1962 definitions under the same name. Unknown attributes in SDP are 1963 simply ignored, but conflicting ones that fragment the protocol are a 1964 serious problem. 1966 New attribute registrations are accepted according to the 1967 "Specification Required" policy of [RFC8126], provided that the 1968 specification includes the following information: 1970 o Contact Name. 1972 o Contact Email Address. 1974 o Attribute Name: The name of the attribute that will appear in 1975 SDP). This MUST conform to the definition of . 1977 o Attribute Syntax: For a value attribute (see clause 5.13), an ABNF 1978 definition of the attribute value syntax (See 1979 Section 9) MUST be provided. The syntax MUST follow the rule form 1980 as per Section 2.2 of [RFC5234] and [RFC7405]. This SHALL define 1981 the allowable values that the attribute might take. It MAY also 1982 define an extension method for the addition of future values. For 1983 a property attribute, the ABNF definition is omitted as the 1984 property attribute takes no values. 1986 o Attribute Semantics: For a value attribute, a semantic description 1987 of the values that the attribute might take MUST be provided. The 1988 usage of a property attribute is described under purpose below. 1990 o Attribute Value: The name of an ABNF syntax rule defining the 1991 syntax of the value. Absence of a rule name indicates that the 1992 attribute takes no values. Enclosing the rule name in "[" and "]" 1993 indicates that a value is optional. 1995 o Usage Level: Usage level(s) of the attribute. One or more of: 1996 session, media, source, dcsa, dcsa(subprotocol). For a definition 1997 of source level attributes, see [RFC5576]. For a definition of 1998 dcsa attributes see: [I-D.ietf-mmusic-data-channel-sdpneg]. 2000 o Charset Dependent: Whether the attribute value is subject to the 2001 charset attribute or not (Yes/No). 2003 o Purpose: An explanation of the purpose and usage of the attribute. 2005 o O/A Procedures: Offer/Answer procedures as explained in [RFC3264]. 2007 o Mux Category: Indication of which multiplexing "category" 2008 [I-D.ietf-mmusic-sdp-mux-attributes] an attribute is associated 2009 with. 2011 o Reference: A reference to the specification defining the 2012 attribute. 2014 The above is the minimum that IANA will accept. Attributes that are 2015 expected to see widespread use and interoperability SHOULD be 2016 documented with a standards-track RFC that specifies the attribute 2017 more precisely. 2019 Submitters of registrations should ensure that the specification is 2020 in the spirit of SDP attributes, most notably that the attribute is 2021 platform independent in the sense that it makes no implicit 2022 assumptions about operating systems and does not name specific pieces 2023 of software in a manner that might inhibit interoperability. 2025 Submitters of registrations should also carefully choose the 2026 attribute usage level. They should not choose only "session" when 2027 the attribute can have different values when media is disaggregated, 2028 i.e., when each m= section has its own IP address on a different 2029 endpoint. In that case the attribute type chosen should be "session, 2030 media" or "media" (depending on desired semantics). The default rule 2031 is that for all new SDP attributes that can occur both in session and 2032 media level, the media level overrides the session level. When this 2033 is not the case for a new SDP attribute, it MUST be explicitly 2034 stated. 2036 IANA has registered the initial set of attribute names ("att-field" 2037 values) with definitions as in Section 6 of this memo (these 2038 definitions replace those in [RFC4566]). 2040 8.2.4.2. Updates to Existing Attributes 2042 Updated attribute registrations are accepted according to the 2043 "Specification Required" policy of [RFC8126], provided that the 2044 specification updating the attribute (for example, by adding a new 2045 value) considers the registration information items from 2046 Section 8.2.4.1 according to the following bullets: 2048 o Contact Name: A name MUST be provided. 2050 o Contact Email Address: An email address MUST be provided. 2052 o Attribute Name: MUST be provided and MUST NOT be changed. 2053 Otherwise it is a new attribute. 2055 o Attribute Syntax: The existing rule syntax with the syntax 2056 extensions MUST be provided if there is a change to the syntax. A 2057 revision to an existing attribute usage MAY extend the syntax of 2058 an attribute, but MUST be backward compatible. 2060 o Attribute Semantics: A semantic description of new additional 2061 attributes values or a semantic extension of existing values. 2062 Existing attribute values semantics MUST only be extended in a 2063 backward compatible manner. 2065 o Usage Level: Updates MAY only add additional levels. 2067 o Charset Dependent: MUST NOT be changed. 2069 o Purpose: MAY be extended according to the updated usage. 2071 o O/A Procedures: MAY be updated in a backward compatible manner 2072 and/or it applies to a new usage level only. 2074 o Mux Category: No change unless from "TBD" to another value (see 2075 [I-D.ietf-mmusic-sdp-mux-attributes]. It MAY also change if 2076 'media' level is being added to the definition of an attribute 2077 that previously did not include it. 2079 o Reference: A new reference MUST be provided. 2081 Items SHOULD be omitted if there is no impact to them as a result of 2082 the attribute update. 2084 8.2.5. Bandwidth Specifiers ("bwtype") 2086 A proliferation of bandwidth specifiers is strongly discouraged. 2088 New bandwidth specifiers ("bwtype" fields) MUST be registered with 2089 IANA. The submission MUST reference a standards-track RFC specifying 2090 the semantics of the bandwidth specifier precisely, and indicating 2091 when it should be used, and why the existing registered bandwidth 2092 specifiers do not suffice. 2094 IANA has registered the bandwidth specifiers "CT" and "AS" with 2095 definitions as in Section 5.8 of this memo (these definitions update 2096 those in [RFC4566]). 2098 8.2.6. Network Types ("nettype") 2100 New network types (the "nettype" sub-field) MUST be registered with 2101 IANA if SDP needs to be used in the context of non-Internet 2102 environments. The registration is subject to the "RFC Required" 2103 policy of [RFC8126]. Although these are not normally the preserve of 2104 IANA, there may be circumstances when an Internet application needs 2105 to interoperate with a non-Internet application, such as when 2106 gatewaying an Internet telephone call into the Public Switched 2107 Telephone Network (PSTN). The number of network types should be 2108 small and should be rarely extended. A new network type cannot be 2109 registered without registering at least one address type to be used 2110 with that network type. A new network type registration MUST 2111 reference an RFC that gives details of the network type and address 2112 type(s) and specifies how and when they would be used. 2114 IANA has registered the network type "IN" to represent the Internet, 2115 with definition as in Sections 5.2 and 5.7 of this memo (these 2116 definitions update those in [RFC4566]). 2118 8.2.7. Address Types ("addrtype") 2120 New address types ("addrtype") MUST be registered with IANA. The 2121 registration is subject to the "RFC Required" policy of [RFC8126]. 2122 An address type is only meaningful in the context of a network type, 2123 and any registration of an address type MUST specify a registered 2124 network type or be submitted along with a network type registration. 2125 A new address type registration MUST reference an RFC giving details 2126 of the syntax of the address type. Address types are not expected to 2127 be registered frequently. 2129 IANA has registered the address types "IP4" and "IP6" with 2130 definitions as in Sections 5.2 and 5.7 of this memo (these 2131 definitions update those in [RFC4566]). 2133 8.2.8. Registration Procedure 2135 In the RFC specifications that register new values for SDP "media", 2136 "proto", "fmt", "bwtype", "nettype", and "addrtype" parameters, the 2137 authors MUST include the following information for IANA to place in 2138 the appropriate registry: 2140 o contact name, email address, and telephone number 2142 o name being registered (as it will appear in SDP) 2144 o long-form name in English 2146 o type of name ("media", "proto", "fmt", "bwtype", "nettype", or 2147 "addrtype") 2149 o a one-paragraph explanation of the purpose of the registered name 2151 o a reference to the specification for the registered name (this 2152 will typically be an RFC number) 2154 In the case of a new addrtype registration, the author has to check 2155 whether the new address type is usable with the existing network 2156 types. If yes, the "nettype" registry MUST be updated accordingly. 2157 In the case of a new nettype registration, the author MUST specify 2158 the usable address type(s). 2160 IANA may refer any registration to the IESG for review, and may 2161 request revisions to be made before a registration will be made. 2163 8.3. Encryption Key Access Methods 2165 The IANA previously maintained a table of SDP encryption key access 2166 method ("enckey") names. This table is obsolete, since the "k=" line 2167 is not extensible. New registrations MUST NOT be accepted. 2169 8.4. Reorganization of the nettype Registry 2171 This document adds a new column in the "nettype" registry with the 2172 title "Usable addrtype Values" and updates the "nettype" registry as 2173 follows: 2175 -------------------------------------------------------------------- 2176 |Type | SDP Name | Usable addrtype Values | Reference | 2177 -------------------------------------------------------------------- 2178 |nettype | IN | IP4, IP6 | [RFCXXXX] | 2179 |nettype | TN | RFC2543 | [RFC2848] | 2180 |nettype | ATM | NSAP, GWID, E164 | [RFC3108] | 2181 |nettype | PSTN | E164 | [RFC7195] | 2182 -------------------------------------------------------------------- 2184 Note that both [RFC7195] and [RFC3108] registered "E164" as an 2185 address type, although [RFC7195] mentions that the "E164" address 2186 type has a different context for ATM and PSTN networks. 2188 8.5. Reorganization of the att-field Registries 2190 This document combines all of the (currently) five "att-field" 2191 registries into one registry called "att-field" registry, and updates 2192 the columns to reflect the name, usage level(s), charset dependency 2193 and reference. As such IANA is requested to create a new combined 2194 registry using the following columns: 2196 Name | Usage Level | Dependent on Charset? | Mux Category | Reference 2198 The "Name" column reflects the attribute name (as it will appear in 2199 the SDP). The "Usage Level" column MUST indicate one or more of the 2200 following: session, media, source, dcsa and dcsa(subprotocol). The 2201 "Dependent on Charset?" column MUST indicate "Yes" or "No" depending 2202 on whether the attribute value is subject to the charset attribute. 2203 The "Mux Category" column MUST indicate one of the following 2204 categories: NORMAL, NOT RECOMMENDED, IDENTICAL, SUM, TRANSPORT, 2205 INHERIT, IDENTICAL-PER-PT, SPECIAL or TBD as defined by 2206 [I-D.ietf-mmusic-sdp-mux-attributes]. Finally, the "Reference" 2207 column indicates the specification(s) where the attribute is defined. 2209 For example, the attribute "setup" which is defined for both session 2210 and media level, will be listed in the new registry as follows: 2212 Name | Usage Level | Dependent on Charset?|Mux Category| Reference | 2213 setup | session,media, | No |IDENTICAL | [RFC4145] | 2214 | dcsa,dcsa(msrp)| | | [RFC6135] | 2215 | | | | [I-D.mmusic 2216 | | | |-msrp-usage- 2217 | | | |data-channel 2218 | | | |] | 2220 9. SDP Grammar 2222 This section provides an Augmented BNF grammar for SDP. ABNF is 2223 defined in [RFC5234] and [RFC7405]. 2225 ; SDP Syntax 2226 session-description = version-field 2227 origin-field 2228 session-name-field 2229 [information-field] 2230 [uri-field] 2231 *email-field 2232 *phone-field 2233 [connection-field] 2234 *bandwidth-field 2235 1*time-description 2236 [key-field] 2237 *attribute-field 2238 *media-description 2240 version-field = %s"v" "=" 1*DIGIT CRLF 2241 ;this memo describes version 0 2243 origin-field = %s"o" "=" username SP sess-id SP sess-version SP 2244 nettype SP addrtype SP unicast-address CRLF 2246 session-name-field = %s"s" "=" text CRLF 2248 information-field = %s"i" "=" text CRLF 2250 uri-field = %s"u" "=" uri CRLF 2252 email-field = %s"e" "=" email-address CRLF 2254 phone-field = %s"p" "=" phone-number CRLF 2256 connection-field = %s"c" "=" nettype SP addrtype SP 2257 connection-address CRLF 2258 ;a connection field must be present 2259 ;in every media description or at the 2260 ;session-level 2262 bandwidth-field = %s"b" "=" bwtype ":" bandwidth CRLF 2264 time-description = time-field 2265 [repeat-description] 2267 repeat-description = 1*repeat-field 2268 [zone-field] 2270 time-field = %s"t" "=" start-time SP stop-time CRLF 2272 repeat-field = %s"r" "=" repeat-interval SP typed-time 2273 1*(SP typed-time) CRLF 2275 zone-field = %s"z" "=" time SP ["-"] typed-time 2276 *(SP time SP ["-"] typed-time) CRLF 2278 key-field = %s"k" "=" key-type CRLF 2280 attribute-field = %s"a" "=" attribute CRLF 2282 media-description = media-field 2283 [information-field] 2284 *connection-field 2285 *bandwidth-field 2286 [key-field] 2287 *attribute-field 2289 media-field = %s"m" "=" media SP port ["/" integer] 2290 SP proto 1*(SP fmt) CRLF 2292 ; sub-rules of 'o=' 2293 username = non-ws-string 2294 ;pretty wide definition, but doesn't 2295 ;include space 2297 sess-id = 1*DIGIT 2298 ;should be unique for this username/host 2300 sess-version = 1*DIGIT 2302 nettype = token 2303 ;typically "IN" 2305 addrtype = token 2306 ;typically "IP4" or "IP6" 2308 ; sub-rules of 'u=' 2309 uri = URI-reference 2310 ; see RFC 3986 2312 ; sub-rules of 'e=', see RFC 5322 for definitions 2313 email-address = address-and-comment / dispname-and-address 2314 / addr-spec 2315 address-and-comment = addr-spec 1*SP "(" 1*email-safe ")" 2316 dispname-and-address = 1*email-safe 1*SP "<" addr-spec ">" 2318 ; sub-rules of 'p=' 2319 phone-number = phone *SP "(" 1*email-safe ")" / 2320 1*email-safe "<" phone ">" / 2321 phone 2323 phone = ["+"] DIGIT 1*(SP / "-" / DIGIT) 2325 ; sub-rules of 'c=' 2326 connection-address = multicast-address / unicast-address 2328 ; sub-rules of 'b=' 2329 bwtype = token 2331 bandwidth = 1*DIGIT 2333 ; sub-rules of 't=' 2334 start-time = time / "0" 2336 stop-time = time / "0" 2338 time = POS-DIGIT 9*DIGIT 2339 ; Decimal representation of NTP time in 2340 ; seconds since 1900. The representation 2341 ; of NTP time is an unbounded length field 2342 ; containing at least 10 digits. Unlike the 2343 ; 64-bit representation used elsewhere, time 2344 ; in SDP does not wrap in the year 2036. 2346 ; sub-rules of 'r=' and 'z=' 2347 repeat-interval = POS-DIGIT *DIGIT [fixed-len-time-unit] 2349 typed-time = 1*DIGIT [fixed-len-time-unit] 2351 fixed-len-time-unit = %s"d" / %s"h" / %s"m" / %s"s" 2352 ; NOTE: These units are case-sensitive. 2354 ; sub-rules of 'k=' 2355 key-type = %s"prompt" / 2356 %s"clear:" text / 2357 %s"base64:" base64 / 2358 %s"uri:" uri 2359 ; NOTE: These names are case-sensitive. 2361 base64 = *base64-unit [base64-pad] 2362 base64-unit = 4base64-char 2363 base64-pad = 2base64-char "==" / 3base64-char "=" 2364 base64-char = ALPHA / DIGIT / "+" / "/" 2366 ; sub-rules of 'a=' 2367 attribute = (att-field ":" att-value) / att-field 2369 att-field = token 2371 att-value = byte-string 2373 ; sub-rules of 'm=' 2374 media = token 2375 ;typically "audio", "video", "text", "image" 2376 ;or "application" 2378 fmt = token 2379 ;typically an RTP payload type for audio 2380 ;and video media 2382 proto = token *("/" token) 2383 ;typically "RTP/AVP" or "udp" 2385 port = 1*DIGIT 2387 ; generic sub-rules: addressing 2388 unicast-address = IP4-address / IP6-address / FQDN / extn-addr 2390 multicast-address = IP4-multicast / IP6-multicast / FQDN 2391 / extn-addr 2393 IP4-multicast = m1 3( "." decimal-uchar ) 2394 "/" ttl [ "/" numaddr ] 2395 ; IP4 multicast addresses may be in the 2396 ; range 224.0.0.0 to 239.255.255.255 2398 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / 2399 ("23" DIGIT ) 2401 IP6-multicast = IP6-address [ "/" numaddr ] 2402 ; IP6 address starting with FF 2404 numaddr = integer 2406 ttl = (POS-DIGIT *2DIGIT) / "0" 2408 FQDN = 4*(alpha-numeric / "-" / ".") 2409 ; fully qualified domain name as specified 2410 ; in RFC 1035 (and updates) 2412 IP4-address = b1 3("." decimal-uchar) 2414 b1 = decimal-uchar 2415 ; less than "224" 2417 IP6-address = 6( h16 ":" ) ls32 2418 / "::" 5( h16 ":" ) ls32 2419 / [ h16 ] "::" 4( h16 ":" ) ls32 2420 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2421 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2422 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2423 / [ *4( h16 ":" ) h16 ] "::" ls32 2424 / [ *5( h16 ":" ) h16 ] "::" h16 2425 / [ *6( h16 ":" ) h16 ] "::" 2427 h16 = 1*4HEXDIG 2429 ls32 = ( h16 ":" h16 ) / IP4-address 2431 ; Generic for other address families 2432 extn-addr = non-ws-string 2434 ; generic sub-rules: datatypes 2435 text = byte-string 2436 ;default is to interpret this as UTF8 text. 2437 ;ISO 8859-1 requires "a=charset:ISO-8859-1" 2438 ;session-level attribute to be used 2440 byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF) 2441 ;any byte except NUL, CR, or LF 2443 non-ws-string = 1*(VCHAR/%x80-FF) 2444 ;string of visible characters 2446 token-char = ALPHA / DIGIT 2447 / "!" / "#" / "$" / "%" / "&" 2448 / "'" ; (single quote) 2449 / "*" / "+" / "-" / "." / "^" / "_" 2450 / "`" ; (Grave accent) 2451 / "{" / "|" / "}" / "~" 2453 token = 1*(token-char) 2455 email-safe = %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF 2456 ;any byte except NUL, CR, LF, or the quoting 2457 ;characters ()<> 2459 integer = POS-DIGIT *DIGIT 2461 zero-based-integer = "0" / integer 2463 non-zero-int-or-real = integer / non-zero-real 2465 non-zero-real = zero-based-integer "." *DIGIT POS-DIGIT 2467 ; generic sub-rules: primitives 2468 alpha-numeric = ALPHA / DIGIT 2470 POS-DIGIT = %x31-39 ; 1 - 9 2472 decimal-uchar = DIGIT 2473 / POS-DIGIT DIGIT 2474 / ("1" 2*(DIGIT)) 2475 / ("2" ("0"/"1"/"2"/"3"/"4") DIGIT) 2476 / ("2" "5" ("0"/"1"/"2"/"3"/"4"/"5")) 2478 ; external references: 2479 ALPHA = 2480 DIGIT = 2481 CRLF = 2482 HEXDIG = 2483 SP = 2484 VCHAR = 2485 URI-reference = 2486 addr-spec = 2488 10. Summary of Changes from RFC 4566 2490 The ABNF rule for IP6-address has been corrected. As a result, the 2491 ABNF rule for IP6-multicast has changed, and the (now unused) rules 2492 for hexpart, hexseq, and hex4 have been removed. 2494 IP4 unicast and multicast addresses in the example SDP descriptions 2495 have been revised per RFCs 5735 and 5771. 2497 Text in Section 5.2 has been revised to clarify the use of local 2498 addresses in case of ICE-like SDP extensions. 2500 Normative and informative references have been updated. 2502 The text regarding the session vs. media-level attribute usage has 2503 been clarified. 2505 The case-insensitivity rules from RFC 4855 have been included in this 2506 document. 2508 11. Acknowledgements 2510 Many people in the IETF Multiparty Multimedia Session Control 2511 (MMUSIC) working group have made comments and suggestions 2512 contributing to this document. 2514 12. References 2516 12.1. Normative References 2518 [E164] International Telecommunication Union, "E.164 : The 2519 international public telecommunication numbering plan", 2520 ITU Recommendation E.164, November 2010. 2522 [I-D.ietf-mmusic-data-channel-sdpneg] 2523 Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak, R., 2524 Marcon, J., and R. Even, "SDP-based Data Channel 2525 Negotiation", draft-ietf-mmusic-data-channel-sdpneg-18 2526 (work in progress), May 2018. 2528 [I-D.ietf-mmusic-sdp-mux-attributes] 2529 Nandakumar, S., "A Framework for SDP Attributes when 2530 Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-17 2531 (work in progress), February 2018. 2533 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 2534 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 2535 . 2537 [RFC1035] Mockapetris, P., "Domain names - implementation and 2538 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 2539 November 1987, . 2541 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2542 Requirement Levels", BCP 14, RFC 2119, 2543 DOI 10.17487/RFC2119, March 1997, 2544 . 2546 [RFC2978] Freed, N. and J. Postel, "IANA Charset Registration 2547 Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978, 2548 October 2000, . 2550 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 2551 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2552 2003, . 2554 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2555 Resource Identifier (URI): Generic Syntax", STD 66, 2556 RFC 3986, DOI 10.17487/RFC3986, January 2005, 2557 . 2559 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 2560 Description Protocol", RFC 4566, DOI 10.17487/RFC4566, 2561 July 2006, . 2563 [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 2564 Specifications: ABNF", STD 68, RFC 5234, 2565 DOI 10.17487/RFC5234, January 2008, 2566 . 2568 [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific 2569 Media Attributes in the Session Description Protocol 2570 (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009, 2571 . 2573 [RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying 2574 Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, 2575 September 2009, . 2577 [RFC5890] Klensin, J., "Internationalized Domain Names for 2578 Applications (IDNA): Definitions and Document Framework", 2579 RFC 5890, DOI 10.17487/RFC5890, August 2010, 2580 . 2582 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for 2583 Writing an IANA Considerations Section in RFCs", BCP 26, 2584 RFC 8126, DOI 10.17487/RFC8126, June 2017, 2585 . 2587 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2588 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2589 May 2017, . 2591 12.2. Informative References 2593 [I-D.ietf-mmusic-sdp-bundle-negotiation] 2594 Holmberg, C., Alvestrand, H., and C. Jennings, 2595 "Negotiating Media Multiplexing Using the Session 2596 Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle- 2597 negotiation-51 (work in progress), May 2018. 2599 [ITU.H332.1998] 2600 International Telecommunication Union, "H.323 extended for 2601 loosely coupled conferences", ITU Recommendation H.332, 2602 September 1998. 2604 [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description 2605 Protocol", RFC 2327, DOI 10.17487/RFC2327, April 1998, 2606 . 2608 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 2609 Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974, 2610 October 2000, . 2612 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 2613 A., Peterson, J., Sparks, R., Handley, M., and E. 2614 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 2615 DOI 10.17487/RFC3261, June 2002, 2616 . 2618 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2619 with Session Description Protocol (SDP)", RFC 3264, 2620 DOI 10.17487/RFC3264, June 2002, 2621 . 2623 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 2624 Jacobson, "RTP: A Transport Protocol for Real-Time 2625 Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, 2626 July 2003, . 2628 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 2629 Video Conferences with Minimal Control", STD 65, RFC 3551, 2630 DOI 10.17487/RFC3551, July 2003, 2631 . 2633 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth 2634 Modifiers for RTP Control Protocol (RTCP) Bandwidth", 2635 RFC 3556, DOI 10.17487/RFC3556, July 2003, 2636 . 2638 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 2639 in Session Description Protocol (SDP)", RFC 3605, 2640 DOI 10.17487/RFC3605, October 2003, 2641 . 2643 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 2644 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 2645 RFC 3711, DOI 10.17487/RFC3711, March 2004, 2646 . 2648 [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, 2649 "Indicating User Agent Capabilities in the Session 2650 Initiation Protocol (SIP)", RFC 3840, 2651 DOI 10.17487/RFC3840, August 2004, 2652 . 2654 [RFC3890] Westerlund, M., "A Transport Independent Bandwidth 2655 Modifier for the Session Description Protocol (SDP)", 2656 RFC 3890, DOI 10.17487/RFC3890, September 2004, 2657 . 2659 [RFC4855] Casner, S., "Media Type Registration of RTP Payload 2660 Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007, 2661 . 2663 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 2664 (ICE): A Protocol for Network Address Translator (NAT) 2665 Traversal for Offer/Answer Protocols", RFC 5245, 2666 DOI 10.17487/RFC5245, April 2010, 2667 . 2669 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 2670 DOI 10.17487/RFC5322, October 2008, 2671 . 2673 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 2674 Protocol (SDP) Grouping Framework", RFC 5888, 2675 DOI 10.17487/RFC5888, June 2010, 2676 . 2678 [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, 2679 "Network Time Protocol Version 4: Protocol and Algorithms 2680 Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010, 2681 . 2683 [RFC6466] Salgueiro, G., "IANA Registration of the 'image' Media 2684 Type for the Session Description Protocol (SDP)", 2685 RFC 6466, DOI 10.17487/RFC6466, December 2011, 2686 . 2688 [RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach, 2689 "TCP Candidates with Interactive Connectivity 2690 Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544, 2691 March 2012, . 2693 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 2694 Specifications and Registration Procedures", BCP 13, 2695 RFC 6838, DOI 10.17487/RFC6838, January 2013, 2696 . 2698 [RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF", 2699 RFC 7405, DOI 10.17487/RFC7405, December 2014, 2700 . 2702 [RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and 2703 B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms 2704 for Real-Time Transport Protocol (RTP) Sources", RFC 7656, 2705 DOI 10.17487/RFC7656, November 2015, 2706 . 2708 [RFC7826] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., 2709 and M. Stiemerling, Ed., "Real-Time Streaming Protocol 2710 Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December 2711 2016, . 2713 Authors' Addresses 2715 Ali Begen 2716 Networked Media 2717 Konya 2718 Turkey 2720 EMail: ali.begen@networked.media 2722 Paul Kyzivat 2723 USA 2725 EMail: pkyzivat@alum.mit.edu 2727 Colin Perkins 2728 University of Glasgow 2729 School of Computing Science 2730 University of Glasgow 2731 Glasgow G12 8QQ 2732 UK 2734 EMail: csp@csperkins.org 2735 Mark Handley 2736 University College London 2737 Department of Computer Science 2738 London WC1E 6BT 2739 UK 2741 EMail: M.Handley@cs.ucl.ac.uk