idnits 2.17.1 draft-ietf-mmusic-rfc4566bis-13.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 1 instance of lines with multicast IPv4 addresses in the document. If these are generic example addresses, they should be changed to use the 233.252.0.x range defined in RFC 5771 Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document seems to contain a disclaimer for pre-RFC5378 work, and may have content which was first submitted before 10 November 2008. The disclaimer is necessary when there are original authors that you have been unable to contact, or if some do not wish to grant the BCP78 rights to the IETF Trust. If you are able to get all authors (current and original) to grant those rights, you can and should remove the disclaimer; otherwise, the disclaimer is needed and you can ignore this comment. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (January 13, 2015) is 3391 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: '0-10' is mentioned on line 1747, but not defined == Unused Reference: 'RFC2365' is defined on line 2583, but no explicit reference was found in the text ** Obsolete normative reference: RFC 5226 (Obsoleted by RFC 8126) ** Obsolete normative reference: RFC 4566 (Obsoleted by RFC 8866) == Outdated reference: A later version (-08) exists of draft-ietf-avtext-rtp-grouping-taxonomy-02 ** Downref: Normative reference to an Informational draft: draft-ietf-avtext-rtp-grouping-taxonomy (ref. 'I-D.ietf-avtext-rtp-grouping-taxonomy') -- Obsolete informational reference (is this intentional?): RFC 2327 (Obsoleted by RFC 4566) -- Obsolete informational reference (is this intentional?): RFC 2326 (Obsoleted by RFC 7826) -- Obsolete informational reference (is this intentional?): RFC 5245 (Obsoleted by RFC 8445, RFC 8839) Summary: 3 errors (**), 0 flaws (~~), 5 warnings (==), 5 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group M. Handley 3 Internet-Draft UCL 4 Obsoletes: 4566 (if approved) V. Jacobson 5 Intended status: Standards Track PARC 6 Expires: July 17, 2015 C.S. Perkins 7 University of Glasgow 8 A. Begen 9 Cisco 10 January 13, 2015 12 SDP: Session Description Protocol 13 draft-ietf-mmusic-rfc4566bis-13 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 http://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 July 17, 2015. 39 Copyright Notice 41 Copyright (c) 2015 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 (http://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 . . . . . . . . . . . . . . . . . . . . 4 71 3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 4 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 . . . . . . . . . . . . . . 5 76 4.1. Media and Transport Information . . . . . . . . . . . . . 6 77 4.2. Timing Information . . . . . . . . . . . . . . . . . . . 7 78 4.3. Private Sessions . . . . . . . . . . . . . . . . . . . . 7 79 4.4. Obtaining Further Information about a Session . . . . . . 8 80 4.5. Categorisation . . . . . . . . . . . . . . . . . . . . . 8 81 4.6. Internationalisation . . . . . . . . . . . . . . . . . . 8 82 5. SDP Specification . . . . . . . . . . . . . . . . . . . . . . 8 83 5.1. Protocol Version ("v=") . . . . . . . . . . . . . . . . . 11 84 5.2. Origin ("o=") . . . . . . . . . . . . . . . . . . . . . . 11 85 5.3. Session Name ("s=") . . . . . . . . . . . . . . . . . . . 13 86 5.4. Session Information ("i=") . . . . . . . . . . . . . . . 13 87 5.5. URI ("u=") . . . . . . . . . . . . . . . . . . . . . . . 13 88 5.6. Email Address and Phone Number ("e=" and "p=") . . . . . 14 89 5.7. Connection Data ("c=") . . . . . . . . . . . . . . . . . 15 90 5.8. Bandwidth ("b=") . . . . . . . . . . . . . . . . . . . . 17 91 5.9. Timing ("t=") . . . . . . . . . . . . . . . . . . . . . . 18 92 5.10. Repeat Times ("r=") . . . . . . . . . . . . . . . . . . . 19 93 5.11. Time Zones ("z=") . . . . . . . . . . . . . . . . . . . . 20 94 5.12. Encryption Keys ("k=") . . . . . . . . . . . . . . . . . 20 95 5.13. Attributes ("a=") . . . . . . . . . . . . . . . . . . . . 22 96 5.14. Media Descriptions ("m=") . . . . . . . . . . . . . . . . 23 98 6. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . . 26 99 6.1. cat (category) . . . . . . . . . . . . . . . . . . . . . 26 100 6.2. keywds (keywords) . . . . . . . . . . . . . . . . . . . . 26 101 6.3. tool . . . . . . . . . . . . . . . . . . . . . . . . . . 27 102 6.4. ptime (packet time) . . . . . . . . . . . . . . . . . . . 28 103 6.5. maxptime (maximum packet time) . . . . . . . . . . . . . 28 104 6.6. rtpmap . . . . . . . . . . . . . . . . . . . . . . . . . 29 105 6.7. Media Direction Attributes . . . . . . . . . . . . . . . 31 106 6.7.1. recvonly (receive-only) . . . . . . . . . . . . . . . 32 107 6.7.2. sendrecv (send-receive) . . . . . . . . . . . . . . . 33 108 6.7.3. sendonly (send-only) . . . . . . . . . . . . . . . . 33 109 6.7.4. inactive . . . . . . . . . . . . . . . . . . . . . . 33 110 6.8. orient (orientation) . . . . . . . . . . . . . . . . . . 34 111 6.9. type (conference type) . . . . . . . . . . . . . . . . . 34 112 6.10. charset (character set) . . . . . . . . . . . . . . . . . 35 113 6.11. sdplang (SDP language) . . . . . . . . . . . . . . . . . 36 114 6.12. lang (language) . . . . . . . . . . . . . . . . . . . . . 37 115 6.13. framerate (frame rate) . . . . . . . . . . . . . . . . . 38 116 6.14. quality . . . . . . . . . . . . . . . . . . . . . . . . . 39 117 6.15. fmtp (format parameters) . . . . . . . . . . . . . . . . 39 118 7. Security Considerations . . . . . . . . . . . . . . . . . . . 40 119 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42 120 8.1. The "application/sdp" Media Type . . . . . . . . . . . . 42 121 8.2. Registration of Parameters . . . . . . . . . . . . . . . 43 122 8.2.1. Media Types ("media") . . . . . . . . . . . . . . . . 43 123 8.2.2. Transport Protocols ("proto") . . . . . . . . . . . . 44 124 8.2.3. Media Formats ("fmt") . . . . . . . . . . . . . . . . 44 125 8.2.4. Attribute Names ("att-field") . . . . . . . . . . . . 45 126 8.2.5. Bandwidth Specifiers ("bwtype") . . . . . . . . . . . 46 127 8.2.6. Network Types ("nettype") . . . . . . . . . . . . . . 47 128 8.2.7. Address Types ("addrtype") . . . . . . . . . . . . . 47 129 8.2.8. Registration Procedure . . . . . . . . . . . . . . . 47 130 8.3. Encryption Key Access Methods . . . . . . . . . . . . . . 48 131 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 . . . . . . . . . . . . . . . . . . . . . . . 57 139 1. Introduction 141 When initiating multimedia teleconferences, voice-over-IP calls, 142 streaming video, or other sessions, there is a requirement to convey 143 media details, transport addresses, and other session description 144 metadata to the participants. 146 SDP provides a standard representation for such information, 147 irrespective of how that information is transported. SDP is purely a 148 format for session description -- it does not incorporate a transport 149 protocol, and it is intended to use different transport protocols as 150 appropriate, including the Session Announcement Protocol [RFC2974], 151 Session Initiation Protocol [RFC3261], Real Time Streaming Protocol 152 [RFC2326], electronic mail using the MIME extensions, and the 153 Hypertext Transport Protocol. 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 term is used in this document and has specific meaning 168 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 The terms "multimedia conference" and "multimedia session" are used 174 in this document as defined in 175 [I-D.ietf-avtext-rtp-grouping-taxonomy]. The terms "session" and 176 "multimedia session" are used interchangeably in this document. 178 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 179 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 180 "OPTIONAL" in this document are to be interpreted as described in 181 [RFC2119]. 183 3. Examples of SDP Usage 185 3.1. Session Initiation 187 The Session Initiation Protocol (SIP) [RFC3261] is an application- 188 layer control protocol for creating, modifying, and terminating 189 sessions such as Internet multimedia conferences, Internet telephone 190 calls, and multimedia distribution. The SIP messages used to create 191 sessions carry session descriptions that allow participants to agree 192 on a set of compatible media types. These session descriptions are 193 commonly formatted using SDP. When used with SIP, the offer/answer 194 model [RFC3264] provides a limited framework for negotiation using 195 SDP. 197 3.2. Streaming Media 199 The Real Time Streaming Protocol (RTSP) [RFC2326], is an application- 200 level protocol for control over the delivery of data with real-time 201 properties. RTSP provides an extensible framework to enable 202 controlled, on-demand delivery of real-time data, such as audio and 203 video. An RTSP client and server negotiate an appropriate set of 204 parameters for media delivery, partially using SDP syntax to describe 205 those parameters. 207 3.3. Email and the World Wide Web 209 Alternative means of conveying session descriptions include 210 electronic mail and the World Wide Web (WWW). For both email and WWW 211 distribution, the media type "application/sdp" is used. This enables 212 the automatic launching of applications for participation in the 213 session from the WWW client or mail reader in a standard manner. 215 Note that announcements of multicast sessions made only via email or 216 the WWW do not have the property that the receiver of a session 217 announcement can necessarily receive the session because the 218 multicast sessions may be restricted in scope, and access to the WWW 219 server or reception of email is possible outside this scope. 221 3.4. Multicast Session Announcement 223 In order to assist the advertisement of multicast multimedia 224 conferences and other multicast sessions, and to communicate the 225 relevant session setup information to prospective participants, a 226 distributed session directory may be used. An instance of such a 227 session directory periodically sends packets containing a description 228 of the session to a well-known multicast group. These advertisements 229 are received by other session directories such that potential remote 230 participants can use the session description to start the tools 231 required to participate in the session. 233 One protocol used to implement such a distributed directory is the 234 Session Announcement Protocol (SAP) [RFC2974]. SDP provides the 235 recommended session description format for such session 236 announcements. 238 4. Requirements and Recommendations 240 The purpose of SDP is to convey information about media streams in 241 multimedia sessions to allow the recipients of a session description 242 to participate in the session. SDP is primarily intended for use in 243 an internetwork, although it is sufficiently general that it can 244 describe multimedia conferences in other network environments. Media 245 streams can be many-to-many. Sessions need not be continually 246 active. 248 Thus far, multicast-based sessions on the Internet have differed from 249 many other forms of conferencing in that anyone receiving the traffic 250 can join the session (unless the session traffic is encrypted). In 251 such an environment, SDP serves two primary purposes. It is a means 252 to communicate the existence of a session, and it is a means to 253 convey sufficient information to enable joining and participating in 254 the session. In a unicast environment, only the latter purpose is 255 likely to be relevant. 257 An SDP session description includes the following: 259 o Session name and purpose 261 o Time(s) the session is active 263 o The media comprising the session 265 o Information needed to receive those media (addresses, ports, 266 formats, etc.) 268 As resources necessary to participate in a session may be limited, 269 some additional information may also be desirable: 271 o Information about the bandwidth to be used by the session 273 o Contact information for the person responsible for the session 275 In general, SDP must convey sufficient information to enable 276 applications to join a session (with the possible exception of 277 encryption keys) and to announce the resources to be used to any non- 278 participants that may need to know. (This latter feature is 279 primarily useful when SDP is used with a multicast session 280 announcement protocol.) 282 4.1. Media and Transport Information 284 An SDP session description includes the following media information: 286 o The type of media (video, audio, etc.) 288 o The transport protocol (RTP/UDP/IP, H.320, etc.) 289 o The format of the media (H.261 video, MPEG video, etc.) 291 In addition to media format and transport protocol, SDP conveys 292 address and port details. For an IP multicast session, these 293 comprise: 295 o The multicast group address for media 297 o The transport port for media 299 This address and port are the destination address and destination 300 port of the multicast stream, whether being sent, received, or both. 302 For unicast IP sessions, the following are conveyed: 304 o The remote address for media 306 o The remote transport port for media 308 The semantics of this address and port depend on the media and 309 transport protocol defined. By default, this SHOULD be the remote 310 address and remote port to which data is sent. Some media types may 311 redefine this behaviour, but this is NOT RECOMMENDED since it 312 complicates implementations (including middleboxes that must parse 313 the addresses to open Network Address Translation (NAT) or firewall 314 pinholes). 316 4.2. Timing Information 318 Sessions may be either bounded or unbounded in time. Whether or not 319 they are bounded, they may be only active at specific times. SDP can 320 convey: 322 o An arbitrary list of start and stop times bounding the session 324 o For each bound, repeat times such as "every Wednesday at 10am for 325 one hour" 327 This timing information is globally consistent, irrespective of local 328 time zone or daylight saving time (see Section 5.9). 330 4.3. Private Sessions 332 It is possible to create both public sessions and private sessions. 333 SDP itself does not distinguish between these; private sessions are 334 typically conveyed by encrypting the session description during 335 distribution. The details of how encryption is performed are 336 dependent on the mechanism used to convey SDP; mechanisms are 337 currently defined for SDP transported using SAP [RFC2974] and SIP 338 [RFC3261], and others may be defined in the future. 340 If a session announcement is private, it is possible to use that 341 private announcement to convey encryption keys necessary to decode 342 each of the media in a multimedia conference, including enough 343 information to know which encryption scheme is used for each media. 345 4.4. Obtaining Further Information about a Session 347 A session description should convey enough information to decide 348 whether or not to participate in a session. SDP may include 349 additional pointers in the form of Uniform Resource Identifiers 350 (URIs) for more information about the session. 352 4.5. Categorisation 354 When many session descriptions are being distributed by SAP, or any 355 other advertisement mechanism, it may be desirable to filter session 356 announcements that are of interest from those that are not. SDP 357 supports a categorisation mechanism for sessions that is capable of 358 being automated (the "a=cat:" attribute; see Section 6). 360 4.6. Internationalisation 362 The SDP specification recommends the use of the ISO 10646 character 363 set in the UTF-8 encoding [RFC3629] to allow many different languages 364 to be represented. However, to assist in compact representations, 365 SDP also allows other character sets such as ISO 8859-1 to be used 366 when desired. Internationalisation only applies to free-text fields 367 (session name and background information), and not to SDP as a whole. 369 5. SDP Specification 371 An SDP session description is denoted by the media type "application/ 372 sdp" (See Section 8). 374 An SDP session description is entirely textual. SDP field names and 375 attribute names use only the US-ASCII subset of UTF-8, but textual 376 fields and attribute values MAY use the full ISO 10646 character set 377 in UTF-8 encoding, or some other character set defined by the 378 "a=charset:" attribute. Field and attribute values that use the full 379 UTF-8 character set are never directly compared, hence there is no 380 requirement for UTF-8 normalisation. The textual form, as opposed to 381 a binary encoding such as ASN.1 or XDR, was chosen to enhance 382 portability, to enable a variety of transports to be used, and to 383 allow flexible, text-based toolkits to be used to generate and 384 process session descriptions. However, since SDP may be used in 385 environments where the maximum permissible size of a session 386 description is limited, the encoding is deliberately compact. Also, 387 since announcements may be transported via very unreliable means or 388 damaged by an intermediate caching server, the encoding was designed 389 with strict order and formatting rules so that most errors would 390 result in malformed session announcements that could be detected 391 easily and discarded. This also allows rapid discarding of encrypted 392 session announcements for which a receiver does not have the correct 393 key. 395 An SDP session description consists of a number of lines of text of 396 the form: 398 = 400 where MUST be exactly one case-significant character and 401 is structured text whose format depends on . In 402 general, is either a number of fields delimited by a single 403 space character or a free format string, and is case-significant 404 unless a specific field defines otherwise. Whitespace MUST NOT be 405 used on either side of the "=" sign. 407 An SDP session description consists of a session-level section 408 followed by zero or more media-level sections. The session-level 409 part starts with a "v=" line and continues to the first media-level 410 section (or the end of the whole description, whichever comes first). 411 Each media-level section starts with an "m=" line and continues to 412 the next media-level section or the end of the whole session 413 description - whichever comes first. In general, session-level 414 values are the default for all media unless overridden by an 415 equivalent media-level value. 417 Some lines in each description are REQUIRED and some are OPTIONAL, 418 but all MUST appear in exactly the order given here (the fixed order 419 greatly enhances error detection and allows for a simple parser). 420 OPTIONAL items are marked with a "*". 422 Session description 423 v= (protocol version) 424 o= (originator and session identifier) 425 s= (session name) 426 i=* (session information) 427 u=* (URI of description) 428 e=* (email address) 429 p=* (phone number) 430 c=* (connection information -- not required if included in 431 all media descriptions) 433 b=* (zero or more bandwidth information lines) 434 One or more time descriptions ("t=" and "r=" lines; see below) 435 z=* (time zone adjustments) 436 k=* (encryption key) 437 a=* (zero or more session attribute lines) 438 Zero or more media descriptions 440 Time description 441 t= (time the session is active) 442 r=* (zero or more repeat times) 444 Media description, if present 445 m= (media name and transport address) 446 i=* (media title) 447 c=* (connection information -- optional if included at 448 session level) 449 b=* (zero or more bandwidth information lines) 450 k=* (encryption key) 451 a=* (zero or more media attribute lines) 453 The set of type letters is deliberately small and not intended to be 454 extensible -- an SDP parser MUST completely ignore any session 455 description that contains a type letter that it does not understand. 456 The attribute mechanism ("a=" described below) is the primary means 457 for extending SDP and tailoring it to particular applications or 458 media. Some attributes (the ones listed in Section 6 of this memo) 459 have a defined meaning, but others may be added on an application-, 460 media-, or session-specific basis. An SDP parser MUST ignore any 461 attribute it doesn't understand. 463 An SDP session description may contain URIs that reference external 464 content in the "u=", "k=", and "a=" lines. These URIs may be 465 dereferenced in some cases, making the session description non-self- 466 contained. 468 The connection ("c=") information in the session-level section 469 applies to all the media of that session unless overridden by 470 connection information in the media description. For instance, in 471 the example below, each audio media behaves as if it were given a 472 "c=IN IP4 233.252.0.2". 474 An example SDP description is: 476 v=0 477 o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1 478 s=SDP Seminar 479 i=A Seminar on the session description protocol 480 u=http://www.example.com/seminars/sdp.pdf 481 e=j.doe@example.com (Jane Doe) 482 c=IN IP4 233.252.0.2 483 t=2873397496 2873404696 484 a=recvonly 485 m=audio 49170 RTP/AVP 0 486 m=audio 49180 RTP/AVP 0 487 m=video 51372 RTP/AVP 99 488 c=IN IP4 233.252.0.1/127 489 a=rtpmap:99 h263-1998/90000 491 Text fields such as the session name and information are octet 492 strings that may contain any octet with the exceptions of 0x00 (Nul), 493 0x0a (ASCII newline), and 0x0d (ASCII carriage return). The sequence 494 CRLF (0x0d0a) is used to end a record, although parsers SHOULD be 495 tolerant and also accept records terminated with a single newline 496 character. If the "a=charset" attribute is not present, these octet 497 strings MUST be interpreted as containing ISO-10646 characters in 498 UTF-8 encoding (the presence of the "a=charset" attribute may force 499 some fields to be interpreted differently). 501 A session description can contain domain names in the "o=", "u=", 502 "e=", "c=", and "a=" lines. Any domain name used in SDP MUST comply 503 with [RFC1034], [RFC1035]. Internationalised domain names (IDNs) 504 MUST be represented using the ASCII Compatible Encoding (ACE) form 505 defined in [RFC5890] and MUST NOT be directly represented in UTF-8 or 506 any other encoding (this requirement is for compatibility with 507 [RFC4566] and other early SDP-related standards, which predate the 508 development of internationalised domain names). 510 5.1. Protocol Version ("v=") 512 v=0 514 The "v=" field gives the version of the Session Description Protocol. 515 This memo defines version 0. There is no minor version number. 517 5.2. Origin ("o=") 519 o= 520 522 The "o=" field gives the originator of the session (her username and 523 the address of the user's host) plus a session identifier and version 524 number: 526 is the user's login on the originating host, or it is "-" 527 if the originating host does not support the concept of user IDs. 528 The MUST NOT contain spaces. 530 is a numeric string such that the tuple of , 531 , , , and forms a 532 globally unique identifier for the session. The method of allocation is up to the creating tool, but it has been 534 suggested that a Network Time Protocol (NTP) format timestamp be 535 used to ensure uniqueness [RFC5905]. 537 is a version number for this session description. 538 Its usage is up to the creating tool, so long as is 539 increased when a modification is made to the session data. Again, 540 it is RECOMMENDED that an NTP format timestamp is used. 542 is a text string giving the type of network. Initially 543 "IN" is defined to have the meaning "Internet", but other values 544 MAY be registered in the future (see Section 8). 546 is a text string giving the type of the address that 547 follows. Initially "IP4" and "IP6" are defined, but other values 548 MAY be registered in the future (see Section 8). 550 is an address of the machine from which the 551 session was created. For an address type of IP4, this is either a 552 fully qualified domain name of the machine or the dotted-decimal 553 representation of an IP version 4 address of the machine. For an 554 address type of IP6, this is either a fully qualified domain name 555 of the machine or the compressed textual representation of an IP 556 version 6 address of the machine. For both IP4 and IP6, the fully 557 qualified domain name is the form that SHOULD be given unless this 558 is unavailable, in which case a globally unique address MAY be 559 substituted. Unless an SDP extension for NAT traversal is used 560 (e.g., ICE [RFC5245], ICE TCP [RFC6544]), a local IP address MUST 561 NOT be used in any context where the SDP description might leave 562 the scope in which the address is meaningful (for example, a local 563 address MUST NOT be included in an application-level referral that 564 might leave the scope). 566 In general, the "o=" field serves as a globally unique identifier for 567 this version of this session description, and the subfields excepting 568 the version taken together identify the session irrespective of any 569 modifications. 571 For privacy reasons, it is sometimes desirable to obfuscate the 572 username and IP address of the session originator. If this is a 573 concern, an arbitrary and private MAY be 574 chosen to populate the "o=" field, provided that these are selected 575 in a manner that does not affect the global uniqueness of the field. 577 5.3. Session Name ("s=") 579 s= 581 The "s=" field is the textual session name. There MUST be one and 582 only one "s=" field per session description. The "s=" field MUST NOT 583 be empty and SHOULD contain ISO 10646 characters (but see also the 584 "a=charset" attribute). If a session has no meaningful name, the 585 value "s= " SHOULD be used (i.e., a single space as the session 586 name). 588 5.4. Session Information ("i=") 590 i= 592 The "i=" field provides textual information about the session. There 593 MUST be at most one session-level "i=" field per session description, 594 and at most one "i=" field per media. If the "a=charset" attribute 595 is present, it specifies the character set used in the "i=" field. 596 If the "a=charset" attribute is not present, the "i=" field MUST 597 contain ISO 10646 characters in UTF-8 encoding. 599 A single "i=" field MAY also be used for each media definition. In 600 media definitions, "i=" fields are primarily intended for labelling 601 media streams. As such, they are most likely to be useful when a 602 single session has more than one distinct media stream of the same 603 media type. An example would be two different whiteboards, one for 604 slides and one for feedback and questions. 606 The "i=" field is intended to provide a free-form human-readable 607 description of the session or the purpose of a media stream. It is 608 not suitable for parsing by automata. 610 5.5. URI ("u=") 612 u= 614 A URI is a Uniform Resource Identifier as used by WWW clients 615 [RFC3986]. The URI should be a pointer to additional information 616 about the session. This field is OPTIONAL, but if it is present it 617 MUST be specified before the first media field. No more than one URI 618 field is allowed per session description. 620 5.6. Email Address and Phone Number ("e=" and "p=") 622 e= 623 p= 625 The "e=" and "p=" lines specify contact information for the person 626 responsible for the session. This is not necessarily the same person 627 that created the session description. 629 Inclusion of an email address or phone number is OPTIONAL. Note that 630 the previous version of SDP specified that either an email field or a 631 phone field MUST be specified, but this was widely ignored. The 632 change brings the specification into line with common usage. 634 If an email address or phone number is present, it MUST be specified 635 before the first media field. More than one email or phone field can 636 be given for a session description. 638 Phone numbers SHOULD be given in the form of an international public 639 telecommunication number (see ITU-T Recommendation E.164) preceded by 640 a "+". Spaces and hyphens may be used to split up a phone field to 641 aid readability if desired. For example: 643 p=+1 617 555-6011 645 Both email addresses and phone numbers can have an OPTIONAL free text 646 string associated with them, normally giving the name of the person 647 who may be contacted. This MUST be enclosed in parentheses if it is 648 present. For example: 650 e=j.doe@example.com (Jane Doe) 652 The alternative [RFC5322] name quoting convention is also allowed for 653 both email addresses and phone numbers. For example: 655 e=Jane Doe 657 The free text string SHOULD be in the ISO-10646 character set with 658 UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if 659 the appropriate session-level "a=charset" attribute is set. 661 5.7. Connection Data ("c=") 663 c= 665 The "c=" field contains connection data. 667 A session description MUST contain either at least one "c=" field in 668 each media description or a single "c=" field at the session level. 669 It MAY contain a single session-level "c=" field and additional "c=" 670 field(s) per media description, in which case the per-media values 671 override the session-level settings for the respective media. 673 The first sub-field ("") is the network type, which is a 674 text string giving the type of network. Initially, "IN" is defined 675 to have the meaning "Internet", but other values MAY be registered in 676 the future (see Section 8). 678 The second sub-field ("") is the address type. This allows 679 SDP to be used for sessions that are not IP based. This memo only 680 defines IP4 and IP6, but other values MAY be registered in the future 681 (see Section 8). 683 The third sub-field ("") is the connection 684 address. OPTIONAL sub-fields MAY be added after the connection 685 address depending on the value of the field. 687 When the is IP4 and IP6, the connection address is defined 688 as follows: 690 o If the session is multicast, the connection address will be an IP 691 multicast group address. If the session is not multicast, then 692 the connection address contains the unicast IP address of the 693 expected data source or data relay or data sink as determined by 694 additional attribute fields. It is not expected that unicast 695 addresses will be given in a session description that is 696 communicated by a multicast announcement, though this is not 697 prohibited. 699 o Sessions using an IP4 multicast connection address MUST also have 700 a time to live (TTL) value present in addition to the multicast 701 address. The TTL and the address together define the scope with 702 which multicast packets sent in this session will be sent. TTL 703 values MUST be in the range 0-255. Although the TTL MUST be 704 specified, its use to scope multicast traffic is deprecated; 705 applications SHOULD use an administratively scoped address 706 instead. 708 The TTL for the session is appended to the address using a slash as a 709 separator. An example is: 711 c=IN IP4 233.252.0.1/127 713 IP6 multicast does not use TTL scoping, and hence the TTL value MUST 714 NOT be present for IP6 multicast. It is expected that IP6 scoped 715 addresses will be used to limit the scope of multimedia conferences. 717 Hierarchical or layered encoding schemes are data streams where the 718 encoding from a single media source is split into a number of layers. 719 The receiver can choose the desired quality (and hence bandwidth) by 720 only subscribing to a subset of these layers. Such layered encodings 721 are normally transmitted in multiple multicast groups to allow 722 multicast pruning. This technique keeps unwanted traffic from sites 723 only requiring certain levels of the hierarchy. For applications 724 requiring multiple multicast groups, we allow the following notation 725 to be used for the connection address: 727 [/]/ 729 If the number of addresses is not given, it is assumed to be one. 730 Multicast addresses so assigned are contiguously allocated above the 731 base address, so that, for example: 733 c=IN IP4 233.252.0.1/127/3 735 would state that addresses 233.252.0.1, 233.252.0.2, and 233.252.0.3 736 are to be used at a TTL of 127. This is semantically identical to 737 including multiple "c=" lines in a media description: 739 c=IN IP4 233.252.0.1/127 740 c=IN IP4 233.252.0.2/127 741 c=IN IP4 233.252.0.3/127 743 Similarly, an IP6 example would be: 745 c=IN IP6 FF15::101/3 747 which is semantically equivalent to: 749 c=IN IP6 FF15::101 750 c=IN IP6 FF15::102 751 c=IN IP6 FF15::103 753 (remembering that the TTL field is not present in IP6 multicast). 755 Multiple addresses or "c=" lines MAY be specified on a per-media 756 basis only if they provide multicast addresses for different layers 757 in a hierarchical or layered encoding scheme. They MUST NOT be 758 specified for a session-level "c=" field. 760 The slash notation for multiple addresses described above MUST NOT be 761 used for IP unicast addresses. 763 5.8. Bandwidth ("b=") 765 b=: 767 This OPTIONAL field denotes the proposed bandwidth to be used by the 768 session or media. The is an alphanumeric modifier giving 769 the meaning of the figure. Two values are defined in 770 this specification, but other values MAY be registered in the future 771 (see Section 8 and [RFC3556], [RFC3890]): 773 CT If the bandwidth of a session or media in a session is different 774 from the bandwidth implicit from the scope, a "b=CT:..." line 775 SHOULD be supplied for the session giving the proposed upper limit 776 to the bandwidth used (the "multimedia conference total" 777 bandwidth). The primary purpose of this is to give an approximate 778 idea as to whether two or more sessions can coexist 779 simultaneously. When using the CT modifier with RTP, if several 780 RTP sessions are part of the multimedia conference, the multimedia 781 conference total refers to total bandwidth of all RTP sessions. 783 AS The bandwidth is interpreted to be application specific (it will 784 be the application's concept of maximum bandwidth). Normally, 785 this will coincide with what is set on the application's "maximum 786 bandwidth" control if applicable. For RTP-based applications, AS 787 gives the RTP "session bandwidth" as defined in Section 6.2 of 788 [RFC3550]. 790 Note that CT gives a total bandwidth figure for all the media at all 791 sites. AS gives a bandwidth figure for a single media at a single 792 site, although there may be many sites sending simultaneously. 794 A prefix "X-" is defined for names. This is intended for 795 experimental purposes only. For example: 797 b=X-YZ:128 799 Use of the "X-" prefix is NOT RECOMMENDED: instead new modifiers 800 SHOULD be registered with IANA in the standard namespace. SDP 801 parsers MUST ignore bandwidth fields with unknown modifiers. 802 Modifiers MUST be alphanumeric and, although no length limit is 803 given, it is recommended that they be short. 805 The is interpreted as kilobits per second by default. 806 The definition of a new modifier MAY specify that the 807 bandwidth is to be interpreted in some alternative unit (the "CT" and 808 "AS" modifiers defined in this memo use the default units). 810 5.9. Timing ("t=") 812 t= 814 The "t=" lines specify the start and stop times for a session. 815 Multiple "t=" lines MAY be used if a session is active at multiple 816 irregularly spaced times; each additional "t=" line specifies an 817 additional period of time for which the session will be active. If 818 the session is active at regular times, an "r=" line (see below) 819 should be used in addition to, and following, a "t=" line -- in which 820 case the "t=" line specifies the start and stop times of the repeat 821 sequence. 823 The first and second sub-fields give the start and stop times, 824 respectively, for the session. These values are the decimal 825 representation of Network Time Protocol (NTP) time values in seconds 826 since 1900 [RFC5905]. To convert these values to UNIX time, subtract 827 decimal 2208988800. 829 NTP timestamps are elsewhere represented by 64-bit values, which wrap 830 sometime in the year 2036. Since SDP uses an arbitrary length 831 decimal representation, this should not cause an issue (SDP 832 timestamps MUST continue counting seconds since 1900, NTP will use 833 the value modulo the 64-bit limit). 835 If the is set to zero, then the session is not bounded, 836 though it will not become active until after the . If 837 the is also zero, the session is regarded as permanent. 839 User interfaces SHOULD strongly discourage the creation of unbounded 840 and permanent sessions as they give no information about when the 841 session is actually going to terminate, and so make scheduling 842 difficult. 844 The general assumption may be made, when displaying unbounded 845 sessions that have not timed out to the user, that an unbounded 846 session will only be active until half an hour from the current time 847 or the session start time, whichever is the later. If behaviour 848 other than this is required, an end-time SHOULD be given and modified 849 as appropriate when new information becomes available about when the 850 session should really end. 852 Permanent sessions may be shown to the user as never being active 853 unless there are associated repeat times that state precisely when 854 the session will be active. 856 5.10. Repeat Times ("r=") 858 r= 860 "r=" fields specify repeat times for a session. For example, if a 861 session is active at 10am on Monday and 11am on Tuesday for one hour 862 each week for three months, then the in the 863 corresponding "t=" field would be the NTP representation of 10am on 864 the first Monday, the would be 1 week, the would be 1 hour, and the offsets would be zero and 25 866 hours. The corresponding "t=" field stop time would be the NTP 867 representation of the end of the last session three months later. By 868 default, all fields are in seconds, so the "r=" and "t=" fields might 869 be the following: 871 t=3034423619 3042462419 872 r=604800 3600 0 90000 874 To make the description more compact, times may also be given in 875 units of days, hours, or minutes. The syntax for these is a number 876 immediately followed by a single case-sensitive character. 877 Fractional units are not allowed -- a smaller unit should be used 878 instead. The following unit specification characters are allowed: 880 d - days (86400 seconds) 881 h - hours (3600 seconds) 882 m - minutes (60 seconds) 883 s - seconds (allowed for completeness) 885 Thus, the above session announcement could also have been written: 887 r=7d 1h 0 25h 888 Monthly and yearly repeats cannot be directly specified with a single 889 SDP repeat time; instead, separate "t=" fields should be used to 890 explicitly list the session times. 892 5.11. Time Zones ("z=") 894 z= .... 896 To schedule a repeated session that spans a change from daylight 897 saving time to standard time or vice versa, it is necessary to 898 specify offsets from the base time. This is required because 899 different time zones change time at different times of day, different 900 countries change to or from daylight saving time on different dates, 901 and some countries do not have daylight saving time at all. 903 Thus, in order to schedule a session that is at the same time winter 904 and summer, it must be possible to specify unambiguously by whose 905 time zone a session is scheduled. To simplify this task for 906 receivers, we allow the sender to specify the NTP time that a time 907 zone adjustment happens and the offset from the time when the session 908 was first scheduled. The "z=" field allows the sender to specify a 909 list of these adjustment times and offsets from the base time. 911 An example might be the following: 913 z=2882844526 -1h 2898848070 0 915 This specifies that at time 2882844526, the time base by which the 916 session's repeat times are calculated is shifted back by 1 hour, and 917 that at time 2898848070, the session's original time base is 918 restored. Adjustments are always relative to the specified start 919 time -- they are not cumulative. Adjustments apply to all "t=" and 920 "r=" lines in a session description. 922 If a session is likely to last several years, it is expected that the 923 session description will be modified periodically rather than 924 transmit several years' worth of adjustments in one session 925 description. 927 5.12. Encryption Keys ("k=") 929 k= 930 k=: 931 If transported over a secure and trusted channel, the Session 932 Description Protocol MAY be used to convey encryption keys. A simple 933 mechanism for key exchange is provided by the key field ("k="), 934 although this is primarily supported for compatibility with older 935 implementations and its use is NOT RECOMMENDED. Work is in progress 936 to define new key exchange mechanisms for use with SDP [RFC4567] 937 [RFC4568], and it is expected that new applications will use those 938 mechanisms. 940 A key field is permitted before the first media entry (in which case 941 it applies to all media in the session), or for each media entry as 942 required. The format of keys and their usage are outside the scope 943 of this document, and the key field provides no way to indicate the 944 encryption algorithm to be used, key type, or other information about 945 the key: this is assumed to be provided by the higher-level protocol 946 using SDP. If there is a need to convey this information within SDP, 947 the extensions mentioned previously SHOULD be used. Many security 948 protocols require two keys: one for confidentiality, another for 949 integrity. This specification does not support transfer of two keys. 951 The method indicates the mechanism to be used to obtain a usable key 952 by external means, or from the encoded encryption key given. The 953 following methods are defined: 955 k=clear: 957 The encryption key is included untransformed in this key field. 958 This method MUST NOT be used unless it can be guaranteed that 959 the SDP is conveyed over a secure channel. The encryption key 960 is interpreted as text according to the charset attribute; use 961 the "k=base64:" method to convey characters that are otherwise 962 prohibited in SDP. 964 k=base64: 966 The encryption key is included in this key field but has been 967 base64 encoded [RFC4648] because it includes characters that 968 are prohibited in SDP. This method MUST NOT be used unless it 969 can be guaranteed that the SDP is conveyed over a secure 970 channel. 972 k=uri: 974 A Uniform Resource Identifier is included in the key field. 975 The URI refers to the data containing the key, and may require 976 additional authentication before the key can be returned. When 977 a request is made to the given URI, the reply should specify 978 the encoding for the key. The URI is often an Secure Socket 979 Layer/Transport Layer Security (SSL/TLS)-protected HTTP URI 980 ("https:"), although this is not required. 982 k=prompt 984 No key is included in this SDP description, but the session or 985 media stream referred to by this key field is encrypted. The 986 user should be prompted for the key when attempting to join the 987 session, and this user-supplied key should then be used to 988 decrypt the media streams. The use of user-specified keys is 989 NOT RECOMMENDED, since such keys tend to have weak security 990 properties. 992 The key field MUST NOT be used unless it can be guaranteed that the 993 SDP is conveyed over a secure and trusted channel. An example of 994 such a channel might be SDP embedded inside an S/MIME message or a 995 TLS-protected HTTP session. It is important to ensure that the 996 secure channel is with the party that is authorised to join the 997 session, not an intermediary: if a caching proxy server is used, it 998 is important to ensure that the proxy is either trusted or unable to 999 access the SDP. 1001 5.13. Attributes ("a=") 1003 a= 1004 a=: 1006 Attributes are the primary means for extending SDP. Attributes may 1007 be defined to be used as "session-level" attributes, "media-level" 1008 attributes, or both. 1010 A media description may have any number of attributes ("a=" fields) 1011 that are media specific. These are referred to as "media-level" 1012 attributes and add information about the media stream. Attribute 1013 fields can also be added before the first media field; these 1014 "session-level" attributes convey additional information that applies 1015 to the session as a whole rather than to individual media. 1017 Attribute fields may be of two forms: 1019 o A property attribute is simply of the form "a=". These are 1020 binary attributes, and the presence of the attribute conveys that 1021 the attribute is a property of the session. An example might be 1022 "a=recvonly". 1024 o A value attribute is of the form "a=:". For 1025 example, a whiteboard could have the value attribute 1026 "a=orient:landscape" 1028 Attribute interpretation depends on the media tool being invoked. 1029 Thus receivers of session descriptions should be configurable in 1030 their interpretation of session descriptions in general and of 1031 attributes in particular. 1033 Attribute names MUST use the US-ASCII subset of ISO-10646/UTF-8. 1035 Attribute values are octet strings, and MAY use any octet value 1036 except 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute 1037 values are to be interpreted as in ISO-10646 character set with UTF-8 1038 encoding. Unlike other text fields, attribute values are NOT 1039 normally affected by the "charset" attribute as this would make 1040 comparisons against known values problematic. However, when an 1041 attribute is defined, it can be defined to be charset dependent, in 1042 which case its value should be interpreted in the session charset 1043 rather than in ISO-10646. 1045 Attributes MUST be registered with IANA (see Section 8). If an 1046 attribute is received that is not understood, it MUST be ignored by 1047 the receiver. 1049 5.14. Media Descriptions ("m=") 1051 m= ... 1053 A session description may contain a number of media descriptions. 1054 Each media description starts with an "m=" field and is terminated by 1055 either the next "m=" field or by the end of the session description. 1056 A media field has several sub-fields: 1058 is the media type. Currently defined media are "audio", 1059 "video", "text", "application", and "message", although this list 1060 may be extended in the future (see Section 8). 1062 is the transport port to which the media stream is sent. The 1063 meaning of the transport port depends on the network being used as 1064 specified in the relevant "c=" field, and on the transport 1065 protocol defined in the sub-field of the media field. 1066 Other ports used by the media application (such as the RTP Control 1067 Protocol (RTCP) port [RFC3550]) MAY be derived algorithmically 1068 from the base media port or MAY be specified in a separate 1069 attribute (for example, "a=rtcp:" as defined in [RFC3605]). 1071 If non-contiguous ports are used or if they don't follow the 1072 parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:" 1073 attribute MUST be used. Applications that are requested to send 1074 media to a that is odd and where the "a=rtcp:" is present 1075 MUST NOT subtract 1 from the RTP port: that is, they MUST send the 1076 RTP to the port indicated in and send the RTCP to the port 1077 indicated in the "a=rtcp" attribute. 1079 For applications where hierarchically encoded streams are being 1080 sent to a unicast address, it may be necessary to specify multiple 1081 transport ports. This is done using a similar notation to that 1082 used for IP multicast addresses in the "c=" field: 1084 m= / ... 1086 In such a case, the ports used depend on the transport protocol. 1087 For RTP, the default is that only the even-numbered ports are used 1088 for data with the corresponding one-higher odd ports used for the 1089 RTCP belonging to the RTP session, and the 1090 denoting the number of RTP sessions. For example: 1092 m=video 49170/2 RTP/AVP 31 1094 would specify that ports 49170 and 49171 form one RTP/RTCP pair 1095 and 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the 1096 transport protocol and 31 is the format (see below). If non- 1097 contiguous ports are required, they must be signalled using a 1098 separate attribute (for example, "a=rtcp:" as defined in 1099 [RFC3605]). 1101 If multiple addresses are specified in the "c=" field and multiple 1102 ports are specified in the "m=" field, a one-to-one mapping from 1103 port to the corresponding address is implied. For example: 1105 c=IN IP4 233.252.0.1/127/2 1106 m=video 49170/2 RTP/AVP 31 1108 would imply that address 233.252.0.1 is used with ports 49170 and 1109 49171, and address 233.252.0.2 is used with ports 49172 and 49173. 1111 The semantics of multiple "m=" lines using the same transport 1112 address are undefined. This implies that, unlike limited past 1113 practice, there is no implicit grouping defined by such means and 1114 an explicit grouping framework (for example, [RFC5888]) should 1115 instead be used to express the intended semantics. 1117 is the transport protocol. The meaning of the transport 1118 protocol is dependent on the address type field in the relevant 1119 "c=" field. Thus a "c=" field of IP4 indicates that the transport 1120 protocol runs over IP4. The following transport protocols are 1121 defined, but may be extended through registration of new protocols 1122 with IANA (see Section 8): 1124 * udp: denotes an unspecified protocol running over UDP. 1126 * RTP/AVP: denotes RTP [RFC3550] used under the RTP Profile for 1127 Audio and Video Conferences with Minimal Control [RFC3551] 1128 running over UDP. 1130 * RTP/SAVP: denotes the Secure Real-time Transport Protocol 1131 [RFC3711] running over UDP. 1133 The main reason to specify the transport protocol in addition to 1134 the media format is that the same standard media formats may be 1135 carried over different transport protocols even when the network 1136 protocol is the same -- a historical example is vat Pulse Code 1137 Modulation (PCM) audio and RTP PCM audio; another might be TCP/RTP 1138 PCM audio. In addition, relays and monitoring tools that are 1139 transport-protocol-specific but format-independent are possible. 1141 is a media format description. The fourth and any subsequent 1142 sub-fields describe the format of the media. The interpretation 1143 of the media format depends on the value of the sub-field. 1145 If the sub-field is "RTP/AVP" or "RTP/SAVP" the sub- 1146 fields contain RTP payload type numbers. When a list of payload 1147 type numbers is given, this implies that all of these payload 1148 formats MAY be used in the session, but the first of these formats 1149 SHOULD be used as the default format for the session. For dynamic 1150 payload type assignments the "a=rtpmap:" attribute (see Section 6) 1151 SHOULD be used to map from an RTP payload type number to a media 1152 encoding name that identifies the payload format. The "a=fmtp:" 1153 attribute MAY be used to specify format parameters (see 1154 Section 6). 1156 If the sub-field is "udp" the sub-fields MUST 1157 reference a media type describing the format under the "audio", 1158 "video", "text", "application", or "message" top-level media 1159 types. The media type registration SHOULD define the packet 1160 format for use with UDP transport. 1162 For media using other transport protocols, the field is 1163 protocol specific. Rules for interpretation of the sub- 1164 field MUST be defined when registering new protocols (see 1165 Section 8.2.2). 1167 Section 3 of [RFC4855] states that the payload format (encoding) 1168 names defined in the RTP Profile are commonly shown in upper case, 1169 while media subtype names are commonly shown in lower case. It 1170 also states that both of these names are case-insensitive in both 1171 places, similar to parameter names which are case-insensitive both 1172 in media type strings and in the default mapping to the SDP a=fmtp 1173 attribute. 1175 6. SDP Attributes 1177 The following attributes are defined. Since application writers may 1178 add new attributes as they are required, this list is not exhaustive. 1179 Registration procedures for new attributes are defined in 1180 Section 8.2.4. 1182 6.1. cat (category) 1184 Name: cat 1186 Value: cat-value 1188 Usage Level: session 1190 Charset Dependent: no 1192 Syntax: 1194 cat-value = category 1195 category = none-ws-string 1197 Example: 1199 a=cat:foo.bar 1201 This attribute gives the dot-separated hierarchical category of the 1202 session. This is to enable a receiver to filter unwanted sessions by 1203 category. There is no central registry of categories. This 1204 attribute is obsoleted. 1206 6.2. keywds (keywords) 1207 Name: keywds 1209 Value: keywds-value 1211 Usage Level: session 1213 Charset Dependent: yes 1215 Syntax: 1217 keywds-value = keywords 1218 keywords = text 1220 Example: 1222 a=keywds:SDP session description protocol 1224 Like the cat attribute, this is to assist identifying wanted sessions 1225 at the receiver. This allows a receiver to select interesting 1226 session based on keywords describing the purpose of the session; 1227 there is no central registry of keywords. Its value should be 1228 interpreted in the charset specified for the session description if 1229 one is specified, or by default in ISO 10646/UTF-8. This attribute 1230 is obsoleted. 1232 6.3. tool 1234 Name: tool 1236 Value: tool-value 1238 Usage Level: session 1240 Charset Dependent: no 1242 Syntax: 1244 tool-value = tool-name-and-version 1245 tool-name-and-version = text 1247 Example: 1249 a=tool:foobar V3.2 1251 This gives the name and version number of the tool used to create the 1252 session description. 1254 6.4. ptime (packet time) 1256 Name: ptime 1258 Value: ptime-value 1260 Usage Level: media 1262 Charset Dependent: no 1264 Syntax: 1266 ptime-value = packet-time 1267 packet-time = integer 1268 ; do we want to define a limited range for this? 1270 Example: 1272 a=ptime:20 1274 This gives the length of time in milliseconds represented by the 1275 media in a packet. This is probably only meaningful for audio data, 1276 but may be used with other media types if it makes sense. It should 1277 not be necessary to know ptime to decode RTP or vat audio, and it is 1278 intended as a recommendation for the encoding/packetisation of audio. 1280 6.5. maxptime (maximum packet time) 1282 Name: maxptime 1284 Value: maxptime-value 1286 Usage Level: media 1288 Charset Dependent: no 1290 Syntax: 1292 maxptime-value = packet-time 1294 Example: 1296 a=maxptime:20 1298 This gives the maximum amount of media that can be encapsulated in 1299 each packet, expressed as time in milliseconds. The time SHALL be 1300 calculated as the sum of the time the media present in the packet 1301 represents. For frame-based codecs, the time SHOULD be an integer 1302 multiple of the frame size. This attribute is probably only 1303 meaningful for audio data, but may be used with other media types if 1304 it makes sense. Note that this attribute was introduced after 1305 [RFC2327], and non-updated implementations will ignore this 1306 attribute. 1308 6.6. rtpmap 1310 Name: rtpmap 1312 Value: rtpmap-value 1314 Usage Level: media 1316 Charset Dependent: no 1318 Syntax: 1320 rtpmap-value = payload-type SP encoding-name 1321 "/" clock-rate [ "/" encoding-params ] 1322 payload-type = integer 1323 encoding-name = token 1324 ; To be matched in a case-insensitive way! 1325 ; RFC4855 seems to be the primary definition for this. 1326 ; It refers both to RFC2045 and RFC4288 for the definition. 1327 ; The definition in RFC2045 is messy, 1328 ; but equivalent to SDP . 1329 ; The definition in RFC4288 () is more 1330 ; restrictive and governs what can be registered. 1331 ; Since ultimately only registered names can be referenced, 1332 ; using here seems safe and easy. 1333 clock-rate = integer 1334 ; do we want to define a limited range for this? 1335 encoding-params = channels 1336 ; 4566 is vague about what this can be. RFC4855 seems to be 1337 ; the authoritative source, and only allows the 1338 ; value of the media subtype "channels" parameter - the 1339 ; number of audio channels. 1340 ; Does anyone think this can be used for something else??? 1341 ; (The implication that multiple parameters might be included 1342 ; seems a misdirection - additional parameters are 1343 ; to go into a=fmtp.) 1344 ; Does anyone have an example of other parameters 1345 ; using this field? 1346 channels = integer 1347 ; Is there any reason to make this less restrictive? 1349 This attribute maps from an RTP payload type number (as used in an 1350 "m=" line) to an encoding name denoting the payload format to be 1351 used. It also provides information on the clock rate and encoding 1352 parameters. Note that the payload type number is indicated in a 1353 7-bit field, limiting the values to incusively between 0 and 127. 1355 Although an RTP profile can make static assignments of payload type 1356 numbers to payload formats, it is more common for that assignment to 1357 be done dynamically using "a=rtpmap:" attributes. As an example of a 1358 static payload type, consider u-law PCM coded single-channel audio 1359 sampled at 8 kHz. This is completely defined in the RTP Audio/Video 1360 profile as payload type 0, so there is no need for an "a=rtpmap:" 1361 attribute, and the media for such a stream sent to UDP port 49232 can 1362 be specified as: 1364 m=audio 49232 RTP/AVP 0 1366 An example of a dynamic payload type is 16-bit linear encoded stereo 1367 audio sampled at 16 kHz. If we wish to use the dynamic RTP/AVP 1368 payload type 98 for this stream, additional information is required 1369 to decode it: 1371 m=audio 49232 RTP/AVP 98 1372 a=rtpmap:98 L16/16000/2 1374 Up to one rtpmap attribute can be defined for each media format 1375 specified. Thus, we might have the following: 1377 m=audio 49230 RTP/AVP 96 97 98 1378 a=rtpmap:96 L8/8000 1379 a=rtpmap:97 L16/8000 1380 a=rtpmap:98 L16/11025/2 1382 RTP profiles that specify the use of dynamic payload types MUST 1383 define the set of valid encoding names and/or a means to register 1384 encoding names if that profile is to be used with SDP. The "RTP/AVP" 1385 and "RTP/SAVP" profiles use media subtypes for encoding names, under 1386 the top-level media type denoted in the "m=" line. In the example 1387 above, the media types are "audio/l8" and "audio/l16". 1389 For audio streams, indicates the number of 1390 audio channels. This parameter is OPTIONAL and may be omitted if the 1391 number of channels is one, provided that no additional parameters are 1392 needed. 1394 For video streams, no encoding parameters are currently specified. 1396 Additional encoding parameters MAY be defined in the future, but 1397 codec-specific parameters SHOULD NOT be added. Parameters added to 1398 an "a=rtpmap:" attribute SHOULD only be those required for a session 1399 directory to make the choice of appropriate media to participate in a 1400 session. Codec-specific parameters should be added in other 1401 attributes (for example, "a=fmtp:"). 1403 Note: RTP audio formats typically do not include information about 1404 the number of samples per packet. If a non-default (as defined in 1405 the RTP Audio/Video Profile) packetisation is required, the "ptime" 1406 attribute is used as given above. 1408 6.7. Media Direction Attributes 1410 At most one of recvonly/sendrecv/sendonly/inactive MAY appear at 1411 session level, and at most one MAY appear in each media section. 1413 If any one of these appears in a media section then it applies for 1414 that media section. If none appear in a media section then the one 1415 from session level, if any, applies to that media section. 1417 If none of the media direction attributes is present at either 1418 session level or media level, "sendrecv" SHOULD be assumed as the 1419 default for sessions that are not of the multimedia conference type 1420 "broadcast" or "H332" (see below). 1422 Within the following SDP example, the "inactive" attribute applies to 1423 audio media and the "recvonly" attribute applies to video media. 1425 v=0 1426 o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1 1427 s=SDP Seminar 1428 i=A Seminar on the session description protocol 1429 u=http://www.example.com/seminars/sdp.pdf 1430 e=j.doe@example.com (Jane Doe) 1431 c=IN IP4 233.252.0.1/127 1432 t=2873397496 2873404696 1433 a=inactive 1434 m=audio 49170 RTP/AVP 0 1435 m=video 51372 RTP/AVP 99 1436 a=rtpmap:99 h263-1998/90000 1437 a=recvonly 1439 6.7.1. recvonly (receive-only) 1441 Name: recvonly 1443 Value: 1445 Usage Level: session, media 1447 Charset Dependent: no 1449 Example: 1451 a=recvonly 1453 This specifies that the tools should be started in receive-only mode 1454 where applicable. Note that recvonly applies to the media only, not 1455 to any associated control protocol (e.g., an RTP-based system in 1456 recvonly mode SHOULD still send RTCP packets). 1458 6.7.2. sendrecv (send-receive) 1460 Name: sendrecv 1462 Value: 1464 Usage Level: session, media 1466 Charset Dependent: no 1468 Example: 1470 a=sendrecv 1472 This specifies that the tools should be started in send and receive 1473 mode. This is necessary for interactive multimedia conferences with 1474 tools that default to receive-only mode. 1476 6.7.3. sendonly (send-only) 1478 Name: sendonly 1480 Value: 1482 Usage Level: session, media 1484 Charset Dependent: no 1486 Example: 1488 a=sendonly 1490 This specifies that the tools should be started in send-only mode. 1491 An example may be where a different unicast address is to be used for 1492 a traffic destination than for a traffic source. In such a case, two 1493 media descriptions may be used, one sendonly and one recvonly. Note 1494 that sendonly applies only to the media, and any associated control 1495 protocol (e.g., RTCP) SHOULD still be received and processed as 1496 normal. 1498 6.7.4. inactive 1500 Name: inactive 1502 Value: 1504 Usage Level: session, media 1506 Charset Dependent: no 1508 Example: 1510 a=inactive 1512 This specifies that the tools should be started in inactive mode. 1513 This is necessary for interactive multimedia conferences where users 1514 can put other users on hold. No media is sent over an inactive media 1515 stream. Note that an RTP-based system SHOULD still send RTCP, even 1516 if started inactive. 1518 6.8. orient (orientation) 1520 Name: orient 1522 Value: orient-value 1524 Usage Level: media 1526 Charset Dependent: no 1528 Syntax: 1530 orient-value = portrait / landscape / seascape 1531 portrait = %x70.6f.72.74.72.61.69.74 ; "portrait" 1532 landscape = %x6c.61.6e.64.73.63.61.70.65 ; "landscape" 1533 seascape = %x73.65.61.73.63.61.70.65 ; "seascape" 1534 ; Note: this assumes the intent was to be case-sensitive 1535 ; Does anything think this should be matched in a 1536 ; case-insensitive way??? 1538 Example: 1540 a=orient:portrait 1542 Normally this is only used for a whiteboard or presentation tool. It 1543 specifies the orientation of a the workspace on the screen. 1544 Permitted values are "portrait", "landscape", and "seascape" (upside- 1545 down landscape). 1547 6.9. type (conference type) 1548 Name: type 1550 Value: type-value 1552 Usage Level: session 1554 Charset Dependent: no 1556 Syntax: 1558 type-value = conference-type 1559 conference-type = broadcast / meeting / moderated / test / H332 1560 broadcast = %x62.72.6f.61.64.63.61.73.74 ; "broadcast" 1561 meeting = %x6d.65.65.74.69.6e.67 ; "meeting" 1562 moderated = %x6d.6f.64.65.72.61.74.65.64 ; "moderated" 1563 test = %x74.65.73.74 ; "test" 1564 H332 = %x48.33.33.32 ; "H332" 1565 ; NOTE: These names are case-sensitive. 1567 Example: 1569 a=type:moderated 1571 This specifies the type of the multimedia conference. Suggested 1572 values are "broadcast", "meeting", "moderated", "test", and "H332". 1573 "recvonly" should be the default for "type:broadcast" sessions, 1574 "type:meeting" should imply "sendrecv", and "type:moderated" should 1575 indicate the use of a floor control tool and that the media tools are 1576 started so as to mute new sites joining the multimedia conference. 1578 Specifying the attribute "type:H332" indicates that this loosely 1579 coupled session is part of an H.332 session as defined in the ITU 1580 H.332 specification [ITU.H332.1998]. Media tools should be started 1581 "recvonly". 1583 Specifying the attribute "type:test" is suggested as a hint that, 1584 unless explicitly requested otherwise, receivers can safely avoid 1585 displaying this session description to users. 1587 6.10. charset (character set) 1589 Name: charset 1591 Value: charset-value 1593 Usage Level: session 1594 Charset Dependent: no 1596 Syntax: 1598 charset-value = mime-charset 1599 mime-charset = 1*mime-charset-chars 1600 mime-charset-chars = ALPHA / DIGIT / 1601 "!" / "#" / "$" / "%" / "&" / 1602 "'" / "+" / "-" / "^" / "_" / 1603 "`" / "{" / "}" / "~" 1605 This specifies the character set to be used to display the session 1606 name and information data. By default, the ISO-10646 character set 1607 in UTF-8 encoding is used. If a more compact representation is 1608 required, other character sets may be used. For example, the ISO 1609 8859-1 is specified with the following SDP attribute: 1611 a=charset:ISO-8859-1 1613 The charset specified MUST be one of those registered in the IANA 1614 Character Sets registry (http://www.iana.org/assignments/character- 1615 sets), such as ISO-8859-1. The character set identifier is a US- 1616 ASCII string and MUST be compared against identifiers from the "Name" 1617 or "Preferred MIME Name" field of the registry using a case- 1618 insensitive comparison. If the identifier is not recognised or not 1619 supported, all strings that are affected by it SHOULD be regarded as 1620 octet strings. 1622 Note that a character set specified MUST still prohibit the use of 1623 bytes 0x00 (Nul), 0x0A (LF), and 0x0d (CR). Character sets requiring 1624 the use of these characters MUST define a quoting mechanism that 1625 prevents these bytes from appearing within text fields. 1627 6.11. sdplang (SDP language) 1629 Name: sdplang 1631 Value: sdplang-value 1633 Usage Level: session, media 1635 Charset Dependent: no 1637 Syntax: 1639 sdplang-value = Language-Tag (as defined in ) 1642 Example: 1644 a=sdplang:fr 1646 This can be a session-level attribute or a media-level attribute. 1647 Multiple sdplang attributes can be provided either at session or 1648 media level if the session description or media use multiple 1649 languages. 1651 As a session-level attribute, it specifies the language for the 1652 session description. As a media-level attribute, it specifies the 1653 language for any media-level SDP information field associated with 1654 that media, overriding any sdplang attributes specified at session- 1655 level. 1657 In general, sending session descriptions consisting of multiple 1658 languages is discouraged. Instead, multiple descriptions SHOULD be 1659 sent describing the session, one in each language. However, this is 1660 not possible with all transport mechanisms, and so multiple sdplang 1661 attributes are allowed although NOT RECOMMENDED. 1663 The "sdplang" attribute value must be a single [RFC5646] language tag 1664 in US-ASCII. An "sdplang" attribute SHOULD be specified when a 1665 session is distributed with sufficient scope to cross geographic 1666 boundaries, where the language of recipients cannot be assumed, or 1667 where the session is in a different language from the locally assumed 1668 norm. 1670 6.12. lang (language) 1672 Name: lang 1674 Value: lang-value 1676 Usage Level: session, media 1678 Charset Dependent: no 1680 Syntax: 1682 lang-value = Language-Tag (as defined in ) 1685 Example: 1687 a=lang:de 1689 Multiple lang attributes can be provided either at session or media 1690 level if the session or media use multiple languages, in which case 1691 the order of the attributes indicates the order of importance of the 1692 various languages in the session or media, from most important to 1693 least important. 1695 As a session-level attribute, it specifies the default language for 1696 the session being described. As a media-level attribute, it 1697 specifies the language for that media, overriding any session-level 1698 languages specified. 1700 The "lang" attribute value must be a single [RFC5646] language tag in 1701 US-ASCII. A "lang" attribute SHOULD be specified when a session is 1702 of sufficient scope to cross geographic boundaries where the language 1703 of recipients cannot be assumed, or where the session is in a 1704 different language from the locally assumed norm. 1706 6.13. framerate (frame rate) 1708 Name: framerate 1710 Value: framerate-value 1712 Usage Level: media 1714 Charset Dependent: no 1716 Syntax: 1718 framerate-value = positive-real-number 1719 positive-real-number = (integer / "0") [ "." integer ] 1720 ; Notes: 1721 ; - this permits a zero value. OK? 1722 ; - do we want to restrict the range or precision? 1724 Example: 1726 a=framerate:60 1728 This gives the maximum video frame rate in frames/sec. It is 1729 intended as a recommendation for the encoding of video data. Decimal 1730 representations of fractional values are allowed. It is defined only 1731 for video media. 1733 6.14. quality 1735 Name: quality 1737 Value: quality-value 1739 Usage Level: media 1741 Charset Dependent: no 1743 Syntax: 1745 quality-value = integer 1746 ; Do we want to restrict the range? 1747 ; The definition above limits the range to [0-10] 1748 ; *for video*, but seems to leave usage open for other media. 1750 Example: 1752 a=quality:10 1754 This gives a suggestion for the quality of the encoding as an integer 1755 value. The intention of the quality attribute for video is to 1756 specify a non-default trade-off between frame-rate and still-image 1757 quality. For video, the value is in the range 0 to 10, with the 1758 following suggested meaning: 1760 10 - the best still-image quality the compression scheme 1761 can give. 1762 5 - the default behaviour given no quality suggestion. 1763 0 - the worst still-image quality the codec designer 1764 thinks is still usable. 1766 6.15. fmtp (format parameters) 1768 Name: fmtp 1770 Value: fmtp-value 1772 Usage Level: media 1774 Charset Dependent: no 1775 Syntax: 1777 fmtp-value = fmt SP format-specific-params 1778 format-specific-params = byte-string 1779 ; Notes: 1780 ; - The format parameters are media type parameters and 1781 need to reflect their syntax. 1783 Example: 1785 a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600 1787 This attribute allows parameters that are specific to a particular 1788 format to be conveyed in a way that SDP does not have to understand 1789 them. The format must be one of the formats specified for the media. 1790 Format-specific parameters may be any set of parameters required to 1791 be conveyed by SDP and given unchanged to the media tool that will 1792 use this format. At most one instance of this attribute is allowed 1793 for each format. 1795 7. Security Considerations 1797 SDP is frequently used with the Session Initiation Protocol [RFC3261] 1798 using the offer/answer model [RFC3264] to agree on parameters for 1799 unicast sessions. When used in this manner, the security 1800 considerations of those protocols apply. 1802 SDP is a session description format that describes multimedia 1803 sessions. Entities receiving and acting upon an SDP message SHOULD 1804 be aware that a session description cannot be trusted unless it has 1805 been obtained by an authenticated transport protocol from a known and 1806 trusted source. Many different transport protocols may be used to 1807 distribute session descriptions, and the nature of the authentication 1808 will differ from transport to transport. For some transports, 1809 security features are often not deployed. In case a session 1810 description has not been obtained in a trusted manner, the endpoint 1811 SHOULD exercise care because, among other attacks, the media sessions 1812 received may not be the intended ones, the destination where media is 1813 sent to may not be the expected one, any of the parameters of the 1814 session may be incorrect, or the media security may be compromised. 1815 It is up to the endpoint to make a sensible decision taking into 1816 account the security risks of the application and the user 1817 preferences and may decide to ask the user whether or not to accept 1818 the session. 1820 One transport that can be used to distribute session descriptions is 1821 the Session Announcement Protocol (SAP). SAP provides both 1822 encryption and authentication mechanisms, but due to the nature of 1823 session announcements it is likely that there are many occasions 1824 where the originator of a session announcement cannot be 1825 authenticated because the originator is previously unknown to the 1826 receiver of the announcement and because no common public key 1827 infrastructure is available. 1829 On receiving a session description over an unauthenticated transport 1830 mechanism or from an untrusted party, software parsing the session 1831 should take a few precautions. Session descriptions contain 1832 information required to start software on the receiver's system. 1833 Software that parses a session description MUST NOT be able to start 1834 other software except that which is specifically configured as 1835 appropriate software to participate in multimedia sessions. It is 1836 normally considered inappropriate for software parsing a session 1837 description to start, on a user's system, software that is 1838 appropriate to participate in multimedia sessions, without the user 1839 first being informed that such software will be started and giving 1840 the user's consent. Thus, a session description arriving by session 1841 announcement, email, session invitation, or WWW page MUST NOT deliver 1842 the user into an interactive multimedia session unless the user has 1843 explicitly pre-authorised such action. As it is not always simple to 1844 tell whether or not a session is interactive, applications that are 1845 unsure should assume sessions are interactive. 1847 In this specification, there are no attributes that would allow the 1848 recipient of a session description to be informed to start multimedia 1849 tools in a mode where they default to transmitting. Under some 1850 circumstances it might be appropriate to define such attributes. If 1851 this is done, an application parsing a session description containing 1852 such attributes SHOULD either ignore them or inform the user that 1853 joining this session will result in the automatic transmission of 1854 multimedia data. The default behaviour for an unknown attribute is 1855 to ignore it. 1857 In certain environments, it has become common for intermediary 1858 systems to intercept and analyse session descriptions contained 1859 within other signalling protocols. This is done for a range of 1860 purposes, including but not limited to opening holes in firewalls to 1861 allow media streams to pass, or to mark, prioritize, or block traffic 1862 selectively. In some cases, such intermediary systems may modify the 1863 session description, for example, to have the contents of the session 1864 description match NAT bindings dynamically created. These behaviours 1865 are NOT RECOMMENDED unless the session description is conveyed in 1866 such a manner that allows the intermediary system to conduct proper 1867 checks to establish the authenticity of the session description, and 1868 the authority of its source to establish such communication sessions. 1869 SDP by itself does not include sufficient information to enable these 1870 checks: they depend on the encapsulating protocol (e.g., SIP or 1871 RTSP). 1873 Use of the "k=" field poses a significant security risk, since it 1874 conveys session encryption keys in the clear. SDP MUST NOT be used 1875 to convey key material, unless it can be guaranteed that the channel 1876 over which the SDP is delivered is both private and authenticated. 1877 Moreover, the "k=" line provides no way to indicate or negotiate 1878 cryptographic key algorithms. As it provides for only a single 1879 symmetric key, rather than separate keys for confidentiality and 1880 integrity, its utility is severely limited. The use of the "k=" line 1881 is NOT RECOMMENDED, as discussed in Section 5.12. 1883 8. IANA Considerations 1885 8.1. The "application/sdp" Media Type 1887 One media type registration from [RFC4566] is to be updated, as 1888 defined below. 1890 To: ietf-types@iana.org 1891 Subject: Registration of media type "application/sdp" 1893 Type name: application 1895 Subtype name: sdp 1897 Required parameters: None. 1899 Optional parameters: None. 1901 Encoding considerations: 1902 SDP files are primarily UTF-8 format text. The "a=charset:" 1903 attribute may be used to signal the presence of other character 1904 sets in certain parts of an SDP file (see Section 6 of RFC 1905 XXXX). Arbitrary binary content cannot be directly 1906 represented in SDP. 1908 Security considerations: 1909 See Section 7 of RFC XXXX. 1911 Interoperability considerations: 1912 See RFC XXXX. 1914 Published specification: 1915 See RFC XXXX. 1917 Applications which use this media type: 1918 Voice over IP, video teleconferencing, streaming media, instant 1919 messaging, among others. See also Section 3 of RFC XXXX. 1921 Additional information: 1923 Magic number(s): None. 1924 File extension(s): The extension ".sdp" is commonly used. 1925 Macintosh File Type Code(s): "sdp " 1927 Person & email address to contact for further information: 1928 IETF MMUSIC working group 1930 Intended usage: COMMON 1932 Author/Change controller: 1933 Authors of RFC XXXX 1934 IETF MMUSIC working group delegated from the IESG 1936 8.2. Registration of Parameters 1938 There are seven field names that may be registered with IANA. Using 1939 the terminology in the SDP specification Backus-Naur Form (BNF), they 1940 are "media", "proto", "fmt", "att-field", "bwtype", "nettype", and 1941 "addrtype". 1943 The contact address for all parameters registered below is: 1945 IETF MMUSIC working group 1947 8.2.1. Media Types ("media") 1949 The set of media types is intended to be small and SHOULD NOT be 1950 extended except under rare circumstances. The same rules should 1951 apply for media names as for top-level media types, and where 1952 possible the same name should be registered for SDP as for MIME. For 1953 media other than existing top-level media types, a Standards Track 1954 RFC MUST be produced for a new top-level media type to be registered, 1955 and the registration MUST provide good justification why no existing 1956 media name is appropriate (the "Standards Action" policy of 1957 [RFC5226]. 1959 This memo registers the media types "audio", "video", "text", 1960 "application", and "message". 1962 Note: The media types "control" and "data" were listed as valid in an 1963 early version of this specification (RFC 2327); however, their 1964 semantics were never fully specified and they are not widely used. 1965 These media types have been removed in this specification, although 1966 they still remain valid media type capabilities for a SIP user agent 1967 as defined in [RFC3840]. If these media types are considered useful 1968 in the future, a Standards Track RFC MUST be produced to document 1969 their use. Until that is done, applications SHOULD NOT use these 1970 types and SHOULD NOT declare support for them in SIP capabilities 1971 declarations (even though they exist in the registry created by 1972 [RFC3840]). 1974 8.2.2. Transport Protocols ("proto") 1976 The "proto" field describes the transport protocol used. This SHOULD 1977 reference a standards-track protocol RFC. This memo registers three 1978 values: "RTP/AVP" is a reference to [RFC3550] used under the RTP 1979 Profile for Audio and Video Conferences with Minimal Control 1980 [RFC3551] running over UDP/IP, "RTP/SAVP" is a reference to the 1981 Secure Real-time Transport Protocol [RFC3711], and "udp" indicates an 1982 unspecified protocol over UDP. 1984 If other RTP profiles are defined in the future, their "proto" name 1985 SHOULD be specified in the same manner. For example, an RTP profile 1986 whose short name is "XYZ" would be denoted by a "proto" field of "RTP 1987 /XYZ". 1989 New transport protocols SHOULD be registered with IANA. 1990 Registrations MUST reference an RFC describing the protocol. Such an 1991 RFC MAY be Experimental or Informational, although it is preferable 1992 that it be Standards Track. Registrations MUST also define the rules 1993 by which their "fmt" namespace is managed (see below). 1995 8.2.3. Media Formats ("fmt") 1997 Each transport protocol, defined by the "proto" field, has an 1998 associated "fmt" namespace that describes the media formats that may 1999 be conveyed by that protocol. Formats cover all the possible 2000 encodings that might want to be transported in a multimedia session. 2002 RTP payload formats under the "RTP/AVP" and "RTP/SAVP" profiles MUST 2003 use the payload type number as their "fmt" value. If the payload 2004 type number is dynamically assigned by this session description, an 2005 additional "rtpmap" attribute MUST be included to specify the format 2006 name and parameters as defined by the media type registration for the 2007 payload format. It is RECOMMENDED that other RTP profiles that are 2008 registered (in combination with RTP) as SDP transport protocols 2009 specify the same rules for the "fmt" namespace. 2011 For the "udp" protocol, new formats SHOULD be registered. Use of an 2012 existing media subtype for the format is encouraged. If no media 2013 subtype exists, it is RECOMMENDED that a suitable one be registered 2014 through the IETF process [RFC6838] by production of, or reference to, 2015 a standards-track RFC that defines the transport protocol for the 2016 format. 2018 For other protocols, formats MAY be registered according to the rules 2019 of the associated "proto" specification. 2021 Registrations of new formats MUST specify which transport protocols 2022 they apply to. 2024 8.2.4. Attribute Names ("att-field") 2026 Attribute field names ("att-field") MUST be registered with IANA and 2027 documented, because of noticeable issues due to conflicting 2028 attributes under the same name. Unknown attributes in SDP are simply 2029 ignored, but conflicting ones that fragment the protocol are a 2030 serious problem. 2032 New attribute registrations are accepted according to the 2033 "Specification Required" policy of [RFC5226], provided that the 2034 specification includes the following information: 2036 o Contact name, email address, and telephone number. 2038 o Attribute name (as it will appear in SDP). This MUST conform to 2039 the definition of . 2041 o Usage level of the attribute. (One or more of: session, media). 2043 o Whether the attribute value is subject to the charset attribute. 2045 o An ABNF definition of the attribute value rule. The rule MUST NOT 2046 match anything that is not also matched by . The rule 2047 name SHOULD [MUST?] NOT be defined as an Incremental Alternative 2048 to . 2050 o An explanation of the purpose and usage of the attribute. 2052 o A specification of appropriate attribute values for this attribute 2053 (If not included in syntax). 2055 The above is the minimum that IANA will accept. Attributes that are 2056 expected to see widespread use and interoperability SHOULD be 2057 documented with a standards-track RFC that specifies the attribute 2058 more precisely. 2060 Submitters of registrations should ensure that the specification is 2061 in the spirit of SDP attributes, most notably that the attribute is 2062 platform independent in the sense that it makes no implicit 2063 assumptions about operating systems and does not name specific pieces 2064 of software in a manner that might inhibit interoperability. 2066 Submitters of registrations should also carefully choose the 2067 attribute usage level. They should not choose only session-level 2068 when the attribute can have different values when media is 2069 disaggregated, i.e., when each m= section has its own IP address on a 2070 different endpoint. In that case the attribute type chosen should be 2071 "session, media". 2073 IANA has registered the following initial set of attribute names 2074 ("att-field" values), with definitions as in Section 6 of this memo 2075 (these definitions replace those in [RFC4566]): 2077 [ED: Do we need the following list? I propose that it be dropped 2078 in favor of simply referencing Section 6.] 2080 Name | Usage Level | Dependent on charset? 2081 ----------+-------------------------+---------------------- 2082 cat | Session | No 2083 keywds | Session | Yes 2084 tool | Session | No 2085 ptime | Media | No 2086 maxptime | Media | No 2087 rtpmap | Media | No 2088 recvonly | Session, Media | No 2089 sendrecv | Session, Media | No 2090 sendonly | Session, Media | No 2091 inactive | Session, Media | No 2092 orient | Media | No 2093 type | Session | No 2094 charset | Session | No 2095 sdplang | Session, Media | No 2096 lang | Session, Media | No 2097 framerate | Media | No 2098 quality | Media | No 2099 fmtp | Media | No 2101 8.2.5. Bandwidth Specifiers ("bwtype") 2103 A proliferation of bandwidth specifiers is strongly discouraged. 2105 New bandwidth specifiers ("bwtype" fields) MUST be registered with 2106 IANA. The submission MUST reference a standards-track RFC specifying 2107 the semantics of the bandwidth specifier precisely, and indicating 2108 when it should be used, and why the existing registered bandwidth 2109 specifiers do not suffice. 2111 IANA has registered the bandwidth specifiers "CT" and "AS" with 2112 definitions as in Section 5.8 of this memo (these definitions update 2113 those in [RFC4566]). 2115 8.2.6. Network Types ("nettype") 2117 New network types (the "nettype" field) may be registered with IANA 2118 if SDP needs to be used in the context of non-Internet environments. 2119 Although these are not normally the preserve of IANA, there may be 2120 circumstances when an Internet application needs to interoperate with 2121 a non-Internet application, such as when gatewaying an Internet 2122 telephone call into the Public Switched Telephone Network (PSTN). 2123 The number of network types should be small and should be rarely 2124 extended. A new network type cannot be registered without 2125 registering at least one address type to be used with that network 2126 type. A new network type registration MUST reference an RFC that 2127 gives details of the network type and address type and specifies how 2128 and when they would be used. 2130 IANA has registered the network type "IN" to represent the Internet, 2131 with definition as in Sections 5.2 and 5.7 of this memo (these 2132 definitions update those in [RFC4566]). 2134 8.2.7. Address Types ("addrtype") 2136 New address types ("addrtype") may be registered with IANA. An 2137 address type is only meaningful in the context of a network type, and 2138 any registration of an address type MUST specify a registered network 2139 type or be submitted along with a network type registration. A new 2140 address type registration MUST reference an RFC giving details of the 2141 syntax of the address type. Address types are not expected to be 2142 registered frequently. 2144 IANA has registered the address types "IP4" and "IP6" with 2145 definitions as in Sections 5.2 and 5.7 of this memo (these 2146 definitions update those in [RFC4566]). 2148 8.2.8. Registration Procedure 2150 In the RFC documentation that registers SDP "media", "proto", "fmt", 2151 "bwtype", "nettype", and "addrtype" fields, the authors MUST include 2152 the following information for IANA to place in the appropriate 2153 registry: 2155 o contact name, email address, and telephone number 2157 o name being registered (as it will appear in SDP) 2159 o long-form name in English 2161 o type of name ("media", "proto", "fmt", "bwtype", "nettype", or 2162 "addrtype") 2164 o a one-paragraph explanation of the purpose of the registered name 2166 o a reference to the specification for the registered name (this 2167 will typically be an RFC number) 2169 IANA may refer any registration to the IESG for review, and may 2170 request revisions to be made before a registration will be made. 2172 8.3. Encryption Key Access Methods 2174 The IANA previously maintained a table of SDP encryption key access 2175 method ("enckey") names. This table is obsolete, since the "k=" line 2176 is not extensible. New registrations MUST NOT be accepted. 2178 9. SDP Grammar 2180 This section provides an Augmented BNF grammar for SDP. ABNF is 2181 defined in [RFC5234]. 2183 ; SDP Syntax 2184 session-description = proto-version 2185 origin-field 2186 session-name-field 2187 information-field 2188 uri-field 2189 email-fields 2190 phone-fields 2191 connection-field 2192 bandwidth-fields 2193 time-fields 2194 key-field 2195 attribute-fields 2196 media-descriptions 2198 proto-version = %x76 "=" 1*DIGIT CRLF 2199 ;this memo describes version 0 2201 origin-field = %x6f "=" username SP sess-id SP sess-version SP 2202 nettype SP addrtype SP unicast-address CRLF 2204 session-name-field = %x73 "=" text CRLF 2206 information-field = [%x69 "=" text CRLF] 2208 uri-field = [%x75 "=" uri CRLF] 2210 email-fields = *(%x65 "=" email-address CRLF) 2212 phone-fields = *(%x70 "=" phone-number CRLF) 2214 connection-field = [%x63 "=" nettype SP addrtype SP 2215 connection-address CRLF] 2216 ;a connection field must be present 2217 ;in every media description or at the 2218 ;session-level 2220 bandwidth-fields = *(%x62 "=" bwtype ":" bandwidth CRLF) 2222 time-fields = 1*( %x74 "=" start-time SP stop-time 2223 *(CRLF repeat-fields) CRLF) 2224 [zone-adjustments CRLF] 2226 repeat-fields = %x72 "=" repeat-interval SP typed-time 2227 1*(SP typed-time) 2229 zone-adjustments = %x7a "=" time SP ["-"] typed-time 2230 *(SP time SP ["-"] typed-time) 2232 key-field = [%x6b "=" key-type CRLF] 2234 attribute-fields = *(%x61 "=" attribute CRLF) 2236 media-descriptions = *( media-field 2237 information-field 2238 *connection-field 2239 bandwidth-fields 2240 key-field 2241 attribute-fields ) 2243 media-field = %x6d "=" media SP port ["/" integer] 2244 SP proto 1*(SP fmt) CRLF 2246 ; sub-rules of 'o=' 2247 username = non-ws-string 2248 ;pretty wide definition, but doesn't 2249 ;include space 2251 sess-id = 1*DIGIT 2252 ;should be unique for this username/host 2254 sess-version = 1*DIGIT 2256 nettype = token 2257 ;typically "IN" 2259 addrtype = token 2260 ;typically "IP4" or "IP6" 2262 ; sub-rules of 'u=' 2263 uri = URI-reference 2264 ; see RFC 3986 2266 ; sub-rules of 'e=', see RFC 5322 for definitions 2267 email-address = address-and-comment / dispname-and-address 2268 / addr-spec 2269 address-and-comment = addr-spec 1*SP "(" 1*email-safe ")" 2270 dispname-and-address = 1*email-safe 1*SP "<" addr-spec ">" 2272 ; sub-rules of 'p=' 2273 phone-number = phone *SP "(" 1*email-safe ")" / 2274 1*email-safe "<" phone ">" / 2275 phone 2277 phone = ["+"] DIGIT 1*(SP / "-" / DIGIT) 2279 ; sub-rules of 'c=' 2280 connection-address = multicast-address / unicast-address 2282 ; sub-rules of 'b=' 2283 bwtype = token 2285 bandwidth = 1*DIGIT 2287 ; sub-rules of 't=' 2288 start-time = time / "0" 2290 stop-time = time / "0" 2292 time = POS-DIGIT 9*DIGIT 2293 ; Decimal representation of NTP time in 2294 ; seconds since 1900. The representation 2295 ; of NTP time is an unbounded length field 2296 ; containing at least 10 digits. Unlike the 2297 ; 64-bit representation used elsewhere, time 2298 ; in SDP does not wrap in the year 2036. 2300 ; sub-rules of 'r=' and 'z=' 2301 repeat-interval = POS-DIGIT *DIGIT [fixed-len-time-unit] 2303 typed-time = 1*DIGIT [fixed-len-time-unit] 2305 fixed-len-time-unit = %x64 / %x68 / %x6d / %x73 2307 ; sub-rules of 'k=' 2308 key-type = %x70 %x72 %x6f %x6d %x70 %x74 / ; "prompt" 2309 %x63 %x6c %x65 %x61 %x72 ":" text / ; "clear:" 2310 %x62 %x61 %x73 %x65 "64:" base64 / ; "base64:" 2311 %x75 %x72 %x69 ":" uri ; "uri:" 2313 base64 = *base64-unit [base64-pad] 2314 base64-unit = 4base64-char 2315 base64-pad = 2base64-char "==" / 3base64-char "=" 2316 base64-char = ALPHA / DIGIT / "+" / "/" 2318 ; sub-rules of 'a=' 2319 attribute = (att-field ":" att-value) / att-field 2321 att-field = token 2323 att-value = byte-string 2325 ; sub-rules of 'm=' 2326 media = token 2327 ;typically "audio", "video", "text", or 2328 ;"application" 2330 fmt = token 2331 ;typically an RTP payload type for audio 2332 ;and video media 2334 proto = token *("/" token) 2335 ;typically "RTP/AVP" or "udp" 2337 port = 1*DIGIT 2339 ; generic sub-rules: addressing 2340 unicast-address = IP4-address / IP6-address / FQDN / extn-addr 2342 multicast-address = IP4-multicast / IP6-multicast / FQDN 2343 / extn-addr 2345 IP4-multicast = m1 3( "." decimal-uchar ) 2346 "/" ttl [ "/" integer ] 2347 ; IP4 multicast addresses may be in the 2348 ; range 224.0.0.0 to 239.255.255.255 2350 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / 2351 ("23" DIGIT ) 2353 IP6-multicast = IP6-address [ "/" integer ] 2354 ; IP6 address starting with FF 2356 ttl = (POS-DIGIT *2DIGIT) / "0" 2358 FQDN = 4*(alpha-numeric / "-" / ".") 2359 ; fully qualified domain name as specified 2360 ; in RFC 1035 (and updates) 2362 IP4-address = b1 3("." decimal-uchar) 2364 b1 = decimal-uchar 2365 ; less than "224" 2367 IP6-address = 6( h16 ":" ) ls32 2368 / "::" 5( h16 ":" ) ls32 2369 / [ h16 ] "::" 4( h16 ":" ) ls32 2370 / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32 2371 / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32 2372 / [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32 2373 / [ *4( h16 ":" ) h16 ] "::" ls32 2374 / [ *5( h16 ":" ) h16 ] "::" h16 2375 / [ *6( h16 ":" ) h16 ] "::" 2377 h16 = 1*4HEXDIG 2379 ls32 = ( h16 ":" h16 ) / IP4-address 2381 ; Generic for other address families 2382 extn-addr = non-ws-string 2384 ; generic sub-rules: datatypes 2385 text = byte-string 2386 ;default is to interpret this as UTF8 text. 2387 ;ISO 8859-1 requires "a=charset:ISO-8859-1" 2388 ;session-level attribute to be used 2390 byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF) 2391 ;any byte except NUL, CR, or LF 2393 non-ws-string = 1*(VCHAR/%x80-FF) 2394 ;string of visible characters 2396 token-char = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39 2397 / %x41-5A / %x5E-7E 2399 token = 1*(token-char) 2401 email-safe = %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF 2402 ;any byte except NUL, CR, LF, or the quoting 2403 ;characters ()<> 2405 integer = POS-DIGIT *DIGIT 2407 ; generic sub-rules: primitives 2408 alpha-numeric = ALPHA / DIGIT 2410 POS-DIGIT = %x31-39 ; 1 - 9 2412 decimal-uchar = DIGIT 2413 / POS-DIGIT DIGIT 2414 / ("1" 2*(DIGIT)) 2415 / ("2" ("0"/"1"/"2"/"3"/"4") DIGIT) 2416 / ("2" "5" ("0"/"1"/"2"/"3"/"4"/"5")) 2418 ; external references: 2419 ; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 5234 2420 ; URI-reference: from RFC 3986 2421 ; addr-spec: from RFC 5322 2423 10. Summary of Changes from RFC 4566 2425 The ABNF rule for IP6-address has been corrected. As a result, the 2426 ABNF rule for IP6-multicast has changed, and the (now unused) rules 2427 for hexpart, hexseq, and hex4 have been removed. 2429 IP4 unicast and multicast addresses in the example SDP descriptions 2430 have been revised per RFCs 5735 and 5771. 2432 Text in Section 5.2 has been revised to clarify the use of local 2433 addresses in case of ICE-like SDP extensions. 2435 Normative and informative references have been updated. 2437 The text regarding the session vs. media-level attribute usage has 2438 been clarified. 2440 The case-insensitivity rules from RFC 4855 have been included in this 2441 document. 2443 11. Acknowledgements 2445 Many people in the IETF Multiparty Multimedia Session Control 2446 (MMUSIC) working group have made comments and suggestions 2447 contributing to this document. 2449 12. References 2451 12.1. Normative References 2453 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 2454 STD 13, RFC 1034, November 1987. 2456 [RFC1035] Mockapetris, P., "Domain names - implementation and 2457 specification", STD 13, RFC 1035, November 1987. 2459 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 2460 Requirement Levels", BCP 14, RFC 2119, March 1997. 2462 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 2463 Specifications: ABNF", STD 68, RFC 5234, January 2008. 2465 [RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 2466 10646", STD 63, RFC 3629, November 2003. 2468 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 2469 Resource Identifier (URI): Generic Syntax", STD 66, RFC 2470 3986, January 2005. 2472 [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an 2473 IANA Considerations Section in RFCs", BCP 26, RFC 5226, 2474 May 2008. 2476 [RFC5646] Phillips, A. and M. Davis, "Tags for Identifying 2477 Languages", BCP 47, RFC 5646, September 2009. 2479 [RFC5890] Klensin, J., "Internationalized Domain Names for 2480 Applications (IDNA): Definitions and Document Framework", 2481 RFC 5890, August 2010. 2483 [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data 2484 Encodings", RFC 4648, October 2006. 2486 [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session 2487 Description Protocol", RFC 4566, July 2006. 2489 [I-D.ietf-avtext-rtp-grouping-taxonomy] 2490 Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro, 2491 "A Taxonomy of Grouping Semantics and Mechanisms for Real- 2492 Time Transport Protocol (RTP) Sources", draft-ietf-avtext- 2493 rtp-grouping-taxonomy-02 (work in progress), June 2014. 2495 12.2. Informative References 2497 [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description 2498 Protocol", RFC 2327, April 1998. 2500 [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network 2501 Time Protocol Version 4: Protocol and Algorithms 2502 Specification", RFC 5905, June 2010. 2504 [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session 2505 Announcement Protocol", RFC 2974, October 2000. 2507 [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, 2508 A., Peterson, J., Sparks, R., Handley, M., and E. 2509 Schooler, "SIP: Session Initiation Protocol", RFC 3261, 2510 June 2002. 2512 [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time 2513 Streaming Protocol (RTSP)", RFC 2326, April 1998. 2515 [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model 2516 with Session Description Protocol (SDP)", RFC 3264, June 2517 2002. 2519 [RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description 2520 Protocol (SDP) Grouping Framework", RFC 5888, June 2010. 2522 [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. 2523 Jacobson, "RTP: A Transport Protocol for Real-Time 2524 Applications", STD 64, RFC 3550, July 2003. 2526 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 2527 Video Conferences with Minimal Control", STD 65, RFC 3551, 2528 July 2003. 2530 [RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth 2531 Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 2532 3556, July 2003. 2534 [RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute 2535 in Session Description Protocol (SDP)", RFC 3605, October 2536 2003. 2538 [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. 2539 Norrman, "The Secure Real-time Transport Protocol (SRTP)", 2540 RFC 3711, March 2004. 2542 [RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, 2543 "Indicating User Agent Capabilities in the Session 2544 Initiation Protocol (SIP)", RFC 3840, August 2004. 2546 [RFC3890] Westerlund, M., "A Transport Independent Bandwidth 2547 Modifier for the Session Description Protocol (SDP)", RFC 2548 3890, September 2004. 2550 [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment 2551 (ICE): A Protocol for Network Address Translator (NAT) 2552 Traversal for Offer/Answer Protocols", RFC 5245, April 2553 2010. 2555 [RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B.B., and A.B. 2556 Roach, "TCP Candidates with Interactive Connectivity 2557 Establishment (ICE)", RFC 6544, March 2012. 2559 [ITU.H332.1998] 2560 International Telecommunication Union, "H.323 extended for 2561 loosely coupled conferences", ITU Recommendation H.332, 2562 September 1998. 2564 [RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E. 2565 Carrara, "Key Management Extensions for Session 2566 Description Protocol (SDP) and Real Time Streaming 2567 Protocol (RTSP)", RFC 4567, July 2006. 2569 [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session 2570 Description Protocol (SDP) Security Descriptions for Media 2571 Streams", RFC 4568, July 2006. 2573 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 2574 October 2008. 2576 [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type 2577 Specifications and Registration Procedures", BCP 13, RFC 2578 6838, January 2013. 2580 [RFC4855] Casner, S., "Media Type Registration of RTP Payload 2581 Formats", RFC 4855, February 2007. 2583 [RFC2365] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, 2584 RFC 2365, July 1998. 2586 Authors' Addresses 2588 Mark Handley 2589 University College London 2590 Department of Computer Science 2591 London WC1E 6BT 2592 UK 2594 EMail: M.Handley@cs.ucl.ac.uk 2596 Van Jacobson 2597 PARC 2598 3333 Coyote Hill Road 2599 Palo Alto, CA 94304 2600 USA 2602 EMail: van@parc.com 2604 Colin Perkins 2605 University of Glasgow 2606 School of Computing Science 2607 University of Glasgow 2608 Glasgow G12 8QQ 2609 UK 2611 EMail: csp@csperkins.org 2613 Ali Begen 2614 Cisco 2615 181 Bay Street 2616 Toronto, ON M5J 2T3 2617 Canada 2619 EMail: abegen@cisco.com