1 2 3 4 5 6 7Network Working Group M. Handley 8Request for Comments: 2327 V. Jacobson 9Category: Standards Track ISI/LBNL 10 April 1998 11 12 13 SDP: Session Description Protocol 14 15Status of this Memo 16 17 This document specifies an Internet standards track protocol for the 18 Internet community, and requests discussion and suggestions for 19 improvements. Please refer to the current edition of the "Internet 20 Official Protocol Standards" (STD 1) for the standardization state 21 and status of this protocol. Distribution of this memo is unlimited. 22 23Copyright Notice 24 25 Copyright (C) The Internet Society (1998). All Rights Reserved. 26 27Abstract 28 29 This document defines the Session Description Protocol, SDP. SDP is 30 intended for describing multimedia sessions for the purposes of 31 session announcement, session invitation, and other forms of 32 multimedia session initiation. 33 34 This document is a product of the Multiparty Multimedia Session 35 Control (MMUSIC) working group of the Internet Engineering Task 36 Force. Comments are solicited and should be addressed to the working 37 group's mailing list at confctrl@isi.edu and/or the authors. 38 391. Introduction 40 41 On the Internet multicast backbone (Mbone), a session directory tool 42 is used to advertise multimedia conferences and communicate the 43 conference addresses and conference tool-specific information 44 necessary for participation. This document defines a session 45 description protocol for this purpose, and for general real-time 46 multimedia session description purposes. This memo does not describe 47 multicast address allocation or the distribution of SDP messages in 48 detail. These are described in accompanying memos. SDP is not 49 intended for negotiation of media encodings. 50 51 52 53 54 55 56 57 58Handley & Jacobson Standards Track [Page 1] 59 60RFC 2327 SDP April 1998 61 62 632. Background 64 65 The Mbone is the part of the internet that supports IP multicast, and 66 thus permits efficient many-to-many communication. It is used 67 extensively for multimedia conferencing. Such conferences usually 68 have the property that tight coordination of conference membership is 69 not necessary; to receive a conference, a user at an Mbone site only 70 has to know the conference's multicast group address and the UDP 71 ports for the conference data streams. 72 73 Session directories assist the advertisement of conference sessions 74 and communicate the relevant conference setup information to 75 prospective participants. SDP is designed to convey such information 76 to recipients. SDP is purely a format for session description - it 77 does not incorporate a transport protocol, and is intended to use 78 different transport protocols as appropriate including the Session 79 Announcement Protocol [4], Session Initiation Protocol [11], Real- 80 Time Streaming Protocol [12], electronic mail using the MIME 81 extensions, and the Hypertext Transport Protocol. 82 83 SDP is intended to be general purpose so that it can be used for a 84 wider range of network environments and applications than just 85 multicast session directories. However, it is not intended to 86 support negotiation of session content or media encodings - this is 87 viewed as outside the scope of session description. 88 893. Glossary of Terms 90 91 The following terms are used in this document, and have specific 92 meaning within the context of this document. 93 94 Conference 95 A multimedia conference is a set of two or more communicating users 96 along with the software they are using to communicate. 97 98 Session 99 A multimedia session is a set of multimedia senders and receivers 100 and the data streams flowing from senders to receivers. A 101 multimedia conference is an example of a multimedia session. 102 103 Session Advertisement 104 See session announcement. 105 106 Session Announcement 107 A session announcement is a mechanism by which a session 108 description is conveyed to users in a proactive fashion, i.e., the 109 session description was not explicitly requested by the user. 110 111 112 113 114Handley & Jacobson Standards Track [Page 2] 115 116RFC 2327 SDP April 1998 117 118 119 Session Description 120 A well defined format for conveying sufficient information to 121 discover and participate in a multimedia session. 122 1233.1. Terminology 124 125 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 126 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 127 document are to be interpreted as described in RFC 2119. 128 1294. SDP Usage 130 1314.1. Multicast Announcements 132 133 SDP is a session description protocol for multimedia sessions. A 134 common mode of usage is for a client to announce a conference session 135 by periodically multicasting an announcement packet to a well known 136 multicast address and port using the Session Announcement Protocol 137 (SAP). 138 139 SAP packets are UDP packets with the following format: 140 141 |--------------------| 142 | SAP header | 143 |--------------------| 144 | text payload | 145 |////////// 146 147 148 The header is the Session Announcement Protocol header. SAP is 149 described in more detail in a companion memo [4] 150 151 The text payload is an SDP session description, as described in this 152 memo. The text payload should be no greater than 1 Kbyte in length. 153 If announced by SAP, only one session announcement is permitted in a 154 single packet. 155 1564.2. Email and WWW Announcements 157 158 Alternative means of conveying session descriptions include 159 electronic mail and the World Wide Web. For both email and WWW 160 distribution, the use of the MIME content type "application/sdp" 161 should be used. This enables the automatic launching of applications 162 for participation in the session from the WWW client or mail reader 163 in a standard manner. 164 165 166 167 168 169 170Handley & Jacobson Standards Track [Page 3] 171 172RFC 2327 SDP April 1998 173 174 175 Note that announcements of multicast sessions made only via email or 176 the World Wide Web (WWW) do not have the property that the receiver 177 of a session announcement can necessarily receive the session because 178 the multicast sessions may be restricted in scope, and access to the 179 WWW server or reception of email is possible outside this scope. SAP 180 announcements do not suffer from this mismatch. 181 1825. Requirements and Recommendations 183 184 The purpose of SDP is to convey information about media streams in 185 multimedia sessions to allow the recipients of a session description 186 to participate in the session. SDP is primarily intended for use in 187 an internetwork, although it is sufficiently general that it can 188 describe conferences in other network environments. 189 190 A multimedia session, for these purposes, is defined as a set of 191 media streams that exist for some duration of time. Media streams 192 can be many-to-many. The times during which the session is active 193 need not be continuous. 194 195 Thus far, multicast based sessions on the Internet have differed from 196 many other forms of conferencing in that anyone receiving the traffic 197 can join the session (unless the session traffic is encrypted). In 198 such an environment, SDP serves two primary purposes. It is a means 199 to communicate the existence of a session, and is a means to convey 200 sufficient information to enable joining and participating in the 201 session. In a unicast environment, only the latter purpose is likely 202 to be relevant. 203 204 Thus SDP includes: 205 206 o Session name and purpose 207 208 o Time(s) the session is active 209 210 o The media comprising the session 211 212 o Information to receive those media (addresses, ports, formats and 213 so on) 214 215 As resources necessary to participate in a session may be limited, 216 some additional information may also be desirable: 217 218 o Information about the bandwidth to be used by the conference 219 220 o Contact information for the person responsible for the session 221 222 223 224 225 226Handley & Jacobson Standards Track [Page 4] 227 228RFC 2327 SDP April 1998 229 230 231 In general, SDP must convey sufficient information to be able to join 232 a session (with the possible exception of encryption keys) and to 233 announce the resources to be used to non-participants that may need 234 to know. 235 2365.1. Media Information 237 238 SDP includes: 239 240 o The type of media (video, audio, etc) 241 242 o The transport protocol (RTP/UDP/IP, H.320, etc) 243 244 o The format of the media (H.261 video, MPEG video, etc) 245 246 For an IP multicast session, the following are also conveyed: 247 248 o Multicast address for media 249 250 o Transport Port for media 251 252 This address and port are the destination address and destination 253 port of the multicast stream, whether being sent, received, or both. 254 255 For an IP unicast session, the following are conveyed: 256 257 o Remote address for media 258 259 o Transport port for contact address 260 261 The semantics of this address and port depend on the media and 262 transport protocol defined. By default, this is the remote address 263 and remote port to which data is sent, and the remote address and 264 local port on which to receive data. However, some media may define 265 to use these to establish a control channel for the actual media 266 flow. 267 2685.2. Timing Information 269 270 Sessions may either be bounded or unbounded in time. Whether or not 271 they are bounded, they may be only active at specific times. 272 273 SDP can convey: 274 275 o An arbitrary list of start and stop times bounding the session 276 277 o For each bound, repeat times such as "every Wednesday at 10am for 278 one hour" 279 280 281 282Handley & Jacobson Standards Track [Page 5] 283 284RFC 2327 SDP April 1998 285 286 287 This timing information is globally consistent, irrespective of local 288 time zone or daylight saving time. 289 2905.3. Private Sessions 291 292 It is possible to create both public sessions and private sessions. 293 Private sessions will typically be conveyed by encrypting the session 294 description to distribute it. The details of how encryption is 295 performed are dependent on the mechanism used to convey SDP - see [4] 296 for how this is done for session announcements. 297 298 If a session announcement is private it is possible to use that 299 private announcement to convey encryption keys necessary to decode 300 each of the media in a conference, including enough information to 301 know which encryption scheme is used for each media. 302 3035.4. Obtaining Further Information about a Session 304 305 A session description should convey enough information to decide 306 whether or not to participate in a session. SDP may include 307 additional pointers in the form of Universal Resources Identifiers 308 (URIs) for more information about the session. 309 3105.5. Categorisation 311 312 When many session descriptions are being distributed by SAP or any 313 other advertisement mechanism, it may be desirable to filter 314 announcements that are of interest from those that are not. SDP 315 supports a categorisation mechanism for sessions that is capable of 316 being automated. 317 3185.6. Internationalization 319 320 The SDP specification recommends the use of the ISO 10646 character 321 sets in the UTF-8 encoding (RFC 2044) to allow many different 322 languages to be represented. However, to assist in compact 323 representations, SDP also allows other character sets such as ISO 324 8859-1 to be used when desired. Internationalization only applies to 325 free-text fields (session name and background information), and not 326 to SDP as a whole. 327 3286. SDP Specification 329 330 SDP session descriptions are entirely textual using the ISO 10646 331 character set in UTF-8 encoding. SDP field names and attributes names 332 use only the US-ASCII subset of UTF-8, but textual fields and 333 attribute values may use the full ISO 10646 character set. The 334 textual form, as opposed to a binary encoding such as ASN/1 or XDR, 335 336 337 338Handley & Jacobson Standards Track [Page 6] 339 340RFC 2327 SDP April 1998 341 342 343 was chosen to enhance portability, to enable a variety of transports 344 to be used (e.g, session description in a MIME email message) and to 345 allow flexible, text-based toolkits (e.g., Tcl/Tk ) to be used to 346 generate and to process session descriptions. However, since the 347 total bandwidth allocated to all SAP announcements is strictly 348 limited, the encoding is deliberately compact. Also, since 349 announcements may be transported via very unreliable means (e.g., 350 email) or damaged by an intermediate caching server, the encoding was 351 designed with strict order and formatting rules so that most errors 352 would result in malformed announcements which could be detected 353 easily and discarded. This also allows rapid discarding of encrypted 354 announcements for which a receiver does not have the correct key. 355 356 An SDP session description consists of a number of lines of text of 357 the form <type>=<value> <type> is always exactly one character and is 358 case-significant. <value> is a structured text string whose format 359 depends on <type>. It also will be case-significant unless a 360 specific field defines otherwise. Whitespace is not permitted either 361 side of the `=' sign. In general <value> is either a number of fields 362 delimited by a single space character or a free format string. 363 364 A session description consists of a session-level description 365 (details that apply to the whole session and all media streams) and 366 optionally several media-level descriptions (details that apply onto 367 to a single media stream). 368 369 An announcement consists of a session-level section followed by zero 370 or more media-level sections. The session-level part starts with a 371 `v=' line and continues to the first media-level section. The media 372 description starts with an `m=' line and continues to the next media 373 description or end of the whole session description. In general, 374 session-level values are the default for all media unless overridden 375 by an equivalent media-level value. 376 377 When SDP is conveyed by SAP, only one session description is allowed 378 per packet. When SDP is conveyed by other means, many SDP session 379 descriptions may be concatenated together (the `v=' line indicating 380 the start of a session description terminates the previous 381 description). Some lines in each description are required and some 382 are optional but all must appear in exactly the order given here (the 383 fixed order greatly enhances error detection and allows for a simple 384 parser). Optional items are marked with a `*'. 385 386Session description 387 v= (protocol version) 388 o= (owner/creator and session identifier). 389 s= (session name) 390 i=* (session information) 391 392 393 394Handley & Jacobson Standards Track [Page 7] 395 396RFC 2327 SDP April 1998 397 398 399 u=* (URI of description) 400 e=* (email address) 401 p=* (phone number) 402 c=* (connection information - not required if included in all media) 403 b=* (bandwidth information) 404 One or more time descriptions (see below) 405 z=* (time zone adjustments) 406 k=* (encryption key) 407 a=* (zero or more session attribute lines) 408 Zero or more media descriptions (see below) 409 410Time description 411 t= (time the session is active) 412 r=* (zero or more repeat times) 413 414Media description 415 m= (media name and transport address) 416 i=* (media title) 417 c=* (connection information - optional if included at session-level) 418 b=* (bandwidth information) 419 k=* (encryption key) 420 a=* (zero or more media attribute lines) 421 422 The set of `type' letters is deliberately small and not intended to 423 be extensible -- SDP parsers must completely ignore any announcement 424 that contains a `type' letter that it does not understand. The 425 `attribute' mechanism ("a=" described below) is the primary means for 426 extending SDP and tailoring it to particular applications or media. 427 Some attributes (the ones listed in this document) have a defined 428 meaning but others may be added on an application-, media- or 429 session-specific basis. A session directory must ignore any 430 attribute it doesn't understand. 431 432 The connection (`c=') and attribute (`a=') information in the 433 session-level section applies to all the media of that session unless 434 overridden by connection information or an attribute of the same name 435 in the media description. For instance, in the example below, each 436 media behaves as if it were given a `recvonly' attribute. 437 438 An example SDP description is: 439 440 v=0 441 o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4 442 s=SDP Seminar 443 i=A Seminar on the session description protocol 444 u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps 445 e=mjh@isi.edu (Mark Handley) 446 c=IN IP4 224.2.17.12/127 447 448 449 450Handley & Jacobson Standards Track [Page 8] 451 452RFC 2327 SDP April 1998 453 454 455 t=2873397496 2873404696 456 a=recvonly 457 m=audio 49170 RTP/AVP 0 458 m=video 51372 RTP/AVP 31 459 m=application 32416 udp wb 460 a=orient:portrait 461 462 Text records such as the session name and information are bytes 463 strings which may contain any byte with the exceptions of 0x00 (Nul), 464 0x0a (ASCII newline) and 0x0d (ASCII carriage return). The sequence 465 CRLF (0x0d0a) is used to end a record, although parsers should be 466 tolerant and also accept records terminated with a single newline 467 character. By default these byte strings contain ISO-10646 468 characters in UTF-8 encoding, but this default may be changed using 469 the `charset' attribute. 470 471 Protocol Version 472 473 v=0 474 475 The "v=" field gives the version of the Session Description Protocol. 476 There is no minor version number. 477 478 Origin 479 480 o=<username> <session id> <version> <network type> <address type> 481 <address> 482 483 The "o=" field gives the originator of the session (their username 484 and the address of the user's host) plus a session id and session 485 version number. 486 487 <username> is the user's login on the originating host, or it is "-" 488 if the originating host does not support the concept of user ids. 489 <username> must not contain spaces. <session id> is a numeric string 490 such that the tuple of <username>, <session id>, <network type>, 491 <address type> and <address> form a globally unique identifier for 492 the session. 493 494 The method of <session id> allocation is up to the creating tool, but 495 it has been suggested that a Network Time Protocol (NTP) timestamp be 496 used to ensure uniqueness [1]. 497 498 <version> is a version number for this announcement. It is needed 499 for proxy announcements to detect which of several announcements for 500 the same session is the most recent. Again its usage is up to the 501 502 503 504 505 506Handley & Jacobson Standards Track [Page 9] 507 508RFC 2327 SDP April 1998 509 510 511 creating tool, so long as <version> is increased when a modification 512 is made to the session data. Again, it is recommended (but not 513 mandatory) that an NTP timestamp is used. 514 515 <network type> is a text string giving the type of network. 516 Initially "IN" is defined to have the meaning "Internet". <address 517 type> is a text string giving the type of the address that follows. 518 Initially "IP4" and "IP6" are defined. <address> is the globally 519 unique address of the machine from which the session was created. 520 For an address type of IP4, this is either the fully-qualified domain 521 name of the machine, or the dotted-decimal representation of the IP 522 version 4 address of the machine. For an address type of IP6, this 523 is either the fully-qualified domain name of the machine, or the 524 compressed textual representation of the IP version 6 address of the 525 machine. For both IP4 and IP6, the fully-qualified domain name is 526 the form that SHOULD be given unless this is unavailable, in which 527 case the globally unique address may be substituted. A local IP 528 address MUST NOT be used in any context where the SDP description 529 might leave the scope in which the address is meaningful. 530 531 In general, the "o=" field serves as a globally unique identifier for 532 this version of this session description, and the subfields excepting 533 the version taken together identify the session irrespective of any 534 modifications. 535 536 Session Name 537 538 s=<session name> 539 540 The "s=" field is the session name. There must be one and only one 541 "s=" field per session description, and it must contain ISO 10646 542 characters (but see also the `charset' attribute below). 543 544 Session and Media Information 545 546 i=<session description> 547 548 The "i=" field is information about the session. There may be at 549 most one session-level "i=" field per session description, and at 550 most one "i=" field per media. Although it may be omitted, this is 551 discouraged for session announcements, and user interfaces for 552 composing sessions should require text to be entered. If it is 553 present it must contain ISO 10646 characters (but see also the 554 `charset' attribute below). 555 556 A single "i=" field can also be used for each media definition. In 557 media definitions, "i=" fields are primarily intended for labeling 558 media streams. As such, they are most likely to be useful when a 559 560 561 562Handley & Jacobson Standards Track [Page 10] 563 564RFC 2327 SDP April 1998 565 566 567 single session has more than one distinct media stream of the same 568 media type. An example would be two different whiteboards, one for 569 slides and one for feedback and questions. 570 571 URI 572 573 u=<URI> 574 575 o A URI is a Universal Resource Identifier as used by WWW clients 576 577 o The URI should be a pointer to additional information about the 578 conference 579 580 o This field is optional, but if it is present it should be specified 581 before the first media field 582 583 o No more than one URI field is allowed per session description 584 585 586 Email Address and Phone Number 587 588 e=<email address> 589 p=<phone number> 590 591 o These specify contact information for the person responsible for 592 the conference. This is not necessarily the same person that 593 created the conference announcement. 594 595 o Either an email field or a phone field must be specified. 596 Additional email and phone fields are allowed. 597 598 o If these are present, they should be specified before the first 599 media field. 600 601 o More than one email or phone field can be given for a session 602 description. 603 604 o Phone numbers should be given in the conventional international 605 606 format - preceded by a "+ and the international country code. 607 There must be a space or a hyphen ("-") between the country code 608 and the rest of the phone number. Spaces and hyphens may be used 609 to split up a phone field to aid readability if desired. For 610 example: 611 612 p=+44-171-380-7777 or p=+1 617 253 6011 613 614 615 616 617 618Handley & Jacobson Standards Track [Page 11] 619 620RFC 2327 SDP April 1998 621 622 623 o Both email addresses and phone numbers can have an optional free 624 text string associated with them, normally giving the name of the 625 person who may be contacted. This should be enclosed in 626 parenthesis if it is present. For example: 627 628 e=mjh@isi.edu (Mark Handley) 629 630 The alternative RFC822 name quoting convention is also allowed for 631 both email addresses and phone numbers. For example, 632 633 e=Mark Handley <mjh@isi.edu> 634 635 The free text string should be in the ISO-10646 character set with 636 UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings 637 if the appropriate charset session-level attribute is set. 638 639 Connection Data 640 641 c=<network type> <address type> <connection address> 642 643 The "c=" field contains connection data. 644 645 A session announcement must contain one "c=" field in each media 646 description (see below) or a "c=" field at the session-level. It may 647 contain a session-level "c=" field and one additional "c=" field per 648 media description, in which case the per-media values override the 649 session-level settings for the relevant media. 650 651 The first sub-field is the network type, which is a text string 652 giving the type of network. Initially "IN" is defined to have the 653 meaning "Internet". 654 655 The second sub-field is the address type. This allows SDP to be used 656 for sessions that are not IP based. Currently only IP4 is defined. 657 658 The third sub-field is the connection address. Optional extra 659 subfields may be added after the connection address depending on the 660 value of the <address type> field. 661 662 For IP4 addresses, the connection address is defined as follows: 663 664 o Typically the connection address will be a class-D IP multicast 665 666 group address. If the session is not multicast, then the 667 connection address contains the fully-qualified domain name or the 668 unicast IP address of the expected data source or data relay or 669 data sink as determined by additional attribute fields. It is not 670 expected that fully-qualified domain names or unicast addresses 671 672 673 674Handley & Jacobson Standards Track [Page 12] 675 676RFC 2327 SDP April 1998 677 678 679 will be given in a session description that is communicated by a 680 multicast announcement, though this is not prohibited. If a 681 unicast data stream is to pass through a network address 682 translator, the use of a fully-qualified domain name rather than an 683 unicast IP address is RECOMMENDED. In other cases, the use of an 684 IP address to specify a particular interface on a multi-homed host 685 might be required. Thus this specification leaves the decision as 686 to which to use up to the individual application, but all 687 applications MUST be able to cope with receiving both formats. 688 689 o Conferences using an IP multicast connection address must also have 690 a time to live (TTL) value present in addition to the multicast 691 address. The TTL and the address together define the scope with 692 which multicast packets sent in this conference will be sent. TTL 693 values must be in the range 0-255. 694 695 The TTL for the session is appended to the address using a slash as 696 a separator. An example is: 697 698 c=IN IP4 224.2.1.1/127 699 700 Hierarchical or layered encoding schemes are data streams where the 701 encoding from a single media source is split into a number of 702 layers. The receiver can choose the desired quality (and hence 703 bandwidth) by only subscribing to a subset of these layers. Such 704 layered encodings are normally transmitted in multiple multicast 705 groups to allow multicast pruning. This technique keeps unwanted 706 traffic from sites only requiring certain levels of the hierarchy. 707 For applications requiring multiple multicast groups, we allow the 708 following notation to be used for the connection address: 709 710 <base multicast address>/<ttl>/<number of addresses> 711 712 If the number of addresses is not given it is assumed to be one. 713 Multicast addresses so assigned are contiguously allocated above 714 the base address, so that, for example: 715 716 c=IN IP4 224.2.1.1/127/3 717 718 would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are 719 to be used at a ttl of 127. This is semantically identical to 720 including multiple "c=" lines in a media description: 721 722 c=IN IP4 224.2.1.1/127 723 c=IN IP4 224.2.1.2/127 724 c=IN IP4 224.2.1.3/127 725 726 727 728 729 730Handley & Jacobson Standards Track [Page 13] 731 732RFC 2327 SDP April 1998 733 734 735 Multiple addresses or "c=" lines can only be specified on a per- 736 media basis, and not for a session-level "c=" field. 737 738 It is illegal for the slash notation described above to be used for 739 IP unicast addresses. 740 741 Bandwidth 742 743 b=<modifier>:<bandwidth-value> 744 745 o This specifies the proposed bandwidth to be used by the session or 746 media, and is optional. 747 748 o <bandwidth-value> is in kilobits per second 749 750 o <modifier> is a single alphanumeric word giving the meaning of the 751 bandwidth figure. 752 753 o Two modifiers are initially defined: 754 755 CT Conference Total: An implicit maximum bandwidth is associated with 756 each TTL on the Mbone or within a particular multicast 757 administrative scope region (the Mbone bandwidth vs. TTL limits are 758 given in the MBone FAQ). If the bandwidth of a session or media in 759 a session is different from the bandwidth implicit from the scope, 760 a `b=CT:...' line should be supplied for the session giving the 761 proposed upper limit to the bandwidth used. The primary purpose of 762 this is to give an approximate idea as to whether two or more 763 conferences can co-exist simultaneously. 764 765 AS Application-Specific Maximum: The bandwidth is interpreted to be 766 application-specific, i.e., will be the application's concept of 767 maximum bandwidth. Normally this will coincide with what is set on 768 the application's "maximum bandwidth" control if applicable. 769 770 Note that CT gives a total bandwidth figure for all the media at 771 all sites. AS gives a bandwidth figure for a single media at a 772 single site, although there may be many sites sending 773 simultaneously. 774 775 o Extension Mechanism: Tool writers can define experimental bandwidth 776 modifiers by prefixing their modifier with "X-". For example: 777 778 b=X-YZ:128 779 780 SDP parsers should ignore bandwidth fields with unknown modifiers. 781 Modifiers should be alpha-numeric and, although no length limit is 782 given, they are recommended to be short. 783 784 785 786Handley & Jacobson Standards Track [Page 14] 787 788RFC 2327 SDP April 1998 789 790 791 Times, Repeat Times and Time Zones 792 793 t=<start time> <stop time> 794 795 o "t=" fields specify the start and stop times for a conference 796 session. Multiple "t=" fields may be used if a session is active 797 at multiple irregularly spaced times; each additional "t=" field 798 specifies an additional period of time for which the session will 799 be active. If the session is active at regular times, an "r=" 800 field (see below) should be used in addition to and following a 801 "t=" field - in which case the "t=" field specifies the start and 802 stop times of the repeat sequence. 803 804 o The first and second sub-fields give the start and stop times for 805 the conference respectively. These values are the decimal 806 representation of Network Time Protocol (NTP) time values in 807 seconds [1]. To convert these values to UNIX time, subtract 808 decimal 2208988800. 809 810 o If the stop-time is set to zero, then the session is not bounded, 811 though it will not become active until after the start-time. If 812 the start-time is also zero, the session is regarded as permanent. 813 814 User interfaces should strongly discourage the creation of 815 unbounded and permanent sessions as they give no information about 816 when the session is actually going to terminate, and so make 817 scheduling difficult. 818 819 The general assumption may be made, when displaying unbounded 820 sessions that have not timed out to the user, that an unbounded 821 session will only be active until half an hour from the current 822 time or the session start time, whichever is the later. If 823 behaviour other than this is required, an end-time should be given 824 and modified as appropriate when new information becomes available 825 about when the session should really end. 826 827 Permanent sessions may be shown to the user as never being active 828 unless there are associated repeat times which state precisely when 829 the session will be active. In general, permanent sessions should 830 not be created for any session expected to have a duration of less 831 than 2 months, and should be discouraged for sessions expected to 832 have a duration of less than 6 months. 833 834 r=<repeat interval> <active duration> <list of offsets from start- 835 time> 836 837 o "r=" fields specify repeat times for a session. For example, if 838 a session is active at 10am on Monday and 11am on Tuesday for one 839 840 841 842Handley & Jacobson Standards Track [Page 15] 843 844RFC 2327 SDP April 1998 845 846 847 hour each week for three months, then the <start time> in the 848 corresponding "t=" field would be the NTP representation of 10am on 849 the first Monday, the <repeat interval> would be 1 week, the 850 <active duration> would be 1 hour, and the offsets would be zero 851 and 25 hours. The corresponding "t=" field stop time would be the 852 NTP representation of the end of the last session three months 853 later. By default all fields are in seconds, so the "r=" and "t=" 854 fields might be: 855 856 t=3034423619 3042462419 857 r=604800 3600 0 90000 858 859 To make announcements more compact, times may also be given in units 860 of days, hours or minutes. The syntax for these is a number 861 immediately followed by a single case-sensitive character. 862 Fractional units are not allowed - a smaller unit should be used 863 instead. The following unit specification characters are allowed: 864 865 d - days (86400 seconds) 866 h - minutes (3600 seconds) 867 m - minutes (60 seconds) 868 s - seconds (allowed for completeness but not recommended) 869 870 Thus, the above announcement could also have been written: 871 872 r=7d 1h 0 25h 873 874 Monthly and yearly repeats cannot currently be directly specified 875 with a single SDP repeat time - instead separate "t" fields should 876 be used to explicitly list the session times. 877 878 z=<adjustment time> <offset> <adjustment time> <offset> .... 879 880 o To schedule a repeated session which spans a change from daylight- 881 saving time to standard time or vice-versa, it is necessary to 882 specify offsets from the base repeat times. This is required 883 because different time zones change time at different times of day, 884 different countries change to or from daylight time on different 885 dates, and some countries do not have daylight saving time at all. 886 887 Thus in order to schedule a session that is at the same time winter 888 and summer, it must be possible to specify unambiguously by whose 889 time zone a session is scheduled. To simplify this task for 890 receivers, we allow the sender to specify the NTP time that a time 891 zone adjustment happens and the offset from the time when the 892 session was first scheduled. The "z" field allows the sender to 893 specify a list of these adjustment times and offsets from the base 894 time. 895 896 897 898Handley & Jacobson Standards Track [Page 16] 899 900RFC 2327 SDP April 1998 901 902 903 An example might be: 904 905 z=2882844526 -1h 2898848070 0 906 907 This specifies that at time 2882844526 the time base by which the 908 session's repeat times are calculated is shifted back by 1 hour, 909 and that at time 2898848070 the session's original time base is 910 restored. Adjustments are always relative to the specified start 911 time - they are not cumulative. 912 913 o If a session is likely to last several years, it is expected 914 that 915 the session announcement will be modified periodically rather than 916 transmit several years worth of adjustments in one announcement. 917 918 Encryption Keys 919 920 k=<method> 921 k=<method>:<encryption key> 922 923 o The session description protocol may be used to convey encryption 924 keys. A key field is permitted before the first media entry (in 925 which case it applies to all media in the session), or for each 926 media entry as required. 927 928 o The format of keys and their usage is outside the scope of this 929 document, but see [3]. 930 931 o The method indicates the mechanism to be used to obtain a usable 932 key by external means, or from the encoded encryption key given. 933 934 The following methods are defined: 935 936 k=clear:<encryption key> 937 The encryption key (as described in [3] for RTP media streams 938 under the AV profile) is included untransformed in this key 939 field. 940 941 k=base64:<encoded encryption key> 942 The encryption key (as described in [3] for RTP media streams 943 under the AV profile) is included in this key field but has been 944 base64 encoded because it includes characters that are 945 prohibited in SDP. 946 947 k=uri:<URI to obtain key> 948 A Universal Resource Identifier as used by WWW clients is 949 included in this key field. The URI refers to the data 950 containing the key, and may require additional authentication 951 952 953 954Handley & Jacobson Standards Track [Page 17] 955 956RFC 2327 SDP April 1998 957 958 959 before the key can be returned. When a request is made to the 960 given URI, the MIME content-type of the reply specifies the 961 encoding for the key in the reply. The key should not be 962 obtained until the user wishes to join the session to reduce 963 synchronisation of requests to the WWW server(s). 964 965 k=prompt 966 No key is included in this SDP description, but the session or 967 media stream referred to by this key field is encrypted. The 968 user should be prompted for the key when attempting to join the 969 session, and this user-supplied key should then be used to 970 decrypt the media streams. 971 972 Attributes 973 974 a=<attribute> 975 a=<attribute>:<value> 976 977 Attributes are the primary means for extending SDP. Attributes may 978 be defined to be used as "session-level" attributes, "media-level" 979 attributes, or both. 980 981 A media description may have any number of attributes ("a=" fields) 982 which are media specific. These are referred to as "media-level" 983 attributes and add information about the media stream. Attribute 984 fields can also be added before the first media field; these 985 "session-level" attributes convey additional information that applies 986 to the conference as a whole rather than to individual media; an 987 example might be the conference's floor control policy. 988 989 Attribute fields may be of two forms: 990 991 o property attributes. A property attribute is simply of the form 992 "a=<flag>". These are binary attributes, and the presence of the 993 attribute conveys that the attribute is a property of the session. 994 An example might be "a=recvonly". 995 996 o value attributes. A value attribute is of the form 997 "a=<attribute>:<value>". An example might be that a whiteboard 998 could have the value attribute "a=orient:landscape" 999 1000 Attribute interpretation depends on the media tool being invoked. 1001 Thus receivers of session descriptions should be configurable in 1002 their interpretation of announcements in general and of attributes in 1003 particular. 1004 1005 Attribute names must be in the US-ASCII subset of ISO-10646/UTF-8. 1006 1007 1008 1009 1010Handley & Jacobson Standards Track [Page 18] 1011 1012RFC 2327 SDP April 1998 1013 1014 1015 Attribute values are byte strings, and MAY use any byte value except 1016 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values 1017 are to be interpreted as in ISO-10646 character set with UTF-8 1018 encoding. Unlike other text fields, attribute values are NOT 1019 normally affected by the `charset' attribute as this would make 1020 comparisons against known values problematic. However, when an 1021 attribute is defined, it can be defined to be charset-dependent, in 1022 which case it's value should be interpreted in the session charset 1023 rather than in ISO-10646. 1024 1025 Attributes that will be commonly used can be registered with IANA 1026 (see Appendix B). Unregistered attributes should begin with "X-" to 1027 prevent inadvertent collision with registered attributes. In either 1028 case, if an attribute is received that is not understood, it should 1029 simply be ignored by the receiver. 1030 1031 Media Announcements 1032 1033 m=<media> <port> <transport> <fmt list> 1034 1035 A session description may contain a number of media descriptions. 1036 Each media description starts with an "m=" field, and is terminated 1037 by either the next "m=" field or by the end of the session 1038 description. A media field also has several sub-fields: 1039 1040 o The first sub-field is the media type. Currently defined media are 1041 "audio", "video", "application", "data" and "control", though this 1042 list may be extended as new communication modalities emerge (e.g., 1043 telepresense). The difference between "application" and "data" is 1044 that the former is a media flow such as whiteboard information, and 1045 the latter is bulk-data transfer such as multicasting of program 1046 executables which will not typically be displayed to the user. 1047 "control" is used to specify an additional conference control 1048 channel for the session. 1049 1050 o The second sub-field is the transport port to which the media 1051 stream will be sent. The meaning of the transport port depends on 1052 the network being used as specified in the relevant "c" field and 1053 on the transport protocol defined in the third sub-field. Other 1054 ports used by the media application (such as the RTCP port, see 1055 [2]) should be derived algorithmically from the base media port. 1056 1057 Note: For transports based on UDP, the value should be in the range 1058 1024 to 65535 inclusive. For RTP compliance it should be an even 1059 number. 1060 1061 1062 1063 1064 1065 1066Handley & Jacobson Standards Track [Page 19] 1067 1068RFC 2327 SDP April 1998 1069 1070 1071 For applications where hierarchically encoded streams are being 1072 sent to a unicast address, it may be necessary to specify multiple 1073 transport ports. This is done using a similar notation to that 1074 used for IP multicast addresses in the "c=" field: 1075 1076 m=<media> <port>/<number of ports> <transport> <fmt list> 1077 1078 In such a case, the ports used depend on the transport protocol. 1079 For RTP, only the even ports are used for data and the 1080 corresponding one-higher odd port is used for RTCP. For example: 1081 1082 m=video 49170/2 RTP/AVP 31 1083 1084 would specify that ports 49170 and 49171 form one RTP/RTCP pair and 1085 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the 1086 transport protocol and 31 is the format (see below). 1087 1088 It is illegal for both multiple addresses to be specified in the 1089 "c=" field and for multiple ports to be specified in the "m=" field 1090 in the same session description. 1091 1092 o The third sub-field is the transport protocol. The transport 1093 protocol values are dependent on the address-type field in the "c=" 1094 fields. Thus a "c=" field of IP4 defines that the transport 1095 protocol runs over IP4. For IP4, it is normally expected that most 1096 media traffic will be carried as RTP over UDP. The following 1097 transport protocols are preliminarily defined, but may be extended 1098 through registration of new protocols with IANA: 1099 1100 - RTP/AVP - the IETF's Realtime Transport Protocol using the 1101 Audio/Video profile carried over UDP. 1102 1103 - udp - User Datagram Protocol 1104 1105 If an application uses a single combined proprietary media format 1106 and transport protocol over UDP, then simply specifying the 1107 transport protocol as udp and using the format field to distinguish 1108 the combined protocol is recommended. If a transport protocol is 1109 used over UDP to carry several distinct media types that need to be 1110 distinguished by a session directory, then specifying the transport 1111 protocol and media format separately is necessary. RTP is an 1112 example of a transport-protocol that carries multiple payload 1113 formats that must be distinguished by the session directory for it 1114 to know how to start appropriate tools, relays, mixers or 1115 recorders. 1116 1117 1118 1119 1120 1121 1122Handley & Jacobson Standards Track [Page 20] 1123 1124RFC 2327 SDP April 1998 1125 1126 1127 The main reason to specify the transport-protocol in addition to 1128 the media format is that the same standard media formats may be 1129 carried over different transport protocols even when the network 1130 protocol is the same - a historical example is vat PCM audio and 1131 RTP PCM audio. In addition, relays and monitoring tools that are 1132 transport-protocol-specific but format-independent are possible. 1133 1134 For RTP media streams operating under the RTP Audio/Video Profile 1135 [3], the protocol field is "RTP/AVP". Should other RTP profiles be 1136 defined in the future, their profiles will be specified in the same 1137 way. For example, the protocol field "RTP/XYZ" would specify RTP 1138 operating under a profile whose short name is "XYZ". 1139 1140 o The fourth and subsequent sub-fields are media formats. For audio 1141 and video, these will normally be a media payload type as defined 1142 in the RTP Audio/Video Profile. 1143 1144 When a list of payload formats is given, this implies that all of 1145 these formats may be used in the session, but the first of these 1146 formats is the default format for the session. 1147 1148 For media whose transport protocol is not RTP or UDP the format 1149 field is protocol specific. Such formats should be defined in an 1150 additional specification document. 1151 1152 For media whose transport protocol is RTP, SDP can be used to 1153 provide a dynamic binding of media encoding to RTP payload type. 1154 The encoding names in the RTP AV Profile do not specify unique 1155 audio encodings (in terms of clock rate and number of audio 1156 channels), and so they are not used directly in SDP format fields. 1157 Instead, the payload type number should be used to specify the 1158 format for static payload types and the payload type number along 1159 with additional encoding information should be used for dynamically 1160 allocated payload types. 1161 1162 An example of a static payload type is u-law PCM coded single 1163 channel audio sampled at 8KHz. This is completely defined in the 1164 RTP Audio/Video profile as payload type 0, so the media field for 1165 such a stream sent to UDP port 49232 is: 1166 1167 m=video 49232 RTP/AVP 0 1168 1169 An example of a dynamic payload type is 16 bit linear encoded 1170 stereo audio sampled at 16KHz. If we wish to use dynamic RTP/AVP 1171 payload type 98 for such a stream, additional information is 1172 required to decode it: 1173 1174 m=video 49232 RTP/AVP 98 1175 1176 1177 1178Handley & Jacobson Standards Track [Page 21] 1179 1180RFC 2327 SDP April 1998 1181 1182 1183 a=rtpmap:98 L16/16000/2 1184 1185 The general form of an rtpmap attribute is: 1186 1187 a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding 1188 parameters>] 1189 1190 For audio streams, <encoding parameters> may specify the number of 1191 audio channels. This parameter may be omitted if the number of 1192 channels is one provided no additional parameters are needed. For 1193 video streams, no encoding parameters are currently specified. 1194 1195 Additional parameters may be defined in the future, but 1196 codecspecific parameters should not be added. Parameters added to 1197 an rtpmap attribute should only be those required for a session 1198 directory to make the choice of appropriate media too to 1199 participate in a session. Codec-specific parameters should be 1200 added in other attributes. 1201 1202 Up to one rtpmap attribute can be defined for each media format 1203 specified. Thus we might have: 1204 1205 m=audio 49230 RTP/AVP 96 97 98 1206 a=rtpmap:96 L8/8000 1207 a=rtpmap:97 L16/8000 1208 a=rtpmap:98 L16/11025/2 1209 1210 RTP profiles that specify the use of dynamic payload types must 1211 define the set of valid encoding names and/or a means to register 1212 encoding names if that profile is to be used with SDP. 1213 1214 Experimental encoding formats can also be specified using rtpmap. 1215 RTP formats that are not registered as standard format names must 1216 be preceded by "X-". Thus a new experimental redundant audio 1217 stream called GSMLPC using dynamic payload type 99 could be 1218 specified as: 1219 1220 m=video 49232 RTP/AVP 99 1221 a=rtpmap:99 X-GSMLPC/8000 1222 1223 Such an experimental encoding requires that any site wishing to 1224 receive the media stream has relevant configured state in its 1225 session directory to know which tools are appropriate. 1226 1227 Note that RTP audio formats typically do not include information 1228 about the number of samples per packet. If a non-default (as 1229 defined in the RTP Audio/Video Profile) packetisation is required, 1230 the "ptime" attribute is used as given below. 1231 1232 1233 1234Handley & Jacobson Standards Track [Page 22] 1235 1236RFC 2327 SDP April 1998 1237 1238 1239 For more details on RTP audio and video formats, see [3]. 1240 1241 o Formats for non-RTP media should be registered as MIME content 1242 types as described in Appendix B. For example, the LBL whiteboard 1243 application might be registered as MIME content-type application/wb 1244 with encoding considerations specifying that it operates over UDP, 1245 with no appropriate file format. In SDP this would then be 1246 expressed using a combination of the "media" field and the "fmt" 1247 field, as follows: 1248 1249 m=application 32416 udp wb 1250 1251 Suggested Attributes 1252 1253 The following attributes are suggested. Since application writers 1254 may add new attributes as they are required, this list is not 1255 exhaustive. 1256 1257 a=cat:<category> 1258 This attribute gives the dot-separated hierarchical category of 1259 the session. This is to enable a receiver to filter unwanted 1260 sessions by category. It would probably have been a compulsory 1261 separate field, except for its experimental nature at this time. 1262 It is a session-level attribute, and is not dependent on charset. 1263 1264 a=keywds:<keywords> 1265 Like the cat attribute, this is to assist identifying wanted 1266 sessions at the receiver. This allows a receiver to select 1267 interesting session based on keywords describing the purpose of 1268 the session. It is a session-level attribute. It is a charset 1269 dependent attribute, meaning that its value should be interpreted 1270 in the charset specified for the session description if one is 1271 specified, or by default in ISO 10646/UTF-8. 1272 1273 a=tool:<name and version of tool> 1274 This gives the name and version number of the tool used to create 1275 the session description. It is a session-level attribute, and is 1276 not dependent on charset. 1277 1278 a=ptime:<packet time> 1279 This gives the length of time in milliseconds represented by the 1280 media in a packet. This is probably only meaningful for audio 1281 data. It should not be necessary to know ptime to decode RTP or 1282 vat audio, and it is intended as a recommendation for the 1283 encoding/packetisation of audio. It is a media attribute, and is 1284 not dependent on charset. 1285 1286 1287 1288 1289 1290Handley & Jacobson Standards Track [Page 23] 1291 1292RFC 2327 SDP April 1998 1293 1294 1295 a=recvonly 1296 This specifies that the tools should be started in receive-only 1297 mode where applicable. It can be either a session or media 1298 attribute, and is not dependent on charset. 1299 1300 a=sendrecv 1301 This specifies that the tools should be started in send and 1302 receive mode. This is necessary for interactive conferences with 1303 tools such as wb which defaults to receive only mode. It can be 1304 either a session or media attribute, and is not dependent on 1305 charset. 1306 1307 a=sendonly 1308 This specifies that the tools should be started in send-only 1309 mode. An example may be where a different unicast address is to 1310 be used for a traffic destination than for a traffic source. In 1311 such a case, two media descriptions may be use, one sendonly and 1312 one recvonly. It can be either a session or media attribute, but 1313 would normally only be used as a media attribute, and is not 1314 dependent on charset. 1315 1316 a=orient:<whiteboard orientation> 1317 Normally this is only used in a whiteboard media specification. 1318 It specifies the orientation of a the whiteboard on the screen. 1319 It is a media attribute. Permitted values are `portrait', 1320 `landscape' and `seascape' (upside down landscape). It is not 1321 dependent on charset 1322 1323 a=type:<conference type> 1324 This specifies the type of the conference. Suggested values are 1325 `broadcast', `meeting', `moderated', `test' and `H332'. 1326 `recvonly' should be the default for `type:broadcast' sessions, 1327 `type:meeting' should imply `sendrecv' and `type:moderated' 1328 should indicate the use of a floor control tool and that the 1329 media tools are started so as to "mute" new sites joining the 1330 conference. 1331 1332 Specifying the attribute type:H332 indicates that this loosely 1333 coupled session is part of a H.332 session as defined in the ITU 1334 H.332 specification [10]. Media tools should be started 1335 `recvonly'. 1336 1337 Specifying the attribute type:test is suggested as a hint that, 1338 unless explicitly requested otherwise, receivers can safely avoid 1339 displaying this session description to users. 1340 1341 The type attribute is a session-level attribute, and is not 1342 dependent on charset. 1343 1344 1345 1346Handley & Jacobson Standards Track [Page 24] 1347 1348RFC 2327 SDP April 1998 1349 1350 1351 a=charset:<character set> 1352 This specifies the character set to be used to display the 1353 session name and information data. By default, the ISO-10646 1354 character set in UTF-8 encoding is used. If a more compact 1355 representation is required, other character sets may be used such 1356 as ISO-8859-1 for Northern European languages. In particular, 1357 the ISO 8859-1 is specified with the following SDP attribute: 1358 1359 a=charset:ISO-8859-1 1360 1361 This is a session-level attribute; if this attribute is present, 1362 it must be before the first media field. The charset specified 1363 MUST be one of those registered with IANA, such as ISO-8859-1. 1364 The character set identifier is a US-ASCII string and MUST be 1365 compared against the IANA identifiers using a case-insensitive 1366 comparison. If the identifier is not recognised or not 1367 supported, all strings that are affected by it SHOULD be regarded 1368 as byte strings. 1369 1370 Note that a character set specified MUST still prohibit the use 1371 of bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets 1372 requiring the use of these characters MUST define a quoting 1373 mechanism that prevents these bytes appearing within text fields. 1374 1375 a=sdplang:<language tag> 1376 This can be a session level attribute or a media level attribute. 1377 As a session level attribute, it specifies the language for the 1378 session description. As a media level attribute, it specifies 1379 the language for any media-level SDP information field associated 1380 with that media. Multiple sdplang attributes can be provided 1381 either at session or media level if multiple languages in the 1382 session description or media use multiple languages, in which 1383 case the order of the attributes indicates the order of 1384 importance of the various languages in the session or media from 1385 most important to least important. 1386 1387 In general, sending session descriptions consisting of multiple 1388 languages should be discouraged. Instead, multiple descriptions 1389 should be sent describing the session, one in each language. 1390 However this is not possible with all transport mechanisms, and 1391 so multiple sdplang attributes are allowed although not 1392 recommended. 1393 1394 The sdplang attribute value must be a single RFC 1766 language 1395 tag in US-ASCII. It is not dependent on the charset attribute. 1396 An sdplang attribute SHOULD be specified when a session is of 1397 1398 1399 1400 1401 1402Handley & Jacobson Standards Track [Page 25] 1403 1404RFC 2327 SDP April 1998 1405 1406 1407 sufficient scope to cross geographic boundaries where the 1408 language of recipients cannot be assumed, or where the session is 1409 in a different language from the locally assumed norm. 1410 1411 a=lang:<language tag> 1412 This can be a session level attribute or a media level attribute. 1413 As a session level attribute, it specifies the default language 1414 for the session being described. As a media level attribute, it 1415 specifies the language for that media, overriding any session- 1416 level language specified. Multiple lang attributes can be 1417 provided either at session or media level if multiple languages 1418 if the session description or media use multiple languages, in 1419 which case the order of the attributes indicates the order of 1420 importance of the various languages in the session or media from 1421 most important to least important. 1422 1423 The lang attribute value must be a single RFC 1766 language tag 1424 in US-ASCII. It is not dependent on the charset attribute. A 1425 lang attribute SHOULD be specified when a session is of 1426 sufficient scope to cross geographic boundaries where the 1427 language of recipients cannot be assumed, or where the session is 1428 in a different language from the locally assumed norm. 1429 1430 a=framerate:<frame rate> 1431 This gives the maximum video frame rate in frames/sec. It is 1432 intended as a recommendation for the encoding of video data. 1433 Decimal representations of fractional values using the notation 1434 "<integer>.<fraction>" are allowed. It is a media attribute, is 1435 only defined for video media, and is not dependent on charset. 1436 1437 a=quality:<quality> 1438 This gives a suggestion for the quality of the encoding as an 1439 integer value. 1440 1441 The intention of the quality attribute for video is to specify a 1442 non-default trade-off between frame-rate and still-image quality. 1443 For video, the value in the range 0 to 10, with the following 1444 suggested meaning: 1445 1446 10 - the best still-image quality the compression scheme can 1447 give. 1448 1449 5 - the default behaviour given no quality suggestion. 1450 1451 0 - the worst still-image quality the codec designer thinks is 1452 still usable. 1453 1454 It is a media attribute, and is not dependent on charset. 1455 1456 1457 1458Handley & Jacobson Standards Track [Page 26] 1459 1460RFC 2327 SDP April 1998 1461 1462 1463 a=fmtp:<format> <format specific parameters> 1464 This attribute allows parameters that are specific to a 1465 particular format to be conveyed in a way that SDP doesn't have 1466 to understand them. The format must be one of the formats 1467 specified for the media. Format-specific parameters may be any 1468 set of parameters required to be conveyed by SDP and given 1469 unchanged to the media tool that will use this format. 1470 1471 It is a media attribute, and is not dependent on charset. 1472 14736.1. Communicating Conference Control Policy 1474 1475 There is some debate over the way conference control policy should be 1476 communicated. In general, the authors believe that an implicit 1477 declarative style of specifying conference control is desirable where 1478 possible. 1479 1480 A simple declarative style uses a single conference attribute field 1481 before the first media field, possibly supplemented by properties 1482 such as `recvonly' for some of the media tools. This conference 1483 attribute conveys the conference control policy. An example might be: 1484 1485 a=type:moderated 1486 1487 In some cases, however, it is possible that this may be insufficient 1488 to communicate the details of an unusual conference control policy. 1489 If this is the case, then a conference attribute specifying external 1490 control might be set, and then one or more "media" fields might be 1491 used to specify the conference control tools and configuration data 1492 for those tools. An example is an ITU H.332 session: 1493 1494 c=IN IP4 224.5.6.7 1495 a=type:H332 1496 m=audio 49230 RTP/AVP 0 1497 m=video 49232 RTP/AVP 31 1498 m=application 12349 udp wb 1499 m=control 49234 H323 mc 1500 c=IN IP4 134.134.157.81 1501 1502 In this example, a general conference attribute (type:H332) is 1503 specified stating that conference control will be provided by an 1504 external H.332 tool, and a contact addresses for the H.323 session 1505 multipoint controller is given. 1506 1507 In this document, only the declarative style of conference control 1508 declaration is specified. Other forms of conference control should 1509 specify an appropriate type attribute, and should define the 1510 implications this has for control media. 1511 1512 1513 1514Handley & Jacobson Standards Track [Page 27] 1515 1516RFC 2327 SDP April 1998 1517 1518 15197. Security Considerations 1520 1521 SDP is a session description format that describes multimedia 1522 sessions. A session description should not be trusted unless it has 1523 been obtained by an authenticated transport protocol from a trusted 1524 source. Many different transport protocols may be used to distribute 1525 session description, and the nature of the authentication will differ 1526 from transport to transport. 1527 1528 One transport that will frequently be used to distribute session 1529 descriptions is the Session Announcement Protocol (SAP). SAP 1530 provides both encryption and authentication mechanisms but due to the 1531 nature of session announcements it is likely that there are many 1532 occasions where the originator of a session announcement cannot be 1533 authenticated because they are previously unknown to the receiver of 1534 the announcement and because no common public key infrastructure is 1535 available. 1536 1537 On receiving a session description over an unauthenticated transport 1538 mechanism or from an untrusted party, software parsing the session 1539 should take a few precautions. Session description contain 1540 information required to start software on the receivers system. 1541 Software that parses a session description MUST not be able to start 1542 other software except that which is specifically configured as 1543 appropriate software to participate in multimedia sessions. It is 1544 normally considered INAPPROPRIATE for software parsing a session 1545 description to start, on a user's system, software that is 1546 appropriate to participate in multimedia sessions, without the user 1547 first being informed that such software will be started and giving 1548 their consent. Thus a session description arriving by session 1549 announcement, email, session invitation, or WWW page SHOULD not 1550 deliver the user into an {it interactive} multimedia session without 1551 the user being aware that this will happen. As it is not always 1552 simple to tell whether a session is interactive or not, applications 1553 that are unsure should assume sessions are interactive. 1554 1555 In this specification, there are no attributes which would allow the 1556 recipient of a session description to be informed to start multimedia 1557 tools in a mode where they default to transmitting. Under some 1558 circumstances it might be appropriate to define such attributes. If 1559 this is done an application parsing a session description containing 1560 such attributes SHOULD either ignore them, or inform the user that 1561 joining this session will result in the automatic transmission of 1562 multimedia data. The default behaviour for an unknown attribute is 1563 to ignore it. 1564 1565 1566 1567 1568 1569 1570Handley & Jacobson Standards Track [Page 28] 1571 1572RFC 2327 SDP April 1998 1573 1574 1575 Session descriptions may be parsed at intermediate systems such as 1576 firewalls for the purposes of opening a hole in the firewall to allow 1577 the participation in multimedia sessions. It is considered 1578 INAPPROPRIATE for a firewall to open such holes for unicast data 1579 streams unless the session description comes in a request from inside 1580 the firewall. 1581 1582 For multicast sessions, it is likely that local administrators will 1583 apply their own policies, but the exclusive use of "local" or "site- 1584 local" administrative scope within the firewall and the refusal of 1585 the firewall to open a hole for such scopes will provide separation 1586 of global multicast sessions from local ones. 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626Handley & Jacobson Standards Track [Page 29] 1627 1628RFC 2327 SDP April 1998 1629 1630 1631Appendix A: SDP Grammar 1632 1633 This appendix provides an Augmented BNF grammar for SDP. ABNF is 1634 defined in RFC 2234. 1635 1636 1637 announcement = proto-version 1638 origin-field 1639 session-name-field 1640 information-field 1641 uri-field 1642 email-fields 1643 phone-fields 1644 connection-field 1645 bandwidth-fields 1646 time-fields 1647 key-field 1648 attribute-fields 1649 media-descriptions 1650 1651 proto-version = "v=" 1*DIGIT CRLF 1652 ;this memo describes version 0 1653 1654 origin-field = "o=" username space 1655 sess-id space sess-version space 1656 nettype space addrtype space 1657 addr CRLF 1658 1659 session-name-field = "s=" text CRLF 1660 1661 information-field = ["i=" text CRLF] 1662 1663 uri-field = ["u=" uri CRLF] 1664 1665 email-fields = *("e=" email-address CRLF) 1666 1667 phone-fields = *("p=" phone-number CRLF) 1668 1669 1670 connection-field = ["c=" nettype space addrtype space 1671 connection-address CRLF] 1672 ;a connection field must be present 1673 ;in every media description or at the 1674 ;session-level 1675 1676 1677 bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF) 1678 1679 1680 1681 1682Handley & Jacobson Standards Track [Page 30] 1683 1684RFC 2327 SDP April 1998 1685 1686 1687 time-fields = 1*( "t=" start-time space stop-time 1688 *(CRLF repeat-fields) CRLF) 1689 [zone-adjustments CRLF] 1690 1691 1692 repeat-fields = "r=" repeat-interval space typed-time 1693 1*(space typed-time) 1694 1695 1696 zone-adjustments = time space ["-"] typed-time 1697 *(space time space ["-"] typed-time) 1698 1699 1700 key-field = ["k=" key-type CRLF] 1701 1702 1703 key-type = "prompt" | 1704 "clear:" key-data | 1705 "base64:" key-data | 1706 "uri:" uri 1707 1708 1709 key-data = email-safe | "~" | " 1710 1711 1712 attribute-fields = *("a=" attribute CRLF) 1713 1714 1715 media-descriptions = *( media-field 1716 information-field 1717 *(connection-field) 1718 bandwidth-fields 1719 key-field 1720 attribute-fields ) 1721 1722 1723 media-field = "m=" media space port ["/" integer] 1724 space proto 1*(space fmt) CRLF 1725 1726 1727 media = 1*(alpha-numeric) 1728 ;typically "audio", "video", "application" 1729 ;or "data" 1730 1731 fmt = 1*(alpha-numeric) 1732 ;typically an RTP payload type for audio 1733 ;and video media 1734 1735 1736 1737 1738Handley & Jacobson Standards Track [Page 31] 1739 1740RFC 2327 SDP April 1998 1741 1742 1743 proto = 1*(alpha-numeric) 1744 ;typically "RTP/AVP" or "udp" for IP4 1745 1746 1747 port = 1*(DIGIT) 1748 ;should in the range "1024" to "65535" inclusive 1749 ;for UDP based media 1750 1751 1752 attribute = (att-field ":" att-value) | att-field 1753 1754 1755 att-field = 1*(alpha-numeric) 1756 1757 1758 att-value = byte-string 1759 1760 1761 sess-id = 1*(DIGIT) 1762 ;should be unique for this originating username/host 1763 1764 1765 sess-version = 1*(DIGIT) 1766 ;0 is a new session 1767 1768 1769 connection-address = multicast-address 1770 | addr 1771 1772 1773 multicast-address = 3*(decimal-uchar ".") decimal-uchar "/" ttl 1774 [ "/" integer ] 1775 ;multicast addresses may be in the range 1776 ;224.0.0.0 to 239.255.255.255 1777 1778 ttl = decimal-uchar 1779 1780 start-time = time | "0" 1781 1782 stop-time = time | "0" 1783 1784 time = POS-DIGIT 9*(DIGIT) 1785 ;sufficient for 2 more centuries 1786 1787 1788 repeat-interval = typed-time 1789 1790 1791 1792 1793 1794Handley & Jacobson Standards Track [Page 32] 1795 1796RFC 2327 SDP April 1998 1797 1798 1799 typed-time = 1*(DIGIT) [fixed-len-time-unit] 1800 1801 1802 fixed-len-time-unit = "d" | "h" | "m" | "s" 1803 1804 1805 bwtype = 1*(alpha-numeric) 1806 1807 bandwidth = 1*(DIGIT) 1808 1809 1810 username = safe 1811 ;pretty wide definition, but doesn't include space 1812 1813 1814 email-address = email | email "(" email-safe ")" | 1815 email-safe "<" email ">" 1816 1817 1818 email = ;defined in RFC822 1819 1820 1821 uri= ;defined in RFC1630 1822 1823 1824 phone-number = phone | phone "(" email-safe ")" | 1825 email-safe "<" phone ">" 1826 1827 1828 phone = "+" POS-DIGIT 1*(space | "-" | DIGIT) 1829 ;there must be a space or hyphen between the 1830 ;international code and the rest of the number. 1831 1832 1833 nettype = "IN" 1834 ;list to be extended 1835 1836 1837 addrtype = "IP4" | "IP6" 1838 ;list to be extended 1839 1840 1841 addr = FQDN | unicast-address 1842 1843 1844 FQDN = 4*(alpha-numeric|"-"|".") 1845 ;fully qualified domain name as specified in RFC1035 1846 1847 1848 1849 1850Handley & Jacobson Standards Track [Page 33] 1851 1852RFC 2327 SDP April 1998 1853 1854 1855 unicast-address = IP4-address | IP6-address 1856 1857 1858 IP4-address = b1 "." decimal-uchar "." decimal-uchar "." b4 1859 b1 = decimal-uchar 1860 ;less than "224"; not "0" or "127" 1861 b4 = decimal-uchar 1862 ;not "0" 1863 1864 IP6-address = ;to be defined 1865 1866 1867 text = byte-string 1868 ;default is to interpret this as IS0-10646 UTF8 1869 ;ISO 8859-1 requires a "a=charset:ISO-8859-1" 1870 ;session-level attribute to be used 1871 1872 1873 byte-string = 1*(0x01..0x09|0x0b|0x0c|0x0e..0xff) 1874 ;any byte except NUL, CR or LF 1875 1876 1877 decimal-uchar = DIGIT 1878 | POS-DIGIT DIGIT 1879 | ("1" 2*(DIGIT)) 1880 | ("2" ("0"|"1"|"2"|"3"|"4") DIGIT) 1881 | ("2" "5" ("0"|"1"|"2"|"3"|"4"|"5")) 1882 1883 1884 integer = POS-DIGIT *(DIGIT) 1885 1886 1887 alpha-numeric = ALPHA | DIGIT 1888 1889 1890 DIGIT = "0" | POS-DIGIT 1891 1892 1893 POS-DIGIT = "1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9" 1894 1895 1896 ALPHA = "a"|"b"|"c"|"d"|"e"|"f"|"g"|"h"|"i"|"j"|"k"| 1897 "l"|"m"|"n"|"o "|"p"|"q"|"r"|"s"|"t"|"u"|"v"| 1898 "w"|"x"|"y"|"z"|"A"|"B"|"C "|"D"|"E"|"F"|"G"| 1899 "H"|"I"|"J"|"K"|"L"|"M"|"N"|"O"|"P"|" Q"|"R"| 1900 "S"|"T"|"U"|"V"|"W"|"X"|"Y"|"Z" 1901 1902 1903 1904 1905 1906Handley & Jacobson Standards Track [Page 34] 1907 1908RFC 2327 SDP April 1998 1909 1910 1911 email-safe = safe | space | tab 1912 1913 1914 safe = alpha-numeric | 1915 "'" | "'" | "-" | "." | "/" | ":" | "?" | """ | 1916 "#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" | 1917 "]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" | 1918 "~" | " 1919 1920 1921 space = %d32 1922 tab = %d9 1923 CRLF = %d13.10 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962Handley & Jacobson Standards Track [Page 35] 1963 1964RFC 2327 SDP April 1998 1965 1966 1967Appendix B: Guidelines for registering SDP names with IANA 1968 1969 There are seven field names that may be registered with IANA. Using 1970 the terminology in the SDP specification BNF, they are "media", 1971 "proto", "fmt", "att-field", "bwtype", "nettype" and "addrtype". 1972 1973 "media" (eg, audio, video, application, data). 1974 1975 Packetized media types, such as those used by RTP, share the 1976 namespace used by media types registry [RFC 2048] (i.e. "MIME 1977 types"). The list of valid media names is the set of top-level 1978 MIME content types. The set of media is intended to be small and 1979 not to be extended except under rare circumstances. (The MIME 1980 subtype corresponds to the "fmt" parameter below). 1981 1982 "proto" 1983 1984 In general this should be an IETF standards-track transport 1985 protocol identifier such as RTP/AVP (rfc 1889 under the rfc 1890 1986 profile). 1987 1988 However, people will want to invent their own proprietary 1989 transport protocols. Some of these should be registered as a 1990 "fmt" using "udp" as the protocol and some of which probably 1991 can't be. 1992 1993 Where the protocol and the application are intimately linked, 1994 such as with the LBL whiteboard wb which used a proprietary and 1995 special purpose protocol over UDP, the protocol name should be 1996 "udp" and the format name that should be registered is "wb". The 1997 rules for formats (see below) apply to such registrations. 1998 1999 Where the proprietary transport protocol really carries many 2000 different data formats, it is possible to register a new protocol 2001 name with IANA. In such a case, an RFC MUST be produced 2002 describing the protocol and referenced in the registration. Such 2003 an RFC MAY be informational, although it is preferable if it is 2004 standards-track. 2005 2006 "fmt" 2007 2008 The format namespace is dependent on the context of the "proto" 2009 field, so a format cannot be registered without specifying one or 2010 more transport protocols that it applies to. 2011 2012 Formats cover all the possible encodings that might want to be 2013 transported in a multimedia session. 2014 2015 2016 2017 2018Handley & Jacobson Standards Track [Page 36] 2019 2020RFC 2327 SDP April 1998 2021 2022 2023 For RTP formats that have been assigned static payload types, the 2024 payload type number is used. For RTP formats using a dynamic 2025 payload type number, the dynamic payload type number is given as 2026 the format and an additional "rtpmap" attribute specifies the 2027 format and parameters. 2028 2029 For non-RTP formats, any unregistered format name may be 2030 registered through the MIME-type registration process [RFC 2048]. 2031 The type given here is the MIME subtype only (the top-level MIME 2032 content type is specified by the media parameter). The MIME type 2033 registration SHOULD reference a standards-track RFC which 2034 describes the transport protocol for this media type. If there 2035 is an existing MIME type for this format, the MIME registration 2036 should be augmented to reference the transport specification for 2037 this media type. If there is not an existing MIME type for this 2038 format, and there exists no appropriate file format, this should 2039 be noted in the encoding considerations as "no appropriate file 2040 format". 2041 2042 "att-field" (Attribute names) 2043 2044 Attribute field names MAY be registered with IANA, although this 2045 is not compulsory, and unknown attributes are simply ignored. 2046 2047 When an attribute is registered, it must be accompanied by a 2048 brief specification stating the following: 2049 2050 o contact name, email address and telephone number 2051 2052 o attribute-name (as it will appear in SDP) 2053 2054 o long-form attribute name in English 2055 2056 o type of attribute (session level, media level, or both) 2057 2058 o whether the attribute value is subject to the charset 2059 attribute. 2060 2061 o a one paragraph explanation of the purpose of the attribute. 2062 2063 o a specification of appropriate attribute values for this 2064 attribute. 2065 2066 IANA will not sanity check such attribute registrations except to 2067 ensure that they do not clash with existing registrations. 2068 2069 2070 2071 2072 2073 2074Handley & Jacobson Standards Track [Page 37] 2075 2076RFC 2327 SDP April 1998 2077 2078 2079 Although the above is the minimum that IANA will accept, if the 2080 attribute is expected to see widespread use and interoperability 2081 is an issue, authors are encouraged to produce a standards-track 2082 RFC that specifies the attribute more precisely. 2083 2084 Submitters of registrations should ensure that the specification 2085 is in the spirit of SDP attributes, most notably that the 2086 attribute is platform independent in the sense that it makes no 2087 implicit assumptions about operating systems and does not name 2088 specific pieces of software in a manner that might inhibit 2089 interoperability. 2090 2091 "bwtype" (bandwidth specifiers) 2092 2093 A proliferation of bandwidth specifiers is strongly discouraged. 2094 2095 New bandwidth specifiers may be registered with IANA. The 2096 submission MUST reference a standards-track RFC specifying the 2097 semantics of the bandwidth specifier precisely, and indicating 2098 when it should be used, and why the existing registered bandwidth 2099 specifiers do not suffice. 2100 2101 "nettype" (Network Type) 2102 2103 New network types may be registered with IANA if SDP needs to be 2104 used in the context of non-internet environments. Whilst these 2105 are not normally the preserve of IANA, there may be circumstances 2106 when an Internet application needs to interoperate with a non- 2107 internet application, such as when gatewaying an internet 2108 telephony call into the PSTN. The number of network types should 2109 be small and should be rarely extended. A new network type 2110 cannot be registered without registering at least one address 2111 type to be used with that network type. A new network type 2112 registration MUST reference an RFC which gives details of the 2113 network type and address type and specifies how and when they 2114 would be used. Such an RFC MAY be Informational. 2115 2116 "addrtype" (Address Type) 2117 2118 New address types may be registered with IANA. An address type 2119 is only meaningful in the context of a network type, and any 2120 registration of an address type MUST specify a registered network 2121 type, or be submitted along with a network type registration. A 2122 new address type registration MUST reference an RFC giving 2123 details of the syntax of the address type. Such an RFC MAY be 2124 Informational. Address types are not expected to be registered 2125 frequently. 2126 2127 2128 2129 2130Handley & Jacobson Standards Track [Page 38] 2131 2132RFC 2327 SDP April 1998 2133 2134 2135 Registration Procedure 2136 2137 To register a name the above guidelines should be followed regarding 2138 the required level of documentation that is required. The 2139 registration itself should be sent to IANA. Attribute registrations 2140 should include the information given above. Other registrations 2141 should include the following additional information: 2142 2143 o contact name, email address and telephone number 2144 2145 o name being registered (as it will appear in SDP) 2146 2147 o long-form name in English 2148 2149 o type of name ("media", "proto", "fmt", "bwtype", "nettype", or 2150 "addrtype") 2151 2152 o a one paragraph explanation of the purpose of the registered name. 2153 2154 o a reference to the specification (eg RFC number) of the registered 2155 name. 2156 2157 IANA may refer any registration to the IESG or to any appropriate 2158 IETF working group for review, and may request revisions to be made 2159 before a registration will be made. 2160 2161 2162 2163 2164 2165 2166 2167 2168 2169 2170 2171 2172 2173 2174 2175 2176 2177 2178 2179 2180 2181 2182 2183 2184 2185 2186Handley & Jacobson Standards Track [Page 39] 2187 2188RFC 2327 SDP April 1998 2189 2190 2191Appendix C: Authors' Addresses 2192 2193 Mark Handley 2194 Information Sciences Institute 2195 c/o MIT Laboratory for Computer Science 2196 545 Technology Square 2197 Cambridge, MA 02139 2198 United States 2199 electronic mail: mjh@isi.edu 2200 2201 Van Jacobson 2202 MS 46a-1121 2203 Lawrence Berkeley Laboratory 2204 Berkeley, CA 94720 2205 United States 2206 electronic mail: van@ee.lbl.gov 2207 2208Acknowledgments 2209 2210 Many people in the IETF MMUSIC working group have made comments and 2211 suggestions contributing to this document. In particular, we would 2212 like to thank Eve Schooler, Steve Casner, Bill Fenner, Allison 2213 Mankin, Ross Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Rob 2214 Lanphier and Steve Hanna. 2215 2216References 2217 2218 [1] Mills, D., "Network Time Protocol (version 3) specification and 2219 implementation", RFC 1305, March 1992. 2220 2221 [2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: 2222 A Transport Protocol for Real-Time Applications", RFC 1889, January 2223 1996. 2224 2225 [3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences 2226 with Minimal Control", RFC 1890, January 1996 2227 2228 [4] Handley, M., "SAP - Session Announcement Protocol", Work in 2229 Progress. 2230 2231 [5] V. Jacobson, S. McCanne, "vat - X11-based audio teleconferencing 2232 tool" vat manual page, Lawrence Berkeley Laboratory, 1994. 2233 2234 [6] The Unicode Consortium, "The Unicode Standard -- Version 2.0", 2235 Addison-Wesley, 1996. 2236 2237 2238 2239 2240 2241 2242Handley & Jacobson Standards Track [Page 40] 2243 2244RFC 2327 SDP April 1998 2245 2246 2247 [7] ISO/IEC 10646-1:1993. International Standard -- Information 2248 technol- ogy -- Universal Multiple-Octet Coded Character Set (UCS) -- 2249 Part 1: Architecture and Basic Multilingual Plane. Five amendments 2250 and a techn- ical corrigendum have been published up to now. UTF-8 2251 is described in Annex R, published as Amendment 2. 2252 2253 [8] Goldsmith, D., and M. Davis, "Using Unicode with MIME", RFC 1641, 2254 July 1994. 2255 2256 [9] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO 2257 10646", RFC 2044, October 1996. 2258 2259 [10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for 2260 Receiving Internet-based H.323 Conferences", ITU, Geneva. 2261 2262 [11] Handley, M., Schooler, E., and H. Schulzrinne, "Session 2263 Initiation Protocol (SIP)", Work in Progress. 2264 2265 [12] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming 2266 Protocol (RTSP)", RFC 2326, April 1998. 2267 2268 2269 2270 2271 2272 2273 2274 2275 2276 2277 2278 2279 2280 2281 2282 2283 2284 2285 2286 2287 2288 2289 2290 2291 2292 2293 2294 2295 2296 2297 2298Handley & Jacobson Standards Track [Page 41] 2299 2300RFC 2327 SDP April 1998 2301 2302 2303Full Copyright Statement 2304 2305 Copyright (C) The Internet Society (1998). All Rights Reserved. 2306 2307 This document and translations of it may be copied and furnished to 2308 others, and derivative works that comment on or otherwise explain it 2309 or assist in its implementation may be prepared, copied, published 2310 and distributed, in whole or in part, without restriction of any 2311 kind, provided that the above copyright notice and this paragraph are 2312 included on all such copies and derivative works. However, this 2313 document itself may not be modified in any way, such as by removing 2314 the copyright notice or references to the Internet Society or other 2315 Internet organizations, except as needed for the purpose of 2316 developing Internet standards in which case the procedures for 2317 copyrights defined in the Internet Standards process must be 2318 followed, or as required to translate it into languages other than 2319 English. 2320 2321 The limited permissions granted above are perpetual and will not be 2322 revoked by the Internet Society or its successors or assigns. 2323 2324 This document and the information contained herein is provided on an 2325 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 2326 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 2327 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 2328 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 2329 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 2330 2331 2332 2333 2334 2335 2336 2337 2338 2339 2340 2341 2342 2343 2344 2345 2346 2347 2348 2349 2350 2351 2352 2353 2354Handley & Jacobson Standards Track [Page 42] 2355 2356