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'1') (Obsoleted by RFC 5905) ** Obsolete normative reference: RFC 1889 (ref. '2') (Obsoleted by RFC 3550) ** Obsolete normative reference: RFC 1890 (ref. '3') (Obsoleted by RFC 3551) ** Downref: Normative reference to an Experimental RFC: RFC 2974 (ref. '4') -- Possible downref: Non-RFC (?) normative reference: ref. '5' -- Possible downref: Non-RFC (?) normative reference: ref. '6' -- Possible downref: Non-RFC (?) normative reference: ref. '7' ** Downref: Normative reference to an Experimental RFC: RFC 1641 (ref. '8') ** Obsolete normative reference: RFC 2279 (ref. '9') (Obsoleted by RFC 3629) -- Possible downref: Non-RFC (?) normative reference: ref. '10' ** Obsolete normative reference: RFC 2326 (ref. '12') (Obsoleted by RFC 7826) -- Possible downref: Non-RFC (?) normative reference: ref. '15' Summary: 14 errors (**), 0 flaws (~~), 11 warnings (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Engineering Task Force MMUSIC WG 2 INTERNET-DRAFT Mark Handley/ACIRI 3 draft-ietf-mmusic-sdp-new-11.txt Van Jacobson/Packet Design 4 Colin Perkins/ISI 5 3 November 2002 6 Expires: May 2003 8 SDP: Session Description Protocol 10 Status of this Memo 12 This document is an Internet-Draft and is in full conformance with all 13 provisions of Section 10 of RFC2026. 15 Internet-Drafts are working documents of the Internet Engineering Task 16 Force (IETF), its areas, and its working groups. Note that other groups 17 may also distribute working documents as Internet-Drafts. 19 Internet-Drafts are draft documents valid for a maximum of six months 20 and may be updated, replaced, or obsoleted by other documents at any 21 time. It is inappropriate to use Internet- Drafts as reference material 22 or to cite them other than as "work in progress." 24 The list of current Internet-Drafts can be accessed at 25 http://www.ietf.org/ietf/1id-abstracts.txt 27 The list of Internet-Draft Shadow Directories can be accessed at 28 http://www.ietf.org/shadow.html. 30 This document is a product of the Multiparty Multimedia Session Control 31 (MMUSIC) working group of the Internet Engineering Task Force. Comments 32 are solicited and should be addressed to the working group's mailing 33 list at confctrl@isi.edu and/or the authors. 35 Abstract 37 This memo defines the Session Description Protocol (SDP). SDP 38 is intended for describing multimedia sessions for the 39 purposes of session announcement, session invitation, and 40 other forms of multimedia session initiation. 42 1. Introduction 44 On the Internet multicast backbone (Mbone), a session directory tool is 45 used to advertise multimedia conferences and communicate the conference 46 addresses and media-specific information necessary for participation. 47 This memo defines a session description protocol for this purpose, and 48 for general real-time multimedia session description purposes; it does 49 not describe multicast address allocation or the distribution of SDP 50 messages. 52 2. Background 54 The Mbone is the part of the Internet that supports IP multicast, and 55 thus permits efficient many-to-many communication. It is used 56 extensively for multimedia conferencing. Such conferences usually have 57 the property that tight coordination of conference membership is not 58 necessary; to receive a conference, a user at an Mbone site only has to 59 know the conference's multicast group address and the UDP ports for the 60 conference data streams. 62 Session directories assist the advertisement of conference sessions and 63 communicate the relevant conference setup information to prospective 64 participants. SDP is designed to convey such information to recipients. 65 SDP is purely a format for session description - it does not incorporate 66 a transport protocol, and is intended to use different transport 67 protocols as appropriate including the Session Announcement Protocol 68 [4], Session Initiation Protocol [11], Real-Time Streaming Protocol 69 [12], electronic mail using the MIME extensions, and the Hypertext 70 Transport Protocol. 72 SDP is intended to be general purpose so that it can be used for a wider 73 range of network environments and applications than just multicast 74 session directories. However, it is not intended to support negotiation 75 of session content or media encodings - this is viewed as outside the 76 scope of session description. 78 3. Glossary of Terms 80 The following terms are used in this document, and have specific meaning 81 within the context of this document. 83 Conference 84 A multimedia conference is a set of two or more communicating users 85 along with the software they are using to communicate. 87 Session 88 A multimedia session is a set of multimedia senders and receivers 89 and the data streams flowing from senders to receivers. A 90 multimedia conference is an example of a multimedia session. 92 Session Advertisement 93 See session announcement. 95 Session Announcement 96 A session announcement is a mechanism by which a session description 97 is conveyed to users in a pro-active fashion, i.e., the session 98 description was not explicitly requested by the user. 100 Session Description 101 A well defined format for conveying sufficient information to 102 discover and participate in a multimedia session. 104 3.1. Terminology 106 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 107 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 108 document are to be interpreted as described in RFC 2119 [13]. 110 4. Examples of SDP Usage 112 4.1. Multicast Announcement 114 In order to assist the advertisement of multicast multimedia conferences 115 and other multicast sessions, and to communicate the relevant session 116 setup information to prospective participants, a distributed session 117 directory may be used. An instance of such a session directory 118 periodically sends packets containing a description of the session to a 119 well known multicast group. These advertisements are received by other 120 session directories such that potential remote participants can use the 121 session description to start the tools required to participate in the 122 session. 124 One protocol commonly used to implement such a distributed directory is 125 the Session Announcement Protocol, SAP [4]. SDP provides the recommended 126 session description format for such announcements. 128 4.2. Session Initiation 130 The Session Initiation Protocol, SIP [11] is an application-layer 131 control protocol for creating, modifying and terminating sessions such 132 as Internet multimedia conferences, Internet telephone calls and 133 multimedia distribution. The SIP messages used to create sessions carry 134 session descriptions which allow participants to agree on a set of 135 compatible media types. These session descriptions are commonly 136 formatted using SDP. When used with SIP, the offer/answer model [14] 137 provides a framework for negotiation using SDP. 139 4.3. Streaming media 141 The Real Time Streaming Protocol, RTSP [12], is an application-level 142 protocol for control over the delivery of data with real-time 143 properties. RTSP provides an extensible framework to enable controlled, 144 on-demand delivery of real-time data, such as audio and video. An RTSP 145 client and server negotiate an appropriate set of parameters for media 146 delivery, partially using SDP syntax to describe those parameters. 148 4.4. Email and the World Wide Web 150 Alternative means of conveying session descriptions include electronic 151 mail and the World Wide Web. For both email and WWW distribution, the 152 use of the MIME content type ``application/sdp'' MUST be used. This 153 enables the automatic launching of applications for participation in the 154 session from the WWW client or mail reader in a standard manner. 156 Note that announcements of multicast sessions made only via email or the 157 World Wide Web (WWW) do not have the property that the receiver of a 158 session announcement can necessarily receive the session because the 159 multicast sessions may be restricted in scope, and access to the WWW 160 server or reception of email is possible outside this scope. SAP 161 announcements do not suffer from this mismatch. 163 5. Requirements and Recommendations 165 The purpose of SDP is to convey information about media streams in 166 multimedia sessions to allow the recipients of a session description to 167 participate in the session. SDP is primarily intended for use in an 168 internetwork, although it is sufficiently general that it can describe 169 conferences in other network environments. 171 A multimedia session, for these purposes, is defined as a set of media 172 streams that exist for some duration of time. Media streams can be 173 many-to-many. The times during which the session is active need not be 174 continuous. 176 Thus far, multicast based sessions on the Internet have differed from 177 many other forms of conferencing in that anyone receiving the traffic 178 can join the session (unless the session traffic is encrypted). In such 179 an environment, SDP serves two primary purposes. It is a means to 180 communicate the existence of a session, and is a means to convey 181 sufficient information to enable joining and participating in the 182 session. In a unicast environment, only the latter purpose is likely to 183 be relevant. 185 Thus SDP includes: 187 o Session name and purpose 189 o Time(s) the session is active 191 o The media comprising the session 193 o Information to receive those media (addresses, ports, formats and so 194 on) 196 As resources necessary to participate in a session may be limited, some 197 additional information may also be desirable: 199 o Information about the bandwidth to be used by the conference 201 o Contact information for the person responsible for the session 203 In general, SDP must convey sufficient information to enable 204 applications to join a session (with the possible exception of 205 encryption keys) and to announce the resources to be used to non- 206 participants that may need to know. 208 5.1. Media Information 210 SDP includes: 212 o The type of media (video, audio, etc) 214 o The transport protocol (RTP/UDP/IP, H.320, etc) 216 o The format of the media (H.261 video, MPEG video, etc) 217 For an IP multicast session, the following are also conveyed: 219 o Multicast address for media 221 o Transport port for media 223 This address and port are the destination address and destination port 224 of the multicast stream, whether being sent, received, or both. 226 For an IP unicast session, the following are conveyed: 228 o Remote address for media 230 o Transport port for media 232 The semantics of this address and port depend on the media and transport 233 protocol defined. By default, this is the remote address and remote port 234 to which data is sent, however some media types may redefine this 235 behaviour. 237 5.2. Timing Information 239 Sessions may either be bounded or unbounded in time. Whether or not 240 they are bounded, they may be only active at specific times. 242 SDP can convey: 244 o An arbitrary list of start and stop times bounding the session 246 o For each bound, repeat times such as "every Wednesday at 10am for 247 one hour" 249 This timing information is globally consistent, irrespective of local 250 time zone or daylight saving time. 252 5.3. Private Sessions 254 It is possible to create both public sessions and private sessions. SDP 255 itself does not distinguish between these: private sessions are 256 typically conveyed by encrypting the session description during 257 distribution. The details of how encryption is performed are dependent 258 on the mechanism used to convey SDP - e.g. mechanisms are defined for 259 SDP transported using SAP [4] and SIP [11]. 261 If a session announcement is private it is possible to use that private 262 announcement to convey encryption keys necessary to decode each of the 263 media in a conference, including enough information to know which 264 encryption scheme is used for each media. 266 5.4. Obtaining Further Information about a Session 268 A session description should convey enough information to decide whether 269 or not to participate in a session. SDP may include additional pointers 270 in the form of Universal Resources Identifiers (URIs) for more 271 information about the session. 273 5.5. Categorisation 275 When many session descriptions are being distributed by SAP, or any 276 other advertisement mechanism, it may be desirable to filter 277 announcements that are of interest from those that are not. SDP 278 supports a categorisation mechanism for sessions that is capable of 279 being automated. 281 5.6. Internationalization 283 The SDP specification recommends the use of the ISO 10646 character sets 284 in the UTF-8 encoding (RFC 2279) to allow many different languages to be 285 represented. However, to assist in compact representations, SDP also 286 allows other character sets such as ISO 8859-1 to be used when desired. 287 Internationalization only applies to free-text fields (session name and 288 background information), and not to SDP as a whole. 290 6. SDP Specification 292 SDP session descriptions are entirely textual using the ISO 10646 293 character set in UTF-8 encoding. SDP field names and attribute names 294 use only the US-ASCII subset of UTF-8, but textual fields and attribute 295 values MAY use the full ISO 10646 character set. The textual form, as 296 opposed to a binary encoding such as ASN.1 or XDR, was chosen to enhance 297 portability, to enable a variety of transports to be used (e.g, session 298 description in a MIME email message) and to allow flexible, text-based 299 toolkits (e.g., Tcl/Tk ) to be used to generate and to process session 300 descriptions. However, since SDP may be used in environments where the 301 maximum permissable size of a session description is limited (e.g. SAP 302 announcements; SIP transported in UDP), the encoding is deliberately 303 compact. Also, since announcements may be transported via very 304 unreliable means or damaged by an intermediate caching server, the 305 encoding was designed with strict order and formatting rules so that 306 most errors would result in malformed announcements which could be 307 detected easily and discarded. This also allows rapid discarding of 308 encrypted announcements for which a receiver does not have the correct 309 key. 311 An SDP session description consists of a number of lines of text of the 312 form: 314 = 316 where MUST be exactly one case-significant character and 317 is structured text whose format depends on . In general 318 is either a number of fields delimited by a single space character, or a 319 free format string. Whitespace MUST NOT be used either side of the `=' 320 sign. 322 A session description consists of a session-level section followed by 323 zero or more media-level sections. The session-level part starts with a 324 `v=' line and continues to the first media-level section. The media 325 description starts with an `m=' line and continues to the next media 326 description or end of the whole session description. In general, 327 session-level values are the default for all media unless overridden by 328 an equivalent media-level value. 330 Some lines in each description are REQUIRED and some are OPTIONAL but 331 all MUST appear in exactly the order given here (the fixed order greatly 332 enhances error detection and allows for a simple parser). OPTIONAL 333 items are marked with a `*'. 335 Session description 336 v= (protocol version) 337 o= (owner/creator and session identifier). 338 s= (session name) 339 i=* (session information) 340 u=* (URI of description) 341 e=* (email address) 342 p=* (phone number) 343 c=* (connection information - not required if included in all media) 344 b=* (bandwidth information) 345 One or more time descriptions (see below) 346 z=* (time zone adjustments) 347 k=* (encryption key) 348 a=* (zero or more session attribute lines) 349 Zero or more media descriptions (see below) 351 Time description 352 t= (time the session is active) 353 r=* (zero or more repeat times) 355 Media description 356 m= (media name and transport address) 357 i=* (media title) 358 c=* (connection information - optional if included at session-level) 359 b=* (bandwidth information) 360 k=* (encryption key) 361 a=* (zero or more media attribute lines) 363 The set of `type' letters is deliberately small and not intended to be 364 extensible -- an SDP parser MUST completely ignore any announcement that 365 contains a `type' letter that it does not understand. The `attribute' 366 mechanism ("a=" described below) is the primary means for extending SDP 367 and tailoring it to particular applications or media. Some attributes 368 (the ones listed in this document) have a defined meaning but others may 369 be added on an application-, media- or session-specific basis. An SDP 370 parser MUST ignore any attribute it doesn't understand. 372 The connection (`c=') and attribute (`a=') information in the session- 373 level section applies to all the media of that session unless overridden 374 by connection information or an attribute of the same name in the media 375 description. For instance, in the example below, each media behaves as 376 if it were given a `recvonly' attribute. 378 An example SDP description is: 380 v=0 381 o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5 382 s=SDP Seminar 383 i=A Seminar on the session description protocol 384 u=http://www.example.com/seminars/sdp.pdf 385 e=j.doe@example.com (Jane Doe) 386 c=IN IP4 224.2.17.12/127 387 t=2873397496 2873404696 388 a=recvonly 389 m=audio 49170 RTP/AVP 0 390 m=video 51372 RTP/AVP 31 391 m=application 32416 udp wb 392 a=orient:portrait 394 Text records such as the session name and information are octet strings 395 which may contain any octet with the exceptions of 0x00 (Nul), 0x0a 396 (ASCII newline) and 0x0d (ASCII carriage return). The sequence CRLF 397 (0x0d0a) is used to end a record, although parsers SHOULD be tolerant 398 and also accept records terminated with a single newline character. By 399 default these byte strings contain ISO-10646 characters in UTF-8 400 encoding, but this default MAY be changed using the `charset' attribute. 402 Protocol Version 404 v=0 406 The ``v='' field gives the version of the Session Description Protocol. 407 There is no minor version number. 409 Origin 411 o=
412
414 The ``o='' field gives the originator of the session (her username and 415 the address of the user's host) plus a session id and session version 416 number. is the user's login on the originating host, or it 417 is ``-'' if the originating host does not support the concept of user 418 ids. MUST NOT contain spaces. is a numeric 419 string such that the tuple of , , , 420
and
form a globally unique identifier for the 421 session. The method of session id allocation is up to the creating 422 tool, but it has been suggested that a Network Time Protocol (NTP) 423 timestamp be used to ensure uniqueness [1]. is a version 424 number for this announcement. It is needed for proxy announcements to 425 detect which of several announcements for the same session is the most 426 recent. Again its usage is up to the creating tool, so long as 427 is increased when a modification is made to the session data. 428 Again, it is RECOMMENDED (but not mandatory) that an NTP timestamp is 429 used. is a text string giving the type of network. 430 Initially ``IN'' is defined to have the meaning ``Internet''.
is a text string giving the type of the address that follows. 432 Initially ``IP4'' and ``IP6'' are defined.
is the globally 433 unique address of the machine from which the session was created. For 434 an address type of IP4, this is either the fully-qualified domain name 435 of the machine, or the dotted-decimal representation of the IP version 4 436 address of the machine. For an address type of IP6, this is either the 437 fully-qualified domain name of the machine, or the compressed textual 438 representation of the IP version 6 address of the machine. For both IP4 439 and IP6, the fully-qualified domain name is the form that SHOULD be 440 given unless this is unavailable, in which case the globally unique 441 address may be substituted. A local IP address MUST NOT be used in any 442 context where the SDP description might leave the scope in which the 443 address is meaningful. 445 In general, the ``o='' field serves as a globally unique identifier for 446 this version of this session description, and the subfields excepting 447 the version taken together identify the session irrespective of any 448 modifications. 450 Session Name 452 s= 454 The ``s='' field is the session name. There MUST be one and only one 455 ``s='' field per session description. The ``s='' field MUST NOT be empty 456 and SHOULD contain ISO 10646 characters (but see also the `charset' 457 attribute below). If a session has no meaningful name, the value ``s= '' 458 SHOULD be used (i.e. a single space as the session name). 460 Session and Media Information 462 i= 464 The ``i='' field is information about the session. There may be at most 465 one session-level ``i='' field per session description, and at most one 466 ``i='' field per media. Although it may be omitted, this is NOT 467 RECOMMENDED for session announcements, and user interfaces for composing 468 sessions should require text to be entered. If it is present it must 469 contain ISO 10646 characters (but see also the `charset' attribute 470 below). 472 A single ``i='' field can also be used for each media definition. In 473 media definitions, ``i='' fields are primarily intended for labeling 474 media streams. As such, they are most likely to be useful when a single 475 session has more than one distinct media stream of the same media type. 476 An example would be two different whiteboards, one for slides and one 477 for feedback and questions. 479 URI 481 u= 483 A URI is a Universal Resource Identifier as used by WWW clients. The URI 484 should be a pointer to additional information about the conference. This 485 field is OPTIONAL, but if it is present it MUST be specified before the 486 first media field. No more than one URI field is allowed per session 487 description. 489 Email Address and Phone Number 491 e= 492 p= 494 These specify contact information for the person responsible for the 495 conference. This is not necessarily the same person that created the 496 conference announcement. 498 Inclusion of an email address or phone number is OPTIONAL. Note that the 499 previous version of SDP specified that either an email field or a phone 500 field MUST be specified, but this was widely ignored. The change brings 501 the specification into line with common usage. 503 If these are present, they MUST be specified before the first media 504 field. More than one email or phone field can be given for a session 505 description. 507 Phone numbers SHOULD be given in the conventional international format - 508 preceded by a ``+'' and the international country code. There must be a 509 space or a hyphen (``-'') between the country code and the rest of the 510 phone number. Spaces and hyphens may be used to split up a phone field 511 to aid readability if desired. For example: 513 p=+44-171-380-7777 or p=+1 617 555 6011 515 Both email addresses and phone numbers can have an optional free text 516 string associated with them, normally giving the name of the person who 517 may be contacted. This should be enclosed in parenthesis if it is 518 present. For example: 520 e=j.doe@example.com (Jane Doe) 522 The alternative RFC822 name quoting convention is also allowed for both 523 email addresses and phone numbers. For example, 525 e=Jane Doe 527 The free text string SHOULD be in the ISO-10646 character set with UTF-8 528 encoding, or alternatively in ISO-8859-1 or other encodings if the 529 appropriate charset session-level attribute is set. 531 Connection Data 533 c=
535 The ``c='' field contains connection data. 537 A session announcement MUST contain either one ``c='' field in each 538 media description (see below) or a ``c='' field at the session-level. 539 It MAY contain a session-level ``c='' field and one additional ``c='' 540 field per media description, in which case the per-media values override 541 the session-level settings for the respective media. 543 The first sub-field is the network type, which is a text string giving 544 the type of network. Initially ``IN'' is defined to have the meaning 545 ``Internet''. 547 The second sub-field is the address type. This allows SDP to be used 548 for sessions that are not IP based. Currently only IP4 and IP6 are 549 defined. 551 The third sub-field is the connection address. Optional extra sub- 552 fields MAY be added after the connection address depending on the value 553 of the
field. 555 For IP4 and IP6 addresses, the connection address is defined as follows: 557 o If the session is multicast, the connection address will be an IP 558 multicast group address. If the conference is not multicast, then 559 the connection address contains the unicast IP address of the 560 expected data source or data relay or data sink as determined by 561 additional attribute fields. It is not expected that unicast 562 addresses will be given in a session description that is 563 communicated by a multicast announcement, though this is not 564 prohibited. 566 o Conferences using an IPv4 multicast connection address MUST also 567 have a time to live (TTL) value present in addition to the multicast 568 address. The TTL and the address together define the scope with 569 which multicast packets sent in this conference will be sent. TTL 570 values MUST be in the range 0-255. 572 The TTL for the session is appended to the address using a slash as 573 a separator. An example is: 575 c=IN IP4 224.2.36.42/127 577 IPv6 multicast does not use TTL scoping, and hence the TTL value 578 MUST NOT be present for IPv6 multicast. It is expected that IPv6 579 scoped addresses will be used to limit the scope of conferences. 581 Hierarchical or layered encoding schemes are data streams where the 582 encoding from a single media source is split into a number of 583 layers. The receiver can choose the desired quality (and hence 584 bandwidth) by only subscribing to a subset of these layers. Such 585 layered encodings are normally transmitted in multiple multicast 586 groups to allow multicast pruning. This technique keeps unwanted 587 traffic from sites only requiring certain levels of the hierarchy. 588 For applications requiring multiple multicast groups, we allow the 589 following notation to be used for the connection address: 591 [/]/ 593 If the number of addresses is not given it is assumed to be one. 594 Multicast addresses so assigned are contiguously allocated above the 595 base address, so that, for example: 597 c=IN IP4 224.2.1.1/127/3 599 would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are to 600 be used at a ttl of 127. This is semantically identical to 601 including multiple ``c='' lines in a media description: 603 c=IN IP4 224.2.1.1/127 604 c=IN IP4 224.2.1.2/127 605 c=IN IP4 224.2.1.3/127 607 Similarly, an IPv6 example would be: 609 c=IN IP6 FF15::101/3 611 which is semantically equivalent to: 613 c=IN IP6 FF15::101 614 c=IN IP6 FF15::102 615 c=IN IP6 FF15::103 617 (remembering that the TTL field is not present in IPv6 multicast). 619 Multiple addresses or ``c='' lines MAY be specified on a per-media 620 basis. They MUST NOT be specified for a session-level ``c='' field. 622 The slash notation described above MUST NOT be used for IP unicast 623 addresses. 625 Bandwidth 627 b=: 629 o This specifies the proposed bandwidth to be used by the session or 630 media, and is OPTIONAL. 632 o is in kilobits per second by default. Modifiers 633 MAY specify that alternative units are to be used (the modifiers 634 defined in this memo use the default units). 636 o is a single alphanumeric word giving the meaning of the 637 bandwidth figure. 639 o Two modifiers are initially defined: 641 CT Conference Total: If the bandwidth of a session or media in a 642 session is different from the bandwidth implicit from the scope, a 643 `b=CT:...' line should be supplied for the session giving the 644 proposed upper limit to the bandwidth used. The primary purpose 645 of this is to give an approximate idea as to whether two or more 646 sessions can co-exist simultaneously. 648 AS Application-Specific Maximum: The bandwidth is interpreted to be 649 application-specific, i.e., will be the application's concept of 650 maximum bandwidth. Normally this will coincide with what is set 651 on the application's ``maximum bandwidth'' control if applicable. 652 For RTP based applications, AS gives the RTP ``session bandwidth'' 653 as defined in section 6.2 of [2]. 655 Note that CT gives a total bandwidth figure for all the media at all 656 sites. AS gives a bandwidth figure for a single media at a single 657 site, although there may be many sites sending simultaneously. 659 o Extension Mechanism: Tool writers can define experimental bandwidth 660 modifiers by prefixing their modifier with ``X-''. For example: 662 b=X-YZ:128 664 Use of the ``X-'' prefix is NOT RECOMMENDED: instead new modifiers 665 SHOULD be registered with IANA in the standard namespace. SDP 666 parsers MUST ignore bandwidth fields with unknown modifiers. 667 Modifiers MUST be alpha-numeric and, although no length limit is 668 given, they are recommended to be short. 670 Times, Repeat Times and Time Zones 672 t= 674 o ``t='' fields specify the start and stop times for a session. 675 Multiple ``t='' fields MAY be used if a session is active at 676 multiple irregularly spaced times; each additional ``t='' field 677 specifies an additional period of time for which the session will be 678 active. If the session is active at regular times, an ``r='' field 679 (see below) should be used in addition to and following a ``t='' 680 field - in which case the ``t='' field specifies the start and stop 681 times of the repeat sequence. 683 o The first and second sub-fields give the start and stop times for 684 the session respectively. These values are the decimal 685 representation of Network Time Protocol (NTP) time values in seconds 686 [1]. To convert these values to UNIX time, subtract decimal 687 2208988800. 689 NTP timestamps are 64 bit values which wrap sometime in the year 690 2036. Since SDP uses an arbitrary length decimal representation, 691 this should not cause an issue (SDP timestamps will continue 692 counting seconds since 1900, NTP will use the value modulo the 64 693 bit limit). 695 o If the stop-time is set to zero, then the session is not bounded, 696 though it will not become active until after the start-time. If the 697 start-time is also zero, the session is regarded as permanent. 699 User interfaces SHOULD strongly discourage the creation of unbounded 700 and permanent sessions as they give no information about when the 701 session is actually going to terminate, and so make scheduling 702 difficult. 704 The general assumption may be made, when displaying unbounded 705 sessions that have not timed out to the user, that an unbounded 706 session will only be active until half an hour from the current time 707 or the session start time, whichever is the later. If behaviour 708 other than this is required, an end-time should be given and 709 modified as appropriate when new information becomes available about 710 when the session should really end. 712 Permanent sessions may be shown to the user as never being active 713 unless there are associated repeat times which state precisely when 714 the session will be active. In general, permanent sessions SHOULD 715 NOT be created for any session expected to have a duration of less 716 than 2 months, and should be discouraged for sessions expected to 717 have a duration of less than 6 months. 719 r= 721 o ``r='' fields specify repeat times for a session. For example, if 722 a session is active at 10am on Monday and 11am on Tuesday for one 723 hour each week for three months, then the in the 724 corresponding ``t='' field would be the NTP representation of 10am 725 on the first Monday, the would be 1 week, the 726 would be 1 hour, and the offsets would be zero and 727 25 hours. The corresponding ``t='' field stop time would be the NTP 728 representation of the end of the last session three months later. By 729 default all fields are in seconds, so the ``r='' and ``t='' fields 730 might be: 732 t=3034423619 3042462419 733 r=604800 3600 0 90000 735 To make description more compact, times may also be given in units 736 of days, hours or minutes. The syntax for these is a number 737 immediately followed by a single case-sensitive character. 738 Fractional units are not allowed - a smaller unit should be used 739 instead. The following unit specification characters are allowed: 741 d - days (86400 seconds) 742 h - hours (3600 seconds) 743 m - minutes (60 seconds) 744 s - seconds (allowed for completeness but not recommended) 746 Thus, the above announcement could also have been written: 748 r=7d 1h 0 25h 750 Monthly and yearly repeats cannot be directly specified with a 751 single SDP repeat time - instead separate "t" fields should be used 752 to explicitly list the session times. 754 z= .... 756 o To schedule a repeated session which spans a change from daylight- 757 saving time to standard time or vice-versa, it is necessary to 758 specify offsets from the base time. This is required because 759 different time zones change time at different times of day, 760 different countries change to or from daylight time on different 761 dates, and some countries do not have daylight saving time at all. 763 Thus in order to schedule a session that is at the same time winter 764 and summer, it must be possible to specify unambiguously by whose 765 time zone a session is scheduled. To simplify this task for 766 receivers, we allow the sender to specify the NTP time that a time 767 zone adjustment happens and the offset from the time when the 768 session was first scheduled. The ``z'' field allows the sender to 769 specify a list of these adjustment times and offsets from the base 770 time. 772 An example might be: 774 z=2882844526 -1h 2898848070 0 776 This specifies that at time 2882844526 the time base by which the 777 session's repeat times are calculated is shifted back by 1 hour, and 778 that at time 2898848070 the session's original time base is 779 restored. Adjustments are always relative to the specified start 780 time - they are not cumulative. Adjustments apply to all ``t='' and 781 ``r='' lines in a session description. 783 o If a session is likely to last several years, it is expected that 784 the session announcement will be modified periodically rather than 785 transmit several years worth of adjustments in one announcement. 787 Encryption Keys 789 k= 790 k=: 792 o The session description protocol MAY be used to convey encryption 793 keys. A key field is permitted before the first media entry (in 794 which case it applies to all media in the session), or for each 795 media entry as required. 797 o The format of keys and their usage is outside the scope of this 798 document, but see [3]. 800 o The method indicates the mechanism to be used to obtain a usable key 801 by external means, or from the encoded encryption key given. The 802 following methods are defined: 804 k=clear: 805 The encryption key (as described in [3] for RTP media streams 806 under the AV profile) is included untransformed in this key 807 field. 809 k=base64: 810 The encryption key (as described in [3] for RTP media streams 811 under the AV profile) is included in this key field but has been 812 base64 encoded because it includes characters that are 813 prohibited in SDP. 815 k=uri: 816 A Universal Resource Identifier as used by WWW clients is 817 included in this key field. The URI refers to the data 818 containing the key, and may require additional authentication 819 before the key can be returned. When a request is made to the 820 given URI, the MIME content-type of the reply specifies the 821 encoding for the key in the reply. The key should not be 822 obtained until the user wishes to join the session to reduce 823 synchronisation of requests to the WWW server(s). 825 k=prompt 826 No key is included in this SDP description, but the session or 827 media stream referred to by this key field is encrypted. The 828 user should be prompted for the key when attempting to join the 829 session, and this user-supplied key should then be used to 830 decrypt the media streams. 832 Attributes 834 a= 835 a=: 837 Attributes are the primary means for extending SDP. Attributes may be 838 defined to be used as "session-level" attributes, "media-level" 839 attributes, or both. 841 A media description may have any number of attributes (``a='' fields) 842 which are media specific. These are referred to as "media-level" 843 attributes and add information about the media stream. Attribute fields 844 can also be added before the first media field; these "session-level" 845 attributes convey additional information that applies to the conference 846 as a whole rather than to individual media; an example might be the 847 conference's floor control policy. 849 Attribute fields may be of two forms: 851 o property attributes. A property attribute is simply of the form 852 ``a=''. These are binary attributes, and the presence of the 853 attribute conveys that the attribute is a property of the session. 854 An example might be ``a=recvonly''. 856 o value attributes. A value attribute is of the form 857 ``a=:''. An example might be that a whiteboard 858 could have the value attribute ``a=orient:landscape'' 860 Attribute interpretation depends on the media tool being invoked. Thus 861 receivers of session descriptions should be configurable in their 862 interpretation of announcements in general and of attributes in 863 particular. 865 Attribute names MUST be in the US-ASCII subset of ISO-10646/UTF-8. 867 Attribute values are octet strings, and MAY use any octet value except 868 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values are 869 to be interpreted as in ISO-10646 character set with UTF-8 encoding. 870 Unlike other text fields, attribute values are NOT normally affected by 871 the `charset' attribute as this would make comparisons against known 872 values problematic. However, when an attribute is defined, it can be 873 defined to be charset-dependent, in which case it's value should be 874 interpreted in the session charset rather than in ISO-10646. 876 Attributes SHOULD be registered with IANA (see Appendix B). Names of 877 unregistered attributes SHOULD begin with "X-" to prevent inadvertent 878 collision with registered attributes, however the use of unregistered 879 attributes is NOT RECOMMENDED. If an attribute is received that is not 880 understood, it MUST be ignored by the receiver. 882 Media Announcements 884 m= 886 A session description may contain a number of media descriptions. Each 887 media description starts with an ``m='' field, and is terminated by 888 either the next ``m='' field or by the end of the session description. 889 A media field also has several sub-fields: 891 o The first sub-field is the media type. Currently defined media are 892 ``audio'', ``video'', ``application'', ``data'' and ``control'', 893 though this list may be extended as new communication modalities 894 emerge (e.g., telepresense). The difference between ``application'' 895 and ``data'' is that the former is a media flow such as whiteboard 896 information, and the latter is bulk-data transfer such as 897 multicasting of program executables which will not typically be 898 displayed to the user. ``control'' is used to specify an additional 899 conference control channel for the session. 901 o The second sub-field is the transport port to which the media stream 902 will be sent. The meaning of the transport port depends on the 903 network being used as specified in the relevant ``c'' field and on 904 the transport protocol defined in the third sub-field. Other ports 905 used by the media application (such as the RTCP port, see [2]) MAY 906 be derived algorithmically from the base media port or MAY be 907 specified in a separate attribute (e.g. ``a=rtcp:'' as defined in 908 [15]). 910 Note: For transports based on UDP, the value should be in the range 911 1024 to 65535 inclusive. For RTP compliance it SHOULD be an even 912 number. 914 For applications where hierarchically encoded streams are being sent 915 to a unicast address, it may be necessary to specify multiple 916 transport ports. This is done using a similar notation to that used 917 for IP multicast addresses in the ``c='' field: 919 m= / 921 In such a case, the ports used depend on the transport protocol. 922 For RTP, only the even ports are used for data and the corresponding 923 one-higher odd port is used for RTCP. For example: 925 m=video 49170/2 RTP/AVP 31 927 would specify that ports 49170 and 49171 form one RTP/RTCP pair and 928 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the 929 transport protocol and 31 is the format (see below). 931 If multiple addresses are specified in the ``c='' field and multiple 932 ports are specified in the ``m='' field, a one-to-one mapping from 933 port to the corresponding address is implied. For example: 935 c=IN IP4 224.2.1.1/127/2 936 m=video 49170/2 RTP/AVP 31 938 would imply that address 224.2.1.1 is used with ports 49170 and 939 49171, and address 224.2.1.2 is used with ports 49172 and 49173. 941 o The third sub-field is the transport protocol. The transport 942 protocol values are dependent on the address-type field in the 943 ``c='' fields. Thus a ``c='' field of IP4 defines that the 944 transport protocol runs over IP4. For IP4, it is normally expected 945 that most media traffic will be carried as RTP over UDP. The 946 following transport protocols are defined, but may be extended 947 through registration of new protocols with IANA (see Appendix B): 949 - RTP/AVP - the IETF's Realtime Transport Protocol using the 950 Audio/Video profile carried over UDP. 952 - udp - User Datagram Protocol 954 If an application uses a single combined proprietary media format 955 and transport protocol over UDP, then simply specifying the 956 transport protocol as udp and using the format field to distinguish 957 the combined protocol is recommended. If a transport protocol is 958 used over UDP to carry several distinct media types that need to be 959 distinguished by a session directory, then specifying the transport 960 protocol and media format separately is necessary. RTP is an 961 example of a transport-protocol that carries multiple payload 962 formats that must be distinguished by the session directory for it 963 to know how to start appropriate tools, relays, mixers or recorders. 965 The main reason to specify the transport-protocol in addition to the 966 media format is that the same standard media formats may be carried 967 over different transport protocols even when the network protocol is 968 the same - a historical example is vat PCM audio and RTP PCM audio. 969 In addition, relays and monitoring tools that are transport- 970 protocol-specific but format-independent are possible. 972 For RTP media streams operating under the RTP Audio/Video Profile 973 [3], the protocol field is ``RTP/AVP''. Should other RTP profiles 974 be defined in the future, their profiles will be specified in the 975 same way. For example, the protocol field ``RTP/XYZ'' would specify 976 RTP operating under a profile whose short name is ``XYZ''. 978 o The fourth and subsequent sub-fields are media formats. For audio 979 and video, these SHOULD reference a MIME sub-type describing the 980 format under the `audio' and `video' top-level MIME types. 982 When a list of payload formats is given, this implies that all of 983 these formats may be used in the session, but the first of these 984 formats SHOULD be used as the default format for the session. 986 For media whose transport protocol is not RTP or UDP the format 987 field is protocol specific. Such formats should be defined in an 988 additional specification document. 990 For media whose transport protocol is RTP, SDP can be used to 991 provide a dynamic binding of media encoding to RTP payload type. The 992 encoding names in the RTP AV Profile do not specify unique audio 993 encodings (in terms of clock rate and number of audio channels), and 994 so they are not used directly in SDP format fields. Instead, the 995 payload type number should be used to specify the format for static 996 payload types and the payload type number along with additional 997 encoding information should be used for dynamically allocated 998 payload types. 1000 An example of a static payload type is u-law PCM coded single 1001 channel audio sampled at 8kHz. This is completely defined in the 1002 RTP Audio/Video profile as payload type 0, so the media field for 1003 such a stream sent to UDP port 49232 is: 1005 m=audio 49232 RTP/AVP 0 1007 An example of a dynamic payload type is 16 bit linear encoded stereo 1008 audio sampled at 16 kHz. If we wish to use dynamic RTP/AVP payload 1009 type 98 for such a stream, additional information is required to 1010 decode it: 1012 m=audio 49232 RTP/AVP 98 1013 a=rtpmap:98 L16/16000/2 1015 The general form of an rtpmap attribute is: 1017 a=rtpmap: /[/] 1019 For audio streams, may specify the number of 1020 audio channels. This parameter may be omitted if the number of 1021 channels is one provided no additional parameters are needed. 1022 For video streams, no encoding parameters are currently specified. 1024 Additional parameters may be defined in the future, but codec- 1025 specific parameters SHOULD NOT be added. Parameters added to an 1026 rtpmap attribute SHOULD only be those required for a session 1027 directory to make the choice of appropriate media to participate in 1028 a session. Codec-specific parameters should be added in other 1029 attributes (for example, ``a=fmtp:''). 1031 Up to one rtpmap attribute can be defined for each media format 1032 specified. Thus we might have: 1034 m=audio 49230 RTP/AVP 96 97 98 1035 a=rtpmap:96 L8/8000 1036 a=rtpmap:97 L16/8000 1037 a=rtpmap:98 L16/11025/2 1039 RTP profiles that specify the use of dynamic payload types MUST 1040 define the set of valid encoding names and/or a means to register 1041 encoding names if that profile is to be used with SDP. 1043 Experimental encoding formats can also be specified using rtpmap. 1044 RTP formats that are not registered as standard format names MUST be 1045 preceded by ``X-''. Use of the ``X-'' prefix is deprecated, and all 1046 new formats SHOULD be registered with IANA. Thus a new experimental 1047 redundant audio stream called GSMLPC using dynamic payload type 99 1048 could be specified as: 1050 m=audio 49232 RTP/AVP 99 1051 a=rtpmap:99 X-GSMLPC/8000 1053 Such an experimental encoding requires that any site wishing to 1054 receive the media stream has relevant configured state in its 1055 session directory to know which tools are appropriate. 1057 Note that RTP audio formats typically do not include information 1058 about the number of samples per packet. If a non-default (as 1059 defined in the RTP Audio/Video Profile) packetisation is required, 1060 the``ptime'' attribute is used as given below. 1062 For more details on RTP audio and video formats, see [3]. 1064 o Predefined formats for UDP protocol non-RTP media are as below. 1066 Application Formats: 1068 wb: LBL Whiteboard (transport: udp) 1070 nt: UCL Network Text Editor (transport: udp) 1072 Suggested Attributes 1074 The following attributes are suggested. Since application writers may 1075 add new attributes as they are required, this list is not exhaustive. 1077 a=cat: 1078 This attribute gives the dot-separated hierarchical category of the 1079 session. This is to enable a receiver to filter unwanted sessions 1080 by category. It would probably have been a compulsory separate 1081 field, except for its experimental nature at this time. It is a 1082 session-level attribute, and is not dependent on charset. 1084 a=keywds: 1085 Like the cat attribute, this is to assist identifying wanted 1086 sessions at the receiver. This allows a receiver to select 1087 interesting session based on keywords describing the purpose of the 1088 session. It is a session-level attribute. It is a charset dependent 1089 attribute, meaning that its value should be interpreted in the 1090 charset specified for the session description if one is specified, 1091 or by default in ISO 10646/UTF-8. 1093 a=tool: 1094 This gives the name and version number of the tool used to create 1095 the session description. It is a session-level attribute, and is 1096 not dependent on charset. 1098 a=ptime: 1099 This gives the length of time in milliseconds represented by the 1100 media in a packet. This is probably only meaningful for audio data, 1101 but may be used with other media types if it makes sense. It should 1102 not be necessary to know ptime to decode RTP or vat audio, and it is 1103 intended as a recommendation for the encoding/packetisation of 1104 audio. It is a media attribute, and is not dependent on charset. 1106 a=maxptime: 1107 The maximum amount of media which can be encapsulated in each 1108 packet, expressed as time in milliseconds. The time SHALL be 1109 calculated as the sum of the time the media present in the packet 1110 represents. The time SHOULD be a multiple of the frame size. This 1111 attribute is probably only meaningful for audio data, but may be 1112 used with other media types if it makes sense. It is a media 1113 attribute, and is not dependent on charset. Note that this 1114 attribute was introduced after RFC 2327, and non updated 1115 implementations will ignore this attribute. 1117 a=rtpmap: /[/] 1119 See the section on Media Announcements (the ``m='' field). This may 1120 be either a session or media attribute. 1122 a=recvonly 1123 This specifies that the tools should be started in receive-only mode 1124 where applicable. It can be either a session or media attribute, and 1125 is not dependent on charset. Note that recvonly applies to the media 1126 only, not to any associated control protocol (e.g. an RTP based 1127 system in recvonly mode SHOULD still send RTCP packets). 1129 a=sendrecv 1130 This specifies that the tools should be started in send and receive 1131 mode. This is necessary for interactive conferences with tools such 1132 as wb which defaults to receive only mode. It can be either a 1133 session or media attribute, and is not dependent on charset. 1135 If none of the attributes "sendonly", "recvonly", "inactive", and 1136 "sendrecv" is present, "sendrecv" SHOULD be assumed as the default 1137 for sessions which are not of the conference type "broadcast" or 1138 "H332" (see below). 1140 a=sendonly 1141 This specifies that the tools should be started in send-only mode. 1142 An example may be where a different unicast address is to be used 1143 for a traffic destination than for a traffic source. In such a case, 1144 two media descriptions may be use, one sendonly and one recvonly. It 1145 can be either a session or media attribute, but would normally only 1146 be used as a media attribute, and is not dependent on charset. Note 1147 that sendonly applies only to the media, and any associated control 1148 protocol (e.g. RTCP) SHOULD still be received and processed as 1149 normal. 1151 a=inactive 1152 This specifies that the tools should be started in inactive mode. 1153 This is necessary for interactive conferences where users can put 1154 other users on hold. No media is sent over an inactive media stream. 1155 Note that an RTP based system SHOULD still send RTCP, even if 1156 started inactive. It can be either a session or media attribute, and 1157 is not dependent on charset. 1159 a=orient: 1160 Normally this is only used in a whiteboard media specification. It 1161 specifies the orientation of a the whiteboard on the screen. It is 1162 a media attribute. Permitted values are `portrait', `landscape' and 1163 `seascape' (upside down landscape). It is not dependent on charset. 1165 a=type: 1166 This specifies the type of the conference. Suggested values are 1167 `broadcast', `meeting', `moderated', `test' and `H332'. `recvonly' 1168 should be the default for `type:broadcast' sessions, `type:meeting' 1169 should imply `sendrecv' and `type:moderated' should indicate the use 1170 of a floor control tool and that the media tools are started so as 1171 to ``mute'' new sites joining the conference. 1173 Specifying the attribute type:H332 indicates that this loosely 1174 coupled session is part of a H.332 session as defined in the ITU 1175 H.332 specification [10]. Media tools should be started `recvonly'. 1177 Specifying the attribute type:test is suggested as a hint that, 1178 unless explicitly requested otherwise, receivers can safely avoid 1179 displaying this session description to users. 1181 The type attribute is a session-level attribute, and is not 1182 dependent on charset. 1184 a=charset: 1185 This specifies the character set to be used to display the session 1186 name and information data. By default, the ISO-10646 character set 1187 in UTF-8 encoding is used. If a more compact representation is 1188 required, other character sets may be used such as ISO-8859-1 for 1189 Northern European languages. In particular, the ISO 8859-1 is 1190 specified with the following SDP attribute: 1192 a=charset:ISO-8859-1 1194 This is a session-level attribute; if this attribute is present, it 1195 must be before the first media field. The charset specified MUST be 1196 one of those registered with IANA, such as ISO-8859-1. The 1197 character set identifier is a US-ASCII string and MUST be compared 1198 against the IANA identifiers using a case-insensitive comparison. 1199 If the identifier is not recognised or not supported, all strings 1200 that are affected by it SHOULD be regarded as octet strings. 1202 Note that a character set specified MUST still prohibit the use of 1203 bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets requiring 1204 the use of these characters MUST define a quoting mechanism that 1205 prevents these bytes appearing within text fields. 1207 a=sdplang: 1208 This can be a session level attribute or a media level attribute. 1209 As a session level attribute, it specifies the language for the 1210 session description. As a media level attribute, it specifies the 1211 language for any media-level SDP information field associated with 1212 that media. Multiple sdplang attributes can be provided either at 1213 session or media level if multiple languages in the session 1214 description or media use multiple languages, in which case the order 1215 of the attributes indicates the order of importance of the various 1216 languages in the session or media from most important to least 1217 important. 1219 In general, sending session descriptions consisting of multiple 1220 languages is discouraged. Instead, multiple descriptions SHOULD be 1221 sent describing the session, one in each language. However this is 1222 not possible with all transport mechanisms, and so multiple sdplang 1223 attributes are allowed although NOT RECOMMENDED. 1225 The sdplang attribute value must be a single RFC 1766 language tag 1226 in US-ASCII. It is not dependent on the charset attribute. An 1227 sdplang attribute SHOULD be specified when a session is of 1228 sufficient scope to cross geographic boundaries where the language 1229 of recipients cannot be assumed, or where the session is in a 1230 different language from the locally assumed norm. 1232 a=lang: 1233 This can be a session level attribute or a media level attribute. 1234 As a session level attribute, it specifies the default language for 1235 the session being described. As a media level attribute, it 1236 specifies the language for that media, overriding any session-level 1237 language specified. Multiple lang attributes can be provided either 1238 at session or media level if multiple languages if the session 1239 description or media use multiple languages, in which case the order 1240 of the attributes indicates the order of importance of the various 1241 languages in the session or media from most important to least 1242 important. 1244 The lang attribute value must be a single RFC 1766 language tag in 1245 US-ASCII. It is not dependent on the charset attribute. A lang 1246 attribute SHOULD be specified when a session is of sufficient scope 1247 to cross geographic boundaries where the language of recipients 1248 cannot be assumed, or where the session is in a different language 1249 from the locally assumed norm. 1251 a=framerate: 1252 This gives the maximum video frame rate in frames/sec. It is 1253 intended as a recommendation for the encoding of video data. 1254 Decimal representations of fractional values using the notation 1255 "." are allowed. It is a media attribute, is 1256 only defined for video media, and is not dependent on charset. 1258 a=quality: 1259 This gives a suggestion for the quality of the encoding as an 1260 integer value. 1262 The intention of the quality attribute for video is to specify a 1263 non-default trade-off between frame-rate and still-image quality. 1264 For video, the value in the range 0 to 10, with the following 1265 suggested meaning: 1267 10 - the best still-image quality the compression scheme can give. 1269 5 - the default behaviour given no quality suggestion. 1271 0 - the worst still-image quality the codec designer thinks is 1272 still usable. 1273 It is a media attribute, and is not dependent on charset. 1275 a=fmtp: 1276 This attribute allows parameters that are specific to a particular 1277 format to be conveyed in a way that SDP doesn't have to understand 1278 them. The format must be one of the formats specified for the 1279 media. Format-specific parameters may be any set of parameters 1280 required to be conveyed by SDP and given unchanged to the media tool 1281 that will use this format. 1283 It is a media attribute, and is not dependent on charset. 1285 6.1. Communicating Conference Control Policy 1287 There is some debate over the way conference control policy should be 1288 communicated. In general, the authors believe that an implicit 1289 declarative style of specifying conference control is desirable where 1290 possible. 1292 A simple declarative style uses a single conference attribute field 1293 before the first media field, possibly supplemented by properties such 1294 as `recvonly' for some of the media tools. This conference attribute 1295 conveys the conference control policy. An example might be: 1297 a=type:moderated 1299 In some cases, however, it is possible that this may be insufficient to 1300 communicate the details of an unusual conference control policy. If 1301 this is the case, then a conference attribute specifying external 1302 control might be set, and then one or more ``media'' fields might be 1303 used to specify the conference control tools and configuration data for 1304 those tools. An example is an ITU H.332 session: 1306 ... 1307 c=IN IP4 224.5.6.7 1308 a=type:H332 1309 m=audio 49230 RTP/AVP 0 1310 m=video 49232 RTP/AVP 31 1311 m=application 12349 udp wb 1312 m=control 49234 H323 mc 1313 c=IN IP4 134.134.157.81 1315 In this example, a general conference attribute (type:H332) is specified 1316 stating that conference control will be provided by an external H.332 1317 tool, and a contact addresses for the H.323 session multipoint 1318 controller is given. 1320 In this document, only the declarative style of conference control 1321 declaration is specified. Other forms of conference control should 1322 specify an appropriate type attribute, and should define the 1323 implications this has for control media. 1325 7. Security Considerations 1327 SDP is a session description format that describes multimedia sessions. 1328 A session description SHOULD NOT be trusted unless it has been obtained 1329 by an authenticated transport protocol from a trusted source. Many 1330 different transport protocols may be used to distribute session 1331 description, and the nature of the authentication will differ from 1332 transport to transport. 1334 One transport that will frequently be used to distribute session 1335 descriptions is the Session Announcement Protocol (SAP). SAP provides 1336 both encryption and authentication mechanisms but due to the nature of 1337 session announcements it is likely that there are many occasions where 1338 the originator of a session announcement cannot be authenticated because 1339 they are previously unknown to the receiver of the announcement and 1340 because no common public key infrastructure is available. 1342 On receiving a session description over an unauthenticated transport 1343 mechanism or from an untrusted party, software parsing the session 1344 should take a few precautions. Session descriptions contain information 1345 required to start software on the receivers system. Software that 1346 parses a session description MUST NOT be able to start other software 1347 except that which is specifically configured as appropriate software to 1348 participate in multimedia sessions. It is normally considered 1349 inappropriate for software parsing a session description to start, on a 1350 user's system, software that is appropriate to participate in multimedia 1351 sessions, without the user first being informed that such software will 1352 be started and giving their consent. Thus a session description 1353 arriving by session announcement, email, session invitation, or WWW page 1354 SHOULD NOT deliver the user into an interactive multimedia session 1355 without the user being aware that this will happen. As it is not always 1356 simple to tell whether a session is interactive or not, applications 1357 that are unsure should assume sessions are interactive. 1359 In this specification, there are no attributes which would allow the 1360 recipient of a session description to be informed to start multimedia 1361 tools in a mode where they default to transmitting. Under some 1362 circumstances it might be appropriate to define such attributes. If 1363 this is done an application parsing a session description containing 1364 such attributes SHOULD either ignore them, or inform the user that 1365 joining this session will result in the automatic transmission of 1366 multimedia data. The default behaviour for an unknown attribute is to 1367 ignore it. 1369 Session descriptions may be parsed at intermediate systems such as 1370 firewalls for the purposes of opening a hole in the firewall to allow 1371 the participation in multimedia sessions. It is considered 1372 inappropriate for a firewall to open such holes for unicast data streams 1373 unless the session description comes in a request from inside the 1374 firewall. For multicast sessions, it is likely that local 1375 administrators will apply their own policies, but the exclusive use of 1376 "local" or "site-local" administrative scope within the firewall and the 1377 refusal of the firewall to open a hole for such scopes will provide 1378 separation of global multicast sessions from local ones. 1380 Appendix A: SDP Grammar 1382 This appendix provides an Augmented BNF grammar for SDP. ABNF is 1383 defined in RFC 2234. 1385 ; SDP Syntax 1386 announcement = proto-version 1387 origin-field 1388 session-name-field 1389 information-field 1390 uri-field 1391 email-fields 1392 phone-fields 1393 connection-field 1394 bandwidth-fields 1395 time-fields 1396 key-field 1397 attribute-fields 1398 media-descriptions 1400 proto-version = "v=" 1*DIGIT CRLF 1401 ;this memo describes version 0 1403 origin-field = "o=" username SP sess-id SP sess-version SP 1404 nettype SP addrtype SP unicast-address CRLF 1406 session-name-field = "s=" text CRLF 1408 information-field = ["i=" text CRLF] 1410 uri-field = ["u=" uri CRLF] 1412 email-fields = *("e=" email-address CRLF) 1414 phone-fields = *("p=" phone-number CRLF) 1415 connection-field = ["c=" nettype SP addrtype SP 1416 connection-address CRLF] 1417 ;a connection field must be present 1418 ;in every media description or at the 1419 ;session-level 1421 bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF) 1423 time-fields = 1*( "t=" start-time SP stop-time 1424 *(CRLF repeat-fields) CRLF) 1425 [zone-adjustments CRLF] 1427 repeat-fields = "r=" repeat-interval SP typed-time 1428 1*(SP typed-time) 1430 zone-adjustments = "z=" time SP ["-"] typed-time 1431 *(SP time SP ["-"] typed-time) 1433 key-field = ["k=" key-type CRLF] 1435 attribute-fields = *("a=" attribute CRLF) 1437 media-descriptions = *( media-field 1438 information-field 1439 connection-field 1440 bandwidth-fields 1441 key-field 1442 attribute-fields ) 1444 media-field = "m=" media SP port ["/" integer] 1445 SP proto 1*(SP fmt) CRLF 1447 ; sub-rules of 'o=' 1448 username = non-ws-string 1449 ;pretty wide definition, but doesn't include space 1451 sess-id = 1*DIGIT 1452 ;should be unique for this originating username/host 1454 sess-version = 1*DIGIT 1455 ;0 is a new session 1457 nettype = token 1458 ;typically "IN" 1460 addrtype = token 1461 ;typically "IP4" or "IP6" 1463 ; sub-rules of 'u=' 1464 uri = URI-reference; defined in RFC1630 and RFC2732 1466 ; sub-rules of 'e=' 1467 email-address = email *SP "(" 1*email-safe ")" / 1468 1*email-safe "<" email ">" / 1469 email 1471 email = addr-spec ; defined in RFC2822 1472 ; modified to remove CFWS 1474 ; sub-rules of 'p=' 1475 phone-number = phone *SP "(" 1*email-safe ")" / 1476 1*email-safe "<" phone ">" / 1477 phone 1479 phone = "+" POS-DIGIT 1*(SP / "-" / DIGIT) 1480 ;there must be a space or hyphen between the 1481 ;international code and the rest of the number. 1483 ; sub-rules of 'c=' 1484 connection-address = multicast-address / unicast-address 1486 ; sub-rules of 'b=' 1487 bwtype = token 1488 bandwidth = 1*DIGIT 1490 ; sub-rules of 't=' 1491 start-time = time / "0" 1493 stop-time = time / "0" 1495 time = POS-DIGIT 9*DIGIT 1496 ; 10-digit NTP time represents times between 1497 ; 1931 and 5068 AD. 9* allows times after that 1498 ; as well. 1500 ; sub-rules of 'r=' and 'z=' 1501 repeat-interval = POS-DIGIT *DIGIT [fixed-len-time-unit] 1503 typed-time = 1*DIGIT [fixed-len-time-unit] 1505 fixed-len-time-unit = "d" / "h" / "m" / "s" 1507 ; sub-rules of 'k=' 1508 key-type = "prompt" / 1509 "clear:" text / 1510 "base64:" base64 / 1511 "uri:" uri / 1512 key-method [ ":" text ] 1514 base64 = *base64-unit [base64-pad] 1515 base64-unit = 4base64-char 1516 base64-pad = 2base64-char "==" / 3base64-char "=" 1517 base64-char = ALPHA / DIGIT / "+" / "/" 1519 key-method = token 1521 ; sub-rules of 'a=' 1522 attribute = (att-field ":" att-value) / att-field 1523 att-field = token 1525 att-value = byte-string 1527 ; sub-rules of 'm=' 1528 media = token 1529 ;typically "audio", "video", "application" 1530 ;or "data" 1532 fmt = token 1533 ;typically an RTP payload type for audio 1534 ;and video media 1536 proto = token "/" token 1537 / token 1538 ;typically "RTP/AVP" or "udp" for IP4 1540 port = 1*DIGIT 1541 ;should be either "0" or in the range "1024" to "65535" 1542 ;inclusive for UDP based media (a value "0" is used to 1543 ;signal special conditions in some uses of SDP) 1545 ; generic sub-rules: addressing 1546 unicast-address = IP4-address / IP6-address / FQDN / extension-addr 1548 multicast-address = IP4-multicast / IP6-multicast 1550 IP4-multicast = m1 3( "." decimal-uchar ) 1551 "/" ttl [ "/" integer ] 1552 ; IPv4 multicast addresses may be in the 1553 ; range 224.0.0.0 to 239.255.255.255 1555 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / 1556 ("23" DIGIT )) 1558 IP6-multicast = hexpart [ "/" integer ] 1559 ; IPv6 address starting with FF 1561 ttl = (POS-DIGIT *2DIGIT) / "0" 1562 FQDN = 4*(alpha-numeric / "-" / ".") 1563 ; fully qualified domain name as specified 1564 ; in RFC1035 1566 IP4-address = b1 3("." decimal-uchar) / "0.0.0.0" 1568 b1 = decimal-uchar 1569 ; less than "224"; not "0" or "127" 1571 ; The following is from RFC2373 Appendix B. It is a direct copy. 1572 IP6-address = hexpart [ ":" IP4-address ] 1574 hexpart = hexseq / hexseq "::" [ hexseq ] / 1575 "::" [ hexseq ] 1577 hexseq = hex4 *( ":" hex4) 1579 hex4 = 1*4HEXDIG 1581 ; Generic for other address families 1582 extension-addr = non-ws-string 1584 ; generic sub-rules: datatypes 1585 text = byte-string 1586 ;default is to interpret this as IS0-10646 UTF8 1587 ;ISO 8859-1 requires a "a=charset:ISO-8859-1" 1588 ;session-level attribute to be used 1590 byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF) 1591 ;any byte except NUL, CR or LF 1593 non-ws-string = 1*(VCHAR/%x80-FF) 1594 ;string of visible US-ASCII, or high-bit, characters 1596 token-char = %x21/%x23-27/%x2A-2B/%x2D-2E/%x30-39/%x41-5A/%x5E-7E 1597 ; definition from RFC 2045 - 1598 ; "any (US-ASCII) CHAR except SPACE, CTLs, 1599 ; or tspecials". 1600 ; the tspecials are ()<>@,;: 1602 token = 1*(token-char) 1604 email-safe = %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF 1605 ;any byte except NUL, CR, LF, or the quoting 1606 ;characters ()<> 1608 integer = POS-DIGIT *DIGIT 1610 ; generic sub-rules: primitives 1611 alpha-numeric = ALPHA / DIGIT 1613 POS-DIGIT = %x31-39 ; 1 - 9 1615 ; external references: 1616 ; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 2234 1617 ; URI-reference: from RFC1630 and RFC2732 1618 ; addr-spec: from RFC 2822 1619 Appendix B: IANA Considerations 1621 There are seven field names that may be registered with IANA. Using the 1622 terminology in the SDP specification BNF, they are "media", "proto", 1623 "fmt", "att-field", "bwtype", "nettype" and "addrtype". 1625 "media" (eg, audio, video, application, data). 1627 The set of media is intended to be small and not to be extended 1628 except under rare circumstances. The same rules should apply for 1629 media names as for top-level MIME content types, and where possible 1630 the same name should be registered for SDP as for MIME. For media 1631 other than existing MIME top-level content types, a standards-track 1632 RFC MUST be produced for a new top-level content type to be 1633 registered, and the registration MUST provide good justification 1634 why no existing media name is appropriate. 1636 "proto" 1638 In general this should be an IETF standards-track transport 1639 protocol identifier such as RTP/AVP (rfc 1889 under the rfc 1890 1640 profile). 1642 However, people will want to invent their own proprietary transport 1643 protocols. Some of these should be registered as a "fmt" using 1644 "udp" as the protocol and some of which probably can't be. 1646 Where the protocol and the application are intimately linked, such 1647 as with the LBL whiteboard wb which used a proprietary and special 1648 purpose protocol over UDP, the protocol name should be "udp" and 1649 the format name that should be registered is "wb". The rules for 1650 formats (see below) apply to such registrations. 1652 Where the proprietary transport protocol really carries many 1653 different data formats, it is possible to register a new protocol 1654 name with IANA. In such a case, an RFC MUST be produced describing 1655 the protocol and referenced in the registration. Such an RFC MAY 1656 be informational, although it is preferable if it is standards- 1657 track. 1659 "fmt" 1661 The format namespace is dependent on the context of the "proto" 1662 field, so a format cannot be registered without specifying one or 1663 more transport protocols that it applies to. 1665 Formats cover all the possible encodings that might want to be 1666 transported in a multimedia session. 1668 For RTP formats that have been assigned static payload types, the 1669 payload type number is used. For RTP formats using a dynamic 1670 payload type number, the dynamic payload type number is given as 1671 the format and an additional "rtpmap" attribute specifies the 1672 format and parameters. 1674 For non-RTP formats, any unregistered format name may be 1675 registered. If there is a suitable mapping from a MIME subtype to 1676 the format, then the MIME subtype name should be registered. If 1677 there is no suitable mapping from a MIME subtype, a new name should 1678 be registered. In either case, unless there are strong reasons not 1679 to do so, a standards-track RFC SHOULD be produced describing the 1680 format and this RFC SHOULD be referenced in the registration. 1682 "att-field" (Attribute names) 1684 Attribute field names SHOULD be registered with IANA, although this 1685 is not compulsory, and unknown attributes are simply ignored. 1687 When an attribute is registered, it must be accompanied by a brief 1688 specification stating the following: 1690 o contact name, email address and telephone number 1692 o attribute-name (as it will appear in SDP) 1694 o long-form attribute name in English 1696 o type of attribute (session level, media level, or both) 1698 o whether the attribute value is subject to the charset 1699 attribute. 1701 o a one paragraph explanation of the purpose of the attribute. 1703 o a specification of appropriate attribute values for this 1704 attribute. 1706 IANA will not sanity check such attribute registrations except to 1707 ensure that they do not clash with existing registrations. 1709 Although the above is the minimum that IANA will accept, if the 1710 attribute is expected to see widespread use and interoperability is 1711 an issue, authors are encouraged to produce a standards-track RFC 1712 that specifies the attribute more precisely. 1714 Submitters of registrations should ensure that the specification is 1715 in the spirit of SDP attributes, most notably that the attribute is 1716 platform independent in the sense that it makes no implicit 1717 assumptions about operating systems and does not name specific 1718 pieces of software in a manner that might inhibit interoperability. 1720 "bwtype" (bandwidth specifiers) 1722 A proliferation of bandwidth specifiers is strongly discouraged. 1724 New bandwidth specifiers may be registered with IANA. The 1725 submission MUST reference a standards-track RFC specifying the 1726 semantics of the bandwidth specifier precisely, and indicating when 1727 it should be used, and why the existing registered bandwidth 1728 specifiers do not suffice. 1730 "nettype" (Network Type) 1732 New network types may be registered with IANA if SDP needs to be 1733 used in the context of non-Internet environments. Whilst these are 1734 not normally the preserve of IANA, there may be circumstances when 1735 an Internet application needs to interoperate with a non-Internet 1736 application, such as when gatewaying an Internet telephony call 1737 into the PSTN. The number of network types should be small and 1738 should be rarely extended. A new network type cannot be registered 1739 without registering at least one address type to be used with that 1740 network type. A new network type registration MUST reference an 1741 RFC which gives details of the network type and address type and 1742 specifies how and when they would be used. Such an RFC MAY be 1743 Informational. 1745 "addrtype" (Address Type) 1747 New address types may be registered with IANA. An address type is 1748 only meaningful in the context of a network type, and any 1749 registration of an address type MUST specify a registered network 1750 type, or be submitted along with a network type registration. A 1751 new address type registration MUST reference an RFC giving details 1752 of the syntax of the address type. Such an RFC MAY be 1753 Informational. Address types are not expected to be registered 1754 frequently. 1756 Registration Procedure 1758 To register a name the above guidelines should be followed regarding the 1759 required level of documentation that is required. The registration 1760 itself should be sent to IANA. Attribute registrations should include 1761 the information given above. Other registrations should include the 1762 following additional information: 1764 o contact name, email address and telephone number 1766 o name being registered (as it will appear in SDP) 1768 o long-form name in English 1770 o type of name ("media", "proto", "fmt", "bwtype", "nettype", or 1771 "addrtype") 1773 o a one paragraph explanation of the purpose of the registered name. 1775 o a reference to the specification (eg RFC number) of the registered 1776 name. 1778 IANA may refer any registration to the IESG or to any appropriate IETF 1779 working group for review, and may request revisions to be made before a 1780 registration will be made. 1782 Appendix C: Changes from RFC 2327 1784 o Clarify that a=recvonly does NOT mean that you don't send RTCP, and 1785 similarly for sendonly and inactive. These only effect the RTP 1786 stream. 1788 o Rewrite the ABNF syntax (thanks to Jonathan Lennox) 1790 o Add a=inactive attribute. 1792 o Add a=maxptime attribute. 1794 o RFC 2327 mandated that either e= or p= was required. Both are now 1795 optional, to reflect actual usage. 1797 o Removed references to "conference" from the description of the t= 1798 line, to make it less SAP oriented. 1800 o Note about wrap-around of NTP timestamps in t= 1802 o Update BNF to support IPv6. 1804 o References have been updated. 1806 o Section 3.1 was replaced with a reference to RFC 2119, and the memo 1807 has been updated to use the RFC 2119 terminology (MUST, SHOULD, 1808 etc). 1810 o Use of "application/sdp" as MIME a type for SDP files is now "MUST" 1811 rather than "SHOULD". 1813 o A number of sections have been updated to be less SAP specific, and 1814 to reference other current uses of SDP such as RTSP and SIP. 1816 o The section on concatenation of session descriptions (which was not 1817 allowed in SAP, but allowed in other cases) has been removed. It is 1818 assumed that transports of SDP specify will specify this. 1820 o The description of the c= line has been updated to reflect common 1821 usage of SDP, rather than Mbone conferencing with SAP. 1823 o The b= line no longer makes a normative reference to the Mbone FAQ 1824 for bandwidth limits at various TTLs. The AS modifier to b= is 1825 noted as being the RTP session bandwidth. 1827 o Define relation between the m= line and MIME types 1828 o Note use of s= in sessions with no meaningful name 1830 o Note that a=rtpmap is a media level attribute 1832 Appendix D: Authors' Addresses 1834 Mark Handley 1835 International Computer Science Institute, 1836 1947 Center Street, Suite 600, 1837 Berkeley, CA 94704 1838 United States 1839 Email: mjh@icir.org 1841 Van Jacobson 1842 Packet Design 1843 2465 Latham Street 1844 Mountain View, CA 94040 1845 United States 1846 Email: van@packetdesign.com 1848 Colin Perkins 1849 USC Information Sciences Institute 1850 3811 N. Fairfax Drive, Suite 200 1851 Arlington, VA 22203 1852 United States 1853 Email: csp@csperkins.org 1855 Acknowledgments 1857 Many people in the IETF MMUSIC working group have made comments and 1858 suggestions contributing to this document. In particular, we would like 1859 to thank Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin, Ross 1860 Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Steve Hanna and 1861 Jonathan Lennox. 1863 References 1865 [1] D. Mills, ``Network Time Protocol (version 3) specification and 1866 implementation", RFC 1305, March 1992. 1868 [2] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson, ``RTP: A 1869 Transport Protocol for Real-Time Applications'', RFC 1889, January 1870 1996. 1872 [3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with 1873 Minimal Control'', RFC 1890, January 1996. 1875 [4] M. Handley, C. Perkins and E. Whelan, ``Session Announcement 1876 Protocol'', RFC 2974, October 2000. 1878 [5] V. Jacobson and S. McCanne, ``vat - X11-based audio teleconferencing 1879 tool'' vat manual page, Lawrence Berkeley Laboratory, 1994. 1881 [6] The Unicode Consortium, "The Unicode Standard -- Version 2.0", 1882 Addison-Wesley, 1996. 1884 [7] ISO/IEC 10646-1:1993. International Standard -- Information 1885 technology -- Universal Multiple-Octet Coded Character Set (UCS) 1886 -- Part 1: Architecture and Basic Multilingual Plane. Five 1887 amendments and a technical corrigendum have been published up 1888 to now. UTF-8 is described in Annex R, published as Amendment 2. 1890 [8] D. Goldsmith and M. Davis, ``Using Unicode with MIME'', RFC1641, 1891 July 1994 1893 [9] F. Yergeau, ``UTF-8, a transformation format of ISO 10646'', 1894 RFC 2279, January 1998 1896 [10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for 1897 Receiving Internet-based H.323 Conferences", ITU, Geneva. 1899 [11] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, 1900 J. Peterson, R. Sparks, M. Handley, E. Schooler ``SIP: Session 1901 Initiatation Protocol'', RFC 3261, May 2002. 1903 [12] H. Schulzrinne, A. Rao and R. Lanphier, ``Real Time Streaming 1904 Protocol (RTSP)'' RFC 2326, April 1998. 1906 [13] S. Bradner, ``Key words for use in RFCs to Indicate Requirement 1907 Levels'', RFC 2119, March 1997. 1909 [14] J. Rosenberg and H. Schulzrinne, ``An Offer/Answer Model with 1910 SDP'', RFC 3264, May 2002. 1912 [15] C. Huitema, ``RTCP Attribute in SDP'', RFC XXXX, May 2002