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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-10.txt Van Jacobson/Packet Design 4 Colin Perkins/ISI 5 27 May 2002 6 Expires: November 2002 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 SDP parsers MUST ignore bandwidth fields with unknown modifiers. 665 Modifiers MUST be alpha-numeric and, although no length limit is 666 given, they are recommended to be short. 668 Times, Repeat Times and Time Zones 670 t= 672 o ``t='' fields specify the start and stop times for a session. 673 Multiple ``t='' fields MAY be used if a session is active at 674 multiple irregularly spaced times; each additional ``t='' field 675 specifies an additional period of time for which the session will be 676 active. If the session is active at regular times, an ``r='' field 677 (see below) should be used in addition to and following a ``t='' 678 field - in which case the ``t='' field specifies the start and stop 679 times of the repeat sequence. 681 o The first and second sub-fields give the start and stop times for 682 the session respectively. These values are the decimal 683 representation of Network Time Protocol (NTP) time values in seconds 684 [1]. To convert these values to UNIX time, subtract decimal 685 2208988800. 687 NTP timestamps are 64 bit values which wrap sometime in the year 688 2036. Since SDP uses an arbitrary length decimal representation, 689 this should not cause an issue (SDP timestamps will continue 690 counting seconds since 1900, NTP will use the value modulo the 64 691 bit limit). 693 o If the stop-time is set to zero, then the session is not bounded, 694 though it will not become active until after the start-time. If the 695 start-time is also zero, the session is regarded as permanent. 697 User interfaces SHOULD strongly discourage the creation of unbounded 698 and permanent sessions as they give no information about when the 699 session is actually going to terminate, and so make scheduling 700 difficult. 702 The general assumption may be made, when displaying unbounded 703 sessions that have not timed out to the user, that an unbounded 704 session will only be active until half an hour from the current time 705 or the session start time, whichever is the later. If behaviour 706 other than this is required, an end-time should be given and 707 modified as appropriate when new information becomes available about 708 when the session should really end. 710 Permanent sessions may be shown to the user as never being active 711 unless there are associated repeat times which state precisely when 712 the session will be active. In general, permanent sessions SHOULD 713 NOT be created for any session expected to have a duration of less 714 than 2 months, and should be discouraged for sessions expected to 715 have a duration of less than 6 months. 717 r= 719 o ``r='' fields specify repeat times for a session. For example, if 720 a session is active at 10am on Monday and 11am on Tuesday for one 721 hour each week for three months, then the in the 722 corresponding ``t='' field would be the NTP representation of 10am 723 on the first Monday, the would be 1 week, the 724 would be 1 hour, and the offsets would be zero and 725 25 hours. The corresponding ``t='' field stop time would be the NTP 726 representation of the end of the last session three months later. By 727 default all fields are in seconds, so the ``r='' and ``t='' fields 728 might be: 730 t=3034423619 3042462419 731 r=604800 3600 0 90000 733 To make description more compact, times may also be given in units 734 of days, hours or minutes. The syntax for these is a number 735 immediately followed by a single case-sensitive character. 736 Fractional units are not allowed - a smaller unit should be used 737 instead. The following unit specification characters are allowed: 739 d - days (86400 seconds) 740 h - hours (3600 seconds) 741 m - minutes (60 seconds) 742 s - seconds (allowed for completeness but not recommended) 744 Thus, the above announcement could also have been written: 746 r=7d 1h 0 25h 748 Monthly and yearly repeats cannot be directly specified with a 749 single SDP repeat time - instead separate "t" fields should be used 750 to explicitly list the session times. 752 z= .... 754 o To schedule a repeated session which spans a change from daylight- 755 saving time to standard time or vice-versa, it is necessary to 756 specify offsets from the base time. This is required because 757 different time zones change time at different times of day, 758 different countries change to or from daylight time on different 759 dates, and some countries do not have daylight saving time at all. 761 Thus in order to schedule a session that is at the same time winter 762 and summer, it must be possible to specify unambiguously by whose 763 time zone a session is scheduled. To simplify this task for 764 receivers, we allow the sender to specify the NTP time that a time 765 zone adjustment happens and the offset from the time when the 766 session was first scheduled. The ``z'' field allows the sender to 767 specify a list of these adjustment times and offsets from the base 768 time. 770 An example might be: 772 z=2882844526 -1h 2898848070 0 774 This specifies that at time 2882844526 the time base by which the 775 session's repeat times are calculated is shifted back by 1 hour, and 776 that at time 2898848070 the session's original time base is 777 restored. Adjustments are always relative to the specified start 778 time - they are not cumulative. Adjustments apply to all ``t='' and 779 ``r='' lines in a session description. 781 o If a session is likely to last several years, it is expected that 782 the session announcement will be modified periodically rather than 783 transmit several years worth of adjustments in one announcement. 785 Encryption Keys 787 k= 788 k=: 790 o The session description protocol MAY be used to convey encryption 791 keys. A key field is permitted before the first media entry (in 792 which case it applies to all media in the session), or for each 793 media entry as required. 795 o The format of keys and their usage is outside the scope of this 796 document, but see [3]. 798 o The method indicates the mechanism to be used to obtain a usable key 799 by external means, or from the encoded encryption key given. The 800 following methods are defined: 802 k=clear: 803 The encryption key (as described in [3] for RTP media streams 804 under the AV profile) is included untransformed in this key 805 field. 807 k=base64: 808 The encryption key (as described in [3] for RTP media streams 809 under the AV profile) is included in this key field but has been 810 base64 encoded because it includes characters that are 811 prohibited in SDP. 813 k=uri: 814 A Universal Resource Identifier as used by WWW clients is 815 included in this key field. The URI refers to the data 816 containing the key, and may require additional authentication 817 before the key can be returned. When a request is made to the 818 given URI, the MIME content-type of the reply specifies the 819 encoding for the key in the reply. The key should not be 820 obtained until the user wishes to join the session to reduce 821 synchronisation of requests to the WWW server(s). 823 k=prompt 824 No key is included in this SDP description, but the session or 825 media stream referred to by this key field is encrypted. The 826 user should be prompted for the key when attempting to join the 827 session, and this user-supplied key should then be used to 828 decrypt the media streams. 830 Attributes 832 a= 833 a=: 835 Attributes are the primary means for extending SDP. Attributes may be 836 defined to be used as "session-level" attributes, "media-level" 837 attributes, or both. 839 A media description may have any number of attributes (``a='' fields) 840 which are media specific. These are referred to as "media-level" 841 attributes and add information about the media stream. Attribute fields 842 can also be added before the first media field; these "session-level" 843 attributes convey additional information that applies to the conference 844 as a whole rather than to individual media; an example might be the 845 conference's floor control policy. 847 Attribute fields may be of two forms: 849 o property attributes. A property attribute is simply of the form 850 ``a=''. These are binary attributes, and the presence of the 851 attribute conveys that the attribute is a property of the session. 852 An example might be ``a=recvonly''. 854 o value attributes. A value attribute is of the form 855 ``a=:''. An example might be that a whiteboard 856 could have the value attribute ``a=orient:landscape'' 858 Attribute interpretation depends on the media tool being invoked. Thus 859 receivers of session descriptions should be configurable in their 860 interpretation of announcements in general and of attributes in 861 particular. 863 Attribute names MUST be in the US-ASCII subset of ISO-10646/UTF-8. 865 Attribute values are octet strings, and MAY use any octet value except 866 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values are 867 to be interpreted as in ISO-10646 character set with UTF-8 encoding. 868 Unlike other text fields, attribute values are NOT normally affected by 869 the `charset' attribute as this would make comparisons against known 870 values problematic. However, when an attribute is defined, it can be 871 defined to be charset-dependent, in which case it's value should be 872 interpreted in the session charset rather than in ISO-10646. 874 Attributes that will be commonly used can be registered with IANA (see 875 Appendix B). Unregistered attributes should begin with "X-" to prevent 876 inadvertent collision with registered attributes. In either case, if an 877 attribute is received that is not understood, it should simply be 878 ignored by the receiver. 880 Media Announcements 882 m= 884 A session description may contain a number of media descriptions. Each 885 media description starts with an ``m='' field, and is terminated by 886 either the next ``m='' field or by the end of the session description. 887 A media field also has several sub-fields: 889 o The first sub-field is the media type. Currently defined media are 890 ``audio'', ``video'', ``application'', ``data'' and ``control'', 891 though this list may be extended as new communication modalities 892 emerge (e.g., telepresense). The difference between ``application'' 893 and ``data'' is that the former is a media flow such as whiteboard 894 information, and the latter is bulk-data transfer such as 895 multicasting of program executables which will not typically be 896 displayed to the user. ``control'' is used to specify an additional 897 conference control channel for the session. 899 o The second sub-field is the transport port to which the media stream 900 will be sent. The meaning of the transport port depends on the 901 network being used as specified in the relevant ``c'' field and on 902 the transport protocol defined in the third sub-field. Other ports 903 used by the media application (such as the RTCP port, see [2]) MAY 904 be derived algorithmically from the base media port or MAY be 905 specified in a separate attribute (e.g. ``a=rtcp:'' as defined in 906 [15]). 908 Note: For transports based on UDP, the value should be in the range 909 1024 to 65535 inclusive. For RTP compliance it SHOULD be an even 910 number. 912 For applications where hierarchically encoded streams are being sent 913 to a unicast address, it may be necessary to specify multiple 914 transport ports. This is done using a similar notation to that used 915 for IP multicast addresses in the ``c='' field: 917 m= / 919 In such a case, the ports used depend on the transport protocol. 920 For RTP, only the even ports are used for data and the corresponding 921 one-higher odd port is used for RTCP. For example: 923 m=video 49170/2 RTP/AVP 31 925 would specify that ports 49170 and 49171 form one RTP/RTCP pair and 926 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the 927 transport protocol and 31 is the format (see below). 929 If multiple addresses are specified in the ``c='' field and multiple 930 ports are specified in the ``m='' field, a one-to-one mapping from 931 port to the corresponding address is implied. For example: 933 c=IN IP4 224.2.1.1/127/2 934 m=video 49170/2 RTP/AVP 31 936 would imply that address 224.2.1.1 is used with ports 49170 and 937 49171, and address 224.2.1.2 is used with ports 49172 and 49173. 939 o The third sub-field is the transport protocol. The transport 940 protocol values are dependent on the address-type field in the 941 ``c='' fields. Thus a ``c='' field of IP4 defines that the 942 transport protocol runs over IP4. For IP4, it is normally expected 943 that most media traffic will be carried as RTP over UDP. The 944 following transport protocols are defined, but may be extended 945 through registration of new protocols with IANA (see Appendix B): 947 - RTP/AVP - the IETF's Realtime Transport Protocol using the 948 Audio/Video profile carried over UDP. 950 - udp - User Datagram Protocol 952 If an application uses a single combined proprietary media format 953 and transport protocol over UDP, then simply specifying the 954 transport protocol as udp and using the format field to distinguish 955 the combined protocol is recommended. If a transport protocol is 956 used over UDP to carry several distinct media types that need to be 957 distinguished by a session directory, then specifying the transport 958 protocol and media format separately is necessary. RTP is an 959 example of a transport-protocol that carries multiple payload 960 formats that must be distinguished by the session directory for it 961 to know how to start appropriate tools, relays, mixers or recorders. 963 The main reason to specify the transport-protocol in addition to the 964 media format is that the same standard media formats may be carried 965 over different transport protocols even when the network protocol is 966 the same - a historical example is vat PCM audio and RTP PCM audio. 967 In addition, relays and monitoring tools that are transport- 968 protocol-specific but format-independent are possible. 970 For RTP media streams operating under the RTP Audio/Video Profile 971 [3], the protocol field is ``RTP/AVP''. Should other RTP profiles 972 be defined in the future, their profiles will be specified in the 973 same way. For example, the protocol field ``RTP/XYZ'' would specify 974 RTP operating under a profile whose short name is ``XYZ''. 976 o The fourth and subsequent sub-fields are media formats. For audio 977 and video, these SHOULD reference a MIME sub-type describing the 978 format under the `audio' and `video' top-level MIME types. 980 When a list of payload formats is given, this implies that all of 981 these formats may be used in the session, but the first of these 982 formats SHOULD be used as the default format for the session. 984 For media whose transport protocol is not RTP or UDP the format 985 field is protocol specific. Such formats should be defined in an 986 additional specification document. 988 For media whose transport protocol is RTP, SDP can be used to 989 provide a dynamic binding of media encoding to RTP payload type. The 990 encoding names in the RTP AV Profile do not specify unique audio 991 encodings (in terms of clock rate and number of audio channels), and 992 so they are not used directly in SDP format fields. Instead, the 993 payload type number should be used to specify the format for static 994 payload types and the payload type number along with additional 995 encoding information should be used for dynamically allocated 996 payload types. 998 An example of a static payload type is u-law PCM coded single 999 channel audio sampled at 8kHz. This is completely defined in the 1000 RTP Audio/Video profile as payload type 0, so the media field for 1001 such a stream sent to UDP port 49232 is: 1003 m=audio 49232 RTP/AVP 0 1005 An example of a dynamic payload type is 16 bit linear encoded stereo 1006 audio sampled at 16KHz. If we wish to use dynamic RTP/AVP payload 1007 type 98 for such a stream, additional information is required to 1008 decode it: 1010 m=video 49232 RTP/AVP 98 1011 a=rtpmap:98 L16/16000/2 1013 The general form of an rtpmap attribute is: 1015 a=rtpmap: /[/] 1017 For audio streams, may specify the number of 1018 audio channels. This parameter may be omitted if the number of 1019 channels is one provided no additional parameters are needed. 1020 For video streams, no encoding parameters are currently specified. 1022 Additional parameters may be defined in the future, but codec- 1023 specific parameters SHOULD NOT be added. Parameters added to an 1024 rtpmap attribute SHOULD only be those required for a session 1025 directory to make the choice of appropriate media to participate in 1026 a session. Codec-specific parameters should be added in other 1027 attributes (for example, ``a=fmtp:''). 1029 Up to one rtpmap attribute can be defined for each media format 1030 specified. Thus we might have: 1032 m=audio 49230 RTP/AVP 96 97 98 1033 a=rtpmap:96 L8/8000 1034 a=rtpmap:97 L16/8000 1035 a=rtpmap:98 L16/11025/2 1037 RTP profiles that specify the use of dynamic payload types MUST 1038 define the set of valid encoding names and/or a means to register 1039 encoding names if that profile is to be used with SDP. 1041 Experimental encoding formats can also be specified using rtpmap. 1042 RTP formats that are not registered as standard format names must be 1043 preceded by ``X-''. Thus a new experimental redundant audio stream 1044 called GSMLPC using dynamic payload type 99 could be specified as: 1046 m=audio 49232 RTP/AVP 99 1047 a=rtpmap:99 X-GSMLPC/8000 1049 Such an experimental encoding requires that any site wishing to 1050 receive the media stream has relevant configured state in its 1051 session directory to know which tools are appropriate. 1053 Note that RTP audio formats typically do not include information 1054 about the number of samples per packet. If a non-default (as 1055 defined in the RTP Audio/Video Profile) packetisation is required, 1056 the``ptime'' attribute is used as given below. 1058 For more details on RTP audio and video formats, see [3]. 1060 o Predefined formats for UDP protocol non-RTP media are as below. 1062 Application Formats: 1064 wb: LBL Whiteboard (transport: udp) 1066 nt: UCL Network Text Editor (transport: udp) 1068 Suggested Attributes 1070 The following attributes are suggested. Since application writers may 1071 add new attributes as they are required, this list is not exhaustive. 1073 a=cat: 1074 This attribute gives the dot-separated hierarchical category of the 1075 session. This is to enable a receiver to filter unwanted sessions 1076 by category. It would probably have been a compulsory separate 1077 field, except for its experimental nature at this time. It is a 1078 session-level attribute, and is not dependent on charset. 1080 a=keywds: 1081 Like the cat attribute, this is to assist identifying wanted 1082 sessions at the receiver. This allows a receiver to select 1083 interesting session based on keywords describing the purpose of the 1084 session. It is a session-level attribute. It is a charset dependent 1085 attribute, meaning that its value should be interpreted in the 1086 charset specified for the session description if one is specified, 1087 or by default in ISO 10646/UTF-8. 1089 a=tool: 1090 This gives the name and version number of the tool used to create 1091 the session description. It is a session-level attribute, and is 1092 not dependent on charset. 1094 a=ptime: 1095 This gives the length of time in milliseconds represented by the 1096 media in a packet. This is probably only meaningful for audio data, 1097 but may be used with other media types if it makes sense. It should 1098 not be necessary to know ptime to decode RTP or vat audio, and it is 1099 intended as a recommendation for the encoding/packetisation of 1100 audio. It is a media attribute, and is not dependent on charset. 1102 a=maxptime: 1103 The maximum amount of media which can be encapsulated in each 1104 packet, expressed as time in milliseconds. The time SHALL be 1105 calculated as the sum of the time the media present in the packet 1106 represents. The time SHOULD be a multiple of the frame size. This 1107 attribute is probably only meaningful for audio data, but may be 1108 used with other media types if it makes sense. It is a media 1109 attribute, and is not dependent on charset. Note that this 1110 attribute was introduced after RFC 2327, and non updated 1111 implementations will ignore this attribute. 1113 a=rtpmap: /[/] 1115 See the section on Media Announcements (the ``m='' field). This may 1116 be either a session or media attribute. 1118 a=recvonly 1119 This specifies that the tools should be started in receive-only mode 1120 where applicable. It can be either a session or media attribute, and 1121 is not dependent on charset. Note that recvonly applies to the media 1122 only, not to any associated control protocol (e.g. an RTP based 1123 system in recvonly mode SHOULD still send RTCP packets). 1125 a=sendrecv 1126 This specifies that the tools should be started in send and receive 1127 mode. This is necessary for interactive conferences with tools such 1128 as wb which defaults to receive only mode. It can be either a 1129 session or media attribute, and is not dependent on charset. 1131 If none of the attributes "sendonly", "recvonly", "inactive", and 1132 "sendrecv" is present, "sendrecv" SHOULD be assumed as the default 1133 for sessions which are not of the conference type "broadcast" or 1134 "H332" (see below). 1136 a=sendonly 1137 This specifies that the tools should be started in send-only mode. 1138 An example may be where a different unicast address is to be used 1139 for a traffic destination than for a traffic source. In such a case, 1140 two media descriptions may be use, one sendonly and one recvonly. It 1141 can be either a session or media attribute, but would normally only 1142 be used as a media attribute, and is not dependent on charset. Note 1143 that sendonly applies only to the media, and any associated control 1144 protocol (e.g. RTCP) SHOULD still be received and processed as 1145 normal. 1147 a=inactive 1148 This specifies that the tools should be started in inactive mode. 1149 This is necessary for interactive conferences where users can put 1150 other users on hold. No media is sent over an inactive media stream. 1151 Note that an RTP based system SHOULD still send RTCP, even if 1152 started inactive. It can be either a session or media attribute, and 1153 is not dependent on charset. 1155 a=orient: 1156 Normally this is only used in a whiteboard media specification. It 1157 specifies the orientation of a the whiteboard on the screen. It is 1158 a media attribute. Permitted values are `portrait', `landscape' and 1159 `seascape' (upside down landscape). It is not dependent on charset. 1161 a=type: 1162 This specifies the type of the conference. Suggested values are 1163 `broadcast', `meeting', `moderated', `test' and `H332'. `recvonly' 1164 should be the default for `type:broadcast' sessions, `type:meeting' 1165 should imply `sendrecv' and `type:moderated' should indicate the use 1166 of a floor control tool and that the media tools are started so as 1167 to ``mute'' new sites joining the conference. 1169 Specifying the attribute type:H332 indicates that this loosely 1170 coupled session is part of a H.332 session as defined in the ITU 1171 H.332 specification [10]. Media tools should be started `recvonly'. 1173 Specifying the attribute type:test is suggested as a hint that, 1174 unless explicitly requested otherwise, receivers can safely avoid 1175 displaying this session description to users. 1177 The type attribute is a session-level attribute, and is not 1178 dependent on charset. 1180 a=charset: 1181 This specifies the character set to be used to display the session 1182 name and information data. By default, the ISO-10646 character set 1183 in UTF-8 encoding is used. If a more compact representation is 1184 required, other character sets may be used such as ISO-8859-1 for 1185 Northern European languages. In particular, the ISO 8859-1 is 1186 specified with the following SDP attribute: 1188 a=charset:ISO-8859-1 1190 This is a session-level attribute; if this attribute is present, it 1191 must be before the first media field. The charset specified MUST be 1192 one of those registered with IANA, such as ISO-8859-1. The 1193 character set identifier is a US-ASCII string and MUST be compared 1194 against the IANA identifiers using a case-insensitive comparison. 1195 If the identifier is not recognised or not supported, all strings 1196 that are affected by it SHOULD be regarded as octet strings. 1198 Note that a character set specified MUST still prohibit the use of 1199 bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets requiring 1200 the use of these characters MUST define a quoting mechanism that 1201 prevents these bytes appearing within text fields. 1203 a=sdplang: 1204 This can be a session level attribute or a media level attribute. 1205 As a session level attribute, it specifies the language for the 1206 session description. As a media level attribute, it specifies the 1207 language for any media-level SDP information field associated with 1208 that media. Multiple sdplang attributes can be provided either at 1209 session or media level if multiple languages in the session 1210 description or media use multiple languages, in which case the order 1211 of the attributes indicates the order of importance of the various 1212 languages in the session or media from most important to least 1213 important. 1215 In general, sending session descriptions consisting of multiple 1216 languages is discouraged. Instead, multiple descriptions SHOULD be 1217 sent describing the session, one in each language. However this is 1218 not possible with all transport mechanisms, and so multiple sdplang 1219 attributes are allowed although NOT RECOMMENDED. 1221 The sdplang attribute value must be a single RFC 1766 language tag 1222 in US-ASCII. It is not dependent on the charset attribute. An 1223 sdplang attribute SHOULD be specified when a session is of 1224 sufficient scope to cross geographic boundaries where the language 1225 of recipients cannot be assumed, or where the session is in a 1226 different language from the locally assumed norm. 1228 a=lang: 1229 This can be a session level attribute or a media level attribute. 1230 As a session level attribute, it specifies the default language for 1231 the session being described. As a media level attribute, it 1232 specifies the language for that media, overriding any session-level 1233 language specified. Multiple lang attributes can be provided either 1234 at session or media level if multiple languages if the session 1235 description or media use multiple languages, in which case the order 1236 of the attributes indicates the order of importance of the various 1237 languages in the session or media from most important to least 1238 important. 1240 The lang attribute value must be a single RFC 1766 language tag in 1241 US-ASCII. It is not dependent on the charset attribute. A lang 1242 attribute SHOULD be specified when a session is of sufficient scope 1243 to cross geographic boundaries where the language of recipients 1244 cannot be assumed, or where the session is in a different language 1245 from the locally assumed norm. 1247 a=framerate: 1248 This gives the maximum video frame rate in frames/sec. It is 1249 intended as a recommendation for the encoding of video data. 1250 Decimal representations of fractional values using the notation 1251 "." are allowed. It is a media attribute, is 1252 only defined for video media, and is not dependent on charset. 1254 a=quality: 1255 This gives a suggestion for the quality of the encoding as an 1256 integer value. 1258 The intention of the quality attribute for video is to specify a 1259 non-default trade-off between frame-rate and still-image quality. 1260 For video, the value in the range 0 to 10, with the following 1261 suggested meaning: 1263 10 - the best still-image quality the compression scheme can give. 1265 5 - the default behaviour given no quality suggestion. 1267 0 - the worst still-image quality the codec designer thinks is 1268 still usable. 1269 It is a media attribute, and is not dependent on charset. 1271 a=fmtp: 1272 This attribute allows parameters that are specific to a particular 1273 format to be conveyed in a way that SDP doesn't have to understand 1274 them. The format must be one of the formats specified for the 1275 media. Format-specific parameters may be any set of parameters 1276 required to be conveyed by SDP and given unchanged to the media tool 1277 that will use this format. 1279 It is a media attribute, and is not dependent on charset. 1281 6.1. Communicating Conference Control Policy 1283 There is some debate over the way conference control policy should be 1284 communicated. In general, the authors believe that an implicit 1285 declarative style of specifying conference control is desirable where 1286 possible. 1288 A simple declarative style uses a single conference attribute field 1289 before the first media field, possibly supplemented by properties such 1290 as `recvonly' for some of the media tools. This conference attribute 1291 conveys the conference control policy. An example might be: 1293 a=type:moderated 1295 In some cases, however, it is possible that this may be insufficient to 1296 communicate the details of an unusual conference control policy. If 1297 this is the case, then a conference attribute specifying external 1298 control might be set, and then one or more ``media'' fields might be 1299 used to specify the conference control tools and configuration data for 1300 those tools. An example is an ITU H.332 session: 1302 ... 1303 c=IN IP4 224.5.6.7 1304 a=type:H332 1305 m=audio 49230 RTP/AVP 0 1306 m=video 49232 RTP/AVP 31 1307 m=application 12349 udp wb 1308 m=control 49234 H323 mc 1309 c=IN IP4 134.134.157.81 1311 In this example, a general conference attribute (type:H332) is specified 1312 stating that conference control will be provided by an external H.332 1313 tool, and a contact addresses for the H.323 session multipoint 1314 controller is given. 1316 In this document, only the declarative style of conference control 1317 declaration is specified. Other forms of conference control should 1318 specify an appropriate type attribute, and should define the 1319 implications this has for control media. 1321 7. Security Considerations 1323 SDP is a session description format that describes multimedia sessions. 1324 A session description SHOULD NOT be trusted unless it has been obtained 1325 by an authenticated transport protocol from a trusted source. Many 1326 different transport protocols may be used to distribute session 1327 description, and the nature of the authentication will differ from 1328 transport to transport. 1330 One transport that will frequently be used to distribute session 1331 descriptions is the Session Announcement Protocol (SAP). SAP provides 1332 both encryption and authentication mechanisms but due to the nature of 1333 session announcements it is likely that there are many occasions where 1334 the originator of a session announcement cannot be authenticated because 1335 they are previously unknown to the receiver of the announcement and 1336 because no common public key infrastructure is available. 1338 On receiving a session description over an unauthenticated transport 1339 mechanism or from an untrusted party, software parsing the session 1340 should take a few precautions. Session descriptions contain information 1341 required to start software on the receivers system. Software that 1342 parses a session description MUST NOT be able to start other software 1343 except that which is specifically configured as appropriate software to 1344 participate in multimedia sessions. It is normally considered 1345 inappropriate for software parsing a session description to start, on a 1346 user's system, software that is appropriate to participate in multimedia 1347 sessions, without the user first being informed that such software will 1348 be started and giving their consent. Thus a session description 1349 arriving by session announcement, email, session invitation, or WWW page 1350 SHOULD NOT deliver the user into an interactive multimedia session 1351 without the user being aware that this will happen. As it is not always 1352 simple to tell whether a session is interactive or not, applications 1353 that are unsure should assume sessions are interactive. 1355 In this specification, there are no attributes which would allow the 1356 recipient of a session description to be informed to start multimedia 1357 tools in a mode where they default to transmitting. Under some 1358 circumstances it might be appropriate to define such attributes. If 1359 this is done an application parsing a session description containing 1360 such attributes SHOULD either ignore them, or inform the user that 1361 joining this session will result in the automatic transmission of 1362 multimedia data. The default behaviour for an unknown attribute is to 1363 ignore it. 1365 Session descriptions may be parsed at intermediate systems such as 1366 firewalls for the purposes of opening a hole in the firewall to allow 1367 the participation in multimedia sessions. It is considered 1368 inappropriate for a firewall to open such holes for unicast data streams 1369 unless the session description comes in a request from inside the 1370 firewall. For multicast sessions, it is likely that local 1371 administrators will apply their own policies, but the exclusive use of 1372 "local" or "site-local" administrative scope within the firewall and the 1373 refusal of the firewall to open a hole for such scopes will provide 1374 separation of global multicast sessions from local ones. 1376 Appendix A: SDP Grammar 1378 This appendix provides an Augmented BNF grammar for SDP. ABNF is 1379 defined in RFC 2234. 1381 ; SDP Syntax 1382 announcement = proto-version 1383 origin-field 1384 session-name-field 1385 information-field 1386 uri-field 1387 email-fields 1388 phone-fields 1389 connection-field 1390 bandwidth-fields 1391 time-fields 1392 key-field 1393 attribute-fields 1394 media-descriptions 1396 proto-version = "v=" 1*DIGIT CRLF 1397 ;this memo describes version 0 1399 origin-field = "o=" username SP sess-id SP sess-version SP 1400 nettype SP addrtype SP unicast-address CRLF 1402 session-name-field = "s=" text CRLF 1404 information-field = ["i=" text CRLF] 1406 uri-field = ["u=" uri CRLF] 1408 email-fields = *("e=" email-address CRLF) 1410 phone-fields = *("p=" phone-number CRLF) 1411 connection-field = ["c=" nettype SP addrtype SP 1412 connection-address CRLF] 1413 ;a connection field must be present 1414 ;in every media description or at the 1415 ;session-level 1417 bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF) 1419 time-fields = 1*( "t=" start-time SP stop-time 1420 *(CRLF repeat-fields) CRLF) 1421 [zone-adjustments CRLF] 1423 repeat-fields = "r=" repeat-interval SP typed-time 1424 1*(SP typed-time) 1426 zone-adjustments = "z=" time SP ["-"] typed-time 1427 *(SP time SP ["-"] typed-time) 1429 key-field = ["k=" key-type CRLF] 1431 attribute-fields = *("a=" attribute CRLF) 1433 media-descriptions = *( media-field 1434 information-field 1435 *connection-field 1436 bandwidth-fields 1437 key-field 1438 attribute-fields ) 1440 media-field = "m=" media SP port ["/" integer] 1441 SP proto 1*(SP fmt) CRLF 1443 ; sub-rules of 'o=' 1444 username = non-ws-string 1445 ;pretty wide definition, but doesn't include space 1447 sess-id = 1*DIGIT 1448 ;should be unique for this originating username/host 1450 sess-version = 1*DIGIT 1451 ;0 is a new session 1453 nettype = token 1454 ;typically "IN" 1456 addrtype = token 1457 ;typically "IP4" or "IP6" 1459 ; sub-rules of 'u=' 1460 uri = URI-reference; defined in RFC1630 and RFC2732 1462 ; sub-rules of 'e=' 1463 email-address = email *SP "(" 1*email-safe ")" / 1464 1*email-safe "<" email ">" / 1465 email 1467 email = addr-spec ; defined in RFC2822 1468 ; modified to remove CFWS 1470 ; sub-rules of 'p=' 1471 phone-number = phone *SP "(" 1*email-safe ")" / 1472 1*email-safe "<" phone ">" / 1473 phone 1475 phone = "+" POS-DIGIT 1*(SP / "-" / DIGIT) 1476 ;there must be a space or hyphen between the 1477 ;international code and the rest of the number. 1479 ; sub-rules of 'c=' 1480 connection-address = multicast-address / unicast-address 1482 ; sub-rules of 'b=' 1483 bwtype = token 1484 bandwidth = 1*DIGIT 1486 ; sub-rules of 't=' 1487 start-time = time / "0" 1489 stop-time = time / "0" 1491 time = POS-DIGIT 9*DIGIT 1492 ; 10-digit NTP time represents times between 1493 ; 1931 and 5068 AD. 9* allows times after that 1494 ; as well. 1496 ; sub-rules of 'r=' and 'z=' 1497 repeat-interval = typed-time 1499 typed-time = POS-DIGIT *DIGIT [fixed-len-time-unit] 1501 fixed-len-time-unit = "d" / "h" / "m" / "s" 1503 ; sub-rules of 'k=' 1504 key-type = "prompt" / 1505 "clear:" text / 1506 "base64:" base64 / 1507 "uri:" uri / 1508 key-method [ ":" text ] 1510 base64 = *base64-unit [base64-pad] 1511 base64-unit = 4base64-char 1512 base64-pad = 2base64-char "==" / 3base64-char "=" 1513 base64-char = ALPHA / DIGIT / "+" / "/" 1515 key-method = token 1517 ; sub-rules of 'a=' 1518 attribute = (att-field ":" att-value) / att-field 1519 att-field = token 1521 att-value = byte-string 1523 ; sub-rules of 'm=' 1524 media = token 1525 ;typically "audio", "video", "application" 1526 ;or "data" 1528 fmt = token 1529 ;typically an RTP payload type for audio 1530 ;and video media 1532 proto = token "/" token 1533 / token 1534 ;typically "RTP/AVP" or "udp" for IP4 1536 port = 1*DIGIT 1537 ;should be either "0" or in the range "1024" to "65535" 1538 ;inclusive for UDP based media (a value "0" is used to 1539 ;signal special conditions in some uses of SDP) 1541 ; generic sub-rules: addressing 1542 unicast-address = IP4-address / IP6-address / FQDN / extension-addr 1544 multicast-address = IP4-multicast / IP6-multicast 1546 IP4-multicast = m1 3( "." decimal-uchar ) 1547 "/" ttl [ "/" integer ] 1548 ; IPv4 multicast addresses may be in the 1549 ; range 224.0.0.0 to 239.255.255.255 1551 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / 1552 ("23" DIGIT )) 1554 IP6-multicast = hexpart [ "/" integer ] 1555 ; IPv6 address starting with FF 1557 ttl = (POS-DIGIT *2DIGIT) / "0" 1558 FQDN = 4*(alpha-numeric / "-" / ".") 1559 ; fully qualified domain name as specified 1560 ; in RFC1035 1562 IP4-address = b1 3("." decimal-uchar) / "0.0.0.0" 1564 b1 = decimal-uchar 1565 ; less than "224"; not "0" or "127" 1567 ; The following is from RFC2373 Appendix B. It is a direct copy. 1568 IP6-address = hexpart [ ":" IP4-address ] 1570 hexpart = hexseq / hexseq "::" [ hexseq ] / 1571 "::" [ hexseq ] 1573 hexseq = hex4 *( ":" hex4) 1575 hex4 = 1*4HEXDIG 1577 ; Generic for other address families 1578 extension-addr = non-ws-string 1580 ; generic sub-rules: datatypes 1581 text = byte-string 1582 ;default is to interpret this as IS0-10646 UTF8 1583 ;ISO 8859-1 requires a "a=charset:ISO-8859-1" 1584 ;session-level attribute to be used 1586 byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF) 1587 ;any byte except NUL, CR or LF 1589 non-ws-string = 1*(VCHAR/%x80-FF) 1590 ;string of visible US-ASCII, or high-bit, characters 1592 token-char = %x21/%x23-27/%x2A-2B/%x2D-2E/%x30-39/%x41-5A/%x5E-7E 1593 ; definition from RFC 2045 - 1594 ; "any (US-ASCII) CHAR except SPACE, CTLs, 1595 ; or tspecials". 1596 ; the tspecials are ()<>@,;: 1598 token = 1*(token-char) 1600 email-safe = 1*(%x01-09/%x0B-0C/%x0E-27/ 1601 %x2A-3B/%x3D/%x3E-FF) 1602 ;any byte except NUL, CR, LF, or the quoting 1603 ;characters ()<> 1605 integer = POS-DIGIT *DIGIT 1607 ; generic sub-rules: primitives 1608 alpha-numeric = ALPHA / DIGIT 1610 POS-DIGIT = %x31-39 ; 1 - 9 1612 ; external references: 1613 ; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 2234 1614 ; URI-reference: from RFC1630 and RFC2732 1615 ; addr-spec: from RFC 2822 1616 Appendix B: IANA Considerations 1618 There are seven field names that may be registered with IANA. Using the 1619 terminology in the SDP specification BNF, they are "media", "proto", 1620 "fmt", "att-field", "bwtype", "nettype" and "addrtype". 1622 "media" (eg, audio, video, application, data). 1624 The set of media is intended to be small and not to be extended 1625 except under rare circumstances. The same rules should apply for 1626 media names as for top-level MIME content types, and where possible 1627 the same name should be registered for SDP as for MIME. For media 1628 other than existing MIME top-level content types, a standards-track 1629 RFC MUST be produced for a new top-level content type to be 1630 registered, and the registration MUST provide good justification 1631 why no existing media name is appropriate. 1633 "proto" 1635 In general this should be an IETF standards-track transport 1636 protocol identifier such as RTP/AVP (rfc 1889 under the rfc 1890 1637 profile). 1639 However, people will want to invent their own proprietary transport 1640 protocols. Some of these should be registered as a "fmt" using 1641 "udp" as the protocol and some of which probably can't be. 1643 Where the protocol and the application are intimately linked, such 1644 as with the LBL whiteboard wb which used a proprietary and special 1645 purpose protocol over UDP, the protocol name should be "udp" and 1646 the format name that should be registered is "wb". The rules for 1647 formats (see below) apply to such registrations. 1649 Where the proprietary transport protocol really carries many 1650 different data formats, it is possible to register a new protocol 1651 name with IANA. In such a case, an RFC MUST be produced describing 1652 the protocol and referenced in the registration. Such an RFC MAY 1653 be informational, although it is preferable if it is standards- 1654 track. 1656 "fmt" 1658 The format namespace is dependent on the context of the "proto" 1659 field, so a format cannot be registered without specifying one or 1660 more transport protocols that it applies to. 1662 Formats cover all the possible encodings that might want to be 1663 transported in a multimedia session. 1665 For RTP formats that have been assigned static payload types, the 1666 payload type number is used. For RTP formats using a dynamic 1667 payload type number, the dynamic payload type number is given as 1668 the format and an additional "rtpmap" attribute specifies the 1669 format and parameters. 1671 For non-RTP formats, any unregistered format name may be 1672 registered. If there is a suitable mapping from a MIME subtype to 1673 the format, then the MIME subtype name should be registered. If 1674 there is no suitable mapping from a MIME subtype, a new name should 1675 be registered. In either case, unless there are strong reasons not 1676 to do so, a standards-track RFC SHOULD be produced describing the 1677 format and this RFC SHOULD be referenced in the registration. 1679 "att-field" (Attribute names) 1681 Attribute field names SHOULD be registered with IANA, although this 1682 is not compulsory, and unknown attributes are simply ignored. 1684 When an attribute is registered, it must be accompanied by a brief 1685 specification stating the following: 1687 o contact name, email address and telephone number 1689 o attribute-name (as it will appear in SDP) 1691 o long-form attribute name in English 1693 o type of attribute (session level, media level, or both) 1695 o whether the attribute value is subject to the charset 1696 attribute. 1698 o a one paragraph explanation of the purpose of the attribute. 1700 o a specification of appropriate attribute values for this 1701 attribute. 1703 IANA will not sanity check such attribute registrations except to 1704 ensure that they do not clash with existing registrations. 1706 Although the above is the minimum that IANA will accept, if the 1707 attribute is expected to see widespread use and interoperability is 1708 an issue, authors are encouraged to produce a standards-track RFC 1709 that specifies the attribute more precisely. 1711 Submitters of registrations should ensure that the specification is 1712 in the spirit of SDP attributes, most notably that the attribute is 1713 platform independent in the sense that it makes no implicit 1714 assumptions about operating systems and does not name specific 1715 pieces of software in a manner that might inhibit interoperability. 1717 "bwtype" (bandwidth specifiers) 1719 A proliferation of bandwidth specifiers is strongly discouraged. 1721 New bandwidth specifiers may be registered with IANA. The 1722 submission MUST reference a standards-track RFC specifying the 1723 semantics of the bandwidth specifier precisely, and indicating when 1724 it should be used, and why the existing registered bandwidth 1725 specifiers do not suffice. 1727 "nettype" (Network Type) 1729 New network types may be registered with IANA if SDP needs to be 1730 used in the context of non-Internet environments. Whilst these are 1731 not normally the preserve of IANA, there may be circumstances when 1732 an Internet application needs to interoperate with a non-Internet 1733 application, such as when gatewaying an Internet telephony call 1734 into the PSTN. The number of network types should be small and 1735 should be rarely extended. A new network type cannot be registered 1736 without registering at least one address type to be used with that 1737 network type. A new network type registration MUST reference an 1738 RFC which gives details of the network type and address type and 1739 specifies how and when they would be used. Such an RFC MAY be 1740 Informational. 1742 "addrtype" (Address Type) 1744 New address types may be registered with IANA. An address type is 1745 only meaningful in the context of a network type, and any 1746 registration of an address type MUST specify a registered network 1747 type, or be submitted along with a network type registration. A 1748 new address type registration MUST reference an RFC giving details 1749 of the syntax of the address type. Such an RFC MAY be 1750 Informational. Address types are not expected to be registered 1751 frequently. 1753 Registration Procedure 1755 To register a name the above guidelines should be followed regarding the 1756 required level of documentation that is required. The registration 1757 itself should be sent to IANA. Attribute registrations should include 1758 the information given above. Other registrations should include the 1759 following additional information: 1761 o contact name, email address and telephone number 1763 o name being registered (as it will appear in SDP) 1765 o long-form name in English 1767 o type of name ("media", "proto", "fmt", "bwtype", "nettype", or 1768 "addrtype") 1770 o a one paragraph explanation of the purpose of the registered name. 1772 o a reference to the specification (eg RFC number) of the registered 1773 name. 1775 IANA may refer any registration to the IESG or to any appropriate IETF 1776 working group for review, and may request revisions to be made before a 1777 registration will be made. 1779 Appendix C: Changes from RFC 2327 1781 o Clarify that a=recvonly does NOT mean that you don't send RTCP, and 1782 similarly for sendonly and inactive. These only effect the RTP 1783 stream. 1785 o Rewrite the ABNF syntax (thanks to Jonathan Lennox) 1787 o Add a=inactive attribute. 1789 o Add a=maxptime attribute. 1791 o RFC 2327 mandated that either e= or p= was required. Both are now 1792 optional, to reflect actual usage. 1794 o Removed references to "conference" from the description of the t= 1795 line, to make it less SAP oriented. 1797 o Note about wrap-around of NTP timestamps in t= 1799 o Update BNF to support IPv6. 1801 o References have been updated. 1803 o Section 3.1 was replaced with a reference to RFC 2119, and the memo 1804 has been updated to use the RFC 2119 terminology (MUST, SHOULD, 1805 etc). 1807 o Use of "application/sdp" as MIME a type for SDP files is now "MUST" 1808 rather than "SHOULD". 1810 o A number of sections have been updated to be less SAP specific, and 1811 to reference other current uses of SDP such as RTSP and SIP. 1813 o The section on concatenation of session descriptions (which was not 1814 allowed in SAP, but allowed in other cases) has been removed. It is 1815 assumed that transports of SDP specify will specify this. 1817 o The description of the c= line has been updated to reflect common 1818 usage of SDP, rather than Mbone conferencing with SAP. 1820 o The b= line no longer makes a normative reference to the Mbone FAQ 1821 for bandwidth limits at various TTLs. The AS modifier to b= is 1822 noted as being the RTP session bandwidth. 1824 o Define relation between the m= line and MIME types 1825 o Note use of s= in sessions with no meaningful name 1827 o Note that a=rtpmap is a media level attribute 1829 Appendix D: Authors' Addresses 1831 Mark Handley 1832 International Computer Science Institute, 1833 1947 Center Street, Suite 600, 1834 Berkeley, CA 94704 1835 United States 1836 Email: mjh@icir.org 1838 Van Jacobson 1839 Packet Design 1840 2465 Latham Street 1841 Mountain View, CA 94040 1842 United States 1843 Email: van@packetdesign.com 1845 Colin Perkins 1846 USC Information Sciences Institute 1847 3811 N. Fairfax Drive, Suite 200 1848 Arlington, VA 22203 1849 United States 1850 Email: csp@isi.edu 1852 Acknowledgments 1854 Many people in the IETF MMUSIC working group have made comments and 1855 suggestions contributing to this document. In particular, we would like 1856 to thank Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin, Ross 1857 Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Steve Hanna and 1858 Jonathan Lennox. 1860 References 1862 [1] D. Mills, ``Network Time Protocol (version 3) specification and 1863 implementation", RFC 1305, March 1992. 1865 [2] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson, ``RTP: A 1866 Transport Protocol for Real-Time Applications'', RFC 1889, January 1867 1996. 1869 [3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with 1870 Minimal Control'', RFC 1890, January 1996. 1872 [4] M. Handley, C. Perkins and E. Whelan, ``Session Announcement 1873 Protocol'', RFC 2974, October 2000. 1875 [5] V. Jacobson and S. McCanne, ``vat - X11-based audio teleconferencing 1876 tool'' vat manual page, Lawrence Berkeley Laboratory, 1994. 1878 [6] The Unicode Consortium, "The Unicode Standard -- Version 2.0", 1879 Addison-Wesley, 1996. 1881 [7] ISO/IEC 10646-1:1993. International Standard -- Information 1882 technology -- Universal Multiple-Octet Coded Character Set (UCS) 1883 -- Part 1: Architecture and Basic Multilingual Plane. Five 1884 amendments and a technical corrigendum have been published up 1885 to now. UTF-8 is described in Annex R, published as Amendment 2. 1887 [8] D. Goldsmith and M. Davis, ``Using Unicode with MIME'', RFC1641, 1888 July 1994 1890 [9] F. Yergeau, ``UTF-8, a transformation format of ISO 10646'', 1891 RFC 2279, January 1998 1893 [10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for 1894 Receiving Internet-based H.323 Conferences", ITU, Geneva. 1896 [11] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, 1897 J. Peterson, R. Sparks, M. Handley, E. Schooler ``SIP: Session 1898 Initiatation Protocol'', RFC 3261, May 2002. 1900 [12] H. Schulzrinne, A. Rao and R. Lanphier, ``Real Time Streaming 1901 Protocol (RTSP)'' RFC 2326, April 1998. 1903 [13] S. Bradner, ``Key words for use in RFCs to Indicate Requirement 1904 Levels'', RFC 2119, March 1997. 1906 [14] J. Rosenberg and H. Schulzrinne, ``An Offer/Answer Model with 1907 SDP'', RFC 3264, May 2002. 1909 [15] C. Huitema, ``RTCP Attribute in SDP'', RFC XXXX, May 2002