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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 2044 (ref. '9') (Obsoleted by RFC 2279) -- Possible downref: Non-RFC (?) normative reference: ref. '10' ** Obsolete normative reference: RFC 2543 (ref. '11') (Obsoleted by RFC 3261, RFC 3262, RFC 3263, RFC 3264, RFC 3265) ** Obsolete normative reference: RFC 2326 (ref. '12') (Obsoleted by RFC 7826) Summary: 15 errors (**), 0 flaws (~~), 11 warnings (==), 6 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-08.txt Van Jacobson/Packet Design 4 Colin Perkins/ISI 5 19 April 2002 6 Expires: October 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 conference tool-specific information necessary for 47 participation. This document defines a session description protocol for 48 this purpose, and for general real-time multimedia session description 49 purposes. This memo does not describe multicast address allocation or 50 the distribution of SDP messages. These are described in accompanying 51 drafts. SDP is not intended for negotiation of media encodings. 53 2. Background 55 The Mbone is the part of the Internet that supports IP multicast, and 56 thus permits efficient many-to-many communication. It is used 57 extensively for multimedia conferencing. Such conferences usually have 58 the property that tight coordination of conference membership is not 59 necessary; to receive a conference, a user at an Mbone site only has to 60 know the conference's multicast group address and the UDP ports for the 61 conference data streams. 63 Session directories assist the advertisement of conference sessions and 64 communicate the relevant conference setup information to prospective 65 participants. SDP is designed to convey such information to recipients. 66 SDP is purely a format for session description - it does not incorporate 67 a transport protocol, and is intended to use different transport 68 protocols as appropriate including the Session Announcement Protocol 69 [4], Session Initiation Protocol [11], Real-Time Streaming Protocol 70 [12], electronic mail using the MIME extensions, and the Hypertext 71 Transport Protocol. 73 SDP is intended to be general purpose so that it can be used for a wider 74 range of network environments and applications than just multicast 75 session directories. However, it is not intended to support negotiation 76 of session content or media encodings - this is viewed as outside the 77 scope of session description. 79 3. Glossary of Terms 81 The following terms are used in this document, and have specific meaning 82 within the context of this document. 84 Conference 85 A multimedia conference is a set of two or more communicating users 86 along with the software they are using to communicate. 88 Session 89 A multimedia session is a set of multimedia senders and receivers 90 and the data streams flowing from senders to receivers. A 91 multimedia conference is an example of a multimedia session. 93 Session Advertisement 94 See session announcement. 96 Session Announcement 97 A session announcement is a mechanism by which a session description 98 is conveyed to users in a pro-active fashion, i.e., the session 99 description was not explicitly requested by the user. 101 Session Description 102 A well defined format for conveying sufficient information to 103 discover and participate in a multimedia session. 105 3.1. Terminology 107 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 108 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 109 document are to be interpreted as described in RFC 2119 [13]. 111 4. Examples of SDP Usage 113 4.1. Multicast Announcement 115 In order to assist the advertisement of multicast multimedia conferences 116 and other multicast sessions, and to communicate the relevant session 117 setup information to prospective participants, a distributed session 118 directory may be used. An instance of such a session directory 119 periodically sends packets containing a description of the session to a 120 well known multicast group. These advertisements are received by other 121 session directories such that potential remote participants can use the 122 session description to start the tools required to participate in the 123 session. 125 One protocol commonly used to implement such a distributed directory is 126 the Session Announcement Protocol, SAP [4]. SDP provides the recommended 127 session description format for such announcements. 129 4.2. Session Initiation 131 The Session Initiation Protocol, SIP [11] is an application-layer 132 control protocol for creating, modifying and terminating sessions such 133 as Internet multimedia conferences, Internet telephone calls and 134 multimedia distribution. The SIP messages used to create sessions carry 135 session descriptions which allow participants to agree on a set of 136 compatible media types. These session descriptions are commonly 137 formatted 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. It is 145 necessary for RTSP to convey a description of the session to be 146 controlled. SDP is often used for this purpose. 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 be able to join a 204 session (with the possible exception of encryption keys) and to announce 205 the resources to be used to non-participants that may need to know. 207 5.1. Media Information 209 SDP includes: 211 o The type of media (video, audio, etc) 213 o The transport protocol (RTP/UDP/IP, H.320, etc) 215 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 contact address 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 234 port to which data is sent, and the remote address and local port on 235 which to receive data. However, some media may define to use these to 236 establish a control channel for the actual media flow. 238 5.2. Timing Information 240 Sessions may either be bounded or unbounded in time. Whether or not 241 they are bounded, they may be only active at specific times. 243 SDP can convey: 245 o An arbitrary list of start and stop times bounding the session 247 o For each bound, repeat times such as "every Wednesday at 10am for 248 one hour" 250 This timing information is globally consistent, irrespective of local 251 time zone or daylight saving time. 253 5.3. Private Sessions 255 It is possible to create both public sessions and private sessions. SDP 256 itself does not distinguish between these: private sessions are 257 typically conveyed by encrypting the session description during 258 distribution. The details of how encryption is performed are dependent 259 on the mechanism used to convey SDP - e.g. mechanisms are defined for 260 SDP transported using SAP [4] and SIP [11]. 262 If a session announcement is private it is possible to use that private 263 announcement to convey encryption keys necessary to decode each of the 264 media in a conference, including enough information to know which 265 encryption scheme is used for each media. 267 5.4. Obtaining Further Information about a Session 269 A session description should convey enough information to decide whether 270 or not to participate in a session. SDP may include additional pointers 271 in the form of Universal Resources Identifiers (URIs) for more 272 information about the session. 274 5.5. Categorisation 276 When many session descriptions are being distributed by SAP, or any 277 other advertisement mechanism, it may be desirable to filter 278 announcements that are of interest from those that are not. SDP 279 supports a categorisation mechanism for sessions that is capable of 280 being automated. 282 5.6. Internationalization 284 The SDP specification recommends the use of the ISO 10646 character sets 285 in the UTF-8 encoding (RFC 2044) to allow many different languages to be 286 represented. However, to assist in compact representations, SDP also 287 allows other character sets such as ISO 8859-1 to be used when desired. 288 Internationalization only applies to free-text fields (session name and 289 background information), and not to SDP as a whole. 291 6. SDP Specification 293 SDP session descriptions are entirely textual using the ISO 10646 294 character set in UTF-8 encoding. SDP field names and attribute names 295 use only the US-ASCII subset of UTF-8, but textual fields and attribute 296 values MAY use the full ISO 10646 character set. The textual form, as 297 opposed to a binary encoding such as ASN/1 or XDR, was chosen to enhance 298 portability, to enable a variety of transports to be used (e.g, session 299 description in a MIME email message) and to allow flexible, text-based 300 toolkits (e.g., Tcl/Tk ) to be used to generate and to process session 301 descriptions. However, since SDP may be used in environments where the 302 maximum permissable size of a session description is limited (e.g. SAP 303 announcements; SIP transported in UDP), the encoding is deliberately 304 compact. Also, since announcements may be transported via very 305 unreliable means or damaged by an intermediate caching server, the 306 encoding was designed with strict order and formatting rules so that 307 most errors would result in malformed announcements which could be 308 detected easily and discarded. This also allows rapid discarding of 309 encrypted announcements for which a receiver does not have the correct 310 key. 312 An SDP session description consists of a number of lines of text of the 313 form 314 = 315 is always exactly one character and is case-significant. 316 is a structured text string whose format depends on . It also 317 will be case-significant unless a specific field defines otherwise. 318 Whitespace MUST NOT be used either side of the `=' sign. In general 319 is either a number of fields delimited by a single space 320 character or a free format string. 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=mhandley 2890844526 2890842807 IN IP4 126.16.64.4 382 s=SDP Seminar 383 i=A Seminar on the session description protocol 384 u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps 385 e=mjh@isi.edu (Mark Handley) 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 (their 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=+1 617 253 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=mjh@isi.edu (Mark Handley) 522 The alternative RFC822 name quoting convention is also allowed for both 523 email addresses and phone numbers. For example, 525 e=Mark Handley 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 relevant 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 can only be specified on a per- 620 media basis, and not 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 repeat times. 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. 780 o If a session is likely to last several years, it is expected that 781 the session announcement will be modified periodically rather than 782 transmit several years worth of adjustments in one announcement. 784 Encryption Keys 786 k= 787 k=: 789 o The session description protocol MAY be used to convey encryption 790 keys. A key field is permitted before the first media entry (in 791 which case it applies to all media in the session), or for each 792 media entry as required. 794 o The format of keys and their usage is outside the scope of this 795 document, but see [3]. 797 o The method indicates the mechanism to be used to obtain a usable key 798 by external means, or from the encoded encryption key given. The 799 following methods are defined: 801 k=clear: 802 The encryption key (as described in [3] for RTP media streams 803 under the AV profile) is included untransformed in this key 804 field. 806 k=base64: 807 The encryption key (as described in [3] for RTP media streams 808 under the AV profile) is included in this key field but has been 809 base64 encoded because it includes characters that are 810 prohibited in SDP. 812 k=uri: 813 A Universal Resource Identifier as used by WWW clients is 814 included in this key field. The URI refers to the data 815 containing the key, and may require additional authentication 816 before the key can be returned. When a request is made to the 817 given URI, the MIME content-type of the reply specifies the 818 encoding for the key in the reply. The key should not be 819 obtained until the user wishes to join the session to reduce 820 synchronisation of requests to the WWW server(s). 822 k=prompt 823 No key is included in this SDP description, but the session or 824 media stream referred to by this key field is encrypted. The 825 user should be prompted for the key when attempting to join the 826 session, and this user-supplied key should then be used to 827 decrypt the media streams. 829 Attributes 831 a= 832 a=: 834 Attributes are the primary means for extending SDP. Attributes may be 835 defined to be used as "session-level" attributes, "media-level" 836 attributes, or both. 838 A media description may have any number of attributes (``a='' fields) 839 which are media specific. These are referred to as "media-level" 840 attributes and add information about the media stream. Attribute fields 841 can also be added before the first media field; these "session-level" 842 attributes convey additional information that applies to the conference 843 as a whole rather than to individual media; an example might be the 844 conference's floor control policy. 846 Attribute fields may be of two forms: 848 o property attributes. A property attribute is simply of the form 849 ``a=''. These are binary attributes, and the presence of the 850 attribute conveys that the attribute is a property of the session. 851 An example might be ``a=recvonly''. 853 o value attributes. A value attribute is of the form 854 ``a=:''. An example might be that a whiteboard 855 could have the value attribute ``a=orient:landscape'' 857 Attribute interpretation depends on the media tool being invoked. Thus 858 receivers of session descriptions should be configurable in their 859 interpretation of announcements in general and of attributes in 860 particular. 862 Attribute names MUST be in the US-ASCII subset of ISO-10646/UTF-8. 864 Attribute values are octet strings, and MAY use any octet value except 865 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values are 866 to be interpreted as in ISO-10646 character set with UTF-8 encoding. 867 Unlike other text fields, attribute values are NOT normally affected by 868 the `charset' attribute as this would make comparisons against known 869 values problematic. However, when an attribute is defined, it can be 870 defined to be charset-dependent, in which case it's value should be 871 interpreted in the session charset rather than in ISO-10646. 873 Attributes that will be commonly used can be registered with IANA (see 874 Appendix B). Unregistered attributes should begin with "X-" to prevent 875 inadvertent collision with registered attributes. In either case, if an 876 attribute is received that is not understood, it should simply be 877 ignored by the receiver. 879 Media Announcements 881 m= 883 A session description may contain a number of media descriptions. Each 884 media description starts with an ``m='' field, and is terminated by 885 either the next ``m='' field or by the end of the session description. 886 A media field also has several sub-fields: 888 o The first sub-field is the media type. Currently defined media are 889 ``audio'', ``video'', ``application'', ``data'' and ``control'', 890 though this list may be extended as new communication modalities 891 emerge (e.g., telepresense). The difference between ``application'' 892 and ``data'' is that the former is a media flow such as whiteboard 893 information, and the latter is bulk-data transfer such as 894 multicasting of program executables which will not typically be 895 displayed to the user. ``control'' is used to specify an additional 896 conference control channel for the session. 898 o The second sub-field is the transport port to which the media stream 899 will be sent. The meaning of the transport port depends on the 900 network being used as specified in the relevant ``c'' field and on 901 the transport protocol defined in the third sub-field. Other ports 902 used by the media application (such as the RTCP port, see [2]) 903 should be derived algorithmically from the base media port. 905 Note: For transports based on UDP, the value should be in the range 906 1024 to 65535 inclusive. For RTP compliance it SHOULD be an even 907 number. 909 For applications where hierarchically encoded streams are being sent 910 to a unicast address, it may be necessary to specify multiple 911 transport ports. This is done using a similar notation to that used 912 for IP multicast addresses in the ``c='' field: 914 m= / 916 In such a case, the ports used depend on the transport protocol. 917 For RTP, only the even ports are used for data and the corresponding 918 one-higher odd port is used for RTCP. For example: 920 m=video 49170/2 RTP/AVP 31 922 would specify that ports 49170 and 49171 form one RTP/RTCP pair and 923 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the 924 transport protocol and 31 is the format (see below). 926 If multiple addresses are specified in the ``c='' field and multiple 927 ports are specified in the ``m='' field, a one-to-one mapping from 928 port to the corresponding address is implied. For example: 930 c=IN IP4 224.2.1.1/127/2 931 m=video 49170/2 RTP/AVP 31 933 would imply that address 224.2.1.1 is used with ports 49170 and 934 49171, and address 224.2.1.2 is used with ports 49172 and 49173. 936 o The third sub-field is the transport protocol. The transport 937 protocol values are dependent on the address-type field in the 938 ``c='' fields. Thus a ``c='' field of IP4 defines that the 939 transport protocol runs over IP4. For IP4, it is normally expected 940 that most media traffic will be carried as RTP over UDP. The 941 following transport protocols are preliminarily defined, but may be 942 extended through registration of new protocols with IANA: 944 - RTP/AVP - the IETF's Realtime Transport Protocol using the 945 Audio/Video profile carried over UDP. 947 - udp - User Datagram Protocol 949 If an application uses a single combined proprietary media format 950 and transport protocol over UDP, then simply specifying the 951 transport protocol as udp and using the format field to distinguish 952 the combined protocol is recommended. If a transport protocol is 953 used over UDP to carry several distinct media types that need to be 954 distinguished by a session directory, then specifying the transport 955 protocol and media format separately is necessary. RTP is an 956 example of a transport-protocol that carries multiple payload 957 formats that must be distinguished by the session directory for it 958 to know how to start appropriate tools, relays, mixers or recorders. 960 The main reason to specify the transport-protocol in addition to the 961 media format is that the same standard media formats may be carried 962 over different transport protocols even when the network protocol is 963 the same - a historical example is vat PCM audio and RTP PCM audio. 964 In addition, relays and monitoring tools that are transport- 965 protocol-specific but format-independent are possible. 967 For RTP media streams operating under the RTP Audio/Video Profile 968 [3], the protocol field is ``RTP/AVP''. Should other RTP profiles 969 be defined in the future, their profiles will be specified in the 970 same way. For example, the protocol field ``RTP/XYZ'' would specify 971 RTP operating under a profile whose short name is ``XYZ''. 973 o The fourth and subsequent sub-fields are media formats. For audio 974 and video, these SHOULD reference a MIME sub-type describing the 975 format under the `audio' and `video' top-level MIME types. 977 When a list of payload formats is given, this implies that all of 978 these formats may be used in the session, but the first of these 979 formats is the default format for the session. 981 For media whose transport protocol is not RTP or UDP the format 982 field is protocol specific. Such formats should be defined in an 983 additional specification document. 985 For media whose transport protocol is RTP, SDP can be used to 986 provide a dynamic binding of media encoding to RTP payload type. 987 The encoding names in the RTP AV Profile do not specify unique audio 988 encodings (in terms of clock rate and number of audio channels), and 989 so they are not used directly in SDP format fields. Instead, the 990 payload type number should be used to specify the format for static 991 payload types and the payload type number along with additional 992 encoding information should be used for dynamically allocated 993 payload types. 995 An example of a static payload type is u-law PCM coded single 996 channel audio sampled at 8kHz. This is completely defined in the 997 RTP Audio/Video profile as payload type 0, so the media field for 998 such a stream sent to UDP port 49232 is: 1000 m=audio 49232 RTP/AVP 0 1002 An example of a dynamic payload type is 16 bit linear encoded stereo 1003 audio sampled at 16KHz. If we wish to use dynamic RTP/AVP payload 1004 type 98 for such a stream, additional information is required to 1005 decode it: 1007 m=video 49232 RTP/AVP 98 1008 a=rtpmap:98 L16/16000/2 1010 The general form of an rtpmap attribute is: 1012 a=rtpmap: /[/] 1014 For audio streams, may specify the number of 1015 audio channels. This parameter may be omitted if the number of 1016 channels is one provided no additional parameters are needed. 1017 For video streams, no encoding parameters are currently specified. 1019 Additional parameters may be defined in the future, but codec- 1020 specific parameters should not be added. Parameters added to an 1021 rtpmap attribute should only be those required for a session 1022 directory to make the choice of appropriate media too to participate 1023 in a session. Codec-specific parameters should be added in other 1024 attributes. 1026 Up to one rtpmap attribute can be defined for each media format 1027 specified. Thus we might have: 1029 m=audio 49230 RTP/AVP 96 97 98 1030 a=rtpmap:96 L8/8000 1031 a=rtpmap:97 L16/8000 1032 a=rtpmap:98 L16/11025/2 1034 RTP profiles that specify the use of dynamic payload types MUST 1035 define the set of valid encoding names and/or a means to register 1036 encoding names if that profile is to be used with SDP. 1038 Experimental encoding formats can also be specified using rtpmap. 1039 RTP formats that are not registered as standard format names must be 1040 preceded by ``X-''. Thus a new experimental redundant audio stream 1041 called GSMLPC using dynamic payload type 99 could be specified as: 1043 m=audio 49232 RTP/AVP 99 1044 a=rtpmap:99 X-GSMLPC/8000 1046 Such an experimental encoding requires that any site wishing to 1047 receive the media stream has relevant configured state in its 1048 session directory to know which tools are appropriate. 1050 Note that RTP audio formats typically do not include information 1051 about the number of samples per packet. If a non-default (as 1052 defined in the RTP Audio/Video Profile) packetisation is required, 1053 the``ptime'' attribute is used as given below. 1055 For more details on RTP audio and video formats, see [3]. 1057 o Predefined formats for UDP protocol non-RTP media are as below. 1059 Application Formats: 1061 wb: LBL Whiteboard (transport: udp) 1063 nt: UCL Network Text Editor (transport: udp) 1065 Suggested Attributes 1067 The following attributes are suggested. Since application writers may 1068 add new attributes as they are required, this list is not exhaustive. 1070 a=cat: 1071 This attribute gives the dot-separated hierarchical category of the 1072 session. This is to enable a receiver to filter unwanted sessions 1073 by category. It would probably have been a compulsory separate 1074 field, except for its experimental nature at this time. It is a 1075 session-level attribute, and is not dependent on charset. 1077 a=keywds: 1078 Like the cat attribute, this is to assist identifying wanted 1079 sessions at the receiver. This allows a receiver to select 1080 interesting session based on keywords describing the purpose of the 1081 session. It is a session-level attribute. It is a charset dependent 1082 attribute, meaning that its value should be interpreted in the 1083 charset specified for the session description if one is specified, 1084 or by default in ISO 10646/UTF-8. 1086 a=tool: 1087 This gives the name and version number of the tool used to create 1088 the session description. It is a session-level attribute, and is 1089 not dependent on charset. 1091 a=ptime: 1092 This gives the length of time in milliseconds represented by the 1093 media in a packet. This is probably only meaningful for audio data, 1094 but may be used with other media types if it makes sense. It should 1095 not be necessary to know ptime to decode RTP or vat audio, and it is 1096 intended as a recommendation for the encoding/packetisation of 1097 audio. It is a media attribute, and is not dependent on charset. 1099 a=maxptime: 1100 The maximum amount of media which can be encapsulated in each 1101 packet, expressed as time in milliseconds. The time SHALL be 1102 calculated as the sum of the time the media present in the packet 1103 represents. The time SHOULD be a multiple of the frame size. This 1104 attribute is probably only meaningful for audio data, but may be 1105 used with other media types if it makes sense. It is a media 1106 attribute, and is not dependent on charset. Note that this 1107 attribute was introduced after RFC 2327, and non updated 1108 implementations will ignore this attribute. 1110 a=rtpmap: /[/] 1112 See the section on Media Announcements (the ``m='' field). This may 1113 be either a session or media attribute. 1115 a=recvonly 1116 This specifies that the tools should be started in receive-only mode 1117 where applicable. It can be either a session or media attribute, and 1118 is not dependent on charset. Note that recvonly applies to the media 1119 only, not to any associated control protocol (e.g. an RTP based 1120 system in recvonly mode SHOULD still send RTCP packets). 1122 a=sendrecv 1123 This specifies that the tools should be started in send and receive 1124 mode. This is necessary for interactive conferences with tools such 1125 as wb which defaults to receive only mode. It can be either a 1126 session or media attribute, and is not dependent on charset. 1128 a=sendonly 1129 This specifies that the tools should be started in send-only mode. 1130 An example may be where a different unicast address is to be used 1131 for a traffic destination than for a traffic source. In such a case, 1132 two media descriptions may be use, one sendonly and one recvonly. It 1133 can be either a session or media attribute, but would normally only 1134 be used as a media attribute, and is not dependent on charset. Note 1135 that sendonly applies only to the media, and any associated control 1136 protocol (e.g. RTCP) SHOULD still be received and processed as 1137 normal. 1139 a=inactive 1140 This specifies that the tools should be started in inactive mode. 1141 This is necessary for interactive conferences where users can put 1142 other users on hold. No media is sent over an inactive media stream. 1143 Note that an RTP based system SHOULD still send RTCP, even if 1144 started inactive. It can be either a session or media attribute, and 1145 is not dependent on charset. 1147 a=orient: 1148 Normally this is only used in a whiteboard media specification. It 1149 specifies the orientation of a the whiteboard on the screen. It is 1150 a media attribute. Permitted values are `portrait', `landscape' and 1151 `seascape' (upside down landscape). It is not dependent on charset 1153 a=type: 1154 This specifies the type of the conference. Suggested values are 1155 `broadcast', `meeting', `moderated', `test' and `H332'. `recvonly' 1156 should be the default for `type:broadcast' sessions, `type:meeting' 1157 should imply `sendrecv' and `type:moderated' should indicate the use 1158 of a floor control tool and that the media tools are started so as 1159 to ``mute'' new sites joining the conference. 1161 Specifying the attribute type:H332 indicates that this loosely 1162 coupled session is part of a H.332 session as defined in the ITU 1163 H.332 specification [10]. Media tools should be started `recvonly'. 1165 Specifying the attribute type:test is suggested as a hint that, 1166 unless explicitly requested otherwise, receivers can safely avoid 1167 displaying this session description to users. 1169 The type attribute is a session-level attribute, and is not 1170 dependent on charset. 1172 a=charset: 1173 This specifies the character set to be used to display the session 1174 name and information data. By default, the ISO-10646 character set 1175 in UTF-8 encoding is used. If a more compact representation is 1176 required, other character sets may be used such as ISO-8859-1 for 1177 Northern European languages. In particular, the ISO 8859-1 is 1178 specified with the following SDP attribute: 1180 a=charset:ISO-8859-1 1182 This is a session-level attribute; if this attribute is present, it 1183 must be before the first media field. The charset specified MUST be 1184 one of those registered with IANA, such as ISO-8859-1. The 1185 character set identifier is a US-ASCII string and MUST be compared 1186 against the IANA identifiers using a case-insensitive comparison. 1187 If the identifier is not recognised or not supported, all strings 1188 that are affected by it SHOULD be regarded as octet strings. 1190 Note that a character set specified MUST still prohibit the use of 1191 bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets requiring 1192 the use of these characters MUST define a quoting mechanism that 1193 prevents these bytes appearing within text fields. 1195 a=sdplang: 1196 This can be a session level attribute or a media level attribute. 1197 As a session level attribute, it specifies the language for the 1198 session description. As a media level attribute, it specifies the 1199 language for any media-level SDP information field associated with 1200 that media. Multiple sdplang attributes can be provided either at 1201 session or media level if multiple languages in the session 1202 description or media use multiple languages, in which case the order 1203 of the attributes indicates the order of importance of the various 1204 languages in the session or media from most important to least 1205 important. 1207 In general, sending session descriptions consisting of multiple 1208 languages should be discouraged. Instead, multiple descriptions 1209 should be sent describing the session, one in each language. 1210 However this is not possible with all transport mechanisms, and so 1211 multiple sdplang attributes are allowed although not recommended. 1213 The sdplang attribute value must be a single RFC 1766 language tag 1214 in US-ASCII. It is not dependent on the charset attribute. An 1215 sdplang attribute SHOULD be specified when a session is of 1216 sufficient scope to cross geographic boundaries where the language 1217 of recipients cannot be assumed, or where the session is in a 1218 different language from the locally assumed norm. 1220 a=lang: 1221 This can be a session level attribute or a media level attribute. 1222 As a session level attribute, it specifies the default language for 1223 the session being described. As a media level attribute, it 1224 specifies the language for that media, overriding any session-level 1225 language specified. Multiple lang attributes can be provided either 1226 at session or media level if multiple languages if the session 1227 description or media use multiple languages, in which case the order 1228 of the attributes indicates the order of importance of the various 1229 languages in the session or media from most important to least 1230 important. 1232 The lang attribute value must be a single RFC 1766 language tag in 1233 US-ASCII. It is not dependent on the charset attribute. A lang 1234 attribute SHOULD be specified when a session is of sufficient scope 1235 to cross geographic boundaries where the language of recipients 1236 cannot be assumed, or where the session is in a different language 1237 from the locally assumed norm. 1239 a=framerate: 1240 This gives the maximum video frame rate in frames/sec. It is 1241 intended as a recommendation for the encoding of video data. 1242 Decimal representations of fractional values using the notation 1243 "." are allowed. It is a media attribute, is 1244 only defined for video media, and is not dependent on charset. 1246 a=quality: 1247 This gives a suggestion for the quality of the encoding as an 1248 integer value. 1250 The intention of the quality attribute for video is to specify a 1251 non-default trade-off between frame-rate and still-image quality. 1252 For video, the value in the range 0 to 10, with the following 1253 suggested meaning: 1255 10 - the best still-image quality the compression scheme can give. 1257 5 - the default behaviour given no quality suggestion. 1259 0 - the worst still-image quality the codec designer thinks is 1260 still usable. 1262 It is a media attribute, and is not dependent on charset. 1264 a=fmtp: 1265 This attribute allows parameters that are specific to a particular 1266 format to be conveyed in a way that SDP doesn't have to understand 1267 them. The format must be one of the formats specified for the 1268 media. Format-specific parameters may be any set of parameters 1269 required to be conveyed by SDP and given unchanged to the media tool 1270 that will use this format. 1272 It is a media attribute, and is not dependent on charset. 1274 6.1. Communicating Conference Control Policy 1276 There is some debate over the way conference control policy should be 1277 communicated. In general, the authors believe that an implicit 1278 declarative style of specifying conference control is desirable where 1279 possible. 1281 A simple declarative style uses a single conference attribute field 1282 before the first media field, possibly supplemented by properties such 1283 as `recvonly' for some of the media tools. This conference attribute 1284 conveys the conference control policy. An example might be: 1286 a=type:moderated 1288 In some cases, however, it is possible that this may be insufficient to 1289 communicate the details of an unusual conference control policy. If 1290 this is the case, then a conference attribute specifying external 1291 control might be set, and then one or more ``media'' fields might be 1292 used to specify the conference control tools and configuration data for 1293 those tools. An example is an ITU H.332 session: 1295 ... 1296 c=IN IP4 224.5.6.7 1297 a=type:H332 1298 m=audio 49230 RTP/AVP 0 1299 m=video 49232 RTP/AVP 31 1300 m=application 12349 udp wb 1301 m=control 49234 H323 mc 1302 c=IN IP4 134.134.157.81 1304 In this example, a general conference attribute (type:H332) is specified 1305 stating that conference control will be provided by an external H.332 1306 tool, and a contact addresses for the H.323 session multipoint 1307 controller is given. 1309 In this document, only the declarative style of conference control 1310 declaration is specified. Other forms of conference control should 1311 specify an appropriate type attribute, and should define the 1312 implications this has for control media. 1314 7. Security Considerations 1316 SDP is a session description format that describes multimedia sessions. 1317 A session description SHOULD NOT be trusted unless it has been obtained 1318 by an authenticated transport protocol from a trusted source. Many 1319 different transport protocols may be used to distribute session 1320 description, and the nature of the authentication will differ from 1321 transport to transport. 1323 One transport that will frequently be used to distribute session 1324 descriptions is the Session Announcement Protocol (SAP). SAP provides 1325 both encryption and authentication mechanisms but due to the nature of 1326 session announcements it is likely that there are many occasions where 1327 the originator of a session announcement cannot be authenticated because 1328 they are previously unknown to the receiver of the announcement and 1329 because no common public key infrastructure is available. 1331 On receiving a session description over an unauthenticated transport 1332 mechanism or from an untrusted party, software parsing the session 1333 should take a few precautions. Session descriptions contain information 1334 required to start software on the receivers system. Software that 1335 parses a session description MUST NOT be able to start other software 1336 except that which is specifically configured as appropriate software to 1337 participate in multimedia sessions. It is normally considered 1338 inappropriate for software parsing a session description to start, on a 1339 user's system, software that is appropriate to participate in multimedia 1340 sessions, without the user first being informed that such software will 1341 be started and giving their consent. Thus a session description 1342 arriving by session announcement, email, session invitation, or WWW page 1343 SHOULD NOT deliver the user into an interactive multimedia session 1344 without the user being aware that this will happen. As it is not always 1345 simple to tell whether a session is interactive or not, applications 1346 that are unsure should assume sessions are interactive. 1348 In this specification, there are no attributes which would allow the 1349 recipient of a session description to be informed to start multimedia 1350 tools in a mode where they default to transmitting. Under some 1351 circumstances it might be appropriate to define such attributes. If 1352 this is done an application parsing a session description containing 1353 such attributes SHOULD either ignore them, or inform the user that 1354 joining this session will result in the automatic transmission of 1355 multimedia data. The default behaviour for an unknown attribute is to 1356 ignore it. 1358 Session descriptions may be parsed at intermediate systems such as 1359 firewalls for the purposes of opening a hole in the firewall to allow 1360 the participation in multimedia sessions. It is considered 1361 inappropriate for a firewall to open such holes for unicast data streams 1362 unless the session description comes in a request from inside the 1363 firewall. For multicast sessions, it is likely that local 1364 administrators will apply their own policies, but the exclusive use of 1365 "local" or "site-local" administrative scope within the firewall and the 1366 refusal of the firewall to open a hole for such scopes will provide 1367 separation of global multicast sessions from local ones. 1369 Appendix A: SDP Grammar 1371 This appendix provides an Augmented BNF grammar for SDP. ABNF is 1372 defined in RFC 2234. 1374 ; SDP Syntax 1375 announcement = proto-version 1376 origin-field 1377 session-name-field 1378 information-field 1379 uri-field 1380 email-fields 1381 phone-fields 1382 connection-field 1383 bandwidth-fields 1384 time-fields 1385 key-field 1386 attribute-fields 1387 media-descriptions 1389 proto-version = "v=" 1*DIGIT CRLF 1390 ;this memo describes version 0 1392 origin-field = "o=" username SP sess-id SP sess-version SP 1393 nettype SP addrtype SP unicast-address CRLF 1395 session-name-field = "s=" text CRLF 1397 information-field = ["i=" text CRLF] 1399 uri-field = ["u=" uri CRLF] 1401 email-fields = *("e=" email-address CRLF) 1403 phone-fields = *("p=" phone-number CRLF) 1404 connection-field = ["c=" nettype SP addrtype SP 1405 connection-address CRLF] 1406 ;a connection field must be present 1407 ;in every media description or at the 1408 ;session-level 1410 bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF) 1412 time-fields = 1*( "t=" start-time SP stop-time 1413 *(CRLF repeat-fields) CRLF) 1414 [zone-adjustments CRLF] 1416 repeat-fields = "r=" repeat-interval SP typed-time 1417 1*(SP typed-time) 1419 zone-adjustments = "z=" time SP ["-"] typed-time 1420 *(SP time SP ["-"] typed-time) 1422 key-field = ["k=" key-type CRLF] 1424 attribute-fields = *("a=" attribute CRLF) 1426 media-descriptions = *( media-field 1427 information-field 1428 *connection-field 1429 bandwidth-fields 1430 key-field 1431 attribute-fields ) 1433 media-field = "m=" media SP port ["/" integer] 1434 SP proto 1*(SP fmt) CRLF 1436 ; sub-rules of 'o=' 1437 username = non-ws-string 1438 ;pretty wide definition, but doesn't include space 1440 sess-id = 1*DIGIT 1441 ;should be unique for this originating username/host 1443 sess-version = 1*DIGIT 1444 ;0 is a new session 1446 nettype = token 1447 ;typically "IN" 1449 addrtype = token 1450 ;typically "IP4" or "IP6" 1452 ; sub-rules of 'u=' 1453 uri = URI-reference; defined in RFC1630 and RFC2732 1455 ; sub-rules of 'e=' 1456 email-address = email *SP "(" 1*email-safe ")" / 1457 1*email-safe "<" email ">" / 1458 email 1460 email = addr-spec ; defined in RFC2822 1461 ; modified to remove CFWS 1463 ; sub-rules of 'p=' 1464 phone-number = phone *SP "(" 1*email-safe ")" / 1465 1*email-safe "<" phone ">" / 1466 phone 1468 phone = "+" POS-DIGIT 1*(SP / "-" / DIGIT) 1469 ;there must be a space or hyphen between the 1470 ;international code and the rest of the number. 1472 ; Should this use the tel: URL syntax? 1474 ; sub-rules of 'c=' 1475 connection-address = multicast-address / unicast-address 1476 ; sub-rules of 'b=' 1477 bwtype = token 1479 bandwidth = 1*DIGIT 1481 ; sub-rules of 't=' 1482 start-time = time / "0" 1484 stop-time = time / "0" 1486 time = POS-DIGIT 9*DIGIT 1487 ; 10-digit NTP time represents times between 1488 ; 1931 and 5068 AD. 9* allows times after that 1489 ; as well. 1491 ; sub-rules of 'r=' and 'z=' 1492 repeat-interval = typed-time 1494 typed-time = POS-DIGIT *DIGIT [fixed-len-time-unit] 1496 fixed-len-time-unit = "d" / "h" / "m" / "s" 1498 ; sub-rules of 'k=' 1499 key-type = "prompt" / 1500 "clear:" text / 1501 "base64:" base64 / 1502 "uri:" uri / 1503 key-method [ ":" text ] 1505 base64 = *base64-unit [base64-pad] 1506 base64-unit = 4base64-char 1507 base64-pad = 2base64-char "==" / 3base64-char "=" 1508 base64-char = ALPHA / DIGIT / "+" / "/" 1510 key-method = token 1511 ; sub-rules of 'a=' 1512 attribute = (att-field ":" att-value) / att-field 1514 att-field = token 1516 att-value = byte-string 1518 ; sub-rules of 'm=' 1519 media = token 1520 ;typically "audio", "video", "application" 1521 ;or "data" 1523 fmt = token 1524 ;typically an RTP payload type for audio 1525 ;and video media 1527 proto = token "/" token 1528 / token 1529 ;typically "RTP/AVP" or "udp" for IP4 1531 port = 1*DIGIT 1532 ;should in the range "1024" to "65535" inclusive 1533 ;for UDP based media 1535 ; generic sub-rules: addressing 1536 unicast-address = IP4-address / IP6-address / FQDN / extension-addr 1538 multicast-address = IP4-multicast / IP6-multicast 1540 IP4-multicast = m1 3*( "." decimal-uchar ) 1541 "/" ttl [ "/" integer ] 1542 ; IPv4 multicast addresses may be in the 1543 ; range 224.0.0.0 to 239.255.255.255 1545 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / 1546 ("23" DIGIT )) 1548 IP6-multicast = hexpart [ "/" integer ] 1549 ; IPv6 address starting with FF 1551 ttl = (POS-DIGIT *2DIGIT) / "0" 1553 FQDN = 4*(alpha-numeric / "-" / ".") 1554 ; fully qualified domain name as specified 1555 ; in RFC1035 1557 IP4-address = b1 3*("." decimal-uchar) / "0.0.0.0" 1559 b1 = decimal-uchar 1560 ; less than "224"; not "0" or "127" 1562 ; The following is from RFC2373 Appendix B. It is a direct copy. 1563 IP6-address = hexpart [ ":" IP4-address ] 1565 hexpart = hexseq / hexseq "::" [ hexseq ] / 1566 "::" [ hexseq ] 1568 hexseq = hex4 *( ":" hex4) 1570 hex4 = 1*4HEXDIG 1572 ; Generic for other address families 1573 extension-addr = non-ws-string 1575 ; generic sub-rules: datatypes 1576 text = byte-string 1577 ;default is to interpret this as IS0-10646 UTF8 1578 ;ISO 8859-1 requires a "a=charset:ISO-8859-1" 1579 ;session-level attribute to be used 1581 byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF) 1582 ;any byte except NUL, CR or LF 1584 non-ws-string = 1*(VCHAR/%x80-FF) 1585 ;string of visible US-ASCII, or high-bit, characters 1587 token-char = %x21/%x23-27/%x2A-2B/%x2D-2E/%x30-39/%x41-5A/%x5E-7E 1588 ; definition from RFC 2045 - 1589 ; "any (US-ASCII) CHAR except SPACE, CTLs, 1590 ; or tspecials". 1591 ; the tspecials are ()<>@,;: 1593 token = 1*(token-char) 1595 email-safe = 1*(%x01-09/%x0B-0C/%x0E-27/ 1596 %x2A-3B/%x3D/%x3E-FF) 1597 ;any byte except NUL, CR, LF, or the quoting 1598 ;characters ()<> 1600 integer = POS-DIGIT *DIGIT 1602 ; generic sub-rules: primitives 1603 alpha-numeric = ALPHA / DIGIT 1605 POS-DIGIT = %x31-39 ; 1 - 9 1607 ; external references: 1608 ; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 2234 1609 ; URI-reference: from RFC1630 and RFC2732 1610 ; addr-spec: from RFC 2822 1611 Appendix B: IANA Considerations 1613 There are seven field names that may be registered with IANA. Using the 1614 terminology in the SDP specification BNF, they are "media", "proto", 1615 "fmt", "att-field", "bwtype", "nettype" and "addrtype". 1617 "media" (eg, audio, video, application, data). 1619 The set of media is intended to be small and not to be extended 1620 except under rare circumstances. The same rules should apply for 1621 media names as for top-level MIME content types, and where possible 1622 the same name should be registered for SDP as for MIME. For media 1623 other than existing MIME top-level content types, a standards-track 1624 RFC MUST be produced for a new top-level content type to be 1625 registered, and the registration MUST provide good justification 1626 why no existing media name is appropriate. 1628 "proto" 1630 In general this should be an IETF standards-track transport 1631 protocol identifier such as RTP/AVP (rfc 1889 under the rfc 1890 1632 profile). 1634 However, people will want to invent their own proprietary transport 1635 protocols. Some of these should be registered as a "fmt" using 1636 "udp" as the protocol and some of which probably can't be. 1638 Where the protocol and the application are intimately linked, such 1639 as with the LBL whiteboard wb which used a proprietary and special 1640 purpose protocol over UDP, the protocol name should be "udp" and 1641 the format name that should be registered is "wb". The rules for 1642 formats (see below) apply to such registrations. 1644 Where the proprietary transport protocol really carries many 1645 different data formats, it is possible to register a new protocol 1646 name with IANA. In such a case, an RFC MUST be produced describing 1647 the protocol and referenced in the registration. Such an RFC MAY 1648 be informational, although it is preferable if it is standards- 1649 track. 1651 "fmt" 1653 The format namespace is dependent on the context of the "proto" 1654 field, so a format cannot be registered without specifying one or 1655 more transport protocols that it applies to. 1657 Formats cover all the possible encodings that might want to be 1658 transported in a multimedia session. 1660 For RTP formats that have been assigned static payload types, the 1661 payload type number is used. For RTP formats using a dynamic 1662 payload type number, the dynamic payload type number is given as 1663 the format and an additional "rtpmap" attribute specifies the 1664 format and parameters. 1666 For non-RTP formats, any unregistered format name may be 1667 registered. If there is a suitable mapping from a MIME subtype to 1668 the format, then the MIME subtype name should be registered. If 1669 there is no suitable mapping from a MIME subtype, a new name should 1670 be registered. In either case, unless there are strong reasons not 1671 to do so, a standards-track RFC SHOULD be produced describing the 1672 format and this RFC SHOULD be referenced in the registration. 1674 "att-field" (Attribute names) 1676 Attribute field names SHOULD be registered with IANA, although this 1677 is not compulsory, and unknown attributes are simply ignored. 1679 When an attribute is registered, it must be accompanied by a brief 1680 specification stating the following: 1682 o contact name, email address and telephone number 1684 o attribute-name (as it will appear in SDP) 1686 o long-form attribute name in English 1688 o type of attribute (session level, media level, or both) 1690 o whether the attribute value is subject to the charset 1691 attribute. 1693 o a one paragraph explanation of the purpose of the attribute. 1695 o a specification of appropriate attribute values for this 1696 attribute. 1698 IANA will not sanity check such attribute registrations except to 1699 ensure that they do not clash with existing registrations. 1701 Although the above is the minimum that IANA will accept, if the 1702 attribute is expected to see widespread use and interoperability is 1703 an issue, authors are encouraged to produce a standards-track RFC 1704 that specifies the attribute more precisely. 1706 Submitters of registrations should ensure that the specification is 1707 in the spirit of SDP attributes, most notably that the attribute is 1708 platform independent in the sense that it makes no implicit 1709 assumptions about operating systems and does not name specific 1710 pieces of software in a manner that might inhibit interoperability. 1712 "bwtype" (bandwidth specifiers) 1714 A proliferation of bandwidth specifiers is strongly discouraged. 1716 New bandwidth specifiers may be registered with IANA. The 1717 submission MUST reference a standards-track RFC specifying the 1718 semantics of the bandwidth specifier precisely, and indicating when 1719 it should be used, and why the existing registered bandwidth 1720 specifiers do not suffice. 1722 "nettype" (Network Type) 1724 New network types may be registered with IANA if SDP needs to be 1725 used in the context of non-Internet environments. Whilst these are 1726 not normally the preserve of IANA, there may be circumstances when 1727 an Internet application needs to interoperate with a non-Internet 1728 application, such as when gatewaying an Internet telephony call 1729 into the PSTN. The number of network types should be small and 1730 should be rarely extended. A new network type cannot be registered 1731 without registering at least one address type to be used with that 1732 network type. A new network type registration MUST reference an 1733 RFC which gives details of the network type and address type and 1734 specifies how and when they would be used. Such an RFC MAY be 1735 Informational. 1737 "addrtype" (Address Type) 1739 New address types may be registered with IANA. An address type is 1740 only meaningful in the context of a network type, and any 1741 registration of an address type MUST specify a registered network 1742 type, or be submitted along with a network type registration. A 1743 new address type registration MUST reference an RFC giving details 1744 of the syntax of the address type. Such an RFC MAY be 1745 Informational. Address types are not expected to be registered 1746 frequently. 1748 Registration Procedure 1750 To register a name the above guidelines should be followed regarding the 1751 required level of documentation that is required. The registration 1752 itself should be sent to IANA. Attribute registrations should include 1753 the information given above. Other registrations should include the 1754 following additional information: 1756 o contact name, email address and telephone number 1758 o name being registered (as it will appear in SDP) 1760 o long-form name in English 1762 o type of name ("media", "proto", "fmt", "bwtype", "nettype", or 1763 "addrtype") 1765 o a one paragraph explanation of the purpose of the registered name. 1767 o a reference to the specification (eg RFC number) of the registered 1768 name. 1770 IANA may refer any registration to the IESG or to any appropriate IETF 1771 working group for review, and may request revisions to be made before a 1772 registration will be made. 1774 Appendix C: Changes from RFC 2327 1776 o Clarify that a=recvonly does NOT mean that you don't send RTCP, and 1777 similarly for sendonly and inactive. These only effect the RTP 1778 stream. 1780 o Rewrite the ABNF syntax (thanks to Jonathan Lennox) 1782 o Add a=inactive attribute. 1784 o Add a=maxptime attribute. 1786 o RFC 2327 mandated that either e= or p= was required. Both are now 1787 optional, to reflect actual usage. 1789 o Removed references to "conference" from the description of the t= 1790 line, to make it less SAP oriented. 1792 o Note about wrap-around of NTP timestamps in t= 1794 o Update BNF to support IPv6. 1796 o References have been updated. 1798 o Section 3.1 was replaced with a reference to RFC 2119, and the memo 1799 has been updated to use the RFC 2119 terminology (MUST, SHOULD, 1800 etc). 1802 o Use of "application/sdp" as MIME a type for SDP files is now "MUST" 1803 rather than "SHOULD". 1805 o A number of sections have been updated to be less SAP specific, and 1806 to reference other current uses of SDP such as RTSP and SIP. 1808 o The section on concatenation of session descriptions (which was not 1809 allowed in SAP, but allowed in other cases) has been removed. It is 1810 assumed that transports of SDP specify will specify this. 1812 o The description of the c= line has been updated to reflect common 1813 usage of SDP, rather than Mbone conferencing with SAP. 1815 o The b= line no longer makes a normative reference to the Mbone FAQ 1816 for bandwidth limits at various TTLs. The AS modifier to b= is 1817 noted as being the RTP session bandwidth. 1819 o Define relation between the m= line and MIME types 1820 o Note use of s= in sessions with no meaningful name 1822 o Note that a=rtpmap is a media level attribute 1824 Appendix D: Authors' Addresses 1826 Mark Handley 1827 AT&T Center for Internet Research at ICSI, 1828 International Computer Science Institute, 1829 1947 Center Street, Suite 600, 1830 Berkeley, CA 94704, USA 1831 Email: mjh@aciri.org 1833 Van Jacobson 1834 MS 46a-1121 1835 Lawrence Berkeley Laboratory 1836 Berkeley, CA 94720 1837 United States 1838 Email: van@ee.lbl.gov 1840 Colin Perkins 1841 USC Information Sciences Institute 1842 3811 N. Fairfax Drive, Suite 200 1843 Arlington, VA 22203 1844 United States 1845 Email: csp@isi.edu 1847 Acknowledgments 1849 Many people in the IETF MMUSIC working group have made comments and 1850 suggestions contributing to this document. In particular, we would like 1851 to thank Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin, Ross 1852 Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Steve Hanna and 1853 Jonathan Lennox. 1855 References 1857 [1] D. Mills, ``Network Time Protocol (version 3) specification and 1858 implementation", RFC 1305, March 1992. 1860 [2] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson, ``RTP: A 1861 Transport Protocol for Real-Time Applications'', RFC 1889, January 1862 1996. 1864 [3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with 1865 Minimal Control'', RFC 1890, January 1996. 1867 [4] M. Handley, C. Perkins and E. Whelan, ``Session Announcement 1868 Protocol'', RFC 2974, October 2000. 1870 [5] V. Jacobson and S. McCanne, ``vat - X11-based audio teleconferencing 1871 tool'' vat manual page, Lawrence Berkeley Laboratory, 1994. 1873 [6] The Unicode Consortium, "The Unicode Standard -- Version 2.0", 1874 Addison-Wesley, 1996. 1876 [7] ISO/IEC 10646-1:1993. International Standard -- Information 1877 technology -- Universal Multiple-Octet Coded Character Set (UCS) 1878 -- Part 1: Architecture and Basic Multilingual Plane. Five 1879 amendments and a technical corrigendum have been published up 1880 to now. UTF-8 is described in Annex R, published as Amendment 2. 1882 [8] D. Goldsmith and M. Davis, ``Using Unicode with MIME'', RFC1641, 1883 July 1994 1885 [9] F. Yergeau, ``UTF-8, a transformation format of Unicode and ISO 1886 10646'', RFC 2044, October 1996 1888 [10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for 1889 Receiving Internet-based H.323 Conferences", ITU, Geneva. 1891 [11] M. Handley, H. Schulzrinne, E. Scholler and J. Rosenberg ``SIP: 1892 Session Initiation Protocol'', RFC 2543, March 1999. 1894 [12] H. Schulzrinne, A. Rao and R. Lanphier, ``Real Time Streaming 1895 Protocol (RTSP)'' RFC 2326, April 1998. 1897 [13] S. Bradner, ``Key words for use in RFCs to Indicate Requirement 1898 Levels'', RFC 2119, March 1997.