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If these are example addresses, they should be changed. == There are 12 instances of lines with multicast IPv4 addresses in the document. If these are generic example addresses, they should be changed to use the 233.252.0.x range defined in RFC 5771 Miscellaneous warnings: ---------------------------------------------------------------------------- == Line 524 has weird spacing: '...7 or p=+...' == Line 773 has weird spacing: '...it must be po...' == The document seems to use 'NOT RECOMMENDED' as an RFC 2119 keyword, but does not include the phrase in its RFC 2119 key words list. == Using lowercase 'not' together with uppercase 'MUST', 'SHALL', 'SHOULD', or 'RECOMMENDED' is not an accepted usage according to RFC 2119. Please use uppercase 'NOT' together with RFC 2119 keywords (if that is what you mean). Found 'MUST not' in this paragraph: On receiving a session description over an unauthenticated transport mechanism or from an untrusted party, software parsing the session should take a few precautions. Session description contain information required to start software on the receivers system. Software that parses a session description MUST not be able to start other software except that which is specifically configured as appropriate software to participate in multimedia sessions. It is normally considered INAPPROPRIATE for software parsing a session description to start, on a user's system, software that is appropriate to participate in multimedia sessions, without the user first being informed that such software will be started and giving their consent. Thus a session description arriving by session announcement, email, sessioR multimedia,session page SHOULD NOT deliver the user into an interactive without the user being aware that this will happen. As it is not always simple to tell whether a session is interactive or not, applications that are unsure should assume sessions are interactive. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (26 March 2002) is 8060 days in the past. Is this intentional? 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'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 (~~), 12 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-07.txt Van Jacobson/Packet Design 4 Colin Perkins/ISI 5 26 March 2002 6 Expires: September 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 draft 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. Session Initiation 115 The Session Initiation Protocol, SIP [11] is an application-layer 116 control protocol for creating, modifying and terminating sessions such 117 as Internet multimedia conferences, Internet telephone calls and 118 multimedia distribution. The SIP messages used to create sessions carry 119 session descriptions which allow participants to agree on a set of 120 compatible media types. These session descriptions are commonly 121 formatted using SDP. 123 4.2. Streaming media 125 The Real Time Streaming Protocol, RTSP [12], is an application-level 126 protocol for control over the delivery of data with real-time 127 properties. RTSP provides an extensible framework to enable controlled, 128 on-demand delivery of real-time data, such as audio and video. It is 129 necessary for RTSP to convey a description of the session to be 130 controlled: SDP is often used for this purpose. 132 4.3. Multicast Announcement 134 In order to assist the advertisement of multicast multimedia conferences 135 and other multicast sessions, and to communicate the relevant session 136 setup information to prospective participants, a distributed session 137 directory may be used. An instance of such a session directory 138 periodically sends packets containing a description of the session to a 139 well known multicast group. These advertisements are received by other 140 session directories such that potential remote participants can use the 141 session description to start the tools required to participate in the 142 session. 144 One protocol commonly used to implement such a distributed directory is 145 the Session Announcement Protocol, SAP [4]. SDP provides the recommended 146 session description format for such announcements. 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 222 This address and port are the destination address and destination port 223 of the multicast stream, whether being sent, received, or both. 225 For an IP unicast session, the following are conveyed: 227 o Remote address for media 229 o Transport port for contact address 231 The semantics of this address and port depend on the media and transport 232 protocol defined. By default, this is the remote address and remote 233 port to which data is sent, and the remote address and local port on 234 which to receive data. However, some media may define to use these to 235 establish a control channel for the actual media flow. 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 2044) 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 attributes 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 (e.g., email) or damaged by an intermediate caching 305 server, the encoding was designed with strict order and formatting rules 306 so that most errors would result in malformed announcements which could 307 be 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 313 = 314 is always exactly one character and is case-significant. 315 is a structured text string whose format depends on . It also 316 will be case-significant unless a specific field defines otherwise. 317 Whitespace MUST NOT be used either side of the `=' sign. In general 318 is either a number of fields delimited by a single space 319 character or a free format string. 321 A session description consists of a session-level description (details 322 that apply to the whole session and all media streams) and optionally 323 several media-level descriptions (details that apply only to a single 324 media stream). 326 An announcement consists of a session-level section followed by zero or 327 more media-level sections. The session-level part starts with a `v=' 328 line and continues to the first media-level section. The media 329 description starts with an `m=' line and continues to the next media 330 description or end of the whole session description. In general, 331 session-level values are the default for all media unless overridden by 332 an equivalent media-level value. 334 Some lines in each description are REQUIRED and some are OPTIONAL but 335 all MUST appear in exactly the order given here (the fixed order greatly 336 enhances error detection and allows for a simple parser). OPTIONAL 337 items are marked with a `*'. 339 Session description 340 v= (protocol version) 341 o= (owner/creator and session identifier). 342 s= (session name) 343 i=* (session information) 344 u=* (URI of description) 345 e=* (email address) 346 p=* (phone number) 347 c=* (connection information - not required if included in all media) 348 b=* (bandwidth information) 349 One or more time descriptions (see below) 350 z=* (time zone adjustments) 351 k=* (encryption key) 352 a=* (zero or more session attribute lines) 353 Zero or more media descriptions (see below) 355 Time description 356 t= (time the session is active) 357 r=* (zero or more repeat times) 359 Media description 360 m= (media name and transport address) 361 i=* (media title) 362 c=* (connection information - optional if included at session-level) 363 b=* (bandwidth information) 364 k=* (encryption key) 365 a=* (zero or more media attribute lines) 367 The set of `type' letters is deliberately small and not intended to be 368 extensible -- an SDP parser MUST completely ignore any announcement that 369 contains a `type' letter that it does not understand. The `attribute' 370 mechanism ("a=" described below) is the primary means for extending SDP 371 and tailoring it to particular applications or media. Some attributes 372 (the ones listed in this document) have a defined meaning but others may 373 be added on an application-, media- or session-specific basis. An SDP 374 parser MUST ignore any attribute it doesn't understand. 376 The connection (`c=') and attribute (`a=') information in the session- 377 level section applies to all the media of that session unless overridden 378 by connection information or an attribute of the same name in the media 379 description. For instance, in the example below, each media behaves as 380 if it were given a `recvonly' attribute. 382 An example SDP description is: 384 v=0 385 o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4 386 s=SDP Seminar 387 i=A Seminar on the session description protocol 388 u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps 389 e=mjh@isi.edu (Mark Handley) 390 c=IN IP4 224.2.17.12/127 391 t=2873397496 2873404696 392 a=recvonly 393 m=audio 49170 RTP/AVP 0 394 m=video 51372 RTP/AVP 31 395 m=application 32416 udp wb 396 a=orient:portrait 398 Text records such as the session name and information are bytes strings 399 which may contain any byte with the exceptions of 0x00 (Nul), 0x0a 400 (ASCII newline) and 0x0d (ASCII carriage return). The sequence CRLF 401 (0x0d0a) is used to end a record, although parsers should be tolerant 402 and also accept records terminated with a single newline character. By 403 default these byte strings contain ISO-10646 characters in UTF-8 404 encoding, but this default may be changed using the `charset' attribute. 406 Protocol Version 408 v=0 410 The ``v='' field gives the version of the Session Description Protocol. 411 There is no minor version number. 413 Origin 415 o=
416
418 The ``o='' field gives the originator of the session (their username and 419 the address of the user's host) plus a session id and session version 420 number. is the user's login on the originating host, or it 421 is ``-'' if the originating host does not support the concept of user 422 ids. MUST NOT contain spaces. is a numeric 423 string such that the tuple of , , , 424
and
form a globally unique identifier for the 425 session. The method of session id allocation is up to the creating 426 tool, but it has been suggested that a Network Time Protocol (NTP) 427 timestamp be used to ensure uniqueness [1]. is a version 428 number for this announcement. It is needed for proxy announcements to 429 detect which of several announcements for the same session is the most 430 recent. Again its usage is up to the creating tool, so long as 431 is increased when a modification is made to the session data. 432 Again, it is RECOMMENDED (but not mandatory) that an NTP timestamp is 433 used. is a text string giving the type of network. 434 Initially ``IN'' is defined to have the meaning ``Internet''.
is a text string giving the type of the address that follows. 436 Initially ``IP4'' and ``IP6'' are defined.
is the globally 437 unique address of the machine from which the session was created. For 438 an address type of IP4, this is either the fully-qualified domain name 439 of the machine, or the dotted-decimal representation of the IP version 4 440 address of the machine. For an address type of IP6, this is either the 441 fully-qualified domain name of the machine, or the compressed textual 442 representation of the IP version 6 address of the machine. For both IP4 443 and IP6, the fully-qualified domain name is the form that SHOULD be 444 given unless this is unavailable, in which case the globally unique 445 address may be substituted. A local IP address MUST NOT be used in any 446 context where the SDP description might leave the scope in which the 447 address is meaningful. 449 In general, the ``o='' field serves as a globally unique identifier for 450 this version of this session description, and the subfields excepting 451 the version taken together identify the session irrespective of any 452 modifications. 454 Session Name 456 s= 458 The ``s='' field is the session name. There MUST be one and only one 459 ``s='' field per session description. The ``s='' field MUST NOT be empty 460 and SHOULD contain ISO 10646 characters (but see also the `charset' 461 attribute below). If a session has no meaningful name, the value ``s= '' 462 SHOULD be used (i.e. a single space as the session name). 464 Session and Media Information 466 i= 468 The ``i='' field is information about the session. There may be at most 469 one session-level ``i='' field per session description, and at most one 470 ``i='' field per media. Although it may be omitted, this is NOT 471 RECOMMENDED for session announcements, and user interfaces for composing 472 sessions should require text to be entered. If it is present it must 473 contain ISO 10646 characters (but see also the `charset' attribute 474 below). 476 A single ``i='' field can also be used for each media definition. In 477 media definitions, ``i='' fields are primarily intended for labeling 478 media streams. As such, they are most likely to be useful when a single 479 session has more than one distinct media stream of the same media type. 480 An example would be two different whiteboards, one for slides and one 481 for feedback and questions. 483 URI 485 u= 487 o A URI is a Universal Resource Identifier as used by WWW clients 489 o The URI should be a pointer to additional information about the 490 conference 492 o This field is OPTIONAL, but if it is present it MUST be specified 493 before the first media field 495 o No more than one URI field is allowed per session description 497 Email Address and Phone Number 499 e= 500 p= 502 o These specify contact information for the person responsible for the 503 conference. This is not necessarily the same person that created 504 the conference announcement. 506 o Inclusion of an email address or phone number is OPTIONAL. Note 507 that the previous version of SDP specified that either an email 508 field or a phone field MUST be specified, but this was widely 509 ignored. The change brings the specification into line with common 510 usage. 512 o If these are present, they should be specified before the first 513 media field. 515 o More than one email or phone field can be given for a session 516 description. 518 o Phone numbers should be given in the conventional international 519 format - preceded by a ``+'' and the international country code. 520 There must be a space or a hyphen (``-'') between the country code 521 and the rest of the phone number. Spaces and hyphens may be used to 522 split up a phone field to aid readability if desired. For example: 524 p=+44-171-380-7777 or p=+1 617 253 6011 526 o Both email addresses and phone numbers can have an optional free 527 text string associated with them, normally giving the name of the 528 person who may be contacted. This should be enclosed in parenthesis 529 if it is present. For example: 531 e=mjh@isi.edu (Mark Handley) 533 The alternative RFC822 name quoting convention is also allowed for 534 both email addresses and phone numbers. For example, 536 e=Mark Handley 538 The free text string should be in the ISO-10646 character set with 539 UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if 540 the appropriate charset session-level attribute is set. 542 Connection Data 544 c=
546 The ``c='' field contains connection data. 548 A session announcement MUST contain either one ``c='' field in each 549 media description (see below) or a ``c='' field at the session-level. 550 It MAY contain a session-level ``c='' field and one additional ``c='' 551 field per media description, in which case the per-media values override 552 the session-level settings for the relevant media. 554 The first sub-field is the network type, which is a text string giving 555 the type of network. Initially ``IN'' is defined to have the meaning 556 ``Internet''. 558 The second sub-field is the address type. This allows SDP to be used 559 for sessions that are not IP based. Currently only IP4 and IP6 are 560 defined. 562 The third sub-field is the connection address. Optional extra sub- 563 fields may be added after the connection address depending on the value 564 of the
field. 566 For IP4 and IP6 addresses, the connection address is defined as follows: 568 o If the session is multicast, the connection address will be an IP 569 multicast group address. If the conference is not multicast, then 570 the connection address contains the unicast IP address of the 571 expected data source or data relay or data sink as determined by 572 additional attribute fields. It is not expected that unicast 573 addresses will be given in a session description that is 574 communicated by a multicast announcement, though this is not 575 prohibited. 577 o Conferences using an IPv4 multicast connection address MUST also 578 have a time to live (TTL) value present in addition to the multicast 579 address. The TTL and the address together define the scope with 580 which multicast packets sent in this conference will be sent. TTL 581 values MUST be in the range 0-255. 583 The TTL for the session is appended to the address using a slash as 584 a separator. An example is: 586 c=IN IP4 224.2.36.42/127 588 IPv6 multicast does not use TTL scoping, and hence the TTL value 589 MUST NOT be present for IPv6 multicast. It is expected that IPv6 590 scoped addresses will be used to limit the scope of conferences. 592 Hierarchical or layered encoding schemes are data streams where the 593 encoding from a single media source is split into a number of 594 layers. The receiver can choose the desired quality (and hence 595 bandwidth) by only subscribing to a subset of these layers. Such 596 layered encodings are normally transmitted in multiple multicast 597 groups to allow multicast pruning. This technique keeps unwanted 598 traffic from sites only requiring certain levels of the hierarchy. 599 For applications requiring multiple multicast groups, we allow the 600 following notation to be used for the connection address: 602 [/]/ 604 If the number of addresses is not given it is assumed to be one. 605 Multicast addresses so assigned are contiguously allocated above the 606 base address, so that, for example: 608 c=IN IP4 224.2.1.1/127/3 610 would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are to 611 be used at a ttl of 127. This is semantically identical to 612 including multiple ``c='' lines in a media description: 614 c=IN IP4 224.2.1.1/127 615 c=IN IP4 224.2.1.2/127 616 c=IN IP4 224.2.1.3/127 618 Similarly, an IPv6 example would be: 620 c=IN IP6 FF15::101/3 622 which is semantically equivalent to: 624 c=IN IP6 FF15::101 625 c=IN IP6 FF15::102 626 c=IN IP6 FF15::103 628 (remembering that the TTL field is not present in IPv6 multicast). 630 Multiple addresses or ``c='' lines can only be specified on a per- 631 media basis, and not for a session-level ``c='' field. 633 The slash notation described above MUST NOT be used for IP unicast 634 addresses. 636 Bandwidth 638 b=: 640 o This specifies the proposed bandwidth to be used by the session or 641 media, and is OPTIONAL. 643 o is in kilobits per second by default. Modifiers 644 may specify that alternative units are to be used (the modifiers 645 defined in this memo use the default units). 647 o is a single alphanumeric word giving the meaning of the 648 bandwidth figure. 650 o Two modifiers are initially defined: 652 CT Conference Total: If the bandwidth of a session or media in a 653 session is different from the bandwidth implicit from the scope, a 654 `b=CT:...' line should be supplied for the session giving the 655 proposed upper limit to the bandwidth used. The primary purpose 656 of this is to give an approximate idea as to whether two or more 657 sessions can co-exist simultaneously. 659 AS Application-Specific Maximum: The bandwidth is interpreted to be 660 application-specific, i.e., will be the application's concept of 661 maximum bandwidth. Normally this will coincide with what is set 662 on the application's ``maximum bandwidth'' control if applicable. 663 For RTP based applications, AS gives the RTP ``session bandwidth'' 664 as defined in section 6.2 of [2]. 666 Note that CT gives a total bandwidth figure for all the media at all 667 sites. AS gives a bandwidth figure for a single media at a single 668 site, although there may be many sites sending simultaneously. 670 o Extension Mechanism: Tool writers can define experimental bandwidth 671 modifiers by prefixing their modifier with ``X-''. For example: 673 b=X-YZ:128 675 SDP parsers MUST ignore bandwidth fields with unknown modifiers. 676 Modifiers MUST be alpha-numeric and, although no length limit is 677 given, they are recommended to be short. 679 Times, Repeat Times and Time Zones 681 t= 683 o ``t='' fields specify the start and stop times for a session. 684 Multiple ``t='' fields MAY be used if a session is active at 685 multiple irregularly spaced times; each additional ``t='' field 686 specifies an additional period of time for which the session will be 687 active. If the session is active at regular times, an ``r='' field 688 (see below) should be used in addition to and following a ``t='' 689 field - in which case the ``t='' field specifies the start and stop 690 times of the repeat sequence. 692 o The first and second sub-fields give the start and stop times for 693 the session respectively. These values are the decimal 694 representation of Network Time Protocol (NTP) time values in seconds 695 [1]. To convert these values to UNIX time, subtract decimal 696 2208988800. 698 NTP timestamps are 64 bit values which wrap sometime in the year 699 2036. Since SDP uses an arbitrary length decimal representation, 700 this should not cause an issue (SDP timestamps will continue 701 counting seconds since 1900, NTP will use the value modulo the 64 702 bit limit). 704 o If the stop-time is set to zero, then the session is not bounded, 705 though it will not become active until after the start-time. If the 706 start-time is also zero, the session is regarded as permanent. 708 User interfaces SHOULD strongly discourage the creation of unbounded 709 and permanent sessions as they give no information about when the 710 session is actually going to terminate, and so make scheduling 711 difficult. 713 The general assumption may be made, when displaying unbounded 714 sessions that have not timed out to the user, that an unbounded 715 session will only be active until half an hour from the current time 716 or the session start time, whichever is the later. If behaviour 717 other than this is required, an end-time should be given and 718 modified as appropriate when new information becomes available about 719 when the session should really end. 721 Permanent sessions may be shown to the user as never being active 722 unless there are associated repeat times which state precisely when 723 the session will be active. In general, permanent sessions SHOULD 724 NOT be created for any session expected to have a duration of less 725 than 2 months, and should be discouraged for sessions expected to 726 have a duration of less than 6 months. 728 r= 730 o ``r='' fields specify repeat times for a session. For example, if 731 a session is active at 10am on Monday and 11am on Tuesday for one 732 hour each week for three months, then the in the 733 corresponding ``t='' field would be the NTP representation of 10am 734 on the first Monday, the would be 1 week, the 735 would be 1 hour, and the offsets would be zero and 736 25 hours. The corresponding ``t='' field stop time would be the NTP 737 representation of the end of the last session three months later. By 738 default all fields are in seconds, so the ``r='' and ``t='' fields 739 might be: 741 t=3034423619 3042462419 742 r=604800 3600 0 90000 744 To make description more compact, times may also be given in units 745 of days, hours or minutes. The syntax for these is a number 746 immediately followed by a single case-sensitive character. 747 Fractional units are not allowed - a smaller unit should be used 748 instead. The following unit specification characters are allowed: 750 d - days (86400 seconds) 751 h - hours (3600 seconds) 752 m - minutes (60 seconds) 753 s - seconds (allowed for completeness but not recommended) 755 Thus, the above announcement could also have been written: 757 r=7d 1h 0 25h 759 Monthly and yearly repeats cannot currently be directly specified 760 with a single SDP repeat time - instead separate "t" fields should 761 be used to explicitly list the session times. 763 z= .... 765 o To schedule a repeated session which spans a change from daylight- 766 saving time to standard time or vice-versa, it is necessary to 767 specify offsets from the base repeat times. This is required because 768 different time zones change time at different times of day, 769 different countries change to or from daylight time on different 770 dates, and some countries do not have daylight saving time at all. 772 Thus in order to schedule a session that is at the same time winter 773 and summer, it must be possible to specify unambiguously by whose 774 time zone a session is scheduled. To simplify this task for 775 receivers, we allow the sender to specify the NTP time that a time 776 zone adjustment happens and the offset from the time when the 777 session was first scheduled. The ``z'' field allows the sender to 778 specify a list of these adjustment times and offsets from the base 779 time. 781 An example might be: 783 z=2882844526 -1h 2898848070 0 785 This specifies that at time 2882844526 the time base by which the 786 session's repeat times are calculated is shifted back by 1 hour, and 787 that at time 2898848070 the session's original time base is 788 restored. Adjustments are always relative to the specified start 789 time - they are not cumulative. 791 o If a session is likely to last several years, it is expected that 792 the session announcement will be modified periodically rather than 793 transmit several years worth of adjustments in one announcement. 795 Encryption Keys 797 k= 798 k=: 799 o The session description protocol MAY be used to convey encryption 800 keys. A key field is permitted before the first media entry (in 801 which case it applies to all media in the session), or for each 802 media entry as required. 804 o The format of keys and their usage is outside the scope of this 805 document, but see [3]. 807 o The method indicates the mechanism to be used to obtain a usable key 808 by external means, or from the encoded encryption key given. The 809 following methods are defined: 811 k=clear: 812 The encryption key (as described in [3] for RTP media streams 813 under the AV profile) is included untransformed in this key 814 field. 816 k=base64: 817 The encryption key (as described in [3] for RTP media streams 818 under the AV profile) is included in this key field but has been 819 base64 encoded because it includes characters that are 820 prohibited in SDP. 822 k=uri: 823 A Universal Resource Identifier as used by WWW clients is 824 included in this key field. The URI refers to the data 825 containing the key, and may require additional authentication 826 before the key can be returned. When a request is made to the 827 given URI, the MIME content-type of the reply specifies the 828 encoding for the key in the reply. The key should not be 829 obtained until the user wishes to join the session to reduce 830 synchronisation of requests to the WWW server(s). 832 k=prompt 833 No key is included in this SDP description, but the session or 834 media stream referred to by this key field is encrypted. The 835 user should be prompted for the key when attempting to join the 836 session, and this user-supplied key should then be used to 837 decrypt the media streams. 839 Attributes 841 a= 842 a=: 844 Attributes are the primary means for extending SDP. Attributes may be 845 defined to be used as "session-level" attributes, "media-level" 846 attributes, or both. 848 A media description may have any number of attributes (``a='' fields) 849 which are media specific. These are referred to as "media-level" 850 attributes and add information about the media stream. Attribute fields 851 can also be added before the first media field; these "session-level" 852 attributes convey additional information that applies to the conference 853 as a whole rather than to individual media; an example might be the 854 conference's floor control policy. 856 Attribute fields may be of two forms: 858 o property attributes. A property attribute is simply of the form 859 ``a=''. These are binary attributes, and the presence of the 860 attribute conveys that the attribute is a property of the session. 861 An example might be ``a=recvonly''. 863 o value attributes. A value attribute is of the form 864 ``a=:''. An example might be that a whiteboard 865 could have the value attribute ``a=orient:landscape'' 867 Attribute interpretation depends on the media tool being invoked. Thus 868 receivers of session descriptions should be configurable in their 869 interpretation of announcements in general and of attributes in 870 particular. 872 Attribute names MUST be in the US-ASCII subset of ISO-10646/UTF-8. 874 Attribute values are byte strings, and MAY use any byte value except 875 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values are 876 to be interpreted as in ISO-10646 character set with UTF-8 encoding. 877 Unlike other text fields, attribute values are NOT normally affected by 878 the `charset' attribute as this would make comparisons against known 879 values problematic. However, when an attribute is defined, it can be 880 defined to be charset-dependent, in which case it's value should be 881 interpreted in the session charset rather than in ISO-10646. 883 Attributes that will be commonly used can be registered with IANA (see 884 Appendix B). Unregistered attributes should begin with "X-" to prevent 885 inadvertent collision with registered attributes. In either case, if an 886 attribute is received that is not understood, it should simply be 887 ignored by the receiver. 889 Media Announcements 891 m= 892 A session description may contain a number of media descriptions. Each 893 media description starts with an ``m='' field, and is terminated by 894 either the next ``m='' field or by the end of the session description. 895 A media field also has several sub-fields: 897 o The first sub-field is the media type. Currently defined media are 898 ``audio'', ``video'', ``application'', ``data'' and ``control'', 899 though this list may be extended as new communication modalities 900 emerge (e.g., telepresense). The difference between ``application'' 901 and ``data'' is that the former is a media flow such as whiteboard 902 information, and the latter is bulk-data transfer such as 903 multicasting of program executables which will not typically be 904 displayed to the user. ``control'' is used to specify an additional 905 conference control channel for the session. 907 o The second sub-field is the transport port to which the media stream 908 will be sent. The meaning of the transport port depends on the 909 network being used as specified in the relevant ``c'' field and on 910 the transport protocol defined in the third sub-field. Other ports 911 used by the media application (such as the RTCP port, see [2]) 912 should be derived algorithmically from the base media port. 914 Note: For transports based on UDP, the value should be in the range 915 1024 to 65535 inclusive. For RTP compliance it SHOULD be an even 916 number. 918 For applications where hierarchically encoded streams are being sent 919 to a unicast address, it may be necessary to specify multiple 920 transport ports. This is done using a similar notation to that used 921 for IP multicast addresses in the ``c='' field: 923 m= / 925 In such a case, the ports used depend on the transport protocol. 926 For RTP, only the even ports are used for data and the corresponding 927 one-higher odd port is used for RTCP. For example: 929 m=video 49170/2 RTP/AVP 31 931 would specify that ports 49170 and 49171 form one RTP/RTCP pair and 932 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the 933 transport protocol and 31 is the format (see below). 935 If multiple addresses are specified in the ``c='' field and multiple 936 ports are specified in the ``m='' field, a one-to-one mapping from 937 port to the corresponding address is implied. For example: 939 c=IN IP4 224.2.1.1/127/2 940 m=video 49170/2 RTP/AVP 31 942 would imply that address 224.2.1.1 is used with ports 49170 and 943 49171, and address 224.2.1.2 is used with ports 49172 and 49173. 945 o The third sub-field is the transport protocol. The transport 946 protocol values are dependent on the address-type field in the 947 ``c='' fields. Thus a ``c='' field of IP4 defines that the 948 transport protocol runs over IP4. For IP4, it is normally expected 949 that most media traffic will be carried as RTP over UDP. The 950 following transport protocols are preliminarily defined, but may be 951 extended through registration of new protocols with IANA: 953 - RTP/AVP - the IETF's Realtime Transport Protocol using the 954 Audio/Video profile carried over UDP. 956 - udp - User Datagram Protocol 958 If an application uses a single combined proprietary media format 959 and transport protocol over UDP, then simply specifying the 960 transport protocol as udp and using the format field to distinguish 961 the combined protocol is recommended. If a transport protocol is 962 used over UDP to carry several distinct media types that need to be 963 distinguished by a session directory, then specifying the transport 964 protocol and media format separately is necessary. RTP is an 965 example of a transport-protocol that carries multiple payload 966 formats that must be distinguished by the session directory for it 967 to know how to start appropriate tools, relays, mixers or recorders. 969 The main reason to specify the transport-protocol in addition to the 970 media format is that the same standard media formats may be carried 971 over different transport protocols even when the network protocol is 972 the same - a historical example is vat PCM audio and RTP PCM audio. 973 In addition, relays and monitoring tools that are transport- 974 protocol-specific but format-independent are possible. 976 For RTP media streams operating under the RTP Audio/Video Profile 977 [3], the protocol field is ``RTP/AVP''. Should other RTP profiles 978 be defined in the future, their profiles will be specified in the 979 same way. For example, the protocol field ``RTP/XYZ'' would specify 980 RTP operating under a profile whose short name is ``XYZ''. 982 o The fourth and subsequent sub-fields are media formats. For audio 983 and video, these SHOULD reference a MIME sub-type describing the 984 format under the `audio' and `video' top-level MIME types. 986 When a list of payload formats is given, this implies that all of 987 these formats may be used in the session, but the first of these 988 formats is the default format for the session. 990 For media whose transport protocol is not RTP or UDP the format 991 field is protocol specific. Such formats should be defined in an 992 additional specification document. 994 For media whose transport protocol is RTP, SDP can be used to 995 provide a dynamic binding of media encoding to RTP payload type. 996 The encoding names in the RTP AV Profile do not specify unique audio 997 encodings (in terms of clock rate and number of audio channels), and 998 so they are not used directly in SDP format fields. Instead, the 999 payload type number should be used to specify the format for static 1000 payload types and the payload type number along with additional 1001 encoding information should be used for dynamically allocated 1002 payload types. 1004 An example of a static payload type is u-law PCM coded single 1005 channel audio sampled at 8kHz. This is completely defined in the 1006 RTP Audio/Video profile as payload type 0, so the media field for 1007 such a stream sent to UDP port 49232 is: 1009 m=audio 49232 RTP/AVP 0 1011 An example of a dynamic payload type is 16 bit linear encoded stereo 1012 audio sampled at 16KHz. If we wish to use dynamic RTP/AVP payload 1013 type 98 for such a stream, additional information is required to 1014 decode it: 1016 m=video 49232 RTP/AVP 98 1017 a=rtpmap:98 L16/16000/2 1019 The general form of an rtpmap attribute is: 1021 a=rtpmap: /[/] 1023 For audio streams, may specify the number of 1024 audio channels. This parameter may be omitted if the number of 1025 channels is one provided no additional parameters are needed. 1026 For video streams, no encoding parameters are currently specified. 1028 Additional parameters may be defined in the future, but codec- 1029 specific parameters should not be added. Parameters added to an 1030 rtpmap attribute should only be those required for a session 1031 directory to make the choice of appropriate media too to participate 1032 in a session. Codec-specific parameters should be added in other 1033 attributes. 1035 Up to one rtpmap attribute can be defined for each media format 1036 specified. Thus we might have: 1038 m=audio 49230 RTP/AVP 96 97 98 1039 a=rtpmap:96 L8/8000 1040 a=rtpmap:97 L16/8000 1041 a=rtpmap:98 L16/11025/2 1043 RTP profiles that specify the use of dynamic payload types must 1044 define the set of valid encoding names and/or a means to register 1045 encoding names if that profile is to be used with SDP. 1047 Experimental encoding formats can also be specified using rtpmap. 1048 RTP formats that are not registered as standard format names must be 1049 preceded by ``X-''. Thus a new experimental redundant audio stream 1050 called GSMLPC using dynamic payload type 99 could be specified as: 1052 m=audio 49232 RTP/AVP 99 1053 a=rtpmap:99 X-GSMLPC/8000 1055 Such an experimental encoding requires that any site wishing to 1056 receive the media stream has relevant configured state in its 1057 session directory to know which tools are appropriate. 1059 Note that RTP audio formats typically do not include information 1060 about the number of samples per packet. If a non-default (as 1061 defined in the RTP Audio/Video Profile) packetisation is required, 1062 the``ptime'' attribute is used as given below. 1064 For more details on RTP audio and video formats, see [3]. 1066 o Predefined formats for UDP protocol non-RTP media are as below. 1068 Application Formats: 1070 wb: LBL Whiteboard (transport: udp) 1072 nt: UCL Network Text Editor (transport: udp) 1074 Suggested Attributes 1076 The following attributes are suggested. Since application writers may 1077 add new attributes as they are required, this list is not exhaustive. 1079 a=cat: 1080 This attribute gives the dot-separated hierarchical category of the 1081 session. This is to enable a receiver to filter unwanted sessions 1082 by category. It would probably have been a compulsory separate 1083 field, except for its experimental nature at this time. It is a 1084 session-level attribute, and is not dependent on charset. 1086 a=keywds: 1087 Like the cat attribute, this is to assist identifying wanted 1088 sessions at the receiver. This allows a receiver to select 1089 interesting session based on keywords describing the purpose of the 1090 session. It is a session-level attribute. It is a charset dependent 1091 attribute, meaning that its value should be interpreted in the 1092 charset specified for the session description if one is specified, 1093 or by default in ISO 10646/UTF-8. 1095 a=tool: 1096 This gives the name and version number of the tool used to create 1097 the session description. It is a session-level attribute, and is 1098 not dependent on charset. 1100 a=ptime: 1101 This gives the length of time in milliseconds represented by the 1102 media in a packet. This is probably only meaningful for audio data. 1103 It should not be necessary to know ptime to decode RTP or vat audio, 1104 and it is intended as a recommendation for the 1105 encoding/packetisation of audio. It is a media attribute, and is 1106 not dependent on charset. 1108 a=maxptime: 1109 The maximum amount of media which can be encapsulated in each 1110 packet, expressed as time in milliseconds. The time shall be 1111 calculated as the sum of the time the media present in the packet 1112 represents. The time SHOULD be a multiple of the frame size. This is 1113 probably only meaningful for audio data. It is a media attribute, 1114 and is not dependent on charset. Note that this attribute was 1115 introduced after RFC 2327, and non updated implementations will 1116 ignore this attribute. 1118 a=rtpmap: /[/] 1120 See the section on Media Announcements (the ``m='' field). This may 1121 be either a session or media attribute. 1123 a=recvonly 1124 This specifies that the tools should be started in receive-only mode 1125 where applicable. It can be either a session or media attribute, and 1126 is not dependent on charset. Note that recvonly applies to the media 1127 only, not to any associated control protocol (e.g. an RTP based 1128 system in recvonly mode SHOULD still send RTCP packets). 1130 a=sendrecv 1131 This specifies that the tools should be started in send and receive 1132 mode. This is necessary for interactive conferences with tools such 1133 as wb which defaults to receive only mode. It can be either a 1134 session or media attribute, and is not dependent on charset. 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 byte 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 should be discouraged. Instead, multiple descriptions 1217 should be sent describing the session, one in each language. 1218 However this is not possible with all transport mechanisms, and so 1219 multiple sdplang 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. 1270 It is a media attribute, and is not dependent on charset. 1272 a=fmtp: 1273 This attribute allows parameters that are specific to a particular 1274 format to be conveyed in a way that SDP doesn't have to understand 1275 them. The format must be one of the formats specified for the 1276 media. Format-specific parameters may be any set of parameters 1277 required to be conveyed by SDP and given unchanged to the media tool 1278 that will use this format. 1280 It is a media attribute, and is not dependent on charset. 1282 6.1. Communicating Conference Control Policy 1284 There is some debate over the way conference control policy should be 1285 communicated. In general, the authors believe that an implicit 1286 declarative style of specifying conference control is desirable where 1287 possible. 1289 A simple declarative style uses a single conference attribute field 1290 before the first media field, possibly supplemented by properties such 1291 as `recvonly' for some of the media tools. This conference attribute 1292 conveys the conference control policy. An example might be: 1294 a=type:moderated 1296 In some cases, however, it is possible that this may be insufficient to 1297 communicate the details of an unusual conference control policy. If 1298 this is the case, then a conference attribute specifying external 1299 control might be set, and then one or more ``media'' fields might be 1300 used to specify the conference control tools and configuration data for 1301 those tools. An example is an ITU H.332 session: 1303 ... 1304 c=IN IP4 224.5.6.7 1305 a=type:H332 1306 m=audio 49230 RTP/AVP 0 1307 m=video 49232 RTP/AVP 31 1308 m=application 12349 udp wb 1309 m=control 49234 H323 mc 1310 c=IN IP4 134.134.157.81 1312 In this example, a general conference attribute (type:H332) is specified 1313 stating that conference control will be provided by an external H.332 1314 tool, and a contact addresses for the H.323 session multipoint 1315 controller is given. 1317 In this document, only the declarative style of conference control 1318 declaration is specified. Other forms of conference control should 1319 specify an appropriate type attribute, and should define the 1320 implications this has for control media. 1322 7. Security Considerations 1324 SDP is a session description format that describes multimedia sessions. 1325 A session description SHOULD NOT be trusted unless it has been obtained 1326 by an authenticated transport protocol from a trusted source. Many 1327 different transport protocols may be used to distribute session 1328 description, and the nature of the authentication will differ from 1329 transport to transport. 1331 One transport that will frequently be used to distribute session 1332 descriptions is the Session Announcement Protocol (SAP). SAP provides 1333 both encryption and authentication mechanisms but due to the nature of 1334 session announcements it is likely that there are many occasions where 1335 the originator of a session announcement cannot be authenticated because 1336 they are previously unknown to the receiver of the announcement and 1337 because no common public key infrastructure is available. 1339 On receiving a session description over an unauthenticated transport 1340 mechanism or from an untrusted party, software parsing the session 1341 should take a few precautions. Session description contain information 1342 required to start software on the receivers system. Software that 1343 parses a session description MUST not be able to start other software 1344 except that which is specifically configured as appropriate software to 1345 participate in multimedia sessions. It is normally considered 1346 INAPPROPRIATE for software parsing a session description to start, on a 1347 user's system, software that is appropriate to participate in multimedia 1348 sessions, without the user first being informed that such software will 1349 be started and giving their consent. Thus a session description 1350 arriving by session announcement, email, sessioR multimedia,session page 1351 SHOULD NOT deliver the user into an interactive 1352 without the user being aware that this will happen. As it is not always 1353 simple to tell whether a session is interactive or not, applications 1354 that are unsure should assume sessions are interactive. 1356 In this specification, there are no attributes which would allow the 1357 recipient of a session description to be informed to start multimedia 1358 tools in a mode where they default to transmitting. Under some 1359 circumstances it might be appropriate to define such attributes. If 1360 this is done an application parsing a session description containing 1361 such attributes SHOULD either ignore them, or inform the user that 1362 joining this session will result in the automatic transmission of 1363 multimedia data. The default behaviour for an unknown attribute is to 1364 ignore it. 1366 Session descriptions may be parsed at intermediate systems such as 1367 firewalls for the purposes of opening a hole in the firewall to allow 1368 the participation in multimedia sessions. It is considered 1369 INAPPROPRIATE for a firewall to open such holes for unicast data streams 1370 unless the session description comes in a request from inside the 1371 firewall. For multicast sessions, it is likely that local 1372 administrators will apply their own policies, but the exclusive use of 1373 "local" or "site-local" administrative scope within the firewall and the 1374 refusal of the firewall to open a hole for such scopes will provide 1375 separation of global multicast sessions from local ones. 1377 Appendix A: SDP Grammar 1379 This appendix provides an Augmented BNF grammar for SDP. ABNF is 1380 defined in RFC 2234. 1382 ; SDP Syntax 1383 announcement = proto-version 1384 origin-field 1385 session-name-field 1386 information-field 1387 uri-field 1388 email-fields 1389 phone-fields 1390 connection-field 1391 bandwidth-fields 1392 time-fields 1393 key-field 1394 attribute-fields 1395 media-descriptions 1397 proto-version = "v=" 1*DIGIT CRLF 1398 ;this memo describes version 0 1400 origin-field = "o=" username SP sess-id SP sess-version SP 1401 nettype SP addrtype SP unicast-address CRLF 1403 session-name-field = "s=" text CRLF 1405 information-field = ["i=" text CRLF] 1407 uri-field = ["u=" uri CRLF] 1409 email-fields = *("e=" email-address CRLF) 1411 phone-fields = *("p=" phone-number CRLF) 1412 connection-field = ["c=" nettype SP addrtype SP 1413 connection-address CRLF] 1414 ;a connection field must be present 1415 ;in every media description or at the 1416 ;session-level 1418 bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF) 1420 time-fields = 1*( "t=" start-time SP stop-time 1421 *(CRLF repeat-fields) CRLF) 1422 [zone-adjustments CRLF] 1424 repeat-fields = "r=" repeat-interval SP typed-time 1425 1*(SP typed-time) 1427 zone-adjustments = "z=" time SP ["-"] typed-time 1428 *(SP time SP ["-"] typed-time) 1430 key-field = ["k=" key-type CRLF] 1432 attribute-fields = *("a=" attribute CRLF) 1434 media-descriptions = *( media-field 1435 information-field 1436 *connection-field 1437 bandwidth-fields 1438 key-field 1439 attribute-fields ) 1441 media-field = "m=" media SP port ["/" integer] 1442 SP proto 1*(SP fmt) CRLF 1444 ; sub-rules of 'o=' 1445 username = non-ws-string 1446 ;pretty wide definition, but doesn't include space 1448 sess-id = 1*DIGIT 1449 ;should be unique for this originating username/host 1451 sess-version = 1*DIGIT 1452 ;0 is a new session 1454 nettype = token 1455 ;typically "IN" 1457 addrtype = token 1458 ;typically "IP4" or "IP6" 1460 ; sub-rules of 'u=' 1461 uri = URI-reference; defined in RFC1630 and RFC2732 1463 ; sub-rules of 'e=' 1464 email-address = email *SP "(" 1*email-safe ")" / 1465 1*email-safe "<" email ">" / 1466 email 1468 email = addr-spec ; defined in RFC2822 1469 ; modified to remove CFWS 1471 ; sub-rules of 'p=' 1472 phone-number = phone *SP "(" 1*email-safe ")" / 1473 1*email-safe "<" phone ">" / 1474 phone 1476 phone = "+" POS-DIGIT 1*(SP / "-" / DIGIT) 1477 ;there must be a space or hyphen between the 1478 ;international code and the rest of the number. 1480 ; Should this use the tel: URL syntax? 1482 ; sub-rules of 'c=' 1483 connection-address = multicast-address / unicast-address 1484 ; sub-rules of 'b=' 1485 bwtype = token 1487 bandwidth = 1*DIGIT 1489 ; sub-rules of 't=' 1490 start-time = time / "0" 1492 stop-time = time / "0" 1494 time = POS-DIGIT 9*DIGIT 1495 ; 10-digit NTP time represents times between 1496 ; 1931 and 5068 AD. 9* allows times after that 1497 ; as well. 1499 ; sub-rules of 'r=' and 'z=' 1500 repeat-interval = typed-time 1502 typed-time = POS-DIGIT *DIGIT [fixed-len-time-unit] 1504 fixed-len-time-unit = "d" / "h" / "m" / "s" 1506 ; sub-rules of 'k=' 1507 key-type = "prompt" / 1508 "clear:" text / 1509 "base64:" base64 / 1510 "uri:" uri / 1511 key-method [ ":" text ] 1513 base64 = *base64-unit [base64-pad] 1514 base64-unit = 4base64-char 1515 base64-pad = 2base64-char "==" / 3base64-char "=" 1516 base64-char = ALPHA / DIGIT / "+" / "/" 1518 key-method = token 1519 ; sub-rules of 'a=' 1520 attribute = (att-field ":" att-value) / att-field 1522 att-field = token 1524 att-value = byte-string 1526 ; sub-rules of 'm=' 1527 media = token 1528 ;typically "audio", "video", "application" 1529 ;or "data" 1531 fmt = token 1532 ;typically an RTP payload type for audio 1533 ;and video media 1535 proto = token "/" token 1536 / token 1537 ;typically "RTP/AVP" or "udp" for IP4 1539 port = 1*DIGIT 1540 ;should in the range "1024" to "65535" inclusive 1541 ;for UDP based media 1543 ; generic sub-rules: addressing 1544 unicast-address = IP4-address / IP6-address / FQDN / extension-addr 1546 multicast-address = IP4-multicast / IP6-multicast 1548 IP4-multicast = m1 3*( "." decimal-uchar ) 1549 "/" ttl [ "/" integer ] 1550 ; IPv4 multicast addresses may be in the 1551 ; range 224.0.0.0 to 239.255.255.255 1553 m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / 1554 ("23" DIGIT )) 1556 IP6-multicast = hexpart [ "/" integer ] 1557 ; IPv6 address starting with FF 1559 ttl = (POS-DIGIT *2DIGIT) / "0" 1561 FQDN = 4*(alpha-numeric / "-" / ".") 1562 ; fully qualified domain name as specified 1563 ; in RFC1035 1565 IP4-address = b1 3*("." decimal-uchar) / "0.0.0.0" 1567 b1 = decimal-uchar 1568 ; less than "224"; not "0" or "127" 1570 ; The following is from RFC2373 Appendix B. It is a direct copy. 1571 IP6-address = hexpart [ ":" IP4-address ] 1573 hexpart = hexseq / hexseq "::" [ hexseq ] / 1574 "::" [ hexseq ] 1576 hexseq = hex4 *( ":" hex4) 1578 hex4 = 1*4HEXDIG 1580 ; Generic for other address families 1581 extension-addr = non-ws-string 1583 ; generic sub-rules: datatypes 1584 text = byte-string 1585 ;default is to interpret this as IS0-10646 UTF8 1586 ;ISO 8859-1 requires a "a=charset:ISO-8859-1" 1587 ;session-level attribute to be used 1589 byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF) 1590 ;any byte except NUL, CR or LF 1592 non-ws-string = 1*(VCHAR/%x80-FF) 1593 ;string of visible US-ASCII, or high-bit, characters 1595 token-char = %x21/%x23-27/%x2A-2B/%x2D-2E/%x30-39/%x41-5A/%x5E-7E 1596 ; definition from RFC 2045 - 1597 ; "any (US-ASCII) CHAR except SPACE, CTLs, 1598 ; or tspecials". 1599 ; the tspecials are ()<>@,;: 1601 token = 1*(token-char) 1603 email-safe = 1*(%x01-09/%x0B-0C/%x0E-27/ 1604 %x2A-3B/%x3D/%x3E-FF) 1605 ;any byte except NUL, CR, LF, or the quoting 1606 ;characters ()<> 1608 integer = POS-DIGIT *DIGIT 1610 ; generic sub-rules: primitives 1611 alpha-numeric = ALPHA / DIGIT 1613 POS-DIGIT = %x31-39 ; 1 - 9 1615 ; external references: 1616 ; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 2234 1617 ; URI-reference: from RFC1630 and RFC2732 1618 ; addr-spec: from RFC 2822 1619 Appendix B: Guidelines for registering SDP names with IANA 1621 There are seven field names that may be registered with IANA. Using the 1622 terminology in the SDP specification BNF, they are "media", "proto", 1623 "fmt", "att-field", "bwtype", "nettype" and "addrtype". 1625 "media" (eg, audio, video, application, data). 1627 The set of media is intended to be small and not to be extended 1628 except under rare circumstances. The same rules should apply for 1629 media names as for top-level MIME content types, and where possible 1630 the same name should be registered for SDP as for MIME. For media 1631 other than existing MIME top-level content types, a standards-track 1632 RFC MUST be produced for a new top-level content type to be 1633 registered, and the registration MUST provide good justification 1634 why no existing media name is appropriate. 1636 "proto" 1638 In general this should be an IETF standards-track transport 1639 protocol identifier such as RTP/AVP (rfc 1889 under the rfc 1890 1640 profile). 1642 However, people will want to invent their own proprietary transport 1643 protocols. Some of these should be registered as a "fmt" using 1644 "udp" as the protocol and some of which probably can't be. 1646 Where the protocol and the application are intimately linked, such 1647 as with the LBL whiteboard wb which used a proprietary and special 1648 purpose protocol over UDP, the protocol name should be "udp" and 1649 the format name that should be registered is "wb". The rules for 1650 formats (see below) apply to such registrations. 1652 Where the proprietary transport protocol really carries many 1653 different data formats, it is possible to register a new protocol 1654 name with IANA. In such a case, an RFC MUST be produced describing 1655 the protocol and referenced in the registration. Such an RFC MAY 1656 be informational, although it is preferable if it is standards- 1657 track. 1659 "fmt" 1661 The format namespace is dependent on the context of the "proto" 1662 field, so a format cannot be registered without specifying one or 1663 more transport protocols that it applies to. 1665 Formats cover all the possible encodings that might want to be 1666 transported in a multimedia session. 1668 For RTP formats that have been assigned static payload types, the 1669 payload type number is used. For RTP formats using a dynamic 1670 payload type number, the dynamic payload type number is given as 1671 the format and an additional "rtpmap" attribute specifies the 1672 format and parameters. 1674 For non-RTP formats, any unregistered format name may be 1675 registered. If there is a suitable mapping from a MIME subtype to 1676 the format, then the MIME subtype name should be registered. If 1677 there is no suitable mapping from a MIME subtype, a new name should 1678 be registered. In either case, unless there are strong reasons not 1679 to do so, a standards-track RFC SHOULD be produced describing the 1680 format and this RFC SHOULD be referenced in the registration. 1682 "att-field" (Attribute names) 1684 Attribute field names MAY be registered with IANA, although this is 1685 not compulsory, and unknown attributes are simply ignored. 1687 When an attribute is registered, it must be accompanied by a brief 1688 specification stating the following: 1690 o contact name, email address and telephone number 1692 o attribute-name (as it will appear in SDP) 1694 o long-form attribute name in English 1696 o type of attribute (session level, media level, or both) 1698 o whether the attribute value is subject to the charset 1699 attribute. 1701 o a one paragraph explanation of the purpose of the attribute. 1703 o a specification of appropriate attribute values for this 1704 attribute. 1706 IANA will not sanity check such attribute registrations except to 1707 ensure that they do not clash with existing registrations. 1709 Although the above is the minimum that IANA will accept, if the 1710 attribute is expected to see widespread use and interoperability is 1711 an issue, authors are encouraged to produce a standards-track RFC 1712 that specifies the attribute more precisely. 1714 Submitters of registrations should ensure that the specification is 1715 in the spirit of SDP attributes, most notably that the attribute is 1716 platform independent in the sense that it makes no implicit 1717 assumptions about operating systems and does not name specific 1718 pieces of software in a manner that might inhibit interoperability. 1720 "bwtype" (bandwidth specifiers) 1722 A proliferation of bandwidth specifiers is strongly discouraged. 1724 New bandwidth specifiers may be registered with IANA. The 1725 submission MUST reference a standards-track RFC specifying the 1726 semantics of the bandwidth specifier precisely, and indicating when 1727 it should be used, and why the existing registered bandwidth 1728 specifiers do not suffice. 1730 "nettype" (Network Type) 1732 New network types may be registered with IANA if SDP needs to be 1733 used in the context of non-internet environments. Whilst these are 1734 not normally the preserve of IANA, there may be circumstances when 1735 an Internet application needs to interoperate with a non-internet 1736 application, such as when gatewaying an internet telephony call 1737 into the PSTN. The number of network types should be small and 1738 should be rarely extended. A new network type cannot be registered 1739 without registering at least one address type to be used with that 1740 network type. A new network type registration MUST reference an 1741 RFC which gives details of the network type and address type and 1742 specifies how and when they would be used. Such an RFC MAY be 1743 Informational. 1745 "addrtype" (Address Type) 1747 New address types may be registered with IANA. An address type is 1748 only meaningful in the context of a network type, and any 1749 registration of an address type MUST specify a registered network 1750 type, or be submitted along with a network type registration. A 1751 new address type registration MUST reference an RFC giving details 1752 of the syntax of the address type. Such an RFC MAY be 1753 Informational. Address types are not expected to be registered 1754 frequently. 1756 Registration Procedure 1758 To register a name the above guidelines should be followed regarding the 1759 required level of documentation that is required. The registration 1760 itself should be sent to IANA. Attribute registrations should include 1761 the information given above. Other registrations should include the 1762 following additional information: 1764 o contact name, email address and telephone number 1766 o name being registered (as it will appear in SDP) 1768 o long-form name in English 1770 o type of name ("media", "proto", "fmt", "bwtype", "nettype", or 1771 "addrtype") 1773 o a one paragraph explanation of the purpose of the registered name. 1775 o a reference to the specification (eg RFC number) of the registered 1776 name. 1778 IANA may refer any registration to the IESG or to any appropriate IETF 1779 working group for review, and may request revisions to be made before a 1780 registration will be made. 1782 Appendix C: Changes from RFC 2327 1784 o Clarify that a=recvonly does NOT mean that you don't send RTCP, and 1785 similarly for sendonly and inactive. These only effect the RTP 1786 stream. 1788 o Rewrite the ABNF syntax (thanks to Jonathan Lennox) 1790 o Add a=inactive attribute. 1792 o Add a=maxptime attribute. 1794 o RFC 2327 mandated that either e= or p= was required. Both are now 1795 optional, to reflect actual usage. 1797 o Removed references to "conference" from the description of the t= 1798 line, to make it less SAP oriented. 1800 o Note about wrap-around of NTP timestamps in t= 1802 o Update BNF to support IPv6. 1804 o References have been updated. 1806 o Section 3.1 was replaced with a reference to RFC 2119, and the memo 1807 has been updated to use the RFC 2119 terminology (MUST, SHOULD, 1808 etc). 1810 o Use of "application/sdp" as MIME a type for SDP files is now "MUST" 1811 rather than "SHOULD". 1813 o A number of sections have been updated to be less SAP specific, and 1814 to reference other current uses of SDP such as RTSP and SIP. 1816 o The section on concatenation of session descriptions (which was not 1817 allowed in SAP, but allowed in other cases) has been removed. It is 1818 assumed that transports of SDP specify will specify this. 1820 o The description of the c= line has been updated to reflect common 1821 usage of SDP, rather than Mbone conferencing with SAP. 1823 o The b= line no longer makes a normative reference to the Mbone FAQ 1824 for bandwidth limits at various TTLs. The AS modifier to b= is 1825 noted as being the RTP session bandwidth. 1827 o Define relation between the m= line and MIME types 1828 o Note use of s= in sessions with no meaningful name 1830 o Note that a=rtpmap is a media level attribute 1832 Appendix D: Authors' Addresses 1834 Mark Handley 1835 AT&T Center for Internet Research at ICSI, 1836 International Computer Science Institute, 1837 1947 Center Street, Suite 600, 1838 Berkeley, CA 94704, USA 1839 Email: mjh@aciri.org 1841 Van Jacobson 1842 MS 46a-1121 1843 Lawrence Berkeley Laboratory 1844 Berkeley, CA 94720 1845 United States 1846 Email: van@ee.lbl.gov 1848 Colin Perkins 1849 USC Information Sciences Institute 1850 3811 N. Fairfax Drive, Suite 200 1851 Arlington, VA 22203 1852 United States 1853 Email: csp@isi.edu 1855 Acknowledgments 1857 Many people in the IETF MMUSIC working group have made comments and 1858 suggestions contributing to this document. In particular, we would like 1859 to thank Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin, Ross 1860 Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Steve Hanna and 1861 Jonathan Lennox. 1863 References 1865 [1] D. Mills, ``Network Time Protocol (version 3) specification and 1866 implementation", RFC 1305, March 1992. 1868 [2] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson, ``RTP: A 1869 Transport Protocol for Real-Time Applications'', RFC 1889, January 1870 1996. 1872 [3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with 1873 Minimal Control'', RFC 1890, January 1996. 1875 [4] M. Handley, C. Perkins and E. Whelan, ``Session Announcement 1876 Protocol'', RFC 2974, October 2000. 1878 [5] V. Jacobson and S. McCanne, ``vat - X11-based audio teleconferencing 1879 tool'' vat manual page, Lawrence Berkeley Laboratory, 1994. 1881 [6] The Unicode Consortium, "The Unicode Standard -- Version 2.0", 1882 Addison-Wesley, 1996. 1884 [7] ISO/IEC 10646-1:1993. International Standard -- Information 1885 technology -- Universal Multiple-Octet Coded Character Set (UCS) 1886 -- Part 1: Architecture and Basic Multilingual Plane. Five 1887 amendments and a technical corrigendum have been published up 1888 to now. UTF-8 is described in Annex R, published as Amendment 2. 1890 [8] D. Goldsmith and M. Davis, ``Using Unicode with MIME'', RFC1641, 1891 July 1994 1893 [9] F. Yergeau, ``UTF-8, a transformation format of Unicode and ISO 1894 10646'', RFC 2044, October 1996 1896 [10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for 1897 Receiving Internet-based H.323 Conferences", ITU, Geneva. 1899 [11] M. Handley, H. Schulzrinne, E. Scholler and J. Rosenberg ``SIP: 1900 Session Initiation Protocol'', RFC 2543, March 1999. 1902 [12] H. Schulzrinne, A. Rao and R. Lanphier, ``Real Time Streaming 1903 Protocol (RTSP)'' RFC 2326, April 1998. 1905 [13] S. Bradner, ``Key words for use in RFCs to Indicate Requirement 1906 Levels'', RFC 2119, March 1997.