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