<pre>Network Working Group                                           M. Handley
Request for Comments: 2327                                     V. Jacobson
Category: Standards Track                                         ISI/LBNL
                                                                April 1998


                   <span class="h1">SDP: Session Description Protocol</span>

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

   This document defines the Session Description Protocol, SDP.  SDP is
   intended for describing multimedia sessions for the purposes of
   session announcement, session invitation, and other forms of
   multimedia session initiation.

   This document is a product of the Multiparty Multimedia Session
   Control (MMUSIC) working group of the Internet Engineering Task
   Force. Comments are solicited and should be addressed to the working
   group's mailing list at confctrl@isi.edu and/or the authors.

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   On the Internet multicast backbone (Mbone), a session directory tool
   is used to advertise multimedia conferences and communicate the
   conference addresses and conference tool-specific information
   necessary for participation.  This document defines a session
   description protocol for this purpose, and for general real-time
   multimedia session description purposes. This memo does not describe
   multicast address allocation or the distribution of SDP messages in
   detail.  These are described in accompanying memos.  SDP is not
   intended for negotiation of media encodings.








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<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Background</span>

   The Mbone is the part of the internet that supports IP multicast, and
   thus permits efficient many-to-many communication.  It is used
   extensively for multimedia conferencing.  Such conferences usually
   have the property that tight coordination of conference membership is
   not necessary; to receive a conference, a user at an Mbone site only
   has to know the conference's multicast group address and the UDP
   ports for the conference data streams.

   Session directories assist the advertisement of conference sessions
   and communicate the relevant conference setup information to
   prospective participants.  SDP is designed to convey such information
   to recipients.  SDP is purely a format for session description - it
   does not incorporate a transport protocol, and is intended to use
   different transport protocols as appropriate including the Session
   Announcement Protocol [<a href="#ref-4" title="&quot;SAP - Session Announcement Protocol&quot;">4</a>], Session Initiation Protocol [<a href="#ref-11" title="&quot;Session Initiation Protocol (SIP)&quot;">11</a>], Real-
   Time Streaming Protocol [<a href="#ref-12" title="&quot;Real Time Streaming Protocol (RTSP)&quot;">12</a>], electronic mail using the MIME
   extensions, and the Hypertext Transport Protocol.

   SDP is intended to be general purpose so that it can be used for a
   wider range of network environments and applications than just
   multicast session directories.  However, it is not intended to
   support negotiation of session content or media encodings - this is
   viewed as outside the scope of session description.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Glossary of Terms</span>

   The following terms are used in this document, and have specific
   meaning within the context of this document.

   Conference
     A multimedia conference is a set of two or more communicating users
     along with the software they are using to communicate.

   Session
     A multimedia session is a set of multimedia senders and receivers
     and the data streams flowing from senders to receivers.  A
     multimedia conference is an example of a multimedia session.

   Session Advertisement
     See session announcement.

   Session Announcement
     A session announcement is a mechanism by which a session
     description is conveyed to users in a proactive fashion, i.e., the
     session description was not explicitly requested by the user.




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   Session Description
     A well defined format for conveying sufficient information to
     discover and participate in a multimedia session.

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Terminology</span>

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in <a href="./rfc2119">RFC 2119</a>.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  SDP Usage</span>

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Multicast Announcements</span>

   SDP is a session description protocol for multimedia sessions. A
   common mode of usage is for a client to announce a conference session
   by periodically multicasting an announcement packet to a well known
   multicast address and port using the Session Announcement Protocol
   (SAP).

   SAP packets are UDP packets with the following format:

         |--------------------|
         | SAP header         |
         |--------------------|
         | text payload       |
         |//////////


   The header is the Session Announcement Protocol header.  SAP is
   described in more detail in a companion memo [<a href="#ref-4" title="&quot;SAP - Session Announcement Protocol&quot;">4</a>]

   The text payload is an SDP session description, as described in this
   memo.  The text payload should be no greater than 1 Kbyte in length.
   If announced by SAP, only one session announcement is permitted in a
   single packet.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Email and WWW Announcements</span>

   Alternative means of conveying session descriptions include
   electronic mail and the World Wide Web. For both email and WWW
   distribution, the use of the MIME content type "application/sdp"
   should be used.  This enables the automatic launching of applications
   for participation in the session from the WWW client or mail reader
   in a standard manner.






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   Note that announcements of multicast sessions made only via email or
   the World Wide Web (WWW) do not have the property that the receiver
   of a session announcement can necessarily receive the session because
   the multicast sessions may be restricted in scope, and access to the
   WWW server or reception of email is possible outside this scope.  SAP
   announcements do not suffer from this mismatch.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Requirements and Recommendations</span>

   The purpose of SDP is to convey information about media streams in
   multimedia sessions to allow the recipients of a session description
   to participate in the session.  SDP is primarily intended for use in
   an internetwork, although it is sufficiently general that it can
   describe conferences in other network environments.

   A multimedia session, for these purposes, is defined as a set of
   media streams that exist for some duration of time.  Media streams
   can be many-to-many.  The times during which the session is active
   need not be continuous.

   Thus far, multicast based sessions on the Internet have differed from
   many other forms of conferencing in that anyone receiving the traffic
   can join the session (unless the session traffic is encrypted).  In
   such an environment, SDP serves two primary purposes.  It is a means
   to communicate the existence of a session, and is a means to convey
   sufficient information to enable joining and participating in the
   session.  In a unicast environment, only the latter purpose is likely
   to be relevant.

   Thus SDP includes:

   o Session name and purpose

   o Time(s) the session is active

   o The media comprising the session

   o Information to receive those media (addresses, ports, formats and
     so on)

   As resources necessary to participate in a session may be limited,
   some additional information may also be desirable:

   o Information about the bandwidth to be used by the conference

   o Contact information for the person responsible for the session





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   In general, SDP must convey sufficient information to be able to join
   a session (with the possible exception of encryption keys) and to
   announce the resources to be used to non-participants that may need
   to know.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Media Information</span>

   SDP includes:

   o The type of media (video, audio, etc)

   o The transport protocol (RTP/UDP/IP, H.320, etc)

   o The format of the media (H.261 video, MPEG video, etc)

   For an IP multicast session, the following are also conveyed:

   o Multicast address for media

   o Transport Port for media

   This address and port are the destination address and destination
   port of the multicast stream, whether being sent, received, or both.

   For an IP unicast session, the following are conveyed:

   o Remote address for media

   o Transport port for contact address

   The semantics of this address and port depend on the media and
   transport protocol defined.  By default, this is the remote address
   and remote port to which data is sent, and the remote address and
   local port on which to receive data.  However, some media may define
   to use these to establish a control channel for the actual media
   flow.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Timing Information</span>

   Sessions may either be bounded or unbounded in time. Whether or not
   they are bounded, they may be only active at specific times.

   SDP can convey:

   o An arbitrary list of start and stop times bounding the session

   o For each bound, repeat times such as "every Wednesday at 10am for
     one hour"



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   This timing information is globally consistent, irrespective of local
   time zone or daylight saving time.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Private Sessions</span>

   It is possible to create both public sessions and private sessions.
   Private sessions will typically be conveyed by encrypting the session
   description to distribute it.  The details of how encryption is
   performed are dependent on the mechanism used to convey SDP - see [<a href="#ref-4" title="&quot;SAP - Session Announcement Protocol&quot;">4</a>]
   for how this is done for session announcements.

   If a session announcement is private it is possible to use that
   private announcement to convey encryption keys necessary to decode
   each of the media in a conference, including enough information to
   know which encryption scheme is used for each media.

<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  Obtaining Further Information about a Session</span>

   A session description should convey enough information to decide
   whether or not to participate in a session.  SDP may include
   additional pointers in the form of Universal Resources Identifiers
   (URIs) for more information about the session.

<span class="h3"><a class="selflink" id="section-5.5" href="#section-5.5">5.5</a>.  Categorisation</span>

   When many session descriptions are being distributed by SAP or any
   other advertisement mechanism, it may be desirable to filter
   announcements that are of interest from those that are not.  SDP
   supports a categorisation mechanism for sessions that is capable of
   being automated.

<span class="h3"><a class="selflink" id="section-5.6" href="#section-5.6">5.6</a>.  Internationalization</span>

   The SDP specification recommends the use of the ISO 10646 character
   sets in the UTF-8 encoding (<a href="./rfc2044">RFC 2044</a>) to allow many different
   languages to be represented.  However, to assist in compact
   representations, SDP also allows other character sets such as ISO
   8859-1 to be used when desired.  Internationalization only applies to
   free-text fields (session name and background information), and not
   to SDP as a whole.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  SDP Specification</span>

   SDP session descriptions are entirely textual using the ISO 10646
   character set in UTF-8 encoding. SDP field names and attributes names
   use only the US-ASCII subset of UTF-8, but textual fields and
   attribute values may use the full ISO 10646 character set.  The
   textual form, as opposed to a binary encoding such as ASN/1 or XDR,



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   was chosen to enhance portability, to enable a variety of transports
   to be used (e.g, session description in a MIME email message) and to
   allow flexible, text-based toolkits (e.g., Tcl/Tk ) to be used to
   generate and to process session descriptions.  However, since the
   total bandwidth allocated to all SAP announcements is strictly
   limited, the encoding is deliberately compact.  Also, since
   announcements may be transported via very unreliable means (e.g.,
   email) or damaged by an intermediate caching server, the encoding was
   designed with strict order and formatting rules so that most errors
   would result in malformed announcements which could be detected
   easily and discarded. This also allows rapid discarding of encrypted
   announcements for which a receiver does not have the correct key.

   An SDP session description consists of a number of lines of text of
   the form &lt;type&gt;=&lt;value&gt; &lt;type&gt; is always exactly one character and is
   case-significant.  &lt;value&gt; is a structured text string whose format
   depends on &lt;type&gt;.  It also will be case-significant unless a
   specific field defines otherwise.  Whitespace is not permitted either
   side of the `=' sign. In general &lt;value&gt; is either a number of fields
   delimited by a single space character or a free format string.

   A session description consists of a session-level description
   (details that apply to the whole session and all media streams) and
   optionally several media-level descriptions (details that apply onto
   to a single media stream).

   An announcement consists of a session-level section followed by zero
   or more media-level sections.  The session-level part starts with a
   `v=' line and continues to the first media-level section.  The media
   description starts with an `m=' line and continues to the next media
   description or end of the whole session description.  In general,
   session-level values are the default for all media unless overridden
   by an equivalent media-level value.

   When SDP is conveyed by SAP, only one session description is allowed
   per packet.  When SDP is conveyed by other means, many SDP session
   descriptions may be concatenated together (the `v=' line indicating
   the start of a session description terminates the previous
   description).  Some lines in each description are required and some
   are optional but all must appear in exactly the order given here (the
   fixed order greatly enhances error detection and allows for a simple
   parser). Optional items are marked with a `*'.

Session description
        v=  (protocol version)
        o=  (owner/creator and session identifier).
        s=  (session name)
        i=* (session information)



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        u=* (URI of description)
        e=* (email address)
        p=* (phone number)
        c=* (connection information - not required if included in all media)
        b=* (bandwidth information)
        One or more time descriptions (see below)
        z=* (time zone adjustments)
        k=* (encryption key)
        a=* (zero or more session attribute lines)
        Zero or more media descriptions (see below)

Time description
        t=  (time the session is active)
        r=* (zero or more repeat times)

Media description
        m=  (media name and transport address)
        i=* (media title)
        c=* (connection information - optional if included at session-level)
        b=* (bandwidth information)
        k=* (encryption key)
        a=* (zero or more media attribute lines)

   The set of `type' letters is deliberately small and not intended to
   be extensible -- SDP parsers must completely ignore any announcement
   that contains a `type' letter that it does not understand. The
   `attribute' mechanism ("a=" described below) is the primary means for
   extending SDP and tailoring it to particular applications or media.
   Some attributes (the ones listed in this document) have a defined
   meaning but others may be added on an application-, media- or
   session-specific basis.  A session directory must ignore any
   attribute it doesn't understand.

   The connection (`c=') and attribute (`a=') information in the
   session-level section applies to all the media of that session unless
   overridden by connection information or an attribute of the same name
   in the media description.  For instance, in the example below, each
   media behaves as if it were given a `recvonly' attribute.

   An example SDP description is:

        v=0
        o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
        s=SDP Seminar
        i=A Seminar on the session description protocol
        u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
        e=mjh@isi.edu (Mark Handley)
        c=IN IP4 224.2.17.12/127



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        t=2873397496 2873404696
        a=recvonly
        m=audio 49170 RTP/AVP 0
        m=video 51372 RTP/AVP 31
        m=application 32416 udp wb
        a=orient:portrait

   Text records such as the session name and information are bytes
   strings which may contain any byte with the exceptions of 0x00 (Nul),
   0x0a (ASCII newline) and 0x0d (ASCII carriage return).  The sequence
   CRLF (0x0d0a) is used to end a record, although parsers should be
   tolerant and also accept records terminated with a single newline
   character.  By default these byte strings contain ISO-10646
   characters in UTF-8 encoding, but this default may be changed using
   the `charset' attribute.

   Protocol Version

   v=0

   The "v=" field gives the version of the Session Description Protocol.
   There is no minor version number.

   Origin

   o=&lt;username&gt; &lt;session id&gt; &lt;version&gt; &lt;network type&gt; &lt;address type&gt;
   &lt;address&gt;

   The "o=" field gives the originator of the session (their username
   and the address of the user's host) plus a session id and session
   version number.

   &lt;username&gt; is the user's login on the originating host, or it is "-"
   if the originating host does not support the concept of user ids.
   &lt;username&gt; must not contain spaces.  &lt;session id&gt; is a numeric string
   such that the tuple of &lt;username&gt;, &lt;session id&gt;, &lt;network type&gt;,
   &lt;address type&gt; and &lt;address&gt; form a globally unique identifier for
   the session.

   The method of &lt;session id&gt; allocation is up to the creating tool, but
   it has been suggested that a Network Time Protocol (NTP) timestamp be
   used to ensure uniqueness [<a href="#ref-1" title="&quot;Network Time Protocol (version 3) specification and implementation&quot;">1</a>].

   &lt;version&gt; is a version number for this announcement.  It is needed
   for proxy announcements to detect which of several announcements for
   the same session is the most recent.  Again its usage is up to the





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   creating tool, so long as &lt;version&gt; is increased when a modification
   is made to the session data.  Again, it is recommended (but not
   mandatory) that an NTP timestamp is used.

   &lt;network type&gt; is a text string giving the type of network.
   Initially "IN" is defined to have the meaning "Internet".  &lt;address
   type&gt; is a text string giving the type of the address that follows.
   Initially "IP4" and "IP6" are defined.  &lt;address&gt; is the globally
   unique address of the machine from which the session was created.
   For an address type of IP4, this is either the fully-qualified domain
   name of the machine, or the dotted-decimal representation of the IP
   version 4 address of the machine.  For an address type of IP6, this
   is either the fully-qualified domain name of the machine, or the
   compressed textual representation of the IP version 6 address of the
   machine.  For both IP4 and IP6, the fully-qualified domain name is
   the form that SHOULD be given unless this is unavailable, in which
   case the globally unique address may be substituted.  A local IP
   address MUST NOT be used in any context where the SDP description
   might leave the scope in which the address is meaningful.

   In general, the "o=" field serves as a globally unique identifier for
   this version of this session description, and the subfields excepting
   the version taken together identify the session irrespective of any
   modifications.

   Session Name

   s=&lt;session name&gt;

   The "s=" field is the session name.  There must be one and only one
   "s=" field per session description, and it must contain ISO 10646
   characters (but see also the `charset' attribute below).

   Session and Media Information

   i=&lt;session description&gt;

   The "i=" field is information about the session.  There may be at
   most one session-level "i=" field per session description, and at
   most one "i=" field per media. Although it may be omitted, this is
   discouraged for session announcements, and user interfaces for
   composing sessions should require text to be entered.  If it is
   present it must contain ISO 10646 characters (but see also the
   `charset' attribute below).

   A single "i=" field can also be used for each media definition.  In
   media definitions, "i=" fields are primarily intended for labeling
   media streams. As such, they are most likely to be useful when a



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   single session has more than one distinct media stream of the same
   media type.  An example would be two different whiteboards, one for
   slides and one for feedback and questions.

   URI

   u=&lt;URI&gt;

   o A URI is a Universal Resource Identifier as used by WWW clients

   o The URI should be a pointer to additional information about the
     conference

   o This field is optional, but if it is present it should be specified
     before the first media field

   o No more than one URI field is allowed per session description


   Email Address and Phone Number

   e=&lt;email address&gt;
   p=&lt;phone number&gt;

   o These specify contact information for the person responsible for
     the conference.  This is not necessarily the same person that
     created the conference announcement.

   o Either an email field or a phone field must be specified.
     Additional email and phone fields are allowed.

   o If these are present, they should be specified before the first
     media field.

   o More than one email or phone field can be given for a session
     description.

   o Phone numbers should be given in the conventional international

     format - preceded by a "+ and the international country code.
     There must be a space or a hyphen ("-") between the country code
     and the rest of the phone number.  Spaces and hyphens may be used
     to split up a phone field to aid readability if desired. For
     example:

                   p=+44-171-380-7777 or p=+1 617 253 6011





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   o Both email addresses and phone numbers can have an optional free
     text string associated with them, normally giving the name of the
     person who may be contacted.  This should be enclosed in
     parenthesis if it is present.  For example:

                        e=mjh@isi.edu (Mark Handley)

     The alternative <a href="./rfc822">RFC822</a> name quoting convention is also allowed for
     both email addresses and phone numbers.  For example,

                        e=Mark Handley &lt;mjh@isi.edu&gt;

     The free text string should be in the ISO-10646 character set with
     UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings
     if the appropriate charset session-level attribute is set.

   Connection Data

   c=&lt;network type&gt; &lt;address type&gt; &lt;connection address&gt;

   The "c=" field contains connection data.

   A session announcement must contain one "c=" field in each media
   description (see below) or a "c=" field at the session-level.  It may
   contain a session-level "c=" field and one additional "c=" field per
   media description, in which case the per-media values override the
   session-level settings for the relevant media.

   The first sub-field is the network type, which is a text string
   giving the type of network.  Initially "IN" is defined to have the
   meaning "Internet".

   The second sub-field is the address type.  This allows SDP to be used
   for sessions that are not IP based.  Currently only IP4 is defined.

   The third sub-field is the connection address.  Optional extra
   subfields may be added after the connection address depending on the
   value of the &lt;address type&gt; field.

   For IP4 addresses, the connection address is defined as follows:

   o Typically the connection address will be a class-D IP multicast

     group address.  If the session is not multicast, then the
     connection address contains the fully-qualified domain name or the
     unicast IP address of the expected data source or data relay or
     data sink as determined by additional attribute fields. It is not
     expected that fully-qualified domain names or unicast addresses



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     will be given in a session description that is communicated by a
     multicast announcement, though this is not prohibited.  If a
     unicast data stream is to pass through a network address
     translator, the use of a fully-qualified domain name rather than an
     unicast IP address is RECOMMENDED.  In other cases, the use of an
     IP address to specify a particular interface on a multi-homed host
     might be required.  Thus this specification leaves the decision as
     to which to use up to the individual application, but all
     applications MUST be able to cope with receiving both formats.

   o Conferences using an IP multicast connection address must also have
     a time to live (TTL) value present in addition to the multicast
     address.  The TTL and the address together define the scope with
     which multicast packets sent in this conference will be sent. TTL
     values must be in the range 0-255.

     The TTL for the session is appended to the address using a slash as
     a separator.  An example is:

                           c=IN IP4 224.2.1.1/127

     Hierarchical or layered encoding schemes are data streams where the
     encoding from a single media source is split into a number of
     layers.  The receiver can choose the desired quality (and hence
     bandwidth) by only subscribing to a subset of these layers.  Such
     layered encodings are normally transmitted in multiple multicast
     groups to allow multicast pruning.  This technique keeps unwanted
     traffic from sites only requiring certain levels of the hierarchy.
     For applications requiring multiple multicast groups, we allow the
     following notation to be used for the connection address:

            &lt;base multicast address&gt;/&lt;ttl&gt;/&lt;number of addresses&gt;

     If the number of addresses is not given it is assumed to be one.
     Multicast addresses so assigned are contiguously allocated above
     the base address, so that, for example:

                          c=IN IP4 224.2.1.1/127/3

     would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are
     to be used at a ttl of 127.  This is semantically identical to
     including multiple "c=" lines in a media description:

                           c=IN IP4 224.2.1.1/127
                           c=IN IP4 224.2.1.2/127
                           c=IN IP4 224.2.1.3/127





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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


     Multiple addresses or "c=" lines can only be specified on a per-
     media basis, and not for a session-level "c=" field.

     It is illegal for the slash notation described above to be used for
     IP unicast addresses.

   Bandwidth

   b=&lt;modifier&gt;:&lt;bandwidth-value&gt;

   o This specifies the proposed bandwidth to be used by the session or
     media, and is optional.

   o &lt;bandwidth-value&gt; is in kilobits per second

   o &lt;modifier&gt; is a single alphanumeric word giving the meaning of the
     bandwidth figure.

   o Two modifiers are initially defined:

   CT Conference Total: An implicit maximum bandwidth is associated with
     each TTL on the Mbone or within a particular multicast
     administrative scope region (the Mbone bandwidth vs. TTL limits are
     given in the MBone FAQ). If the bandwidth of a session or media in
     a session is different from the bandwidth implicit from the scope,
     a `b=CT:...' line should be supplied for the session giving the
     proposed upper limit to the bandwidth used. The primary purpose of
     this is to give an approximate idea as to whether two or more
     conferences can co-exist simultaneously.

   AS Application-Specific Maximum: The bandwidth is interpreted to be
     application-specific, i.e., will be the application's concept of
     maximum bandwidth.  Normally this will coincide with what is set on
     the application's "maximum bandwidth" control if applicable.

     Note that CT gives a total bandwidth figure for all the media at
     all sites.  AS gives a bandwidth figure for a single media at a
     single site, although there may be many sites sending
     simultaneously.

   o Extension Mechanism: Tool writers can define experimental bandwidth
     modifiers by prefixing their modifier with "X-". For example:

                                 b=X-YZ:128

     SDP parsers should ignore bandwidth fields with unknown modifiers.
     Modifiers should be alpha-numeric and, although no length limit is
     given, they are recommended to be short.



<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   Times, Repeat Times and Time Zones

   t=&lt;start time&gt;  &lt;stop time&gt;

   o "t=" fields specify the start and stop times for a conference
     session.  Multiple "t=" fields may be used if a session is active
     at multiple irregularly spaced times; each additional "t=" field
     specifies an additional period of time for which the session will
     be active.  If the session is active at regular times, an "r="
     field (see below) should be used in addition to and following a
     "t=" field - in which case the "t=" field specifies the start and
     stop times of the repeat sequence.

   o The first and second sub-fields give the start and stop times for
     the conference respectively.  These values are the decimal
     representation of Network Time Protocol (NTP) time values in
     seconds [<a href="#ref-1" title="&quot;Network Time Protocol (version 3) specification and implementation&quot;">1</a>].  To convert these values to UNIX time, subtract
     decimal 2208988800.

   o If the stop-time is set to zero, then the session is not bounded,
     though it will not become active until after the start-time.  If
     the start-time is also zero, the session is regarded as permanent.

     User interfaces should strongly discourage the creation of
     unbounded and permanent sessions as they give no information about
     when the session is actually going to terminate, and so make
     scheduling difficult.

     The general assumption may be made, when displaying unbounded
     sessions that have not timed out to the user, that an unbounded
     session will only be active until half an hour from the current
     time or the session start time, whichever is the later.  If
     behaviour other than this is required, an end-time should be given
     and modified as appropriate when new information becomes available
     about when the session should really end.

     Permanent sessions may be shown to the user as never being active
     unless there are associated repeat times which state precisely when
     the session will be active.  In general, permanent sessions should
     not be created for any session expected to have a duration of less
     than 2 months, and should be discouraged for sessions expected to
     have a duration of less than 6 months.

     r=&lt;repeat interval&gt; &lt;active duration&gt; &lt;list of offsets from start-
     time&gt;

   o "r=" fields specify repeat times for a session.  For example, if
     a session is active at 10am on Monday and 11am on Tuesday for one



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


     hour each week for three months, then the &lt;start time&gt; in the
     corresponding "t=" field would be the NTP representation of 10am on
     the first Monday, the &lt;repeat interval&gt; would be 1 week, the
     &lt;active duration&gt; would be 1 hour, and the offsets would be zero
     and 25 hours. The corresponding "t=" field stop time would be the
     NTP representation of the end of the last session three months
     later. By default all fields are in seconds, so the "r=" and "t="
     fields might be:

                           t=3034423619 3042462419
                            r=604800 3600 0 90000

    To make announcements more compact, times may also be given in units
    of days, hours or minutes. The syntax for these is a number
    immediately followed by a single case-sensitive character.
    Fractional units are not allowed - a smaller unit should be used
    instead.  The following unit specification characters are allowed:

                         d - days (86400 seconds)
                        h - minutes (3600 seconds)
                         m - minutes (60 seconds)
         s - seconds (allowed for completeness but not recommended)

   Thus, the above announcement could also have been written:

                               r=7d 1h 0 25h

     Monthly and yearly repeats cannot currently be directly specified
     with a single SDP repeat time - instead separate "t" fields should
     be used to explicitly list the session times.

        z=&lt;adjustment time&gt; &lt;offset&gt; &lt;adjustment time&gt; &lt;offset&gt; ....

   o To schedule a repeated session which spans a change from daylight-
     saving time to standard time or vice-versa, it is necessary to
     specify offsets from the base repeat times. This is required
     because different time zones change time at different times of day,
     different countries change to or from daylight time on different
     dates, and some countries do not have daylight saving time at all.

     Thus in order to schedule a session that is at the same time winter
     and summer, it must be possible to specify unambiguously by whose
     time zone a session is scheduled.  To simplify this task for
     receivers, we allow the sender to specify the NTP time that a time
     zone adjustment happens and the offset from the time when the
     session was first scheduled.  The "z" field allows the sender to
     specify a list of these adjustment times and offsets from the base
     time.



<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


     An example might be:

                        z=2882844526 -1h 2898848070 0

     This specifies that at time 2882844526 the time base by which the
     session's repeat times are calculated is shifted back by 1 hour,
     and that at time 2898848070 the session's original time base is
     restored. Adjustments are always relative to the specified start
     time - they are not cumulative.

   o    If a session is likely to last several years, it is  expected
   that
     the session announcement will be modified periodically rather than
     transmit several years worth of adjustments in one announcement.

   Encryption Keys

   k=&lt;method&gt;
   k=&lt;method&gt;:&lt;encryption key&gt;

   o The session description protocol may be used to convey encryption
     keys.  A key field is permitted before the first media entry (in
     which case it applies to all media in the session), or for each
     media entry as required.

   o The format of keys and their usage is outside the scope of this
     document, but see [<a href="#ref-3" title="&quot;RTP Profile for Audio and Video Conferences with Minimal Control&quot;">3</a>].

   o The method indicates the mechanism to be used to obtain a usable
     key by external means, or from the encoded encryption key given.

     The following methods are defined:

      k=clear:&lt;encryption key&gt;
        The encryption key (as described in [<a href="#ref-3" title="&quot;RTP Profile for Audio and Video Conferences with Minimal Control&quot;">3</a>] for  RTP  media  streams
        under  the  AV  profile)  is  included untransformed in this key
        field.

      k=base64:&lt;encoded encryption key&gt;
        The encryption key (as described in [<a href="#ref-3" title="&quot;RTP Profile for Audio and Video Conferences with Minimal Control&quot;">3</a>] for RTP media streams
        under the AV profile) is included in this key field but has been
        base64 encoded because it includes characters that are
        prohibited in SDP.

      k=uri:&lt;URI to obtain key&gt;
        A Universal Resource Identifier as used by WWW clients is
        included in this key field.  The URI refers to the data
        containing the key, and may require additional authentication



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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


        before the key can be returned.  When a request is made to the
        given URI, the MIME content-type of the reply specifies the
        encoding for the key in the reply.  The key should not be
        obtained until the user wishes to join the session to reduce
        synchronisation of requests to the WWW server(s).

      k=prompt
        No key is included in this SDP description, but the session or
        media stream referred to by this key field is encrypted.  The
        user should be prompted for the key when attempting to join the
        session, and this user-supplied key should then be used to
        decrypt the media streams.

   Attributes

   a=&lt;attribute&gt;
   a=&lt;attribute&gt;:&lt;value&gt;

   Attributes are the primary means for extending SDP.  Attributes may
   be defined to be used as "session-level" attributes, "media-level"
   attributes, or both.

   A media description may have any number of attributes ("a=" fields)
   which are media specific.  These are referred to as "media-level"
   attributes and add information about the media stream.  Attribute
   fields can also be added before the first media field; these
   "session-level" attributes convey additional information that applies
   to the conference as a whole rather than to individual media; an
   example might be the conference's floor control policy.

   Attribute fields may be of two forms:

   o property attributes.  A property attribute is simply of the form
     "a=&lt;flag&gt;".  These are binary attributes, and the presence of the
     attribute conveys that the attribute is a property of the session.
     An example might be "a=recvonly".

   o value attributes.  A value attribute is of the form
     "a=&lt;attribute&gt;:&lt;value&gt;".  An example might be that a whiteboard
     could have the value attribute "a=orient:landscape"

   Attribute interpretation depends on the media tool being invoked.
   Thus receivers of session descriptions should be configurable in
   their interpretation of announcements in general and of attributes in
   particular.

   Attribute names must be in the US-ASCII subset of ISO-10646/UTF-8.




<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   Attribute values are byte strings, and MAY use any byte value except
   0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values
   are to be interpreted as in ISO-10646 character set with UTF-8
   encoding.  Unlike other text fields, attribute values are NOT
   normally affected by the `charset' attribute as this would make
   comparisons against known values problematic.  However, when an
   attribute is defined, it can be defined to be charset-dependent, in
   which case it's value should be interpreted in the session charset
   rather than in ISO-10646.

   Attributes that will be commonly used can be registered with IANA
   (see <a href="#appendix-B">Appendix B</a>).  Unregistered attributes should begin with "X-" to
   prevent inadvertent collision with registered attributes.  In either
   case, if an attribute is received that is not understood, it should
   simply be ignored by the receiver.

   Media Announcements

   m=&lt;media&gt; &lt;port&gt; &lt;transport&gt; &lt;fmt list&gt;

   A session description may contain a number of media descriptions.
   Each media description starts with an "m=" field, and is terminated
   by either the next "m=" field or by the end of the session
   description.  A media field also has several sub-fields:

   o The first sub-field is the media type.  Currently defined media are
     "audio", "video", "application", "data" and "control", though this
     list may be extended as new communication modalities emerge (e.g.,
     telepresense).  The difference between "application" and "data" is
     that the former is a media flow such as whiteboard information, and
     the latter is bulk-data transfer such as multicasting of program
     executables which will not typically be displayed to the user.
     "control" is used to specify an additional conference control
     channel for the session.

   o The second sub-field is the transport port to which the media
     stream will be sent.  The meaning of the transport port depends on
     the network being used as specified in the relevant "c" field and
     on the transport protocol defined in the third sub-field.  Other
     ports used by the media application (such as the RTCP port, see
     [<a href="#ref-2" title="&quot;RTP: A Transport Protocol for Real-Time Applications&quot;">2</a>]) should be derived algorithmically from the base media port.

     Note: For transports based on UDP, the value should be in the range
     1024 to 65535 inclusive.  For RTP compliance it should be an even
     number.






<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


     For applications where hierarchically encoded streams are being
     sent to a unicast address, it may be necessary to specify multiple
     transport ports.  This is done using a similar notation to that
     used for IP multicast addresses in the "c=" field:

          m=&lt;media&gt; &lt;port&gt;/&lt;number of ports&gt; &lt;transport&gt; &lt;fmt list&gt;

     In such a case, the ports used depend on the transport protocol.
     For RTP, only the even ports are used for data and the
     corresponding one-higher odd port is used for RTCP.  For example:

                         m=video 49170/2 RTP/AVP 31

     would specify that ports 49170 and 49171 form one RTP/RTCP pair and
     49172 and 49173 form the second RTP/RTCP pair.  RTP/AVP is the
     transport protocol and 31 is the format (see below).

     It is illegal for both multiple addresses to be specified in the
     "c=" field and for multiple ports to be specified in the "m=" field
     in the same session description.

   o The third sub-field is the transport protocol.  The transport
     protocol values are dependent on the address-type field in the "c="
     fields.  Thus a "c=" field of IP4 defines that the transport
     protocol runs over IP4.  For IP4, it is normally expected that most
     media traffic will be carried as RTP over UDP.  The following
     transport protocols are preliminarily defined, but may be extended
     through registration of new protocols with IANA:

     - RTP/AVP - the IETF's Realtime Transport Protocol using the
       Audio/Video profile carried over UDP.

     - udp - User Datagram Protocol

     If an application uses a single combined proprietary media format
     and transport protocol over UDP, then simply specifying the
     transport protocol as udp and using the format field to distinguish
     the combined protocol is recommended.  If a transport protocol is
     used over UDP to carry several distinct media types that need to be
     distinguished by a session directory, then specifying the transport
     protocol and media format separately is necessary. RTP is an
     example of a transport-protocol that carries multiple payload
     formats that must be distinguished by the session directory for it
     to know how to start appropriate tools, relays, mixers or
     recorders.






<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


     The main reason to specify the transport-protocol in addition to
     the media format is that the same standard media formats may be
     carried over different transport protocols even when the network
     protocol is the same - a historical example is vat PCM audio and
     RTP PCM audio.  In addition, relays and monitoring tools that are
     transport-protocol-specific but format-independent are possible.

     For RTP media streams operating under the RTP Audio/Video Profile
     [<a href="#ref-3" title="&quot;RTP Profile for Audio and Video Conferences with Minimal Control&quot;">3</a>], the protocol field is "RTP/AVP".  Should other RTP profiles be
     defined in the future, their profiles will be specified in the same
     way.  For example, the protocol field "RTP/XYZ" would specify RTP
     operating under a profile whose short name is "XYZ".

   o The fourth and subsequent sub-fields are media formats.  For audio
     and video, these will normally be a media payload type as defined
     in the RTP Audio/Video Profile.

     When a list of payload formats is given, this implies that all of
     these formats may be used in the session, but the first of these
     formats is the default format for the session.

     For media whose transport protocol is not RTP or UDP the format
     field is protocol specific.  Such formats should be defined in an
     additional specification document.

     For media whose transport protocol is RTP, SDP can be used to
     provide a dynamic binding of media encoding to RTP payload type.
     The encoding names in the RTP AV Profile do not specify unique
     audio encodings (in terms of clock rate and number of audio
     channels), and so they are not used directly in SDP format fields.
     Instead, the payload type number should be used to specify the
     format for static payload types and the payload type number along
     with additional encoding information should be used for dynamically
     allocated payload types.

     An example of a static payload type is u-law PCM coded single
     channel audio sampled at 8KHz.  This is completely defined in the
     RTP Audio/Video profile as payload type 0, so the media field for
     such a stream sent to UDP port 49232 is:

                           m=video 49232 RTP/AVP 0

     An example of a dynamic payload type is 16 bit linear encoded
     stereo audio sampled at 16KHz.  If we wish to use dynamic RTP/AVP
     payload type 98 for such a stream, additional information is
     required to decode it:

                          m=video 49232 RTP/AVP 98



<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


                           a=rtpmap:98 L16/16000/2

     The general form of an rtpmap attribute is:

     a=rtpmap:&lt;payload type&gt; &lt;encoding name&gt;/&lt;clock rate&gt;[/&lt;encoding
     parameters&gt;]

     For audio streams, &lt;encoding parameters&gt; may specify the number of
     audio channels.  This parameter may be omitted if the number of
     channels is one provided no additional parameters are needed.  For
     video streams, no encoding parameters are currently specified.

     Additional parameters may be defined in the future, but
     codecspecific parameters should not be added.  Parameters added to
     an rtpmap attribute should only be those required for a session
     directory to make the choice of appropriate media too to
     participate in a session.  Codec-specific parameters should be
     added in other attributes.

     Up to one rtpmap attribute can be defined for each media format
     specified. Thus we might have:

                       m=audio 49230 RTP/AVP 96 97 98
                             a=rtpmap:96 L8/8000
                            a=rtpmap:97 L16/8000
                           a=rtpmap:98 L16/11025/2

     RTP profiles that specify the use of dynamic payload types must
     define the set of valid encoding names and/or a means to register
     encoding names if that profile is to be used with SDP.

     Experimental encoding formats can also be specified using rtpmap.
     RTP formats that are not registered as standard format names must
     be preceded by "X-".  Thus a new experimental redundant audio
     stream called GSMLPC using dynamic payload type 99 could be
     specified as:

                          m=video 49232 RTP/AVP 99
                          a=rtpmap:99 X-GSMLPC/8000

     Such an experimental encoding requires that any site wishing to
     receive the media stream has relevant configured state in its
     session directory to know which tools are appropriate.

     Note that RTP audio formats typically do not include information
     about the number of samples per packet.  If a non-default (as
     defined in the RTP Audio/Video Profile) packetisation is required,
     the "ptime" attribute is used as given below.



<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


     For more details on RTP audio and video formats, see [<a href="#ref-3" title="&quot;RTP Profile for Audio and Video Conferences with Minimal Control&quot;">3</a>].

   o Formats for non-RTP media should be registered as MIME content
     types as described in <a href="#appendix-B">Appendix B</a>.  For example, the LBL whiteboard
     application might be registered as MIME content-type application/wb
     with encoding considerations specifying that it operates over UDP,
     with no appropriate file format.  In SDP this would then be
     expressed using a combination of the "media" field and the "fmt"
     field, as follows:

                         m=application 32416 udp wb

   Suggested Attributes

   The following attributes are suggested.  Since application writers
   may add new attributes as they are required, this list is not
   exhaustive.

   a=cat:&lt;category&gt;
       This attribute gives the dot-separated hierarchical category of
       the session.  This is to enable a receiver to filter unwanted
       sessions by category.  It would probably have been a compulsory
       separate field, except for its experimental nature at this time.
       It is a session-level attribute, and is not dependent on charset.

   a=keywds:&lt;keywords&gt;
       Like the cat attribute, this is to assist identifying wanted
       sessions at the receiver.  This allows a receiver to select
       interesting session based on keywords describing the purpose of
       the session.  It is a session-level attribute. It is a charset
       dependent attribute, meaning that its value should be interpreted
       in the charset specified for the session description if one is
       specified, or by default in ISO 10646/UTF-8.

   a=tool:&lt;name and version of tool&gt;
       This gives the name and version number of the tool used to create
       the session description.  It is a session-level attribute, and is
       not dependent on charset.

   a=ptime:&lt;packet time&gt;
       This gives the length of time in milliseconds represented by the
       media in a packet. This is probably only meaningful for audio
       data.  It should not be necessary to know ptime to decode RTP or
       vat audio, and it is intended as a recommendation for the
       encoding/packetisation of audio.  It is a media attribute, and is
       not dependent on charset.





<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   a=recvonly
       This specifies that the tools should be started in receive-only
       mode where applicable. It can be either a session or media
       attribute, and is not dependent on charset.

   a=sendrecv
       This specifies that the tools should be started in send and
       receive mode.  This is necessary for interactive conferences with
       tools such as wb which defaults to receive only mode. It can be
       either a session or media attribute, and is not dependent on
       charset.

   a=sendonly
       This specifies that the tools should be started in send-only
       mode.  An example may be where a different unicast address is to
       be used for a traffic destination than for a traffic source. In
       such a case, two media descriptions may be use, one sendonly and
       one recvonly. It can be either a session or media attribute, but
       would normally only be used as a media attribute, and is not
       dependent on charset.

   a=orient:&lt;whiteboard orientation&gt;
       Normally this is only used in a whiteboard media specification.
       It specifies the orientation of a the whiteboard on the screen.
       It is a media attribute. Permitted values are `portrait',
       `landscape' and `seascape' (upside down landscape). It is not
       dependent on charset

   a=type:&lt;conference type&gt;
       This specifies the type of the conference.  Suggested values are
       `broadcast', `meeting', `moderated', `test' and `H332'.
       `recvonly' should be the default for `type:broadcast' sessions,
       `type:meeting' should imply `sendrecv' and `type:moderated'
       should indicate the use of a floor control tool and that the
       media tools are started so as to "mute" new sites joining the
       conference.

       Specifying the attribute type:H332 indicates that this loosely
       coupled session is part of a H.332 session as defined in the ITU
       H.332 specification [<a href="#ref-10" title="&quot;Multimedia Terminal for Receiving Internet-based H.323 Conferences&quot;">10</a>].  Media tools should be started
       `recvonly'.

       Specifying the attribute type:test is suggested as a hint that,
       unless explicitly requested otherwise, receivers can safely avoid
       displaying this session description to users.

       The type attribute is a session-level attribute, and is not
       dependent on charset.



<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   a=charset:&lt;character set&gt;
       This specifies the character set to be used to display the
       session name and information data.  By default, the ISO-10646
       character set in UTF-8 encoding is used. If a more compact
       representation is required, other character sets may be used such
       as ISO-8859-1 for Northern European languages.  In particular,
       the ISO 8859-1 is specified with the following SDP attribute:

                             a=charset:ISO-8859-1

       This is a session-level attribute; if this attribute is present,
       it must be before the first media field.  The charset specified
       MUST be one of those registered with IANA, such as ISO-8859-1.
       The character set identifier is a US-ASCII string and MUST be
       compared against the IANA identifiers using a case-insensitive
       comparison.  If the identifier is not recognised or not
       supported, all strings that are affected by it SHOULD be regarded
       as byte strings.

       Note that a character set specified MUST still prohibit the use
       of bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets
       requiring the use of these characters MUST define a quoting
       mechanism that prevents these bytes appearing within text fields.

   a=sdplang:&lt;language tag&gt;
       This can be a session level attribute or a media level attribute.
       As a session level attribute, it specifies the language for the
       session description.  As a media level attribute, it specifies
       the language for any media-level SDP information field associated
       with that media.  Multiple sdplang attributes can be provided
       either at session or media level if multiple languages in the
       session description or media use multiple languages, in which
       case the order of the attributes indicates the order of
       importance of the various languages in the session or media from
       most important to least important.

       In general, sending session descriptions consisting of multiple
       languages should be discouraged.  Instead, multiple descriptions
       should be sent describing the session, one in each language.
       However this is not possible with all transport mechanisms, and
       so multiple sdplang attributes are allowed although not
       recommended.

       The sdplang attribute value must be a single <a href="./rfc1766">RFC 1766</a> language
       tag in US-ASCII.  It is not dependent on the charset attribute.
       An sdplang attribute SHOULD be specified when a session is of





<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


       sufficient scope to cross geographic boundaries where the
       language of recipients cannot be assumed, or where the session is
       in a different language from the locally assumed norm.

   a=lang:&lt;language tag&gt;
       This can be a session level attribute or a media level attribute.
       As a session level attribute, it specifies the default language
       for the session being described.  As a media level attribute, it
       specifies the language for that media, overriding any session-
       level language specified.  Multiple lang attributes can be
       provided either at session or media level if multiple languages
       if the session description or media use multiple languages, in
       which case the order of the attributes indicates the order of
       importance of the various languages in the session or media from
       most important to least important.

       The lang attribute value must be a single <a href="./rfc1766">RFC 1766</a> language tag
       in US-ASCII. It is not dependent on the charset attribute.  A
       lang attribute SHOULD be specified when a session is of
       sufficient scope to cross geographic boundaries where the
       language of recipients cannot be assumed, or where the session is
       in a different language from the locally assumed norm.

   a=framerate:&lt;frame rate&gt;
       This gives the maximum video frame rate in frames/sec.  It is
       intended as a recommendation for the encoding of video data.
       Decimal representations of fractional values using the notation
       "&lt;integer&gt;.&lt;fraction&gt;" are allowed.  It is a media attribute, is
       only defined for video media, and is not dependent on charset.

   a=quality:&lt;quality&gt;
       This gives a suggestion for the quality of the encoding as an
       integer value.

       The intention of the quality attribute for video is to specify a
       non-default trade-off between frame-rate and still-image quality.
       For video, the value in the range 0 to 10, with the following
       suggested meaning:

       10 - the best still-image quality the compression scheme can
       give.

       5 - the default behaviour given no quality suggestion.

       0 - the worst still-image quality the codec designer thinks is
           still usable.

       It is a media attribute, and is not dependent on charset.



<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   a=fmtp:&lt;format&gt; &lt;format specific parameters&gt;
       This attribute allows parameters that are specific to a
       particular format to be conveyed in a way that SDP doesn't have
       to understand them.  The format must be one of the formats
       specified for the media.  Format-specific parameters may be any
       set of parameters required to be conveyed by SDP and given
       unchanged to the media tool that will use this format.

       It is a media attribute, and is not dependent on charset.

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Communicating Conference Control Policy</span>

   There is some debate over the way conference control policy should be
   communicated.  In general, the authors believe that an implicit
   declarative style of specifying conference control is desirable where
   possible.

   A simple declarative style uses a single conference attribute field
   before the first media field, possibly supplemented by properties
   such as `recvonly' for some of the media tools.  This conference
   attribute conveys the conference control policy. An example might be:

                             a=type:moderated

   In some cases, however, it is possible that this may be insufficient
   to communicate the details of an unusual conference control policy.
   If this is the case, then a conference attribute specifying external
   control might be set, and then one or more "media" fields might be
   used to specify the conference control tools and configuration data
   for those tools. An example is an ITU H.332 session:

                c=IN IP4 224.5.6.7
                a=type:H332
                m=audio 49230 RTP/AVP 0
                m=video 49232 RTP/AVP 31
                m=application 12349 udp wb
                m=control 49234 H323 mc
                c=IN IP4 134.134.157.81

   In this example, a general conference attribute (type:H332) is
   specified stating that conference control will be provided by an
   external H.332 tool, and a contact addresses for the H.323 session
   multipoint controller is given.

   In this document, only the declarative style of conference control
   declaration is specified.  Other forms of conference control should
   specify an appropriate type attribute, and should define the
   implications this has for control media.



<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Security Considerations</span>

   SDP is a session description format that describes multimedia
   sessions.  A session description should not be trusted unless it has
   been obtained by an authenticated transport protocol from a trusted
   source.  Many different transport protocols may be used to distribute
   session description, and the nature of the authentication will differ
   from transport to transport.

   One transport that will frequently be used to distribute session
   descriptions is the Session Announcement Protocol (SAP).  SAP
   provides both encryption and authentication mechanisms but due to the
   nature of session announcements it is likely that there are many
   occasions where the originator of a session announcement cannot be
   authenticated because they are previously unknown to the receiver of
   the announcement and because no common public key infrastructure is
   available.

   On receiving a session description over an unauthenticated transport
   mechanism or from an untrusted party, software parsing the session
   should take a few precautions. Session description contain
   information required to start software on the receivers system.
   Software that parses a session description MUST not be able to start
   other software except that which is specifically configured as
   appropriate software to participate in multimedia sessions.  It is
   normally considered INAPPROPRIATE for software parsing a session
   description to start, on a user's system, software that is
   appropriate to participate in multimedia sessions, without the user
   first being informed that such software will be started and giving
   their consent.  Thus a session description arriving by session
   announcement, email, session invitation, or WWW page SHOULD not
   deliver the user into an {it interactive} multimedia session without
   the user being aware that this will happen.  As it is not always
   simple to tell whether a session is interactive or not, applications
   that are unsure should assume sessions are interactive.

   In this specification, there are no attributes which would allow the
   recipient of a session description to be informed to start multimedia
   tools in a mode where they default to transmitting.  Under some
   circumstances it might be appropriate to define such attributes.  If
   this is done an application parsing a session description containing
   such attributes SHOULD either ignore them, or inform the user that
   joining this session will result in the automatic transmission of
   multimedia data.  The default behaviour for an unknown attribute is
   to ignore it.






<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   Session descriptions may be parsed at intermediate systems such as
   firewalls for the purposes of opening a hole in the firewall to allow
   the participation in multimedia sessions.  It is considered
   INAPPROPRIATE for a firewall to open such holes for unicast data
   streams unless the session description comes in a request from inside
   the firewall.

   For multicast sessions, it is likely that local administrators will
   apply their own policies, but the exclusive use of "local" or "site-
   local" administrative scope within the firewall and the refusal of
   the firewall to open a hole for such scopes will provide separation
   of global multicast sessions from local ones.







































<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


Appendix A: SDP Grammar

   This appendix provides an Augmented BNF grammar for SDP. ABNF is
   defined in <a href="./rfc2234">RFC 2234</a>.


   announcement =        proto-version
                         origin-field
                         session-name-field
                         information-field
                         uri-field
                         email-fields
                         phone-fields
                         connection-field
                         bandwidth-fields
                         time-fields
                         key-field
                         attribute-fields
                         media-descriptions

   proto-version =       "v=" 1*DIGIT CRLF
                         ;this memo describes version 0

   origin-field =        "o=" username space
                         sess-id space sess-version space
                         nettype space addrtype space
                         addr CRLF

   session-name-field =  "s=" text CRLF

   information-field =   ["i=" text CRLF]

   uri-field =           ["u=" uri CRLF]

   email-fields =        *("e=" email-address CRLF)

   phone-fields =        *("p=" phone-number CRLF)


   connection-field =    ["c=" nettype space addrtype space
                         connection-address CRLF]
                         ;a connection field must be present
                         ;in every media description or at the
                         ;session-level


   bandwidth-fields =    *("b=" bwtype ":" bandwidth CRLF)




<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   time-fields =         1*( "t=" start-time space stop-time
                         *(CRLF repeat-fields) CRLF)
                         [zone-adjustments CRLF]


   repeat-fields =       "r=" repeat-interval space typed-time
                         1*(space typed-time)


   zone-adjustments =    time space ["-"] typed-time
                         *(space time space ["-"] typed-time)


   key-field =           ["k=" key-type CRLF]


   key-type =            "prompt" |
                         "clear:" key-data |
                         "base64:" key-data |
                         "uri:" uri


   key-data =            email-safe | "~" | "


   attribute-fields =    *("a=" attribute CRLF)


   media-descriptions =  *( media-field
                         information-field
                         *(connection-field)
                         bandwidth-fields
                         key-field
                         attribute-fields )


   media-field =         "m=" media space port ["/" integer]
                         space proto 1*(space fmt) CRLF


   media =               1*(alpha-numeric)
                         ;typically "audio", "video", "application"
                         ;or "data"

   fmt =                 1*(alpha-numeric)
                         ;typically an RTP payload type for audio
                         ;and video media




<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   proto =               1*(alpha-numeric)
                         ;typically "RTP/AVP" or "udp" for IP4


   port =                1*(DIGIT)
                         ;should in the range "1024" to "65535" inclusive
                         ;for UDP based media


   attribute =           (att-field ":" att-value) | att-field


   att-field =           1*(alpha-numeric)


   att-value =           byte-string


   sess-id =             1*(DIGIT)
                         ;should be unique for this originating username/host


   sess-version =        1*(DIGIT)
                         ;0 is a new session


   connection-address =  multicast-address
                         | addr


   multicast-address =   3*(decimal-uchar ".") decimal-uchar "/" ttl
                         [ "/" integer ]
                         ;multicast addresses may be in the range
                         ;224.0.0.0 to 239.255.255.255

   ttl =                 decimal-uchar

   start-time =          time | "0"

   stop-time =           time | "0"

   time =                POS-DIGIT 9*(DIGIT)
                         ;sufficient for 2 more centuries


   repeat-interval =     typed-time





<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   typed-time =          1*(DIGIT) [fixed-len-time-unit]


   fixed-len-time-unit = "d" | "h" | "m" | "s"


   bwtype =              1*(alpha-numeric)

   bandwidth =           1*(DIGIT)


   username =            safe
                         ;pretty wide definition, but doesn't include space


   email-address =       email | email "(" email-safe ")" |
                         email-safe "&lt;" email "&gt;"


   email =               ;defined in <a href="./rfc822">RFC822</a>


   uri=                  ;defined in <a href="./rfc1630">RFC1630</a>


   phone-number =        phone | phone "(" email-safe ")" |
                         email-safe "&lt;" phone "&gt;"


   phone =               "+" POS-DIGIT 1*(space | "-" | DIGIT)
                         ;there must be a space or hyphen between the
                         ;international code and the rest of the number.


   nettype =             "IN"
                         ;list to be extended


   addrtype =            "IP4" | "IP6"
                         ;list to be extended


   addr =                FQDN | unicast-address


   FQDN =                4*(alpha-numeric|"-"|".")
                         ;fully qualified domain name as specified in <a href="./rfc1035">RFC1035</a>




<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 33]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-34" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   unicast-address =     IP4-address | IP6-address


   IP4-address =         b1 "." decimal-uchar "." decimal-uchar "." b4
   b1 =                  decimal-uchar
                         ;less than "224"; not "0" or "127"
   b4 =                  decimal-uchar
                         ;not "0"

   IP6-address =         ;to be defined


   text =                byte-string
                         ;default is to interpret this as IS0-10646 UTF8
                         ;ISO 8859-1 requires a "a=charset:ISO-8859-1"
                         ;session-level attribute to be used


   byte-string =         1*(0x01..0x09|0x0b|0x0c|0x0e..0xff)
                         ;any byte except NUL, CR or LF


   decimal-uchar =       DIGIT
                         | POS-DIGIT DIGIT
                         | ("1" 2*(DIGIT))
                         | ("2" ("0"|"1"|"2"|"3"|"4") DIGIT)
                         | ("2" "5" ("0"|"1"|"2"|"3"|"4"|"5"))


   integer =             POS-DIGIT *(DIGIT)


   alpha-numeric =       ALPHA | DIGIT


   DIGIT =               "0" | POS-DIGIT


   POS-DIGIT =           "1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"


   ALPHA =               "a"|"b"|"c"|"d"|"e"|"f"|"g"|"h"|"i"|"j"|"k"|
                         "l"|"m"|"n"|"o "|"p"|"q"|"r"|"s"|"t"|"u"|"v"|
                         "w"|"x"|"y"|"z"|"A"|"B"|"C "|"D"|"E"|"F"|"G"|
                         "H"|"I"|"J"|"K"|"L"|"M"|"N"|"O"|"P"|" Q"|"R"|
                         "S"|"T"|"U"|"V"|"W"|"X"|"Y"|"Z"





<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 34]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-35" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   email-safe =          safe | space | tab


   safe =                alpha-numeric |
                         "'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
                         "#" | "$" | "&amp;" | "*" | ";" | "=" | "@" | "[" |
                         "]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
                         "~" | "


   space =               %d32
   tab =                 %d9
   CRLF =                %d13.10






































<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 35]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-36" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


Appendix B: Guidelines for registering SDP names with IANA

   There are seven field names that may be registered with IANA. Using
   the terminology in the SDP specification BNF, they are "media",
   "proto", "fmt", "att-field", "bwtype", "nettype" and "addrtype".

   "media" (eg, audio, video, application, data).

       Packetized media types, such as those used by RTP, share the
       namespace used by media types registry [<a href="./rfc2048">RFC 2048</a>] (i.e. "MIME
       types").  The list of valid media names is the set of top-level
       MIME content types.  The set of media is intended to be small and
       not to be extended except under rare circumstances.  (The MIME
       subtype corresponds to the "fmt" parameter below).

   "proto"

       In general this should be an IETF standards-track transport
       protocol identifier such as RTP/AVP (<a href="./rfc1889">rfc 1889</a> under the <a href="./rfc1890">rfc 1890</a>
       profile).

       However, people will want to invent their own proprietary
       transport protocols.  Some of these should be registered as a
       "fmt" using "udp" as the protocol and some of which probably
       can't be.

       Where the protocol and the application are intimately linked,
       such as with the LBL whiteboard wb which used a proprietary and
       special purpose protocol over UDP, the protocol name should be
       "udp" and the format name that should be registered is "wb".  The
       rules for formats (see below) apply to such registrations.

       Where the proprietary transport protocol really carries many
       different data formats, it is possible to register a new protocol
       name with IANA. In such a case, an RFC MUST be produced
       describing the protocol and referenced in the registration.  Such
       an RFC MAY be informational, although it is preferable if it is
       standards-track.

   "fmt"

       The format namespace is dependent on the context of the "proto"
       field, so a format cannot be registered without specifying one or
       more transport protocols that it applies to.

       Formats cover all the possible encodings that might want to be
       transported in a multimedia session.




<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 36]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-37" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


       For RTP formats that have been assigned static payload types, the
       payload type number is used.  For RTP formats using a dynamic
       payload type number, the dynamic payload type number is given as
       the format and an additional "rtpmap" attribute specifies the
       format and parameters.

       For non-RTP formats, any unregistered format name may be
       registered through the MIME-type registration process [<a href="./rfc2048">RFC 2048</a>].
       The type given here is the MIME subtype only (the top-level MIME
       content type is specified by the media parameter).  The MIME type
       registration SHOULD reference a standards-track RFC which
       describes the transport protocol for this media type.  If there
       is an existing MIME type for this format, the MIME registration
       should be augmented to reference the transport specification for
       this media type.  If there is not an existing MIME type for this
       format, and there exists no appropriate file format, this should
       be noted in the encoding considerations as "no appropriate file
       format".

   "att-field" (Attribute names)

       Attribute field names MAY be registered with IANA, although this
       is not compulsory, and unknown attributes are simply ignored.

       When an attribute is registered, it must be accompanied by a
       brief specification stating the following:

       o contact name, email address and telephone number

       o attribute-name (as it will appear in SDP)

       o long-form attribute name in English

       o type of attribute (session level, media level, or both)

       o whether the attribute value is subject to the charset
       attribute.

       o a one paragraph explanation of the purpose of the attribute.

       o a specification of appropriate attribute values for this
         attribute.

       IANA will not sanity check such attribute registrations except to
       ensure that they do not clash with existing registrations.






<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 37]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-38" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


       Although the above is the minimum that IANA will accept, if the
       attribute is expected to see widespread use and interoperability
       is an issue, authors are encouraged to produce a standards-track
       RFC that specifies the attribute more precisely.

       Submitters of registrations should ensure that the specification
       is in the spirit of SDP attributes, most notably that the
       attribute is platform independent in the sense that it makes no
       implicit assumptions about operating systems and does not name
       specific pieces of software in a manner that might inhibit
       interoperability.

   "bwtype" (bandwidth specifiers)

       A proliferation of bandwidth specifiers is strongly discouraged.

       New bandwidth specifiers may be registered with IANA.  The
       submission MUST reference a standards-track RFC specifying the
       semantics of the bandwidth specifier precisely, and indicating
       when it should be used, and why the existing registered bandwidth
       specifiers do not suffice.

   "nettype" (Network Type)

       New network types may be registered with IANA if SDP needs to be
       used in the context of non-internet environments. Whilst these
       are not normally the preserve of IANA, there may be circumstances
       when an Internet application needs to interoperate with a non-
       internet application, such as when gatewaying an internet
       telephony call into the PSTN.  The number of network types should
       be small and should be rarely extended.  A new network type
       cannot be registered without registering at least one address
       type to be used with that network type.  A new network type
       registration MUST reference an RFC which gives details of the
       network type and address type and specifies how and when they
       would be used.  Such an RFC MAY be Informational.

   "addrtype" (Address Type)

       New address types may be registered with IANA.  An address type
       is only meaningful in the context of a network type, and any
       registration of an address type MUST specify a registered network
       type, or be submitted along with a network type registration.  A
       new address type registration MUST reference an RFC giving
       details of the syntax of the address type.  Such an RFC MAY be
       Informational.  Address types are not expected to be registered
       frequently.




<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 38]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-39" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   Registration Procedure

   To register a name the above guidelines should be followed regarding
   the required  level  of  documentation  that  is required.  The
   registration itself should be sent to IANA.  Attribute registrations
   should  include the  information  given  above.   Other registrations
   should include the following additional information:

   o contact name, email address and telephone number

   o name being registered (as it will appear in SDP)

   o long-form name in English

   o type of name ("media", "proto", "fmt", "bwtype", "nettype", or
     "addrtype")

   o a one paragraph explanation of the purpose of the registered name.

   o a reference to the specification (eg RFC number) of the registered
     name.

   IANA may refer any registration to the IESG or to any appropriate
   IETF working group for review, and may request revisions to be made
   before a registration will be made.


























<span class="grey">Handley &amp; Jacobson          Standards Track                    [Page 39]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-40" ></span>
<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


Appendix C: Authors' Addresses

   Mark Handley
   Information Sciences Institute
   c/o MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139
   United States
   electronic mail: mjh@isi.edu

   Van Jacobson
   MS 46a-1121
   Lawrence Berkeley Laboratory
   Berkeley, CA 94720
   United States
   electronic mail: van@ee.lbl.gov

Acknowledgments

   Many people in the IETF MMUSIC working group have made comments and
   suggestions contributing to this document.  In particular, we would
   like to thank Eve Schooler, Steve Casner, Bill Fenner, Allison
   Mankin, Ross Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Rob
   Lanphier and Steve Hanna.

References

   [<a id="ref-1">1</a>] Mills, D., "Network Time Protocol (version 3) specification and
   implementation", <a href="./rfc1305">RFC 1305</a>, March 1992.

   [<a id="ref-2">2</a>] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
   A Transport Protocol for Real-Time Applications", <a href="./rfc1889">RFC 1889</a>, January
   1996.

   [<a id="ref-3">3</a>] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
   with Minimal Control", <a href="./rfc1890">RFC 1890</a>, January 1996

   [<a id="ref-4">4</a>] Handley, M., <a style="text-decoration: none" href='https://www.google.com/search?sitesearch=datatracker.ietf.org%2Fdoc%2Fhtml%2F&amp;q=inurl:draft-+%22SAP+-+Session+Announcement+Protocol%22'>"SAP - Session Announcement Protocol"</a>, Work in
   Progress.

   [<a id="ref-5">5</a>] V. Jacobson, S. McCanne, "vat - X11-based audio teleconferencing
   tool" vat manual page, Lawrence Berkeley Laboratory, 1994.

   [<a id="ref-6">6</a>] The Unicode Consortium, "The Unicode Standard -- Version 2.0",
   Addison-Wesley, 1996.






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<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


   [<a id="ref-7">7</a>] ISO/IEC 10646-1:1993. International Standard -- Information
   technol- ogy -- Universal Multiple-Octet Coded Character Set (UCS) --
   Part 1: Architecture and Basic Multilingual Plane.  Five amendments
   and a techn- ical  corrigendum  have been published up to now.  UTF-8
   is described in Annex R, published as Amendment 2.

   [<a id="ref-8">8</a>] Goldsmith, D., and M. Davis, "Using Unicode with MIME", <a href="./rfc1641">RFC 1641</a>,
   July 1994.

   [<a id="ref-9">9</a>] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
   10646", <a href="./rfc2044">RFC 2044</a>, October 1996.

   [<a id="ref-10">10</a>] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for
   Receiving Internet-based H.323 Conferences", ITU, Geneva.

   [<a id="ref-11">11</a>] Handley, M., Schooler, E., and H. Schulzrinne, "Session
   Initiation Protocol (SIP)", Work in Progress.

   [<a id="ref-12">12</a>] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
   Protocol (RTSP)", <a href="./rfc2326">RFC 2326</a>, April 1998.































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<span class="grey"><a href="./rfc2327">RFC 2327</a>                          SDP                         April 1998</span>


Full Copyright Statement

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