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rfc:rfc2327

Network Working Group M. Handley Request for Comments: 2327 V. Jacobson Category: Standards Track ISI/LBNL

                                                              April 1998
                 SDP: Session Description Protocol

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.

1. Introduction

 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.

Handley & Jacobson Standards Track [Page 1] RFC 2327 SDP April 1998

2. Background

 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 [4], Session Initiation Protocol [11], Real-
 Time Streaming Protocol [12], 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.

3. Glossary of Terms

 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.

Handley & Jacobson Standards Track [Page 2] RFC 2327 SDP April 1998

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

3.1. Terminology

 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 RFC 2119.

4. SDP Usage

4.1. Multicast Announcements

 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 [4]
 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.

4.2. Email and WWW Announcements

 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.

Handley & Jacobson Standards Track [Page 3] RFC 2327 SDP April 1998

 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.

5. Requirements and Recommendations

 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

Handley & Jacobson Standards Track [Page 4] RFC 2327 SDP April 1998

 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.

5.1. Media Information

 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.

5.2. Timing Information

 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"

Handley & Jacobson Standards Track [Page 5] RFC 2327 SDP April 1998

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

5.3. Private Sessions

 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 [4]
 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.

5.4. Obtaining Further Information about a Session

 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.

5.5. Categorisation

 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.

5.6. Internationalization

 The SDP specification recommends the use of the ISO 10646 character
 sets in the UTF-8 encoding (RFC 2044) 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.

6. SDP Specification

 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,

Handley & Jacobson Standards Track [Page 6] RFC 2327 SDP April 1998

 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 <type>=<value> <type> is always exactly one character and is
 case-significant.  <value> is a structured text string whose format
 depends on <type>.  It also will be case-significant unless a
 specific field defines otherwise.  Whitespace is not permitted either
 side of the `=' sign. In general <value> 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)

Handley & Jacobson Standards Track [Page 7] RFC 2327 SDP April 1998

      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

Handley & Jacobson Standards Track [Page 8] RFC 2327 SDP April 1998

      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=<username> <session id> <version> <network type> <address type>
 <address>
 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.
 <username> is the user's login on the originating host, or it is "-"
 if the originating host does not support the concept of user ids.
 <username> must not contain spaces.  <session id> is a numeric string
 such that the tuple of <username>, <session id>, <network type>,
 <address type> and <address> form a globally unique identifier for
 the session.
 The method of <session id> allocation is up to the creating tool, but
 it has been suggested that a Network Time Protocol (NTP) timestamp be
 used to ensure uniqueness [1].
 <version> 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

Handley & Jacobson Standards Track [Page 9] RFC 2327 SDP April 1998

 creating tool, so long as <version> is increased when a modification
 is made to the session data.  Again, it is recommended (but not
 mandatory) that an NTP timestamp is used.
 <network type> is a text string giving the type of network.
 Initially "IN" is defined to have the meaning "Internet".  <address
 type> is a text string giving the type of the address that follows.
 Initially "IP4" and "IP6" are defined.  <address> 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=<session name>
 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=<session description>
 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

Handley & Jacobson Standards Track [Page 10] RFC 2327 SDP April 1998

 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=<URI>
 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=<email address>
 p=<phone number>
 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

Handley & Jacobson Standards Track [Page 11] RFC 2327 SDP April 1998

 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 RFC822 name quoting convention is also allowed for
   both email addresses and phone numbers.  For example,
                      e=Mark Handley <mjh@isi.edu>
   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=<network type> <address type> <connection address>
 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 <address type> 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

Handley & Jacobson Standards Track [Page 12] RFC 2327 SDP April 1998

   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:
          <base multicast address>/<ttl>/<number of addresses>
   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

Handley & Jacobson Standards Track [Page 13] RFC 2327 SDP April 1998

   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=<modifier>:<bandwidth-value>
 o This specifies the proposed bandwidth to be used by the session or
   media, and is optional.
 o <bandwidth-value> is in kilobits per second
 o <modifier> 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.

Handley & Jacobson Standards Track [Page 14] RFC 2327 SDP April 1998

 Times, Repeat Times and Time Zones
 t=<start time>  <stop time>
 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 [1].  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=<repeat interval> <active duration> <list of offsets from start-
   time>
 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

Handley & Jacobson Standards Track [Page 15] RFC 2327 SDP April 1998

   hour each week for three months, then the <start time> in the
   corresponding "t=" field would be the NTP representation of 10am on
   the first Monday, the <repeat interval> would be 1 week, the
   <active duration> 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=<adjustment time> <offset> <adjustment time> <offset> ....
 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.

Handley & Jacobson Standards Track [Page 16] RFC 2327 SDP April 1998

   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=<method>
 k=<method>:<encryption key>
 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 [3].
 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:<encryption key>
      The encryption key (as described in [3] for  RTP  media  streams
      under  the  AV  profile)  is  included untransformed in this key
      field.
    k=base64:<encoded encryption key>
      The encryption key (as described in [3] 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:<URI to obtain key>
      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

Handley & Jacobson Standards Track [Page 17] RFC 2327 SDP April 1998

      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=<attribute>
 a=<attribute>:<value>
 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=<flag>".  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=<attribute>:<value>".  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.

Handley & Jacobson Standards Track [Page 18] RFC 2327 SDP April 1998

 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 Appendix B).  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=<media> <port> <transport> <fmt list>
 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
   [2]) 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.

Handley & Jacobson Standards Track [Page 19] RFC 2327 SDP April 1998

   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=<media> <port>/<number of ports> <transport> <fmt list>
   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:
  1. RTP/AVP - the IETF's Realtime Transport Protocol using the

Audio/Video profile carried over UDP.

  1. 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.

Handley & Jacobson Standards Track [Page 20] RFC 2327 SDP April 1998

   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
   [3], 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

Handley & Jacobson Standards Track [Page 21] RFC 2327 SDP April 1998

                         a=rtpmap:98 L16/16000/2
   The general form of an rtpmap attribute is:
   a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
   parameters>]
   For audio streams, <encoding parameters> 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.

Handley & Jacobson Standards Track [Page 22] RFC 2327 SDP April 1998

   For more details on RTP audio and video formats, see [3].
 o Formats for non-RTP media should be registered as MIME content
   types as described in Appendix B.  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:<category>
     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:<keywords>
     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:<name and version of tool>
     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:<packet time>
     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.

Handley & Jacobson Standards Track [Page 23] RFC 2327 SDP April 1998

 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:<whiteboard orientation>
     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:<conference type>
     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 [10].  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.

Handley & Jacobson Standards Track [Page 24] RFC 2327 SDP April 1998

 a=charset:<character set>
     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:<language tag>
     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 RFC 1766 language
     tag in US-ASCII.  It is not dependent on the charset attribute.
     An sdplang attribute SHOULD be specified when a session is of

Handley & Jacobson Standards Track [Page 25] RFC 2327 SDP April 1998

     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:<language tag>
     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 RFC 1766 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:<frame rate>
     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
     "<integer>.<fraction>" are allowed.  It is a media attribute, is
     only defined for video media, and is not dependent on charset.
 a=quality:<quality>
     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.

Handley & Jacobson Standards Track [Page 26] RFC 2327 SDP April 1998

 a=fmtp:<format> <format specific parameters>
     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.

6.1. Communicating Conference Control Policy

 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.

Handley & Jacobson Standards Track [Page 27] RFC 2327 SDP April 1998

7. Security Considerations

 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.

Handley & Jacobson Standards Track [Page 28] RFC 2327 SDP April 1998

 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.

Handley & Jacobson Standards Track [Page 29] RFC 2327 SDP April 1998

Appendix A: SDP Grammar

 This appendix provides an Augmented BNF grammar for SDP. ABNF is
 defined in RFC 2234.
 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)

Handley & Jacobson Standards Track [Page 30] RFC 2327 SDP April 1998

 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

Handley & Jacobson Standards Track [Page 31] RFC 2327 SDP April 1998

 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

Handley & Jacobson Standards Track [Page 32] RFC 2327 SDP April 1998

 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 "<" email ">"
 email =               ;defined in RFC822
 uri=                  ;defined in RFC1630
 phone-number =        phone | phone "(" email-safe ")" |
                       email-safe "<" phone ">"
 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 RFC1035

Handley & Jacobson Standards Track [Page 33] RFC 2327 SDP April 1998

 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"

Handley & Jacobson Standards Track [Page 34] RFC 2327 SDP April 1998

 email-safe =          safe | space | tab
 safe =                alpha-numeric |
                       "'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
                       "#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" |
                       "]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
                       "~" | "
 space =               %d32
 tab =                 %d9
 CRLF =                %d13.10

Handley & Jacobson Standards Track [Page 35] RFC 2327 SDP April 1998

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 [RFC 2048] (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 (rfc 1889 under the rfc 1890
     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.

Handley & Jacobson Standards Track [Page 36] RFC 2327 SDP April 1998

     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 [RFC 2048].
     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.

Handley & Jacobson Standards Track [Page 37] RFC 2327 SDP April 1998

     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.

Handley & Jacobson Standards Track [Page 38] RFC 2327 SDP April 1998

 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.

Handley & Jacobson Standards Track [Page 39] RFC 2327 SDP April 1998

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

 [1] Mills, D., "Network Time Protocol (version 3) specification and
 implementation", RFC 1305, March 1992.
 [2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
 A Transport Protocol for Real-Time Applications", RFC 1889, January
 1996.
 [3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
 with Minimal Control", RFC 1890, January 1996
 [4] Handley, M., "SAP - Session Announcement Protocol", Work in
 Progress.
 [5] V. Jacobson, S. McCanne, "vat - X11-based audio teleconferencing
 tool" vat manual page, Lawrence Berkeley Laboratory, 1994.
 [6] The Unicode Consortium, "The Unicode Standard -- Version 2.0",
 Addison-Wesley, 1996.

Handley & Jacobson Standards Track [Page 40] RFC 2327 SDP April 1998

 [7] 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.
 [8] Goldsmith, D., and M. Davis, "Using Unicode with MIME", RFC 1641,
 July 1994.
 [9] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
 10646", RFC 2044, October 1996.
 [10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for
 Receiving Internet-based H.323 Conferences", ITU, Geneva.
 [11] Handley, M., Schooler, E., and H. Schulzrinne, "Session
 Initiation Protocol (SIP)", Work in Progress.
 [12] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
 Protocol (RTSP)", RFC 2326, April 1998.

Handley & Jacobson Standards Track [Page 41] RFC 2327 SDP April 1998

Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Handley & Jacobson Standards Track [Page 42]

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