Network Working Group                                         M. Handley
Internet-Draft                                                       UCL
Obsoletes: 2327, 3266 (if                                    V. Jacobson
approved)                                                  Packet Design
Expires: March 4, 2004                                        C. Perkins
                                                   University of Glasgow
                                                       September 4, 2003

                   SDP: Session Description Protocol

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on March 4, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   This memo 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.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.   Glossary of Terms  . . . . . . . . . . . . . . . . . . . . .   4
   3.   Examples of SDP Usage  . . . . . . . . . . . . . . . . . . .   5
   3.1  Multicast Announcement . . . . . . . . . . . . . . . . . . .   5
   3.2  Session Initiation . . . . . . . . . . . . . . . . . . . . .   5
   3.3  Streaming media  . . . . . . . . . . . . . . . . . . . . . .   5
   3.4  Email and the World Wide Web . . . . . . . . . . . . . . . .   5
   4.   Requirements and Recommendations . . . . . . . . . . . . . .   6
   4.1  Media Information  . . . . . . . . . . . . . . . . . . . . .   7
   4.2  Timing Information . . . . . . . . . . . . . . . . . . . . .   7
   4.3  Private Sessions . . . . . . . . . . . . . . . . . . . . . .   8
   4.4  Obtaining Further Information about a Session  . . . . . . .   8
   4.5  Categorisation . . . . . . . . . . . . . . . . . . . . . . .   8
   4.6  Internationalization . . . . . . . . . . . . . . . . . . . .   8
   5.   SDP Specification  . . . . . . . . . . . . . . . . . . . . .   8
   5.1  Protocol Version ("v=")  . . . . . . . . . . . . . . . . . .  11
   5.2  Origin ("o=")  . . . . . . . . . . . . . . . . . . . . . . .  11
   5.3  Session Name ("s=")  . . . . . . . . . . . . . . . . . . . .  12
   5.4  Session and Media Information ("i=") . . . . . . . . . . . .  12
   5.5  URI ("u=") . . . . . . . . . . . . . . . . . . . . . . . . .  13
   5.6  Email Address and Phone Number ("e=" and "p=") . . . . . . .  13
   5.7  Connection Data ("c=") . . . . . . . . . . . . . . . . . . .  14
   5.8  Bandwidth ("b=") . . . . . . . . . . . . . . . . . . . . . .  16
   5.9  Timing ("t=")  . . . . . . . . . . . . . . . . . . . . . . .  17
   5.10 Repeat Times ("r=")  . . . . . . . . . . . . . . . . . . . .  18
   5.11 Time Zones ("z=")  . . . . . . . . . . . . . . . . . . . . .  18
   5.12 Encryption Keys ("k=") . . . . . . . . . . . . . . . . . . .  19
   5.13 Attributes ("a=")  . . . . . . . . . . . . . . . . . . . . .  20
   5.14 Media Announcements ("m=") . . . . . . . . . . . . . . . . .  21
   6.   Suggested Attributes . . . . . . . . . . . . . . . . . . . .  25
   7.   Communicating Conference Control Policy  . . . . . . . . . .  30
   8.   Security Considerations  . . . . . . . . . . . . . . . . . .  31
   9.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  32
   9.1  Media types ("media")  . . . . . . . . . . . . . . . . . . .  32
   9.2  Transport protocols ("proto")  . . . . . . . . . . . . . . .  32
   9.3  Media formats ("fmt")  . . . . . . . . . . . . . . . . . . .  33
   9.4  Attribute names ("att-field")  . . . . . . . . . . . . . . .  33
   9.5  Bandwidth specifiers ("bwtype")  . . . . . . . . . . . . . .  34
   9.6  Network types ("nettype")  . . . . . . . . . . . . . . . . .  34
   9.7  Address types ("addrtype") . . . . . . . . . . . . . . . . .  35
   9.8  Registration Procedure . . . . . . . . . . . . . . . . . . .  35
   A.   SDP Grammar  . . . . . . . . . . . . . . . . . . . . . . . .  35
   B.   Changes from RFC 2327  . . . . . . . . . . . . . . . . . . .  41
   C.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  42
        Normative References . . . . . . . . . . . . . . . . . . . .  42
        Informative References . . . . . . . . . . . . . . . . . . .  42

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        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  43
        Intellectual Property and Copyright Statements . . . . . . .  45

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1. Introduction

   When initiating multimedia teleconferences, voice-over-IP calls,
   streaming video, or other real-time sessions, there is a requirement
   to convey media details, transport addresses, and other session
   description metadata to the participants.

   SDP provides a standard representation for such information,
   irrespective of how that information is transported.  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 [RFC2974],
   Session Initiation Protocol [RFC3261], Real-Time Streaming Protocol
   [RFC2326], electronic mail using the MIME extensions, and the
   Hypertext Transport Protocol.

   SDP is intended to be general purpose so that it can be used in a
   wide range of network environments and applications.  However, it is
   not intended to support negotiation of session content or media
   encodings: this is viewed as outside the scope of session

2. 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

   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 pro-active
      fashion, i.e., the session description was not explicitly
      requested by the user.

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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3. Examples of SDP Usage

3.1 Multicast Announcement

   In order to assist the advertisement of multicast multimedia
   conferences and other multicast sessions, and to communicate the
   relevant session setup information to prospective participants, a
   distributed session directory may be used. An instance of such a
   session directory periodically sends packets containing a description
   of the session to a well known multicast group. These advertisements
   are received by other session directories such that potential remote
   participants can use the session description to start the tools
   required to participate in the session.

   One protocol commonly used to implement such a distributed directory
   is the Session Announcement Protocol, SAP [RFC2974]. SDP provides the
   recommended session description format for such announcements.

3.2 Session Initiation

   The Session Initiation Protocol, SIP [RFC3261] is an application
   layer control protocol for creating, modifying and terminating
   sessions such as Internet multimedia conferences, Internet telephone
   calls and multimedia distribution.  The SIP messages used to create
   sessions carry session descriptions which allow participants to agree
   on a set of compatible media types. These session descriptions are
   commonly formatted using SDP. When used with SIP, the offer/answer
   model [RFC3264] provides a limited framework for negotiation using

3.3 Streaming media

   The Real Time Streaming Protocol, RTSP [RFC2326], is an application-
   level protocol for control over the delivery of data with real-time
   properties.  RTSP provides an extensible framework to enable
   controlled, on-demand delivery of real-time data, such as audio and
   video. An RTSP client and server negotiate an appropriate set of
   parameters for media delivery, partially using SDP syntax to describe
   those parameters.

3.4 Email and the World Wide Web

   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" MUST
   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.

4. 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 needed 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

   In general, SDP must convey sufficient information to enable

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   applications 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.

4.1 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 media

   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, however some media types may
   redefine this behaviour.

4.2 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"

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

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4.3 Private Sessions

   It is possible to create both public sessions and private sessions.
   SDP itself does not distinguish between these: private sessions are
   typically conveyed by encrypting the session description during
   distribution.  The details of how encryption is performed are
   dependent on the mechanism used to convey SDP - e.g. mechanisms are
   defined for SDP transported using SAP [RFC2974] and SIP [RFC3261].

   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.

4.4 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.

4.5 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.

4.6 Internationalization

   The SDP specification recommends the use of the ISO 10646 character
   sets in the UTF-8 encoding [RFC2279] 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.

5. SDP Specification

   SDP session descriptions are entirely textual using the ISO 10646
   character set in UTF-8 encoding.  SDP field names and attribute 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,
   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

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   allow flexible, text-based toolkits (e.g., Tcl/Tk) to be used to
   generate and to process session descriptions.  However, since SDP may
   be used in environments where the maximum permissable size of a
   session description is limited (e.g. SAP announcements; SIP
   transported in UDP), the encoding is deliberately compact.  Also,
   since announcements may be transported via very unreliable means 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:


   where <type> MUST be exactly one case-significant character and
   <value> is structured text whose format depends on <type>.  In
   general <value> is either a number of fields delimited by a single
   space character, or a free format string. Whitespace MUST NOT be used
   either side of the "=" sign.

   A session description 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.

   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)
         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)

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         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
         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 -- an SDP parser 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.  An SDP parser 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:

      o=jdoe 2890844526 2890842807 IN IP4
      s=SDP Seminar
      i=A Seminar on the session description protocol
      u= (Jane Doe)
      c=IN IP4
      t=2873397496 2873404696
      m=audio 49170 RTP/AVP 0
      m=video 51372 RTP/AVP 31
      m=application 32416 udp wb

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   Text records such as the session name and information are octet
   strings which may contain any octet 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.

5.1 Protocol Version ("v=")


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

5.2 Origin ("o=")

      o=<username> <session id> <version> <network type> <address type>

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

   <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) format timestamp be
      used to ensure uniqueness [RFC1305].

   <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
      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 format timestamp is used.

   <network type> is a text string giving the type of network.
      Initially "IN" is defined to have the meaning "Internet".

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   <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

5.3 Session Name ("s=")

      s=<session name>

   The "s=" field is the session name.  There MUST be one and only one
   "s=" field per session description. The "s=" field MUST NOT be empty
   and SHOULD contain ISO 10646 characters (but see also the "a=charset"
   attribute). If a session has no meaningful name, the value "s= "
   SHOULD be used (i.e. a single space as the session name).

5.4 Session and Media Information ("i=")

      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
   NOT RECOMMENDED 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
   "a=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
   single session has more than one distinct media stream of the same
   media type.  An example would be two different whiteboards, one for

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   slides and one for feedback and questions.

5.5 URI ("u=")


   A URI is a Universal Resource Identifier as used by WWW clients. The
   URI should be a pointer to additional information about the
   conference. This field is OPTIONAL, but if it is present it MUST be
   specified before the first media field. No more than one URI field is
   allowed per session description.

5.6 Email Address and Phone Number ("e=" and "p=")

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

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

   Inclusion of an email address or phone number is OPTIONAL. Note that
   the previous version of SDP specified that either an email field or a
   phone field MUST be specified, but this was widely ignored.  The
   change brings the specification into line with common usage.

   If the email addres or phone number are present, they MUST be
   specified before the first media field.  More than one email or phone
   field can be given for a session description.

   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=+1 617 555 6011

   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: (Jane Doe)

   The alternative RFC822 name quoting convention is also allowed for
   both email addresses and phone numbers.  For example:

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      e=Jane Doe <>

   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.

5.7 Connection Data ("c=")

      c=<network type> <address type> <connection address>

   The "c=" field contains connection data.

   A session announcement MUST contain either at least one "c=" field in
   each media description (see below) or a single "c=" field at the
   session-level.  It MAY contain a single session-level "c=" field and
   additional "c=" field(s) per media description, in which case the
   per-media values override the session-level settings for the
   respective 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 and IP6 are

   The third sub-field is the connection address.  Optional extra
   sub-fields MAY be added after the connection address depending on the
   value of the <address type> field.

   For IP4 and IP6 addresses, the connection address is defined as

   o  If the session is multicast, the connection address will be an IP
      multicast group address.  If the session is not multicast, then
      the connection address contains 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 unicast
      addresses will be given in a session description that is
      communicated by a multicast announcement, though this is not

   o  Conferences using an IPv4 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.

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

      c=IN IP4

   IPv6 multicast does not use TTL scoping, and hence the TTL value MUST
   NOT be present for IPv6 multicast. It is expected that IPv6 scoped
   addresses will be used to limit the scope of conferences.

   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

   would state that addresses, and 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
      c=IN IP4
      c=IN IP4

   Similarly, an IPv6 example would be:

      c=IN IP6 FF15::101/3

   which is semantically equivalent to:

      c=IN IP6 FF15::101
      c=IN IP6 FF15::102
      c=IN IP6 FF15::103

   (remembering that the TTL field is not present in IPv6 multicast).

   Multiple addresses or "c=" lines MAY be specified on a per-media

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   basis. They MUST NOT be specified for a session-level "c=" field.

   The slash notation described above MUST NOT be used for IP unicast

5.8 Bandwidth ("b=")


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

   The <bandwidth-value> is in kilobits per second by default. Modifiers
   MAY specify that alternative units are to be used (the modifiers
   defined in this memo use the default units).

   The <modifier> is a single alphanumeric word giving the meaning of
   the bandwidth figure. Two modifiers are initially defined:

      CT  Conference Total
          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 sessions can co-exist simultaneously.

      AS  Application-Specific Maximum
          The bandwidth is interpreted to be application-specific (it
          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.
          For RTP based applications, AS gives the RTP "session
          bandwidth" as defined in section 6.2 of [RFC1889].

   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.

   Tool writers MAY define experimental bandwidth modifiers by prefixing
   their modifier with "X-".  For example:


   Use of the "X-" prefix is NOT RECOMMENDED: instead new modifiers
   SHOULD be registered with IANA in the standard namespace. SDP parsers
   MUST ignore bandwidth fields with unknown modifiers. Modifiers MUST
   be alpha-numeric and, although no length limit is given, they are

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   recommended to be short.

5.9 Timing ("t=")

      t=<start time> <stop time>

   "t=" fields specify the start and stop times for a 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

   The first and second sub-fields give the start and stop times for the
   session respectively. These values are the decimal representation of
   Network Time Protocol (NTP) time values in seconds [RFC1305]. To
   convert these values to UNIX time, subtract decimal 2208988800.

   NTP timestamps are 64 bit values which wrap sometime in the year
   2036.  Since SDP uses an arbitrary length decimal representation,
   this should not cause an issue (SDP timestamps will continue counting
   seconds since 1900, NTP will use the value modulo the 64 bit limit).

   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

   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.

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5.10 Repeat Times ("r=")

      r=<repeat interval> <active duration> <offsets from start-time>

   "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 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

      t=3034423619 3042462419
      r=604800 3600 0 90000

   To make description 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 - hours (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 be directly specified with a single
   SDP repeat time - instead separate "t=" fields should be used to
   explicitly list the session times.

5.11 Time Zones ("z=")

      z=<adjustment time> <offset> <adjustment time> <offset> ....

   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 time. 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.

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   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.

   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.  Adjustments apply to all "t=" and "r=" lines in
   a session description.

   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.

5.12 Encryption Keys ("k=")

      k=<method>:<encryption key>

   If transported over a secure and trusted channel, 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

   The format of keys and their usage is outside the scope of this
   document, but see [RFC1890] for an example.

   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 is included untransformed in this key field.
         This method MUST NOT be used unless it can be guaranteed that
         the SDP is conveyed over a secure channel.

      k=base64:<encoded encryption key>

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         The encryption key is included in this key field but has been
         base64 encoded because it includes characters that are
         prohibited in SDP.  This method MUST NOT be used unless it can
         be guaranteed that the SDP is conveyed over a secure channel.

      k=uri:<URI to obtain key>

         A Universal Resource Identifier is included in the key field.
         The URI refers to the data containing the key, and may require
         additional authentication 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.


         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.  The use of user-specified keys is
         NOT RECOMMENDED, since such keys tend to have weak security

   The key field MUST NOT be used unless it can be guaranteed that the
   SDP is conveyed over a secure and trusted channel. An example of such
   a channel might be SDP embedded inside an S/MIME message.

5.13 Attributes ("a=")


   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>".

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      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>".
      For example, a whiteboard could have the value attribute

   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

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

   Attribute values are octet strings, and MAY use any octet 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 MUST be registered with IANA (see Section 9). If an
   attribute is received that is not understood, it MUST be ignored by
   the receiver.

5.14 Media Announcements ("m=")

      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 has several sub-fields.

   The first sub-field is the media type.  Currently defined media are
   "audio", "video", "application", "data" and "control", though this
   list may be extended in future.  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.

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   The second sub-field is the transport port to which the media stream
   is 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 [RFC1889]) MAY be
   derived algorithmically from the base media port or MAY be specified
   in a separate attribute (e.g. "a=rtcp:" as defined in [RTCPSDP]).

   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, the default is that only the even numbered ports are used for
   data and the corresponding one-higher odd port is used for RTCP.  For

      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). If non-
   contiguous ports are required, they must be signalled using a
   separate attribute (e.g. "a=rtcp:" as defined in [RTCPSDP]).

   If multiple addresses are specified in the "c=" field and multiple
   ports are specified in the "m=" field, a one-to-one mapping from port
   to the corresponding address is implied. For example:

      c=IN IP4
      m=video 49170/2 RTP/AVP 31

   would imply that address is used with ports 49170 and
   49171, and address is used with ports 49172 and 49173.

   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 defined, but may be extended through registration of
   new protocols with IANA (see Section 9):

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

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      udp     - User Datagram Protocol
      TCP     - Transmission Control 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.

   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
   [RFC1890], 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".

   The fourth and subsequent sub-fields are media formats.  For audio
   and video, these SHOULD reference a MIME sub-type describing the
   format under the "audio" and "video" top-level MIME types.

   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 SHOULD be used as 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.

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   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=audio 49232 RTP/AVP 0

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

      m=audio 49232 RTP/AVP 98
      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 codec-
   specific 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 to participate in a
   session.  Codec-specific parameters should be added in other
   attributes (for example, "a=fmtp:").

   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.

   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

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   "ptime" attribute is used as given below.

   For more details on RTP audio and video formats, see [RFC1890].

   Predefined application formats for the UDP protocol with non-RTP
   media are as below:

      wb: LBL Whiteboard (transport: udp)
      nt: UCL Network Text Editor (transport: udp)

6. Suggested Attributes

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


         This attribute gives the dot-separated hierarchical category
         of the session.  This is to enable a receiver to filter
         unwanted sessions by category.  It is a session-level
         attribute, and is not dependent on charset.


         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

      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, but may be used with other media types if it makes
         sense.  It should not be necessary to know ptime to decode RTP
         or vat audio, and it is intended as a recommendation for the

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         encoding/packetisation of audio.  It is a media attribute, and
         is not dependent on charset.

      a=maxptime:<maximum packet time>

         The maximum amount of media which can be encapsulated in each
         packet, expressed as time in milliseconds. The time SHALL be
         calculated as the sum of the time the media present in the
         packet represents. The time SHOULD be a multiple of the frame
         size. This attribute is probably only meaningful for audio
         data, but may be used with other media types if it makes
         sense.  It is a media attribute, and is not dependent on
         charset.  Note that this attribute was introduced after RFC
         2327, and non updated implementations will ignore this

      a=rtpmap:<payload type> <encoding name>/<clock rate>
        [/<encoding parameters>]

         See Section 5.14. This may be a session or media attribute.


         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. Note that
         recvonly applies to the media only, not to any associated
         control protocol (e.g. an RTP based system in recvonly mode
         SHOULD still send RTCP packets).


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

         If none of the attributes "sendonly", "recvonly", "inactive",
         and "sendrecv" is present, "sendrecv" SHOULD be assumed as the
         default for sessions which are not of the conference type
         "broadcast" or "H332" (see below).


         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,

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         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. Note that sendonly
         applies only to the media, and any associated control protocol
         (e.g. RTCP) SHOULD still be received and processed as normal.


         This specifies that the tools should be started in inactive
         mode.  This is necessary for interactive conferences where
         users can put other users on hold. No media is sent over an
         inactive media stream.  Note that an RTP based system SHOULD
         still send RTCP, even if started inactive. It can be either a
         session or 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 [H.332].  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.

      a=charset:<character set>

         This specifies the character set to be used to display the

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         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:


         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 octet 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

         In general, sending session descriptions consisting of
         multiple languages is 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 3066
         language tag in US-ASCII [RFC3066].  It is not dependent on
         the charset attribute.  An "sdplang" attribute SHOULD be

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         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=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 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 3066 language
         tag in US-ASCII [RFC3066].  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, defined only for video media, and is not
         dependent on charset.


         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.

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

      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

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

7. 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

   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


   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
      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

   In this example, a general conference attribute (type:H332) is

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   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.

8. 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

   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 descriptions 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 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

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   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.

   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.

   Use of the "k=" field poses a significant security risk, since it
   conveys session encryption keys in the clear.  SDP MUST NOT be used
   to convey key material, unless it can be guaranteed that the channel
   over which the SDP is delivered is both private and authenticated.

9. IANA Considerations

   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".

9.1 Media types ("media")

   The set of media types is intended to be small and SHOULD NOT be
   extended except under rare circumstances.  The same rules should
   apply for media names as for top-level MIME content types, and where
   possible the same name should be registered for SDP as for MIME.  For
   media other than existing MIME top-level content types, a
   standards-track RFC MUST be produced for a new top-level content type
   to be registered, and the registration MUST provide good
   justification why no existing media name is appropriate (the
   "Standards Action" policy of RFC 2434 [RFC2434]).

9.2 Transport protocols ("proto")

   The "proto" field describes the transport protocol used. This SHOULD
   reference a standards-track protocol RFC.  This memo registers three
   values: "RTP/AVP" is a reference to RTP [RFC1889] used under the RTP
   Profile for Audio and Video Conferences with Minimal Control
   [RFC1890]) running over UDP/IP; "TCP" denotes an unspecified format

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   over TCP; and "udp" indicates an unspecified format over UDP.

   New transport protocols MAY be registered with IANA. Registrations
   MUST reference an RFC describing the protocol.  Such an RFC MAY be
   Experimental or Informational, although it is preferable if it is
   Standards-Track. Registrations MUST also define the rules by which
   their "fmt" namespace is managed (see below).

9.3 Media formats ("fmt")

   Each transport protocol, defined by the "proto" field, has an
   associated "fmt" namespace that describes the media formats which may
   conveyed by that protocol.  Formats cover all the possible encodings
   that might want to be transported in a multimedia session.

   RTP payload formats under the "RTP/AVP" protocol that have been
   assigned static payload types MUST use the static payload type as
   their "fmt" value.  For payload formats under "RTP/AVP" that have a
   dynamic payload type number, the dynamic payload type number is given
   as the "fmt" and an additional "rtpmap" attribute specifies the
   format name and parameters as defined by the MIME type registration
   for the payload format.

   For "TCP" and "udp" protocols, new formats SHOULD be registered. Use
   of an existing MIME subtype for the format is encouraged. If no MIME
   subtype exists, it is RECOMMENDED that a suitable one is registered
   through the IETF process (RFC 2048) by production of, or reference
   to, a standards-track RFC. If a MIME subtype is for some reason
   inappropriate, an RFC publication describing the format MUST be
   referenced in the registration, but it may be Informational or
   Experimental if the protocol is not deemed to be of widespread

   For other protocols, formats MAY be registered according to the rules
   of the associated "proto" specification.

   Registrations of new formats MUST specify which transport protocols
   they apply to.

9.4 Attribute names ("att-field")

   Attribute field names ("att-field") MUST be registered with IANA and
   documented, because of noticeable issues due to conflicting
   attributes under the same name.  Unknown attributes in SDP are simply
   ignored, but conflicting ones that fragment the protocol are a
   serious problem.

   New attribute registerations are accepted according to the

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   "Specification Required" policy of RFC 2434, provided that the
   specification includes the following information:

   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

   The above is the minimum that IANA will accept.  Attributes that are
   expected to see widespread use and interoperability, SHOULD be
   documented with 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.

9.5 Bandwidth specifiers ("bwtype")

   A proliferation of bandwidth specifiers is strongly discouraged.

   New bandwidth specifiers ("bwtype" fields) MUST 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.

9.6 Network types ("nettype")

   New network types (the "nettype" field) 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

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   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.

9.7 Address types ("addrtype")

   New address types ("addrtype") 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.

9.8 Registration Procedure

   In the RFC documentation that registers SDP "media", "proto", "fmt",
   "bwtype", "nettype" and "addrtype" fields, the authors MUST include
   the following information for IANA to place in the appropriate

   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

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

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

   IANA may refer any registration to the IESG Transport Area Directors
   for review, and may request revisions to be made before a
   registration will be made.

Appendix A. SDP Grammar

   This appendix provides an Augmented BNF grammar for SDP.  ABNF is
   defined in [RFC2234].

      ; SDP Syntax
      announcement =        proto-version

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

      origin-field =        "o=" username SP sess-id SP sess-version SP
                            nettype SP addrtype SP unicast-address 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 SP addrtype SP
                            connection-address CRLF]
                            ;a connection field must be present
                            ;in every media description or at the

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

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

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

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

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

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

      media-descriptions =  *( media-field
                            attribute-fields )

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

      ; sub-rules of 'o='
      username =            non-ws-string
                            ;pretty wide definition, but doesn't
                            ;include space

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

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

      nettype =             token
                            ;typically "IN"

      addrtype =            token
                            ;typically "IP4" or "IP6"

      ; sub-rules of 'u='
      uri =                 URI-reference; see RFC1630 and RFC2732

      ; sub-rules of 'e='
      email-address =       email *SP "(" 1*email-safe ")" /
                            1*email-safe "<" email ">" /

      email =               addr-spec ; defined in RFC2822
                                      ; modified to remove CFWS

      ; sub-rules of 'p='
      phone-number =        phone *SP "(" 1*email-safe ")" /
                            1*email-safe "<" phone ">" /

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

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                            ;the number.

      ; sub-rules of 'c='
      connection-address =  multicast-address / unicast-address

      ; sub-rules of 'b='
      bwtype =              token

      bandwidth =           1*DIGIT

      ; sub-rules of 't='
      start-time =          time / "0"

      stop-time =           time / "0"

      time =                POS-DIGIT 9*DIGIT
                            ; 10-digit NTP time represents times between
                            ; 1931 and 5068 AD.  9* allows times after
                            ; that as well.

      ; sub-rules of 'r=' and 'z='
      repeat-interval =     POS-DIGIT *DIGIT [fixed-len-time-unit]

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

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

      ; sub-rules of 'k='
      key-type =            "prompt" /
                            "clear:" text /
                            "base64:" base64 /
                            "uri:" uri /
                            key-method [ ":" text ]

      base64      =         *base64-unit [base64-pad]
      base64-unit =         4base64-char
      base64-pad  =         2base64-char "==" / 3base64-char "="
      base64-char =         ALPHA / DIGIT / "+" / "/"

      key-method =          token

      ; sub-rules of 'a='
      attribute =           (att-field ":" att-value) / att-field

      att-field =           token

      att-value =           byte-string

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      ; sub-rules of 'm='
      media =               token
                            ;typically "audio", "video", "application"
                            ;or "data"

      fmt =                 token
                            ;typically an RTP payload type for audio
                            ;and video media

      proto  =              token "/" token
                            / token
                            ;typically "RTP/AVP" or "udp" for IP4

      port =                1*DIGIT
                            ;should be either "0" or in the range "1024"
                            ;to "65535" inclusive for UDP based media
                            ;(a value of "0" is used to signal special
                            ;conditions in some uses of SDP)

      ; generic sub-rules: addressing
      unicast-address =     IP4-address / IP6-address / FQDN / extn-addr

      multicast-address =   IP4-multicast / IP6-multicast

      IP4-multicast =       m1 3( "." decimal-uchar )
                            "/" ttl [ "/" integer ]
                            ; IPv4 multicast addresses may be in the
                            ; range to

      m1 =                  ("22" ("4"/"5"/"6"/"7"/"8"/"9")) /
                            ("23" DIGIT ))

      IP6-multicast =       hexpart [ "/" integer ]
                            ; IPv6 address starting with FF

      ttl =                 (POS-DIGIT *2DIGIT) / "0"

      FQDN =                4*(alpha-numeric / "-" / ".")
                            ; fully qualified domain name as specified
                            ; in RFC1035

      IP4-address =         b1 3("." decimal-uchar) / ""

      b1 =                  decimal-uchar
                            ; less than "224"; not "0" or "127"

      ; The following is from RFC2373 Appendix B. It is a direct copy.
      IP6-address =         hexpart [ ":" IP4-address ]

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      hexpart =             hexseq / hexseq "::" [ hexseq ] /
                            "::" [ hexseq ]

      hexseq  =             hex4 *( ":" hex4)

      hex4    =             1*4HEXDIG

      ; Generic for other address families
      extn-addr =      non-ws-string

      ; generic sub-rules: datatypes
      text =                byte-string
                            ;default is to interpret this as UTF8 text.
                            ;ISO 8859-1 requires "a=charset:ISO-8859-1"
                            ;session-level attribute to be used

      byte-string =         1*(%x01-09/%x0B-0C/%x0E-FF)
                            ;any byte except NUL, CR or LF

      non-ws-string =       1*(VCHAR/%x80-FF)
                            ;string of visible characters

      token-char =          %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39
                            / %x41-5A / %x5E-7E
                            ; definition from RFC 2045 -
                            ; "any (US-ASCII) CHAR except SPACE, CTLs,
                            ; or tspecials".
                            ; the tspecials are ()<>@,;:

      token =               1*(token-char)

      email-safe =          %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF
                            ;any byte except NUL, CR, LF, or the quoting
                            ;characters ()<>

      integer =             POS-DIGIT *DIGIT

      ; generic sub-rules: primitives
      alpha-numeric =       ALPHA / DIGIT

      POS-DIGIT =           %x31-39 ; 1 - 9

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

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      ; external references:
      ; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 2234
      ; URI-reference: from RFC1630 and RFC2732
      ; addr-spec: from RFC 2822

Appendix B. Changes from RFC 2327

   o  Deprecate X- notation for experimental parameters

   o  Clarify that a=recvonly does NOT mean that you don't send RTCP,
      and similarly for sendonly and inactive. These only effect the RTP

   o  Rewrite and correct the ABNF syntax (thanks to Jonathan Lennox)

   o  Update BNF to support IPv6.

   o  Add a=inactive attribute.

   o  Add a=maxptime attribute.

   o  RFC 2327 mandated that either e= or p= was required. Both are now
      optional, to reflect actual usage.

   o  Removed references to "conference" from the description of the t=
      line, to make it less SAP oriented.

   o  Note about wrap-around of NTP timestamps in t=

   o  References have been updated and split into normative and
      informative sections.

   o  Section 3.1 was replaced with a reference to RFC 2119, and the
      memo has been updated to use the RFC 2119 terminology (MUST,
      SHOULD, etc).

   o  Use of "application/sdp" as MIME a type for SDP files is now
      "MUST" rather than "SHOULD".

   o  Many sections have been updated to be less SAP specific, and to
      reference other current uses of SDP such as RTSP and SIP.

   o  The introduction and background has been rewritten, to remove
      references to the Mbone, reflecting current use of SDP.

   o  The section on concatenation of session descriptions (which was
      not allowed in SAP, but allowed in other cases) has been removed.

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      It is assumed that transports of SDP specify will specify this.

   o  The description of the c= line has been updated to reflect common
      usage of SDP, rather than Mbone conferencing with SAP.

   o  The b= line no longer makes a normative reference to the Mbone FAQ
      for bandwidth limits at various TTLs. The AS modifier to b= is
      noted as being the RTP session bandwidth.

   o  Define relation between the m= line and MIME types

   o  Note use of s= in sessions with no meaningful name

   o  Allow a=rtpmap to be a session level attribute, in addition to a
      media level attribute

   o  Clarify the limitations of the k= field

   o  Clarify IANA considerations

Appendix C. 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,
   Steve Hanna and Jonathan Lennox.

Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [3]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

   [4]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

   [5]  Alvestrand, H., "Tags for the Identification of Languages", BCP
        47, RFC 3066, January 2001.

Informative References

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   [6]   Mills, D., "Network Time Protocol (Version 3) Specification,
         Implementation", RFC 1305, March 1992.

   [7]   Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", RFC
         1889, January 1996.

   [8]   Schulzrinne, H., "RTP Profile for Audio and Video Conferences
         with Minimal Control", RFC 1890, January 1996.

   [9]   Handley, M., Perkins, C. and E. Whelan, "Session Announcement
         Protocol", RFC 2974, October 2000.

   [10]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [11]  Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", RFC 2326, April 1998.

   [12]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [13]  Huitema, C., "RTCP attribute in SDP",
         draft-ietf-mmusic-sdp4nat-05 (work in progress), June 2003.

Authors' Addresses

   Mark Handley
   University College London
   Gower Street
   London  WC1E 6BT


   Van Jacobson
   Packet Design
   2465 Latham Street
   Mountain View, CA  94040


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   Colin Perkins
   University of Glasgow
   17 Lilybank Gardens
   Glasgow  G12 8QQ


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Internet-Draft                    SDP                     September 2003



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