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Internet Engineering Task Force                                MMUSIC WG
INTERNET-DRAFT                                        Mark Handley/ACIRI
draft-ietf-mmusic-sdp-new-12.txt              Van Jacobson/Packet Design
                                                       Colin Perkins/ISI
                                                            2 March 2003
                                                 Expires: September 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://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   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 mmusic@ietf.org and/or the authors.

Copyright Notice

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

Abstract

   This memo defines the Session Description Protocol (SDP). SDP is
   intended for describing multimedia sessions for the purposes of



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   session announcement, session invitation, and other forms of
   multimedia session initiation.


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
   description.


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



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


2.1.  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 [RFC2119].


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 framework for negotiation using SDP.

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



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   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.

   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



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     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

   In general, SDP must convey sufficient information to enable
   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.






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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.


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.





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4.6.  Internationalization

   The SDP specification recommends the use of the ISO 10646 character
   sets in the UTF-8 encoding (RFC 2279) 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
   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:

      <type>=<value>

   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



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   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)
         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 -- 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



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   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=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
      s=SDP Seminar
      i=A Seminar on the session description protocol
      u=http://www.example.com/seminars/sdp.pdf
      e=j.doe@example.com (Jane Doe)
      c=IN IP4 224.2.17.12/127
      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 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.

   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 (her 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



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         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".

         <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. The "s=" field MUST NOT be



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      empty and SHOULD contain ISO 10646 characters (but see also the
      "a=charset" attribute below). If a session has no meaningful name,
      the value "s= " SHOULD be used (i.e.  a single space as the
      session name).

   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
      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 slides and one for feedback and questions.

   URI

      u=<URI>

      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.

   Email Address and Phone Number

      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.



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      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:

         e=j.doe@example.com (Jane Doe)

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

         e=Jane Doe <j.doe@example.com>

      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 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



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      IP6 are defined.

      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
      follows:

        o  If the session is multicast, the connection address will be
           an IP multicast group address.  If the conference 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 prohibited.

        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.

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

         c=IN IP4 224.2.36.42/127

      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



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      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

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

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

   Bandwidth

      b=<modifier>:<bandwidth-value>

      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



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            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:

         b=X-YZ:128

      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 recommended to be short.

   Times, Repeat Times and Time Zones

      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 sequence.

      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.



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      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 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>

      "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 be:

         t=3034423619 3042462419
         r=604800 3600 0 90000





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      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.

      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.

      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.




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      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>

      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
      required.

      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>

         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.

         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



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         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
         properties.

      The key field MUST NOT be used unless it can be guaranteed that
      the SDP is conveyed over a secure and trusted channel. Examples of
      such channels might include an SSL encrypted SIP or HTTP session
      where any intermediate proxies are trusted, or SDP embedded inside
      an encrypted S/MIME message.

   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>".
           For example, 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.



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      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 SHOULD be registered with IANA (see Appendix B). Names
      of unregistered attributes SHOULD begin with "X-" to prevent
      inadvertent collision with registered attributes, however the use
      of unregistered attributes is NOT RECOMMENDED. If an attribute is
      received that is not understood, it MUST 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 has several four 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.

      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:



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         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).

      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 224.2.1.1/127/2
         m=video 49170/2 RTP/AVP 31

      would imply that address 224.2.1.1 is used with ports 49170 and
      49171, and address 224.2.1.2 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 Appendix B):

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



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      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.

      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



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         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.

      Experimental encoding formats can also be specified using rtpmap.
      RTP formats that are not registered as standard format names MUST
      be preceded by "X-". Use of the ``X-'' prefix is deprecated, and
      all new formats SHOULD be registered with IANA.  Thus a new
      experimental redundant audio stream called GSMLPC using dynamic
      payload type 99 could be specified as:

         m=audio 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



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      defined in the RTP Audio/Video Profile) packetisation is required,
      the"ptime" attribute is used as given below.

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

      Predefined applicarion formats for the UDP protocol with non-RTP
      media are as below:
         wb: LBL Whiteboard (transport: udp)
         nt: UCL Network Text Editor (transport: udp)

   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, but may be used with other media types if it makes
         sense.  It should not be necessary to know ptime to decode RTP



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         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.

      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 attribute.

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

         See the section on Media Announcements (the "m=" field). This
         may be either a session or media attribute.

      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. 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).

      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.

         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).

      a=sendonly

         This specifies that the tools should be started in send-only
         mode.  An example may be where a different unicast address is



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

      a=inactive

         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>



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         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 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 important.

         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 1766
         language tag in US-ASCII.  It is not dependent on the charset
         attribute.  An "sdplang" attribute SHOULD be specified when a



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         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
         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.




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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 format.

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

5.1.  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.



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   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.


6.  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 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
   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



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   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.
































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Appendix A: SDP Grammar

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

      ; SDP Syntax
      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 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)









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


      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]


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


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






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      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; defined in RFC1630 and RFC2732


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


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


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


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




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      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






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      att-field =           token


      att-value =           byte-string


      ; 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
                            ;"0" is used to signal special conditions in some
                            ;uses of SDP)


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


      multicast-address =   IP4-multicast / IP6-multicast

      IP4-multicast =       m1 3( "." decimal-uchar )
                            "/" ttl [ "/" integer ]
                            ; IPv4 multicast addresses may be in the
                            ; range 224.0.0.0 to 239.255.255.255

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

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






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


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


      IP4-address =         b1 3("." decimal-uchar) / "0.0.0.0"

      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 ]

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

      hexseq  =             hex4 *( ":" hex4)

      hex4    =             1*4HEXDIG


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


      ; generic sub-rules: datatypes
      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*(%x01-09/%x0B-0C/%x0E-FF)
                            ;any byte except NUL, CR or LF


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










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      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"))


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



Appendix B: 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".

   "media" (e.g., audio, video, application, data).

      The set of media is intended to be small and SHOULD NOT be



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      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]).

   "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 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).

      Application-specific proprietary protocols that run over an
      existing transport protocol SHOULD be registered as a "fmt". The
      rules for formats (see below) apply to such registrations. An
      example is the LBL whiteboard application, which uses the proto
      "udp" with "wb" as the format.

   "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.

      For "TCP" and "udp" protocols, new formats may be registered. If
      there is a suitable mapping from a MIME subtype to the format, it



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      is RECOMMENDED that the MIME subtype name be used as the "fmt"
      name. If there is no suitable mapping from a MIME subtype, a new
      name should be registered. In either case, a standards-track RFC
      MUST be produced describing the format and this RFC MUST be
      referenced in the registration.

      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.

   "att-field" (Attribute names)

      Attribute field names 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 attributes MAY be registered according to the "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
           attribute.

      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



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      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 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.

   "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.

   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
   registry:

     o  contact name, email address and telephone number




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     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 (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 C: Changes from RFC 2327


     o  Deprecate X- notation for experimental parameters

     o  Correct example on page 23

     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 stream.

     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.




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     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. 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 D: Authors' Addresses













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   Mark Handley
   International Computer Science Institute,
   1947 Center Street, Suite 600,
   Berkeley, CA 94704
   United States
   Email: mjh@icir.org

   Van Jacobson
   Packet Design
   2465 Latham Street
   Mountain View, CA 94040
   United States
   Email: van@packetdesign.com

   Colin Perkins
   USC Information Sciences Institute
   3811 N. Fairfax Drive, Suite 200
   Arlington, VA 22203
   United States
   Email: csp@csperkins.org


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.


Full Copyright Statement

   Copyright (C) The Internet Society 2003. 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 implmentation 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.



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   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."


Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; neither does it represent that it
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   proprietary rights by implementors or users of this specification can
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   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
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   Director.


Normative References

   [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, March 1997.

   [RFC2434] T. Narten and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", RFC 2434, October
             1998.

Informative References

   [RFC1305] D. Mills, "Network Time Protocol (version 3) specification
             and implementation", RFC 1305, March 1992.




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   [RFC1889] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson,
             "RTP: A Transport Protocol for Real-Time Applications",
             RFC 1889, January 1996.

   [RFC1890] H. Schulzrinne, "RTP Profile for Audio and Video
             Conferences with Minimal Control", RFC 1890, January
             1996.

   [RFC2974] M. Handley, C. Perkins and E. Whelan, "Session
             Announcement Protocol", RFC 2974, October 2000.

   [H.332]   ITU-T Recommendation H.332 (1998): "Multimedia Terminal for
             Receiving Internet-based H.323 Conferences", ITU, Geneva.

   [RFC3261] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
             Peterson, R. Sparks, M. Handley, E. Schooler "SIP: Session
             Initiatation Protocol", RFC 3261, May 2002.

   [RFC2326] H. Schulzrinne, A. Rao and R. Lanphier, "Real Time
             Streaming Protocol (RTSP)" RFC 2326, April 1998.

   [RFC3264] J. Rosenberg and H. Schulzrinne, "An Offer/Answer Model
             with SDP", RFC 3264, May 2002.

   [RTCPSDP] C. Huitema, "RTCP Attribute in SDP", RFC XXXX, May 2002


























Handley/Jacobson/Perkins                                       [Page 46]


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