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Versions: (RFC 4566) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22

Network Working Group                                         M. Handley
Internet-Draft                                                       UCL
Obsoletes: 4566 (if approved)                                V. Jacobson
Intended status: Standards Track
Expires: December 19, 2017                                    C. Perkins
                                                   University of Glasgow
                                                                A. Begen
                                                         Networked Media
                                                           June 17, 2017


                   SDP: Session Description Protocol
                    draft-ietf-mmusic-rfc4566bis-19

Abstract

   This memo defines the Session Description Protocol (SDP).  SDP is
   intended for describing multimedia sessions for the purposes of
   session announcement, session invitation, and other forms of
   multimedia session initiation.  This document obsoletes RFC 4566.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on December 19, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Glossary of Terms . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Examples of SDP Usage . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Session Initiation  . . . . . . . . . . . . . . . . . . .   4
     3.2.  Streaming Media . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Email and the World Wide Web  . . . . . . . . . . . . . .   5
     3.4.  Multicast Session Announcement  . . . . . . . . . . . . .   5
   4.  Requirements and Recommendations  . . . . . . . . . . . . . .   5
     4.1.  Media and Transport Information . . . . . . . . . . . . .   6
     4.2.  Timing Information  . . . . . . . . . . . . . . . . . . .   7
     4.3.  Obtaining Further Information about a Session . . . . . .   7
     4.4.  Categorisation  . . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Internationalisation  . . . . . . . . . . . . . . . . . .   8
   5.  SDP Specification . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Protocol Version ("v=") . . . . . . . . . . . . . . . . .  11
     5.2.  Origin ("o=") . . . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Session Name ("s=") . . . . . . . . . . . . . . . . . . .  13
     5.4.  Session Information ("i=")  . . . . . . . . . . . . . . .  13
     5.5.  URI ("u=")  . . . . . . . . . . . . . . . . . . . . . . .  14
     5.6.  Email Address and Phone Number ("e=" and "p=")  . . . . .  14
     5.7.  Connection Data ("c=")  . . . . . . . . . . . . . . . . .  15
     5.8.  Bandwidth ("b=")  . . . . . . . . . . . . . . . . . . . .  17
     5.9.  Timing ("t=") . . . . . . . . . . . . . . . . . . . . . .  18
     5.10. Repeat Times ("r=") . . . . . . . . . . . . . . . . . . .  19
     5.11. Time Zones ("z=") . . . . . . . . . . . . . . . . . . . .  19
     5.12. Encryption Keys ("k=")  . . . . . . . . . . . . . . . . .  20
     5.13. Attributes ("a=") . . . . . . . . . . . . . . . . . . . .  22
     5.14. Media Descriptions ("m=") . . . . . . . . . . . . . . . .  23
   6.  SDP Attributes  . . . . . . . . . . . . . . . . . . . . . . .  25



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     6.1.  cat (category)  . . . . . . . . . . . . . . . . . . . . .  26
     6.2.  keywds (keywords) . . . . . . . . . . . . . . . . . . . .  26
     6.3.  tool  . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     6.4.  ptime (packet time) . . . . . . . . . . . . . . . . . . .  27
     6.5.  maxptime (maximum packet time)  . . . . . . . . . . . . .  28
     6.6.  rtpmap  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     6.7.  Media Direction Attributes  . . . . . . . . . . . . . . .  30
       6.7.1.  recvonly (receive-only) . . . . . . . . . . . . . . .  31
       6.7.2.  sendrecv (send-receive) . . . . . . . . . . . . . . .  31
       6.7.3.  sendonly (send-only)  . . . . . . . . . . . . . . . .  32
       6.7.4.  inactive  . . . . . . . . . . . . . . . . . . . . . .  32
     6.8.  orient (orientation)  . . . . . . . . . . . . . . . . . .  33
     6.9.  type (conference type)  . . . . . . . . . . . . . . . . .  33
     6.10. charset (character set) . . . . . . . . . . . . . . . . .  34
     6.11. sdplang (SDP language)  . . . . . . . . . . . . . . . . .  35
     6.12. lang (language) . . . . . . . . . . . . . . . . . . . . .  36
     6.13. framerate (frame rate)  . . . . . . . . . . . . . . . . .  37
     6.14. quality . . . . . . . . . . . . . . . . . . . . . . . . .  38
     6.15. fmtp (format parameters)  . . . . . . . . . . . . . . . .  38
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  39
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
     8.1.  The "application/sdp" Media Type  . . . . . . . . . . . .  41
     8.2.  Registration of Parameters  . . . . . . . . . . . . . . .  43
       8.2.1.  Media Types ("media") . . . . . . . . . . . . . . . .  43
       8.2.2.  Transport Protocols ("proto") . . . . . . . . . . . .  43
       8.2.3.  Media Formats ("fmt") . . . . . . . . . . . . . . . .  44
       8.2.4.  Attribute Names ("att-field") . . . . . . . . . . . .  44
       8.2.5.  Bandwidth Specifiers ("bwtype") . . . . . . . . . . .  47
       8.2.6.  Network Types ("nettype") . . . . . . . . . . . . . .  47
       8.2.7.  Address Types ("addrtype")  . . . . . . . . . . . . .  48
       8.2.8.  Registration Procedure  . . . . . . . . . . . . . . .  48
     8.3.  Encryption Key Access Methods . . . . . . . . . . . . . .  49
     8.4.  Reorganization of the nettype Registry  . . . . . . . . .  49
     8.5.  Reorganization of the att-field Registries  . . . . . . .  49
   9.  SDP Grammar . . . . . . . . . . . . . . . . . . . . . . . . .  50
   10. Summary of Changes from RFC 4566  . . . . . . . . . . . . . .  55
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  56
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  56
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  56
     12.2.  Informative References . . . . . . . . . . . . . . . . .  57
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  60

1.  Introduction

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



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   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 it is intended to use different transport protocols as
   appropriate, including the Session Announcement Protocol (SAP)
   [RFC2974], Session Initiation Protocol (SIP) [RFC3261], Real Time
   Streaming Protocol (RTSP) [RFC7826], electronic mail using the MIME
   extensions, and the Hypertext Transport Protocol (HTTP).

   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.

   This memo obsoletes [RFC4566].  The changes relative to [RFC4566] are
   limited to essential corrections, and are outlined in Section 10 of
   this memo.

2.  Glossary of Terms

   The following term is used in this document and has specific meaning
   within the context of this document.

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

   The terms "multimedia conference" and "multimedia session" are used
   in this document as defined in [RFC7656].  The terms "session" and
   "multimedia session" are used interchangeably in this document.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

3.  Examples of SDP Usage

3.1.  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 that 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




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   model [RFC3264] provides a limited framework for negotiation using
   SDP.

3.2.  Streaming Media

   The Real Time Streaming Protocol (RTSP) [RFC7826], is an application-
   level protocol for control over the delivery of data with real-time
   properties.  RTSP provides an extensible framework to enable
   controlled, on-demand delivery of real-time data, such as audio and
   video.  An RTSP client and server negotiate an appropriate set of
   parameters for media delivery, partially using SDP syntax to describe
   those parameters.

3.3.  Email and the World Wide Web

   Alternative means of conveying session descriptions include
   electronic mail and the World Wide Web (WWW).  For both email and WWW
   distribution, the media type "application/sdp" is 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 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.

3.4.  Multicast Session 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 used to implement such a distributed directory is the
   SAP [RFC2974].  SDP provides the recommended session description
   format for such session announcements.

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



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   an internetwork, although it is sufficiently general that it can
   describe multimedia conferences in other network environments.  Media
   streams can be many-to-many.  Sessions need not be continually
   active.

   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 it 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.

   An SDP description includes the following:

   o  Session name and purpose

   o  Time(s) the session is active

   o  The media comprising the session

   o  Information needed to receive those media (addresses, ports,
      formats, etc.)

   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 session

   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 any non-
   participants that may need to know.  (This latter feature is
   primarily useful when SDP is used with a multicast session
   announcement protocol.)

4.1.  Media and Transport Information

   An SDP description includes the following media information:

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

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

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



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   In addition to media format and transport protocol, SDP conveys
   address and port details.  For an IP multicast session, these
   comprise:

   o  The multicast group address for media

   o  The 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 unicast IP sessions, the following are conveyed:

   o  The remote address for media

   o  The remote transport port for media

   The semantics of this address and port depend on the media and
   transport protocol defined.  By default, this SHOULD be the remote
   address and remote port to which data is sent.  Some media types may
   redefine this behaviour, but this is NOT RECOMMENDED since it
   complicates implementations (including middleboxes that must parse
   the addresses to open Network Address Translation (NAT) or firewall
   pinholes).

4.2.  Timing Information

   Sessions may be either 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 (see Section 5.9).

4.3.  Obtaining Further Information about a Session

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






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4.4.  Categorisation

   When many session descriptions are being distributed by SAP, or any
   other advertisement mechanism, it may be desirable to filter session
   announcements that are of interest from those that are not.  SDP
   supports a categorisation mechanism for sessions that is capable of
   being automated (the "a=cat:" attribute; see Section 6).

4.5.  Internationalisation

   The SDP specification recommends the use of the ISO 10646 character
   set in the UTF-8 encoding [RFC3629] 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.  Internationalisation only applies to free-text fields
   (session name and background information), and not to SDP as a whole.

5.  SDP Specification

   An SDP description is denoted by the media type "application/sdp"
   (See Section 8).

   An SDP description is entirely textual.  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
   in UTF-8 encoding, or some other character set defined by the
   "a=charset:" attribute.  Field and attribute values that use the full
   UTF-8 character set are never directly compared, hence there is no
   requirement for UTF-8 normalisation.  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, and to
   allow flexible, text-based toolkits to be used to generate and
   process session descriptions.  However, since SDP may be used in
   environments where the maximum permissible size of a session
   description is limited, 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 session announcements that could be detected
   easily and discarded.  This also allows rapid discarding of encrypted
   session announcements for which a receiver does not have the correct
   key.

   An SDP description consists of a number of lines of text of the form:

      <type>=<value>





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   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, and is case-significant
   unless a specific field defines otherwise.  Whitespace separators
   MUST NOT be used on either side of the "=" sign, however, if the
   value can contain a leading whitespace as part of its syntax, i.e.,
   that whitespace is part of the value.

   An SDP 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 (or
   the end of the whole description, whichever comes first).  Each
   media-level section starts with an "m=" line and continues to the
   next media-level section or the end of the whole session description
   - whichever comes first.  In general, session-level values are the
   default for all media unless overridden by an equivalent media-level
   value.

   Some lines in each description are REQUIRED and some are OPTIONAL,
   but all MUST appear in exactly the order given here (the fixed order
   greatly enhances error detection and allows for a simple parser).
   OPTIONAL items are marked with a "*".




























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      Session description
         v=  (protocol version)
         o=  (originator 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 descriptions)
         b=* (zero or more bandwidth information lines)
         One or more time descriptions ("t=" and "r=" lines; see below)
         z=* (time zone adjustments)
         k=* (encryption key)
         a=* (zero or more session attribute lines)
         Zero or more media descriptions

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

      Media description, if present
         m=  (media name and transport address)
         i=* (media title)
         c=* (connection information -- optional if included at
              session level)
         b=* (zero or more bandwidth information lines)
         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 session
   description 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 Section 6 of this memo)
   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.

   An SDP description may contain URIs that reference external content
   in the "u=", "k=", and "a=" lines.  These URIs may be dereferenced in
   some cases, making the session description non-self- contained.

   The connection ("c=") information in the session-level section
   applies to all the media of that session unless overridden by
   connection information in the media description.  For instance, in




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   the example below, each audio media description behaves as if it were
   given a "c=IN IP4 233.252.0.2".

   An example SDP description is:

             v=0
             o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1
             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 233.252.0.2
             t=2873397496 2873404696
             a=recvonly
             m=audio 49170 RTP/AVP 0
             m=audio 49180 RTP/AVP 0
             m=video 51372 RTP/AVP 99
             c=IN IP4 233.252.0.1/127
             a=rtpmap:99 h263-1998/90000


   Text fields such as the session name and information are octet
   strings that 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.  If the "a=charset" attribute is not present, these octet
   strings MUST be interpreted as containing ISO-10646 characters in
   UTF-8 encoding (the presence of the "a=charset" attribute may force
   some fields to be interpreted differently).

   A session description can contain domain names in the "o=", "u=",
   "e=", "c=", and "a=" lines.  Any domain name used in SDP MUST comply
   with [RFC1034], [RFC1035].  Internationalised domain names (IDNs)
   MUST be represented using the ASCII Compatible Encoding (ACE) form
   defined in [RFC5890] and MUST NOT be directly represented in UTF-8 or
   any other encoding (this requirement is for compatibility with
   [RFC2327] and other early SDP-related standards, which predate the
   development of internationalised domain names).

5.1.  Protocol Version ("v=")

      v=0

   The "v=" line gives the version of the Session Description Protocol.
   This memo defines version 0.  There is no minor version number.





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5.2.  Origin ("o=")

      o=<username> <sess-id> <sess-version> <nettype> <addrtype>
        <unicast-address>

   The "o=" line gives the originator of the session (her username and
   the address of the user's host) plus a session identifier and version
   number:

   <username>  is the user's login on the originating host, or it is "-"
      if the originating host does not support the concept of user IDs.
      The <username> MUST NOT contain spaces.

   <sess-id>  is a numeric string such that the tuple of <username>,
      <sess-id>, <nettype>, <addrtype>, and <unicast-address> forms a
      globally unique identifier for the session.  The method of <sess-
      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 [RFC5905].

   <sess-version>  is a version number for this session description.
      Its usage is up to the creating tool, so long as <sess-version> is
      increased when a modification is made to the session data.  Again,
      it is RECOMMENDED that an NTP format timestamp is used.

   <nettype>  is a text string giving the type of network.  Initially
      "IN" is defined to have the meaning "Internet", but other values
      MAY be registered in the future (see Section 8).

   <addrtype>  is a text string giving the type of the address that
      follows.  Initially "IP4" and "IP6" are defined, but other values
      MAY be registered in the future (see Section 8).

   <unicast-address>  is an address of the machine from which the
      session was created.  For an address type of IP4, this is either a
      fully qualified domain name of the machine or the dotted-decimal
      representation of an IP version 4 address of the machine.  For an
      address type of IP6, this is either a fully qualified domain name
      of the machine or the compressed textual representation of an 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 a globally unique address MAY be
      substituted.  Unless an SDP extension for NAT traversal is used
      (e.g., ICE [RFC5245], ICE TCP [RFC6544]), 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 (for example, a local
      address MUST NOT be included in an application-level referral that
      might leave the scope).



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   In general, the "o=" line 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.

   For privacy reasons, it is sometimes desirable to obfuscate the
   username and IP address of the session originator.  If this is a
   concern, an arbitrary <username> and private <unicast-address> MAY be
   chosen to populate the "o=" line, provided that these are selected in
   a manner that does not affect the global uniqueness of the field.

5.3.  Session Name ("s=")

      s=<session name>

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

5.4.  Session Information ("i=")

      i=<session description>

   The "i=" line provides textual information about the session.  There
   MUST be at most one session-level "i=" line per session description,
   and at most one "i=" line per media description/definition.  Unless a
   media level "i="" line is used, the session-level "i="" line applies
   to that media description.  If the "a=charset" attribute is present,
   it specifies the character set used in the "i=" line.  If the
   "a=charset" attribute is not present, the "i=" line MUST contain ISO
   10646 characters in UTF-8 encoding.

   A single "i=" line can be used for each media definition.  In media
   definitions, "i=" lines are primarily intended for labelling 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.

   The "i=" line is intended to provide a free-form human-readable
   description of the session or the purpose of a media stream.  It is
   not suitable for parsing by automata.






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5.5.  URI ("u=")

      u=<uri>

   A URI is a Uniform Resource Identifier as used by WWW clients
   [RFC3986].  The URI should be a pointer to additional information
   about the session.  This line is OPTIONAL.  No more than one URI line
   is allowed per session description.

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

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

   The "e=" and "p=" lines specify contact information for the person
   responsible for the session.  This is not necessarily the same person
   that created the session description.

   Inclusion of an email address or phone number is OPTIONAL.

   If an email address or phone number is present, it MUST be specified
   before the first media field.  More than one email or phone line can
   be given for a session description.

   Phone numbers SHOULD be given in the form of an international public
   telecommunication number (see ITU-T Recommendation E.164) preceded by
   a "+".  Spaces and hyphens may be used to split up a phone field to
   aid readability if desired.  For example:

      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 MUST be enclosed in parentheses if it is
   present.  For example:

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

   The alternative [RFC5322] 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 session-level "a=charset" attribute is set.





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5.7.  Connection Data ("c=")

      c=<nettype> <addrtype> <connection-address>

   The "c=" line contains connection data.

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

   The first sub-field ("<nettype>") is the network type, which is a
   text string giving the type of network.  Initially, "IN" is defined
   to have the meaning "Internet", but other values MAY be registered in
   the future (see Section 8).

   The second sub-field ("<addrtype>") is the address type.  This allows
   SDP to be used for sessions that are not IP based.  This memo only
   defines IP4 and IP6, but other values MAY be registered in the future
   (see Section 8).

   The third sub-field ("<connection-address>") is the connection
   address.  OPTIONAL sub-fields MAY be added after the connection
   address depending on the value of the <addrtype> field.

   When the <addrtype> is IP4 and IP6, 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 session is not multicast, then
      the connection address contains the unicast IP address of the
      expected data source or data relay or data sink as determined by
      additional attribute fields.  It is not expected that unicast
      addresses will be given in a session description that is
      communicated by a multicast announcement, though this is not
      prohibited.

   o  Sessions using an IP4 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 session will be sent.  TTL
      values MUST be in the range 0-255.  Although the TTL MUST be
      specified, its use to scope multicast traffic is deprecated;
      applications SHOULD use an administratively scoped address
      instead.





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

      c=IN IP4 233.252.0.1/127

   IP6 multicast does not use TTL scoping, and hence the TTL value MUST
   NOT be present for IP6 multicast.  It is expected that IP6 scoped
   addresses will be used to limit the scope of multimedia conferences.

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

      <base multicast address>[/<ttl>]/<number of addresses>

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

      c=IN IP4 233.252.0.1/127/3

   would state that addresses 233.252.0.1, 233.252.0.2, and 233.252.0.3
   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 233.252.0.1/127
      c=IN IP4 233.252.0.2/127
      c=IN IP4 233.252.0.3/127

   Similarly, an IP6 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 IP6 multicast).





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   Multiple addresses or "c=" lines MAY be specified on a per-media
   basis only if they provide multicast addresses for different layers
   in a hierarchical or layered encoding scheme.  They MUST NOT be
   specified for a session-level "c=" line.

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

5.8.  Bandwidth ("b=")

      b=<bwtype>:<bandwidth>

   This OPTIONAL line denotes the proposed bandwidth to be used by the
   session or media.  The <bwtype> is an alphanumeric modifier giving
   the meaning of the <bandwidth> figure.  Two values are defined in
   this specification, but other values MAY be registered in the future
   (see Section 8 and [RFC3556], [RFC3890]):

   CT If the bandwidth of a session is different from the bandwidth
      implicit from the scope, a "b=CT:..." line SHOULD be supplied for
      the session giving the proposed upper limit to the bandwidth used
      (the "conference total" bandwidth).  Similarly, if the bandwidth
      of bundled media streams in an m line is different from the
      implicit value from the scope, a "b=CT:..." line SHOULD be
      supplied in the media level.  The primary purpose of this is to
      give an approximate idea as to whether two or more sessions (or
      bundled media streams) can coexist simultaneously.  Note that CT
      gives a total bandwidth figure for all the media at all endpoints.

   AS 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
      [RFC3550].  Note that AS gives a bandwidth figure for a single
      media at a single endpoint, although there may be many endpoints
      sending simultaneously.

   A prefix "X-" is defined for <bwtype> names.  This is intended for
   experimental purposes only.  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 alphanumeric and, although no length limit is
   given, it is recommended that they be short.



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   The <bandwidth> is interpreted as kilobits per second by default.
   The definition of a new <bwtype> modifier MAY specify that the
   bandwidth is to be interpreted in some alternative unit (the "CT" and
   "AS" modifiers defined in this memo use the default units).

5.9.  Timing ("t=")

      t=<start-time> <stop-time>

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

   The first and second sub-fields give the start and stop times,
   respectively, for the session.  These values are the decimal
   representation of Network Time Protocol (NTP) time values in seconds
   since 1900 [RFC5905].  To convert these values to UNIX time, subtract
   decimal 2208988800.

   NTP timestamps are elsewhere represented by 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 MUST 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.





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   Permanent sessions may be shown to the user as never being active
   unless there are associated repeat times that state precisely when
   the session will be active.

5.10.  Repeat Times ("r=")

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

   "r=" line specifies 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=" line 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=" line 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=" lines might
   be the following:

      t=3034423619 3042462419
      r=604800 3600 0 90000

   To make the 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)

   Thus, the above session 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=" lines should be used to
   explicitly list the session times.

5.11.  Time Zones ("z=")

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

   To schedule a repeated session that 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



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   different time zones change time at different times of day, different
   countries change to or from daylight saving 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=" line allows the sender to specify a
   list of these adjustment times and offsets from the base time.

   An example might be the following:

      z=2882844526 -1h 2898848070 0

   This specifies that at time 2882844526, the time base by which the
   session's repeat times are calculated is shifted back by 1 hour, and
   that at time 2898848070, the session's original time base is
   restored.  Adjustments are always relative to the specified start
   time -- they are not cumulative.  Adjustments apply to all "t=" and
   "r=" lines in a session description.

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

5.12.  Encryption Keys ("k=")

      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 simple
   mechanism for key exchange is provided by the key line ("k="),
   although this is primarily supported for compatibility with older
   implementations and its use is NOT RECOMMENDED.  Work is in progress
   to define new key exchange mechanisms for use with SDP [RFC4567]
   [RFC4568], and it is expected that new applications will use those
   mechanisms.

   A key line 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 are outside the scope
   of this document, and the key field provides no way to indicate the
   encryption algorithm to be used, key type, or other information about
   the key: this is assumed to be provided by the higher-level protocol



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   using SDP.  If there is a need to convey this information within SDP,
   the extensions mentioned previously SHOULD be used.  Many security
   protocols require two keys: one for confidentiality, another for
   integrity.  This specification does not support transfer of two keys.

   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 line.
         This method MUST NOT be used unless it can be guaranteed that
         the SDP is conveyed over a secure channel.  The encryption key
         is interpreted as text according to the charset attribute; use
         the "k=base64:" method to convey characters that are otherwise
         prohibited in SDP.

      k=base64:<encoded encryption key>

         The encryption key is included in this key line but has been
         base64 encoded [RFC4648] 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 Uniform Resource Identifier is included in the key line.  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 reply should specify
         the encoding for the key.  The URI is often an Secure Socket
         Layer/Transport Layer Security (SSL/TLS)-protected HTTP URI
         ("https:"), although this is not required.

      k=prompt

         No key is included in this SDP description, but the session or
         media stream referred to by this key line is encrypted.  The
         user should be prompted for the key when attempting to join the
         session, and this user-supplied key should then be used to
         decrypt the media streams.  The use of user-specified keys is
         NOT RECOMMENDED, since such keys tend to have weak security
         properties.

   The key line MUST NOT be used unless it can be guaranteed that the
   SDP is conveyed over a secure and trusted channel.  An example of



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   such a channel might be SDP embedded inside an S/MIME message or a
   TLS-protected HTTP session.  It is important to ensure that the
   secure channel is with the party that is authorised to join the
   session, not an intermediary: if a caching proxy server is used, it
   is important to ensure that the proxy is either trusted or unable to
   access the SDP.

5.13.  Attributes ("a=")

      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=" lines)
   that are media specific.  These are referred to as "media-level"
   attributes and add information about the media stream.  Attribute
   lines can also be added before the first media field; these "session-
   level" attributes convey additional information that applies to the
   session as a whole rather than to individual media.

   Attribute lines may be of two forms:

   o  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  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 session descriptions in general and of
   attributes in particular.

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

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



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   which case its value should be interpreted in the session charset
   rather than in ISO-10646.

   Attributes MUST be registered with IANA (see Section 8).  If an
   attribute is received that is not understood, it MUST be ignored by
   the receiver.

5.14.  Media Descriptions ("m=")

      m=<media> <port> <proto> <fmt> ...

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

   <media>  is the media type.  This document defines the values
      "audio", "video", "text", "application", and "message".  This list
      is extended and may be further extended by other memos registering
      media types in the future (see Section 8).

   <port>  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=" line, and on the transport protocol
      defined in the <proto> sub-field of the media field.  Other ports
      used by the media application (such as the RTP Control Protocol
      (RTCP) port [RFC3550]) MAY be derived algorithmically from the
      base media port or MAY be specified in a separate attribute (for
      example, "a=rtcp:" as defined in [RFC3605]).

      If non-contiguous ports are used or if they don't follow the
      parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:"
      attribute MUST be used.  Applications that are requested to send
      media to a <port> that is odd and where the "a=rtcp:" is present
      MUST NOT subtract 1 from the RTP port: that is, they MUST send the
      RTP to the port indicated in <port> and send the RTCP to the port
      indicated in the "a=rtcp" attribute.

      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=" line:

          m=<media> <port>/<number of ports> <proto> <fmt> ...

      In such a case, the ports used depend on the transport protocol.
      For RTP, the default is that only the even-numbered ports are used
      for data with the corresponding one-higher odd ports used for the



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      RTCP belonging to the RTP session, and the <number of ports>
      denoting the number of RTP sessions.  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 non-
      contiguous ports are required, they must be signalled using a
      separate attribute (for example, "a=rtcp:" as defined in
      [RFC3605]).

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

          c=IN IP4 233.252.0.1/127/2
          m=video 49170/2 RTP/AVP 31

      would imply that address 233.252.0.1 is used with ports 49170 and
      49171, and address 233.252.0.2 is used with ports 49172 and 49173.

      The semantics of multiple "m=" lines using the same transport
      address are undefined.  This implies that, unlike limited past
      practice, there is no implicit grouping defined by such means and
      an explicit grouping framework (for example, [RFC5888]) should
      instead be used to express the intended semantics.

   <proto>  is the transport protocol.  The meaning of the transport
      protocol is dependent on the address type field in the relevant
      "c=" line.  Thus a "c=" field of IP4 indicates that the transport
      protocol runs over IP4.  The following transport protocols are
      defined, but may be extended through registration of new protocols
      with IANA (see Section 8):

      *  udp: denotes an unspecified protocol running over UDP.

      *  RTP/AVP: denotes RTP [RFC3550] used under the RTP Profile for
         Audio and Video Conferences with Minimal Control [RFC3551]
         running over UDP.

      *  RTP/SAVP: denotes the Secure Real-time Transport Protocol
         [RFC3711] running over UDP.

      The main reason to specify the transport protocol in addition to
      the media format is that the same standard media formats may be
      carried over different transport protocols even when the network
      protocol is the same -- a historical example is vat Pulse Code



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      Modulation (PCM) audio and RTP PCM audio; another might be TCP/RTP
      PCM audio.  In addition, relays and monitoring tools that are
      transport-protocol-specific but format-independent are possible.

   <fmt>  is a media format description.  The fourth and any subsequent
      sub-fields describe the format of the media.  The interpretation
      of the media format depends on the value of the <proto> sub-field.

      If the <proto> sub-field is "RTP/AVP" or "RTP/SAVP" the <fmt> sub-
      fields contain RTP payload type numbers.  When a list of payload
      type numbers is given, this implies that all of these payload
      formats MAY be used in the session, but the first of these formats
      SHOULD be used as the default format for the session.  For dynamic
      payload type assignments the "a=rtpmap:" attribute (see Section 6)
      SHOULD be used to map from an RTP payload type number to a media
      encoding name that identifies the payload format.  The "a=fmtp:"
      attribute MAY be used to specify format parameters (see
      Section 6).

      If the <proto> sub-field is "udp" the <fmt> sub-fields MUST
      reference a media type describing the format under the "audio",
      "video", "text", "application", or "message" top-level media
      types.  The media type registration SHOULD define the packet
      format for use with UDP transport.

      For media using other transport protocols, the <fmt> field is
      protocol specific.  Rules for interpretation of the <fmt> sub-
      field MUST be defined when registering new protocols (see
      Section 8.2.2).

      Section 3 of [RFC4855] states that the payload format (encoding)
      names defined in the RTP Profile are commonly shown in upper case,
      while media subtype names are commonly shown in lower case.  It
      also states that both of these names are case-insensitive in both
      places, similar to parameter names which are case-insensitive both
      in media type strings and in the default mapping to the SDP a=fmtp
      attribute.

6.  SDP Attributes

   The following attributes are defined.  Since application writers may
   add new attributes as they are required, this list is not exhaustive.
   Registration procedures for new attributes are defined in
   Section 8.2.4.







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6.1.  cat (category)

   Name: cat

   Value: cat-value

   Usage Level: session

   Charset Dependent: no

   Syntax:

         cat-value = category
         category = non-ws-string

   Example:

         a=cat:foo.bar

   This attribute gives the dot-separated hierarchical category of the
   session.  This is to enable a receiver to filter unwanted sessions by
   category.  There is no central registry of categories.  This
   attribute is obsoleted.

6.2.  keywds (keywords)

   Name: keywds

   Value: keywds-value

   Usage Level: session

   Charset Dependent: yes

   Syntax:

         keywds-value = keywords
         keywords = text

   Example:

         a=keywds:SDP session description protocol

   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;
   there is no central registry of keywords.  Its value should be
   interpreted in the charset specified for the session description if



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   one is specified, or by default in ISO 10646/UTF-8.  This attribute
   is obsoleted.

6.3.  tool

   Name: tool

   Value: tool-value

   Usage Level: session

   Charset Dependent: no

   Syntax:

         tool-value = tool-name-and-version
         tool-name-and-version = text

   Example:

         a=tool:foobar V3.2

   This gives the name and version number of the tool used to create the
   session description.

6.4.  ptime (packet time)

   Name: ptime

   Value: ptime-value

   Usage Level: media

   Charset Dependent: no

   Syntax:

         ptime-value = non-zero-int-or-real

   Example:

         a=ptime:20

   This gives the length of time in milliseconds represented by the
   media in a packet.  This is probably only meaningful for audio data,
   but may be used with other media types if it makes sense.  It should
   not be necessary to know ptime to decode RTP or vat audio, and it is
   intended as a recommendation for the encoding/packetisation of audio.



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6.5.  maxptime (maximum packet time)

   Name: maxptime

   Value: maxptime-value

   Usage Level: media

   Charset Dependent: no

   Syntax:

         maxptime-value = non-zero-int-or-real

   Example:

         a=maxptime:20

   This gives the maximum amount of media that 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.  For frame-based codecs, the time SHOULD be an integer
   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.  Note that this attribute was introduced after
   [RFC2327], and non-updated implementations will ignore this
   attribute.

6.6.  rtpmap

   Name: rtpmap

   Value: rtpmap-value

   Usage Level: media

   Charset Dependent: no














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   Syntax:

         rtpmap-value = payload-type SP encoding-name
           "/" clock-rate [ "/" encoding-params ]
         payload-type = zero-based-integer
         encoding-name = token
         clock-rate = integer
           ; do we want to define a limited range for this?
         encoding-params = channels
           ; 4566 is vague about what this can be. RFC4855 seems to be
           ; the authoritative source, and only allows the
           ; value of the media subtype "channels" parameter - the
           ; number of audio channels.
           ; Does anyone think this can be used for something else???
           ; (The implication that multiple parameters might be included
           ; seems a misdirection - additional parameters are
           ; to go into a=fmtp.)
           ; Does anyone have an example of other parameters
           ; using this field?
         channels = integer
           ; Is there any reason to make this less restrictive?

   This attribute maps from an RTP payload type number (as used in an
   "m=" line) to an encoding name denoting the payload format to be
   used.  It also provides information on the clock rate and encoding
   parameters.  Note that the payload type number is indicated in a
   7-bit field, limiting the values to incusively between 0 and 127.

   Although an RTP profile can make static assignments of payload type
   numbers to payload formats, it is more common for that assignment to
   be done dynamically using "a=rtpmap:" attributes.  As an example of a
   static payload type, consider u-law PCM coded single-channel audio
   sampled at 8 kHz.  This is completely defined in the RTP Audio/Video
   profile as payload type 0, so there is no need for an "a=rtpmap:"
   attribute, and the media for such a stream sent to UDP port 49232 can
   be specified as:

             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 the dynamic RTP/AVP
   payload type 98 for this stream, additional information is required
   to decode it:

             m=audio 49232 RTP/AVP 98
             a=rtpmap:98 L16/16000/2





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

             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.  The "RTP/AVP"
   and "RTP/SAVP" profiles use media subtypes for encoding names, under
   the top-level media type denoted in the "m=" line.  In the example
   above, the media types are "audio/l8" and "audio/l16".

   For audio streams, <encoding parameters> indicates the number of
   audio channels.  This parameter is OPTIONAL and may be omitted if the
   number of channels is one, provided that no additional parameters are
   needed.

   For video streams, no encoding parameters are currently specified.

   Additional encoding parameters MAY be defined in the future, but
   codec-specific parameters SHOULD NOT be added.  Parameters added to
   an "a=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:").

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

6.7.  Media Direction Attributes

   At most one of recvonly/sendrecv/sendonly/inactive MAY appear at
   session level, and at most one MAY appear in each media section.

   If any one of these appears in a media section then it applies for
   that media section.  If none appear in a media section then the one
   from session level, if any, applies to that media section.

   If none of the media direction attributes is present at either
   session level or media level, "sendrecv" SHOULD be assumed as the
   default for sessions that are not of the multimedia conference type
   "broadcast" or "H332" (see below).




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   Within the following SDP example, the "inactive" attribute applies to
   audio media and the "recvonly" attribute applies to video media.

             v=0
             o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1
             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 233.252.0.1/127
             t=2873397496 2873404696
             a=inactive
             m=audio 49170 RTP/AVP 0
             m=video 51372 RTP/AVP 99
             a=rtpmap:99 h263-1998/90000
             a=recvonly

6.7.1.  recvonly (receive-only)

   Name: recvonly

   Value:

   Usage Level: session, media

   Charset Dependent: no

   Example:

         a=recvonly

   This specifies that the tools should be started in receive-only mode
   where applicable.  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).

6.7.2.  sendrecv (send-receive)

   Name: sendrecv

   Value:

   Usage Level: session, media

   Charset Dependent: no






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   Example:

         a=sendrecv

   This specifies that the tools should be started in send and receive
   mode.  This is necessary for interactive multimedia conferences with
   tools that default to receive-only mode.

6.7.3.  sendonly (send-only)

   Name: sendonly

   Value:

   Usage Level: session, media

   Charset Dependent: no

   Example:

         a=sendonly

   This specifies that the tools should be started in send-only mode.
   An example may be where a different unicast address is to be used for
   a traffic destination than for a traffic source.  In such a case, two
   media descriptions may be used, one sendonly and one recvonly.  Note
   that sendonly applies only to the media, and any associated control
   protocol (e.g., RTCP) SHOULD still be received and processed as
   normal.

6.7.4.  inactive

   Name: inactive

   Value:

   Usage Level: session, media

   Charset Dependent: no

   Example:

         a=inactive

   This specifies that the tools should be started in inactive mode.
   This is necessary for interactive multimedia conferences where users
   can put other users on hold.  No media is sent over an inactive media




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   stream.  Note that an RTP-based system MUST still send RTCP (if RTCP
   is used), even if started inactive.

6.8.  orient (orientation)

   Name: orient

   Value: orient-value

   Usage Level: media

   Charset Dependent: no

   Syntax:

         orient-value = portrait / landscape / seascape
         portrait  = %s"portrait"
         landscape = %s"landscape"
         seascape  = %s"seascape"
            ; NOTE: These names are case-sensitive.

   Example:

         a=orient:portrait

   Normally this is only used for a whiteboard or presentation tool.  It
   specifies the orientation of the workspace on the screen.  Permitted
   values are "portrait", "landscape", and "seascape" (upside-down
   landscape).

6.9.  type (conference type)

   Name: type

   Value: type-value

   Usage Level: session

   Charset Dependent: no












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   Syntax:

             type-value = conference-type
             conference-type = broadcast / meeting / moderated / test /
             H332
             broadcast = %s"broadcast"
             meeting   = %s"meeting"
             moderated = %s"moderated"
             test      = %s"test"
             H332      = %s"H332"
                ; NOTE: These names are case-sensitive.

   Example:

         a=type:moderated

   This specifies the type of the multimedia 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 multimedia conference.

   Specifying the attribute "type:H332" indicates that this loosely
   coupled session is part of an H.332 session as defined in the ITU
   H.332 specification [ITU.H332.1998].  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.

6.10.  charset (character set)

   Name: charset

   Value: charset-value

   Usage Level: session

   Charset Dependent: no

   Syntax:

             charset-value = mime-charset
             (as defined in [RFC 2978])





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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.  For example, the ISO
   8859-1 is specified with the following SDP attribute:

         a=charset:ISO-8859-1

   The charset specified MUST be one of those registered in the IANA
   Character Sets registry (http://www.iana.org/assignments/character-
   sets), such as ISO-8859-1.  The character set identifier is a US-
   ASCII string and MUST be compared against identifiers from the "Name"
   or "Preferred MIME Name" field of the registry 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 from appearing within text fields.

6.11.  sdplang (SDP language)

   Name: sdplang

   Value: sdplang-value

   Usage Level: session, media

   Charset Dependent: no

   Syntax:

             sdplang-value = Language-Tag
             ; Language-Tag defined in RFC5646

   Example:

         a=sdplang:fr

   Multiple sdplang attributes can be provided either at session or
   media level if the session description or media use multiple
   languages.

   As a session-level attribute, it specifies the language for the
   session description (not the language of the media).  As a media-
   level attribute, it specifies the language for any media-level SDP



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   information field associated with that media (again not the language
   of the media), overriding any sdplang attributes specified at
   session-level.

   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 [RFC5646] language tag
   in US-ASCII.  An "sdplang" attribute SHOULD be specified when a
   session is distributed with 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.

6.12.  lang (language)

   Name: lang

   Value: lang-value

   Usage Level: session, media

   Charset Dependent: no

   Syntax:

         lang-value = Language-Tag
         ; Language-Tag defined in RFC5646

   Example:

         a=lang:de

   Multiple lang attributes can be provided either at session or media
   level if the session or media has capabilities in more than one
   language, in which case the order of the attributes indicates the
   order of preference of the various languages in the session or media,
   from most preferred to least preferred.

   As a session-level attribute, lang specifies a language capability
   for the session being described.  As a media-level attribute, it
   specifies a language capability for that media, overriding any
   session-level language(s) specified.





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   The "lang" attribute value must be a single [RFC5646] language tag in
   US-ASCII.  A "lang" attribute SHOULD be specified when a session is
   of sufficient scope to cross geographic boundaries where the language
   of participants cannot be assumed, or where the session has
   capabilities in languages different from the locally assumed norm.

   The "lang" attribute is supposed to be used for settling the initial
   language(s) used in the session.  Events during the session may
   influence which language(s) are used, and the participants are not
   strictly bound to only use the declared languages.

   Most real-time use cases start with just one language used, while
   other cases involve a range of languages, e.g. an interpreted or
   subtitled session.  When more than one 'lang' attribute is specified,
   the "lang" attribute itself does not provide any information about if
   multiple languages are intended to be used during the session, or if
   the intention is to only select one language.  Other attributes or
   the semantics in which the "lang" attributes are used might indicate
   such conditions.  Without such indications of usage intent, it is
   assumed that for a negotiated session one of the declared languages
   will be selected and used.

6.13.  framerate (frame rate)

   Name: framerate

   Value: framerate-value

   Usage Level: media

   Charset Dependent: no

   Syntax:

         framerate-value = non-zero-int-or-real

   Example:

         a=framerate:60

   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 are allowed.  It is defined only
   for video media.







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6.14.  quality

   Name: quality

   Value: quality-value

   Usage Level: media

   Charset Dependent: no

   Syntax:

         quality-value = zero-based-integer

   Example:

         a=quality:10

   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 is 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.

   Editor's note: The usage should be checked with the SIP Forum to see
   whether anybody is using this.  Otherwise, the recommendation will be
   not to use this attribute and the receiver ignores it upon reception.

6.15.  fmtp (format parameters)

   Name: fmtp

   Value: fmtp-value

   Usage Level: media

   Charset Dependent: no








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   Syntax:

         fmtp-value = fmt SP format-specific-params
         format-specific-params = byte-string
           ; Notes:
           ; - The format parameters are media type parameters and
           need to reflect their syntax.

   Example:

         a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600

   This attribute allows parameters that are specific to a particular
   format to be conveyed in a way that SDP does not 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.  At most one instance of this attribute is allowed
   for each format.

7.  Security Considerations

   SDP is frequently used with the Session Initiation Protocol [RFC3261]
   using the offer/answer model [RFC3264] to agree on parameters for
   unicast sessions.  When used in this manner, the security
   considerations of those protocols apply.

   SDP is a session description format that describes multimedia
   sessions.  Entities receiving and acting upon an SDP message SHOULD
   be aware that a session description cannot be trusted unless it has
   been obtained by an authenticated transport protocol from a known and
   trusted source.  Many different transport protocols may be used to
   distribute session descriptions, and the nature of the authentication
   will differ from transport to transport.  For some transports,
   security features are often not deployed.  In case a session
   description has not been obtained in a trusted manner, the endpoint
   SHOULD exercise care because, among other attacks, the media sessions
   received may not be the intended ones, the destination where media is
   sent to may not be the expected one, any of the parameters of the
   session may be incorrect, or the media security may be compromised.
   It is up to the endpoint to make a sensible decision taking into
   account the security risks of the application and the user
   preferences and may decide to ask the user whether or not to accept
   the session.

   One transport that can be used to distribute session descriptions is
   the SAP.  SAP provides both encryption and authentication mechanisms,
   but due to the nature of session announcements it is likely that



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   there are many occasions where the originator of a session
   announcement cannot be authenticated because the originator is
   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 receiver's 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
   the user's consent.  Thus, a session description arriving by session
   announcement, email, session invitation, or WWW page MUST NOT deliver
   the user into an interactive multimedia session unless the user has
   explicitly pre-authorised such action.  As it is not always simple to
   tell whether or not a session is interactive, applications that are
   unsure should assume sessions are interactive.

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

   In certain environments, it has become common for intermediary
   systems to intercept and analyse session descriptions contained
   within other signalling protocols.  This is done for a range of
   purposes, including but not limited to opening holes in firewalls to
   allow media streams to pass, or to mark, prioritize, or block traffic
   selectively.  In some cases, such intermediary systems may modify the
   session description, for example, to have the contents of the session
   description match NAT bindings dynamically created.  These behaviours
   are NOT RECOMMENDED unless the session description is conveyed in
   such a manner that allows the intermediary system to conduct proper
   checks to establish the authenticity of the session description, and
   the authority of its source to establish such communication sessions.
   SDP by itself does not include sufficient information to enable these
   checks: they depend on the encapsulating protocol (e.g., SIP or
   RTSP).



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   Use of the "k=" line 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.
   Moreover, the "k=" line provides no way to indicate or negotiate
   cryptographic key algorithms.  As it provides for only a single
   symmetric key, rather than separate keys for confidentiality and
   integrity, its utility is severely limited.  The use of the "k=" line
   is NOT RECOMMENDED, as discussed in Section 5.12.

8.  IANA Considerations

8.1.  The "application/sdp" Media Type

   One media type registration from [RFC4566] is to be updated, as
   defined below.



































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      To: ietf-types@iana.org
      Subject: Registration of media type "application/sdp"

      Type name: application

      Subtype name: sdp

      Required parameters: None.

      Optional parameters: None.

      Encoding considerations:
         SDP files are primarily UTF-8 format text. The "a=charset:"
         attribute may be used to signal the presence of other character
         sets in certain parts of an SDP file (see Section 6 of RFC
         XXXX).  Arbitrary binary content cannot be directly
         represented in SDP.

      Security considerations:
         See Section 7 of RFC XXXX.

      Interoperability considerations:
         See RFC XXXX.

      Published specification:
         See RFC XXXX.

      Applications which use this media type:
         Voice over IP, video teleconferencing, streaming media, instant
         messaging, among others.  See also Section 3 of RFC XXXX.

      Additional information:

      Magic number(s):   None.
      File extension(s): The extension ".sdp" is commonly used.
      Macintosh File Type Code(s): "sdp "

      Person & email address to contact for further information:
         IETF MMUSIC working group <mmusic@ietf.org>

      Intended usage: COMMON

      Author/Change controller:
         Authors of RFC XXXX
         IETF MMUSIC working group delegated from the IESG






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8.2.  Registration of Parameters

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

   The contact address for all parameters registered below is:

      IETF MMUSIC working group <mmusic@ietf.org>

8.2.1.  Media Types ("media")

   The set of media types is intended to be small and SHOULD NOT be
   extended except under rare circumstances.  The same rules should
   apply for media names as for top-level media types, and where
   possible the same name should be registered for SDP as for MIME.  For
   media other than existing top-level media types, a Standards Track
   RFC MUST be produced for a new top-level media type to be registered,
   and the registration MUST provide good justification why no existing
   media name is appropriate (the "Standards Action" policy of
   [RFC5226].

   This memo registers the media types "audio", "video", "text",
   "application", and "message".

   Note: The media types "control" and "data" were listed as valid in an
   early version of this specification (RFC 2327); however, their
   semantics were never fully specified and they are not widely used.
   These media types have been removed in this specification, although
   they still remain valid media type capabilities for a SIP user agent
   as defined in [RFC3840].  If these media types are considered useful
   in the future, a Standards Track RFC MUST be produced to document
   their use.  Until that is done, applications SHOULD NOT use these
   types and SHOULD NOT declare support for them in SIP capabilities
   declarations (even though they exist in the registry created by
   [RFC3840]).

8.2.2.  Transport Protocols ("proto")

   The "proto" field describes the transport protocol used.  This SHOULD
   reference a standards-track protocol RFC.  This memo registers three
   values: "RTP/AVP" is a reference to [RFC3550] used under the RTP
   Profile for Audio and Video Conferences with Minimal Control
   [RFC3551] running over UDP/IP, "RTP/SAVP" is a reference to the
   Secure Real-time Transport Protocol [RFC3711], and "udp" indicates an
   unspecified protocol over UDP.




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   If other RTP profiles are defined in the future, their "proto" name
   SHOULD be specified in the same manner.  For example, an RTP profile
   whose short name is "XYZ" would be denoted by a "proto" field of
   "RTP/XYZ".

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

8.2.3.  Media Formats ("fmt")

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

   RTP payload formats under the "RTP/AVP" and "RTP/SAVP" profiles MUST
   use the payload type number as their "fmt" value.  If the payload
   type number is dynamically assigned by this session description, an
   additional "rtpmap" attribute MUST be included to specify the format
   name and parameters as defined by the media type registration for the
   payload format.  It is RECOMMENDED that other RTP profiles that are
   registered (in combination with RTP) as SDP transport protocols
   specify the same rules for the "fmt" namespace.

   For the "udp" protocol, new formats SHOULD be registered.  Use of an
   existing media subtype for the format is encouraged.  If no media
   subtype exists, it is RECOMMENDED that a suitable one be registered
   through the IETF process [RFC6838] by production of, or reference to,
   a standards-track RFC that defines the transport protocol for the
   format.

   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.

8.2.4.  Attribute Names ("att-field")

8.2.4.1.  New Attributes

   Attribute field names ("att-field") MUST be registered with IANA and
   documented, because of noticeable issues due to conflicting
   attributes under the same name.  Unknown attributes in SDP are simply




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   ignored, but conflicting ones that fragment the protocol are a
   serious problem.

   New attribute registrations are accepted according to the
   "Specification Required" policy of [RFC5226], provided that the
   specification includes the following information:

   o  Contact Name.

   o  Contact Email Address.

   o  Attribute Name: The name of the attribute that will appear in
      SDP).  This MUST conform to the definition of <att-field>.

   o  Attribute Syntax: For a value attribute (see clause 5.13), an ABNF
      definition of the attribute value <att-value> syntax (See
      Section Section 9) MUST be provided.  The syntax MUST follow the
      rule form as per Section 2.2 of [RFC5234].  This SHALL define the
      allowable values that the attribute might take.  It MAY also
      define an extension method for the addition of future values.  For
      a property attribute, the ABNF definition is omitted as the
      property attribute takes no values.

   o  Attribute Semantics: For a value attribute, a semantic description
      of the values that the attribute might take MUST be provided.  The
      usage of a property attribute is described under purpose below.

   o  Attribute Value: The name of an ABNF syntax rule defining the
      syntax of the value.  Absence of a rule name indicates that the
      attribute takes no values.  Enclosing the rule name in "[" and "]"
      indicates that a value is optional.

   o  Usage Level: Usage level(s) of the attribute.  One or more of:
      session, media, source, dcsa, dcsa(subprotocol).  For a definition
      of source level attributes, see [RFC5576].  For a definition of
      dcsa attributes see: [I-D.ietf-mmusic-data-channel-sdpneg].

   o  Charset Dependent: Whether the attribute value is subject to the
      charset attribute or not (Yes/No).

   o  Purpose: An explanation of the purpose and usage of the attribute.

   o  O/A Procedures: Offer/Answer procedures as explained in [RFC3264].

   o  Mux Category: Indication of which multiplexing "category"
      [I-D.ietf-mmusic-sdp-mux-attributes] an attribute is associated
      with.




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   o  Reference: A reference to the specification defining the
      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 specific pieces
   of software in a manner that might inhibit interoperability.

   Submitters of registrations should also carefully choose the
   attribute usage level.  They should not choose only "session" when
   the attribute can have different values when media is disaggregated,
   i.e., when each m= section has its own IP address on a different
   endpoint.  In that case the attribute type chosen should be "session,
   media".  The default rule is that for all new SDP attributes that can
   occur both in session and media level, the media level overrides the
   session level.  When this is not the case for a new SDP attribute, it
   MUST be explicitly stated.

   IANA has registered the initial set of attribute names ("att-field"
   values), with definitions as in Section 6 of this memo (these
   definitions replace those in [RFC4566]).

8.2.4.2.  Updates to Existing Attributes

   Updated attribute registrations are accepted according to the
   "Specification Required" policy of [RFC5226], provided that the
   specification updating the attribute (for example, by adding a new
   value) considers the registration information items from
   Section Section 8.2.4.1 according to the following bullets:

   o  Contact Name: A name MUST be provided.

   o  Contact Email Address: An email address MUST be provided.

   o  Attribute Name: MUST be provided and MUST NOT be changed.
      Otherwise it is a new attribute.

   o  Attribute Syntax: The existing rule syntax with the syntax
      extensions MUST be provided if there is a change to the syntax.  A
      revision to an existing attribute usage MAY extend the syntax of
      an attribute, but MUST be backward compatible.




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   o  Attribute Semantics: A semantic description of new additional
      attributes values or a semantic extension of existing values.
      Existing attribute values semantics MUST only be extended in a
      backward compatible manner.

   o  Usage Level: Updates MAY only add additional levels.

   o  Charset Dependent: MUST NOT be changed.

   o  Purpose: MAY be extended according to the updated usage.

   o  O/A Procedures: MAY be updated in a backward compatible manner
      and/or it applies to a new usage level only.

   o  Mux Category: No change unless from TBD to another value.  It MAY
      also change if 'media' level is being added to the definition of
      an attribute that previously did not include it.

   o  Reference: A new reference MUST be provided.

   Items SHOULD be omitted if there is no impact to them as a result of
   the attribute update.

8.2.5.  Bandwidth Specifiers ("bwtype")

   A proliferation of bandwidth specifiers is strongly discouraged.

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

   IANA has registered the bandwidth specifiers "CT" and "AS" with
   definitions as in Section 5.8 of this memo (these definitions update
   those in [RFC4566]).

8.2.6.  Network Types ("nettype")

   New network types (the "nettype" field) MUST be registered with IANA
   if SDP needs to be used in the context of non-Internet environments.
   The registration is subject to the RFC Required - RFC publication
   policy of [RFC5226].  Although 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 telephone call into the Public Switched
   Telephone Network (PSTN).  The number of network types should be
   small and should be rarely extended.  A new network type cannot be



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   registered without registering at least one address type to be used
   with that network type.  A new network type registration MUST
   reference an RFC that gives details of the network type and address
   type(s) and specifies how and when they would be used.

   IANA has registered the network type "IN" to represent the Internet,
   with definition as in Sections 5.2 and 5.7 of this memo (these
   definitions update those in [RFC4566]).

8.2.7.  Address Types ("addrtype")

   New address types ("addrtype") MUST be registered with IANA.  The
   registration is subject to the RFC Required - RFC publication policy
   of [RFC5226].  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.  Address types are
   not expected to be registered frequently.

   IANA has registered the address types "IP4" and "IP6" with
   definitions as in Sections 5.2 and 5.7 of this memo (these
   definitions update those in [RFC4566]).

8.2.8.  Registration Procedure

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

   o  contact name, email address, and telephone number

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

   o  long-form name in English

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

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

   o  a reference to the specification for the registered name (this
      will typically be an RFC number)

   In the case of a new addrtype registration, the author has to check
   whether the new address type is usable with the existing network
   types.  If yes, the "nettype" registry MUST be updated accordingly.



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   In the case of a new nettype registration, the author MUST specify
   the usable address type(s).

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

8.3.  Encryption Key Access Methods

   The IANA previously maintained a table of SDP encryption key access
   method ("enckey") names.  This table is obsolete, since the "k=" line
   is not extensible.  New registrations MUST NOT be accepted.

8.4.  Reorganization of the nettype Registry

   This document adds a new column in the "nettype" registry with the
   title "Usable addrtype Values" and updates the "nettype" registry as
   follows:

   --------------------------------------------------------------------
   |Type      | SDP Name | Usable addrtype Values | Reference         |
   --------------------------------------------------------------------
   |nettype   | IN       | IP4, IP6               | [RFC4566]         |
   |nettype   | TN       | RFC2543                | [RFC2848]         |
   |nettype   | ATM      | NSAP, GWID, E164       | [RFC3108]         |
   |nettype   | PSTN     | E164                   | [RFC7195]         |
   --------------------------------------------------------------------

   Note that both [RFC7195] and [RFC3108] registered "E164" as an
   address type, although [RFC7195] mentions that the "E164" address
   type has a different context for ATM and PSTN networks.

8.5.  Reorganization of the att-field Registries

   This document combines all of the (currently) five "att-field"
   registries into one registry called "att-field" registry, and updates
   the columns to reflect the name, usage level(s), charset dependency
   and reference.  As such IANA is requested to create a new combined
   registry using the following columns:

   Name | Usage Level | Dependent on Charset? | Mux Category | Reference

   The "Name" column reflects the attribute name (as it will appear in
   the SDP).  The "Usage Level" column MUST indicate one or more of the
   following: session, media, source, dcsa and dcsa(subprotocol).  The
   "Dependent on Charset?" column MUST indicate "Yes" or "No" depending
   on whether the attribute value is subject to the charset attribute.
   The "Mux Category" column MUST indicate one of the following
   categories: NORMAL, NOT RECOMMENDED, IDENTICAL, SUM, TRANSPORT,



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   INHERIT, IDENTICAL-PER-PT, SPECIAL or TBD as defined by
   [I-D.ietf-mmusic-sdp-mux-attributes].  Finally, the "Reference"
   column indicates the specification(s) where the attribute is defined.

   For example, the attribute "setup" which is defined for both session
   and media level, will be listed in the new registry as follows:

Name  | Usage Level    | Dependent on Charset?|Mux Category| Reference |
setup | session,media, | No                   |IDENTICAL   | [RFC4145] |
      | dcsa,dcsa(msrp)|                      |            | [RFC6135] |
      |                |                      |            | [I-D.mmusic
      |                |                      |            |-msrp-usage-
      |                |                      |            |data-channel
      |                |                      |            |]          |

9.  SDP Grammar

   This section provides an Augmented BNF grammar for SDP.  ABNF is
   defined in [RFC5234] and [RFC7405].

   ; SDP Syntax
   session-description = proto-version
                         origin-field
                         session-name-field
                         [information-field]
                         [uri-field]
                         *email-fields
                         *phone-fields
                         [connection-field]
                         *bandwidth-fields
                         1*time-fields
                         [key-field]
                         *attribute-fields
                         *media-descriptions

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

   origin-field =       %s"o" "=" username SP sess-id SP sess-version SP
                            nettype SP addrtype SP unicast-address CRLF

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

   information-field =   %s"i" "=" text CRLF

   uri-field =           %s"u" "=" uri CRLF

   email-fields =        %s"e" "=" email-address CRLF



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   phone-fields =        %s"p" "=" phone-number CRLF

   connection-field =    %s"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 =    %s"b" "=" bwtype ":" bandwidth CRLF

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

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

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

   key-field =           %s"k" "=" key-type CRLF

   attribute-fields =    %s"a" "=" attribute CRLF

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

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

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

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

   sess-version =        1*DIGIT

   nettype =             token
                         ;typically "IN"

   addrtype =            token



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                         ;typically "IP4" or "IP6"

   ; sub-rules of 'u='
   uri =                 URI-reference
                         ; see RFC 3986

   ; sub-rules of 'e=', see RFC 5322 for definitions
   email-address        = address-and-comment / dispname-and-address
                          / addr-spec
   address-and-comment  = addr-spec 1*SP "(" 1*email-safe ")"
   dispname-and-address = 1*email-safe 1*SP "<" addr-spec ">"

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

   phone =               ["+"] DIGIT 1*(SP / "-" / DIGIT)

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

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

   bandwidth =           1*DIGIT

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

   stop-time =           time / "0"

   time =                POS-DIGIT 9*DIGIT
                         ; Decimal representation of NTP time in
                         ; seconds since 1900.  The representation
                         ; of NTP time is an unbounded length field
                         ; containing at least 10 digits.  Unlike the
                         ; 64-bit representation used elsewhere, time
                         ; in SDP does not wrap in the year 2036.

   ; 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 = %s"d" / %s"h" / %s"m" / %s"s"
   ; NOTE: These units are case-sensitive.




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   ; sub-rules of 'k='
   key-type =            %s"prompt"
                         %s"clear:"
                         %s"base64:"
                         %s"uri:"
                         ; NOTE: These names are case-sensitive.

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

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

   att-field =           token

   att-value =           byte-string

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

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

   proto  =              token *("/" token)
                         ;typically "RTP/AVP" or "udp"

   port =                1*DIGIT

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

   multicast-address =   IP4-multicast / IP6-multicast / FQDN
                         / extn-addr

   IP4-multicast =       m1 3( "." decimal-uchar )
                         "/" ttl [ "/" integer ]
                         ; IP4 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 =       IP6-address [ "/" integer ]



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                         ; IP6 address starting with FF

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

   FQDN =                4*(alpha-numeric / "-" / ".")
                         ; fully qualified domain name as specified
                         ; in RFC 1035 (and updates)

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

   b1 =                  decimal-uchar
                         ; less than "224"

   IP6-address =                                      6( h16 ":" ) ls32
                         /                       "::" 5( h16 ":" ) ls32
                         / [               h16 ] "::" 4( h16 ":" ) ls32
                         / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
                         / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
                         / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
                         / [ *4( h16 ":" ) h16 ] "::"              ls32
                         / [ *5( h16 ":" ) h16 ] "::"              h16
                         / [ *6( h16 ":" ) h16 ] "::"

   h16 =                 1*4HEXDIG

   ls32 =                ( h16 ":" h16 ) / IP4-address

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

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

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

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

    token-char =          ALPHA / DIGIT
                                 / "!" / "#" / "$" / "%" / "&"
                                 / "'" ; (single quote)
                                 / "*" / "+" / "-" / "." / "^" / "_"
                                 / "`" ; (Grave accent)
                                 / "{" / "|" / "}" / "~"



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   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

   zero-based-integer = "0" / integer

   non-zero-int-or-real = integer / non-zero-real

   non-zero-real = zero-based-integer "." *DIGIT POS-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 5234
   ; URI-reference: from RFC 3986
   ; addr-spec: from RFC 5322

10.  Summary of Changes from RFC 4566

   The ABNF rule for IP6-address has been corrected.  As a result, the
   ABNF rule for IP6-multicast has changed, and the (now unused) rules
   for hexpart, hexseq, and hex4 have been removed.

   IP4 unicast and multicast addresses in the example SDP descriptions
   have been revised per RFCs 5735 and 5771.

   Text in Section 5.2 has been revised to clarify the use of local
   addresses in case of ICE-like SDP extensions.

   Normative and informative references have been updated.

   The text regarding the session vs. media-level attribute usage has
   been clarified.





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   The case-insensitivity rules from RFC 4855 have been included in this
   document.

11.  Acknowledgements

   Many people in the IETF Multiparty Multimedia Session Control
   (MMUSIC) working group have made comments and suggestions
   contributing to this document.

12.  References

12.1.  Normative References

   [I-D.ietf-mmusic-data-channel-sdpneg]
              Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak, R.,
              and J. Marcon, "SDP-based Data Channel Negotiation",
              draft-ietf-mmusic-data-channel-sdpneg-12 (work in
              progress), March 2017.

   [I-D.ietf-mmusic-sdp-mux-attributes]
              Nandakumar, S., "A Framework for SDP Attributes when
              Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
              (work in progress), December 2016.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2978]  Freed, N. and J. Postel, "IANA Charset Registration
              Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
              October 2000, <http://www.rfc-editor.org/info/rfc2978>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <http://www.rfc-editor.org/info/rfc3629>.







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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <http://www.rfc-editor.org/info/rfc4566>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
              <http://www.rfc-editor.org/info/rfc5576>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <http://www.rfc-editor.org/info/rfc5646>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

12.2.  Informative References

   [ITU.H332.1998]
              International Telecommunication Union, "H.323 extended for
              loosely coupled conferences", ITU Recommendation H.332,
              September 1998.

   [RFC2327]  Handley, M. and V. Jacobson, "SDP: Session Description
              Protocol", RFC 2327, DOI 10.17487/RFC2327, April 1998,
              <http://www.rfc-editor.org/info/rfc2327>.




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   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
              October 2000, <http://www.rfc-editor.org/info/rfc2974>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <http://www.rfc-editor.org/info/rfc3261>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <http://www.rfc-editor.org/info/rfc3264>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <http://www.rfc-editor.org/info/rfc3550>.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
              <http://www.rfc-editor.org/info/rfc3551>.

   [RFC3556]  Casner, S., "Session Description Protocol (SDP) Bandwidth
              Modifiers for RTP Control Protocol (RTCP) Bandwidth",
              RFC 3556, DOI 10.17487/RFC3556, July 2003,
              <http://www.rfc-editor.org/info/rfc3556>.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              DOI 10.17487/RFC3605, October 2003,
              <http://www.rfc-editor.org/info/rfc3605>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <http://www.rfc-editor.org/info/rfc3711>.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840,
              DOI 10.17487/RFC3840, August 2004,
              <http://www.rfc-editor.org/info/rfc3840>.






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   [RFC3890]  Westerlund, M., "A Transport Independent Bandwidth
              Modifier for the Session Description Protocol (SDP)",
              RFC 3890, DOI 10.17487/RFC3890, September 2004,
              <http://www.rfc-editor.org/info/rfc3890>.

   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, DOI 10.17487/RFC4567, July
              2006, <http://www.rfc-editor.org/info/rfc4567>.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
              <http://www.rfc-editor.org/info/rfc4568>.

   [RFC4855]  Casner, S., "Media Type Registration of RTP Payload
              Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
              <http://www.rfc-editor.org/info/rfc4855>.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <http://www.rfc-editor.org/info/rfc5245>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <http://www.rfc-editor.org/info/rfc5322>.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,
              <http://www.rfc-editor.org/info/rfc5888>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

   [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
              "TCP Candidates with Interactive Connectivity
              Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
              March 2012, <http://www.rfc-editor.org/info/rfc6544>.







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   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <http://www.rfc-editor.org/info/rfc7405>.

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <http://www.rfc-editor.org/info/rfc7656>.

   [RFC7826]  Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
              and M. Stiemerling, Ed., "Real-Time Streaming Protocol
              Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December
              2016, <http://www.rfc-editor.org/info/rfc7826>.

Authors' Addresses

   Mark Handley
   University College London
   Department of Computer Science
   London  WC1E 6BT
   UK

   EMail: M.Handley@cs.ucl.ac.uk


   Van Jacobson
   USA

   EMail: van@parc.com


   Colin Perkins
   University of Glasgow
   School of Computing Science
   University of Glasgow
   Glasgow  G12 8QQ
   UK

   EMail: csp@csperkins.org






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   Ali Begen
   Networked Media
   Konya
   Turkey

   EMail: ali.begen@networked.media













































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