path: root/doc/rfc/rfc8866.txt

Internet Engineering Task Force (IETF)                          A. Begen
Request for Comments: 8866                               Networked Media
Obsoletes: 4566                                               P. Kyzivat
Category: Standards Track                                               
ISSN: 2070-1721                                               C. Perkins
                                                   University of Glasgow
                                                              M. Handley
                                                                     UCL
                                                            January 2021


                   SDP: Session Description Protocol

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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8866.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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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   than English.

Table of Contents

   1.  Introduction
   2.  Glossary of Terms
   3.  Examples of SDP Usage
     3.1.  Session Initiation
     3.2.  Streaming Media
     3.3.  Email and the World Wide Web
     3.4.  Multicast Session Announcement
   4.  Requirements and Recommendations
     4.1.  Media and Transport Information
     4.2.  Timing Information
     4.3.  Obtaining Further Information about a Session
     4.4.  Internationalization
   5.  SDP Specification
     5.1.  Protocol Version ("v=")
     5.2.  Origin ("o=")
     5.3.  Session Name ("s=")
     5.4.  Session Information ("i=")
     5.5.  URI ("u=")
     5.6.  Email Address and Phone Number ("e=" and "p=")
     5.7.  Connection Information ("c=")
     5.8.  Bandwidth Information ("b=")
     5.9.  Time Active ("t=")
     5.10. Repeat Times ("r=")
     5.11. Time Zone Adjustment ("z=")
     5.12. Encryption Keys ("k=")
     5.13. Attributes ("a=")
     5.14. Media Descriptions ("m=")
   6.  SDP Attributes
     6.1.  cat (Category)
     6.2.  keywds (Keywords)
     6.3.  tool
     6.4.  ptime (Packet Time)
     6.5.  maxptime (Maximum Packet Time)
     6.6.  rtpmap
     6.7.  Media Direction Attributes
       6.7.1.  recvonly (Receive-Only)
       6.7.2.  sendrecv (Send-Receive)
       6.7.3.  sendonly (Send-Only)
       6.7.4.  inactive
     6.8.  orient (Orientation)
     6.9.  type (Conference Type)
     6.10. charset (Character Set)
     6.11. sdplang (SDP Language)
     6.12. lang (Language)
     6.13. framerate (Frame Rate)
     6.14. quality
     6.15. fmtp (Format Parameters)
   7.  Security Considerations
   8.  IANA Considerations
     8.1.  The "application/sdp" Media Type
     8.2.  Registration of SDP Parameters with IANA
       8.2.1.  Registration Procedure
       8.2.2.  Media Types (<media>)
       8.2.3.  Transport Protocols (<proto>)
       8.2.4.  Attribute Names (<attribute-name>)
       8.2.5.  Bandwidth Specifiers (<bwtype>)
       8.2.6.  Network Types (<nettype>)
       8.2.7.  Address Types (<addrtype>)
     8.3.  Encryption Key Access Methods (OBSOLETE)
   9.  SDP Grammar
   10. Summary of Changes from RFC 4566
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgements
   Authors' Addresses

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.

   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 [RFC5322] using
   the MIME extensions [RFC2045], and the Hypertext Transport Protocol
   (HTTP) [RFC7230].

   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
   outlined in Section 10 of this memo.

2.  Glossary of Terms

   The following terms are used in this document and have 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.

   Media Description:  A Media Description contains the information
      needed for one party to establish an application-layer network
      protocol connection to another party.  It starts with an "m=" line
      and is terminated by either the next "m=" line or by the end of
      the session description.

   Session-Level Section:  This refers to the parts that are not media
      descriptions, whereas the session description refers to the whole
      body that includes the session-level section and the media
      description(s).

   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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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 [RFC6838].  These session
   descriptions are commonly formatted using SDP.  When used with SIP,
   the offer/answer 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 descriptions of multicast sessions sent only via email or
   the WWW do not have the property that the receiver of a session
   description 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 possibly 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
   with Internet protocols, 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:

   *  Session name and purpose

   *  Time(s) the session is active

   *  The media comprising the session

   *  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:

   *  Information about the bandwidth to be used by the session

   *  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
   nonparticipants 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:

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

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

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

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

   *  The multicast group address for media

   *  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:

   *  The remote address for media

   *  The remote transport port for media

   The semantics of the address and port depend on context.  Typically,
   this SHOULD be the remote address and remote port to which media is
   to be sent or received.  Details may differ based on the network
   type, address type, protocol, and media specified, and whether the
   SDP is being distributed as an advertisement or negotiated in an
   offer/answer [RFC3264] exchange.  (E.g., Some address types or
   protocols may not have a notion of port.)  Deviating from typical
   behavior should be done cautiously since this 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:

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

   *  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) [RFC3986] for more information about the session.  (Note that
   use of URIs to indicate remote resources is subject to the security
   considerations from [RFC3986].)

4.4.  Internationalization

   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.8859-1.1998] to be
   used when desired.  Internationalization only applies to free-text
   subfields (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 [RFC3629], 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 (Section 6.10).  Field and attribute
   values that use the full UTF-8 character set are never directly
   compared, hence there is no requirement for UTF-8 normalization.  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 descriptions 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 descriptions that could be detected
   easily and discarded.

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

      <type>=<value>

   where <type> is exactly one case-significant character and <value> is
   structured text whose format depends on <type>.  In general, <value>
   is either a number of subfields delimited by a single space character
   or a free format string, and is case-significant unless a specific
   field defines otherwise.  Whitespace separators are not used on
   either side of the "=" sign, however, the value can contain a leading
   whitespace as part of its syntax, i.e., that whitespace is part of
   the value.

   An SDP description MUST conform to the syntax defined in Section 9.
   The following is an overview of the syntax.

   An SDP description consists of a session-level section followed by
   zero or more media descriptions.  The session-level section starts
   with a "v=" line and continues to the first media description (or the
   end of the whole description, whichever comes first).  Each media
   description starts with an "m=" line and continues to the next media
   description 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 when present, they must appear in exactly the order given here.
   (The fixed order greatly enhances error detection and allows for a
   simple parser).  In the following overview, optional items are marked
   with a "*".

      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=", "r=" and "z=" lines; see below)
         k=* (obsolete)
         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)
         z=* (optional time zone offset line)

      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=* (obsolete)
         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 or reject any
   session description that contains a type letter that it does not
   understand.  The attribute mechanism ("a=", described in
   Section 5.13) is the primary means for extending SDP and tailoring it
   to particular applications or media.  Some attributes (the ones
   listed in Section 6) have a defined meaning, but others may be added
   on a media- or session-specific basis.  (Attribute scopes in addition
   to media-specific and session-specific scopes may also be defined in
   extensions to this document, e.g., [RFC5576] and [RFC8864].)  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 descriptions of that session unless
   overridden by connection information in the media description.  For
   instance, in the example below, each audio media description behaves
   as if it were given a "c=IN IP4 198.51.100.1".

   An example SDP description is:

         v=0
         o=jdoe 3724394400 3724394405 IN IP4 198.51.100.1
         s=Call to John Smith
         i=SDP Offer #1
         u=http://www.jdoe.example.com/home.html
         e=Jane Doe <jane@jdoe.example.com>
         p=+1 617 555-6011
         c=IN IP4 198.51.100.1
         t=0 0
         m=audio 49170 RTP/AVP 0
         m=audio 49180 RTP/AVP 0
         m=video 51372 RTP/AVP 99
         c=IN IP6 2001:db8::2
         a=rtpmap:99 h263-1998/90000

   Text-containing fields such as the session-name-field and
   information-field 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 line,
   although parsers SHOULD be tolerant and also accept lines 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.  When the "a=charset:"
   attribute is present the session-name-field, information-field, and
   some attribute fields are interpreted according to the selected
   character set.

   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] and [RFC1035].  Internationalized 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 internationalized domain names).

5.1.  Protocol Version ("v=")

         v=0

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

5.2.  Origin ("o=")

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

   The "o=" line (origin-field) 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 a timestamp, in
      seconds since January 1, 1900 UTC, is recommended to ensure
      uniqueness.

   <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 description.
      Again, as with <sess-id> it is RECOMMENDED that a timestamp be
      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 address of the machine represented as
      specified in Section 4 of [RFC5952].  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.

   In general, the "o=" line serves as a globally unique identifier for
   this version of the 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 (session-name-field) is the textual session name.
   There MUST be one and only one "s=" line per session description.
   The "s=" line MUST NOT be empty.  If a session has no meaningful
   name, then "s= " or "s=-" (i.e., a single space or dash as the
   session name) is RECOMMENDED.  If a session-level "a=charset:"
   attribute is present, it specifies the character set used in the "s="
   field.  If a session-level "a=charset:" attribute is not present, the
   "s=" field MUST contain ISO 10646 characters in UTF-8 encoding.

5.4.  Session Information ("i=")

      i=<session information>

   The "i=" line (information-field) provides textual information about
   the session.  There can be at most one session-level "i=" line per
   session description, and at most one "i=" line in each media
   description.  Unless a media-level "i=" line is provided, 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.

   At most one "i=" line can be used for each media description.  In
   media definitions, "i=" lines are primarily intended for labeling
   media streams.  As such, they are most likely to be useful when a
   single session has more than one distinct media stream of the same
   media type.  An example would be two different whiteboards, one for
   slides and one for feedback and questions.

   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.

5.5.  URI ("u=")

      u=<uri>

   The "u=" line (uri-field) provides a URI (Uniform Resource
   Identifier) [RFC3986].  The URI should be a pointer to additional
   human readable information about the session.  This line is OPTIONAL.
   No more than one "u=" line is allowed per session description.

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

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

   The "e=" line (email-field) and "p=" line (phone-field) 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 description.  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 [E164])
   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.

5.7.  Connection Information ("c=")

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

   The "c=" line (connection-field) contains information necessary to
   establish a network connection.

   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 media-
   level "c=" line(s) per-media-description, in which case the media-
   level values override the session-level settings for the respective
   media.

   The first subfield (<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 subfield (<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 subfield (<connection-address>) is the connection address.
   Additional subfields MAY be added after the connection address
   depending on the value of the <addrtype> subfield.

   When the <addrtype> is "IP4" or "IP6", the connection address is
   defined as follows:

   *  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, data relay, or data sink as determined by
      additional attribute-fields (Section 5.13).  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.

   *  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.

   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 IPv6
   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 with 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 IPv6 example would be:

      c=IN IP6 ff00::db8:0:101/3

   which is semantically equivalent to:

      c=IN IP6 ff00::db8:0:101
      c=IN IP6 ff00::db8:0:102
      c=IN IP6 ff00::db8:0:103

   (remember that the TTL subfield is not present in "IP6" multicast).

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

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

5.8.  Bandwidth Information ("b=")

      b=<bwtype>:<bandwidth>

   The OPTIONAL "b=" line (bandwidth-field) denotes the proposed
   bandwidth to be used by the session or media description.  The
   <bwtype> is an alphanumeric modifier that provides the meaning of the
   <bandwidth> number.  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 [RFC8843] 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 a
      "b=CT:" line gives a total bandwidth figure for all the media at
      all endpoints.

      The Mux Category for "b=CT:" is NORMAL.  This is discussed in
      [RFC8859].

   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, the
      "b=AS:" line gives the RTP "session bandwidth" as defined in
      Section 6.2 of [RFC3550].  Note that a "b=AS:" line gives a
      bandwidth figure for a single media at a single endpoint, although
      there may be many endpoints sending simultaneously.

      The Mux Category for "b=AS:" is SUM.  This is discussed in
      [RFC8859].

   [RFC4566] defined an "X-" prefix for <bwtype> names.  This was
   intended for experimental purposes only.  For example:

      b=X-YZ:128

   Use of the "X-" prefix is NOT RECOMMENDED.  Instead new (non "X-"
   prefix) <bwtype> names SHOULD be defined, and then MUST be registered
   with IANA in the standard namespace.  SDP parsers MUST ignore
   bandwidth-fields with unknown <bwtype> names.  The <bwtype> names
   MUST be alphanumeric and, although no length limit is given, it is
   recommended that they be short.

   The <bandwidth> is interpreted as kilobits per second by default
   (including the transport and network-layer, but not the link-layer,
   overhead).  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.  Time Active ("t=")

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

   A "t=" line (time-field) begins a time description that specifies the
   start and stop times for a session.  Multiple time descriptions MAY
   be used if a session is active at multiple irregularly spaced times;
   each additional time description specifies additional periods of time
   for which the session will be active.  If the session is active at
   regular repeat times, a repeat description, begun by an "r=" line
   (see Section 5.10) can be included following the time-field -- in
   which case the time-field specifies the start and stop times of the
   entire repeat sequence.

   The following example specifies two active intervals:

      t=3724394400 3724398000 ; Mon 8-Jan-2018 10:00-11:00 UTC
      t=3724484400 3724488000 ; Tue 9-Jan-2018 11:00-12:00 UTC

   The first and second subfields of the time-field give the start and
   stop times, respectively, for the session.  These are the decimal
   representation of time values in seconds since January 1, 1900 UTC.
   To convert these values to Unix time (UTC), subtract decimal
   2208988800.

   Some time representations will wrap in the year 2036.  Because SDP
   uses an arbitrary length decimal representation, it does not have
   this issue.  Implementations of SDP need to be prepared to handle
   these larger values.

   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 behavior other
   than this is required, a <stop-time> SHOULD be given and modified as
   appropriate when new information becomes available about when the
   session should really end.

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

5.10.  Repeat Times ("r=")

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

   An"r=" line (repeat-field) specifies repeat times for a session.  If
   needed to express complex schedules, multiple repeat-fields may be
   included.  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
   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 representation of the end of the last session three
   months later.  By default, all subfields are in seconds, so the "r="
   and "t=" lines might be the following:

      t=3724394400 3730536000 ; Mon 8-Jan-2018 10:00-11:00 UTC
                              ; Tues 20-Mar-2018 12:00 UTC
      r=604800 3600 0 90000   ; 1 week, 1 hour, zero, 25 hours

   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) |
                +---+------------------------------------+

                     Table 1: Time Unit Specification
                                Characters

   Thus, the above repeat-field 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 time-descriptions should be used
   to explicitly list the session times.

5.11.  Time Zone Adjustment ("z=")

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

   A "z=" line (zone-field) is an optional modifier to the repeat-fields
   it immediately follows.  It does not apply to any other fields.

   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
   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 time (represented as
   seconds since January 1, 1900 UTC) 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:

      t=3724394400 3754123200       ; Mon 8-Jan-2018 10:00 UTC
                                    ; Tues 18-Dec-2018 12:00 UTC
      r=604800 3600 0 90000         ; 1 week, 1 hour, zero, 25 hours
      z=3730928400 -1h 3749680800 0 ; Sun 25-Mar-2018 1:00 UTC,
                                    ; offset 1 hour,
                                    ; Sun 28-Oct-2018 2:00 UTC,
                                    ; no offset

   This specifies that at time 3730928400 (Sun 25-Mar-2018 1:00 UTC, the
   onset of British Summer Time) the time base by which the session's
   repeat times are calculated is shifted back by 1 hour, and that at
   time 3749680800 (Sun 28-Oct-2018 2:00 UTC, the end of British Summer
   Time) the session's original time base is restored.  Adjustments are
   always relative to the specified start time -- they are not
   cumulative.

   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>

   The "k=" line (key-field) is obsolete and MUST NOT be used.  It is
   included in this document for legacy reasons.  One MUST NOT include a
   "k=" line in an SDP, and MUST discard it if it is received in an SDP.

5.13.  Attributes ("a=")

      a=<attribute-name>
      a=<attribute-name>:<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.  (Attribute scopes in addition to media-level
   and session-level scopes may also be defined in extensions to this
   document, e.g., [RFC5576] and [RFC8864].)

   A media description may contain any number of "a=" lines (attribute-
   fields) that are media description specific.  These are referred to
   as media-level attributes and add information about the media
   description.  Attribute-fields can also be added before the first
   media description; these session-level attributes convey additional
   information that applies to the session as a whole rather than to
   individual media descriptions.

   Attribute-fields may be of two forms:

   *  A property attribute is simply of the form "a=<attribute-name>".
      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".

   *  A value attribute is of the form "a=<attribute-name>:<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 "a=charset:" attribute as this would make
   comparisons against known values problematic.  However, when an
   attribute is defined, it can be defined to be charset dependent, in
   which case 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 (media-field) and is
   terminated by either the next "m=" line or by the end of the session
   description.  A media-field has several subfields:

   <media>  is the media type.  This document defines the values
      "audio", "video", "text", "application", and "message".  This list
      is extended by other memos and may be further extended by
      additional memos registering media types in the future (see
      Section 8).  For example, [RFC6466] defined the "image" media
      type.

   <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> subfield 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, the "a=rtcp:" attribute as defined in [RFC3605]).

      If noncontiguous 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:" attribute 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
      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 the description of
      <fmt> below).

      This document does not include a mechanism for declaring
      hierarchically encoded streams using noncontiguous ports.  (There
      is currently no attribute defined that can accomplish this.  The
      "a=rtcp:" attribute defined in [RFC3605] does not handle
      hierarchical encoding.)  If a need arises to declare noncontiguous
      ports then it will be necessary to define a new attribute to do
      so.

      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:

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

      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 mapping is similar if multiple addresses are specified using
      multiple "c=" lines.  For example:

             m=video 49170/2 RTP/AVP 31
             c=IN IP6 ff00::db8:0:101
             c=IN IP6 ff00::db8:0:102

      would imply that address ff00::db8:0:101 is used with ports 49170
      and 49171, and address ff00::db8:0:102 is used with ports 49172
      and 49173.

      This document gives no meaning to assigning the same media address
      to multiple media descriptions.  Doing so does not implicitly
      group those media descriptions in any way.  An explicit grouping
      framework (for example, [RFC5888]) should instead be used to
      express the intended semantics.  For instance, see [RFC8843].

   <proto>  is the transport protocol.  The meaning of the transport
      protocol is dependent on the address type subfield in the relevant
      "c=" line.  Thus a "c=" line with an address type of "IP4"
      indicates that the transport protocol runs over IPv4.  The
      following transport protocols are defined, but may be extended
      through registration of new protocols with IANA (see Section 8):

      *  udp: denotes that the data is transported directly in UDP with
         no additional framing.

      *  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.

      *  RTP/SAVPF: denotes SRTP with the Extended SRTP Profile for
         RTCP-Based Feedback [RFC5124] 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 (MBone's
      popular multimedia audio tool) Pulse Code 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
      subfields describe the format of the media.  The interpretation of
      the media format depends on the value of the <proto> subfield.

      If the <proto> subfield is "RTP/AVP" or "RTP/SAVP", the <fmt>
      subfields 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, and these payload
      formats are listed in order of preference, with the first format
      listed being preferred.  When multiple payload formats are listed,
      the first acceptable payload format from the beginning of the list
      SHOULD be used for the session.  For dynamic payload type
      assignments, the "a=rtpmap:" attribute (see Section 6.6) 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 6.15).

      If the <proto> subfield is "udp", the <fmt> subfields 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> subfield is
      protocol specific.  Rules for interpretation of the <fmt> subfield
      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.  Syntax is provided using ABNF [RFC7405] with some of
   the rules defined further in Section 9.

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 obsolete and SHOULD NOT be used.  It SHOULD be ignored
   if received.

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 "a=cat:" attribute, this was intended to assist identifying
   wanted sessions at the receiver, and to allow a receiver to select
   interesting sessions based on keywords describing the purpose of the
   session; however, there is no central registry of keywords.  Its
   value should be interpreted in the charset specified for the session
   description if one is specified, or by default in ISO 10646/UTF-8.
   This attribute is obsolete and SHOULD NOT be used.  It SHOULD be
   ignored if received.

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 "a=ptime:" to decode RTP or vat audio, and
   it is intended as a recommendation for the encoding/packetization of
   audio.

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 implementations that have not been updated will ignore
   this attribute.

6.6.  rtpmap

   Name:  rtpmap

   Value:  rtpmap-value

   Usage Level:  media

   Charset Dependent:  no

   Syntax:

         rtpmap-value = payload-type SP encoding-name
           "/" clock-rate [ "/" encoding-params ]
         payload-type = zero-based-integer
         encoding-name = token
         clock-rate = integer
         encoding-params = channels
         channels = integer

   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 inclusively 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 encoded 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

   Up to one "a=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-params 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 [RFC3551]) packetization is required, the
   "a=ptime:" attribute is used as given in Section 6.4.

6.7.  Media Direction Attributes

   At most one occurrence of "a=recvonly", "a=sendrecv", "a=sendonly",
   or "a=inactive" MAY appear at session level, and at most one MAY
   appear in each media description.

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

   If none of the media direction attributes is present at either
   session level or media level, "a=sendrecv" SHOULD be assumed as the
   default.

   Within the following SDP example, the "a=sendrecv" attribute applies
   to the first audio media and the "a=inactive" attribute applies to
   the others.

         v=0
         o=jdoe 3724395000 3724395001 IN IP6 2001:db8::1
         s=-
         c=IN IP6 2001:db8::1
         t=0 0
         a=inactive
         m=audio 49170 RTP/AVP 0
         a=sendrecv
         m=audio 49180 RTP/AVP 0
         m=video 51372 RTP/AVP 99
         a=rtpmap:99 h263-1998/90000

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 receive-only mode applies to the media
   only, not to any associated control protocol.  An RTP-based system in
   receive-only mode MUST still send RTCP packets as described in
   [RFC3550], Section 6.

6.7.2.  sendrecv (Send-Receive)

   Name:  sendrecv

   Value:

   Usage Level:  session, media

   Charset Dependent:  no

   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 in send-only mode and one in
   receive-vonly mode.  Note that send-only mode 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
   stream.  Note that an RTP-based system MUST still send RTCP (if RTCP
   is used), even if started in inactive mode.

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

   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.  Allowed values
   are "broadcast", "meeting", "moderated", "test", and "H332".  These
   values have implications for other options that are likely to be
   appropriate:

   *  When "a=type:broadcast" is specified, "a=recvonly" is probably
      appropriate for those connecting.

   *  When "a=type:meeting" is specified, "a=sendrecv" is likely to be
      appropriate.

   *  "a=type:moderated" suggests the use of a floor control tool and
      that the media tools be started so as to mute new sites joining
      the multimedia conference.

   *  Specifying "a=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
      using "a=recvonly".

   *  Specifying "a=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 = <defined in [RFC2978]>

   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 string
   that 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 recognized or not supported,
   all strings that are affected by it SHOULD be regarded as octet
   strings.

   Charset-dependent fields MUST contain only sequences of bytes that
   are valid according to the definition of the selected character set.
   Furthermore, charset-dependent fields MUST NOT contain the bytes 0x00
   (Nul), 0x0A (LF), and 0x0d (CR).

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 RFC 5646

   Example:

         a=sdplang:fr

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

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

   The "a=sdplang:" attribute value must be a single language tag
   [RFC5646].  An "a=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 RFC 5646

   Example:

         a=lang:de

   Multiple "a=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, "a=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.

   The "a=lang:" attribute value must be a single [RFC5646] language
   tag.  An "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 "a=lang:" attribute is supposed to be used for setting 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 "a=lang:" attribute is
   specified, the "a=lang:" attribute itself does not provide any
   information about multiple languages being intended to be used during
   the session, or if the intention is to only select one of the
   languages.  If needed, a new attribute can be defined and used to
   indicate such intentions.  Without such semantics, 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.

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 behavior given no quality  |
              |    | suggestion.                            |
              +----+----------------------------------------+
              | 0  | the worst still-image quality the      |
              |    | codec designer thinks is still usable. |
              +----+----------------------------------------+

                      Table 2: Encoding Quality Values

6.15.  fmtp (Format Parameters)

   Name:  fmtp

   Value:  fmtp-value

   Usage Level:  media

   Charset Dependent:  no

   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, semicolon separated, 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.

   The "a=fmtp:" attribute may be used to specify parameters for any
   protocol and format that defines use of such parameters.

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 and integrity-protected 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 and integrity protection 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 to where the media is sent 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 - the
   endpoint may decide to ask the user whether or not to accept the
   session.

   On receiving a session description over an unauthenticated transport
   mechanism or from an untrusted party, software parsing the session
   description should take a few precautions.  Similar concerns apply if
   integrity protection is not in place.  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-authorized 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.  Software processing
   URLs contained in session descriptions should also heed the security
   considerations identified in [RFC3986].

   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 behavior for an unknown attribute is to
   ignore it.

   In certain environments, it has become common for intermediary
   systems to intercept and analyze session descriptions contained
   within other signaling 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 behaviors
   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).  The use of some procedures and SDP extensions (e.g.,
   Interactive Connectivity Establishment (ICE) [RFC8445] and ICE-SIP-
   SDP [RFC8839]) may avoid the need for intermediaries to modify SDP.

   SDP MUST NOT be used to convey keying material (e.g., using the
   "a=crypto:" attribute [RFC4568]) unless it can be guaranteed that the
   channel over which the SDP is delivered is both private and
   authenticated.

8.  IANA Considerations

8.1.  The "application/sdp" Media Type

   One media type registration from [RFC4566] has been updated, as
   defined below.

   Type name:  application

   Subtype name:  sdp

   Required parameters:  None.

   Optional parameters:  None.

   Encoding considerations:  8-bit text.  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 8866).  Arbitrary binary content cannot be
      directly represented in SDP.

   Security considerations:  See Section 7 of RFC 8866.

   Interoperability considerations:  See RFC 8866.

   Published specification:  See RFC 8866.

   Applications which use this media type:
      Voice over IP, video teleconferencing, streaming media, instant
      messaging, among others.  See also Section 3 of RFC 8866.

   Fragment identifier considerations:  None

   Additional information:

      Deprecated alias names for this type:  N/A
      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

   Restrictions on usage:  None

   Author/Change controller:
      Authors of RFC 8866
      IETF MMUSIC working group delegated from the IESG

8.2.  Registration of SDP Parameters with IANA

   This document specifies IANA parameter registries for six named SDP
   subfields.  Using the terminology in the SDP specification Augmented
   Backus-Naur Form (ABNF), they are <media>, <proto>, <attribute-name>,
   <bwtype>, <nettype>, and <addrtype>.

   This document also replaces and updates the definitions of all those
   parameters previously defined by [RFC4566].

   IANA has changed all references to RFC 4566 in these registries to
   instead refer to this document.

   The contact name and email address for all parameters registered in
   this document is:

      The IETF MMUSIC working group <mmusic@ietf.org> or its successor
      as designated by the IESG.

   All of these registries have a common format:

             +======+==========+================+===========+
             | Type | SDP Name | [other fields] | Reference |
             +======+==========+================+===========+

                Table 3: Common Format for SDP Registries

8.2.1.  Registration Procedure

   A specification document that defines values for SDP <media>,
   <proto>, <attribute-name>, <bwtype>, <nettype>, and <addrtype>
   parameters MUST include the following information:

   *  Contact name

   *  Contact email address

   *  Name being defined (as it will appear in SDP)

   *  Type of name (<media>, <proto>, <attribute-name>, <bwtype>,
      <nettype>, or <addrtype>)

   *  A description of the purpose of the defined name

   *  A stable reference to the document containing this information and
      the definition of the value.  (This will typically be an RFC
      number.)

   The subsections below specify what other information (if any) must be
   specified for particular parameters, and what other fields are to be
   included in the registry.

8.2.2.  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 well 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
   [RFC8126]).

   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 [RFC2327]; 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]).  Also note that [RFC6466] defined the "image" media type.

8.2.3.  Transport Protocols (<proto>)

   The <proto> subfield describes the transport protocol used.  The
   registration procedure for this registry is "RFC Required".

   This document registers two 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.

   *  "udp" indicates direct use of UDP.

   New transport protocols MAY be defined, and MUST 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.  The RFC defining a new
   protocol MUST define the rules by which the <fmt> (see below)
   namespace is managed.

   <proto> names starting with "RTP/" MUST only be used for defining
   transport protocols that are profiles of RTP.  For example, a profile
   whose short name is "XYZ" would be denoted by a <proto> subfield of
   "RTP/XYZ".

   Each transport protocol, defined by the <proto> subfield, 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 other "RTP/*" 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 "a=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, the allowed <fmt> values are media subtypes
   from the IANA Media Types registry.  The media type and subtype
   combination <media>/<fmt> specifies the format of the body of UDP
   packets.  Use of an existing media subtype for the format is
   encouraged.  If no suitable media subtype exists, it is RECOMMENDED
   that a new one be registered through the IETF process [RFC6838] by
   production of, or reference to, a Standards Track RFC that defines
   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 (<attribute-name>)

   Attribute-field names (<attribute-name>) MUST be registered with IANA
   and documented to avoid any issues due to conflicting attribute
   definitions under the same name.  (While unknown attributes in SDP
   are simply ignored, conflicting ones that fragment the protocol are a
   serious problem.)

   The format of the <attribute-name> registry is:

       +======+==========+=============+==============+===========+
       | Type | SDP Name | Usage Level | Mux Category | Reference |
       +======+==========+=============+==============+===========+

             Table 4: Format of the <attribute-name> Registry

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

   +===========+==========+================+==============+===========+
   | Type      | SDP Name | Usage Level    | Mux Category | Reference |
   +===========+==========+================+==============+===========+
   | attribute | lang     | session, media | TRANSPORT    | [RFC8866] |
   |           |          |                |              | [RFC8859] |
   +-----------+----------+----------------+--------------+-----------+

                Table 5: <attribute-name> Registry Example

   This one <attribute-name> registry combines all of the previous
   usage-level-specific "att-field" registries, including updates made
   by [RFC8859], and renames the "att-field" registry to the "attribute-
   name (formerly "att-field")" registry.  IANA has completed the
   necessary reformatting.

   Section 6 of this document replaces the initial set of attribute
   definitions made by [RFC4566].  IANA has updated the registry
   accordingly.

   Documents can define new attributes and can also extend the
   definitions of previously defined attributes.

8.2.4.1.  New Attributes

   New attribute registrations are accepted according to the
   "Specification Required" policy of [RFC8126], provided that the
   specification includes the following information:

   *  Contact name

   *  Contact email address

   *  Attribute name: the name of the attribute that will appear in SDP.
      This MUST conform to the definition of <attribute-name>.

   *  Attribute syntax: for a value attribute (see Section 5.13), an
      ABNF definition of the attribute value <attribute-value> syntax
      (see Section 9) MUST be provided.  The syntax MUST follow the rule
      form per Section 2.2 of [RFC5234] and [RFC7405].  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.

   *  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.

   *  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.

   *  Usage level: the usage level(s) of the attribute.  This MUST be
      one or more of the following: session, media, source, dcsa, and
      dcsa(subprotocol).  For a definition of source-level attributes,
      see [RFC5576].  For a definition of dcsa attributes see [RFC8864].

   *  Charset dependent: this MUST be "Yes" or "No" depending on whether
      the attribute value is subject to the "a=charset:" attribute.

   *  Purpose: an explanation of the purpose and usage of the attribute.

   *  O/A procedures: offer/answer procedures as explained in [RFC3264].

   *  Mux Category: this MUST indicate one of the following categories:
      NORMAL, NOT RECOMMENDED, IDENTICAL, SUM, TRANSPORT, INHERIT,
      IDENTICAL-PER-PT, SPECIAL, or TBD as defined by [RFC8859].

   *  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" or "media" (depending on desired semantics).  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 (<attribute-
   name> 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 [RFC8126].

   The Designated Expert reviewing the update is requested to evaluate
   whether the update is compatible with the prior intent and use of the
   attribute, and whether the new document is of sufficient maturity and
   authority in relation to the prior document.

   The specification updating the attribute (for example, by adding a
   new value) MUST update registration information items from
   Section 8.2.4.1 according to the following constraints:

   *  Contact name: a name for an entity responsible for the update MUST
      be provided.

   *  Contact email address: an email address for an entity responsible
      for the update MUST be provided.

   *  Attribute name: MUST be provided and MUST NOT be changed.
      Otherwise it is a new attribute.

   *  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.

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

   *  Usage level: updates MAY only add additional levels.

   *  Charset dependent: MUST NOT be changed.

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

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

   *  Mux Category: no change unless from "TBD" to another value (see
      [RFC8859].  It MAY also change if media level is being added to
      the definition of an attribute that previously did not include it.

   *  Reference: a new (additional or replacement) 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> subfield values) 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.

   The RFC MUST specify the Mux Category for this value as defined by
   [RFC8859].

   The format of the <bwtype> registry is:

              +======+==========+==============+===========+
              | Type | SDP Name | Mux Category | Reference |
              +======+==========+==============+===========+

                 Table 6: Format of the <bwtype> Registry

   IANA has updated the <bwtype> registry entries for the bandwidth
   specifiers "CT" and "AS" with the definitions in Section 5.8 of this
   memo (these definitions replace those in [RFC4566]).

8.2.6.  Network Types (<nettype>)

   Network type "IN", representing the Internet, is defined in
   Section 5.2 and Section 5.7 of this memo (this definition replaces
   that in [RFC4566]).

   To enable SDP to reference a new non-Internet environment, a new
   network type (<nettype> subfield value) MUST be registered with IANA.
   The registration is subject to the "RFC Required" policy of
   [RFC8126].  Although non-Internet environments 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
   registration MUST reference an RFC that gives details of the network
   type and the address type(s) that may be used with it.

   The format of the <nettype> registry is:

         +======+==========+========================+===========+
         | Type | SDP Name | Usable addrtype Values | Reference |
         +======+==========+========================+===========+

                Table 7: Format of the <nettype> Registry

   IANA has updated the <nettype> registry to this new format.  The
   following is the updated content of the registry:

        +=========+==========+========================+===========+
        | Type    | SDP Name | Usable addrtype Values | Reference |
        +=========+==========+========================+===========+
        | nettype | IN       | IP4, IP6               | [RFC8866] |
        +---------+----------+------------------------+-----------+
        | nettype | TN       | RFC2543                | [RFC2848] |
        +---------+----------+------------------------+-----------+
        | nettype | ATM      | NSAP, GWID, E164       | [RFC3108] |
        +---------+----------+------------------------+-----------+
        | nettype | PSTN     | E164                   | [RFC7195] |
        +---------+----------+------------------------+-----------+

                 Table 8: Content of the <nettype> registry

   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.

8.2.7.  Address Types (<addrtype>)

   New address types (<addrtype>) MUST be registered with IANA.  The
   registration is subject to the "RFC Required" policy of [RFC8126].  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.

   Section 5.7 of this document gives new definitions of address types
   "IP4" and "IP6".

8.3.  Encryption Key Access Methods (OBSOLETE)

   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.

9.  SDP Grammar

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

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

   version-field =       %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-field =         %s"e" "=" email-address CRLF

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

   time-description =    time-field
                             [repeat-description]

   repeat-description =  1*repeat-field
                             [zone-field]

   time-field =          %s"t" "=" start-time SP stop-time CRLF

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

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

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

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

   media-description =   media-field
                         [information-field]
                         *connection-field
                         *bandwidth-field
                         [key-field]
                         *attribute-field

   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
                         ;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 time in
                         ; seconds since January 1, 1900 UTC.
                         ; The representation is an unbounded
                         ; length field containing at least
                         ; 10 digits. Unlike some representations
                         ; 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.

   ; sub-rules of 'k='
   key-type =            %s"prompt" /
                         %s"clear:" text /
                         %s"base64:" base64 /
                         %s"uri:" 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 =           (attribute-name ":" attribute-value) /
                         attribute-name

   attribute-name =      token

   attribute-value =     byte-string

   att-field =           attribute-name ; for backward compatibility

   ; 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", "RTP/SAVP", "udp",
                         ;or "RTP/SAVPF"

   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 [ "/" numaddr ]
                         ; 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 [ "/" numaddr ]
                         ; IP6 address starting with FF

   numaddr =             integer

   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)
                                 / "{" / "|" / "}" / "~"

   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 =               <ALPHA definition from RFC 5234>
   DIGIT =               <DIGIT definition from RFC 5234>
   CRLF =                <CRLF definition from RFC 5234>
   HEXDIG =              <HEXDIG definition from RFC 5234>
   SP =                  <SP definition from RFC 5234>
   VCHAR =               <VCHAR definition from RFC 5234>
   URI-reference =       <URI-reference definition from RFC 3986>
   addr-spec =           <addr-spec definition from RFC 5322>

10.  Summary of Changes from RFC 4566

   *  Generally clarified and refined terminology.  Aligned terms used
      in text with the ABNF.  The terms <attribute>, <att-field>, and
      "att-field" are now <attribute-name>.  The terms <value> and <att-
      value> are now <attribute-value>.  The term "media" is now
      <media>.

   *  Identified now-obsolete items: "a=cat:" (Section 6.1), "a=keywds:"
      (Section 6.2), and "k=" (Section 5.12).

   *  Updated normative and informative references, and added references
      to additional relevant related RFCs.

   *  Reformatted the SDP Attributes section (Section 6) for
      readability.  The syntax of attribute values is now given in ABNF.

   *  Made mandatory the sending of RTCP with inactive media streams
      (Section 6.7.4).

   *  Removed the section "Private Sessions".  That section dated back
      to a time when the primary use of SDP was with SAP (Session
      Announcement Protocol), which has fallen out of use.  Now the vast
      majority of uses of SDP is for establishment of private sessions.
      The considerations for that are covered in Section 7.

   *  Expanded and clarified the specification of the "a=lang:"
      (Section 6.12) and "a=sdplang:" (Section 6.11) attributes.

   *  Removed some references to SAP because it is no longer in
      widespread use.

   *  Changed the way <fmt> values for UDP transport are registered
      (Section 8.2.3).

   *  Changed the mechanism and documentation required for registering
      new attributes (Section 8.2.4.1).

   *  Tightened up IANA registration procedures for extensions.  Removed
      phone number and long-form name (Section 8.2).

   *  Expanded the IANA <nettype> registry to identify valid <addrtype>
      subfields (Section 8.2.6).

   *  Reorganized the several IANA "att-field" registries into a single
      <attribute-name> registry (Section 8.2.4).

   *  Revised ABNF syntax (Section 9) for clarity and for alignment with
      text.  Backward compatibility is maintained with a few exceptions.
      Of particular note:

      -  Revised the syntax of time descriptions ("t=", "r=", "z=") to
         remove ambiguities.  Clarified that "z=" only modifies the
         immediately preceding "r=" lines.  Made "z=" without a
         preceding "r=" a syntax error (Section 5.11).  (This is
         incompatible with certain aberrant usage.)

      -  Updated the "IP6-address" and "IP6-multicast" rules, consistent
         with the syntax in [RFC3986], mirroring a bug fix made to
         [RFC3261] by [RFC5954].  Removed rules that were unused as a
         result of this change.

      -  The "att-field" rule has been renamed "attribute-name" because
         elsewhere "*-field" always refers to a complete line.  However,
         the rulename "att-field" remains defined as a synonym for
         backward compatibility with references from other RFCs.

      -  The "att-value" rule has been renamed "attribute-value".

   *  Revised normative statements that were redundant with ABNF syntax,
      making the text non-normative.

   *  Revised IPv4 unicast and multicast addresses in the example SDP
      descriptions per [RFC5735] and [RFC5771].

   *  Changed some examples to use IPv6 addresses, and added additional
      examples using IPv6.

   *  Incorporated case-insensitivity rules from [RFC4855].

   *  Revised sections that incorrectly referenced NTP (Section 5.2,
      Section 5.9, Section 5.10, and Section 5.11).

   *  Clarified the explanation of the impact and use of the
      "a=charset:" attribute (Section 6.10).

   *  Revised the description of the "a=type:" attribute to remove
      implication that it sometimes changes the default media direction
      to something other than "a=sendrecv" (Section 6.9).

11.  References

11.1.  Normative References

   [E164]     International Telecommunication Union, "E.164 : The
              international public telecommunication numbering plan",
              ITU Recommendation E.164, November 2010,
              <https://www.itu.int/rec/T-REC-E.164-201011-I/en>.

   [ISO.8859-1.1998]
              International Organization for Standardization,
              "Information technology - 8-bit single byte coded graphic
              - character sets - Part 1: Latin alphabet No. 1, JTC1/
              SC2", ISO/IEC Standard 8859-1, 1998.

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

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2848]  Petrack, S. and L. Conroy, "The PINT Service Protocol:
              Extensions to SIP and SDP for IP Access to Telephone Call
              Services", RFC 2848, DOI 10.17487/RFC2848, June 2000,
              <https://www.rfc-editor.org/info/rfc2848>.

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

   [RFC3108]  Kumar, R. and M. Mostafa, "Conventions for the use of the
              Session Description Protocol (SDP) for ATM Bearer
              Connections", RFC 3108, DOI 10.17487/RFC3108, May 2001,
              <https://www.rfc-editor.org/info/rfc3108>.

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

   [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,
              <https://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, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://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,
              <https://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, <https://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,
              <https://www.rfc-editor.org/info/rfc5890>.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,
              <https://www.rfc-editor.org/info/rfc5952>.

   [RFC7195]  Garcia-Martin, M. and S. Veikkolainen, "Session
              Description Protocol (SDP) Extension for Setting Audio and
              Video Media Streams over Circuit-Switched Bearers in the
              Public Switched Telephone Network (PSTN)", RFC 7195,
              DOI 10.17487/RFC7195, May 2014,
              <https://www.rfc-editor.org/info/rfc7195>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8859]  Nandakumar, S., "A Framework for Session Description
              Protocol (SDP) Attributes When Multiplexing", RFC 8859,
              DOI 10.17487/RFC8859, January 2021,
              <https://www.rfc-editor.org/info/rfc8859>.

   [RFC8864]  Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.
              Even, Ed., "Negotiation Data Channels Using the Session
              Description Protocol (SDP)", RFC 8864,
              DOI 10.17487/RFC8864, January 2021,
              <https://www.rfc-editor.org/info/rfc8864>.

11.2.  Informative References

   [ITU.H332.1998]
              International Telecommunication Union, "H.332 : H.323
              extended for loosely coupled conferences", ITU
              Recommendation H.332, September 1998,
              <https://www.itu.int/rec/T-REC-H.332-199809-I/en>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

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

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
              October 2000, <https://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,
              <https://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,
              <https://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, <https://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,
              <https://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,
              <https://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,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc3840>.

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

   [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,
              <https://www.rfc-editor.org/info/rfc4568>.

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

   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <https://www.rfc-editor.org/info/rfc5124>.

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

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              RFC 5735, DOI 10.17487/RFC5735, January 2010,
              <https://www.rfc-editor.org/info/rfc5735>.

   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
              DOI 10.17487/RFC5771, March 2010,
              <https://www.rfc-editor.org/info/rfc5771>.

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

   [RFC5954]  Gurbani, V., Ed., Carpenter, B., Ed., and B. Tate, Ed.,
              "Essential Correction for IPv6 ABNF and URI Comparison in
              RFC 3261", RFC 5954, DOI 10.17487/RFC5954, August 2010,
              <https://www.rfc-editor.org/info/rfc5954>.

   [RFC6466]  Salgueiro, G., "IANA Registration of the 'image' Media
              Type for the Session Description Protocol (SDP)",
              RFC 6466, DOI 10.17487/RFC6466, December 2011,
              <https://www.rfc-editor.org/info/rfc6466>.

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

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://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,
              <https://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, <https://www.rfc-editor.org/info/rfc7826>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8839]  Petit-Huguenin, M., Nandakumar, S., Holmberg, C., Keränen,
              A., and R. Shpount, "Session Description Protocol (SDP)
              Offer/Answer Procedures for Interactive Connectivity
              Establishment (ICE)", RFC 8839, DOI 10.17487/RFC8839,
              January 2021, <https://www.rfc-editor.org/info/rfc8839>.

   [RFC8843]  Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", RFC 8843,
              DOI 10.17487/RFC8843, January 2021,
              <https://www.rfc-editor.org/info/rfc8843>.

Acknowledgements

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

   In particular, we would like to thank the following people who
   contributed to the creation of this document or one of its
   predecessor documents: Adam Roach, Allison Mankin, Bernie Hoeneisen,
   Bill Fenner, Carsten Bormann, Eve Schooler, Flemming Andreasen,
   Gonzalo Camarillo, Jörg Ott, John Elwell, Jon Peterson, Jonathan
   Lennox, Jonathan Rosenberg, Keith Drage, Peter Parnes, Rob Lanphier,
   Ross Finlayson, Sean Olson, Spencer Dawkins, Steve Casner, Steve
   Hanna, Van Jacobson.

Authors' Addresses

   Ali Begen
   Networked Media
   Turkey

   Email: ali.begen@networked.media


   Paul Kyzivat
   United States of America

   Email: pkyzivat@alum.mit.edu


   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow
   G12 8QQ
   United Kingdom

   Email: csp@csperkins.org


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

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