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rfc:rfc4566

Network Working Group M. Handley Request for Comments: 4566 UCL Obsoletes: 2327, 3266 V. Jacobson Category: Standards Track Packet Design

                                                            C. Perkins
                                                 University of Glasgow
                                                             July 2006
                 SDP: Session Description Protocol

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

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.

Table of Contents

 1. Introduction ....................................................3
 2. Glossary of Terms ...............................................3
 3. Examples of SDP Usage ...........................................4
    3.1. Session Initiation .........................................4
    3.2. Streaming Media ............................................4
    3.3. Email and the World Wide Web ...............................4
    3.4. Multicast Session Announcement .............................4
 4. Requirements and Recommendations ................................5
    4.1. Media and Transport Information ............................6
    4.2. Timing Information .........................................6
    4.3. Private Sessions ...........................................7
    4.4. Obtaining Further Information about a Session ..............7
    4.5. Categorisation .............................................7
    4.6. Internationalisation .......................................7

Handley, et al. Standards Track [Page 1] RFC 4566 SDP July 2006

 5. SDP Specification ...............................................7
    5.1. Protocol Version ("v=") ...................................10
    5.2. Origin ("o=") .............................................11
    5.3. Session Name ("s=") .......................................12
    5.4. Session Information ("i=") ................................12
    5.5. URI ("u=") ................................................13
    5.6. Email Address and Phone Number ("e=" and "p=") ............13
    5.7. Connection Data ("c=") ....................................14
    5.8. Bandwidth ("b=") ..........................................16
    5.9. Timing ("t=") .............................................17
    5.10. Repeat Times ("r=") ......................................18
    5.11. Time Zones ("z=") ........................................19
    5.12. Encryption Keys ("k=") ...................................19
    5.13. Attributes ("a=") ........................................21
    5.14. Media Descriptions ("m=") ................................22
 6. SDP Attributes .................................................24
 7. Security Considerations ........................................31
 8. IANA Considerations ............................................33
    8.1. The "application/sdp" Media Type ..........................33
    8.2. Registration of Parameters ................................34
         8.2.1. Media Types ("media") ..............................34
         8.2.2. Transport Protocols ("proto") ......................34
         8.2.3. Media Formats ("fmt") ..............................35
         8.2.4. Attribute Names ("att-field") ......................36
         8.2.5. Bandwidth Specifiers ("bwtype") ....................37
         8.2.6. Network Types ("nettype") ..........................37
         8.2.7. Address Types ("addrtype") .........................38
         8.2.8. Registration Procedure .............................38
    8.3. Encryption Key Access Methods .............................39
 9. SDP Grammar ....................................................39
 10. Summary of Changes from RFC 2327 ..............................44
 11. Acknowledgements ..............................................45
 12. References ....................................................45
    12.1. Normative References .....................................45
    12.2. Informative References ...................................46

Handley, et al. Standards Track [Page 2] RFC 4566 SDP July 2006

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 [14],
 Session Initiation Protocol [15], Real Time Streaming Protocol [16],
 electronic mail using the MIME extensions, and the Hypertext
 Transport Protocol.
 SDP is intended to be general purpose so that it can be used in a
 wide range of network environments and applications.  However, it is
 not intended to support negotiation of session content or media
 encodings: this is viewed as outside the scope of session
 description.
 This memo obsoletes RFC 2327 [6] and RFC 3266 [10].  Section 10
 outlines the changes introduced in this memo.

2. Glossary of Terms

 The following terms are used in this document and have specific
 meaning within the context of this document.
 Conference: A multimedia conference is a set of two or more
    communicating users along with the software they are using to
    communicate.
 Session: A multimedia session is a set of multimedia senders and
    receivers and the data streams flowing from senders to receivers.
    A multimedia conference is an example of a multimedia session.
 Session Description: A well-defined format for conveying sufficient
    information to discover and participate in a multimedia session.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [3].

Handley, et al. Standards Track [Page 3] RFC 4566 SDP July 2006

3. Examples of SDP Usage

3.1. Session Initiation

 The Session Initiation Protocol (SIP) [15] is an application-layer
 control protocol for creating, modifying, and terminating sessions
 such as Internet multimedia conferences, Internet telephone calls,
 and multimedia distribution.  The SIP messages used to create
 sessions carry session descriptions that allow participants to agree
 on a set of compatible media types.  These session descriptions are
 commonly formatted using SDP.  When used with SIP, the offer/answer
 model [17] provides a limited framework for negotiation using SDP.

3.2. Streaming Media

 The Real Time Streaming Protocol (RTSP) [16], is an application-level
 protocol for control over the delivery of data with real-time
 properties.  RTSP provides an extensible framework to enable
 controlled, on-demand delivery of real-time data, such as audio and
 video.  An RTSP client and server negotiate an appropriate set of
 parameters for media delivery, partially using SDP syntax to describe
 those parameters.

3.3. Email and the World Wide Web

 Alternative means of conveying session descriptions include
 electronic mail and the World Wide Web (WWW).  For both email and WWW
 distribution, the media type "application/sdp" is used.  This enables
 the automatic launching of applications for participation in the
 session from the WWW client or mail reader in a standard manner.
 Note that announcements of multicast sessions made only via email or
 the WWW do not have the property that the receiver of a session
 announcement can necessarily receive the session because the
 multicast sessions may be restricted in scope, and access to the WWW
 server or reception of email is possible outside this scope.

3.4. Multicast Session Announcement

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

Handley, et al. Standards Track [Page 4] RFC 4566 SDP July 2006

 One protocol used to implement such a distributed directory is the
 Session Announcement Protocol (SAP) [14].  SDP provides the
 recommended session description format for such session
 announcements.

4. Requirements and Recommendations

 The purpose of SDP is to convey information about media streams in
 multimedia sessions to allow the recipients of a session description
 to participate in the session.  SDP is primarily intended for use in
 an internetwork, although it is sufficiently general that it can
 describe 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 session description includes the following:
 o  Session name and purpose
 o  Time(s) the session is active
 o  The media comprising the session
 o  Information needed to receive those media (addresses, ports,
    formats, etc.)
 As resources necessary to participate in a session may be limited,
 some additional information may also be desirable:
 o  Information about the bandwidth to be used by the session
 o  Contact information for the person responsible for the session
 In general, SDP must convey sufficient information to enable
 applications to join a session (with the possible exception of
 encryption keys) and to announce the resources to be used to any
 non-participants that may need to know.  (This latter feature is
 primarily useful when SDP is used with a multicast session
 announcement protocol.)

Handley, et al. Standards Track [Page 5] RFC 4566 SDP July 2006

4.1. Media and Transport Information

 An SDP session description includes the following media information:
 o  The type of media (video, audio, etc.)
 o  The transport protocol (RTP/UDP/IP, H.320, etc.)
 o  The format of the media (H.261 video, MPEG video, etc.)
 In addition to media format and transport protocol, SDP conveys
 address and port details.  For an IP multicast session, these
 comprise:
 o  The multicast group address for media
 o  The transport port for media
 This address and port are the destination address and destination
 port of the multicast stream, whether being sent, received, or both.
 For unicast IP sessions, the following are conveyed:
 o  The remote address for media
 o  The remote transport port for media
 The semantics of this address and port depend on the media and
 transport protocol defined.  By default, this SHOULD be the remote
 address and remote port to which data is sent.  Some media types may
 redefine this behaviour, but this is NOT RECOMMENDED since it
 complicates implementations (including middleboxes that must parse
 the addresses to open Network Address Translation (NAT) or firewall
 pinholes).

4.2. Timing Information

 Sessions may be either bounded or unbounded in time.  Whether or not
 they are bounded, they may be only active at specific times.  SDP can
 convey:
 o  An arbitrary list of start and stop times bounding the session
 o  For each bound, repeat times such as "every Wednesday at 10am for
    one hour"
 This timing information is globally consistent, irrespective of local
 time zone or daylight saving time (see Section 5.9).

Handley, et al. Standards Track [Page 6] RFC 4566 SDP July 2006

4.3. Private Sessions

 It is possible to create both public sessions and private sessions.
 SDP itself does not distinguish between these; private sessions are
 typically conveyed by encrypting the session description during
 distribution.  The details of how encryption is performed are
 dependent on the mechanism used to convey SDP; mechanisms are
 currently defined for SDP transported using SAP [14] and SIP [15],
 and others may be defined in the future.
 If a session announcement is private, it is possible to use that
 private announcement to convey encryption keys necessary to decode
 each of the media in a conference, including enough information to
 know which encryption scheme is used for each media.

4.4. Obtaining Further Information about a Session

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

4.5. Categorisation

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

4.6. Internationalisation

 The SDP specification recommends the use of the ISO 10646 character
 sets in the UTF-8 encoding [5] to allow many different languages to
 be represented.  However, to assist in compact representations, SDP
 also allows other character sets such as ISO 8859-1 to be used when
 desired.  Internationalisation only applies to free-text fields
 (session name and background information), and not to SDP as a whole.

5. SDP Specification

 An SDP session description is denoted by the media type
 "application/sdp" (See Section 8).
 An SDP session description is entirely textual using the ISO 10646
 character set in UTF-8 encoding.  SDP field names and attribute names
 use only the US-ASCII subset of UTF-8, but textual fields and
 attribute values MAY use the full ISO 10646 character set.  Field and

Handley, et al. Standards Track [Page 7] RFC 4566 SDP July 2006

 attribute values that use the full UTF-8 character set are never
 directly compared, hence there is no requirement for UTF-8
 normalisation.  The textual form, as opposed to a binary encoding
 such as ASN.1 or XDR, was chosen to enhance portability, to enable a
 variety of transports to be used, and to allow flexible, text-based
 toolkits to be used to generate and process session descriptions.
 However, since SDP may be used in environments where the maximum
 permissible size of a session description is limited, the encoding is
 deliberately compact.  Also, since announcements may be transported
 via very unreliable means or damaged by an intermediate caching
 server, the encoding was designed with strict order and formatting
 rules so that most errors would result in malformed session
 announcements that could be detected easily and discarded.  This also
 allows rapid discarding of encrypted session announcements for which
 a receiver does not have the correct key.
 An SDP session description consists of a number of lines of text of
 the form:
    <type>=<value>
 where <type> MUST be exactly one case-significant character and
 <value> is structured text whose format depends on <type>.  In
 general, <value> is either a number of fields delimited by a single
 space character or a free format string, and is case-significant
 unless a specific field defines otherwise.  Whitespace MUST NOT be
 used on either side of the "=" sign.
 An SDP session description consists of a session-level section
 followed by zero or more media-level sections.  The session-level
 part starts with a "v=" line and continues to the first media-level
 section.  Each media-level section starts with an "m=" line and
 continues to the next media-level section or end of the whole session
 description.  In general, session-level values are the default for
 all media unless overridden by an equivalent media-level value.
 Some lines in each description are REQUIRED and some are OPTIONAL,
 but all MUST appear in exactly the order given here (the fixed order
 greatly enhances error detection and allows for a simple parser).
 OPTIONAL items are marked with a "*".

Handley, et al. Standards Track [Page 8] RFC 4566 SDP July 2006

    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)
       b=* (zero or more bandwidth information lines)
       One or more time descriptions ("t=" and "r=" lines; see below)
       z=* (time zone adjustments)
       k=* (encryption key)
       a=* (zero or more session attribute lines)
       Zero or more media descriptions
    Time description
       t=  (time the session is active)
       r=* (zero or more repeat times)
    Media description, if present
       m=  (media name and transport address)
       i=* (media title)
       c=* (connection information -- optional if included at
            session level)
       b=* (zero or more bandwidth information lines)
       k=* (encryption key)
       a=* (zero or more media attribute lines)
 The set of type letters is deliberately small and not intended to be
 extensible -- an SDP parser MUST completely ignore any session
 description that contains a type letter that it does not understand.
 The attribute mechanism ("a=" described below) is the primary means
 for extending SDP and tailoring it to particular applications or
 media.  Some attributes (the ones listed in Section 6 of this memo)
 have a defined meaning, but others may be added on an application-,
 media-, or session-specific basis.  An SDP parser MUST ignore any
 attribute it doesn't understand.
 An SDP session 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.

Handley, et al. Standards Track [Page 9] RFC 4566 SDP July 2006

 The connection ("c=") and attribute ("a=") information in the
 session-level section applies to all the media of that session unless
 overridden by connection information or an attribute of the same name
 in the media description.  For instance, in the example below, each
 media behaves as if it were given a "recvonly" attribute.
 An example SDP description is:
    v=0
    o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
    s=SDP Seminar
    i=A Seminar on the session description protocol
    u=http://www.example.com/seminars/sdp.pdf
    e=j.doe@example.com (Jane Doe)
    c=IN IP4 224.2.17.12/127
    t=2873397496 2873404696
    a=recvonly
    m=audio 49170 RTP/AVP 0
    m=video 51372 RTP/AVP 99
    a=rtpmap:99 h263-1998/90000
 Text fields such as the session name and information are octet
 strings that may contain any octet with the exceptions of 0x00 (Nul),
 0x0a (ASCII newline), and 0x0d (ASCII carriage return).  The sequence
 CRLF (0x0d0a) is used to end a record, although parsers SHOULD be
 tolerant and also accept records terminated with a single newline
 character.  If the "a=charset" attribute is not present, these octet
 strings MUST be interpreted as containing ISO-10646 characters in
 UTF-8 encoding (the presence of the "a=charset" attribute may force
 some fields to be interpreted differently).
 A session description can contain domain names in the "o=", "u=",
 "e=", "c=", and "a=" lines.  Any domain name used in SDP MUST comply
 with [1], [2].  Internationalised domain names (IDNs) MUST be
 represented using the ASCII Compatible Encoding (ACE) form defined in
 [11] and MUST NOT be directly represented in UTF-8 or any other
 encoding (this requirement is for compatibility with RFC 2327 and
 other SDP-related standards, which predate the development of
 internationalised domain names).

5.1. Protocol Version ("v=")

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

Handley, et al. Standards Track [Page 10] RFC 4566 SDP July 2006

5.2. Origin ("o=")

    o=<username> <sess-id> <sess-version> <nettype> <addrtype>
      <unicast-address>
 The "o=" 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 it has been
    suggested that a Network Time Protocol (NTP) format timestamp be
    used to ensure uniqueness [13].
 <sess-version> is a version number for this session description.  Its
    usage is up to the creating tool, so long as <sess-version> is
    increased when a modification is made to the session data.  Again,
    it is RECOMMENDED that an NTP format timestamp is used.
 <nettype> is a text string giving the type of network.  Initially
    "IN" is defined to have the meaning "Internet", but other values
    MAY be registered in the future (see Section 8).
 <addrtype> is a text string giving the type of the address that
    follows.  Initially "IP4" and "IP6" are defined, but other values
    MAY be registered in the future (see Section 8).
 <unicast-address> is the address of the machine from which the
    session was created.  For an address type of IP4, this is either
    the fully qualified domain name of the machine or the dotted-
    decimal representation of the IP version 4 address of the machine.
    For an address type of IP6, this is either the fully qualified
    domain name of the machine or the compressed textual
    representation of the IP version 6 address of the machine.  For
    both IP4 and IP6, the fully qualified domain name is the form that
    SHOULD be given unless this is unavailable, in which case the
    globally unique address MAY be substituted.  A local IP address
    MUST NOT be used in any context where the SDP description might
    leave the scope in which the address is meaningful (for example, a
    local address MUST NOT be included in an application-level
    referral that might leave the scope).

Handley, et al. Standards Track [Page 11] RFC 4566 SDP July 2006

 In general, the "o=" field serves as a globally unique identifier for
 this version of this session description, and the subfields excepting
 the version taken together identify the session irrespective of any
 modifications.
 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=" field, 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=" field is the textual session name.  There MUST be one and
 only one "s=" field per session description.  The "s=" field MUST NOT
 be empty and SHOULD contain ISO 10646 characters (but see also the
 "a=charset" attribute).  If a session has no meaningful name, the
 value "s= " SHOULD be used (i.e., a single space as the session
 name).

5.4. Session Information ("i=")

    i=<session description>
 The "i=" field provides textual information about the session.  There
 MUST be at most one session-level "i=" field per session description,
 and at most one "i=" field per media.  If the "a=charset" attribute
 is present, it specifies the character set used in the "i=" field.
 If the "a=charset" attribute is not present, the "i=" field MUST
 contain ISO 10646 characters in UTF-8 encoding.
 A single "i=" field MAY also be used for each media definition.  In
 media definitions, "i=" fields are primarily intended for labelling
 media streams.  As such, they are most likely to be useful when a
 single session has more than one distinct media stream of the same
 media type.  An example would be two different whiteboards, one for
 slides and one for feedback and questions.
 The "i=" field 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.

Handley, et al. Standards Track [Page 12] RFC 4566 SDP July 2006

5.5. URI ("u=")

    u=<uri>
 A URI is a Uniform Resource Identifier as used by WWW clients [7].
 The URI should be a pointer to additional information about the
 session.  This field is OPTIONAL, but if it is present it MUST be
 specified before the first media field.  No more than one URI field
 is allowed per session description.

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

    e=<email-address>
    p=<phone-number>
 The "e=" and "p=" lines specify contact information for the person
 responsible for the conference.  This is not necessarily the same
 person that created the conference announcement.
 Inclusion of an email address or phone number is OPTIONAL.  Note that
 the previous version of SDP specified that either an email field or a
 phone field MUST be specified, but this was widely ignored.  The
 change brings the specification into line with common usage.
 If an email address or phone number is present, it MUST be specified
 before the first media field.  More than one email or phone field can
 be given for a session description.
 Phone numbers SHOULD be given in the form of an international public
 telecommunication number (see ITU-T Recommendation E.164) preceded by
 a "+".  Spaces and hyphens may be used to split up a phone field to
 aid readability if desired.  For example:
    p=+1 617 555-6011
 Both email addresses and phone numbers can have an OPTIONAL free text
 string associated with them, normally giving the name of the person
 who may be contacted.  This MUST be enclosed in parentheses if it is
 present.  For example:
    e=j.doe@example.com (Jane Doe)
 The alternative RFC 2822 [29] name quoting convention is also allowed
 for both email addresses and phone numbers.  For example:
    e=Jane Doe <j.doe@example.com>

Handley, et al. Standards Track [Page 13] RFC 4566 SDP July 2006

 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 Data ("c=")

    c=<nettype> <addrtype> <connection-address>
 The "c=" field contains connection data.
 A session description MUST contain either at least one "c=" field in
 each media description or a single "c=" field at the session level.
 It MAY contain a single session-level "c=" field and additional "c="
 field(s) per media description, in which case the per-media values
 override the session-level settings for the respective media.
 The first sub-field ("<nettype>") is the network type, which is a
 text string giving the type of network.  Initially, "IN" is defined
 to have the meaning "Internet", but other values MAY be registered in
 the future (see Section 8).
 The second sub-field ("<addrtype>") is the address type.  This allows
 SDP to be used for sessions that are not IP based.  This memo only
 defines IP4 and IP6, but other values MAY be registered in the future
 (see Section 8).
 The third sub-field ("<connection-address>") is the connection
 address.  OPTIONAL sub-fields MAY be added after the connection
 address depending on the value of the <addrtype> field.
 When the <addrtype> is IP4 and IP6, the connection address is defined
 as follows:
 o  If the session is multicast, the connection address will be an IP
    multicast group address.  If the session is not multicast, then
    the connection address contains the unicast IP address of the
    expected data source or data relay or data sink as determined by
    additional attribute fields.  It is not expected that unicast
    addresses will be given in a session description that is
    communicated by a multicast announcement, though this is not
    prohibited.
 o  Sessions using an IPv4 multicast connection address MUST also have
    a time to live (TTL) value present in addition to the multicast
    address.  The TTL and the address together define the scope with
    which multicast packets sent in this conference will be sent.  TTL
    values MUST be in the range 0-255.  Although the TTL MUST be
    specified, its use to scope multicast traffic is deprecated;

Handley, et al. Standards Track [Page 14] RFC 4566 SDP July 2006

    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 224.2.36.42/127
 IPv6 multicast does not use TTL scoping, and hence the TTL value MUST
 NOT be present for IPv6 multicast.  It is expected that IPv6 scoped
 addresses will be used to limit the scope of conferences.
 Hierarchical or layered encoding schemes are data streams where the
 encoding from a single media source is split into a number of layers.
 The receiver can choose the desired quality (and hence bandwidth) by
 only subscribing to a subset of these layers.  Such layered encodings
 are normally transmitted in multiple multicast groups to allow
 multicast pruning.  This technique keeps unwanted traffic from sites
 only requiring certain levels of the hierarchy.  For applications
 requiring multiple multicast groups, we allow the following notation
 to be used for the connection address:
    <base multicast address>[/<ttl>]/<number of addresses>
 If the number of addresses is not given, it is assumed to be one.
 Multicast addresses so assigned are contiguously allocated above the
 base address, so that, for example:
    c=IN IP4 224.2.1.1/127/3
 would state that addresses 224.2.1.1, 224.2.1.2, and 224.2.1.3 are to
 be used at a TTL of 127.  This is semantically identical to including
 multiple "c=" lines in a media description:
    c=IN IP4 224.2.1.1/127
    c=IN IP4 224.2.1.2/127
    c=IN IP4 224.2.1.3/127
 Similarly, an IPv6 example would be:
    c=IN IP6 FF15::101/3
 which is semantically equivalent to:
    c=IN IP6 FF15::101
    c=IN IP6 FF15::102
    c=IN IP6 FF15::103

Handley, et al. Standards Track [Page 15] RFC 4566 SDP July 2006

 (remembering that the TTL field is not present in IPv6 multicast).
 Multiple addresses or "c=" lines MAY be specified on a per-media
 basis only if they provide multicast addresses for different layers
 in a hierarchical or layered encoding scheme.  They MUST NOT be
 specified for a session-level "c=" field.
 The slash notation for multiple addresses described above MUST NOT be
 used for IP unicast addresses.

5.8. Bandwidth ("b=")

    b=<bwtype>:<bandwidth>
 This OPTIONAL field denotes the proposed bandwidth to be used by the
 session or media.  The <bwtype> is an alphanumeric modifier giving
 the meaning of the <bandwidth> figure.  Two values are defined in
 this specification, but other values MAY be registered in the future
 (see Section 8 and [21], [25]):
 CT If the bandwidth of a session or media in a session is different
    from the bandwidth implicit from the scope, a "b=CT:..." line
    SHOULD be supplied for the session giving the proposed upper limit
    to the bandwidth used (the "conference total" bandwidth).  The
    primary purpose of this is to give an approximate idea as to
    whether two or more sessions can coexist simultaneously.  When
    using the CT modifier with RTP, if several RTP sessions are part
    of the conference, the conference total refers to total bandwidth
    of all RTP sessions.
 AS The bandwidth is interpreted to be application specific (it will
    be the application's concept of maximum bandwidth).  Normally,
    this will coincide with what is set on the application's "maximum
    bandwidth" control if applicable.  For RTP-based applications, AS
    gives the RTP "session bandwidth" as defined in Section 6.2 of
    [19].
 Note that CT gives a total bandwidth figure for all the media at all
 sites.  AS gives a bandwidth figure for a single media at a single
 site, although there may be many sites sending simultaneously.
 A prefix "X-" is defined for <bwtype> names.  This is intended for
 experimental purposes only.  For example:
    b=X-YZ:128

Handley, et al. Standards Track [Page 16] RFC 4566 SDP July 2006

 Use of the "X-" prefix is NOT RECOMMENDED: instead new modifiers
 SHOULD be registered with IANA in the standard namespace.  SDP
 parsers MUST ignore bandwidth fields with unknown modifiers.
 Modifiers MUST be alphanumeric and, although no length limit is
 given, it is recommended that they be short.
 The <bandwidth> is interpreted as kilobits per second by default.
 The definition of a new <bwtype> modifier MAY specify that the
 bandwidth is to be interpreted in some alternative unit (the "CT" and
 "AS" modifiers defined in this memo use the default units).

5.9. Timing ("t=")

    t=<start-time> <stop-time>
 The "t=" lines specify the start and stop times for a session.
 Multiple "t=" lines MAY be used if a session is active at multiple
 irregularly spaced times; each additional "t=" line specifies an
 additional period of time for which the session will be active.  If
 the session is active at regular times, an "r=" line (see below)
 should be used in addition to, and following, a "t=" line -- in which
 case the "t=" line specifies the start and stop times of the repeat
 sequence.
 The first and second sub-fields give the start and stop times,
 respectively, for the session.  These values are the decimal
 representation of Network Time Protocol (NTP) time values in seconds
 since 1900 [13].  To convert these values to UNIX time, subtract
 decimal 2208988800.
 NTP timestamps are elsewhere represented by 64-bit values, which wrap
 sometime in the year 2036.  Since SDP uses an arbitrary length
 decimal representation, this should not cause an issue (SDP
 timestamps MUST continue counting seconds since 1900, NTP will use
 the value modulo the 64-bit limit).
 If the <stop-time> is set to zero, then the session is not bounded,
 though it will not become active until after the <start-time>.  If
 the <start-time> is also zero, the session is regarded as permanent.
 User interfaces SHOULD strongly discourage the creation of unbounded
 and permanent sessions as they give no information about when the
 session is actually going to terminate, and so make scheduling
 difficult.
 The general assumption may be made, when displaying unbounded
 sessions that have not timed out to the user, that an unbounded
 session will only be active until half an hour from the current time

Handley, et al. Standards Track [Page 17] RFC 4566 SDP July 2006

 or the session start time, whichever is the later.  If behaviour
 other than this is required, an end-time SHOULD be given and modified
 as appropriate when new information becomes available about when the
 session should really end.
 Permanent sessions may be shown to the user as never being active
 unless there are associated repeat times that state precisely when
 the session will be active.

5.10. Repeat Times ("r=")

    r=<repeat interval> <active duration> <offsets from start-time>
 "r=" fields specify repeat times for a session.  For example, if a
 session is active at 10am on Monday and 11am on Tuesday for one hour
 each week for three months, then the <start-time> in the
 corresponding "t=" field would be the NTP representation of 10am on
 the first Monday, the <repeat interval> would be 1 week, the <active
 duration> would be 1 hour, and the offsets would be zero and 25
 hours.  The corresponding "t=" field stop time would be the NTP
 representation of the end of the last session three months later.  By
 default, all fields are in seconds, so the "r=" and "t=" fields might
 be the following:
    t=3034423619 3042462419
    r=604800 3600 0 90000
 To make description more compact, times may also be given in units of
 days, hours, or minutes.  The syntax for these is a number
 immediately followed by a single case-sensitive character.
 Fractional units are not allowed -- a smaller unit should be used
 instead.  The following unit specification characters are allowed:
    d - days (86400 seconds)
    h - hours (3600 seconds)
    m - minutes (60 seconds)
    s - seconds (allowed for completeness)
 Thus, the above session announcement could also have been written:
    r=7d 1h 0 25h
 Monthly and yearly repeats cannot be directly specified with a single
 SDP repeat time; instead, separate "t=" fields should be used to
 explicitly list the session times.

Handley, et al. Standards Track [Page 18] RFC 4566 SDP July 2006

5.11. Time Zones ("z=")

    z=<adjustment time> <offset> <adjustment time> <offset> ....
 To schedule a repeated session that spans a change from daylight
 saving time to standard time or vice versa, it is necessary to
 specify offsets from the base time.  This is required because
 different time zones change time at different times of day, different
 countries change to or from daylight saving time on different dates,
 and some countries do not have daylight saving time at all.
 Thus, in order to schedule a session that is at the same time winter
 and summer, it must be possible to specify unambiguously by whose
 time zone a session is scheduled.  To simplify this task for
 receivers, we allow the sender to specify the NTP time that a time
 zone adjustment happens and the offset from the time when the session
 was first scheduled.  The "z=" field allows the sender to specify a
 list of these adjustment times and offsets from the base time.
 An example might be the following:
    z=2882844526 -1h 2898848070 0
 This specifies that at time 2882844526, the time base by which the
 session's repeat times are calculated is shifted back by 1 hour, and
 that at time 2898848070, the session's original time base is
 restored.  Adjustments are always relative to the specified start
 time -- they are not cumulative.  Adjustments apply to all "t=" and
 "r=" lines in a session description.
 If a session is likely to last several years, it is expected that the
 session announcement will be modified periodically rather than
 transmit several years' worth of adjustments in one session
 announcement.

5.12. Encryption Keys ("k=")

    k=<method>
    k=<method>:<encryption key>
 If transported over a secure and trusted channel, the Session
 Description Protocol MAY be used to convey encryption keys.  A simple
 mechanism for key exchange is provided by the key field ("k="),
 although this is primarily supported for compatibility with older
 implementations and its use is NOT RECOMMENDED.  Work is in progress
 to define new key exchange mechanisms for use with SDP [27] [28], and
 it is expected that new applications will use those mechanisms.

Handley, et al. Standards Track [Page 19] RFC 4566 SDP July 2006

 A key field is permitted before the first media entry (in which case
 it applies to all media in the session), or for each media entry as
 required.  The format of keys and their usage are outside the scope
 of this document, and the key field provides no way to indicate the
 encryption algorithm to be used, key type, or other information about
 the key: this is assumed to be provided by the higher-level protocol
 using SDP.  If there is a need to convey this information within SDP,
 the extensions mentioned previously SHOULD be used.  Many security
 protocols require two keys: one for confidentiality, another for
 integrity.  This specification does not support transfer of two keys.
 The method indicates the mechanism to be used to obtain a usable key
 by external means, or from the encoded encryption key given.  The
 following methods are defined:
    k=clear:<encryption key>
       The encryption key is included untransformed in this key field.
       This method MUST NOT be used unless it can be guaranteed that
       the SDP is conveyed over a secure channel.  The encryption key
       is interpreted as text according to the charset attribute; use
       the "k=base64:" method to convey characters that are otherwise
       prohibited in SDP.
    k=base64:<encoded encryption key>
       The encryption key is included in this key field but has been
       base64 encoded [12] because it includes characters that are
       prohibited in SDP.  This method MUST NOT be used unless it can
       be guaranteed that the SDP is conveyed over a secure channel.
    k=uri:<URI to obtain key>
       A Uniform Resource Identifier is included in the key field.
       The URI refers to the data containing the key, and may require
       additional authentication before the key can be returned.  When
       a request is made to the given URI, the reply should specify
       the encoding for the key.  The URI is often an Secure Socket
       Layer/Transport Layer Security (SSL/TLS)-protected HTTP URI
       ("https:"), although this is not required.
    k=prompt
       No key is included in this SDP description, but the session or
       media stream referred to by this key field is encrypted.  The
       user should be prompted for the key when attempting to join the
       session, and this user-supplied key should then be used to

Handley, et al. Standards Track [Page 20] RFC 4566 SDP July 2006

       decrypt the media streams.  The use of user-specified keys is
       NOT RECOMMENDED, since such keys tend to have weak security
       properties.
 The key field MUST NOT be used unless it can be guaranteed that the
 SDP is conveyed over a secure and trusted channel.  An example of
 such a channel might be SDP embedded inside an S/MIME message or a
 TLS-protected HTTP session.  It is important to ensure that the
 secure channel is with the party that is authorised to join the
 session, not an intermediary: if a caching proxy server is used, it
 is important to ensure that the proxy is either trusted or unable to
 access the SDP.

5.13. Attributes ("a=")

    a=<attribute>
    a=<attribute>:<value>
 Attributes are the primary means for extending SDP.  Attributes may
 be defined to be used as "session-level" attributes, "media-level"
 attributes, or both.
 A media description may have any number of attributes ("a=" fields)
 that are media specific.  These are referred to as "media-level"
 attributes and add information about the media stream.  Attribute
 fields can also be added before the first media field; these
 "session-level" attributes convey additional information that applies
 to the conference as a whole rather than to individual media.
 Attribute fields may be of two forms:
 o  A property attribute is simply of the form "a=<flag>".  These are
    binary attributes, and the presence of the attribute conveys that
    the attribute is a property of the session.  An example might be
    "a=recvonly".
 o  A value attribute is of the form "a=<attribute>:<value>".  For
    example, a whiteboard could have the value attribute "a=orient:
    landscape"
 Attribute interpretation depends on the media tool being invoked.
 Thus receivers of session descriptions should be configurable in
 their interpretation of session descriptions in general and of
 attributes in particular.
 Attribute names MUST use the US-ASCII subset of ISO-10646/UTF-8.

Handley, et al. Standards Track [Page 21] RFC 4566 SDP July 2006

 Attribute values are octet strings, and MAY use any octet value
 except 0x00 (Nul), 0x0A (LF), and 0x0D (CR).  By default, attribute
 values are to be interpreted as in ISO-10646 character set with UTF-8
 encoding.  Unlike other text fields, attribute values are NOT
 normally affected by the "charset" attribute as this would make
 comparisons against known values problematic.  However, when an
 attribute is defined, it can be defined to be charset dependent, in
 which case 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=" field and is terminated by
 either the next "m=" field or by the end of the session description.
 A media field has several sub-fields:
 <media> is the media type.  Currently defined media are "audio",
    "video", "text", "application", and "message", although this list
    may be extended in the future (see Section 8).
 <port> is the transport port to which the media stream is sent.  The
    meaning of the transport port depends on the network being used as
    specified in the relevant "c=" field, and on the transport
    protocol defined in the <proto> sub-field of the media field.
    Other ports used by the media application (such as the RTP Control
    Protocol (RTCP) port [19]) MAY be derived algorithmically from the
    base media port or MAY be specified in a separate attribute (for
    example, "a=rtcp:" as defined in [22]).
    If non-contiguous ports are used or if they don't follow the
    parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:"
    attribute MUST be used.  Applications that are requested to send
    media to a <port> that is odd and where the "a=rtcp:" is present
    MUST NOT subtract 1 from the RTP port: that is, they MUST send the
    RTP to the port indicated in <port> and send the RTCP to the port
    indicated in the "a=rtcp" attribute.
    For applications where hierarchically encoded streams are being
    sent to a unicast address, it may be necessary to specify multiple
    transport ports.  This is done using a similar notation to that
    used for IP multicast addresses in the "c=" field:

Handley, et al. Standards Track [Page 22] RFC 4566 SDP July 2006

       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 below).  If non-
    contiguous ports are required, they must be signalled using a
    separate attribute (for example, "a=rtcp:" as defined in [22]).
    If multiple addresses are specified in the "c=" field and multiple
    ports are specified in the "m=" field, a one-to-one mapping from
    port to the corresponding address is implied.  For example:
       c=IN IP4 224.2.1.1/127/2
       m=video 49170/2 RTP/AVP 31
    would imply that address 224.2.1.1 is used with ports 49170 and
    49171, and address 224.2.1.2 is used with ports 49172 and 49173.
    The semantics of multiple "m=" lines using the same transport
    address are undefined.  This implies that, unlike limited past
    practice, there is no implicit grouping defined by such means and
    an explicit grouping framework (for example, [18]) should instead
    be used to express the intended semantics.
 <proto> is the transport protocol.  The meaning of the transport
    protocol is dependent on the address type field in the relevant
    "c=" field.  Thus a "c=" field of IP4 indicates that the transport
    protocol runs over IP4.  The following transport protocols are
    defined, but may be extended through registration of new protocols
    with IANA (see Section 8):
  • udp: denotes an unspecified protocol running over UDP.
  • RTP/AVP: denotes RTP [19] used under the RTP Profile for Audio

and Video Conferences with Minimal Control [20] running over

       UDP.
  • RTP/SAVP: denotes the Secure Real-time Transport Protocol [23]

running over UDP.

Handley, et al. Standards Track [Page 23] RFC 4566 SDP July 2006

    The main reason to specify the transport protocol in addition to
    the media format is that the same standard media formats may be
    carried over different transport protocols even when the network
    protocol is the same -- a historical example is vat Pulse Code
    Modulation (PCM) audio and RTP PCM audio; another might be TCP/RTP
    PCM audio.  In addition, relays and monitoring tools that are
    transport-protocol-specific but format-independent are possible.
 <fmt> is a media format description.  The fourth and any subsequent
    sub-fields describe the format of the media.  The interpretation
    of the media format depends on the value of the <proto> sub-field.
    If the <proto> sub-field is "RTP/AVP" or "RTP/SAVP" the <fmt>
    sub-fields contain RTP payload type numbers.  When a list of
    payload type numbers is given, this implies that all of these
    payload formats MAY be used in the session, but the first of these
    formats SHOULD be used as the default format for the session.  For
    dynamic payload type assignments the "a=rtpmap:" attribute (see
    Section 6) SHOULD be used to map from an RTP payload type number
    to a media encoding name that identifies the payload format.  The
    "a=fmtp:"  attribute MAY be used to specify format parameters (see
    Section 6).
    If the <proto> sub-field is "udp" the <fmt> sub-fields MUST
    reference a media type describing the format under the "audio",
    "video", "text", "application", or "message" top-level media
    types.  The media type registration SHOULD define the packet
    format for use with UDP transport.
    For media using other transport protocols, the <fmt> field is
    protocol specific.  Rules for interpretation of the <fmt> sub-
    field MUST be defined when registering new protocols (see Section
    8.2.2).

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.
    a=cat:<category>
       This attribute gives the dot-separated hierarchical category of
       the session.  This is to enable a receiver to filter unwanted
       sessions by category.  There is no central registry of
       categories.  It is a session-level attribute, and it is not
       dependent on charset.

Handley, et al. Standards Track [Page 24] RFC 4566 SDP July 2006

    a=keywds:<keywords>
       Like the cat attribute, this is to assist identifying wanted
       sessions at the receiver.  This allows a receiver to select
       interesting session based on keywords describing the purpose of
       the session; there is no central registry of keywords.  It is a
       session-level attribute.  It is a charset-dependent attribute,
       meaning that its value should be interpreted in the charset
       specified for the session description if one is specified, or
       by default in ISO 10646/UTF-8.
    a=tool:<name and version of tool>
       This gives the name and version number of the tool used to
       create the session description.  It is a session-level
       attribute, and it is not dependent on charset.
    a=ptime:<packet time>
       This gives the length of time in milliseconds represented by
       the media in a packet.  This is probably only meaningful for
       audio data, but may be used with other media types if it makes
       sense.  It should not be necessary to know ptime to decode RTP
       or vat audio, and it is intended as a recommendation for the
       encoding/packetisation of audio.  It is a media-level
       attribute, and it is not dependent on charset.
    a=maxptime:<maximum packet time>
       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.  It is a media-level
       attribute, and it is not dependent on charset.  Note that this
       attribute was introduced after RFC 2327, and non-updated
       implementations will ignore this attribute.
    a=rtpmap:<payload type> <encoding name>/<clock rate> [/<encoding
       parameters>]
       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.  It is a media-level attribute that is not
       dependent on charset.

Handley, et al. Standards Track [Page 25] RFC 4566 SDP July 2006

       Although an RTP profile may make static assignments of payload
       type numbers to payload formats, it is more common for that
       assignment to be done dynamically using "a=rtpmap:" attributes.
       As an example of a static payload type, consider u-law PCM
       coded single-channel audio sampled at 8 kHz.  This is
       completely defined in the RTP Audio/Video profile as payload
       type 0, so there is no need for an "a=rtpmap:" attribute, and
       the media for such a stream sent to UDP port 49232 can be
       specified as:
          m=audio 49232 RTP/AVP 0
       An example of a dynamic payload type is 16-bit linear encoded
       stereo audio sampled at 16 kHz.  If we wish to use the dynamic
       RTP/AVP payload type 98 for this stream, additional information
       is required to decode it:
          m=audio 49232 RTP/AVP 98
          a=rtpmap:98 L16/16000/2
       Up to one rtpmap attribute can be defined for each media format
       specified.  Thus, we might have the following:
          m=audio 49230 RTP/AVP 96 97 98
          a=rtpmap:96 L8/8000
          a=rtpmap:97 L16/8000
          a=rtpmap:98 L16/11025/2
       RTP profiles that specify the use of dynamic payload types MUST
       define the set of valid encoding names and/or a means to
       register encoding names if that profile is to be used with SDP.
       The "RTP/AVP" and "RTP/SAVP" profiles use media subtypes for
       encoding names, under the top-level media type denoted in the
       "m=" line.  In the example above, the media types are
       "audio/l8" and "audio/l16".
       For audio streams, <encoding parameters> indicates the number
       of audio channels.  This parameter is OPTIONAL and may be
       omitted if the number of channels is one, provided that no
       additional parameters are needed.
       For video streams, no encoding parameters are currently
       specified.
       Additional encoding parameters MAY be defined in the future,
       but codec-specific parameters SHOULD NOT be added.  Parameters
       added to an "a=rtpmap:" attribute SHOULD only be those required
       for a session directory to make the choice of appropriate media

Handley, et al. Standards Track [Page 26] RFC 4566 SDP July 2006

       to participate in a session.  Codec-specific parameters should
       be added in other attributes (for example, "a=fmtp:").
       Note: RTP audio formats typically do not include information
       about the number of samples per packet.  If a non-default (as
       defined in the RTP Audio/Video Profile) packetisation is
       required, the "ptime" attribute is used as given above.
    a=recvonly
       This specifies that the tools should be started in receive-only
       mode where applicable.  It can be either a session- or media-
       level attribute, and it is not dependent on charset.  Note that
       recvonly applies to the media only, not to any associated
       control protocol (e.g., an RTP-based system in recvonly mode
       SHOULD still send RTCP packets).
    a=sendrecv
       This specifies that the tools should be started in send and
       receive mode.  This is necessary for interactive conferences
       with tools that default to receive-only mode.  It can be either
       a session or media-level attribute, and it is not dependent on
       charset.
       If none of the attributes "sendonly", "recvonly", "inactive",
       and "sendrecv" is present, "sendrecv" SHOULD be assumed as the
       default for sessions that are not of the conference type
       "broadcast" or "H332" (see below).
    a=sendonly
       This specifies that the tools should be started in send-only
       mode.  An example may be where a different unicast address is
       to be used for a traffic destination than for a traffic source.
       In such a case, two media descriptions may be used, one
       sendonly and one recvonly.  It can be either a session- or
       media-level attribute, but would normally only be used as a
       media attribute.  It is not dependent on charset.  Note that
       sendonly applies only to the media, and any associated control
       protocol (e.g., RTCP) SHOULD still be received and processed as
       normal.
    a=inactive
       This specifies that the tools should be started in inactive
       mode.  This is necessary for interactive conferences where
       users can put other users on hold.  No media is sent over an

Handley, et al. Standards Track [Page 27] RFC 4566 SDP July 2006

       inactive media stream.  Note that an RTP-based system SHOULD
       still send RTCP, even if started inactive.  It can be either a
       session or media-level attribute, and it is not dependent on
       charset.
    a=orient:<orientation>
       Normally this is only used for a whiteboard or presentation
       tool.  It specifies the orientation of a the workspace on the
       screen.  It is a media-level attribute.  Permitted values are
       "portrait", "landscape", and "seascape" (upside-down
       landscape).  It is not dependent on charset.
    a=type:<conference type>
       This specifies the type of the conference.  Suggested values
       are "broadcast", "meeting", "moderated", "test", and "H332".
       "recvonly" should be the default for "type:broadcast" sessions,
       "type:meeting" should imply "sendrecv", and "type:moderated"
       should indicate the use of a floor control tool and that the
       media tools are started so as to mute new sites joining the
       conference.
       Specifying the attribute "type:H332" indicates that this
       loosely coupled session is part of an H.332 session as defined
       in the ITU H.332 specification [26].  Media tools should be
       started "recvonly".
       Specifying the attribute "type:test" is suggested as a hint
       that, unless explicitly requested otherwise, receivers can
       safely avoid displaying this session description to users.
       The type attribute is a session-level attribute, and it is not
       dependent on charset.
    a=charset:<character set>
       This specifies the character set to be used to display the
       session name and information data.  By default, the ISO-10646
       character set in UTF-8 encoding is used.  If a more compact
       representation is required, other character sets may be used.
       For example, the ISO 8859-1 is specified with the following SDP
       attribute:
          a=charset:ISO-8859-1

Handley, et al. Standards Track [Page 28] RFC 4566 SDP July 2006

       This is a session-level attribute and is not dependent on
       charset.  The charset specified MUST be one of those registered
       with IANA, such as ISO-8859-1.  The character set identifier is
       a US-ASCII string and MUST be compared against the IANA
       identifiers using a case-insensitive comparison.  If the
       identifier is not recognised or not supported, all strings that
       are affected by it SHOULD be regarded as octet strings.
       Note that a character set specified MUST still prohibit the use
       of bytes 0x00 (Nul), 0x0A (LF), and 0x0d (CR).  Character sets
       requiring the use of these characters MUST define a quoting
       mechanism that prevents these bytes from appearing within text
       fields.
    a=sdplang:<language tag>
       This can be a session-level attribute or a media-level
       attribute.  As a session-level attribute, it specifies the
       language for the session description.  As a media-level
       attribute, it specifies the language for any media-level SDP
       information field associated with that media.  Multiple sdplang
       attributes can be provided either at session or media level if
       multiple languages in the session description or media use
       multiple languages, in which case the order of the attributes
       indicates the order of importance of the various languages in
       the session or media from most important to least important.
       In general, sending session descriptions consisting of multiple
       languages is discouraged.  Instead, multiple descriptions
       SHOULD be sent describing the session, one in each language.
       However, this is not possible with all transport mechanisms,
       and so multiple sdplang attributes are allowed although NOT
       RECOMMENDED.
       The "sdplang" attribute value must be a single RFC 3066
       language tag in US-ASCII [9].  It is not dependent on the
       charset attribute.  An "sdplang" attribute SHOULD be specified
       when a session is of sufficient scope to cross geographic
       boundaries where the language of recipients cannot be assumed,
       or where the session is in a different language from the
       locally assumed norm.
    a=lang:<language tag>
       This can be a session-level attribute or a media-level
       attribute.  As a session-level attribute, it specifies the
       default language for the session being described.  As a media-
       level attribute, it specifies the language for that media,

Handley, et al. Standards Track [Page 29] RFC 4566 SDP July 2006

       overriding any session-level language specified.  Multiple lang
       attributes can be provided either at session or media level if
       the session description or media use multiple languages, in
       which case the order of the attributes indicates the order of
       importance of the various languages in the session or media
       from most important to least important.
       The "lang" attribute value must be a single RFC 3066 language
       tag in US-ASCII [9].  It is not dependent on the charset
       attribute.  A "lang" attribute SHOULD be specified when a
       session is of sufficient scope to cross geographic boundaries
       where the language of recipients cannot be assumed, or where
       the session is in a different language from the locally assumed
       norm.
    a=framerate:<frame rate>
       This gives the maximum video frame rate in frames/sec.  It is
       intended as a recommendation for the encoding of video data.
       Decimal representations of fractional values using the notation
       "<integer>.<fraction>" are allowed.  It is a media-level
       attribute, defined only for video media, and it is not
       dependent on charset.
    a=quality:<quality>
       This gives a suggestion for the quality of the encoding as an
       integer value.  The intention of the quality attribute for
       video is to specify a non-default trade-off between frame-rate
       and still-image quality.  For video, the value is in the range
       0 to 10, with the following suggested meaning:
          10 - the best still-image quality the compression scheme can
               give.
          5  - the default behaviour given no quality suggestion.
          0  - the worst still-image quality the codec designer thinks
               is still usable.
       It is a media-level attribute, and it is not dependent on
       charset.
    a=fmtp:<format> <format specific parameters>
       This attribute allows parameters that are specific to a
       particular format to be conveyed in a way that SDP does not
       have to understand them.  The format must be one of the formats
       specified for the media.  Format-specific parameters may be any
       set of parameters required to be conveyed by SDP and given

Handley, et al. Standards Track [Page 30] RFC 4566 SDP July 2006

       unchanged to the media tool that will use this format.  At most
       one instance of this attribute is allowed for each format.
       It is a media-level attribute, and it is not dependent on
       charset.

7. Security Considerations

 SDP is frequently used with the Session Initiation Protocol [15]
 using the offer/answer model [17] to agree on parameters for unicast
 sessions.  When used in this manner, the security considerations of
 those protocols apply.
 SDP is a session description format that describes multimedia
 sessions.  Entities receiving and acting upon an SDP message SHOULD
 be aware that a session description cannot be trusted unless it has
 been obtained by an authenticated transport protocol from a known and
 trusted source.  Many different transport protocols may be used to
 distribute session description, and the nature of the authentication
 will differ from transport to transport.  For some transports,
 security features are often not deployed.  In case a session
 description has not been obtained in a trusted manner, the endpoint
 SHOULD exercise care because, among other attacks, the media sessions
 received may not be the intended ones, the destination where media is
 sent to may not be the expected one, any of the parameters of the
 session may be incorrect, or the media security may be compromised.
 It is up to the endpoint to make a sensible decision taking into
 account the security risks of the application and the user
 preferences and may decide to ask the user whether or not to accept
 the session.
 One transport that can be used to distribute session descriptions is
 the Session Announcement Protocol (SAP).  SAP provides both
 encryption and authentication mechanisms, but due to the nature of
 session announcements it is likely that there are many occasions
 where the originator of a session announcement cannot be
 authenticated because the originator is previously unknown to the
 receiver of the announcement and because no common public key
 infrastructure is available.
 On receiving a session description over an unauthenticated transport
 mechanism or from an untrusted party, software parsing the session
 should take a few precautions.  Session descriptions contain
 information required to start software on the receiver's system.
 Software that parses a session description MUST NOT be able to start
 other software except that which is specifically configured as
 appropriate software to participate in multimedia sessions.  It is
 normally considered inappropriate for software parsing a session

Handley, et al. Standards Track [Page 31] RFC 4566 SDP July 2006

 description to start, on a user's system, software that is
 appropriate to participate in multimedia sessions, without the user
 first being informed that such software will be started and giving
 the user's consent.  Thus, a session description arriving by session
 announcement, email, session invitation, or WWW page MUST NOT deliver
 the user into an interactive multimedia session unless the user has
 explicitly pre-authorised such action.  As it is not always simple to
 tell whether or not a session is interactive, applications that are
 unsure should assume sessions are interactive.
 In this specification, there are no attributes that would allow the
 recipient of a session description to be informed to start multimedia
 tools in a mode where they default to transmitting.  Under some
 circumstances it might be appropriate to define such attributes.  If
 this is done, an application parsing a session description containing
 such attributes SHOULD either ignore them or inform the user that
 joining this session will result in the automatic transmission of
 multimedia data.  The default behaviour for an unknown attribute is
 to ignore it.
 In certain environments, it has become common for intermediary
 systems to intercept and analyse session descriptions contained
 within other signalling protocols.  This is done for a range of
 purposes, including but not limited to opening holes in firewalls to
 allow media streams to pass, or to mark, prioritize, or block traffic
 selectively.  In some cases, such intermediary systems may modify the
 session description, for example, to have the contents of the session
 description match NAT bindings dynamically created.  These behaviours
 are NOT RECOMMENDED unless the session description is conveyed in
 such a manner that allows the intermediary system to conduct proper
 checks to establish the authenticity of the session description, and
 the authority of its source to establish such communication sessions.
 SDP by itself does not include sufficient information to enable these
 checks: they depend on the encapsulating protocol (e.g., SIP or
 RTSP).
 Use of the "k=" field poses a significant security risk, since it
 conveys session encryption keys in the clear.  SDP MUST NOT be used
 to convey key material, unless it can be guaranteed that the channel
 over which the SDP is delivered is both private and authenticated.
 Moreover, the "k=" line provides no way to indicate or negotiate
 cryptographic key algorithms.  As it provides for only a single
 symmetric key, rather than separate keys for confidentiality and
 integrity, its utility is severely limited.  The use of the "k=" line
 is NOT RECOMMENDED, as discussed in Section 5.12.

Handley, et al. Standards Track [Page 32] RFC 4566 SDP July 2006

8. IANA Considerations

8.1. The "application/sdp" Media Type

 One media type registration from RFC 2327 is to be updated, as
 defined below.
    To: ietf-types@iana.org
    Subject: Registration of media type "application/sdp"
    Type name: application
    Subtype name: sdp
    Required parameters: None.
    Optional parameters: None.
    Encoding considerations:
       SDP files are primarily UTF-8 format text.  The "a=charset:"
       attribute may be used to signal the presence of other
       character sets in certain parts of an SDP file (see
       Section 6 of RFC 4566).  Arbitrary binary content cannot
       be directly represented in SDP.
    Security considerations:
       See Section 7 of RFC 4566
    Interoperability considerations:
       See RFC 4566
    Published specification:
       See RFC 4566
    Applications which use this media type:
       Voice over IP, video teleconferencing, streaming media, instant
       messaging, among others.  See also Section 3 of RFC 4566.
    Additional information:
    Magic number(s):   None.
    File extension(s): The extension ".sdp" is commonly used.
    Macintosh File Type Code(s): "sdp "
    Person & email address to contact for further information:
       Mark Handley  <M.Handley@cs.ucl.ac.uk>
       Colin Perkins <csp@csperkins.org>
       IETF MMUSIC working group <mmusic@ietf.org>

Handley, et al. Standards Track [Page 33] RFC 4566 SDP July 2006

    Intended usage: COMMON
    Author/Change controller:
       Authors of RFC 4566
       IETF MMUSIC working group delegated from the IESG

8.2. Registration of Parameters

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

8.2.1. Media Types ("media")

 The set of media types is intended to be small and SHOULD NOT be
 extended except under rare circumstances.  The same rules should
 apply for media names as for top-level media content types, and where
 possible the same name should be registered for SDP as for MIME.  For
 media other than existing top-level media content types, a Standards
 Track RFC MUST be produced for a new top-level content type to be
 registered, and the registration MUST provide good justification why
 no existing media name is appropriate (the "Standards Action" policy
 of RFC 2434 [8].
 This memo registers the media types "audio", "video", "text",
 "application", and "message".
 Note: The media types "control" and "data" were listed as valid in
 the previous version of this specification [6]; 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 RFC 3840 [24].  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 RFC 3840).

8.2.2. Transport Protocols ("proto")

 The "proto" field describes the transport protocol used.  This SHOULD
 reference a standards-track protocol RFC.  This memo registers three
 values: "RTP/AVP" is a reference to RTP [19] used under the RTP
 Profile for Audio and Video Conferences with Minimal Control [20]

Handley, et al. Standards Track [Page 34] RFC 4566 SDP July 2006

 running over UDP/IP, "RTP/SAVP" is a reference to the Secure Real-
 time Transport Protocol [23], and "udp" indicates an unspecified
 protocol over UDP.
 If other RTP profiles are defined in the future, their "proto" name
 SHOULD be specified in the same manner.  For example, an RTP profile
 whose short name is "XYZ" would be denoted by a "proto" field of
 "RTP/XYZ".
 New transport protocols SHOULD be registered with IANA.
 Registrations MUST reference an RFC describing the protocol.  Such an
 RFC MAY be Experimental or Informational, although it is preferable
 that it be Standards Track.  Registrations MUST also define the rules
 by which their "fmt" namespace is managed (see below).

8.2.3. Media Formats ("fmt")

 Each transport protocol, defined by the "proto" field, has an
 associated "fmt" namespace that describes the media formats that may
 be conveyed by that protocol.  Formats cover all the possible
 encodings that might want to be transported in a multimedia session.
 RTP payload formats under the "RTP/AVP" and "RTP/SAVP" profiles MUST
 use the payload type number as their "fmt" value.  If the payload
 type number is dynamically assigned by this session description, an
 additional "rtpmap" attribute MUST be included to specify the format
 name and parameters as defined by the media type registration for the
 payload format.  It is RECOMMENDED that other RTP profiles that are
 registered (in combination with RTP) as SDP transport protocols
 specify the same rules for the "fmt" namespace.
 For the "udp" protocol, new formats SHOULD be registered.  Use of an
 existing media subtype for the format is encouraged.  If no media
 subtype exists, it is RECOMMENDED that a suitable one be registered
 through the IETF process [31] by production of, or reference to, a
 standards-track RFC that defines the transport protocol for the
 format.
 For other protocols, formats MAY be registered according to the rules
 of the associated "proto" specification.
 Registrations of new formats MUST specify which transport protocols
 they apply to.

Handley, et al. Standards Track [Page 35] RFC 4566 SDP July 2006

8.2.4. Attribute Names ("att-field")

 Attribute field names ("att-field") MUST be registered with IANA and
 documented, because of noticeable issues due to conflicting
 attributes under the same name.  Unknown attributes in SDP are simply
 ignored, but conflicting ones that fragment the protocol are a
 serious problem.
 New attribute registrations are accepted according to the
 "Specification Required" policy of RFC 2434, provided that the
 specification includes the following information:
 o  contact name, email address, and telephone number
 o  attribute name (as it will appear in SDP)
 o  long-form attribute name in English
 o  type of attribute (session level, media level, or both)
 o  whether the attribute value is subject to the charset attribute
 o  a one-paragraph explanation of the purpose of the attribute
 o  a specification of appropriate attribute values for this attribute
 The above is the minimum that IANA will accept.  Attributes that are
 expected to see widespread use and interoperability SHOULD be
 documented with a standards-track RFC that specifies the attribute
 more precisely.
 Submitters of registrations should ensure that the specification is
 in the spirit of SDP attributes, most notably that the attribute is
 platform independent in the sense that it makes no implicit
 assumptions about operating systems and does not name specific pieces
 of software in a manner that might inhibit interoperability.
 IANA has registered the following initial set of attribute names
 ("att-field" values), with definitions as in Section 6 of this memo
 (these definitions update those in RFC 2327):

Handley, et al. Standards Track [Page 36] RFC 4566 SDP July 2006

    Name      | Session or Media level? | Dependent on charset?
    ----------+-------------------------+----------------------
    cat       | Session                 | No
    keywds    | Session                 | Yes
    tool      | Session                 | No
    ptime     | Media                   | No
    maxptime  | Media                   | No
    rtpmap    | Media                   | No
    recvonly  | Either                  | No
    sendrecv  | Either                  | No
    sendonly  | Either                  | No
    inactive  | Either                  | No
    orient    | Media                   | No
    type      | Session                 | No
    charset   | Session                 | No
    sdplang   | Either                  | No
    lang      | Either                  | No
    framerate | Media                   | No
    quality   | Media                   | No
    fmtp      | Media                   | No

8.2.5. Bandwidth Specifiers ("bwtype")

 A proliferation of bandwidth specifiers is strongly discouraged.
 New bandwidth specifiers ("bwtype" fields) MUST be registered with
 IANA.  The submission MUST reference a standards-track RFC specifying
 the semantics of the bandwidth specifier precisely, and indicating
 when it should be used, and why the existing registered bandwidth
 specifiers do not suffice.
 IANA has registered the bandwidth specifiers "CT" and "AS" with
 definitions as in Section 5.8 of this memo (these definitions update
 those in RFC 2327).

8.2.6. Network Types ("nettype")

 New network types (the "nettype" field) may be registered with IANA
 if SDP needs to be used in the context of non-Internet environments.
 Although these are not normally the preserve of IANA, there may be
 circumstances when an Internet application needs to interoperate with
 a non-Internet application, such as when gatewaying an Internet
 telephone call into the Public Switched Telephone Network (PSTN).
 The number of network types should be small and should be rarely
 extended.  A new network type cannot be registered without
 registering at least one address type to be used with that network

Handley, et al. Standards Track [Page 37] RFC 4566 SDP July 2006

 type.  A new network type registration MUST reference an RFC that
 gives details of the network type and address type and specifies how
 and when they would be used.
 IANA has registered the network type "IN" to represent the Internet,
 with definition as in Sections 5.2 and 5.7 of this memo (these
 definitions update those in RFC 2327).

8.2.7. Address Types ("addrtype")

 New address types ("addrtype") may be registered with IANA.  An
 address type is only meaningful in the context of a network type, and
 any registration of an address type MUST specify a registered network
 type or be submitted along with a network type registration.  A new
 address type registration MUST reference an RFC giving details of the
 syntax of the address type.  Address types are not expected to be
 registered frequently.
 IANA has registered the address types "IP4" and "IP6" with
 definitions as in Sections 5.2 and 5.7 of this memo (these
 definitions update those in RFC 2327).

8.2.8. Registration Procedure

 In the RFC documentation that registers SDP "media", "proto", "fmt",
 "bwtype", "nettype", and "addrtype" fields, the authors MUST include
 the following information for IANA to place in the appropriate
 registry:
 o  contact name, email address, and telephone number
 o  name being registered (as it will appear in SDP)
 o  long-form name in English
 o  type of name ("media", "proto", "fmt", "bwtype", "nettype", or
    "addrtype")
 o  a one-paragraph explanation of the purpose of the registered name
 o  a reference to the specification for the registered name (this
    will typically be an RFC number)
 IANA may refer any registration to the IESG for review, and may
 request revisions to be made before a registration will be made.

Handley, et al. Standards Track [Page 38] RFC 4566 SDP July 2006

8.3. Encryption Key Access Methods

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

9. SDP Grammar

 This section provides an Augmented BNF grammar for SDP.  ABNF is
 defined in [4].
 ; SDP Syntax
 session-description = proto-version
                       origin-field
                       session-name-field
                       information-field
                       uri-field
                       email-fields
                       phone-fields
                       connection-field
                       bandwidth-fields
                       time-fields
                       key-field
                       attribute-fields
                       media-descriptions
 proto-version =       %x76 "=" 1*DIGIT CRLF
                       ;this memo describes version 0
 origin-field =        %x6f "=" username SP sess-id SP sess-version SP
                       nettype SP addrtype SP unicast-address CRLF
 session-name-field =  %x73 "=" text CRLF
 information-field =   [%x69 "=" text CRLF]
 uri-field =           [%x75 "=" uri CRLF]
 email-fields =        *(%x65 "=" email-address CRLF)
 phone-fields =        *(%x70 "=" phone-number CRLF)
 connection-field =    [%x63 "=" nettype SP addrtype SP
                       connection-address CRLF]
                       ;a connection field must be present
                       ;in every media description or at the
                       ;session-level

Handley, et al. Standards Track [Page 39] RFC 4566 SDP July 2006

 bandwidth-fields =    *(%x62 "=" bwtype ":" bandwidth CRLF)
 time-fields =         1*( %x74 "=" start-time SP stop-time
                       *(CRLF repeat-fields) CRLF)
                       [zone-adjustments CRLF]
 repeat-fields =       %x72 "=" repeat-interval SP typed-time
                       1*(SP typed-time)
 zone-adjustments =    %x7a "=" time SP ["-"] typed-time
                       *(SP time SP ["-"] typed-time)
 key-field =           [%x6b "=" key-type CRLF]
 attribute-fields =    *(%x61 "=" attribute CRLF)
 media-descriptions =  *( media-field
                       information-field
                       *connection-field
                       bandwidth-fields
                       key-field
                       attribute-fields )
 media-field =         %x6d "=" 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

Handley, et al. Standards Track [Page 40] RFC 4566 SDP July 2006

 ; sub-rules of 'e=', see RFC 2822 for definitions
 email-address        = address-and-comment / dispname-and-address
                        / addr-spec
 address-and-comment  = addr-spec 1*SP "(" 1*email-safe ")"
 dispname-and-address = 1*email-safe 1*SP "<" addr-spec ">"
 ; sub-rules of 'p='
 phone-number =        phone *SP "(" 1*email-safe ")" /
                       1*email-safe "<" phone ">" /
                       phone
 phone =               ["+"] DIGIT 1*(SP / "-" / DIGIT)
 ; sub-rules of 'c='
 connection-address =  multicast-address / unicast-address
 ; sub-rules of 'b='
 bwtype =              token
 bandwidth =           1*DIGIT
 ; sub-rules of 't='
 start-time =          time / "0"
 stop-time =           time / "0"
 time =                POS-DIGIT 9*DIGIT
                       ; Decimal representation of NTP time in
                       ; seconds since 1900.  The representation
                       ; of NTP time is an unbounded length field
                       ; containing at least 10 digits.  Unlike the
                       ; 64-bit representation used elsewhere, time
                       ; in SDP does not wrap in the year 2036.
 ; sub-rules of 'r=' and 'z='
 repeat-interval =     POS-DIGIT *DIGIT [fixed-len-time-unit]
 typed-time =          1*DIGIT [fixed-len-time-unit]
 fixed-len-time-unit = %x64 / %x68 / %x6d / %x73
 ; sub-rules of 'k='
 key-type =            %x70 %x72 %x6f %x6d %x70 %x74 /     ; "prompt"
                       %x63 %x6c %x65 %x61 %x72 ":" text / ; "clear:"
                       %x62 %x61 %x73 %x65 "64:" base64 /  ; "base64:"
                       %x75 %x72 %x69 ":" uri              ; "uri:"
 base64      =         *base64-unit [base64-pad]

Handley, et al. Standards Track [Page 41] RFC 4566 SDP July 2006

 base64-unit =         4base64-char
 base64-pad  =         2base64-char "==" / 3base64-char "="
 base64-char =         ALPHA / DIGIT / "+" / "/"
 ; sub-rules of 'a='
 attribute =           (att-field ":" att-value) / att-field
 att-field =           token
 att-value =           byte-string
 ; sub-rules of 'm='
 media =               token
                       ;typically "audio", "video", "text", or
                       ;"application"
 fmt =                 token
                       ;typically an RTP payload type for audio
                       ;and video media
 proto  =              token *("/" token)
                       ;typically "RTP/AVP" or "udp"
 port =                1*DIGIT
 ; generic sub-rules: addressing
 unicast-address =     IP4-address / IP6-address / FQDN / extn-addr
 multicast-address =   IP4-multicast / IP6-multicast / FQDN
                       / extn-addr
 IP4-multicast =       m1 3( "." decimal-uchar )
                       "/" ttl [ "/" integer ]
                       ; IPv4 multicast addresses may be in the
                       ; range 224.0.0.0 to 239.255.255.255
 m1 =                  ("22" ("4"/"5"/"6"/"7"/"8"/"9")) /
                       ("23" DIGIT )
 IP6-multicast =       hexpart [ "/" integer ]
                       ; IPv6 address starting with FF
 ttl =                 (POS-DIGIT *2DIGIT) / "0"
 FQDN =                4*(alpha-numeric / "-" / ".")
                       ; fully qualified domain name as specified
                       ; in RFC 1035 (and updates)

Handley, et al. Standards Track [Page 42] RFC 4566 SDP July 2006

 IP4-address =         b1 3("." decimal-uchar)
 b1 =                  decimal-uchar
                       ; less than "224"
 ; The following is consistent with RFC 2373 [30], Appendix B.
 IP6-address =         hexpart [ ":" IP4-address ]
 hexpart =             hexseq / hexseq "::" [ hexseq ] /
                       "::" [ hexseq ]
 hexseq  =             hex4 *( ":" hex4)
 hex4    =             1*4HEXDIG
 ; Generic for other address families
 extn-addr =           non-ws-string
 ; generic sub-rules: datatypes
 text =                byte-string
                       ;default is to interpret this as UTF8 text.
                       ;ISO 8859-1 requires "a=charset:ISO-8859-1"
                       ;session-level attribute to be used
 byte-string =         1*(%x01-09/%x0B-0C/%x0E-FF)
                       ;any byte except NUL, CR, or LF
 non-ws-string =       1*(VCHAR/%x80-FF)
                       ;string of visible characters
 token-char =          %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39
                       / %x41-5A / %x5E-7E
 token =               1*(token-char)
 email-safe =          %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF
                       ;any byte except NUL, CR, LF, or the quoting
                       ;characters ()<>
 integer =             POS-DIGIT *DIGIT
 ; generic sub-rules: primitives
 alpha-numeric =       ALPHA / DIGIT
 POS-DIGIT =           %x31-39 ; 1 - 9

Handley, et al. Standards Track [Page 43] RFC 4566 SDP July 2006

 decimal-uchar =       DIGIT
                       / POS-DIGIT DIGIT
                       / ("1" 2*(DIGIT))
                       / ("2" ("0"/"1"/"2"/"3"/"4") DIGIT)
                       / ("2" "5" ("0"/"1"/"2"/"3"/"4"/"5"))
 ; external references:
       ; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 4234
       ; URI-reference: from RFC 3986
       ; addr-spec: from RFC 2822

10. Summary of Changes from RFC 2327

 The memo has been significantly restructured, incorporating a large
 number of clarifications to the specification in light of use.  With
 the exception of those items noted below, the changes to the memo are
 intended to be backward-compatible clarifications.  However, due to
 inconsistencies and unclear definitions in RFC 2327 it is likely that
 some implementations interpreted that memo in ways that differ from
 this version of SDP.
 The ABNF grammar in Section 9 has been extensively revised and
 updated, correcting a number of mistakes and incorporating the RFC
 3266 IPv6 extensions.  Known inconsistencies between the grammar and
 the specification text have been resolved.
 A media type registration for SDP is included.  Requirements for the
 registration of attributes and other parameters with IANA have been
 clarified and tightened (Section 8).  It is noted that "text" and
 "message" are valid media types for use with SDP, but that "control"
 and "data" are under-specified and deprecated.
 RFC 2119 terms are now used throughout to specify requirements
 levels.  Certain of those requirements, in particular in relation to
 parameter registration, are stricter than those in RFC 2327.
 The "RTP/SAVP" RTP profile and its "fmt" namespace are registered.
 The attributes "a=inactive" and "a=maxptime" have been added.
 RFC 2327 mandated that either "e=" or "p=" was required.  Both are
 now optional, to reflect actual usage.
 The significant limitations of the "k=" field are noted, and its use
 is deprecated.
 Most uses of the "x-" prefix notation for experimental parameters are
 disallowed and the other uses are deprecated.

Handley, et al. Standards Track [Page 44] RFC 4566 SDP July 2006

11. 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
 Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin, Ross
 Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Steve Hanna,
 Jonathan Lennox, Keith Drage, Sean Olson, Bernie Hoeneisen, Jonathan
 Rosenberg, John Elwell, Flemming Andreasen, Jon Peterson, and Spencer
 Dawkins.

12. References

12.1. Normative References

 [1]   Mockapetris, P., "Domain names - concepts and facilities", STD
       13, RFC 1034, November 1987.
 [2]   Mockapetris, P., "Domain names - implementation and
       specification", STD 13, RFC 1035, November 1987.
 [3]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [4]   Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
       Specifications: ABNF", RFC 4234, October 2005.
 [5]   Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
       63, RFC 3629, November 2003.
 [6]   Handley, M. and V. Jacobson, "SDP: Session Description
       Protocol", RFC 2327, April 1998.
 [7]   Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
       Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
       January 2005.
 [8]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
       Considerations Section in RFCs", BCP 26, RFC 2434, October
       1998.
 [9]   Alvestrand, H., "Tags for the Identification of Languages", BCP
       47, RFC 3066, January 2001.
 [10]  Olson, S., Camarillo, G., and A. Roach, "Support for IPv6 in
       Session Description Protocol (SDP)", RFC 3266, June 2002.

Handley, et al. Standards Track [Page 45] RFC 4566 SDP July 2006

 [11]  Faltstrom, P., Hoffman, P., and A. Costello,
       "Internationalizing Domain Names in Applications (IDNA)", RFC
       3490, March 2003.
 [12]  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
       RFC 3548, July 2003.

12.2. Informative References

 [13]  Mills, D., "Network Time Protocol (Version 3) Specification,
       Implementation", RFC 1305, March 1992.
 [14]  Handley, M., Perkins, C., and E. Whelan, "Session Announcement
       Protocol", RFC 2974, October 2000.
 [15]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
       Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
       Session Initiation Protocol", RFC 3261, June 2002.
 [16]  Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
       Protocol (RTSP)", RFC 2326, April 1998.
 [17]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
       Session Description Protocol (SDP)", RFC 3264, June 2002.
 [18]  Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
       "Grouping of Media Lines in the Session Description Protocol
       (SDP)", RFC 3388, December 2002.
 [19]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", STD 64,
       RFC 3550, July 2003.
 [20]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
       Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
 [21]  Casner, S., "Session Description Protocol (SDP) Bandwidth
       Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
       July 2003.
 [22]  Huitema, C., "Real Time Control Protocol (RTCP) attribute in
       Session Description Protocol (SDP)", RFC 3605, October 2003.
 [23]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
       Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
       3711, March 2004.

Handley, et al. Standards Track [Page 46] RFC 4566 SDP July 2006

 [24]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
       User Agent Capabilities in the Session Initiation Protocol
       (SIP)", RFC 3840, August 2004.
 [25]  Westerlund, M., "A Transport Independent Bandwidth Modifier for
       the Session Description Protocol (SDP)", RFC 3890, September
       2004.
 [26]  International Telecommunication Union, "H.323 extended for
       loosely coupled conferences", ITU Recommendation H.332,
       September 1998.
 [27]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
       Norrman, "Key Management Extensions for Session Description
       Protocol (SDP) and Real Time Streaming Protocol (RTSP)", RFC
       4567, July 2006.
 [28]  Andreasen, F., Baugher, M., and D. Wing, "Session Description
       Protocol (SDP) Security Descriptions for Media Streams", RFC
       4568, July 2006.
 [29]  Resnick, P., "Internet Message Format", RFC 2822, April 2001.
 [30]  Hinden, R. and S. Deering, "IP Version 6 Addressing
       Architecture", RFC 2373, July 1998.
 [31]  Freed, N. and J. Klensin, "Media Type Specifications and
       Registration Procedures", BCP 13, RFC 4288, December 2005.

Handley, et al. Standards Track [Page 47] RFC 4566 SDP July 2006

Authors' Addresses

 Mark Handley
 University College London
 Department of Computer Science
 Gower Street
 London  WC1E 6BT
 UK
 EMail: M.Handley@cs.ucl.ac.uk
 Van Jacobson
 Packet Design
 2465 Latham Street
 Mountain View, CA  94040
 USA
 EMail: van@packetdesign.com
 Colin Perkins
 University of Glasgow
 Department of Computing Science
 17 Lilybank Gardens
 Glasgow  G12 8QQ
 UK
 EMail: csp@csperkins.org

Handley, et al. Standards Track [Page 48] RFC 4566 SDP July 2006

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Handley, et al. Standards Track [Page 49]

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