Internet Engineering Task Force                                MMUSIC WG
INTERNET-DRAFT                                        Mark Handley/ACIRI
draft-ietf-mmusic-sdp-new-05.txt              Van Jacobson/Packet Design
                                                       Colin Perkins/ISI
                                                        21 November 2001
                                                         8 February 2002
                                                    Expires: May August 2002

                   SDP: Session Description Protocol

Status of this Memo

provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
may also distribute working documents as Internet-Drafts.


The list of current Internet-Drafts can be accessed at

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

This document is a product of the Multiparty Multimedia Session Control
are solicited and should be addressed to the working group's mailing
list at confctrl@isi.edu and/or the authors.


     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.

1.  Introduction

used to advertise multimedia conferences and communicate the conference
addresses and conference tool-specific information necessary for
participation.  This document defines a session description protocol for
this purpose, and for general real-time multimedia session description
purposes.  This draft does not describe multicast address allocation or
the distribution of SDP messages.  These are described in accompanying
drafts.  SDP is not intended for negotiation of media encodings.

2.  Background

extensively for multimedia conferencing.  Such conferences usually have
the property that tight coordination of conference membership is not
know the conference's multicast group address and the UDP ports for the
conference data streams.

Session directories assist the advertisement of conference sessions and
communicate the relevant conference setup information to prospective
participants.  SDP is designed to convey such information to recipients.
a transport protocol, and is intended to use different transport
protocols as appropriate including the Session Announcement Protocol
[4], Session Initiation Protocol [11], Real-Time Streaming Protocol
[12], electronic mail using the MIME extensions, and the Hypertext
Transport Protocol.

range of network environments and applications than just multicast
session directories.  However, it is not intended to support negotiation
of session content or media encodings - this is viewed as outside the
scope of session description.

3.  Glossary of Terms

within the context of this document.

    A multimedia conference is a set of two or more communicating users
    along with the software they are using to communicate.

    A multimedia session is a set of multimedia senders and receivers
    and the data streams flowing from senders to receivers.  A
    multimedia conference is an example of a multimedia session.

Session Advertisement
    See session announcement.

Session Announcement
    A session announcement is a mechanism by which a session description
    description was not explicitly requested by the user.

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

3.1.  Terminology


4.  Examples of SDP Usage

4.1.  Session Initiation

The Session Initiation Protocol, SIP [11] is an application-layer
control protocol for creating, modifying and terminating sessions such
as Internet multimedia conferences, Internet telephone calls and
multimedia distribution.  The SIP messages used to create sessions carry
session descriptions which allow participants to agree on a set of
compatible media types. These session descriptions are commonly
formatted using SDP.

4.2.  Streaming media

The Real Time Streaming Protocol, RTSP [12], 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. It is
necessary for RTSP to convey a description of the session to be
controlled: SDP is often used for this purpose.

4.3.  Multicast Announcement

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

One protocol commonly used to implement such a distributed directory is
the Session Announcement Protocol, SAP [4]. SDP provides the recommended
session description format for such announcements.

4.4.  Email and the World Wide Web

Alternative means of conveying session descriptions include electronic
use of the MIME content type ``application/sdp'' MUST be used.  This
enables the automatic launching of applications for participation in the

Note that announcements of multicast sessions made only via email or the
session announcement can necessarily receive the session because the
server or reception of email is possible outside this scope.  SAP
announcements do not suffer from this mismatch.

5.  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
internetwork, although it is sufficiently general that it can describe
conferences in other network environments.


Thus far, multicast based sessions on the Internet have differed from

many other forms of conferencing in that anyone receiving the traffic
sufficient information to enable joining and participating in the
be relevant.

Thus SDP includes:

o   Session name and purpose

o   Time(s) the session is active

o   The media comprising the session

o   Information to receive those media (addresses, ports, formats and so

additional information may also be desirable:

o   Information about the bandwidth to be used by the conference

o   Contact information for the person responsible for the session


5.1.  Media Information

SDP includes:

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



For an IP multicast session, the following are also conveyed:

o   Multicast address for media

o   Transport Port for media

This address and port are the destination address and destination port

For an IP unicast session, the following are conveyed:

o   Remote address for media

o   Transport port for contact address

establish a control channel for the actual media flow.

5.2.  Timing Information


SDP can convey:

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

     one hour"

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

5.3.  Private Sessions

It is possible to create both public sessions and private sessions. SDP
itself does not distinguish between these: private sessions are
typically conveyed by encrypting the session description during
distribution.  The details of how encryption is performed are dependent
on the mechanism used to convey SDP - e.g. mechanisms are defined for
SDP transported using SAP [4] and SIP [11].

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.

5.4.  Obtaining Further Information about a Session

A session description should convey enough information to decide whether
in the form of Universal Resources Identifiers (URIs) for more
information about the session.

5.5.  Categorisation

When many session descriptions are being distributed by SAP, or any
other advertisement mechanism, it may be desirable to filter
announcements that are of interest from those that are not.  SDP
supports a categorisation mechanism for sessions that is capable of
being automated.

5.6.  Internationalization

The SDP specification recommends the use of the ISO 10646 character sets
represented.  However, to assist in compact representations, SDP also
Internationalization only applies to free-text fields (session name and
background information), and not to SDP as a whole.

6.  SDP Specification

SDP session descriptions are entirely textual using the ISO 10646
maximum permissable size of a session description is limited (e.g. SAP
announcements; SIP transported in UDP), the encoding is deliberately
compact.  Also, since announcements may be transported via very
unreliable means (e.g., email) or damaged by an intermediate caching
server, the encoding was designed with strict order and formatting rules
so that most errors would result in malformed announcements which could
be detected easily and discarded.  This also allows rapid discarding of
encrypted announcements for which a receiver does not have the correct

will be case-significant unless a specific field defines otherwise.
Whitespace MUST NOT be used either side of the `=' sign. In general
character or a free format string.

A session description consists of a session-level description (details
several media-level descriptions (details that apply only to a single
media stream).

description or end of the whole session description.  In general,
an equivalent media-level value.

Some lines in each description are REQUIRED and some are OPTIONAL but
enhances error detection and allows for a simple parser).  OPTIONAL
items are marked with a `*'.

     Session description
          v=  (protocol version)
          o=  (owner/creator and session identifier).
          s=  (session name)
          i=* (session information)
          u=* (URI of description)
          e=* (email address)
          p=* (phone number)
          b=* (bandwidth information)
             One or more time descriptions (see below)
          z=* (time zone adjustments)
          k=* (encryption key)
          a=* (zero or more session attribute lines)
             Zero or more media descriptions (see below)

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

     Media description
          m=  (media name and transport address)
          i=* (media title)
          b=* (bandwidth information)
          k=* (encryption key)
          a=* (zero or more media attribute lines)

extensible -- an SDP parser MUST completely ignore any announcement that
and tailoring it to particular applications or media.  Some attributes
be added on an application-, media- or session-specific basis.  An SDP
parser MUST ignore any attribute it doesn't understand.

level section applies to all the media of that session unless overridden
if it were given a `recvonly' attribute.

An example SDP description is:

     o=mhandley 2890844526 2890842807 IN IP4
     s=SDP Seminar
     i=A Seminar on the session description protocol
     e=mjh@isi.edu (Mark Handley)
     c=IN IP4
     t=2873397496 2873404696
     m=audio 49170 RTP/AVP 0
     m=video 51372 RTP/AVP 31
     m=application 32416 udp wb

Text records such as the session name and information are bytes strings
and also accept records terminated with a single newline character.  By
encoding, but this default may be changed using the `charset' attribute.

Protocol Version


There is no minor version number.



ids.  <username> MUST NOT contain spaces.  <session id> is a numeric
number for this announcement.  It is needed for proxy announcements to
detect which of several announcements for the same session is the most

<version> is increased when a modification is made to the session data.
Again, it is RECOMMENDED (but not mandatory) that an NTP timestamp is
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.

this version of this session description, and the subfields excepting
the version taken together identify the session irrespective of any

Session Name

s=<session name>

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

Session and Media Information

i=<session description>

RECOMMENDED for session announcements, and user interfaces for composing
sessions should require text to be entered.  If it is present it must
contain ISO 10646 characters (but see also the `charset' attribute


for feedback and questions.




o   The URI should be a pointer to additional information about the

o   This field is OPTIONAL, but if it is present it MUST be specified
    before the first media field

o   No more than one URI field is allowed per session description

Email Address and Phone Number

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

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

o   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

    media field.


o   Phone numbers should be given in the conventional international


o   Both email addresses and phone numbers can have an optional free
    text string associated with them, normally giving the name of the
    person who may be contacted.  This should be enclosed in parenthesis
    if it is present.  For example:

        e=mjh@isi.edu (Mark Handley)

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

        e=Mark Handley <mjh@isi.edu>

    the appropriate charset session-level attribute is set.

Connection Data


The ``c='' field contains connection data.

A session announcement MUST contain either one ``c='' field in each
field per media description, in which case the per-media values override
the session-level settings for the relevant media.


for sessions that are not IP based.  Currently only IP4 and IP6 are

fields may be added after the connection address depending on the value
of the <address type> field.

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

    multicast group address.  If the conference is not multicast, then
    the connection address contains the unicast IP address of the
    expected data source or data relay or data sink as determined by
    additional attribute fields.  It is not expected that unicast
    addresses will be given in a session description that is
    communicated by a multicast announcement, though this is not

o   Conferences using an IP multicast connection address MUST also have
    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.

    a separator.  An example is:

         c=IN IP6 FF00:03AD::7F2E:172A:1E24/127

    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
    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:


    Multicast addresses so assigned are contiguously allocated above the
    base address, so that, for example:

         c=IN IP4

    would state that addresses, and are to
    including multiple ``c='' lines in a media description:

         c=IN IP4
         c=IN IP4
         c=IN IP4


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



    media, and is OPTIONAL.

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

    bandwidth figure.

o   Two modifiers are initially defined:

CT    Conference Total: If the bandwidth of a session or media in a
      session is different from the bandwidth implicit from the scope, a
      proposed upper limit to the bandwidth used.  The primary purpose
      sessions can co-exist simultaneously.

AS    Application-Specific Maximum: The bandwidth is interpreted to be
      maximum bandwidth.  Normally this will coincide with what is set
      For RTP based applications, AS gives the RTP ``session bandwidth''
      as defined in section 6.2 of [2].

    site, although there may be many sites sending simultaneously.

o   Extension Mechanism: Tool writers can define experimental bandwidth


    SDP parsers MUST ignore bandwidth fields with unknown modifiers.
    given, they are recommended to be short.

Times, Repeat Times and Time Zones

t=<start time>  <stop time>

o   ``t='' fields specify the start and stop times for a session.
    Multiple ``t='' fields MAY be used if a session is active at
    specifies an additional period of time for which the session will be
    times of the repeat sequence.

    the session respectively.  These values are the decimal
    representation of Network Time Protocol (NTP) time values in seconds

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


    User interfaces SHOULD strongly discourage the creation of unbounded
    and permanent sessions as they give no information about when the
    session is actually going to terminate, and so make scheduling

    The general assumption may be made, when displaying unbounded
    other than this is required, an end-time should be given and
    modified as appropriate when new information becomes available about
    when the session should really end.

    Permanent sessions may be shown to the user as never being active
    unless there are associated repeat times which state precisely when
    the session will be active.  In general, permanent sessions SHOULD
    than 2 months, and should be discouraged for sessions expected to
    have a duration of less than 6 months.


    might be:

        t=3034423619 3042462419
        r=604800 3600 0 90000

    immediately followed by a single case-sensitive character.
    Fractional units are not allowed - a smaller unit should be used
    instead.  The following unit specification characters are allowed:

         d - days (86400 seconds)
         h - hours (3600 seconds)
         m - minutes (60 seconds)
         s - seconds (allowed for completeness but not recommended)

    Thus, the above announcement could also have been written:

        r=7d 1h 0 25h

    Monthly and yearly repeats cannot currently be directly specified
    be used to explicitly list the session times.


    specify offsets from the base repeat times. This is required because
    different time zones change time at different times of day,
    different countries change to or from daylight time on different

    and summer, it must  be possible to specify unambiguously by whose
    time zone a session is scheduled.  To simplify this task for
    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

    An example might be:

    z=2882844526 -1h 2898848070 0

    This specifies that at time 2882844526 the time base by which the
    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.

    the session announcement will be modified periodically rather than
    transmit several years worth of adjustments in one announcement.

Encryption Keys

k=<method>:<encryption key>

o   The session description protocol MAY be used to convey encryption
    media entry as required.

    document, but see [3].

    following methods are defined:

    k=clear:<encryption key>
        under the AV profile) is included untransformed in this key

    k=base64:<encoded encryption key>
        base64 encoded because it includes characters that are
        prohibited in SDP.

    k=uri:<URI to obtain key>
        A Universal Resource Identifier as used by WWW clients is
        included in this key field.  The URI refers to the data
        containing the key, and may require additional authentication
        obtained until the user wishes to join the session to reduce
        synchronisation of requests to the WWW server(s).

        media stream referred to by this key field is encrypted.  The
        user should be prompted for the key when attempting to join the
        decrypt the media streams.



Attributes are the primary means for extending SDP.  Attributes may be
attributes, or both.

attributes and add information about the media stream.  Attribute fields
can also be added before the first media field; these "session-level"
attributes convey additional information that applies to the conference
as a whole rather than to individual media; an example might be the
conference's floor control policy.

Attribute fields may be of two forms:

o   property attributes.  A property attribute is simply of the form
    attribute conveys that the attribute is a property of the session.
    An example might be ``a=recvonly''.

o   value attributes.  A value attribute is of the form
    could have the value attribute ``a=orient:landscape''

Attribute interpretation depends on the media tool being invoked.  Thus
receivers of session descriptions should be configurable in their
interpretation of announcements in general and of attributes in

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

Unlike other text fields, attribute values are NOT normally affected by
the `charset' attribute as this would make comparisons against known
interpreted in the session charset rather than in ISO-10646.

inadvertent collision with registered attributes.  In either case, if an
attribute is received that is not understood, it should simply be
ignored by the receiver.

Media Announcements


A session description may contain a number of media descriptions.  Each
A media field also has several sub-fields:

    though this list may be extended as new communication modalities
    information, and the latter is bulk-data transfer such as
    multicasting of program executables which will not typically be
    displayed to the user.  ``control'' is used to specify an additional
    conference control channel for the session.

o   The second sub-field is the transport port to which the media stream
    will be sent.  The meaning of the transport port depends on the
    network being used as specified in the relevant ``c'' field and on
    the transport protocol defined in the third sub-field.  Other ports
    should be derived algorithmically from the base media port.

    1024 to 65535 inclusive.  For RTP compliance it SHOULD be an even

    For applications where hierarchically encoded streams are being sent
    to a unicast address, it may be necessary to specify multiple
    for IP multicast addresses in the ``c='' field:



         m=video 49170/2 RTP/AVP 31

    transport protocol and 31 is the format (see below).

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

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

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

    protocol values are dependent on the address-type field in the
    ``c='' fields.  Thus a ``c='' field of IP4 defines that the
    transport protocol runs over IP4.  For IP4, it is normally expected
    that most media traffic will be carried as RTP over UDP.  The
    following transport protocols are preliminarily defined, but may be
    extended through registration of new protocols with IANA:

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

    - udp  - User Datagram Protocol
    If an application uses a single combined proprietary media format
    and transport protocol over UDP, then simply specifying the
    transport protocol as udp and using the format field to distinguish
    the combined protocol is recommended.  If a transport protocol is
    distinguished by a session directory, then specifying the transport
    protocol and media format separately is necessary.  RTP is an
    example of a transport-protocol that carries multiple payload
    formats that must be distinguished by the session directory for it
    to know how to start appropriate tools, relays, mixers or recorders.

    media format is that the same standard media formats may be carried
    over different transport protocols even when the network protocol is
    In addition, relays and monitoring tools that are transport-
    protocol-specific but format-independent are possible.

    For RTP media streams operating under the RTP Audio/Video Profile

    and video, these SHOULD reference a MIME sub-type describing the
    format under the `audio' and `video' top-level MIME types.

    formats is the default format for the session.

    For media whose transport protocol is not RTP or UDP the format
    field is protocol specific.  Such formats should be defined in an
    additional specification document.

    provide a dynamic binding of media encoding to RTP payload type.
    The encoding names in the RTP AV Profile do not specify unique audio
    payload type number should be used to specify the format for static
    payload types and the payload type number along with additional
    encoding information should be used for dynamically allocated
    payload types.

    channel audio sampled at 8KHz. 8kHz.  This is completely defined in the
    such a stream sent to UDP port 49232 is:


         m=audio 49232 RTP/AVP 0

    type 98 for such a stream, additional information is required to
    decode it:

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

    The general form of an rtpmap attribute is:


    audio channels.  This parameter may be omitted if the number of
    channels is one provided no additional parameters are needed.
    For video streams, no encoding parameters are currently specified.

    Additional parameters may be defined in the future, but codec-
    specific parameters should not be added.  Parameters added to an
    rtpmap attribute should only be those required for a session
    directory to make the choice of appropriate media too to participate

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

         m=audio 49230 RTP/AVP 96 97 98
         a=rtpmap:96 L8/8000
         a=rtpmap:97 L16/8000
         a=rtpmap:98 L16/11025/2

    RTP profiles that specify the use of dynamic payload types must
    encoding names if that profile is to be used with SDP.

    Experimental encoding formats can also be specified using rtpmap.
    RTP formats that are not registered as standard format names must be
    called GSMLPC using dynamic payload type 99 could be specified as:

         m=audio 49232 RTP/AVP 99
         a=rtpmap:99 X-GSMLPC/8000

    Such an experimental encoding requires that any site wishing to
    receive the media stream has relevant configured state in its
    session directory to know which tools are appropriate.

    Note that RTP audio formats typically do not include information



    Application Formats:

    wb:   LBL Whiteboard (transport: udp)

    nt:   UCL Network Text Editor (transport: udp)

Suggested Attributes

The following attributes are suggested.  Since application writers may

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

    Like the cat attribute, this is to assist identifying wanted
    sessions at the receiver.  This allows a receiver to select
    interesting session based on keywords describing the purpose of the
    session.  It is a session-level attribute. It is a charset dependent
    attribute, meaning that its value should be interpreted in the
    charset specified for the session description if one is specified,
    or by default in ISO 10646/UTF-8.

a=tool:<name and version of tool>
    not dependent on charset.

a=ptime:<packet time>
    This gives the length of time in milliseconds represented by the
    and it is intended as a recommendation for the
    not dependent on charset.

a=maxptime:<maximum packet time>
    The maximum amount of media which can be encapsulated in each
    packet, expressed as time in milliseconds. The time shall be
    represents. The time SHOULD be a multiple of the frame size. This is
    probably only meaningful for audio data. It is a media attribute,
    and is not dependent on charset.  Note that this attribute was
    introduced after RFC 2327, and non updated implementations will
    ignore this attribute.

    See the section on Media Announcements (the ``m='' field). This is a
    media attribute.

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

    This specifies that the tools should be started in send and receive
    mode.  This is necessary for interactive conferences with tools such
    session or media attribute, and is not dependent on charset.

    be used as a media attribute, and is not dependent on charset. Note
    that sendonly applies only to the media, and any associated control
    protocol (e.g. RTCP) SHOULD still be received and processed as

    This specifies that the tools should be started in inactive mode.
    This is necessary for interactive conferences where users can put
    Note that an RTP based system SHOULD still send RTCP, even if
    started inactive. It can be either a session or media attribute, and
    is not dependent on charset.

a=orient:<whiteboard orientation>
    Normally this is only used in a whiteboard media specification. It

a=type:<conference type>
    This specifies the type of the conference.  Suggested values are
    to ``mute'' new sites joining the conference.

    Specifying the attribute type:H332 indicates that this loosely
    H.332 specification [10].  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.

    dependent on charset.

a=charset:<character set>
    This specifies the character set to be used to display the session
    specified with the following SDP attribute:


    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 byte strings.

    Note that a character set specified MUST still prohibit the use of
    the use of these characters MUST define a quoting mechanism that
    prevents these bytes appearing within text fields.

a=sdplang:<language tag>
    As a session level attribute, it specifies the language for the
    session description.  As a media level attribute, it specifies the
    language for any media-level SDP information field associated with
    that media.  Multiple sdplang attributes can be provided either at
    session or media level if multiple languages in the session
    description or media use multiple languages, in which case the order
    of the attributes indicates the order of importance of the various
    languages in the session or media from most important to least

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

    The sdplang attribute value must be a single RFC 1766 language tag
    sdplang attribute SHOULD be specified when a session is of
    sufficient scope to cross geographic boundaries where the language
    different language from the locally assumed norm.

a=lang:<language tag>
    As a session level attribute, it specifies the default language for
    the session being described.  As a media level attribute, it
    specifies the language for that media, overriding any session-level
    language specified.  Multiple lang attributes can be provided either
    at session or media level if multiple languages if the session
    description or media use multiple languages, in which case the order
    of the attributes indicates the order of importance of the various
    languages in the session or media from most important to least

    attribute SHOULD be specified when a session is of sufficient scope
    to cross geographic boundaries where the language of recipients
    from the locally assumed norm.

a=framerate:<frame rate>
    intended as a recommendation for the encoding of video data.
    Decimal representations of fractional values using the notation

    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
    suggested meaning:


    5   - the default behaviour given no quality suggestion.

        still usable.

a=fmtp:<format> <format specific parameters>
    This attribute allows parameters that are specific to a particular
    media.  Format-specific parameters may be any set of parameters
    that will use this format.


6.1.  Communicating Conference Control Policy

There is some debate over the way conference control policy should be
communicated.  In general, the authors believe that an implicit

declarative style of specifying conference control is desirable where

A simple declarative style uses a single conference attribute field
before the first media field, possibly supplemented by properties such
conveys the conference control policy.  An example might be:


communicate the details of an unusual conference control policy.  If
this is the case, then a conference attribute specifying external
used to specify the conference control tools and configuration data for

          c=IN IP4
          m=audio 49230 RTP/AVP 0
          m=video 49232 RTP/AVP 31
          m=application 12349 udp wb
          m=control 49234 H323 mc
          c=IN IP4

controller is given.

In this document, only the declarative style of conference control
declaration is specified.  Other forms of conference control should
specify an appropriate type attribute, and should define the
implications this has for control media.

7.  Security Considerations

SDP is a session description format that describes multimedia sessions.
A session description SHOULD NOT be trusted unless it has been obtained
by an authenticated transport protocol from a trusted source.  Many
different transport protocols may be used to distribute session
description, and the nature of the authentication will differ from
transport to transport.

One transport that will frequently be used to distribute session
descriptions is the Session Announcement Protocol (SAP).  SAP provides
both encryption and authentication mechanisms but due to the nature of
session announcements it is likely that there are many occasions where
the originator of a session announcement cannot be authenticated because
they are previously unknown to the receiver of the announcement and
because no common public key infrastructure is available.

On receiving a session description over an unauthenticated transport
mechanism or from an untrusted party, software parsing the session
should take a few precautions.  Session description contain information
required to start software on the receivers system.  Software that
parses a session description MUST not be able to start other software
except that which is specifically configured as appropriate software to
participate in multimedia sessions.  It is normally considered
INAPPROPRIATE for software parsing a session description to start, on a
user's system, software that is appropriate to participate in multimedia
sessions, without the user first being informed that such software will
be started and giving their consent.  Thus a session description
arriving by session announcement, email, sessioR multimedia,session page
SHOULD NOT deliver the user into an interactive
simple to tell whether a session is interactive or not, applications
that are unsure should assume sessions are interactive.

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

Session descriptions may be parsed at intermediate systems such as
the participation in multimedia sessions.  It is considered
INAPPROPRIATE for a firewall to open such holes for unicast data streams

unless the session description comes in a request from inside the
firewall.  For multicast sessions, it is likely that local
administrators will apply their own policies, but the exclusive use of
"local" or "site-local" administrative scope within the firewall and the
refusal of the firewall to open a hole for such scopes will provide
separation of global multicast sessions from local ones.

Appendix A: SDP Grammar

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

   ; SDP Syntax
   announcement =        proto-version

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

                         nettype SP addrtype SP addr CRLF

   session-name-field =  ["s="  "s=" text CRLF] CRLF

   information-field =   ["i=" text CRLF]

   uri-field =           ["u=" uri CRLF]


                         connection-address CRLF]
                         ;a connection field must be present
                         ;in every media description or at the


                         *(CRLF repeat-fields) CRLF)
                         [zone-adjustments CRLF]

                         1*(SP typed-time)




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

                         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 originating username/host

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

   nettype =             token
                         ;typically "IN"

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

   ; sub-rules of 'u='
   uri =                 URI-reference; defined in RFC2396/2732

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

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

   ; sub-rules of 'p='
                         1*email-safe "<" phone ">" |

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

                         ; Should this use the tel: URL syntax?

   ; sub-rules of 'c='
   connection-address =  multicast-address
                         | addr unicast-address
   ; sub-rules of 'b='
   bwtype =              token

   bandwidth =           1*DIGIT

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

   stop-time =           time | "0"

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

   repeat-interval =     typed-time

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


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

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

   key-method =          token
   ; sub-rules of 'a='

   att-field =           token

   att-value =           byte-string

   ; sub-rules of 'm='
   media =               token
                         ;or "data"

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

   proto  =              token "/" token
                         | token

   port =                1*DIGIT
                         ;for UDP based media

   ; generic sub-rules: addressing
   multicast-address =   addr "/" ttl [ "/" integer ]
                         ;IPv4 multicast addresses must be in the range
                         ; to
                         ;IPv6 multicast addresses must begin with the byte
                         ;FF or include an IPv4 multicast address

   unicast-address =      addr
                         ; a unicast address or domain name

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

   addr =                IPv4address | IPv6address | FQDN | extension-addr
   FQDN =                *( domainlabel "." ) toplabel

   domainlabel =         alpha-numeric restoflabel

   toplabel =            ALPHA restoflabel

   restoflabel =         *(*("-") alpha-numeric)

   extension-addr =      non-ws-string

   ; generic sub-rules: datatypes
   text =                byte-string
                         ;default is to interpret this as IS0-10646 UTF8
                         ;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 US-ASCII, or high-bit, characters

   token-char =          %x21|%x23-27|%x2a-2b|%x2d-2e|%x30-39|
                         ; definition from RFC 2045 -
                         ; "any (US-ASCII) CHAR except SPACE, CTLs,
                         ; or tspecials".
                         ; the tspecials are ()<>@,;:

   token =               1*(token-char)

   email-safe =          1*(%x01-09|%x0b-0c|%x0e-27|
                         ;characters ()<>
   integer =             POS-DIGIT *DIGIT

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

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

   ; external references:
   ; IPv4address, IPv6address: From RFC 2373
   ; URI-reference: from RFC 2396, as modified by RFC 2732
   ; addr-spec: from RFC 2822

Appendix B: Guidelines for registering SDP names with IANA

There are seven field names that may be registered with IANA. Using the


     except under rare circumstances.  The same rules should apply for
     registered, and the registration MUST provide good justification
     why no existing media name is appropriate.


     In general this should be an IETF standards-track transport

     However, people will want to invent their own proprietary transport

     Where the protocol and the application are intimately linked, such
     as with the LBL whiteboard wb which used a proprietary and special
     formats (see below) apply to such registrations.

     Where the proprietary transport protocol really carries many
     different data formats, it is possible to register a new protocol
     the protocol and referenced in the registration.  Such an RFC MAY
     be informational, although it is preferable if it is standards-


     field, so a format cannot be registered without specifying one or
     more transport protocols that it applies to.

     Formats cover all the possible encodings that might want to be
     transported in a multimedia session.

     For RTP formats that have been assigned static payload types, the
     payload type number is used.  For RTP formats using a dynamic
     payload type number, the dynamic payload type number is given as
     the format and an additional "rtpmap" attribute specifies the
     format and parameters.

     For non-RTP formats, any unregistered format name may be
     registered.  If there is a suitable mapping from a MIME subtype to
     the format, then the MIME subtype name should be registered.  If
     there is no suitable mapping from a MIME subtype, a new name should
     be registered.  In either case, unless there are strong reasons not
     to do so, a standards-track RFC SHOULD be produced describing the
     format and this RFC SHOULD be referenced in the registration.

"att-field" (Attribute names)

     Attribute field names MAY be registered with IANA, although this is
     not compulsory, and unknown attributes are simply ignored.

     specification stating the following:

     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    whether the attribute value is subject to the charset

     o    a one paragraph explanation of the purpose of the attribute.

     o    a specification of appropriate attribute values for this

     IANA will not sanity check such attribute registrations except to
     ensure that they do not clash with existing registrations.

     attribute is expected to see widespread use and interoperability is
     that specifies the attribute more precisely.

     Submitters of registrations should ensure that the specification 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.

"bwtype" (bandwidth specifiers)

     A proliferation of bandwidth specifiers is strongly discouraged.

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

"nettype" (Network Type)

     used in the context of non-internet environments.  Whilst these are
     not normally the preserve of IANA, there may be circumstances when
     an Internet application needs to interoperate with a non-internet
     application, such as when gatewaying an internet telephony call
     into the PSTN.  The number of network types should be small and
     should be rarely extended.  A new network type cannot be registered
     without registering at least one address type to be used with that
     network type.  A new network type registration MUST reference an
     RFC which gives details of the network type and address type and
     specifies how and when they would be used.  Such an RFC MAY be

"addrtype" (Address Type)

     New address types may be registered with IANA.  An address type is
     only meaningful in the context of a network type, and any
     registration of an address type MUST specify a registered network
     new address type registration MUST reference an RFC giving details
     Informational.  Address types are not expected to be registered

Registration Procedure

To register a name the above guidelines should be followed regarding the
required level of documentation that is required.  The registration
itself should be sent to IANA.  Attribute registrations should include
the information given above.  Other registrations should include the
following additional information:

o    contact name, email address and telephone number

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

o    long-form name in English




registration will be made.

Appendix C: Changes from RFC 2327

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

o    Rewrite the ABNF syntax (thanks to Jonathan Lennox)

o    Add a=inactive attribute.

o    Add a=maxptime attribute.

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

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

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

o    Update BNF to support IPv6.

o    References have been updated.

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

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

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

o    The section on concatenation of session descriptions (which was not
     allowed in SAP, but allowed in other cases) has been removed. It is
     assumed that transports of SDP specify will specify this.

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

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

o    Define relation between the m= line and MIME types

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

o    Note that a=rtpmap is a media level attribute

Appendix D: Authors' Addresses

Mark Handley
AT&T Center for Internet Research at ICSI,
International Computer Science Institute,
1947 Center Street, Suite 600,
Berkeley, CA 94704, USA
Email: mjh@aciri.org

Van Jacobson
MS 46a-1121
Lawrence Berkeley Laboratory
Berkeley, CA 94720
United States
Email: van@ee.lbl.gov

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


Many people in the IETF MMUSIC working group have made comments and
suggestions contributing to this document.  In particular, we would like
Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Steve Hanna and
Jonathan Lennox.


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

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

    Minimal Control'', RFC 1890, January 1996.

[4] M. Handley, C. Perkins and E. Whelan, ``Session Announcement
    Protocol'', RFC 2974, October 2000.

[5] V. Jacobson and S. McCanne, ``vat - X11-based audio teleconferencing

    Addison-Wesley, 1996.

    -- Part 1: Architecture and Basic Multilingual Plane. Five
    amendments and a technical corrigendum have been published up

[8] D. Goldsmith and M. Davis, ``Using Unicode with MIME'', RFC1641,
    July 1994

    10646'', RFC 2044, October 1996


[11] M. Handley, H. Schulzrinne, E. Scholler and J. Rosenberg ``SIP:
     Session Initiation Protocol'', RFC 2543, March 1999.

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

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