draft-ietf-mmusic-sdp-06.txt   rfc2327.txt 
Internet Engineering Task Force MMUSIC WG Network Working Group M. Handley
INTERNET-DRAFT Mark Handley/Van Jacobson Request for Comments: 2327 V. Jacobson
draft-ietf-mmusic-sdp-06.ps ISI/LBNL Category: Standards Track ISI/LBNL
22nd Jan 1997 April 1998
Expires: 22nd Jul 1997
SDP: Session Description Protocol SDP: Session Description Protocol
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu- This document specifies an Internet standards track protocol for the
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Distribution of this document is unlimited. Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract Abstract
This document defines the Session Description Protocol, SDP. This document defines the Session Description Protocol, SDP. SDP is
SDP is intended for describing multimedia sessions for the intended for describing multimedia sessions for the purposes of
purposes of session announcement, session invitation, and session announcement, session invitation, and other forms of
other forms of multimedia session initiation. multimedia session initiation.
This document is a product of the Multiparty Multimedia Session Control This document is a product of the Multiparty Multimedia Session
(MMUSIC) working group of the Internet Engineering Task Force. Comments Control (MMUSIC) working group of the Internet Engineering Task
are solicited and should be addressed to the working group's mailing Force. Comments are solicited and should be addressed to the working
list at confctrl@isi.edu and/or the authors. group's mailing list at confctrl@isi.edu and/or the authors.
1. Introduction 1. Introduction
On the Internet multicast backbone (Mbone), a session directory tool is On the Internet multicast backbone (Mbone), a session directory tool
used to advertise multimedia conferences and communicate the conference is used to advertise multimedia conferences and communicate the
addresses and conference tool-specific information necessary for parti- conference addresses and conference tool-specific information
cipation. This document defines a session description protocol for this necessary for participation. This document defines a session
purpose, and for general real-time multimedia session description pur- description protocol for this purpose, and for general real-time
poses. This draft does not describe multicast address allocation or the multimedia session description purposes. This memo does not describe
distribution of SDP messages in detail. These are described in accom- multicast address allocation or the distribution of SDP messages in
panying drafts. SDP is not intended for negotiation of media encodings. detail. These are described in accompanying memos. SDP is not
intended for negotiation of media encodings.
2. Background 2. Background
The Mbone is the part of the internet that supports IP multicast, and The Mbone is the part of the internet that supports IP multicast, and
thus permits efficient many-to-many communication. It is used exten- thus permits efficient many-to-many communication. It is used
sively for multimedia conferencing. Such conferences usually have the extensively for multimedia conferencing. Such conferences usually
property that tight coordination of conference membership is not neces- have the property that tight coordination of conference membership is
sary; to receive a conference, a user at an Mbone site only has to know not necessary; to receive a conference, a user at an Mbone site only
the conference's multicast group address and the UDP ports for the has to know the conference's multicast group address and the UDP
conference data streams. ports for the conference data streams.
Session directories assist the advertisement of conference sessions and Session directories assist the advertisement of conference sessions
communicate the relevant conference setup information to prospective and communicate the relevant conference setup information to
participants. SDP is designed to convey such information to recipients. prospective participants. SDP is designed to convey such information
SDP is purely a format for session description - it does not incorporate to recipients. SDP is purely a format for session description - it
a transport protocol, and is intended to use different transport proto- does not incorporate a transport protocol, and is intended to use
cols as appropriate including the Session Announcement Protocol [4], different transport protocols as appropriate including the Session
Session Initiation Protocol [11], Real-Time Streaming Protocol [12], Announcement Protocol [4], Session Initiation Protocol [11], Real-
electronic mail using the MIME extensions, and the Hypertext Transport Time Streaming Protocol [12], electronic mail using the MIME
Protocol. extensions, and the Hypertext Transport Protocol.
SDP is intended to be general purpose so that it can be used for a wider SDP is intended to be general purpose so that it can be used for a
range of network environments and applications than just multicast ses- wider range of network environments and applications than just
sion directories. However, it is not intended to support negotiation of multicast session directories. However, it is not intended to
session content or media encodings - this is viewed as outside the scope support negotiation of session content or media encodings - this is
of session description. viewed as outside the scope of session description.
3. Glossary of Terms 3. Glossary of Terms
The following terms are used in this document, and have specific meaning The following terms are used in this document, and have specific
within the context of this document. meaning within the context of this document.
Conference Conference
A multimedia conference is a set of two or more communicating users A multimedia conference is a set of two or more communicating users
along with the software they are using to communicate. along with the software they are using to communicate.
Session Session
A multimedia session is a set of multimedia senders and receivers A multimedia session is a set of multimedia senders and receivers
and the data streams flowing from senders to receivers. A mul- and the data streams flowing from senders to receivers. A
timedia conference is an example of a multimedia session. multimedia conference is an example of a multimedia session.
Session Advertisement Session Advertisement
See session announcement. See session announcement.
Session Announcement Session Announcement
A session announcement is a mechanism by which a session description A session announcement is a mechanism by which a session
is conveyed to users in a pro-active fashion, i.e., the session description is conveyed to users in a proactive fashion, i.e., the
description was not explicitly requested by the user. session description was not explicitly requested by the user.
Session Description Session Description
A well defined format for conveying sufficient information to dis- A well defined format for conveying sufficient information to
cover and participate in a multimedia session. discover and participate in a multimedia session.
4. SDP Usage 3.1. Terminology
4.1. Multicast Announcements 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.
SDP is a session description protocol for multimedia sessions. A common 4. SDP Usage
mode of usage is for a client to announce a conference session by
periodically multicasting an announcement packet to a well known multi-
cast address and port using the Session Announcement Protocol (SAP).
SAP packets are UDP packets with the following format: 4.1. Multicast Announcements
0 31 SDP is a session description protocol for multimedia sessions. A
|--------------------| common mode of usage is for a client to announce a conference session
| SAP header | by periodically multicasting an announcement packet to a well known
|--------------------| multicast address and port using the Session Announcement Protocol
| text payload | (SAP).
|//////////
The header is the Session Announcement Protocol header. SAP is SAP packets are UDP packets with the following format:
described in more detail in a companion draft [4]
The text payload is an SDP session description, as described in this |--------------------|
draft. The text payload should be no greater than 1 Kbyte in length. | SAP header |
If announced by SAP, only one session announcement is permitted in a |--------------------|
single packet. | text payload |
|//////////
4.2. Email and WWW Announcements The header is the Session Announcement Protocol header. SAP is
described in more detail in a companion memo [4]
Alternative means of conveying session descriptions include electronic The text payload is an SDP session description, as described in this
mail and the World Wide Web. For both email and WWW distribution, the memo. The text payload should be no greater than 1 Kbyte in length.
use of the MIME content type ``application/sdp'' should be used. This If announced by SAP, only one session announcement is permitted in a
enables the automatic launching of applications for participation in the single packet.
session from the WWW client or mail reader in a standard manner.
Note that announcements of multicast sessions made only via email or the 4.2. Email and WWW Announcements
World Wide Web (WWW) do not have the property that the receiver of a
session announcement can necessarily receive the session because the
multicast sessions may be restricted in scope, and access to the WWW
server or reception of email is possible outside this scope. SAP
announcements do not suffer from this mismatch.
5. Requirements and Recommendations Alternative means of conveying session descriptions include
electronic mail and the World Wide Web. For both email and WWW
distribution, the use of the MIME content type "application/sdp"
should be used. This enables the automatic launching of applications
for participation in the session from the WWW client or mail reader
in a standard manner.
The purpose of SDP is to convey information about media streams in mul- Note that announcements of multicast sessions made only via email or
timedia sessions to allow the recipients of a session description to the World Wide Web (WWW) do not have the property that the receiver
participate in the session. SDP is primarily intended for use in an of a session announcement can necessarily receive the session because
internetwork, although it is sufficiently general that it can describe the multicast sessions may be restricted in scope, and access to the
conferences in other network environments. WWW server or reception of email is possible outside this scope. SAP
announcements do not suffer from this mismatch.
A multimedia session, for these purposes, is defined as a set of media 5. Requirements and Recommendations
streams that exist for some duration of time. Media streams can be
many-to-many. The times during which the session is active need not be
continuous.
Thus far, multicast based sessions on the Internet have differed from The purpose of SDP is to convey information about media streams in
many other forms of conferencing in that anyone receiving the traffic multimedia sessions to allow the recipients of a session description
can join the session (unless the session traffic is encrypted). In such to participate in the session. SDP is primarily intended for use in
an environment, SDP serves two primary purposes. It is a means to com- an internetwork, although it is sufficiently general that it can
municate the existence of a session, and is a means to convey sufficient describe conferences in other network environments.
information to enable joining and participating in the session. In a
unicast environment, only the latter purpose is likely to be relevant.
Thus SDP includes: A multimedia session, for these purposes, is defined as a set of
media streams that exist for some duration of time. Media streams
can be many-to-many. The times during which the session is active
need not be continuous.
o Session name and purpose Thus far, multicast based sessions on the Internet have differed from
many other forms of conferencing in that anyone receiving the traffic
can join the session (unless the session traffic is encrypted). In
such an environment, SDP serves two primary purposes. It is a means
to communicate the existence of a session, and is a means to convey
sufficient information to enable joining and participating in the
session. In a unicast environment, only the latter purpose is likely
to be relevant.
o Time(s) the session is active Thus SDP includes:
o The media comprising the session o Session name and purpose
o Information to receive those media (addresses, ports, formats and o Time(s) the session is active
so on)
As resources necessary to participate in a session may be limited, some o The media comprising the session
additional information may also be desirable:
o Information about the bandwidth to be used by the conference o Information to receive those media (addresses, ports, formats and
so on)
o Contact information for the person responsible for the session As resources necessary to participate in a session may be limited,
some additional information may also be desirable:
In general, SDP must convey sufficient information to be able to join a o Information about the bandwidth to be used by the conference
session (with the possible exception of encryption keys) and to announce
the resources to be used to non-participants that may need to know. o Contact information for the person responsible for the session
In general, SDP must convey sufficient information to be able to join
a session (with the possible exception of encryption keys) and to
announce the resources to be used to non-participants that may need
to know.
5.1. Media Information 5.1. Media Information
SDP includes: SDP includes:
o The type of media (video, audio, etc) o The type of media (video, audio, etc)
o The transport protocol (RTP/UDP/IP, H.320, etc) o The transport protocol (RTP/UDP/IP, H.320, etc)
o The format of the media (H.261 video, MPEG video, etc) o The format of the media (H.261 video, MPEG video, etc)
For an IP multicast session, the following are also conveyed: For an IP multicast session, the following are also conveyed:
o Multicast address for media o Multicast address for media
o Transport Port for media o Transport Port for media
This address and port are the destination address and destination port This address and port are the destination address and destination
of the multicast stream, whether being sent, received, or both. port of the multicast stream, whether being sent, received, or both.
For an IP unicast session, the following are conveyed: For an IP unicast session, the following are conveyed:
o Remote address for media o Remote address for media
o Transport port for contact address o Transport port for contact address
The semantics of this address and port depend on the media and transport The semantics of this address and port depend on the media and
protocol defined. By default, this is the remote address and remote transport protocol defined. By default, this is the remote address
port to which data is sent, and the remote address and local port on and remote port to which data is sent, and the remote address and
which to receive data. However, some media may define to use these to local port on which to receive data. However, some media may define
establish a control channel for the actual media flow. to use these to establish a control channel for the actual media
flow.
5.2. Timing Information 5.2. Timing Information
Sessions may either be bounded or unbounded in time. Whether or not Sessions may either be bounded or unbounded in time. Whether or not
they are bounded, they may be only active at specific times. they are bounded, they may be only active at specific times.
SDP can convey: SDP can convey:
o An arbitrary list of start and stop times bounding the session 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
o For each bound, repeat times such as "every Wednesday at 10am for
one hour" one hour"
This timing information is globally consistent, irrespective of local This timing information is globally consistent, irrespective of local
time zone or daylight saving time. time zone or daylight saving time.
5.3. Private Sessions 5.3. Private Sessions
It is possible to create both public sessions and private sessions. It is possible to create both public sessions and private sessions.
Private sessions will typically be conveyed by encrypting the session Private sessions will typically be conveyed by encrypting the session
description to distribute it. The details of how encryption is per- description to distribute it. The details of how encryption is
formed are dependent on the mechanism used to convey SDP - see [4] for performed are dependent on the mechanism used to convey SDP - see [4]
how this is done for session announcements. for how this is done for session announcements.
If a session announcement is private it is possible to use that private If a session announcement is private it is possible to use that
announcement to convey encryption keys necessary to decode each of the private announcement to convey encryption keys necessary to decode
media in a conference, including enough information to know which each of the media in a conference, including enough information to
encryption scheme is used for each media. know which encryption scheme is used for each media.
5.4. Obtaining Further Information about a Session 5.4. Obtaining Further Information about a Session
A session description should convey enough information to decide whether A session description should convey enough information to decide
or not to participate in a session. SDP may include additional pointers whether or not to participate in a session. SDP may include
in the form of Universal Resources Identifiers (URIs) for more informa- additional pointers in the form of Universal Resources Identifiers
tion about the session. (URIs) for more information about the session.
5.5. Categorisation 5.5. Categorisation
When many session descriptions are being distributed by SAP or any other When many session descriptions are being distributed by SAP or any
advertisement mechanism, it may be desirable to filter announcements other advertisement mechanism, it may be desirable to filter
that are of interest from those that are not. SDP supports a categori- announcements that are of interest from those that are not. SDP
sation mechanism for sessions that is capable of being automated. supports a categorisation mechanism for sessions that is capable of
being automated.
5.6. Internationalization 5.6. Internationalization
The SDP specification recommends the use of the ISO 10646 character sets The SDP specification recommends the use of the ISO 10646 character
in the UTF-8 encoding (RFC 2044) to allow many different languages to be sets in the UTF-8 encoding (RFC 2044) to allow many different
represented. However, to assist in compact representations, SDP also languages to be represented. However, to assist in compact
allows other character sets such as ISO 8859-1 to be used when desired. representations, SDP also allows other character sets such as ISO
Internationalization only applies to free-text fields (session name and 8859-1 to be used when desired. Internationalization only applies to
background information), and not to SDP as a whole. free-text fields (session name and background information), and not
to SDP as a whole.
6. SDP Specification 6. SDP Specification
SDP session descriptions are entirely textual using the ISO 10646 char- SDP session descriptions are entirely textual using the ISO 10646
acter set in UTF-8 encoding. SDP field names and attributes names use character set in UTF-8 encoding. SDP field names and attributes names
only the US-ASCII subset of UTF-8, but textual fields and attribute use only the US-ASCII subset of UTF-8, but textual fields and
values may use the full ISO 10646 character set. The textual form, as attribute values may use the full ISO 10646 character set. The
opposed to a binary encoding such as ASN/1 or XDR, was chosen to enhance textual form, as opposed to a binary encoding such as ASN/1 or XDR,
portability, to enable a variety of transports to be used (e.g, session was chosen to enhance portability, to enable a variety of transports
description in a MIME email message) and to allow flexible, text-based to be used (e.g, session description in a MIME email message) and to
toolkits (e.g., Tcl/Tk ) to be used to generate and to process session allow flexible, text-based toolkits (e.g., Tcl/Tk ) to be used to
descriptions. However, since the total bandwidth allocated to all SAP generate and to process session descriptions. However, since the
announcements is strictly limited, the encoding is deliberately compact. total bandwidth allocated to all SAP announcements is strictly
Also, since announcements may be transported via very unreliable means limited, the encoding is deliberately compact. Also, since
(e.g., email) or damaged by an intermediate caching server, the encoding announcements may be transported via very unreliable means (e.g.,
was designed with strict order and formatting rules so that most errors email) or damaged by an intermediate caching server, the encoding was
would result in malformed announcements which could be detected easily designed with strict order and formatting rules so that most errors
and discarded. This also allows rapid discarding of encrypted announce- would result in malformed announcements which could be detected
ments for which a receiver does not have the correct key. easily and discarded. This also allows rapid discarding of encrypted
announcements for which a receiver does not have the correct key.
An SDP session description consists of a number of lines of text of the An SDP session description consists of a number of lines of text of
form the form <type>=<value> <type> is always exactly one character and is
<type>=<value> case-significant. <value> is a structured text string whose format
<type> is always exactly one character and is case-significant. <value> depends on <type>. It also will be case-significant unless a
is a structured text string whose format depends on <type>. It also specific field defines otherwise. Whitespace is not permitted either
will be case-significant unless a specific field defines otherwise. side of the `=' sign. In general <value> is either a number of fields
Whitespace is not permitted either side of the `=' sign. In general delimited by a single space character or a free format string.
<value> is either a number of fields delimited by a single space charac-
ter or a free format string.
A session description consists of a session-level description (details A session description consists of a session-level description
that apply to the whole session and all media streams) and optionally (details that apply to the whole session and all media streams) and
several media-level descriptions (details that apply onto to a single optionally several media-level descriptions (details that apply onto
media stream). to a single media stream).
An announcement consists of a session-level section followed by zero or An announcement consists of a session-level section followed by zero
more media-level sections. The session-level part starts with a `v=' or more media-level sections. The session-level part starts with a
line and continues to the first media-level section. The media descrip- `v=' line and continues to the first media-level section. The media
tion starts with an `m=' line and continues to the next media descrip- description starts with an `m=' line and continues to the next media
tion or end of the whole session description. In general, session-level description or end of the whole session description. In general,
values are the default for all media unless overridden by an equivalent session-level values are the default for all media unless overridden
media-level value. by an equivalent media-level value.
When SDP is conveyed by SAP, only one session description is allowed per When SDP is conveyed by SAP, only one session description is allowed
packet. When SDP is conveyed by other means, many SDP session descrip- per packet. When SDP is conveyed by other means, many SDP session
tions may be concatenated together (the `v=' line indicating the start descriptions may be concatenated together (the `v=' line indicating
of a session description terminates the previous description). Some the start of a session description terminates the previous
lines in each description are required and some are optional but all description). Some lines in each description are required and some
must appear in exactly the order given here (the fixed order greatly are optional but all must appear in exactly the order given here (the
enhances error detection and allows for a simple parser). Optional fixed order greatly enhances error detection and allows for a simple
items are marked with a `*'. parser). Optional items are marked with a `*'.
Session description Session description
v= (protocol version) v= (protocol version)
o= (owner/creator and session identifier). o= (owner/creator and session identifier).
s= (session name) s= (session name)
i=* (session information) i=* (session information)
u=* (URI of description) u=* (URI of description)
e=* (email address) e=* (email address)
p=* (phone number) p=* (phone number)
c=* (connection information - not required if included in all media) c=* (connection information - not required if included in all media)
b=* (bandwidth information) b=* (bandwidth information)
One or more time descriptions (see below) One or more time descriptions (see below)
z=* (time zone adjustments) z=* (time zone adjustments)
k=* (encryption key) k=* (encryption key)
a=* (zero or more session attribute lines) a=* (zero or more session attribute lines)
Zero or more media descriptions (see below) Zero or more media descriptions (see below)
Time description Time description
t= (time the session is active) t= (time the session is active)
r=* (zero or more repeat times) r=* (zero or more repeat times)
Media description Media description
m= (media name and transport address) m= (media name and transport address)
i=* (media title) i=* (media title)
c=* (connection information - optional if included at session-level) c=* (connection information - optional if included at session-level)
b=* (bandwidth information) b=* (bandwidth information)
k=* (encryption key) k=* (encryption key)
a=* (zero or more media attribute lines) a=* (zero or more media attribute lines)
The set of `type' letters is deliberately small and not intended to be The set of `type' letters is deliberately small and not intended to
extensible -- SDP parsers must completely ignore any announcement that be extensible -- SDP parsers must completely ignore any announcement
contains a `type' letter that it does not understand. The `attribute' that contains a `type' letter that it does not understand. The
mechanism ("a=" described below) is the primary means for extending SDP `attribute' mechanism ("a=" described below) is the primary means for
and tailoring it to particular applications or media. Some attributes extending SDP and tailoring it to particular applications or media.
(the ones listed in this document) have a defined meaning but others may Some attributes (the ones listed in this document) have a defined
be added on an application-, media- or session-specific basis. A ses- meaning but others may be added on an application-, media- or
sion directory must ignore any attribute it doesn't understand. session-specific basis. A session directory must ignore any
attribute it doesn't understand.
The connection (`c=') and attribute (`a=') information in the session- The connection (`c=') and attribute (`a=') information in the
level section applies to all the media of that session unless overridden session-level section applies to all the media of that session unless
by connection information or an attribute of the same name in the media overridden by connection information or an attribute of the same name
description. For instance, in the example below, each media behaves as in the media description. For instance, in the example below, each
if it were given a `recvonly' attribute. media behaves as if it were given a `recvonly' attribute.
An example SDP description is: An example SDP description is:
v=0 v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4 o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar s=SDP Seminar
i=A Seminar on the session description protocol i=A Seminar on the session description protocol
u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
e=mjh@isi.edu (Mark Handley) e=mjh@isi.edu (Mark Handley)
c=IN IP4 224.2.17.12/127 c=IN IP4 224.2.17.12/127
t=2873397496 2873404696 t=2873397496 2873404696
a=recvonly a=recvonly
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 31 m=video 51372 RTP/AVP 31
m=application 32416 udp wb m=application 32416 udp wb
a=orient:portrait a=orient:portrait
Text records such as the session name and information are bytes strings Text records such as the session name and information are bytes
which may contain any byte with the exceptions of 0x00 (Nul), 0x0a strings which may contain any byte with the exceptions of 0x00 (Nul),
(ASCII newline) and 0x0d (ASCII carriage return). The sequence CRLF 0x0a (ASCII newline) and 0x0d (ASCII carriage return). The sequence
(0x0d0a) is used to end a record, although parsers should be tolerant CRLF (0x0d0a) is used to end a record, although parsers should be
and also accept records terminated with a single newline character. By tolerant and also accept records terminated with a single newline
default these byte strings contain ISO-10646 characters in UTF-8 encod- character. By default these byte strings contain ISO-10646
ing, but this default may be changed using the `charset' attribute. characters in UTF-8 encoding, but this default may be changed using
the `charset' attribute.
Protocol Version Protocol Version
v=0 v=0
The ``v='' field gives the version of the Session Description Protocol. The "v=" field gives the version of the Session Description Protocol.
There is no minor version number. There is no minor version number.
Origin Origin
o=<username> <session id> <version> <network type> <address type> o=<username> <session id> <version> <network type> <address type>
<address> <address>
The ``o='' field gives the originator of the session (their username and The "o=" field gives the originator of the session (their username
the address of the user's host) plus a session id and session version and the address of the user's host) plus a session id and session
number. <username> is the user's login on the originating host, or it version number.
is ``-'' if the originating host does not support the concept of user
ids. <username> must not contain spaces. <session id> is a numeric
string such that the tuple of <username>, <session id>, <network type>,
<address type> and <address> form a globally unique identifier for the
session. The method of session id allocation is up to the creating
tool, but it has been suggested that a Network Time Protocol (NTP)
timestamp be used to ensure uniqueness [1]. <version> is a version
number for this announcement. It is needed for proxy announcements to
detect which of several announcements for the same session is the most
recent. Again its usage is up to the creating tool, so long as <ver-
sion> is increased when a modification is made to the session data.
Again, it is recommended (but not mandatory) that an NTP timestamp is
used. <network type> is a text string giving the type of network. Ini-
tially ``IN'' is defined to have the meaning ``Internet''. <address
type> is a text string giving the type of the address that follows.
Initially ``IP4'' and ``IP6'' are defined. <address> is the globally
unique address of the machine from which the session was created. For
an address type of IP4, this is the dotted-decimal representation of the
IP version 4 address of the machine. For an address type of IP6, this
is the compressed textual representation of the IP version 6 address of
the machine.
In general, the ``o='' field serves as a globally unique identifier for <username> is the user's login on the originating host, or it is "-"
this version of this session description, and the subfields excepting if the originating host does not support the concept of user ids.
the version taken together identify the session irrespective of any <username> must not contain spaces. <session id> is a numeric string
modifications. such that the tuple of <username>, <session id>, <network type>,
<address type> and <address> form a globally unique identifier for
the session.
Session Name The method of <session id> allocation is up to the creating tool, but
it has been suggested that a Network Time Protocol (NTP) timestamp be
used to ensure uniqueness [1].
s=<session name> <version> is a version number for this announcement. It is needed
for proxy announcements to detect which of several announcements for
the same session is the most recent. Again its usage is up to the
creating tool, so long as <version> is increased when a modification
is made to the session data. Again, it is recommended (but not
mandatory) that an NTP timestamp is used.
The ``s='' field is the session name. There must be one and only one <network type> is a text string giving the type of network.
``s='' field per session description, and it must contain ISO 10646 Initially "IN" is defined to have the meaning "Internet". <address
characters (but see also the `charset' attribute below). type> is a text string giving the type of the address that follows.
Initially "IP4" and "IP6" are defined. <address> is the globally
unique address of the machine from which the session was created.
For an address type of IP4, this is either the fully-qualified domain
name of the machine, or the dotted-decimal representation of the IP
version 4 address of the machine. For an address type of IP6, this
is either the fully-qualified domain name of the machine, or the
compressed textual representation of the IP version 6 address of the
machine. For both IP4 and IP6, the fully-qualified domain name is
the form that SHOULD be given unless this is unavailable, in which
case the globally unique address may be substituted. A local IP
address MUST NOT be used in any context where the SDP description
might leave the scope in which the address is meaningful.
Session and Media Information 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.
i=<session description> Session Name
The ``i='' field is information about the session. There may be at most s=<session name>
one session-level ``i='' field per session description, and at most one
``i='' field per media. Although it may be omitted, this is discouraged
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 below).
A single ``i='' field can also be used for each media definition. In The "s=" field is the session name. There must be one and only one
media definitions, ``i='' fields are primarily intended for labeling "s=" field per session description, and it must contain ISO 10646
media streams. As such, they are most likely to be useful when a single characters (but see also the `charset' attribute below).
session has more than one distinct media stream of the same media type.
An example would be two different whiteboards, one for slides and one
for feedback and questions.
URI Session and Media Information
u=<URI>
o A URI is a Universal Resource Identifier as used by WWW clients i=<session description>
o The URI should be a pointer to additional information about the The "i=" field is information about the session. There may be at
conference most one session-level "i=" field per session description, and at
most one "i=" field per media. Although it may be omitted, this is
discouraged 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 below).
o This field is optional, but if it is present it should be specified A single "i=" field can also be used for each media definition. In
before the first media field media definitions, "i=" fields are primarily intended for labeling
media streams. As such, they are most likely to be useful when a
single session has more than one distinct media stream of the same
media type. An example would be two different whiteboards, one for
slides and one for feedback and questions.
o No more than one URI field is allowed per session description URI
Email Address and Phone Number u=<URI>
e=<email address> o A URI is a Universal Resource Identifier as used by WWW clients
p=<phone number>
o These specify contact information for the person responsible for o The URI should be a pointer to additional information about the
the conference. This is not necessarily the same person that conference
created the conference announcement.
o Either an email field or a phone field must be specified. Addi- o This field is optional, but if it is present it should be specified
tional email and phone fields are allowed. before the first media field
o If these are present, they should be specified before the first o No more than one URI field is allowed per session description
media field.
o More than one email or phone field can be given for a session Email Address and Phone Number
description.
o Phone numbers should be given in the conventional international e=<email address>
format - preceded by a ``+'' and the international country code. p=<phone number>
There must be a space or a hyphen (``-'') between the country code
and the rest of the phone number. Spaces and hyphens may be used to
split up a phone field to aid readability if desired. For example:
p=+44-171-380-7777 or p=+1 617 253 6011 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 Both email addresses and phone numbers can have an optional free o Either an email field or a phone field must be specified.
text string associated with them, normally giving the name of the Additional email and phone fields are allowed.
person who may be contacted. This should be enclosed in parenthesis
if it is present. For example:
e=mjh@isi.edu (Mark Handley) o If these are present, they should be specified before the first
The alternative RFC822 name quoting convention is also allowed for media field.
both email addresses and phone numbers. For example,
e=Mark Handley <mjh@isi.edu> o More than one email or phone field can be given for a session
description.
The free text string should be in the ISO-10646 character set with o Phone numbers should be given in the conventional international
UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if
the appropriate charset session-level attribute is set.
Connection Data format - preceded by a "+ and the international country code.
There must be a space or a hyphen ("-") between the country code
and the rest of the phone number. Spaces and hyphens may be used
to split up a phone field to aid readability if desired. For
example:
c=<network type> <address type> <connection address> p=+44-171-380-7777 or p=+1 617 253 6011
The ``c='' field contains connection data. 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:
A session announcement must contain one ``c='' field in each media e=mjh@isi.edu (Mark Handley)
description (see below) or a ``c='' field at the session-level. It may
contain a session-level ``c='' field and one additional ``c='' field per
media description, in which case the per-media values override the
session-level settings for the relevant media.
The first sub-field is the network type, which is a text string giving The alternative RFC822 name quoting convention is also allowed for
the type of network. Initially ``IN'' is defined to have the meaning both email addresses and phone numbers. For example,
``Internet''.
The second sub-field is the address type. This allows SDP to be used e=Mark Handley <mjh@isi.edu>
for sessions that are not IP based. Currently only IP4 is defined.
The third sub-field is the connection address. Optional extra sub- The free text string should be in the ISO-10646 character set with
fields may be added after the connection address depending on the value UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings
of the <address type> field. if the appropriate charset session-level attribute is set.
For IP4 addresses, the connection address is defined as follows: Connection Data
o Typically the connection address will be a class-D IP multicast c=<network type> <address type> <connection address>
group address. If the conference is not multicast, then the connec-
tion address contains the unicast IP address of the expected data
source or data relay or data sink as determined by additional attri-
bute fields. It is not expected that unicast addresses will be
given in a session description that is communicated by a multicast
announcement, though this is not prohibited.
o Conferences using an IP multicast connection address must also have The "c=" field contains connection data.
a time to live (TTL) value present in addition to the multicast
address. The TTL and the address together define the scope with
which multicast packets sent in this conference will be sent. TTL
values must be in the range 0-255.
The TTL for the session is appended to the address using a slash as A session announcement must contain one "c=" field in each media
a separator. An example is: description (see below) or a "c=" field at the session-level. It may
contain a session-level "c=" field and one additional "c=" field per
media description, in which case the per-media values override the
session-level settings for the relevant media.
c=IN IP4 224.2.1.1/127 The first sub-field is the network type, which is a text string
giving the type of network. Initially "IN" is defined to have the
meaning "Internet".
Hierarchical or layered encoding schemes are data streams where the The second sub-field is the address type. This allows SDP to be used
encoding from a single media source is split into a number of for sessions that are not IP based. Currently only IP4 is defined.
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> The third sub-field is the connection address. Optional extra
subfields may be added after the connection address depending on the
value of the <address type> field.
If the number of addresses is not given it is assumed to be one. For IP4 addresses, the connection address is defined as follows:
Multicast addresses so assigned are contiguously allocated above the
base address, so that, for example:
c=IN IP4 224.2.1.1/127/3 o Typically the connection address will be a class-D IP multicast
would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are to group address. If the session is not multicast, then the
be used at a ttl of 127. This is semantically identical to includ- connection address contains the fully-qualified domain name or the
ing multiple ``c='' lines in a media description: 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 fully-qualified domain names or unicast addresses
will be given in a session description that is communicated by a
multicast announcement, though this is not prohibited. If a
unicast data stream is to pass through a network address
translator, the use of a fully-qualified domain name rather than an
unicast IP address is RECOMMENDED. In other cases, the use of an
IP address to specify a particular interface on a multi-homed host
might be required. Thus this specification leaves the decision as
to which to use up to the individual application, but all
applications MUST be able to cope with receiving both formats.
c=IN IP4 224.2.1.1/127 o Conferences using an IP multicast connection address must also have
c=IN IP4 224.2.1.2/127 a time to live (TTL) value present in addition to the multicast
c=IN IP4 224.2.1.3/127 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.
Multiple addresses or ``c='' lines can only be specified on a per- The TTL for the session is appended to the address using a slash as
media basis, and not for a session-level ``c='' field. a separator. An example is:
It is illegal for the slash notation described above to be used for c=IN IP4 224.2.1.1/127
IP unicast addresses.
Bandwidth 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:
b=<modifier>:<bandwidth-value> <base multicast address>/<ttl>/<number of addresses>
o This specifies the proposed bandwidth to be used by the session or If the number of addresses is not given it is assumed to be one.
media, and is optional. Multicast addresses so assigned are contiguously allocated above
the base address, so that, for example:
o <bandwidth-value> is in kilobits per second c=IN IP4 224.2.1.1/127/3
o <modifier> is a single alphanumeric word giving the meaning of the
bandwidth figure.
o Two modifiers are initially defined: 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:
CT Conference Total: An implicit maximum bandwidth is associated with c=IN IP4 224.2.1.1/127
each TTL on the Mbone or within a particular multicast administra- c=IN IP4 224.2.1.2/127
tive scope region (the Mbone bandwidth vs. TTL limits are given in c=IN IP4 224.2.1.3/127
the MBone FAQ). If the bandwidth of a session or media in a ses-
sion is different from the bandwidth implicit from the scope, a
`b=CT:...' line should be supplied for the session giving the pro-
posed upper limit to the bandwidth used. The primary purpose of
this is to give an approximate idea as to whether two or more
conferences can co-exist simultaneously.
AS Application-Specific Maximum: The bandwidth is interpreted to be Multiple addresses or "c=" lines can only be specified on a per-
application-specific, i.e., will be the application's concept of media basis, and not for a session-level "c=" field.
maximum bandwidth. Normally this will coincide with what is set
on the application's ``maximum bandwidth'' control if applicable.
Note that CT gives a total bandwidth figure for all the media at all It is illegal for the slash notation described above to be used for
sites. AS gives a bandwidth figure for a single media at a single IP unicast addresses.
site, although there may be many sites sending simultaneously.
o Extension Mechanism: Tool writers can define experimental bandwidth Bandwidth
modifiers by prefixing their modifier with ``X-''. For example:
b=X-YZ:128 b=<modifier>:<bandwidth-value>
SDP parsers should ignore bandwidth fields with unknown modifiers. o This specifies the proposed bandwidth to be used by the session or
Modifiers should be alpha-numeric and, although no length limit is media, and is optional.
given, they are recommended to be short.
Times, Repeat Times and Time Zones o <bandwidth-value> is in kilobits per second
t=<start time> <stop time> o <modifier> is a single alphanumeric word giving the meaning of the
bandwidth figure.
o ``t='' fields specify the start and stop times for a conference o Two modifiers are initially defined:
session. Multiple ``t='' fields may be used if a session is active
at multiple irregularly spaced times; each additional ``t='' field
specifies an additional period of time for which the session will be
active. If the session is active at regular times, an ``r='' field
(see below) should be used in addition to and following a ``t=''
field - in which case the ``t='' field specifies the start and stop
times of the repeat sequence.
o The first and second sub-fields give the start and stop times for CT Conference Total: An implicit maximum bandwidth is associated with
the conference respectively. These values are the decimal each TTL on the Mbone or within a particular multicast
representation of Network Time Protocol (NTP) time values in seconds administrative scope region (the Mbone bandwidth vs. TTL limits are
[1]. To convert these values to UNIX time, subtract decimal given in the MBone FAQ). If the bandwidth of a session or media in
2208988800. 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 primary purpose of
this is to give an approximate idea as to whether two or more
conferences can co-exist simultaneously.
o If the stop-time is set to zero, then the session is not bounded, AS Application-Specific Maximum: The bandwidth is interpreted to be
though it will not become active until after the start-time. If the application-specific, i.e., will be the application's concept of
start-time is also zero, the session is regarded as permanent. maximum bandwidth. Normally this will coincide with what is set on
the application's "maximum bandwidth" control if applicable.
User interfaces should strongly discourage the creation of unbounded Note that CT gives a total bandwidth figure for all the media at
and permanent sessions as they give no information about when the all sites. AS gives a bandwidth figure for a single media at a
session is actually going to terminate, and so make scheduling dif- single site, although there may be many sites sending
ficult. simultaneously.
The general assumption may be made, when displaying unbounded ses- o Extension Mechanism: Tool writers can define experimental bandwidth
sions that have not timed out to the user, that an unbounded session modifiers by prefixing their modifier with "X-". For example:
will only be active until half an hour from the current time or the
session start time, whichever is the later. If behaviour other than
this is required, an end-time should be given and modified as
appropriate when new information becomes available about when the
session should really end.
Permanent sessions may be shown to the user as never being active b=X-YZ:128
unless there are associated repeat times which state precisely when
the session will be active. In general, permanent sessions should
not be created for any session expected to have a duration of less
than 2 months, and should be discouraged for sessions expected to
have a duration of less than 6 months.
r=<repeat interval> <active duration> <list of offsets from start-time> SDP parsers should ignore bandwidth fields with unknown modifiers.
Modifiers should be alpha-numeric and, although no length limit is
given, they are recommended to be short.
o ``r='' fields specify repeat times for a session. For example, if Times, Repeat Times and Time Zones
a session is active at 10am on Monday and 11am on Tuesday for one
hour each week for three months, then the <start time> in the
corresponding ``t='' field would be the NTP representation of 10am
on the first Monday, the <repeat interval> would be 1 week, the
<active duration> would be 1 hour, and the offsets would be zero and
25 hours. The corresponding ``t='' field stop time would be the NTP
representation of the end of the last session three months later. By
default all fields are in seconds, so the ``r='' and ``t='' fields
might be:
t=3034423619 3042462419 t=<start time> <stop time>
r=604800 3600 0 90000
To make announcements more compact, times may also be given in o "t=" fields specify the start and stop times for a conference
units of days, hours or minutes. The syntax for these is a number session. Multiple "t=" fields may be used if a session is active
immediately followed by a single case-sensitive character. at multiple irregularly spaced times; each additional "t=" field
specifies an additional period of time for which the session will
be active. If the session is active at regular times, an "r="
field (see below) should be used in addition to and following a
"t=" field - in which case the "t=" field specifies the start and
stop times of the repeat sequence.
Fractional units are not allowed - a smaller unit should be used o The first and second sub-fields give the start and stop times for
the conference respectively. These values are the decimal
representation of Network Time Protocol (NTP) time values in
seconds [1]. To convert these values to UNIX time, subtract
decimal 2208988800.
o If the stop-time is set to zero, then the session is not bounded,
though it will not become active until after the start-time. If
the start-time is also zero, the session is regarded as permanent.
User interfaces should strongly discourage the creation of
unbounded and permanent sessions as they give no information about
when the session is actually going to terminate, and so make
scheduling difficult.
The general assumption may be made, when displaying unbounded
sessions that have not timed out to the user, that an unbounded
session will only be active until half an hour from the current
time or the session start time, whichever is the later. If
behaviour other than this is required, an end-time should be given
and modified as appropriate when new information becomes available
about when the session should really end.
Permanent sessions may be shown to the user as never being active
unless there are associated repeat times which state precisely when
the session will be active. In general, permanent sessions should
not be created for any session expected to have a duration of less
than 2 months, and should be discouraged for sessions expected to
have a duration of less than 6 months.
r=<repeat interval> <active duration> <list of offsets from start-
time>
o "r=" fields specify repeat times for a session. For example, if
a session is active at 10am on Monday and 11am on Tuesday for one
hour each week for three months, then the <start time> in the
corresponding "t=" field would be the NTP representation of 10am on
the first Monday, the <repeat interval> would be 1 week, the
<active duration> would be 1 hour, and the offsets would be zero
and 25 hours. The corresponding "t=" field stop time would be the
NTP representation of the end of the last session three months
later. By default all fields are in seconds, so the "r=" and "t="
fields might be:
t=3034423619 3042462419
r=604800 3600 0 90000
To make announcements 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: instead. The following unit specification characters are allowed:
d - days (86400 seconds) d - days (86400 seconds)
h - minutes (3600 seconds) h - minutes (3600 seconds)
m - minutes (60 seconds) m - minutes (60 seconds)
s - seconds (allowed for completeness but not recommended) s - seconds (allowed for completeness but not recommended)
Thus, the above announcement could also have been written: Thus, the above announcement could also have been written:
r=7d 1h 0 25h r=7d 1h 0 25h
Monthly and yearly repeats cannot currently be directly specified Monthly and yearly repeats cannot currently be directly specified
with a single SDP repeat time - instead separate "t" fields should with a single SDP repeat time - instead separate "t" fields should
be used to explicitly list the session times. be used to explicitly list the session times.
z=<adjustment time> <offset> <adjustment time> <offset> .... z=<adjustment time> <offset> <adjustment time> <offset> ....
o To schedule a repeated session which spans a change from daylight- o To schedule a repeated session which spans a change from daylight-
saving time to standard time or vice-versa, it is necessary to saving time to standard time or vice-versa, it is necessary to
specify offsets from the base repeat times. This is required because specify offsets from the base repeat times. This is required
different time zones change time at different times of day, dif- because different time zones change time at different times of day,
ferent countries change to or from daylight time on different dates, different countries change to or from daylight time on different
and some countries do not have daylight saving time at all. 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 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 and summer, it must be possible to specify unambiguously by whose
time zone a session is scheduled. To simplify this task for time zone a session is scheduled. To simplify this task for
receivers, we allow the sender to specify the NTP time that a time receivers, we allow the sender to specify the NTP time that a time
zone adjustment happens and the offset from the time when the ses- zone adjustment happens and the offset from the time when the
sion was first scheduled. The ``z'' field allows the sender to session was first scheduled. The "z" field allows the sender to
specify a list of these adjustment times and offsets from the base specify a list of these adjustment times and offsets from the base
time. time.
An example might be: An example might be:
z=2882844526 -1h 2898848070 0 z=2882844526 -1h 2898848070 0
This specifies that at time 2882844526 the time base by which the 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 session's repeat times are calculated is shifted back by 1 hour,
that at time 2898848070 the session's original time base is and that at time 2898848070 the session's original time base is
restored. Adjustments are always relative to the specified start restored. Adjustments are always relative to the specified start
time - they are not cumulative. time - they are not cumulative.
o If a session is likely to last several years, it is expected that o If a session is likely to last several years, it is expected
the session announcement will be modified periodically rather than that
transmit several years worth of adjustments in one announcement. the session announcement will be modified periodically rather than
transmit several years worth of adjustments in one announcement.
Encryption Keys Encryption Keys
k=<method> k=<method>
k=<method>:<encryption key> k=<method>:<encryption key>
o The session description protocol may be used to convey encryption o The session description protocol may be used to convey encryption
keys. A key field is permitted before the first media entry (in keys. A key field is permitted before the first media entry (in
which case it applies to all media in the session), or for each which case it applies to all media in the session), or for each
media entry as required. media entry as required.
o The format of keys and their usage is outside the scope of this o The format of keys and their usage is outside the scope of this
document, but see [3]. document, but see [3].
o The method indicates the mechanism to be used to obtain a usable o The method indicates the mechanism to be used to obtain a usable
key by external means, or from the encoded encryption key given. key by external means, or from the encoded encryption key given.
The following methods are defined:
k=clear:<encryption key> The following methods are defined:
The encryption key (as described in [3] for RTP media streams
under the AV profile) is included untransformed in this key
field.
k=base64:<encoded encryption key> k=clear:<encryption key>
The encryption key (as described in [3] for RTP media streams The encryption key (as described in [3] for RTP media streams
under the AV profile) is included in this key field but has been under the AV profile) is included untransformed in this key
base64 encoded because it includes characters that are prohi- field.
bited in SDP.
k=uri:<URI to obtain key> k=base64:<encoded encryption key>
A Universal Resource Identifier as used by WWW clients is The encryption key (as described in [3] for RTP media streams
included in this key field. The URI refers to the data contain- under the AV profile) is included in this key field but has been
ing the key, and may require additional authentication before base64 encoded because it includes characters that are
the key can be returned. When a request is made to the given prohibited in SDP.
URI, the MIME content-type of the reply specifies the encoding
for the key in the reply. The key should not be obtained until
the user wishes to join the session to reduce synchronisation of
requests to the WWW server(s).
k=prompt k=uri:<URI to obtain key>
No key is included in this SDP description, but the session or A Universal Resource Identifier as used by WWW clients is
media stream referred to by this key field is encrypted. The included in this key field. The URI refers to the data
user should be prompted for the key when attempting to join the containing the key, and may require additional authentication
session, and this user-supplied key should then be used to before the key can be returned. When a request is made to the
decrypt the media streams. given URI, the MIME content-type of the reply specifies the
encoding for the key in the reply. The key should not be
obtained until the user wishes to join the session to reduce
synchronisation of requests to the WWW server(s).
Attributes 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
decrypt the media streams.
a=<attribute> Attributes
a=<attribute>:<value>
Attributes are the primary means for extending SDP. Attributes may be a=<attribute>
defined to be used as "session-level" attributes, "media-level" attri- a=<attribute>:<value>
butes, or both.
A media description may have any number of attributes (``a='' fields) Attributes are the primary means for extending SDP. Attributes may
which are media specific. These are refered to as "media-level" attri- be defined to be used as "session-level" attributes, "media-level"
butes and add information about the media stream. Attribute fields can attributes, or both.
also be added before the first media field; these "session-level" attri-
butes 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: A media description may have any number of attributes ("a=" fields)
which are media specific. These are referred to as "media-level"
attributes and add information about the media stream. Attribute
fields can also be added before the first media field; these
"session-level" attributes convey additional information that applies
to the conference as a whole rather than to individual media; an
example might be the conference's floor control policy.
o property attributes. A property attribute is simply of the form Attribute fields may be of two forms:
``a=<flag>''. These are binary attributes, and the presence of the
attribute conveys that the attribute is a property of the session.
An example might be ``a=recvonly''.
o value attributes. A value attribute is of the form o property attributes. A property attribute is simply of the form
``a=<attribute>:<value>''. An example might be that a whiteboard "a=<flag>". These are binary attributes, and the presence of the
could have the value attribute ``a=orient:landscape'' attribute conveys that the attribute is a property of the session.
An example might be "a=recvonly".
Attribute interpretation depends on the media tool being invoked. Thus o value attributes. A value attribute is of the form
receivers of session descriptions should be configurable in their "a=<attribute>:<value>". An example might be that a whiteboard
interpretation of announcements in general and of attributes in particu- could have the value attribute "a=orient:landscape"
lar.
Attribute names must be in the US-ASCII subset of ISO-10646/UTF-8. Attribute interpretation depends on the media tool being invoked.
Thus receivers of session descriptions should be configurable in
their interpretation of announcements in general and of attributes in
particular.
Attribute values are byte strings, and MAY use any byte value except Attribute names must be in the US-ASCII subset of ISO-10646/UTF-8.
0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values are
to be interpreted as in ISO-10646 character set with UTF-8 encoding.
Unlike other text fields, attribute values are NOT normally affected by
the `charset' attribute as this would make comparisons against known
values problematic. However, when an attribute is defined, it can be
defined to be charset-dependent, in which case it's value should be
interpreted in the session charset rather than in ISO-10646.
Attributes that will be commonly used can be registered with IANA (see Attribute values are byte strings, and MAY use any byte value except
Appendix B). Unregistered attributes should begin with "X-" to prevent 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values
inadvertant collision with registered attributes. In either case, if an are to be interpreted as in ISO-10646 character set with UTF-8
attribute is received that is not understood, it should simply be encoding. Unlike other text fields, attribute values are NOT
ignored by the receiver. normally affected by the `charset' attribute as this would make
comparisons against known values problematic. However, when an
attribute is defined, it can be defined to be charset-dependent, in
which case it's value should be interpreted in the session charset
rather than in ISO-10646.
Media Announcements Attributes that will be commonly used can be registered with IANA
(see Appendix B). Unregistered attributes should begin with "X-" to
prevent 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.
m=<media> <port> <transport> <fmt list> Media Announcements
A session description may contain a number of media descriptions. Each m=<media> <port> <transport> <fmt list>
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 also has several sub-fields:
o The first sub-field is the media type. Currently defined media are A session description may contain a number of media descriptions.
``audio'', ``video'', ``application'', ``data'' and ``control'', Each media description starts with an "m=" field, and is terminated
though this list may be extended as new communication modalities by either the next "m=" field or by the end of the session
emerge (e.g., telepresense). The difference between ``application'' description. A media field also has several sub-fields:
and ``data'' is that the former is a media flow such as whiteboard
information, and the latter is bulk-data transfer such as multicast-
ing 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 o The first sub-field is the media type. Currently defined media are
stream will be sent. The meaning of the transport port depends on "audio", "video", "application", "data" and "control", though this
the network being used as specified in the relevant ``c'' field and list may be extended as new communication modalities emerge (e.g.,
on the transport protocol defined in the third sub-field. Other telepresense). The difference between "application" and "data" is
ports used by the media application (such as the RTCP port, see [2]) that the former is a media flow such as whiteboard information, and
should be derived algorithmically from the base media port. 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.
Note: For transports based on UDP, the value should be in the range o The second sub-field is the transport port to which the media
1024 to 65535 inclusive. For RTP compliance it should be an even stream will be sent. The meaning of the transport port depends on
number. the network being used as specified in the relevant "c" field and
on the transport protocol defined in the third sub-field. Other
ports used by the media application (such as the RTCP port, see
[2]) should be derived algorithmically from the base media port.
For applications where hierarchically encoded streams are being sent Note: For transports based on UDP, the value should be in the range
to a unicast address, it may be necessary to specify multiple tran- 1024 to 65535 inclusive. For RTP compliance it should be an even
sport ports. This is done using a similar notation to that used for number.
IP multicast addresses in the ``c='' field:
m=<media> <port>/<number of ports> <transport> <fmt list> 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:
In such a case, the ports used depend on the transport protocol. m=<media> <port>/<number of ports> <transport> <fmt list>
For RTP, only the even ports are used for data and the corresponding
one-higher odd port is used for RTCP. For example:
m=video 49170/2 RTP/AVP 31 In such a case, the ports used depend on the transport protocol.
For RTP, only the even ports are used for data and the
corresponding one-higher odd port is used for RTCP. For example:
would specify that ports 49170 and 49171 form one RTP/RTCP pair and m=video 49170/2 RTP/AVP 31
49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the tran-
sport protocol and 31 is the format (see below).
It is illegal for both multiple addresses to be specified in the would specify that ports 49170 and 49171 form one RTP/RTCP pair and
``c='' field and for multiple ports to be specified in the ``m='' 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the
field in the same session description. transport protocol and 31 is the format (see below).
o The third sub-field is the transport protocol. The transport pro- It is illegal for both multiple addresses to be specified in the
tocol values are dependent on the address-type field in the ``c='' "c=" field and for multiple ports to be specified in the "m=" field
fields. Thus a ``c='' field of IP4 defines that the transport pro- in the same session description.
tocol runs over IP4. For IP4, it is normally expected that most
media traffic will be carried as RTP over UDP. The following tran-
sport 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 o The third sub-field is the transport protocol. The transport
Audio/Video profile carried over UDP. 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:
- udp - User Datagram Protocol - RTP/AVP - the IETF's Realtime Transport Protocol using the
Audio/Video profile carried over UDP.
If an application uses a single combined proprietary media format - udp - User Datagram Protocol
and transport protocol over UDP, then simply specifying the tran-
sport protocol as udp and using the format field to distinguish the
combined protocol is recommended. If a transport protocol is used
over UDP to carry several distinct media types that need to be dis-
tinguished by a session directory, then specifying the transport
protocol and media format separately is necessary. RTP is an exam-
ple of a transport-protocol that carries multiple payload formats
that must be distinguished by the session directory for it to know
how to start appropriate tools, relays, mixers or recorders.
The main reason to specify the transport-protocol in addition to the If an application uses a single combined proprietary media format
media format is that the same standard media formats may be carried and transport protocol over UDP, then simply specifying the
over different transport protocols even when the network protocol is transport protocol as udp and using the format field to distinguish
the same - a historical example is vat PCM audio and RTP PCM audio. the combined protocol is recommended. If a transport protocol is
In addition, relays and monitoring tools that are transport- used over UDP to carry several distinct media types that need to be
protocol-specific but format-independent are possible. 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.
For RTP media streams operating under the RTP Audio/Video Profile The main reason to specify the transport-protocol in addition to
[3], the protocol field is ``RTP/AVP''. Should other RTP profiles the media format is that the same standard media formats may be
be defined in the future, their profiles will be specified in the carried over different transport protocols even when the network
same way. For example, the protocol field ``RTP/XYZ'' would specify protocol is the same - a historical example is vat PCM audio and
RTP operating under a profile whose short name is ``XYZ''. RTP PCM audio. In addition, relays and monitoring tools that are
transport-protocol-specific but format-independent are possible.
o The fourth and subsequent sub-fields are media formats. For audio For RTP media streams operating under the RTP Audio/Video Profile
and video, these will normally be a media payload type as defined in [3], the protocol field is "RTP/AVP". Should other RTP profiles be
the RTP Audio/Video Profile. defined in the future, their profiles will be specified in the same
way. For example, the protocol field "RTP/XYZ" would specify RTP
operating under a profile whose short name is "XYZ".
When a list of payload formats is given, this implies that all of o The fourth and subsequent sub-fields are media formats. For audio
these formats may be used in the session, but the first of these and video, these will normally be a media payload type as defined
formats is the default format for the session. in the RTP Audio/Video Profile.
For media whose transport protocol is not RTP or UDP the format When a list of payload formats is given, this implies that all of
field is protocol specific. Such formats should be defined in an these formats may be used in the session, but the first of these
additional specification document. formats is the default format for the session.
For media whose transport protocol is RTP, SDP can be used to pro- For media whose transport protocol is not RTP or UDP the format
vide a dynamic binding of media encoding to RTP payload type. The field is protocol specific. Such formats should be defined in an
encoding names in the RTP AV Profile do not specify unique audio additional specification document.
encodings (in terms of clock rate and number of audio channels), and
so they are not used directly in SDP format fields. Instead, the
payload type number should be used to specify the format for static
payload types and the payload type number along with additional
encoding information should be used for dynamically allocated pay-
load types.
An example of a static payload type is u-law PCM coded single chan- For media whose transport protocol is RTP, SDP can be used to
nel audio sampled at 8KHz. This is completely defined in the RTP provide a dynamic binding of media encoding to RTP payload type.
Audio/Video profile as payload type 0, so the media field for such a The encoding names in the RTP AV Profile do not specify unique
stream sent to UDP port 49232 is: audio encodings (in terms of clock rate and number of audio
channels), and so they are not used directly in SDP format fields.
Instead, the payload type number should be used to specify the
format for static payload types and the payload type number along
with additional encoding information should be used for dynamically
allocated payload types.
m=video 49232 RTP/AVP 0 An example of a static payload type is u-law PCM coded single
channel audio sampled at 8KHz. This is completely defined in the
RTP Audio/Video profile as payload type 0, so the media field for
such a stream sent to UDP port 49232 is:
An example of a dynamic payload type is 16 bit linear encoded stereo m=video 49232 RTP/AVP 0
audio sampled at 16KHz. If we wish to use dynamic RTP/AVP payload
type 98 for such a stream, additional information is required to
decode it:
m=video 49232 RTP/AVP 98 An example of a dynamic payload type is 16 bit linear encoded
a=rtpmap:98 L16/16000/2 stereo audio sampled at 16KHz. If we wish to use dynamic RTP/AVP
payload type 98 for such a stream, additional information is
required to decode it:
The general form of an rtpmap attribute is: m=video 49232 RTP/AVP 98
a=rtpmap:98 L16/16000/2
a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding parameters>] The general form of an rtpmap attribute is:
For audio streams, <encoding parameters> may specify the number of a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
audio channels. This parameter may be omitted if the number of parameters>]
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- For audio streams, <encoding parameters> may specify the number of
specific parameters should not be added. Parameters added to an audio channels. This parameter may be omitted if the number of
rtpmap attribute should only be those required for a session channels is one provided no additional parameters are needed. For
directory to make the choice of appropriate media too to participate video streams, no encoding parameters are currently specified.
in a session. Codec-specific parameters should be added in other
attributes.
Up to one rtpmap attribute can be defined for each media format Additional parameters may be defined in the future, but
specified. Thus we might have: codecspecific 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 in a session. Codec-specific parameters should be
added in other attributes.
m=audio 49230 RTP/AVP 96 97 98 Up to one rtpmap attribute can be defined for each media format
a=rtpmap:96 L8/8000 specified. Thus we might have:
a=rtpmap:97 L16/8000
a=rtpmap:98 L16/11025/2
RTP profiles that specify the use of dynamic payload types must m=audio 49230 RTP/AVP 96 97 98
define the set of valid encoding names and/or a means to register a=rtpmap:96 L8/8000
encoding names if that profile is to be used with SDP. a=rtpmap:97 L16/8000
a=rtpmap:98 L16/11025/2
Experimental encoding formats can also be specified using rtpmap. RTP profiles that specify the use of dynamic payload types must
RTP formats that are not registered as standard format names must be define the set of valid encoding names and/or a means to register
preceded by ``X-''. Thus a new experimental redundant audio stream encoding names if that profile is to be used with SDP.
called GSMLPC using dynamic payload type 99 could be specified as:
m=video 49232 RTP/AVP 99 Experimental encoding formats can also be specified using rtpmap.
a=rtpmap:99 X-GSMLPC/8000 RTP formats that are not registered as standard format names must
be preceded by "X-". Thus a new experimental redundant audio
stream called GSMLPC using dynamic payload type 99 could be
specified as:
Such an experimental encoding requires that any site wishing to m=video 49232 RTP/AVP 99
receive the media stream has relevant configured state in its ses- a=rtpmap:99 X-GSMLPC/8000
sion directory to know which tools are appropriate.
Note that RTP audio formats typically do not include information Such an experimental encoding requires that any site wishing to
about the number of samples per packet. If a non-default (as receive the media stream has relevant configured state in its
defined in the RTP Audio/Video Profile) packetisation is required, session directory to know which tools are appropriate.
the``ptime'' attribute is used as given below.
For more details on RTP audio and video formats, see [3]. Note that 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 below.
o Predefined formats for UDP protocol non-RTP media are as below. For more details on RTP audio and video formats, see [3].
Application Formats: o Formats for non-RTP media should be registered as MIME content
types as described in Appendix B. For example, the LBL whiteboard
application might be registered as MIME content-type application/wb
with encoding considerations specifying that it operates over UDP,
with no appropriate file format. In SDP this would then be
expressed using a combination of the "media" field and the "fmt"
field, as follows:
wb: LBL Whiteboard (transport: udp) m=application 32416 udp wb
nt: UCL Network Text Editor (transport: udp) Suggested Attributes
Suggested Attributes The following attributes are suggested. Since application writers
The following attributes are suggested. Since application writers may may add new attributes as they are required, this list is not
add new attributes as they are required, this list is not exhaustive. exhaustive.
a=cat:<category> a=cat:<category>
This attribute gives the dot-separated hierarchical category of the This attribute gives the dot-separated hierarchical category of
session. This is to enable a receiver to filter unwanted sessions the session. This is to enable a receiver to filter unwanted
by category. It would probably have been a compulsory separate sessions by category. It would probably have been a compulsory
field, except for its experimental nature at this time. It is a separate field, except for its experimental nature at this time.
session-level attribute, and is not dependent on charset. It is a session-level attribute, and is not dependent on charset.
a=keywds:<keywords> a=keywds:<keywords>
Like the cat attribute, this is to assist identifying wanted ses- Like the cat attribute, this is to assist identifying wanted
sions at the receiver. This allows a receiver to select interesting sessions at the receiver. This allows a receiver to select
session based on keywords describing the purpose of the session. It interesting session based on keywords describing the purpose of
is a session-level attribute. It is a charset dependent attribute, the session. It is a session-level attribute. It is a charset
meaning that its value should be interpreted in the charset speci- dependent attribute, meaning that its value should be interpreted
fied for the session description if one is specified, or by default in the charset specified for the session description if one is
in ISO 10646/UTF-8. specified, or by default in ISO 10646/UTF-8.
a=tool:<name and version of tool> a=tool:<name and version of tool>
This gives the name and version number of the tool used to create This gives the name and version number of the tool used to create
the session description. It is a session-level attribute, and is the session description. It is a session-level attribute, and is
not dependent on charset. not dependent on charset.
a=ptime:<packet time> a=ptime:<packet time>
This gives the length of time in milliseconds represented by the This gives the length of time in milliseconds represented by the
media in a packet. This is probably only meaningful for audio data. media in a packet. This is probably only meaningful for audio
It should not be necessary to know ptime to decode RTP or vat audio, data. It should not be necessary to know ptime to decode RTP or
and it is intended as a recommendation for the vat audio, and it is intended as a recommendation for the
encoding/packetisation of audio. It is a media attribute, and is encoding/packetisation of audio. It is a media attribute, and is
not dependent on charset. not dependent on charset.
a=recvonly a=recvonly
This specifies that the tools should be started in receive-only mode This specifies that the tools should be started in receive-only
where applicable. It can be either a session or media attribute, and mode where applicable. It can be either a session or media
is not dependent on charset. attribute, and is not dependent on charset.
a=sendrecv a=sendrecv
This specifies that the tools should be started in send and receive This specifies that the tools should be started in send and
mode. This is necessary for interactive conferences with tools such receive mode. This is necessary for interactive conferences with
as wb which defaults to receive only mode. It can be either a ses- tools such as wb which defaults to receive only mode. It can be
sion or media attribute, and is not dependent on charset. either a session or media attribute, and is not dependent on
charset.
a=sendonly a=sendonly
This specifies that the tools should be started in send-only mode. This specifies that the tools should be started in send-only
An example may be where a different unicast address is to be used mode. An example may be where a different unicast address is to
for a traffic destination than for a traffic source. In such a case, be used for a traffic destination than for a traffic source. In
two media descriptions may be use, one sendonly and one recvonly. It such a case, two media descriptions may be use, one sendonly and
can be either a session or media attribute, but would normally only one recvonly. It can be either a session or media attribute, but
be used as a media attribute, and is not dependent on charset. would normally only be used as a media attribute, and is not
dependent on charset.
a=orient:<whiteboard orientation> a=orient:<whiteboard orientation>
Normally this is only used in a whiteboard media specification. It Normally this is only used in a whiteboard media specification.
specifies the orientation of a the whiteboard on the screen. It is It specifies the orientation of a the whiteboard on the screen.
a media attribute. Permitted values are `portrait', `landscape' and It is a media attribute. Permitted values are `portrait',
`seascape' (upside down landscape). It is not dependent on charset `landscape' and `seascape' (upside down landscape). It is not
dependent on charset
a=type:<conference type> a=type:<conference type>
This specifies the type of the conference. Suggested values are This specifies the type of the conference. Suggested values are
`broadcast', `meeting', `moderated', `test' and `H332'. `recvonly' `broadcast', `meeting', `moderated', `test' and `H332'.
should be the default for `type:broadcast' sessions, `type:meeting' `recvonly' should be the default for `type:broadcast' sessions,
should imply `sendrecv' and `type:moderated' should indicate the use `type:meeting' should imply `sendrecv' and `type:moderated'
of a floor control tool and that the media tools are started so as should indicate the use of a floor control tool and that the
to ``mute'' new sites joining the conference. media tools are started so as to "mute" new sites joining the
conference.
Specifying the attribute type:H332 indicates that this loosely cou- Specifying the attribute type:H332 indicates that this loosely
pled session is part of a H.332 session as defined in the ITU H.332 coupled session is part of a H.332 session as defined in the ITU
specification [10]. Media tools should be started `recvonly'. H.332 specification [10]. Media tools should be started
`recvonly'.
Specifying the attribute type:test is suggested as a hint that, Specifying the attribute type:test is suggested as a hint that,
unless explicitly requested otherwise, receivers can safely avoid unless explicitly requested otherwise, receivers can safely avoid
displaying this session description to users. displaying this session description to users.
The type attribute is a session-level attribute, and is not depen- The type attribute is a session-level attribute, and is not
dent on charset. dependent on charset.
a=charset:<character set> a=charset:<character set>
This specifies the character set to be used to display the session This specifies the character set to be used to display the
name and information data. By default, the ISO-10646 character set session name and information data. By default, the ISO-10646
in UTF-8 encoding is used. If a more compact representation is character set in UTF-8 encoding is used. If a more compact
required, other character sets may be used such as ISO-8859-1 for representation is required, other character sets may be used such
Northern European languages. In particular, the ISO 8859-1 is as ISO-8859-1 for Northern European languages. In particular,
specified with the following SDP attribute: the ISO 8859-1 is specified with the following SDP attribute:
a=charset:ISO-8859-1 a=charset:ISO-8859-1
This is a session-level attribute; if this attribute is present, it This is a session-level attribute; if this attribute is present,
must be before the first media field. The charset specified MUST be it must be before the first media field. The charset specified
one of those registered with IANA, such as ISO-8859-1. The charac- MUST be one of those registered with IANA, such as ISO-8859-1.
ter set identifier is a US-ASCII string and MUST be compared against The character set identifier is a US-ASCII string and MUST be
the IANA identifiers using a case-insensitive comparison. If the compared against the IANA identifiers using a case-insensitive
identifier is not recognised or not supported, all strings that are comparison. If the identifier is not recognised or not
affected by it SHOULD be regarded as byte strings. 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 Note that a character set specified MUST still prohibit the use
bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets requiring of bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets
the use of these characters MUST define a quoting mechanism that requiring the use of these characters MUST define a quoting
prevents these bytes appearing within text fields. mechanism that prevents these bytes appearing within text fields.
a=sdplang:<language tag> a=sdplang:<language tag>
This can be a session level attribute or a media level attribute. This can be a session level attribute or a media level attribute.
As a session level attribute, it specifies the language for the ses- As a session level attribute, it specifies the language for the
sion description. As a media level attribute, it specifies the session description. As a media level attribute, it specifies
language for any media-level SDP information field associated with the language for any media-level SDP information field associated
that media. Multiple sdplang attributes can be provided either at with that media. Multiple sdplang attributes can be provided
session or media level if multiple languages if the session descrip- either at session or media level if multiple languages in the
tion or media use multiple languages, in which case the order of the session description or media use multiple languages, in which
attributes indicates the order of importance of the various case the order of the attributes indicates the order of
languages in the session or media from most important to least importance of the various languages in the session or media from
important. most important to least important.
The sdplang attribute value must be a single RFC 1766 language tag In general, sending session descriptions consisting of multiple
in US-ASCII. It is not dependent on the charset attribute. An languages should be discouraged. Instead, multiple descriptions
sdplang attribute SHOULD be specified when a session is of suffi- should be sent describing the session, one in each language.
cient scope to cross geographic boundaries where the language of However this is not possible with all transport mechanisms, and
recipients cannot be assumed, or where the session is in a different so multiple sdplang attributes are allowed although not
language from the locally assumed norm. recommended.
a=lang:<language tag> The sdplang attribute value must be a single RFC 1766 language
This can be a session level attribute or a media level attribute. tag in US-ASCII. It is not dependent on the charset attribute.
As a session level attribute, it specifies the default language for An sdplang attribute SHOULD be specified when a session is of
the session being described. As a media level attribute, it speci- sufficient scope to cross geographic boundaries where the
fies the language for that media, overriding any session-level language of recipients cannot be assumed, or where the session is
language specified. Multiple lang attributes can be provided either in a different language from the locally assumed norm.
at session or media level if multiple languages if the session
description or media use multiple languages, in which case the order
of the attributes indicates the order of importance of the various
languages in the session or media from most important to least
important.
The lang attribute value must be a single RFC 1766 language tag in a=lang:<language tag>
US-ASCII. It is not dependent on the charset attribute. A lang This can be a session level attribute or a media level attribute.
attribute SHOULD be specified when a session is of sufficient scope As a session level attribute, it specifies the default language
to cross geographic boundaries where the language of recipients can- for the session being described. As a media level attribute, it
not be assumed, or where the session is in a different language from specifies the language for that media, overriding any session-
the locally assumed norm. level language specified. Multiple lang attributes can be
provided either at session or media level if multiple languages
if the session description or media use multiple languages, in
which case the order of the attributes indicates the order of
importance of the various languages in the session or media from
most important to least important.
a=framerate:<frame rate> The lang attribute value must be a single RFC 1766 language tag
This gives the maximum video frame rate in frames/sec. It is in US-ASCII. It is not dependent on the charset attribute. A
intended as a recommendation for the encoding of video data. lang attribute SHOULD be specified when a session is of
Decimal representations of fractional values using the notation sufficient scope to cross geographic boundaries where the
"<integer>.<fraction>" are allowed. It is a media attribute, is language of recipients cannot be assumed, or where the session is
only defined for video media, and is not dependent on charset. in a different language from the locally assumed norm.
a=quality:<quality> a=framerate:<frame rate>
This gives a suggestion for the quality of the encoding as an This gives the maximum video frame rate in frames/sec. It is
integer value. intended as a recommendation for the encoding of video data.
Decimal representations of fractional values using the notation
"<integer>.<fraction>" are allowed. It is a media attribute, is
only defined for video media, and is not dependent on charset.
The intention of the quality attribute for video is to specify a a=quality:<quality>
non-default trade-off between frame-rate and still-image quality. This gives a suggestion for the quality of the encoding as an
For video, the value in the range 0 to 10, with the following sug- integer value.
gested meaning:
10 - the best still-image quality the compression scheme can give. The intention of the quality attribute for video is to specify a
non-default trade-off between frame-rate and still-image quality.
For video, the value in the range 0 to 10, with the following
suggested meaning:
5 - the default behaviour given no quality suggestion. 10 - the best still-image quality the compression scheme can
give.
0 - the worst still-image quality the codec designer thinks is 5 - the default behaviour given no quality suggestion.
still usable.
It is a media attribute, and is not dependent on charset. 0 - the worst still-image quality the codec designer thinks is
still usable.
a=fmtp:<format> <format specific parameters> It is a media attribute, and is not dependent on charset.
This attribute allows parameters that are specific to a particular
format to be conveyed in a way that SDP doesn't have to understand
them. The format must be one of the formats specified for the
media. Format-specific parameters may be any set of parameters
required to be conveyed by SDP and given unchanged to the media tool
that will use this format.
It is a media attribute, and is not dependent on charset. a=fmtp:<format> <format specific parameters>
This attribute allows parameters that are specific to a
particular format to be conveyed in a way that SDP doesn't have
to understand them. The format must be one of the formats
specified for the media. Format-specific parameters may be any
set of parameters required to be conveyed by SDP and given
unchanged to the media tool that will use this format.
It is a media attribute, and is not dependent on charset.
6.1. Communicating Conference Control Policy 6.1. Communicating Conference Control Policy
There is some debate over the way conference control policy should be There is some debate over the way conference control policy should be
communicated. In general, the authors believe that an implicit declara- communicated. In general, the authors believe that an implicit
tive style of specifying conference control is desirable where possible. declarative style of specifying conference control is desirable where
possible.
A simple declarative style uses a single conference attribute field A simple declarative style uses a single conference attribute field
before the first media field, possibly supplemented by properties such before the first media field, possibly supplemented by properties
as `recvonly' for some of the media tools. This conference attribute such as `recvonly' for some of the media tools. This conference
conveys the conference control policy. An example might be: attribute conveys the conference control policy. An example might be:
a=type:moderated a=type:moderated
In some cases, however, it is possible that this may be insufficient to
communicate the details of an unusual conference control policy. If
this is the case, then a conference attribute specifying external con-
trol might be set, and then one or more ``media'' fields might be used
to specify the conference control tools and configuration data for those
tools. An example is an ITU H.332 session:
... In some cases, however, it is possible that this may be insufficient
c=IN IP4 224.5.6.7 to communicate the details of an unusual conference control policy.
a=type:H332 If this is the case, then a conference attribute specifying external
m=audio 49230 RTP/AVP 0 control might be set, and then one or more "media" fields might be
m=video 49232 RTP/AVP 31 used to specify the conference control tools and configuration data
m=application 12349 udp wb for those tools. An example is an ITU H.332 session:
m=control 49234 H323 mc
c=IN IP4 134.134.157.81
In this example, a general conference attribute (type:H332) is specified c=IN IP4 224.5.6.7
stating that conference control will be provided by an external H.332 a=type:H332
tool, and a contact addresses for the H.323 session multipoint con- m=audio 49230 RTP/AVP 0
troller is given. m=video 49232 RTP/AVP 31
m=application 12349 udp wb
m=control 49234 H323 mc
c=IN IP4 134.134.157.81
In this document, only the declarative style of conference control In this example, a general conference attribute (type:H332) is
declaration is specified. Other forms of conference control should specified stating that conference control will be provided by an
specify an appropriate type attribute, and should define the implica- external H.332 tool, and a contact addresses for the H.323 session
tions this has for control media. multipoint 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 7. Security Considerations
SDP is a session description format that describes multimedia sessions. SDP is a session description format that describes multimedia
A session description should not be trusted unless it has been obtained sessions. A session description should not be trusted unless it has
by an authenticated transport protocol from a trusted source. Many dif- been obtained by an authenticated transport protocol from a trusted
ferent transport protocols may be used to distribute session descrip- source. Many different transport protocols may be used to distribute
tion, and the nature of the authentication will differ from transport to session description, and the nature of the authentication will differ
transport. from transport to transport.
One transport that will frequently be used to distribute session One transport that will frequently be used to distribute session
descriptions is the Session Announcement Protocol (SAP). SAP provides descriptions is the Session Announcement Protocol (SAP). SAP
both encryption and authentication mechanisms but due to the nature of provides both encryption and authentication mechanisms but due to the
session announcements it is likely that there are many occasions where nature of session announcements it is likely that there are many
the originator of a session announcement cannot be authenticated because occasions where the originator of a session announcement cannot be
they are previously unknown to the receiver of the announcement and authenticated because they are previously unknown to the receiver of
because no common public key infrastructure is available. the announcement and because no common public key infrastructure is
available.
On receiving a session description over an unauthenticated transport On receiving a session description over an unauthenticated transport
mechanism or from an untrusted party, software parsing the session mechanism or from an untrusted party, software parsing the session
should take a few precautions. Session description contain information should take a few precautions. Session description contain
required to start software on the receivers system. Software that information required to start software on the receivers system.
parses a session description MUST not be able to start other software Software that parses a session description MUST not be able to start
except that which is specifically configured as appropriate software to other software except that which is specifically configured as
participate in multimedia sessions. It is normally considered INAP- appropriate software to participate in multimedia sessions. It is
PROPRIATE for software parsing a session description to start, on a normally considered INAPPROPRIATE for software parsing a session
user's system, software that is appropriate to participate in multimedia description to start, on a user's system, software that is
sessions, without the user first being informed that such software will appropriate to participate in multimedia sessions, without the user
be started and giving their consent. Thus a session description arriv- first being informed that such software will be started and giving
ing by session announcement, email, session invitation, or WWW page their consent. Thus a session description arriving by session
SHOULD not deliver the user into an {it interactive} multimedia session announcement, email, session invitation, or WWW page SHOULD not
without the user being aware that this will happen. As it is not always deliver the user into an {it interactive} multimedia session without
simple to tell whether a session is interactive or not, applications the user being aware that this will happen. As it is not always
that are unsure should assume sessions are 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 In this specification, there are no attributes which would allow the
recipient of a session description to be informed to start multimedia recipient of a session description to be informed to start multimedia
tools in a mode where they default to transmitting. Under some cir- tools in a mode where they default to transmitting. Under some
cumstances it might be appropriate to define such attributes. If this circumstances it might be appropriate to define such attributes. If
is done an application parsing a session description containing such this is done an application parsing a session description containing
attributes SHOULD either ignore them, or inform the user that joining such attributes SHOULD either ignore them, or inform the user that
this session will result in the automatic transmission of multimedia joining this session will result in the automatic transmission of
data. The default behaviour for an unknown attribute is to ignore it. multimedia data. The default behaviour for an unknown attribute is
to ignore it.
Session descriptions may be parsed at intermediate systems such as Session descriptions may be parsed at intermediate systems such as
firewalls for the purposes of opening a hole in the firewall to allow firewalls for the purposes of opening a hole in the firewall to allow
the participation in multimedia sessions. It is considered INAPPROPRI- the participation in multimedia sessions. It is considered
ATE for a firewall to open such holes for unicast data streams unless INAPPROPRIATE for a firewall to open such holes for unicast data
the session description comes in a request from inside the firewall. streams unless the session description comes in a request from inside
the firewall.
For multicast sessions, it is likely that local administrators will For multicast sessions, it is likely that local administrators will
apply their own policies, but the exclusive use of "local" or "site- apply their own policies, but the exclusive use of "local" or "site-
local" administrative scope within the firewall and the refusal of the local" administrative scope within the firewall and the refusal of
firewall to open a hole for such scopes will provide separation of glo- the firewall to open a hole for such scopes will provide separation
bal multicast sessions from local ones. of global multicast sessions from local ones.
Appendix A: SDP Grammar Appendix A: SDP Grammar
This appendix provides an Augmented BNF grammar for SDP. This appendix provides an Augmented BNF grammar for SDP. ABNF is
defined in RFC 2234.
announcement ::= proto-version announcement = proto-version
origin-field origin-field
session-name-field session-name-field
information-field information-field
uri-field uri-field
email-fields email-fields
phone-fields phone-fields
connection-field connection-field
bandwidth-fields bandwidth-fields
time-fields time-fields
key-field key-field
attribute-fields attribute-fields
media-descriptions media-descriptions
proto-version ::= "v=" 1*(DIGIT) CRLF proto-version = "v=" 1*DIGIT CRLF
;this draft describes version 0 ;this memo describes version 0
origin-field ::= "o=" username space origin-field = "o=" username space
sess-id space sess-version space sess-id space sess-version space
nettype space addrtype space nettype space addrtype space
addr CRLF addr CRLF
session-name-field ::= "s=" text CRLF session-name-field = "s=" text CRLF
information-field ::= ["i=" text CRLF] information-field = ["i=" text CRLF]
uri-field ::= ["u=" uri CRLF] uri-field = ["u=" uri CRLF]
email-fields ::= *("e=" email-address CRLF) email-fields = *("e=" email-address CRLF)
phone-fields ::= *("p=" phone-number CRLF) phone-fields = *("p=" phone-number CRLF)
connection-field ::= ["c=" nettype space addrtype space connection-field = ["c=" nettype space addrtype space
connection-address CRLF] connection-address CRLF]
;a connection field must be present ;a connection field must be present
;in every media description or at the ;in every media description or at the
;session-level ;session-level
bandwidth-fields ::= *("b=" bwtype ":" bandwidth CRLF) bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF)
time-fields ::= 1*( "t=" start-time space stop-time time-fields = 1*( "t=" start-time space stop-time
*(CRLF repeat-fields) CRLF) *(CRLF repeat-fields) CRLF)
[zone-adjustments CRLF] [zone-adjustments CRLF]
repeat-fields ::= "r=" repeat-interval space typed-time repeat-fields = "r=" repeat-interval space typed-time
1*(space typed-time) 1*(space typed-time)
zone-adjustments ::= time space [``-''] typed-time zone-adjustments = time space ["-"] typed-time
*(space time space [``-''] typed-time) *(space time space ["-"] typed-time)
key-field ::= ["k=" key-type CRLF] key-field = ["k=" key-type CRLF]
key-type ::= "prompt" | key-type = "prompt" |
"clear:" key-data | "clear:" key-data |
"base64:" key-data | "base64:" key-data |
"uri:" uri "uri:" uri
key-data ::= email-safe | "~" | " key-data = email-safe | "~" | "
attribute-fields ::= *("a=" attribute CRLF) attribute-fields = *("a=" attribute CRLF)
media-descriptions ::= *( media-field media-descriptions = *( media-field
information-field information-field
*(connection-field) *(connection-field)
bandwidth-fields bandwidth-fields
key-field key-field
attribute-fields ) attribute-fields )
media-field ::= "m=" media space port ["/" integer] media-field = "m=" media space port ["/" integer]
space proto (space fmt)+ CRLF space proto 1*(space fmt) CRLF
media ::= 1*(alpha-numeric) media = 1*(alpha-numeric)
;typically "audio", "video", "application" ;typically "audio", "video", "application"
;or "data" ;or "data"
fmt ::= 1*(alpha-numeric)
;typically an RTP payload type for audio
;and video media
proto ::= 1*(alpha-numeric) fmt = 1*(alpha-numeric)
;typically "RTP/AVP" or "udp" for IP4 ;typically an RTP payload type for audio
;and video media
port ::= 1*(DIGIT) proto = 1*(alpha-numeric)
;should in the range "1024" to "65535" inclusive ;typically "RTP/AVP" or "udp" for IP4
;for UDP based media
attribute ::= (att-field ":" att-value) | att-field port = 1*(DIGIT)
;should in the range "1024" to "65535" inclusive
;for UDP based media
att-field ::= 1*(alpha-numeric) attribute = (att-field ":" att-value) | att-field
att-value ::= byte-string att-field = 1*(alpha-numeric)
sess-id ::= 1*(DIGIT) att-value = byte-string
;should be unique for this originating username/host
sess-version ::= 1*(DIGIT) sess-id = 1*(DIGIT)
;0 is a new session ;should be unique for this originating username/host
connection-address ::= multicast-address sess-version = 1*(DIGIT)
| unicast-address ;0 is a new session
multicast-address ::= connection-address = multicast-address
3*(decimal_uchar ".") decimal_uchar "/" ttl | addr
[ "/" integer ]
;multicast addresses may be in the range
;224.0.0.0 to 239.255.255.255
ttl ::= decimal_uchar
start-time ::= time | "0" multicast-address = 3*(decimal-uchar ".") decimal-uchar "/" ttl
[ "/" integer ]
;multicast addresses may be in the range
;224.0.0.0 to 239.255.255.255
stop-time ::= time | "0" ttl = decimal-uchar
time ::= POS-DIGIT 9*(DIGIT) start-time = time | "0"
;sufficient for 2 more centuries
repeat-interval ::= typed-time stop-time = time | "0"
typed-time ::= 1*(DIGIT) [fixed-len-time-unit] time = POS-DIGIT 9*(DIGIT)
;sufficient for 2 more centuries
fixed-len-time-unit ::= ``d'' | ``h'' | ``m'' | ``s'' repeat-interval = typed-time
typed-time = 1*(DIGIT) [fixed-len-time-unit]
bwtype ::= 1*(alpha-numeric) fixed-len-time-unit = "d" | "h" | "m" | "s"
bandwidth ::= 1*(DIGIT) bwtype = 1*(alpha-numeric)
username ::= safe bandwidth = 1*(DIGIT)
;pretty wide definition, but doesn't include space
email-address ::= email | email "(" email-safe ")" | username = safe
email-safe "<" email ">" ;pretty wide definition, but doesn't include space
email ::= ;defined in RFC822 email-address = email | email "(" email-safe ")" |
email-safe "<" email ">"
uri::= ;defined in RFC1630 email = ;defined in RFC822
phone-number ::= phone | phone "(" email-safe ")" | uri= ;defined in RFC1630
email-safe "<" phone ">"
phone ::= "+" POS-DIGIT 1*(space | "-" | DIGIT) phone-number = phone | phone "(" email-safe ")" |
;there must be a space or hyphen between the email-safe "<" phone ">"
;international code and the rest of the number.
nettype ::= "IN" phone = "+" POS-DIGIT 1*(space | "-" | DIGIT)
;list to be extended ;there must be a space or hyphen between the
;international code and the rest of the number.
addrtype ::= "IP4" | "IP6" nettype = "IN"
;list to be extended ;list to be extended
addr ::= unicast-address addrtype = "IP4" | "IP6"
;list to be extended
unicast-address ::= IP4-address | IP6-address addr = FQDN | unicast-address
IP4-address ::= b1 "." decimal_uchar "." decimal_uchar "." b4 FQDN = 4*(alpha-numeric|"-"|".")
b1 ::= decimal_uchar ;fully qualified domain name as specified in RFC1035
;less than "224"; not "0" or "127"
b4 ::= decimal_uchar
;not "0"
IP6-address ::= ;to be defined unicast-address = IP4-address | IP6-address
text ::= byte-string IP4-address = b1 "." decimal-uchar "." decimal-uchar "." b4
;default is to interpret this as IS0-10646 UTF8 b1 = decimal-uchar
;ISO 8859-1 requires a "a=charset:ISO-8859-1" ;less than "224"; not "0" or "127"
;session-level attribute to be used b4 = decimal-uchar
;not "0"
byte-string ::= 1*(0x01..0x09|0x0b|0x0c|0x0e..0xff) IP6-address = ;to be defined
;any byte except NUL, CR or LF
decimal_uchar ::= DIGIT text = byte-string
| POS-DIGIT DIGIT ;default is to interpret this as IS0-10646 UTF8
| (1 2*(DIGIT)) ;ISO 8859-1 requires a "a=charset:ISO-8859-1"
| (2 (0|1|2|3|4) DIGIT) ;session-level attribute to be used
| (2 5 (0|1|2|3|4|5))
integer ::= POS-DIGIT *(DIGIT) byte-string = 1*(0x01..0x09|0x0b|0x0c|0x0e..0xff)
;any byte except NUL, CR or LF
alpha-numeric ::= ALPHA | DIGIT decimal-uchar = DIGIT
| POS-DIGIT DIGIT
| ("1" 2*(DIGIT))
| ("2" ("0"|"1"|"2"|"3"|"4") DIGIT)
| ("2" "5" ("0"|"1"|"2"|"3"|"4"|"5"))
DIGIT ::= 0 | POS-DIGIT integer = POS-DIGIT *(DIGIT)
POS-DIGIT ::= 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
ALPHA ::= a | b | c | d | e | f | g | h | i | j | k | alpha-numeric = ALPHA | DIGIT
l | m | n | o | p | q | r | s | t | u | v |
w | x | y | z | A | B | C | D | E | F | G |
H | I | J | K | L | M | N | O | P | Q | R |
S | T | U | V | W | X | Y | Z
email-safe ::= safe | space | tab DIGIT = "0" | POS-DIGIT
safe ::= alpha-numeric | POS-DIGIT = "1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"
"'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
"#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" | ALPHA = "a"|"b"|"c"|"d"|"e"|"f"|"g"|"h"|"i"|"j"|"k"|
"]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" | "l"|"m"|"n"|"o "|"p"|"q"|"r"|"s"|"t"|"u"|"v"|
"~" | " "w"|"x"|"y"|"z"|"A"|"B"|"C "|"D"|"E"|"F"|"G"|
"H"|"I"|"J"|"K"|"L"|"M"|"N"|"O"|"P"|" Q"|"R"|
"S"|"T"|"U"|"V"|"W"|"X"|"Y"|"Z"
email-safe = safe | space | tab
safe = alpha-numeric |
"'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
"#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" |
"]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
"~" | "
space = %d32
tab = %d9
CRLF = %d13.10
space ::= ;ascii code 32
tab ::= ;ascii code 9
CRLF ::= ;ascii code 13 followed by ascii code 10
Appendix B: Guidelines for registering SDP names with IANA Appendix B: Guidelines for registering SDP names with IANA
There are five field names that may be registered with IANA. Using the There are seven field names that may be registered with IANA. Using
terminology in the SDP specification BNF, they are "media", "proto", the terminology in the SDP specification BNF, they are "media",
"fmt", "att-field" and "bwtype". "proto", "fmt", "att-field", "bwtype", "nettype" and "addrtype".
"media" (eg, audio, video, application, data). "media" (eg, audio, video, application, data).
The set of media is intended to be small and not to be extended Packetized media types, such as those used by RTP, share the
except under rare circumstances. The same rules should apply for namespace used by media types registry [RFC 2048] (i.e. "MIME
media names as for top-level MIME content types, and where possible types"). The list of valid media names is the set of top-level
the same name should be registered for SDP as for MIME. For media MIME content types. The set of media is intended to be small and
other than existing MIME top-level content types, a standards-track not to be extended except under rare circumstances. (The MIME
RFC MUST be produced for a new top-level content type to be subtype corresponds to the "fmt" parameter below).
registered, and the registration MUST provide good justification why
no existing media name is appropriate.
"proto" "proto"
In general this should be an IETF standards-track transport protocol In general this should be an IETF standards-track transport
identifier such as RTP/AVP (rfc 1889 under the rfc 1890 profile). protocol identifier such as RTP/AVP (rfc 1889 under the rfc 1890
profile).
However, people will want to invent their own proprietary transport However, people will want to invent their own proprietary
protocols. Some of these should be registered as a "fmt" using transport protocols. Some of these should be registered as a
"udp" as the protocol and some of which probably can't be. "fmt" using "udp" as the protocol and some of which probably
can't be.
Where the protocol and the application are intimately linked, such Where the protocol and the application are intimately linked,
as with the LBL whiteboard wb which used a proprietary and special such as with the LBL whiteboard wb which used a proprietary and
purpose protocol over UDP, the protocol name should be "udp" and the special purpose protocol over UDP, the protocol name should be
format name that should be registered is "wb". The rules for for- "udp" and the format name that should be registered is "wb". The
mats (see below) apply to such registrations. rules for formats (see below) apply to such registrations.
Where the proprietary transport protocol really carries many dif- Where the proprietary transport protocol really carries many
ferent data formats, it is possible to register a new protocol name different data formats, it is possible to register a new protocol
with IANA. In such a case, an RFC MUST be produced describing the name with IANA. In such a case, an RFC MUST be produced
protocol and referenced in the registration. Such an RFC MAY be describing the protocol and referenced in the registration. Such
informational, although it is preferable if it is standards-track. an RFC MAY be informational, although it is preferable if it is
standards-track.
"fmt" "fmt"
The format namespace is dependent on the context of the "proto" The format namespace is dependent on the context of the "proto"
field, so a format cannot be registered without specifying one or field, so a format cannot be registered without specifying one or
more transport protocols that it applies to. more transport protocols that it applies to.
Formats cover all the possible encodings that might want to be tran- Formats cover all the possible encodings that might want to be
sported in a multimedia session. transported in a multimedia session.
For RTP formats that have been assigned static payload types, the For RTP formats that have been assigned static payload types, the
payload type number is used. For RTP formats using a dynamic pay- payload type number is used. For RTP formats using a dynamic
load type number, the dynamic payload type number is given as the payload type number, the dynamic payload type number is given as
format and an additional "rtpmap" attribute specifies the format and the format and an additional "rtpmap" attribute specifies the
parameters. format and parameters.
For non-RTP formats, any unregistered format name may be registered. For non-RTP formats, any unregistered format name may be
If there is a suitable mapping from a MIME subtype to the format, registered through the MIME-type registration process [RFC 2048].
then the MIME subtype name should be registered. If there is no The type given here is the MIME subtype only (the top-level MIME
suitable mapping from a MIME subtype, a new name should be content type is specified by the media parameter). The MIME type
registered. In either case, unless there are strong reasons not to registration SHOULD reference a standards-track RFC which
do so, a standards-track RFC SHOULD be produced describing the for- describes the transport protocol for this media type. If there
mat and this RFC SHOULD be referenced in the registration. is an existing MIME type for this format, the MIME registration
should be augmented to reference the transport specification for
this media type. If there is not an existing MIME type for this
format, and there exists no appropriate file format, this should
be noted in the encoding considerations as "no appropriate file
format".
"att-field" (Attribute names) "att-field" (Attribute names)
Attribute field names MAY be registered with IANA, although this is Attribute field names MAY be registered with IANA, although this
not compulsory, and unknown attributes are simply ignored. is not compulsory, and unknown attributes are simply ignored.
When an attribute is registered, it must be accompanied by a brief When an attribute is registered, it must be accompanied by a
specification stating the following: brief specification stating the following:
o contact name, email address and telephone number o contact name, email address and telephone number
o attribute-name (as it will appear in SDP) o attribute-name (as it will appear in SDP)
o long-form attribute name in English o long-form attribute name in English
o type of attribute (session level, media level, or both) o type of attribute (session level, media level, or both)
o a one paragraph explanation of the purpose of the attribute. o whether the attribute value is subject to the charset
attribute.
o a specification of appropriate attribute values for this o a one paragraph explanation of the purpose of the attribute.
attribute.
IANA will not sanity check such attribute registrations except to o a specification of appropriate attribute values for this
ensure that they do not clash with existing registrations. attribute.
Although the above is the minimum that IANA will accept, if the IANA will not sanity check such attribute registrations except to
attribute is expected to see widespread use and interoperability is ensure that they do not clash with existing registrations.
an issue, authors are encouraged to produce a standards-track RFC
that specifies the attribute more precisely.
Submitters of registrations should ensure that the specification is Although the above is the minimum that IANA will accept, if the
in the spirit of SDP attributes, most notably that the attribute is attribute is expected to see widespread use and interoperability
platform independent in the sense that it makes no implicit assump- is an issue, authors are encouraged to produce a standards-track
tions about operating systems and does not name specific pieces of RFC that specifies the attribute more precisely.
software in a manner that might inhibit interoperability.
"bwtype" (bandwidth specifiers) 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.
A proliferation of bandwidth specifiers is strongly discouraged. "bwtype" (bandwidth specifiers)
New bandwidth specifiers may be registered with IANA. The submis- A proliferation of bandwidth specifiers is strongly discouraged.
sion 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.
Registration Procedure 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.
To register a name the above guidelines should be followed regarding the "nettype" (Network Type)
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 New network types may be registered with IANA if SDP needs to be
used in the context of non-internet environments. Whilst these
are not normally the preserve of IANA, there may be circumstances
when an Internet application needs to interoperate with a non-
internet application, such as when gatewaying an internet
telephony call into the PSTN. The number of network types should
be small and should be rarely extended. A new network type
cannot be registered without registering at least one address
type to be used with that network type. A new network type
registration MUST reference an RFC which gives details of the
network type and address type and specifies how and when they
would be used. Such an RFC MAY be Informational.
o name being registered (as it will appear in SDP) "addrtype" (Address Type)
o long-form name in English New address types may be registered with IANA. An address type
is only meaningful in the context of a network type, and any
registration of an address type MUST specify a registered network
type, or be submitted along with a network type registration. A
new address type registration MUST reference an RFC giving
details of the syntax of the address type. Such an RFC MAY be
Informational. Address types are not expected to be registered
frequently.
o type of name ("media", "proto", "fmt" or "bwtype") Registration Procedure
o a one paragraph explanation of the purpose of the registered name. 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 a reference to the specification (eg RFC number) of the registered 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 (eg RFC number) of the registered
name. name.
IANA may refer any registration to the IESG or to any appropriate IETF IANA may refer any registration to the IESG or to any appropriate
working group for review, and may request revisions to be made before a IETF working group for review, and may request revisions to be made
registration will be made. before a registration will be made.
Appendix C: Authors' Addresses Appendix C: Authors' Addresses
Mark Handley Mark Handley
Information Sciences Institute Information Sciences Institute
c/o MIT Laboratory for Computer Science c/o MIT Laboratory for Computer Science
545 Technology Square 545 Technology Square
Cambridge, MA 02139 Cambridge, MA 02139
United States United States
electronic mail: mjh@isi.edu electronic mail: mjh@isi.edu
Van Jacobson Van Jacobson
MS 46a-1121 MS 46a-1121
Lawrence Berkeley Laboratory Lawrence Berkeley Laboratory
Berkeley, CA 94720 Berkeley, CA 94720
United States United States
electronic mail: van@ee.lbl.gov electronic mail: van@ee.lbl.gov
Acknowledgments Acknowledgments
Many people in the IETF MMUSIC working group have made comments and Many people in the IETF MMUSIC working group have made comments and
suggestions contributing to this document. In particular, we would like suggestions contributing to this document. In particular, we would
to thank Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin, Ross like to thank Eve Schooler, Steve Casner, Bill Fenner, Allison
Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann and Steve Hanna. Mankin, Ross Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Rob
Lanphier and Steve Hanna.
References References
[1] D. Mills, ``Network Time Protocol version 2 specification and imple- [1] Mills, D., "Network Time Protocol (version 3) specification and
mentation", RFC1119, 1st Sept 1989. implementation", RFC 1305, March 1992.
[2] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, ``RTP: A Tran- [2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
sport Protocol for Real-Time Applications'', RFC 1889 A Transport Protocol for Real-Time Applications", RFC 1889, January
1996.
[3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with [3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
Minimal Control'', RFC 1890 with Minimal Control", RFC 1890, January 1996
[4] M. Handley, ``SAP - Session Announcement Protocol'', INTERNET-DRAFT, [4] Handley, M., "SAP - Session Announcement Protocol", Work in
November 25th 1996. Progress.
[5] V. Jacobson, S. McCanne, ``vat - X11-based audio teleconferencing [5] V. Jacobson, S. McCanne, "vat - X11-based audio teleconferencing
tool'' vat manual page, Lawrence Berkeley Laboratory, 1994. tool" vat manual page, Lawrence Berkeley Laboratory, 1994.
[6] ``The Unicode Standard, Version 1.1'': Version 1.0, Volume 1 (ISBN [6] The Unicode Consortium, "The Unicode Standard -- Version 2.0",
0-201-56788-1), Version 1.0, Volume 2 (ISBN 0-201-60845-6), and "Unicode Addison-Wesley, 1996.
Technical Report #4, The Unicode Standard, Version 1.1" (available from
The Unicode Consortium, and soon to be published by Addison- Wesley).
[7] ISO/IEC 10646-1:1993(E) Information Technology--Universal Multiple- [7] ISO/IEC 10646-1:1993. International Standard -- Information
octet Coded Character Set (UCS). technol- ogy -- Universal Multiple-Octet Coded Character Set (UCS) --
Part 1: Architecture and Basic Multilingual Plane. Five amendments
and a techn- ical corrigendum have been published up to now. UTF-8
is described in Annex R, published as Amendment 2.
[8] D. Goldsmith, M. Davis, ``Using Unicode with MIME'', RFC1641, July [8] Goldsmith, D., and M. Davis, "Using Unicode with MIME", RFC 1641,
1994 July 1994.
[9] F. Yergeau, ``UTF-8, a transformation format of Unicode and ISO [9] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
10646'', RFC 2044, Oct 30th 1996 10646", RFC 2044, October 1996.
[10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for Receiv- [10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for
ing Internet-based H.323 Conferences", ITU, Geneva. Receiving Internet-based H.323 Conferences", ITU, Geneva.
[11] M. Handley, E. Schooler, H. Schulzrinne, ``Session Initiation Pro- [11] Handley, M., Schooler, E., and H. Schulzrinne, "Session
tocol (SIP)'' Internet Draft, Nov 1997. Initiation Protocol (SIP)", Work in Progress.
[12] H. Schulzrinne, A. Rao, R. Lanphier, ``Real Time Streaming Protocol [12] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
(RTSP)'' Internet Draft, Jan 1998. Protocol (RTSP)", RFC 2326, April 1998.
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