draft-ietf-mmusic-sdp-02.txt   draft-ietf-mmusic-sdp-03.txt 
Internet Engineering Task Force MMUSIC WG Internet Engineering Task Force MMUSIC WG
INTERNET-DRAFT Mark Handley/Van Jacobson INTERNET-DRAFT Mark Handley/Van Jacobson
draft-ietf-mmusic-sdp-02.txt ISI/LBNL draft-ietf-mmusic-sdp-03.txt ISI/LBNL
21st Nov 1996 26th March 1997
Expires: 21st May 1997 Expires: 26th Sept 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 is an Internet-Draft. Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and its ments of the Internet Engineering Task Force (IETF), its areas, and its
working groups. Note that other groups may also distribute working working groups. Note that other groups may also distribute working
documents as Internet-Drafts. documents as Internet-Drafts.
skipping to change at page 1, line 33 skipping to change at page 1, line 33
To learn the current status of any Internet-Draft, please check the To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast). ftp.isi.edu (US West Coast).
Distribution of this document is unlimited. Distribution of this document is unlimited.
Abstract Abstract
This document defined the Session Description Protocol, SDP. This document defines the Session Description Protocol, SDP.
SDP is intended for describing multimedia sessions for the SDP is intended for describing multimedia sessions for the
purposes of session announcement, session invitation, and purposes of session announcement, session invitation, and
other forms of session initiation. other forms of session initiation.
This document is a product of the Multiparty Multimedia Session Control This document is a product of the Multiparty Multimedia Session Control
(MMUSIC) working group of the Internet Engineering Task Force. Comments (MMUSIC) working group of the Internet Engineering Task Force. Comments
are solicited and should be addressed to the working group's mailing are solicited and should be addressed to the working group's mailing
list at confctrl@isi.edu and/or the authors. 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 is
used to advertise multimedia conferences and communicate the conference used to advertise multimedia conferences and communicate the conference
addresses and conference tool-specific information necessary for parti- addresses and conference tool-specific information necessary for parti-
cipation. This document defines a session description protocol for this cipation. This document defines a session description protocol for this
purpose, and for general real-time multimedia session description pur- purpose, and for general real-time multimedia session description pur-
poses. This draft does not describe multicast address allocation or the poses. This draft does not describe multicast address allocation or the
distribution of SDP messages in detail. These are described in accom- distribution of SDP messages in detail. These are described in accom-
panying drafts. panying drafts. SDP is not intended for negotitation of media encod-
ings.
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 exten-
sively for multimedia conferencing. Such conferences usually have the sively for multimedia conferencing. Such conferences usually have the
property that tight coordination of conference membership is not neces- property that tight coordination of conference membership is not neces-
sary; to receive a conference, a user at an Mbone site only has to know sary; to receive a conference, a user at an Mbone site only has to know
the conference's multicast group address and the UDP ports for the the conference's multicast group address and the UDP ports for the
conference data streams. conference data streams.
Session directories assist the advertisement of conference sessions and Session directories assist the advertisement of conference sessions and
communicate the relevant conference setup information to prospective communicate the relevant conference setup information to prospective
participants. participants. SDP is designed to convey such information to recipients.
SDP is purely a format for session description - it does not incorpor-
This document defines a session description protocol that is used to tate a transport protocol, and is intended to use different transport
define multimedia sessions. It is also an attempt to provide guidelines protocols as appropriate including the Session Announcement Protocol
to the writers of such announcement tools to protect the Mbone from [4], electronic mail using the MIME extensions, and the Hypertext Tran-
misuse and to preserve the inherent scalability of the original LBNL sd sport Protocol.
session directory program whilst enhancing its functionality.
We have also attempted to enhance the generality of SDP so that it can SDP is intended to be general purpose so that it can be used for a wider
be used for a wider range of network environments and applications than range of network environments and applications than just multicast ses-
just multicast session directories. sion directories. However, it is not intended to support negotiation of
session content or media encodings - this is viewed as outside the scope
of session description.
3. Glossary of Terms 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 meaning
within the context of this document. 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 a multimedia senders and receivers and A multimedia session is a set of multimedia senders and receivers
the data streams flowing from senders to receivers. A multimedia and the data streams flowing from senders to receivers. A mul-
conference is an example of a multimedia session. timedia 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 description
is conveyed to users is a pro-active fashion, i.e., the session is conveyed to users in a pro-active fashion, i.e., the session
description was not explicitly requested by the user. 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 dis-
cover and participate in a multimedia session. cover and participate in a multimedia session.
4. SDP Usage 4. SDP Usage
4.1. Multicast Announcements 4.1. Multicast Announcements
SDP is a session description protocol for multimedia sessions. Normally SDP is a session description protocol for multimedia sessions. A common
it is used by an SDP client that announces a conference session by mode of usage is for a client to announce a conference session by
periodically multicasting an announcement packet on a well known multi- periodically multicasting an announcement packet to a well known multi-
cast address and port. With the advent of administrative scoping in the cast address and port using the Session Announcement Protocol (SAP).
Mbone, it is likely that session directory clients will need to be able
to listen for such announcements on multiple addresses. This multicast-
ing of announcements is performed by the Session Announcement Protocol
(SAP).
SAP packets are UDP packets with the following format: SAP packets are UDP packets with the following format:
0 31 0 31
|--------------------| |--------------------|
| SAP header | | SAP header |
|--------------------| |--------------------|
| text payload | | text payload |
|/\/\/\/\/\/\/\/\/\/\| |/\/\/\/\/\/\/\/\/\/\|
The header is the Session Announcement Protocol header. SAP is The header is the Session Announcement Protocol header. SAP is
described in more detail in a companion draft [4] described in more detail in a companion draft [4]
The text payload is an SDP session description, as described in this 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. draft. The text payload should be no greater than 1 Kbyte in length.
If announced by SAP, only one session announcement is permitted in a If announced by SAP, only one session announcement is permitted in a
single packet. single packet.
4.2. Email and WWW Announcements 4.2. Email and WWW Announcements
For both email and WWW distribution, the use of the MIME content type Alternative means of conveying session descriptions include electronic
``application/x-sdp'' is currently suggested. This enables the mail and the World Wide Web. For both email and WWW distribution, the
automatic launching of applications from the WWW client or mail reader
in a standard manner. 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.
Note that announcements of multicast sessions made only via email or the Note that announcements of multicast sessions made only via email or the
World Wide Web (WWW) do not have the property that the receiver of a World Wide Web (WWW) do not have the property that the receiver of a
session announcement can necessarily receive the session, nor do they session announcement can necessarily receive the session because the
allow scalable dynamic multicast address allocation. 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 5. Requirements and Recommendations
The purpose of SDP is to convey information about media streams in mul- The purpose of SDP is to convey information about media streams in mul-
timedia sessions to allow the recipients of a session description to timedia sessions to allow the recipients of a session description to
participate in the session. SDP is primarily intended for use in an participate in the session. SDP is primarily intended for use in an
internetwork, although it is sufficiently general that it can describe internetwork, although it is sufficiently general that it can describe
conferences in other network environments. conferences in other network environments.
A multimedia session, for these purposes, is defined as a set of media A multimedia session, for these purposes, is defined as a set of media
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Thus far, multicast based sessions on the internet have differed from Thus far, multicast based sessions on the internet have differed from
many other forms of conferencing in that anyone receiving the traffic many other forms of conferencing in that anyone receiving the traffic
can join the session (unless the session traffic is encrypted). In such can join the session (unless the session traffic is encrypted). In such
an environment, SDP serves two primary purposes. It is a means to com- an environment, SDP serves two primary purposes. It is a means to com-
municate the existence of a session, and is a means to convey sufficient municate the existence of a session, and is a means to convey sufficient
information to enable joining and participating in the session. In a information to enable joining and participating in the session. In a
unicast environment, only the latter purpose is likely to be relevant. unicast environment, only the latter purpose is likely to be relevant.
Thus SDP includes: Thus SDP includes:
+ Session name and purpose o Session name and purpose
+ Time(s) the session is active o Time(s) the session is active
+ The media comprising the session o The media comprising the session
+ Information to receive those media (addresses, ports, formats and so o Information to receive those media (addresses, ports, formats and
on) so on)
As resources necessary to participate in a session may be limited, some As resources necessary to participate in a session may be limited, some
additional information may also be desirable: additional information may also be desirable:
+ Information about the bandwidth to be used by the conference o Information about the bandwidth to be used by the conference
+ Contact information for the person responsible for the session o Contact information for the person responsible for the session
In general, SDP must convey sufficient information to be able to join a In general, SDP must convey sufficient information to be able to join a
session (with the possible exception of encryption keys) and to announce session (with the possible exception of encryption keys) and to announce
the resources to be used to non-participants that may need to know. 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:
+ The type of media (video, audio, etc) o The type of media (video, audio, etc)
+ The transport protocol (RTP/UDP/IP, H.320, etc) o The transport protocol (RTP/UDP/IP, H.320, etc)
+ 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:
+ Multicast address for media o Multicast address for media
+ Transport Port for media o Transport Port for media
For an IP unicast session, the following are conveyed: This address and port are the destination address and destination port
of the multicast stream, whether being sent, received, or both.
+ Contact address for media For an IP unicast session, the following are conveyed:
+ Transport port for contact address o Remote address for media
This may or may not be be the source and destination of the media o Transport port for contact address
stream.
Sessions being conveyed over other types of network will have their own The semantics of this address and port depend on the media and transport
specific requirements - SDP must be extensible for these. protocol defined. By default, this is the remote address and remote
port to send data to, and the remote address and local port for receiv-
ing data. However, some media may define 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:
+ An arbitrary list of start and stop times bounding the session o An arbitrary list of start and stop times bounding the session
+ 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 per-
formed are dependent on the mechanism used to convey SDP - see [4] for formed are dependent on the mechanism used to convey SDP - see [4] for
how this is done for session announcements. 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 private
announcement to convey encryption keys necessary to decode each of the announcement to convey encryption keys necessary to decode each of the
media in a conference, including enough information to know which media in a conference, including enough information to know which
encryption scheme is used for each media. encryption scheme is used for each media.
5.4. Further Information 5.4. Obtaining Further Information about a Session
SDP should convey enough information to decide whether or not to parti- A session description should convey enough information to decide whether
cipate in a session. It should include additional pointers in the form or not to participate in a session. SDP may include additional pointers
of Universal Resources Identifiers (URIs) for in the form of Universal Resources Identifiers (URIs) for more informa-
more information about the session. tion 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 other
advertisement mechanism, it may be desirable to filter announcements advertisement mechanism, it may be desirable to filter announcements
that are of interest from those that are not. SDP should support a that are of interest from those that are not. SDP supports a categori-
categorisation mechanism for sessions that is capable of being sation mechanism for sessions that is capable of being automated.
automated.
5.6. Internationalization 5.6. Internationalization
The SDP specification recommends the use of 8-bit ISO 8859-1 character The SDP specification recommends the use of 8-bit ISO 8859-1 character
sets to allow the extended ASCII characters used by many western and sets to allow the extended ASCII characters used by many western and
northern European languages to be represented. However, there are many northern European languages to be represented. However, there are many
languages that cannot be represented in an ISO 8859-1 character set. languages that cannot be represented in an ISO 8859-1 character set.
SDP should also allow extensions to allow other font types to be used SDP allows extensions to allow other character sets to be used when
when required. required.
6. SDP Specification 6. SDP Specification
SDP session descriptions are entirely textual. The textual form, as SDP session descriptions are entirely textual. The textual form, as
opposed to a binary encoding such as ASN/1 or XDR, was chosen to enhance opposed to a binary encoding such as ASN/1 or XDR, was chosen to enhance
portability, to enable a variety of transports to be used (e.g, session portability, to enable a variety of transports to be used (e.g, session
description in a MIME email message) and to allow flexible, text-based description in a MIME email message) and to allow flexible, text-based
toolkits (e.g., Tcl/Tk ) to be used to generate and to process session toolkits (e.g., Tcl/Tk ) to be used to generate and to process session
descriptions. However, since the total bandwidth allocated to all SAP descriptions. However, since the total bandwidth allocated to all SAP
announcements is strictly limited, the encoding is deliberately compact. announcements is strictly limited, the encoding is deliberately compact.
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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 the
form form
<type>=<value> <type>=<value>
<type> is always exactly one character and is case-significant. <value> <type> is always exactly one character and is case-significant. <value>
is a structured text string whose format depends on <type>. Whitespace is a structured text string whose format depends on <type>. Whitespace
is not permitted either side of the `=' sign. In general <value> is is not permitted either side of the `=' sign. In general <value> is
either a number of fields delimited by a single space character or a either a number of fields delimited by a single space character or a
free format string. free format string.
Each announcement consists of a session description section followed by A session description consists of a session-level descriptions (details
zero or more `media' description sections. The session description that apply to the whole session and all media streams) and optionally
starts with a `v=' line and continues to the first media description or several media-level descriptions (details that apply onto to a single
the next session description. The media description starts with an `m=' media stream).
line and continues to the next media description or session description.
An announcement consists of a session-level section followed by zero or
more media-level sections. The session-level part starts with a `v='
line and continues to the first media-level section. The media descrip-
tion starts with an `m=' line and continues to the next media descrip-
tion or end of the whole session description. In general, session-level
values are the default for all media unless overridden by an equivalent
media-level value.
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 per
packet. When SDP is conveyed by other means, many SDP session descrip- packet. When SDP is conveyed by other means, many SDP session descrip-
tions may be concatenated together. Some lines in each description are tions may be concatenated together (the `v=' line indicating the start
required and some are optional but all must appear in exactly the order of a session description terminates the previous description). Some
given here (the fixed order greatly enhances error detection and allows lines in each description are required and some are optional but all
for a simple parser). Optional items are marked with a `*'. must appear in exactly the order given here (the fixed order greatly
enhances error detection and allows for a simple parser). Optional
items are marked with a `*'.
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)
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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 be
extensible -- SDP parsers must completely ignore any announcement that extensible -- SDP parsers must completely ignore any announcement that
contains a `type' letter that it does not understand. The `attribute' contains a `type' letter that it does not understand. The `attribute'
mechanism (described below) is the primary means for extending SDP and mechanism (described below) is the primary means for extending SDP and
tailoring it to particular applications or media. Some attributes (the tailoring it to particular applications or media. Some attributes (the
ones listed in this document) have a defined meaning but others may be ones listed in this document) have a defined meaning but others may be
added on an application-, media- or session-specific basis. A session added on an application-, media- or session-specific basis. A session
directory must ignore any attribute it doesn't understand. 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 session-
section applies to all the media of that session unless overridden by level section applies to all the media of that session unless overridden
connection information or an attribute of the same name in the media by connection information or an attribute of the same name in the media
description. For instance, in the example below, each media behaves as description. For instance, in the example below, each media behaves as
if it were given a `recvonly' attribute. 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 3456 RTP/AVP 0 m=audio 3456 RTP/AVP 0
m=video 2232 RTP/AVP 31 m=video 2232 RTP/AVP 31
m=whiteboard 32416 UDP WB m=whiteboard 32416 udp wb
a=orient:portrait a=orient:portrait
Text records such as the session name and information may contain any Text records such as the session name and information may contain any
printable 8 bit ISO 8859-1 character with the exceptions of 0x0a (new- printable 8 bit ISO 8859-1 character with the exceptions of 0x0a (new-
line) and 0x0d (carriage return). Carriage Return is prohibited, and line) and 0x0d (carriage return). Carriage Return is prohibited, and
Newline is used to end a record. Newline is used to end a record.
Protocol Version Protocol Version
v=0 v=0
skipping to change at page 9, line 41 skipping to change at page 9, line 41
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 and
the address of the user's host) plus a session id and session version the address of the user's host) plus a session id and session version
number. <username> is the user's login on the originating host, or it number. <username> is the user's login on the originating host, or it
is ``-'' if the originating host does not support the concept of user is ``-'' if the originating host does not support the concept of user
ids. <username> must not contain spaces. <session id> is a numeric ids. <username> must not contain spaces. <session id> is a numeric
string such that the triple of <username>, <session id> and <address> string such that the tuple of <username>, <session id>, <network type>,
form a globally unique identifier for the session. The method of ses- <address type> and <address> form a globally unique identifier for the
sion id allocation is up to the creating tool, but it has been suggested session. The method of session id allocation is up to the creating
that a Network Time Protocol (NTP) timestamp be used to ensure unique- tool, but it has been suggested that a Network Time Protocol (NTP)
ness [1]. <version> is a version number for this announcement. It is timestamp be used to ensure uniqueness [1]. <version> is a version
needed for proxy announcements to detect which of several announcements number for this announcement. It is needed for proxy announcements to
for the same session is the most recent. Again its usage is up to the detect which of several announcements for the same session is the most
creating tool, so long as <version> is increased when a modification is recent. Again its usage is up to the creating tool, so long as <ver-
made to the session data. Again, it is recommended (but not mandatory) sion> is increased when a modification is made to the session data.
that an NTP timestamp is used. <network type> is a text string giving Again, it is recommended (but not mandatory) that an NTP timestamp is
the type of network. Initially ``IN'' is defined to have the meaning used. <network type> is a text string giving the type of network. Ini-
``Internet''. <address type> is a text string giving the type of the tially ``IN'' is defined to have the meaning ``Internet''. <address
address that follows. Initially ``IP4'' and ``IP6'' are defined. type> is a text string giving the type of the address that follows.
<address> is the globally unique address of the machine from which the Initially ``IP4'' and ``IP6'' are defined. <address> is the globally
session was created. For an address type of IP4, this is the dotted- unique address of the machine from which the session was created. For
decimal representation of the IP version 4 address of the machine. 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.
Session Name Session Name
s=<session name> s=<session name>
The ``s'' field is the session name. There must be one and only one The ``s'' field is the session name. There must be one and only one
``s'' field per announcement, and it must contain printable ISO 8859-1 ``s'' field per announcement, and it must contain printable ISO 8859-1
characters (but see also the `charset' attribute below). characters (but see also the `charset' attribute below).
Session and Media Information Session and Media Information
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media definitions, ``i'' fields are primarily intended for labeling media definitions, ``i'' fields are primarily intended for labeling
media streams. As such, they are most likely to be useful when a single 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. 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 An example would be two different whiteboards, one for slides and one
for feedback and questions. for feedback and questions.
URI URI
u=<URI> u=<URI>
+ A URI is a Universal Resource Identifier as used by WWW clients o A URI is a Universal Resource Identifier as used by WWW clients
+ The URI should be a pointer to additional information about the o The URI should be a pointer to additional information about the
conference conference
+ This field is optional, but if it is present it should be specified o This field is optional, but if it is present it should be specified
before the first media field before the first media field
+ No more than one URI field is allowed per session description o No more than one URI field is allowed per session description
Email Address and Phone Number Email Address and Phone Number
e=<email address> e=<email address>
p=<phone number> p=<phone number>
+ These specify contact information for the person responsible for the o These specify contact information for the person responsible for
conference. This is not necessarily the same person that created the conference. This is not necessarily the same person that
the conference announcement. created the conference announcement.
+ Either an email field or a phone field must be specified. Addi- o Either an email field or a phone field must be specified. Addi-
tional email and phone fields are allowed. tional email and phone fields are allowed.
+ If these are present, they should be specified before the first o If these are present, they should be specified before the first
media field. media field.
+ More than one email or phone field can be given for a session o More than one email or phone field can be given for a session
description. description.
+ Phone numbers should be given in the conventional international for- o Phone numbers should be given in the conventional international
mat - preceded by a ``+'' and the international country code. There format - preceded by a ``+'' and the international country code.
must be a space or a hyphen (``-'') between the country code and the There must be a space or a hyphen (``-'') between the country code
rest of the phone number. Spaces and hyphens may be used to split and the rest of the phone number. Spaces and hyphens may be used to
up a phone field to aid readability if desired. For example: split up a phone field to aid readability if desired. For example:
p=+44-171-380-7777 or p=+1 617 253 6011 p=+44-171-380-7777 or p=+1 617 253 6011
+ Both email addresses and phone numbers can have an optional free o Both email addresses and phone numbers can have an optional free
text string associated with them, normally giving the name of the text string associated with them, normally giving the name of the
person who may be contacted. This should be enclosed in parenthesis person who may be contacted. This should be enclosed in parenthesis
if it is present. For example: if it is present. For example:
e=mjh@isi.edu (Mark Handley) e=mjh@isi.edu (Mark Handley)
The alternative RFC822 name quoting convention is also allowed for The alternative RFC822 name quoting convention is also allowed for
both email addresses and phone numbers. For example, both email addresses and phone numbers. For example,
e=Mark Handley <mjh@isi.edu> e=Mark Handley <mjh@isi.edu>
The free text string should be in an IS0-8859-1 character set, or The free text string should be in the ISO 8859-1 character set, or
alternatively in unicode UTF-7 encoding if the appropriate charset alternatively in unicode UTF-7 encoding if the appropriate charset
session-level attribute is set. session-level attribute is set.
Connection Data Connection Data
c=<network type> <address type> <connection address> c=<network type> <address type> <connection address>
The ``c'' field contains connection data. The ``c'' field contains connection data.
The first sub-field is the network type, which is a text string giving The first sub-field is the network type, which is a text string giving
skipping to change at page 12, line 22 skipping to change at page 12, line 24
The second sub-field is the address type. This allows SDP to be used The second sub-field is the address type. This allows SDP to be used
for sessions that are not IP based. Currently only IP4 is defined. for sessions that are not IP based. Currently only IP4 is defined.
The third sub-field is the connection address. Optional extra sub- The third sub-field is the connection address. Optional extra sub-
fields may be added after the connection address depending on the value fields may be added after the connection address depending on the value
of the <address type> field. of the <address type> field.
For IP4 addresses, the connection address is defined as follows: For IP4 addresses, the connection address is defined as follows:
+ Typically the connection address will be a class-D IP multicast o Typically the connection address will be a class-D IP multicast
group address. If the conference is not multicast, then the connec- group address. If the conference is not multicast, then the connec-
tion address contains the unicast IP address of the expected data tion address contains the unicast IP address of the expected data
source or data relay or data sink as determined by additional attri- source or data relay or data sink as determined by additional attri-
bute fields. It is not expected that unicast addresses will be bute fields. It is not expected that unicast addresses will be
given in a session description that is communicated by a multicast given in a session description that is communicated by a multicast
announcement, though this is not prohibited. announcement, though this is not prohibited.
+ Conferences using an IP multicast connection address must also have o Conferences using an IP multicast connection address must also have
a time to live (TTL) value present in addition to the multicast a time to live (TTL) value present in addition to the multicast
address. The TTL defines the scope with which multicast packets address. The TTL defines the scope with which multicast packets
sent in this conference should be sent. TTL values must be in the sent in this conference should be sent. TTL values must be in the
range 0-255. The Mbone usage guidelines (currently available at range 0-255. The Mbone usage guidelines (currently available at
ftp://ftp.isi.edu/mbone/faq.txt) define several standard settings ftp://ftp.isi.edu/mbone/faq.txt) define several standard settings
for TTL: for TTL:
local net: 1 local net: 1
site: 15 site: 15
region: 63 region: 63
skipping to change at page 13, line 38 skipping to change at page 13, line 41
A session announcement must contain one ``c'' field in each media A session announcement must contain one ``c'' field in each media
description (see below) or a ``c'' field at the session-level. It description (see below) or a ``c'' field at the session-level. It
may contain a session-level ``c'' field and one additional ``c'' may contain a session-level ``c'' field and one additional ``c''
field per media description, in which case the per-media values field per media description, in which case the per-media values
override the session-level settings for the relevant media. override the session-level settings for the relevant media.
Bandwidth Bandwidth
b=<modifier>:<bandwidth-value> b=<modifier>:<bandwidth-value>
+ This specifies the proposed bandwidth to be used by the session or o This specifies the proposed bandwidth to be used by the session or
media, and is optional. media, and is optional.
+ <bandwidth-value> is in kilobits per second o <bandwidth-value> is in kilobits per second
+ <modifier> is an single alphanumeric word giving the meaning of the o <modifier> is an single alphanumeric word giving the meaning of
bandwidth figure. the bandwidth figure.
+ Two modifiers are initially defined: o Two modifiers are initially defined:
CT Conference Total: An implicit maximum bandwidth is associated with CT Conference Total: An implicit maximum bandwidth is associated with
each TTL on the Mbone or within a particular multicast each TTL on the Mbone or within a particular multicast administra-
administrative scope region (the Mbone bandwidth vs. TTL limits tive scope region (the Mbone bandwidth vs. TTL limits are given in
are given in the MBone FAQ). If the bandwidth of a session or the MBone FAQ). If the bandwidth of a session or media in a ses-
media in a session is different from the bandwidth implicit from sion is different from the bandwidth implicit from the scope, a
the scope, a `b=CT:...' line should be supplied for the session `b=CT:...' line should be supplied for the session giving the pro-
giving the proposed upper limit to the bandwidth used. The pri- posed upper limit to the bandwidth used. The primary purpose of
mary purpose of this is to give an approximate idea as to whether this is to give an approximate idea as to whether two or more
two or more conferences can co-exist simultaneously. conferences can co-exist simultaneously.
AS Application Specific Maximum: The bandwidth is interpreted to be AS Application Specific Maximum: The bandwidth is interpreted to be
application specific, i.e., will be the application's concept of application specific, i.e., will be the application's concept of
maximum bandwidth. Normally this will coincide with what is set maximum bandwidth. Normally this will coincide with what is set
on the application's ``maximum bandwidth'' control if applicable. on the application's ``maximum bandwidth'' control if applicable.
Note that CT gives a total bandwidth figure for all the media at all Note that CT gives a total bandwidth figure for all the media at all
sites. AS gives a bandwidth figure for a single media at a single sites. AS gives a bandwidth figure for a single media at a single
site, although there may be many sites sending simultaneously. site, although there may be many sites sending simultaneously.
+ Extension Mechanism: Tool writers can define experimental bandwidth o Extension Mechanism: Tool writers can define experimental bandwidth
modifiers by prefixing their modifier with ``X-''. For example: modifiers by prefixing their modifier with ``X-''. For example:
b=X-YZ:128 b=X-YZ:128
SDP parsers should ignore bandwidth fields with unknown modifiers. SDP parsers should ignore bandwidth fields with unknown modifiers.
Modifiers should be alpha-numeric and, although no length limit is Modifiers should be alpha-numeric and, although no length limit is
given, they are recommended to be short. given, they are recommended to be short.
Times, Repeat Times and Time Zones Times, Repeat Times and Time Zones
t=<start time> <stop time> t=<start time> <stop time>
+ ``t'' fields specify the start and stop times for a conference ses- o ``t'' fields specify the start and stop times for a conference ses-
sion. Multiple ``t'' fields may be used if a session is active at sion. Multiple ``t'' fields may be used if a session is active at
multiple irregularly spaced times; each additional ``t'' field multiple irregularly spaced times; each additional ``t'' field
specifies an additional period of time for which the session will be specifies an additional period of time for which the session will be
active. If the session is active at regular times, an ``r'' field active. If the session is active at regular times, an ``r'' field
(see below) should be used in addition to and following a ``t'' (see below) should be used in addition to and following a ``t''
field - in which case the ``t'' field specifies the start and stop field - in which case the ``t'' field specifies the start and stop
times of the repeat sequence. times of the repeat sequence.
+ The first and second sub-fields give the start and stop times for o The first and second sub-fields give the start and stop times for
the conference respectively. These values are the decimal represen- the conference respectively. These values are the decimal represen-
tation of Network Time Protocol (NTP) time values in seconds [1]. tation of Network Time Protocol (NTP) time values in seconds [1].
To convert these values to UNIX time, subtract decimal 2208988800. To convert these values to UNIX time, subtract decimal 2208988800.
+ If the stop-time is set to zero, then the session is not bounded, 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 though it will not become active until after the start-time. If the
start-time is also zero, the session is regarded as permanent. start-time is also zero, the session is regarded as permanent.
User interfaces should strongly discourage the creation of unbounded User interfaces should strongly discourage the creation of unbounded
and permanent sessions as they give no information about when the and permanent sessions as they give no information about when the
session is actually going to terminate, and so make scheduling dif- session is actually going to terminate, and so make scheduling dif-
ficult. ficult.
The general assumption may be made, when displaying unbounded ses- The general assumption may be made, when displaying unbounded ses-
sions that have not timed out to the user, that an unbounded session sions that have not timed out to the user, that an unbounded session
skipping to change at page 15, line 25 skipping to change at page 15, line 29
appropriate when new information becomes available about when the appropriate when new information becomes available about when the
session should really end. session should really end.
Permanent sessions may be shown to the user as never being active Permanent sessions may be shown to the user as never being active
unless there are associated repeat times which state precisely when unless there are associated repeat times which state precisely when
the session will be active. In general, permanent sessions should the session will be active. In general, permanent sessions should
not be created for any session expected to have a duration of less not be created for any session expected to have a duration of less
than 2 months, and should be discouraged for sessions expected to than 2 months, and should be discouraged for sessions expected to
have a duration of less than 6 months. have a duration of less than 6 months.
+ It is prohibited for the start time to be after the stop time.
r=<repeat interval> <active duration> <list of offsets from start-time> r=<repeat interval> <active duration> <list of offsets from start-time>
+ ``r'' fields specify repeat times for a session. For example, if a o ``r'' fields specify repeat times for a session. For example, if
session is active at 10am on Monday and 11am on Tuesday for one hour a session is active at 10am on Monday and 11am on Tuesday for one
each week for three months, then the <start time> in the correspond- hour each week for three months, then the <start time> in the
ing ``t'' field would be the NTP representation of 10am on the first corresponding ``t'' field would be the NTP representation of 10am on
Monday, the <repeat interval> would be 1 week, the <active duration> the first Monday, the <repeat interval> would be 1 week, the <active
would be 1 hour, and the offsets would be zero and 25 hours. The duration> would be 1 hour, and the offsets would be zero and 25
corresponding ``t'' field stop time would be the NTP representation hours. The corresponding ``t'' field stop time would be the NTP
of the end of the last session three months later. By default all representation of the end of the last session three months later. By
fields are in seconds, so the ``r'' and ``t'' fields might be: default all fields are in seconds, so the ``r'' and ``t'' fields
might be:
t=3034423619 3042462419 t=3034423619 3042462419
r=604800 3600 0 90000 r=604800 3600 0 90000
To make announcements more compact, times may also be given in To make announcements more compact, times may also be given in
units of days, hours or minutes. The syntax for these is a number units of days, hours or minutes. The syntax for these is a number
immediately followed by a single case-sensitive character. Frac- immediately followed by a single case-sensitive character. Frac-
tional units are not allowed - a smaller unit should be used tional 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:
skipping to change at page 16, line 4 skipping to change at page 16, line 9
To make announcements more compact, times may also be given in To make announcements more compact, times may also be given in
units of days, hours or minutes. The syntax for these is a number units of days, hours or minutes. The syntax for these is a number
immediately followed by a single case-sensitive character. Frac- immediately followed by a single case-sensitive character. Frac-
tional units are not allowed - a smaller unit should be used tional 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=1d 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> ....
+ Should it be necessary to schedule a repeated session which spans a o To schedule a repeated session which spans a change from daylight-
change from daylight-saving time to standard time or vice-versa, it saving time to standard time or vice-versa, it is necessary to
is necessary to specify offsets from the base repeat times. This is specify offsets from the base repeat times. This is required because
necessary because different time zones change time at different different time zones change time at different times of day, dif-
times of day, different countries change to or from daylight time on ferent countries change to or from daylight time on different dates,
different dates, and some countries to not have daylight saving time and some countries do not have daylight saving time at all.
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 ses-
sion was first scheduled. The ``z'' field allows the sender to sion 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, and
that at time 2898848070 the session's original time base is 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.
+ 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 that
the session announcement will be modified periodically rather than the session announcement will be modified periodically rather than
transmit several years worth of adjustments in one announcement. 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>
+ In countries where encrypted sessions are not prohibited by law, the o The session description protocol may be used to convey encryption
session description protocol may be used to convey encryption keys. 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
+ A key field is permitted before the first media entry (in which case media entry as required.
it applies to all media in the session), or for each media entry as
required.
+ The format of keys and their usage is outside the scope of this 4 The format of keys and their usage is outside the scope of this
document, but see [3]. document, but see [3].
+ The method indicates the mechanism to be used to obtain a usable key o The method indicates the mechanism to be used to obtain a usable
by external means, or from the encoded encryption key given. The key by external means, or from the encoded encryption key given.
following methods are defined: The following methods are defined:
k=clear:<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 untransformed in this key under the AV profile) is included untransformed in this key
field. field.
k=base64:<encoded encryption key> k=base64:<encoded 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 in this key field but has been
base64 encoded because it includes characters that are prohi- base64 encoded because it includes characters that are prohi-
bited in SDP. bited in SDP.
k=uri:<URI to obtain key> k=uri:<URI to obtain key>
A Universal Resource Identifier as used by WWW clients is A Universal Resource Identifier as used by WWW clients is
included in this key field. The URI refers to the data contain- included in this key field. The URI refers to the data contain-
ing the key, and may require additional authentication before ing the key, and may require additional authentication before
the key can be returned. The key should not be obtained until the key can be returned. When a request is made to the 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 the user wishes to join the session to reduce synchronisation of
requests to the WWW server(s). requests to the WWW server(s).
k=prompt k=prompt
No key is included in this SDP description, but the session or No key is included in this SDP description, but the session or
media stream referred to by this key field is encrypted. The media stream referred to by this key field is encrypted. The
user should be prompted for the key when attempting to join the user should be prompted for the key when attempting to join the
session, and this user-supplied key should then used to decrypt session, and this user-supplied key should then used to decrypt
the media streams. the media streams.
Attributes Attributes
a=<flag> a=<attribute>
a=<attribute>:<value> a=<attribute>:<value>
A media field may also have any number of attributes (``a'' fields) A media field may also have any number of attributes (``a'' fields)
which are media specific. Attribute fields may be of two forms: which are media specific. Attribute fields may be of two forms:
+ flag attributes. A flag attribute is simply of the form o property attributes. A property attribute is simply of the form
``a=<flag>''. These are binary attributes, and the presence of the ``a=<flag>''. These are binary attributes, and the presence of the
attribute conveys that the attribute is ``true''. An example might attribute conveys that the attribute is a property of the session.
be ``a=recvonly''. An example might be ``a=recvonly''.
+ value attributes. A value attribute is of the form o value attributes. A value attribute is of the form
``a=<attribute>:<value>''. An example might be that a whiteboard ``a=<attribute>:<value>''. An example might be that a whiteboard
could have the value attribute ``a=orient:landscape'' could have the value attribute ``a=orient:landscape''
Attribute interpretation depends on the media tool being invoked. Thus Attribute interpretation depends on the media tool being invoked. Thus
receivers of session descriptions should be configurable in their receivers of session descriptions should be configurable in their
interpretation of announcements in general and of attributes in particu- interpretation of announcements in general and of attributes in particu-
lar. lar.
Attribute fields (``a'' fields) can also be added before the first media Attribute fields (``a'' fields) can also be added before the first media
field. These attributes would convey additional information that field. These attributes would convey additional information that
skipping to change at page 18, line 36 skipping to change at page 18, line 41
Media Announcements Media Announcements
m=<media> <port> <transport> <fmt list> m=<media> <port> <transport> <fmt list>
A session announcement may contain a number of media announcements. A session announcement may contain a number of media announcements.
Each media announcement starts with an ``m'' field, and is terminated by Each media announcement starts with an ``m'' field, and is terminated by
either the next ``m'' field or by the end of the session announcement. either the next ``m'' field or by the end of the session announcement.
A media field also has several sub-fields: A media field also has several sub-fields:
+ The first sub-field is the media type. Currently defined media are o The first sub-field is the media type. Currently defined media are
``audio'', ``video'', ``whiteboard'', ``text'' and ``data'', though ``audio'', ``video'', ``whiteboard'', ``text'' and ``data'', though
this list may be extended as new communication modalities emerge this list may be extended as new communication modalities emerge
(e.g., telepresense or conference control). (e.g., telepresence or conference control).
+ The second sub-field is the transport port to which the media stream o The second sub-field is the transport port to which the media
will be sent. The meaning of the transport port depends on the net- stream will be sent. The meaning of the transport port depends on
work being used as specified in the relevant ``c'' field and on the the network being used as specified in the relevant ``c'' field and
transport protocol defined in the third sub-field. Other ports used on the transport protocol defined in the third sub-field. Other
by the media application (such as the RTCP port, see [2]) should be ports used by the media application (such as the RTCP port, see [2])
derived algorithmically from the base media port. should be derived algorithmically from the base media port.
For transports based on UDP, the value should be in the range 1024 Note: For transports based on UDP, the value should be in the range
to 65535 inclusive. For RTP compliance it should be an even number. 1024 to 65535 inclusive. For RTP compliance it should be an even
If the port is allocated randomly by the creating application, it is number. If the port is allocated randomly by the creating applica-
recommended that ports above 5000 are chosen as, on Unix systems, tion, it is recommended that ports above 5000 are chosen as, on Unix
ports below 5000 may be allocated automatically by the operating systems, ports below 5000 may be allocated automatically by the
system. operating system.
For applications where hierarchically encoded streams are being sent For applications where hierarchically encoded streams are being sent
to a unicast address, it may be necessary to specify multiple tran- to a unicast address, it may be necessary to specify multiple tran-
sport ports. This is done using a similar notation to that used for sport ports. This is done using a similar notation to that used for
IP multicast addresses in the ``c'' field: IP multicast addresses in the ``c'' field:
m=<media> <port>/<number of ports> <transport> <fmt list> m=<media> <port>/<number of ports> <transport> <fmt list>
In such a case, the ports used depend on the transport protocol. 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 For RTP, only the even ports are used for data and the corresponding
skipping to change at page 19, line 29 skipping to change at page 19, line 34
m=video 3456/2 RTP/AVP 31 m=video 3456/2 RTP/AVP 31
would specify that ports 3456 and 3457 form one RTP/RTCP pair and would specify that ports 3456 and 3457 form one RTP/RTCP pair and
3458 and 3459 form the second RTP/RTCP pair. RTP/AVP is the tran- 3458 and 3459 form the second RTP/RTCP pair. RTP/AVP is the tran-
sport protocol and 31 is the format (see below). sport protocol and 31 is the format (see below).
It is illegal for both multiple addresses to be specified in the It is illegal for both multiple addresses to be specified in the
``c'' field and for multiple ports to be specified in the ``m'' ``c'' field and for multiple ports to be specified in the ``m''
field in the same session announcement. field in the same session announcement.
+ The third sub-field is the transport protocol. The transport proto- o The third sub-field is the transport protocol. The transport pro-
col values are dependent on the address-type field in the ``c'' tocol values are dependent on the address-type field in the ``c''
fields. Thus a ``c'' field of IP4 defines that the transport proto- fields. Thus a ``c'' field of IP4 defines that the transport proto-
col runs over IP4. For IP4, it is normally expected that most col runs over IP4. For IP4, it is normally expected that most media
media traffic will be carried as RTP over UDP. However, some com- traffic will be carried as RTP over UDP. The following transport
monly used applications such as vat [5] do not use RTP. Thus the protocols are preliminarily defined, but may be extended through
following transport protocols are defined: registration of new protocols with IANA:
- RTP/AVP - the IETF's Realtime Transport Protocol using the - RTP/AVP - the IETF's Realtime Transport Protocol using the
Audio/Video profile carried over UDP. Audio/Video profile carried over UDP.
- VAT - LBL's Visual Audio Tool packet format carried over UDP. - udp - User Datagram Protocol
- UDP - User Datagram Protocol
If an application uses a single combined propriety media format and If an application uses a single combined proprietary media format
transport protocol over UDP, then simply specifying the transport and transport protocol over UDP, then simply specifying the
protocol as UDP and using the format field to distinguish the com- transport protocol as udp and using the format field to distinguish
bined protocol is recommended. If a transport protocol is used over the combined protocol is recommended. If a transport protocol is
UDP to carry several distinct media types that need to be used over UDP to carry several distinct media types that need to be
distinguished by a session directory, then specifying the transport distinguished by a session directory, then specifying the transport
protocol and media format separately is necessary. VAT and RTP are protocol and media format separately is necessary. RTP is an exam-
examples of transport protocols that carry multiple payload formats ple of a transport protocols that carry multiple payload formats
that must be distinguished by the session directory for it to know that must be distinguished by the session directory for it to know
how to start appropriate tools, relays, mixers or recorders. how to start appropriate tools, relays, mixers or recorders.
The main reason to specify the transport protocol in addition to the The main reason to specify the transport protocol in addition to the
media format is that the same standard media formats may be carried media format is that the same standard media formats may be carried
over different transport protocols even when the network protocol is over different transport protocols even when the network protocol is
the same - for example vat PCM audio and RTP PCM audio. In addi- the same - a historical example is vat PCM audio and RTP PCM audio.
tion, relays and monitoring tools that are transport protocol In addition, relays and monitoring tools that are transport protocol
specific but format independent are possible. specific but format independent are possible.
For RTP media streams operating under the RTP Audio/Video Profile For RTP media streams operating under the RTP Audio/Video Profile
[3], the protocol field is ``RTP/AVP''. Should other RTP profiles [3], the protocol field is ``RTP/AVP''. Should other RTP profiles
be defined in the future, their profiles will be specified in the be defined in the future, their profiles will be specified in the
same way. For example, the protocol field ``RTP/XYZ'' would specify same way. For example, the protocol field ``RTP/XYZ'' would specify
RTP operating under a profile whose short name is ``XYZ''. RTP operating under a profile whose short name is ``XYZ''.
+ The fourth and subsequent sub-fields are media formats. For audio o The fourth and subsequent sub-fields are media formats. For audio
and video, these will normally be a media payload type as defined in and video, these will normally be a media payload type as defined in
the RTP Audio/Video Profile. the RTP Audio/Video Profile.
When a list of payload formats is given, this implies that all of When a list of payload formats is given, this implies that all of
these formats may be used in the session, but the first of these these formats may be used in the session, but the first of these
formats is the default format for the session. formats is the default format for the session.
For media whose transport protocol is not RTP or UDP the format For media whose transport protocol is not RTP or UDP the format
field is protocol specific. Such formats should be defined in an field is protocol specific. Such formats should be defined in an
additional specification document. additional specification document.
skipping to change at page 21, line 16 skipping to change at page 21, line 19
type 98 for such a stream, additional information is required to type 98 for such a stream, additional information is required to
decode it: decode it:
m=video 3456 RTP/AVP 98 m=video 3456 RTP/AVP 98
a=rtpmap:98 L16/16000/2 a=rtpmap:98 L16/16000/2
The general form of an rtpmap attribute is: The general form of an rtpmap attribute is:
a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding parameters>] a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding parameters>]
For audio streams, <encoding parameters> optionally specifies number For audio streams, <encoding parameters> may specify the number of
of audio channels. For video streams, no encoding parameters are audio channels. This parameter may be omitted if the number of
currently specified. channels is one provided no additional parameters are needed.
For video streams, no encoding parameters are currently specified.
Additional parameters may be defined in the future, but codec-
specific parameters should not be added. Parameters added to an
rtpmap attribute should only be those required for a session direc-
tory to make the choice of appropriate media too to participate in a
session. Codec-specific parameters should be added in other attri-
butes.
Up to one rtpmap attribute can be define for each media format Up to one rtpmap attribute can be define for each media format
specified. Thus we might have: specified. Thus we might have:
m=audio 12345 RTP/AVP 96 97 98 m=audio 12345 RTP/AVP 96 97 98
a=rtpmap:96 L8/8000 a=rtpmap:96 L8/8000
a=rtpmap:97 L16/8000 a=rtpmap:97 L16/8000
a=rtpmap:98 L16/11025/2 a=rtpmap:98 L16/11025/2
Experimental encoding formats can also be specified in this way. Experimental encoding formats can also be specified in this way.
skipping to change at page 21, line 48 skipping to change at page 22, line 12
receive the media stream has relevant configured state in its ses- receive the media stream has relevant configured state in its ses-
sion directory to know which tools are appropriate. sion directory to know which tools are appropriate.
Note that RTP audio formats typically do not include information Note that RTP audio formats typically do not include information
about the number of samples per packet. If a non-default (as about the number of samples per packet. If a non-default (as
defined in the RTP Audio/Video Profile) packetisation is required, defined in the RTP Audio/Video Profile) packetisation is required,
the``ptime'' attribute is used as given below. the``ptime'' attribute is used as given below.
For more details on RTP audio and video formats, see [3]. For more details on RTP audio and video formats, see [3].
+ Predefined formats for UDP protocol non-RTP media are as below. o Predefined formats for UDP protocol non-RTP media are as below.
Whiteboard Formats: Whiteboard Formats:
WB: LBL Whiteboard (transport: UDP) wb: LBL Whiteboard (transport: udp)
Text Formats: Text Formats:
NT: UCL Network Text Editor (transport: UDP) nt: UCL Network Text Editor (transport: udp)
Suggested Attributes Suggested Attributes
The following attributes are suggested. Since application writers may The following attributes are suggested. Since application writers may
add new attributes as they are required, this list is not exhaustive. add new attributes as they are required, this list is not 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 the
session. This is to enable a receiver to filter unwanted sessions session. This is to enable a receiver to filter unwanted sessions
by category. It would probably have been a compulsory separate by category. It would probably have been a compulsory separate
field, except for its experimental nature at this time. It is a field, except for its experimental nature at this time. It is a
session attribute. session-level attribute.
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 ses-
sions at the receiver. It is a session attribute. sions at the receiver. This allows a receiver to select interesting
session based on keywords describing the purpose of the session. It
is a session-level attribute.
a=tool:<name and version of tool>
This gives the name and version number of the tool used to create
the session description. It is a session-level attribute.
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 data.
It should not be necessary to know ptime to decode RTP or vat audio, It should not be necessary to know ptime to decode RTP or vat audio,
and it is intended as a recommendation for the and it is intended as a recommendation for the
encoding/packetisation of audio. It is a media attribute. encoding/packetisation of audio. It is a media attribute.
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 mode
where applicable. It can be either a session or media attribute. where applicable. It can be either a session or media attribute.
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 receive
mode. This is necessary for interactive conferences with tools such mode. This is necessary for interactive conferences with tools such
as wb which defaults to receive only mode. It can be either a ses- as wb which defaults to receive only mode. It can be either a ses-
sion or media attribute. sion or media attribute.
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 mode.
Typically this may be used where a different unicast address is to An example may be where a different unicast address is to be used
be used for a traffic destination than for a traffic source. It can for a traffic destination than for a traffic source. In such a case,
be either a session or media attribute, but would normally only be two media descriptions may be use, one sendonly and one recvonly. It
used as a media attribute. can be either a session or media attribute, but would normally only
be used as a media attribute.
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. It
specifies the orientation of a the whiteboard on the screen. It is specifies the orientation of a the whiteboard on the screen. It is
a media attribute. Permitted values are `portrait', `landscape' and a media attribute. Permitted values are `portrait', `landscape' and
`seascape' (upside down landscape). `seascape' (upside down landscape).
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', and `moderated'. `recvonly' should be the `broadcast', `meeting', `moderated', `test' and `H332'. `recvonly'
default for `type:broadcast' sessions, `type:meeting' should imply should be the default for `type:broadcast' sessions, `type:meeting'
`sendrecv' and `type:moderated' should imply the use of a floor con- should imply `sendrecv' and `type:moderated' should indicate the use
trol tool and that the media tools are started so as to ``mute'' new of a floor control tool and that the media tools are started so as
sites joining the conference. to ``mute'' new sites joining the conference.
Specifying the attribute type:test is also suggested as a hint that, Specifying the attribute type:H332 indicates that this loosely cou-
pled session is part of a H.332 session as defined in the ITU H.332
specification. Media tools should be started `recvonly'.
Specifying the attribute type:test is suggested as a hint that,
unless explicitly requested otherwise, receivers can safely avoid unless explicitly requested otherwise, receivers can safely avoid
displaying this session description to users. displaying this session description to users.
type is a session attribute. type is a session-level attribute.
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 session
name and information data. By default, an ISO 8859-1 character set name and information data. By default, the ISO 8859-1 character set
is used. If an ISO 8859-1 character set is not suitable, the use of is used. If the ISO 8859-1 character set is not suitable, the use
unicode (ISO 10646) [6],[7], as specified in RFC1641 [8] is sug- of unicode (ISO 10646) [6],[7], as specified in RFC1641 [8] is sug-
gested. In particular, the UTF-7 (RFC1642) [9] encoding is sug- gested. In particular, the UTF-7 (RFC1642) [9] encoding is sug-
gested with the following SDP attribute: gested with the following SDP attribute:
a=charset:unicode-1-1-utf-7 a=charset:unicode-1-1-utf-7
This is a session attribute; if this attribute is present, it must This is a session-level attribute; if this attribute is present, it
be before the first media field. must be before the first media field.
a=framerate:<frame rate> a=framerate:<frame rate>
This gives the maximum video frame rate in frames/sec. It is This gives the maximum video frame rate in frames/sec. It is
intended as a recommendation for the encoding of video data. intended as a recommendation for the encoding of video data.
Decimal representations of fractional values using the notation Decimal representations of fractional values using the notation
"<integer>.<fraction>" are allowed. It is a media attribute, and is "<integer>.<fraction>" are allowed. It is a media attribute, and is
only defined for video media. only defined for video media.
a=quality:<quality> a=quality:<quality>
This gives a suggestion for the quality of the encoding as an This gives a suggestion for the quality of the encoding as an
skipping to change at page 24, line 17 skipping to change at page 24, line 38
For video, the value in the range 0 to 10, with the following sug- For video, the value in the range 0 to 10, with the following sug-
gested meaning: gested meaning:
10 - the best still-image quality the compression scheme can give. 10 - the best still-image quality the compression scheme can give.
5 - the default behaviour given no quality suggestion. 5 - the default behaviour given no quality suggestion.
0 - the worst still-image quality the codec designer thinks is 0 - the worst still-image quality the codec designer thinks is
still usable. still usable.
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 a the media tool
that will use this format.
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 declara-
tive style of specifying conference control is desirable where possible. tive 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 flags such as before the first media field, possibly supplemented by properties such
`recvonly' for some of the media tools. This conference attribute con- as `recvonly' for some of the media tools. This conference attribute
veys the conference control policy. An example might be: 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 In some cases, however, it is possible that this may be insufficient to
communicate the details of an unusual conference control policy. If communicate the details of an unusual conference control policy. If
this is the case, then a conference attribute specifying external con- 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 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 to specify the conference control tools and configuration data for those
tools. A fictional example might be: tools. An example is an ITU H.332 session:
... ...
a=type:external-control c=IN IP4 224.5.6.7
a=type:H332
m=audio 12345 RTP/AVP 0 m=audio 12345 RTP/AVP 0
m=video 12347 RTP/AVP 31 m=video 12347 RTP/AVP 31
m=whiteboard 12349 UDP WB m=whiteboard 12349 udp wb
m=control 12341 UDP SCCP m=control 12341 H323 mc
a=mode:chaired c=IN IP4 134.134.157.81
a=chair:128.16.64.2
a=video:follows-audio
a=audio:on-demand
a=audio:chair-mutes-mike
a=whiteboard:chaired
In this fictional example (i.e., this is not implemented anywhere), a
general conference attribute is specified stating that conference
control will be provided by an external tool, and specific attributes In this example, a general conference attribute (type:H332) is specified
are given to specify the control policy that tool should use. stating that conference control will be provided by an external H.332
tool, and a contact addresses for the H.323 session multipoint con-
troller is given.
In this document, only the former style of conference control declara- In this document, only the declaritive style of conference control
tion is specified, though we recognise that some variant on the latter declaration is specified. Other forms of conference control should
may also be used eventually. specify an appropriate type attribute, and should define the implica-
tions this has for control media.
Appendix A: SDP Grammar Appendix A: SDP Grammar
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
skipping to change at page 27, line 41 skipping to change at page 27, line 41
media ::= (alpha-numeric)+ media ::= (alpha-numeric)+
;typically "audio", "video", "whiteboard" ;typically "audio", "video", "whiteboard"
;or "text" ;or "text"
fmt ::= (alpha-numeric)+ fmt ::= (alpha-numeric)+
;typically an RTP payload type for audio ;typically an RTP payload type for audio
;and video media ;and video media
proto ::= (alpha-numeric)+ proto ::= (alpha-numeric)+
;typically "RTP/AVP", "VAT", or "UDP" for IP4 ;typically "RTP/AVP" or "udp" for IP4
port ::= (DIGIT)+ port ::= (DIGIT)+
;should in the range "1024" to "65535" inclusive ;should in the range "1024" to "65535" inclusive
;for UDP based media ;random allocation should ;for UDP based media ;random allocation should
;only assign above UDP port "5000". ;only assign above UDP port "5000".
attribute ::= att-field ":" att-value | att-field attribute ::= att-field ":" att-value | att-field
att-field ::= (ALPHA)+ att-field ::= (ALPHA)+
skipping to change at page 29, line 28 skipping to change at page 29, line 28
interval-time ::= (DIGIT)+ variable-len-time-unit interval-time ::= (DIGIT)+ variable-len-time-unit
fixed-len-time-unit ::= ``d'' | ``h'' | ``m'' | ``s'' fixed-len-time-unit ::= ``d'' | ``h'' | ``m'' | ``s''
variable-len-time-unit ::= ``Y'' | ``M'' variable-len-time-unit ::= ``Y'' | ``M''
bwtype ::= (alpha-numeric)+ bwtype ::= (alpha-numeric)+
bandwidth ::= (DIGIT)+ bandwidth ::= (DIGIT)+
username ::= alpha-numeric username ::= safe
;perhaps this is too restrictive... ;pretty wide definition, but doesn't include space
email-address ::= email | email "(" text ")" | text "<" email ">" email-address ::= email | email "(" email-safe ")" |
email-safe "<" email ">"
email ::= ;defined in RFC822 email ::= ;defined in RFC822
uri::= ;defined in RFC1630 uri::= ;defined in RFC1630
phone-number ::= phone | phone "(" text ")" | phone-number ::= phone | phone "(" email-safe ")" |
text "<" phone ">" email-safe "<" phone ">"
phone ::= "+" POS-DIGIT (space | "-" | DIGIT)+ phone ::= "+" POS-DIGIT (space | "-" | DIGIT)+
;there must be a space or hyphen between the ;there must be a space or hyphen between the
;international code and the rest of the number. ;international code and the rest of the number.
nettype ::= "IN" nettype ::= "IN"
;list to be extended ;list to be extended
addrtype ::= "IP4" | "IP6" addrtype ::= "IP4" | "IP6"
;list to be extended ;list to be extended
skipping to change at page 31, line 21 skipping to change at page 31, line 21
DIGIT ::= 0 | POS-DIGIT DIGIT ::= 0 | POS-DIGIT
POS-DIGIT ::= 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 POS-DIGIT ::= 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
ALPHA ::= a | b | c | d | e | f | g | h | i | j | k | ALPHA ::= a | b | c | d | e | f | g | h | i | j | k |
l | m | n | o | p | q | r | s | t | u | v | l | m | n | o | p | q | r | s | t | u | v |
w | x | y | z | A | B | C | D | E | F | G | w | x | y | z | A | B | C | D | E | F | G |
H | I | J | K | L | M | N | O | P | Q | R | H | I | J | K | L | M | N | O | P | Q | R |
S | T | U | V | W | X | Y | Z S | T | U | V | W | X | Y | Z
unicode-safe ::= alpha-numeric | unicode-safe ::= email-safe | "(" | ")" | "<" | ">"
"'" | "(" | ")" | "'" | "-" | "." | "/" | ":" |
"?" | """ | "#" | "$" | "&" | "*" | ";" | "<" |
"=" | ">" | "@" | "[" | "]" | "^" | "_" | "`" |
"{" | "|" | "}" | "+" | space | tab
;although unicode allows newline and carriage ;although unicode allows newline and carriage
;return, we don't here. ;return, we don't here.
email-safe ::= safe | space | tab
safe ::= alpha-numeric |
"'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
"#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" |
"]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
"~" | "
space ::= ;ascii code 32 space ::= ;ascii code 32
tab ::= ;ascii code 9 tab ::= ;ascii code 9
newline ::= ;ascii code 10 newline ::= ;ascii code 10
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
skipping to change at page 32, line 26 skipping to change at page 32, line 26
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 like
to thank Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin and to thank Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin, Ross
Ross Finlayson. Finlayson, Peter Parnes, Joerg Ott and Carsten Bormann.
References References
[1] D. Mills, ``Network Time Protocol version 2 specification and imple- [1] D. Mills, ``Network Time Protocol version 2 specification and imple-
mentation", RFC1119, 1st Sept 1989. mentation", RFC1119, 1st Sept 1989.
[2] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, ``RTP: A Tran- [2] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, ``RTP: A Tran-
sport Protocol for Real-Time Applications'', RFC 1889 sport Protocol for Real-Time Applications'', RFC 1889
[3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with [3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with
 End of changes. 

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