draft-ietf-mmusic-sdp-03.txt   draft-ietf-mmusic-sdp-04.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-03.txt ISI/LBNL draft-ietf-mmusic-sdp-04.ps ISI/LBNL
26th March 1997 2nd Sept 1997
Expires: 26th Sept 1997 Expires: 2nd Apr 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 36 skipping to change at page 1, line 36
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 defines 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 multimedia 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
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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. SDP is designed to convey such information to recipients. participants. SDP is designed to convey such information to recipients.
SDP is purely a format for session description - it does not incorpor- SDP is purely a format for session description - it does not incorpor-
tate a transport protocol, and is intended to use different transport tate a transport protocol, and is intended to use different transport
protocols as appropriate including the Session Announcement Protocol protocols as appropriate including the Session Announcement Protocol
[4], electronic mail using the MIME extensions, and the Hypertext Tran- [4], Session Inititation Protocol [11], Real-Time Streaming Protocol
[12], electronic mail using the MIME extensions, and the Hypertext Tran-
sport Protocol. sport 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 wider
range of network environments and applications than just multicast ses- range of network environments and applications than just multicast ses-
sion directories. However, it is not intended to support negotiation of sion directories. However, it is not intended to support negotiation of
session content or media encodings - this is viewed as outside the scope session content or media encodings - this is viewed as outside the scope
of session description. of session description.
3. Glossary of Terms 3. Glossary of Terms
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periodically multicasting an announcement packet to a well known multi- periodically multicasting an announcement packet to a well known multi-
cast address and port using the Session Announcement Protocol (SAP). cast address and port using 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
Alternative means of conveying session descriptions include electronic Alternative means of conveying session descriptions include electronic
mail and the World Wide Web. For both email and WWW distribution, the
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 use of the MIME content type ``application/sdp'' should be used. This
enables the automatic launching of applications for participation in the enables the automatic launching of applications for participation in the
session from the WWW client or mail reader in a standard manner. 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 because the session announcement can necessarily receive the session because the
multicast sessions may be restricted in scope, and access to the WWW multicast sessions may be restricted in scope, and access to the WWW
server or reception of email is possible outside this scope. SAP server or reception of email is possible outside this scope. SAP
announcements do not suffer from this mismatch. announcements do not suffer from this mismatch.
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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
streams that exist for some duration of time. Media streams can be 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 many-to-many. The times during which the session is active need not be
continuous. continuous.
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:
o Session name and purpose o Session name and purpose
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of the multicast stream, whether being sent, received, or both. 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 transport
protocol defined. By default, this is the remote address and remote 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- port to which data is sent, and the remote address and local port on
ing data. However, some media may define to use these to establish a which to receive data. However, some media may define to use these to
control channel for the actual media flow. 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
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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 supports a categori- that are of interest from those that are not. SDP supports a categori-
sation mechanism for sessions that is capable of being automated. sation mechanism for sessions that is capable of being 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 the ISO 10646 character sets
sets to allow the extended ASCII characters used by many western and in the UTF-8 encoding to allow many different languages to be
northern European languages to be represented. However, there are many represented. However, to assist in compact representations, SDP also
languages that cannot be represented in an ISO 8859-1 character set. allows other character sets such as ISO 8859-1 to be used when desired.
SDP allows extensions to allow other character sets to be used when
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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(e.g., email) or damaged by an intermediate caching server, the encoding (e.g., email) or damaged by an intermediate caching server, the encoding
was designed with strict order and formatting rules so that most errors was designed with strict order and formatting rules so that most errors
would result in malformed announcements which could be detected easily would result in malformed announcements which could be detected easily
and discarded. This also allows rapid discarding of encrypted announce- and discarded. This also allows rapid discarding of encrypted announce-
ments for which a receiver does not have the correct key. ments 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 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>. It also
is not permitted either side of the `=' sign. In general <value> is will be case-significant unless a specific field defines otherwise.
either a number of fields delimited by a single space character or a Whitespace is not permitted either side of the `=' sign. In general
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 descriptions (details A session description consists of a session-level description (details
that apply to the whole session and all media streams) and optionally that apply to the whole session and all media streams) and optionally
several media-level descriptions (details that apply onto to a single several media-level descriptions (details that apply onto to a single
media stream). 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 or
more media-level sections. The session-level part starts with a `v=' more media-level sections. The session-level part starts with a `v='
line and continues to the first media-level section. The media descrip- 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 starts with an `m=' line and continues to the next media descrip-
tion or end of the whole session description. In general, session-level tion or end of the whole session description. In general, session-level
values are the default for all media unless overridden by an equivalent values are the default for all media unless overridden by an equivalent
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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 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 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 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 ("a=" described below) is the primary means for extending SDP
tailoring it to particular applications or media. Some attributes (the and tailoring it to particular applications or media. Some attributes
ones listed in this document) have a defined meaning but others may be (the ones listed in this document) have a defined meaning but others may
added on an application-, media- or session-specific basis. A session be added on an application-, media- or session-specific basis. A ses-
directory must ignore any attribute it doesn't understand. sion 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-
level section applies to all the media of that session unless overridden level section applies to all the media of that session unless overridden
by connection information or an attribute of the same name in the media 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=application 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 10646 character with the exceptions of 0x0a (ASCII
line) and 0x0d (carriage return). Carriage Return is prohibited, and newline) and 0x0d (ASCII carriage return). The sequence CRLF (0x0a0d)
Newline is used to end a record. is used to end a record, although parsers should be tolerant and also
accept records terminated with a single newline character.
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 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 tuple of <username>, <session id>, <network type>, string such that the tuple of <username>, <session id>, <network type>,
<address type> and <address> form a globally unique identifier for the <address type> and <address> form a globally unique identifier for the
session. The method of session id allocation is up to the creating session. The method of session id allocation is up to the creating
tool, but it has been suggested that a Network Time Protocol (NTP) tool, but it has been suggested that a Network Time Protocol (NTP)
timestamp be used to ensure uniqueness [1]. <version> is a version timestamp be used to ensure uniqueness [1]. <version> is a version
number for this announcement. It is needed for proxy announcements to number for this announcement. It is needed for proxy announcements to
detect which of several announcements for the same session is the most 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- recent. Again its usage is up to the creating tool, so long as
sion> is increased when a modification is made to the session data.
<version> is increased when a modification is made to the session data.
Again, it is recommended (but not mandatory) that an NTP timestamp is 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- used. <network type> is a text string giving the type of network. Ini-
tially ``IN'' is defined to have the meaning ``Internet''. <address tially ``IN'' is defined to have the meaning ``Internet''. <address
type> is a text string giving the type of the address that follows. type> is a text string giving the type of the address that follows.
Initially ``IP4'' and ``IP6'' are defined. <address> is the globally Initially ``IP4'' and ``IP6'' are defined. <address> is the globally
unique address of the machine from which the session was created. For 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 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 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 is the compressed textual representation of the IP version 6 address of
the machine. the machine.
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.
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 session description, and it must contain printable ISO
characters (but see also the `charset' attribute below). 10646 characters (but see also the `charset' attribute below).
Session and Media Information Session and Media Information
i=<session description> i=<session description>
The ``i'' field is information about the session. There must be no more The ``i='' field is information about the session. There may be at most
than one session-level ``i'' field per session announcement. Although it one session-level ``i='' field per session description, and at most one
may be omitted, this is discouraged, and user interfaces for composing ``i='' field per media. Although it may be omitted, this is discouraged
sessions should require text to be entered. If it is present it must for session announcements, and user interfaces for composing sessions
contain printable ISO 8859-1 characters (but see also the `charset' should require text to be entered. If it is present it must contain
attribute below). printable ISO 10646 characters (but see also the `charset' attribute
below).
A single ``i'' field can also be used for each media definition. In A single ``i='' field can also be used for each media definition. In
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>
o 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
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o 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 the ISO 8859-1 character set, or The free text string should be in the ISO-10646 character set with
alternatively in unicode UTF-7 encoding if the appropriate charset UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if
session-level attribute is set. the appropriate charset 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.
A session announcement must contain one ``c='' field in each media
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 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 the type of network. Initially ``IN'' is defined to have the meaning
``Internet'' ``Internet''.
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:
o 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.
o 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 and the address together define the scope with
sent in this conference should be sent. TTL values must be in the which multicast packets sent in this conference will be sent. TTL
range 0-255. The Mbone usage guidelines (currently available at values must be in the range 0-255.
ftp://ftp.isi.edu/mbone/faq.txt) define several standard settings
for TTL:
local net: 1
site: 15
region: 63
world: 127
Other settings may have local meaning (e.g., 31 for all sites within
an organization).
The TTL for the session is appended to the address using a slash as The TTL for the session is appended to the address using a slash as
a separator. An example is: a separator. An example is:
c=IN IP4 224.2.1.1/127 c=IN IP4 224.2.1.1/127
Hierarchical or layered encoding schemes are data streams where the Hierarchical or layered encoding schemes are data streams where the
encoding from a single media source is split into a number of encoding from a single media source is split into a number of
layers. The receiver can choose the desired quality (and hence layers. The receiver can choose the desired quality (and hence
bandwidth) by only subscribing to a subset of these layers. Such bandwidth) by only subscribing to a subset of these layers. Such
skipping to change at page 13, line 26 skipping to change at page 13, line 26
<base multicast address>/<ttl>/<number of addresses> <base multicast address>/<ttl>/<number of addresses>
If the number of addresses is not given it is assumed to be one. If the number of addresses is not given it is assumed to be one.
Multicast addresses so assigned are contiguously allocated above the Multicast addresses so assigned are contiguously allocated above the
base address, so that, for example: base address, so that, for example:
c=IN IP4 224.2.1.1/127/3 c=IN IP4 224.2.1.1/127/3
would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are to 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. be used at a ttl of 127. This is semantically identical to includ-
ing multiple ``c='' lines in a media description:
c=IN IP4 224.2.1.1/127
c=IN IP4 224.2.1.2/127
c=IN IP4 224.2.1.3/127
Multiple addresses or ``c='' lines can only be specified on a per-
media basis, and not for a session-level ``c='' field.
It is illegal for the slash notation described above to be used for It is illegal for the slash notation described above to be used for
IP unicast addresses. IP unicast addresses.
A session announcement must contain one ``c'' field in each media
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.
Bandwidth Bandwidth
b=<modifier>:<bandwidth-value> b=<modifier>:<bandwidth-value>
o 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.
o <bandwidth-value> is in kilobits per second o <bandwidth-value> is in kilobits per second
o <modifier> is an single alphanumeric word giving the meaning of o <modifier> is a single alphanumeric word giving the meaning of the
the bandwidth figure. bandwidth figure.
o 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 administra- each TTL on the Mbone or within a particular multicast administra-
tive scope region (the Mbone bandwidth vs. TTL limits are given in tive scope region (the Mbone bandwidth vs. TTL limits are given in
the MBone FAQ). If the bandwidth of a session or media in a ses- 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 sion is different from the bandwidth implicit from the scope, a
`b=CT:...' line should be supplied for the session giving the pro- `b=CT:...' line should be supplied for the session giving the pro-
posed upper limit to the bandwidth used. The primary purpose of posed upper limit to the bandwidth used. The primary purpose of
this is to give an approximate idea as to whether two or more this is to give an approximate idea as to whether 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.
o 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>
o ``t'' fields specify the start and stop times for a conference ses- o ``t='' fields specify the start and stop times for a conference
sion. Multiple ``t'' fields may be used if a session is active at session. Multiple ``t='' fields may be used if a session is active
multiple irregularly spaced times; each additional ``t'' field at 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.
o 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.
o 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.
skipping to change at page 15, line 31 skipping to change at page 15, line 33
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.
r=<repeat interval> <active duration> <list of offsets from start-time> r=<repeat interval> <active duration> <list of offsets from start-time>
o ``r'' fields specify repeat times for a session. For example, if 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 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 hour each week for three months, then the <start time> in the
corresponding ``t'' field would be the NTP representation of 10am on corresponding ``t='' field would be the NTP representation of 10am
the first Monday, the <repeat interval> would be 1 week, the <active on the first Monday, the <repeat interval> would be 1 week, the
duration> would be 1 hour, and the offsets would be zero and 25 <active duration> would be 1 hour, and the offsets would be zero and
hours. The corresponding ``t'' field stop time would be the NTP 25 hours. The corresponding ``t='' field stop time would be the NTP
representation of the end of the last session three months later. By representation of the end of the last session three months later. By
default all fields are in seconds, so the ``r'' and ``t'' fields default all fields are in seconds, so the ``r='' and ``t='' fields
might be: 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 17, line 15 skipping to change at page 17, line 15
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.
4 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: 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.
skipping to change at page 17, line 47 skipping to change at page 17, line 47
the key can be returned. When a request is made to the given the key can be returned. When a request is made to the given
URI, the MIME content-type of the reply specifies the encoding URI, the MIME content-type of the reply specifies the encoding
for the key in the reply. The key should not be obtained until 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 be used to
the media streams. decrypt the media streams.
Attributes Attributes
a=<attribute> 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 can also be added before the
first media field. These attributes would convey additional information
that applies to the conference as a whole rather than to individual
media; an example might be the conference's floor control policy.
Attribute fields may be of two forms:
o property attributes. A property attribute is simply of the form 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 a property of the session. attribute conveys that the attribute is a property of the session.
An example might be ``a=recvonly''. An example might be ``a=recvonly''.
o 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
field. These attributes would 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.
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
either the next ``m'' field or by the end of the session announcement. by either the next ``m='' field or by the end of the session announce-
A media field also has several sub-fields: ment. A media field also has several sub-fields:
o 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'', ``application'', ``data'' and ``control'',
this list may be extended as new communication modalities emerge though this list may be extended as new communication modalities
(e.g., telepresence or conference control). emerge (e.g., telepresence). The difference between ``application''
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 second sub-field is the transport port to which the media
stream will be sent. The meaning of the transport port depends on stream will be sent. The meaning of the transport port depends on
the network being used as specified in the relevant ``c'' field and the network being used as specified in the relevant ``c'' field and
on the transport protocol defined in the third sub-field. Other on the transport protocol defined in the third sub-field. Other
ports used by the media application (such as the RTCP port, see [2]) ports used by the media application (such as the RTCP port, see [2])
should be derived algorithmically from the base media port. should be derived algorithmically from the base media port.
Note: For transports based on UDP, the value should be in the range Note: For transports based on UDP, the value should be in the range
1024 to 65535 inclusive. For RTP compliance it should be an even 1024 to 65535 inclusive. For RTP compliance it should be an even
number. If the port is allocated randomly by the creating applica- number.
tion, it is recommended that ports above 5000 are chosen as, on Unix
systems, ports below 5000 may be allocated automatically by the
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
one-higher odd port is used for RTCP. For example: one-higher odd port is used for RTCP. For example:
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.
o The third sub-field is the transport protocol. The transport pro- o The third sub-field is the transport protocol. The transport pro-
tocol 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 pro-
col runs over IP4. For IP4, it is normally expected that most media tocol runs over IP4. For IP4, it is normally expected that most
traffic will be carried as RTP over UDP. The following transport media traffic will be carried as RTP over UDP. The following tran-
protocols are preliminarily defined, but may be extended through sport protocols are preliminarily defined, but may be extended
registration of new protocols with IANA: through 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.
- udp - User Datagram Protocol - udp - User Datagram Protocol
If an application uses a single combined proprietary media format If an application uses a single combined proprietary media format
and transport protocol over UDP, then simply specifying the and transport protocol over UDP, then simply specifying the tran-
transport protocol as udp and using the format field to distinguish sport protocol as udp and using the format field to distinguish the
the combined protocol is recommended. If a transport protocol is combined protocol is recommended. If a transport protocol is used
used over UDP to carry several distinct media types that need to be over UDP to carry several distinct media types that need to be dis-
distinguished by a session directory, then specifying the transport tinguished by a session directory, then specifying the transport
protocol and media format separately is necessary. RTP is an exam- protocol and media format separately is necessary. RTP is an exam-
ple of a transport protocols that carry multiple payload formats ple of a transport-protocol that carries 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 - a historical example is vat PCM audio and RTP PCM audio. the same - a historical example is vat PCM audio and RTP PCM audio.
In addition, relays and monitoring tools that are transport protocol In addition, relays and monitoring tools that are transport-
specific but format independent are possible. protocol-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''.
o 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.
skipping to change at page 22, line 14 skipping to change at page 22, line 16
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].
o 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: Application Formats:
wb: LBL Whiteboard (transport: udp) wb: LBL Whiteboard (transport: udp)
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
skipping to change at page 23, line 41 skipping to change at page 23, line 40
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'. `recvonly'
should be the default for `type:broadcast' sessions, `type:meeting' should be the default for `type:broadcast' sessions, `type:meeting'
should imply `sendrecv' and `type:moderated' should indicate the use should imply `sendrecv' and `type:moderated' should indicate the use
of a floor control tool and that the media tools are started so as of a floor control tool and that the media tools are started so as
to ``mute'' new sites joining the conference. 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 cou-
pled session is part of a H.332 session as defined in the ITU H.332 pled session is part of a H.332 session as defined in the ITU H.332
specification. Media tools should be started `recvonly'. 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.
type is a session-level attribute. The type attribute 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, the ISO 8859-1 character set name and information data. By default, the ISO-10646 character set
is used. If the ISO 8859-1 character set is not suitable, the use in UTF-8 encoding is used. If a more compact representation is
of unicode (ISO 10646) [6],[7], as specified in RFC1641 [8] is sug- required, other character sets may be used such as ISO-8859-1 for
gested. In particular, the UTF-7 (RFC1642) [9] encoding is sug- Northern European languages. In particular, the ISO 8859-1 is
gested with the following SDP attribute: specified with the following SDP attribute:
a=charset:unicode-1-1-utf-7 a=charset:iso8859-1
This is a session-level attribute; if this attribute is present, it This is a session-level attribute; if this attribute is present, it
must 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.
skipping to change at page 24, line 43 skipping to change at page 24, line 42
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> a=fmtp:<format> <format specific parameters>
This attribute allows parameters that are specific to a particular This attribute allows parameters that are specific to a particular
format to be conveyed in a way that SDP doesn't have to understand 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 them. The format must be one of the formats specified for the
media. Format-specific parameters may be any set of parameters media. Format-specific parameters may be any set of parameters
required to be conveyed by SDP and given unchanged a the media tool required to be conveyed by SDP and given unchanged to the media tool
that will use this format. 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 properties such before the first media field, possibly supplemented by properties such
skipping to change at page 25, line 30 skipping to change at page 25, line 30
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. An example is an ITU H.332 session: tools. An example is an ITU H.332 session:
... ...
c=IN IP4 224.5.6.7 c=IN IP4 224.5.6.7
a=type:H332 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=application 12349 udp wb
m=control 12341 H323 mc m=control 12341 H323 mc
c=IN IP4 134.134.157.81 c=IN IP4 134.134.157.81
In this example, a general conference attribute (type:H332) is specified In this example, a general conference attribute (type:H332) is specified
stating that conference control will be provided by an external H.332 stating that conference control will be provided by an external H.332
tool, and a contact addresses for the H.323 session multipoint con- tool, and a contact addresses for the H.323 session multipoint con-
troller is given. troller is given.
In this document, only the declaritive style of conference control In this document, only the declaritive style of conference control
declaration is specified. Other forms of conference control should declaration is specified. Other forms of conference control should
specify an appropriate type attribute, and should define the implica- specify an appropriate type attribute, and should define the implica-
tions this has for control media. tions this has for control media.
7. Security Considerations
SDP is a session description format that describes multimedia sessions.
A session description should not be trusted unless it has been obtained
by an authenticated transport protocol from a trusted source. Many dif-
ferent transport protocols may be used to distribute session descrip-
tion, and the nature of the authentication will differ from transport to
transport.
One transport that will frequently be used to distribute session
descriptions is the Session Announcement Protocol (SAP). SAP provides
both encryption and authentication mechanisms but due to the nature of
session announcements it is likely that there are many occasions where
the originator of a session announcement cannot be authenticated because
they are previously unknown to the receiver of the announcement and
because no common public key infrastructure is available.
On receiving a session description over an unauthenticated transport
mechanism or from an untrusted party, software parsing the session
should take a few precautions. Session description contain information
required to start software on the receivers system. Software that
parses a session description MUST not be able to start other software
except that which is specifically configured as appropriate software to
participate in multimedia sessions. It is normally considered INAP-
PROPRIATE for software parsing a session description to start, on a
user's system, software that is appropriate to participate in multimedia
sessions, without the user first being informed that such software will
be started and giving their consent. Thus a session description arriv-
ing by session announcement, email, session invitation, or WWW page
SHOULD not deliver the user into an {it interactive} multimedia session
without the user being aware that this will happen. As it is not always
simple to tell whether a session is interactive or not, applications
that are unsure should assume sessions are interactive.
In this specification, there are no attributes which would allow the
recipient of a session description to be informed to start multimedia
tools in a mode where they default to transmitting. Under some cir-
cumstances it might be appropriate to define such attributes. If this
is done an application parsing a session description containing such
attributes SHOULD either ignore them, or inform the user that joining
this session will result in the automatic transmission of multimedia
data. The default behaviour for an unknown attribute is to ignore it.
Session descriptions may be parsed at intermediate systems such as
firewalls for the purposes of opening a hole in the firewall to allow
the participation in multimedia sessions. It is considered INAPPROPRI-
ATE for a firewall to open such holes for unicast data streams unless
the session description comes in a request from inside the firewall.
For multicast sessions, it is likely that local administrators will
apply their own policies, but the exclusive use of "local" or "site-
local" administrative scope within the firewall and the refusal of the
firewall to open a hole for such scopes will provide separation of glo-
bal multicast sessions from local ones.
Appendix A: SDP Grammar Appendix A: SDP Grammar
This appendix provides an Augmented BNF grammar for SDP.
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=" (DIGIT)+ proto-version ::= "v=" 1*(DIGIT) CRLF
;this draft describes version 0 ;this draft 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 newline addr CRLF
session-name-field ::= "s=" text session-name-field ::= "s=" text CRLF
information-field ::= ["i=" text newline] information-field ::= ["i=" text CRLF]
uri-field ::= ["u=" uri newline] uri-field ::= ["u=" uri CRLF]
email-fields ::= ("e=" email-address newline)* email-fields ::= *("e=" email-address CRLF)
phone-fields ::= ("p=" phone-number newline)* phone-fields ::= *("p=" phone-number CRLF)
connection-field ::= ["c=" nettype space addrtype space connection-field ::= ["c=" nettype space addrtype space
connection-address newline] 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 newline)* bandwidth-fields ::= *("b=" bwtype ":" bandwidth CRLF)
time-fields ::= ( "t=" start-time space stop-time time-fields ::= 1*( "t=" start-time space stop-time
(newline repeat-fields)* newline)+ *(CRLF repeat-fields) CRLF)
[zone-adjustments newline] [zone-adjustments CRLF]
repeat-fields ::= "r=" repeat-interval space typed-time repeat-fields ::= "r=" repeat-interval space typed-time
(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 newline] 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 ::= printable-ascii key-data ::= printable-ascii
attribute-fields ::= ("a=" attribute newline)* 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)+ newline space proto (space fmt)+ CRLF
media ::= (alpha-numeric)+ media ::= 1*(alpha-numeric)
;typically "audio", "video", "whiteboard" ;typically "audio", "video", "application"
;or "text" ;or "data"
fmt ::= (alpha-numeric)+ fmt ::= 1*(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 ::= 1*(alpha-numeric)
;typically "RTP/AVP" or "udp" for IP4 ;typically "RTP/AVP" or "udp" for IP4
port ::= (DIGIT)+ port ::= 1*(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
;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 ::= 1*(ALPHA)
att-value ::= (att-char)+ att-value ::= 1*(att-char)
att-char ::= alpha-numeric | "-" att-char ::= alpha-numeric | "-"
;is this too tight a restriction ;is this too tight a restriction
sess-id ::= (DIGIT)+ sess-id ::= 1*(DIGIT)
;should be unique for this originating username/host ;should be unique for this originating username/host
sess-version ::= (DIGIT)+ sess-version ::= 1*(DIGIT)
;0 is a new session ;0 is a new session
connection-address ::= multicast-conf-address | multicast-scoped-address connection-address ::= multicast-address
| unicast-address | unicast-address
multicast-conf-address ::= multicast-address ::=
"224.2." decimal_uchar "." decimal_uchar "/" ttl 3*(decimal_uchar ".") decimal_uchar "/" ttl
[ "/" integer ] [ "/" integer ]
;multicast addresses may be in a larger range ;multicast addresses may be in the range
;but only these should be assigned by an sdp tool ;224.0.0.0 to 239.255.255.255
multicast-scoped-address ::=
"239." decimal_uchar "." decimal_uchar "."
decimal_uchar "/" ttl [ "/" integer ]
ttl ::= decimal_uchar ttl ::= decimal_uchar
start-time ::= time | "0" start-time ::= time | "0"
stop-time ::= time | "0" stop-time ::= time | "0"
time ::= POS-DIGIT 9*DIGIT time ::= POS-DIGIT 9*(DIGIT)
;sufficient for 2 more centuries ;sufficient for 2 more centuries
repeat-interval ::= typed-time | interval-time repeat-interval ::= typed-time
typed-time ::= (DIGIT)+ [fixed-len-time-unit]
interval-time ::= (DIGIT)+ variable-len-time-unit typed-time ::= 1*(DIGIT) [fixed-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'' bwtype ::= 1*(alpha-numeric)
bwtype ::= (alpha-numeric)+
bandwidth ::= (DIGIT)+ bandwidth ::= 1*(DIGIT)
username ::= safe username ::= safe
;pretty wide definition, but doesn't include space ;pretty wide definition, but doesn't include space
email-address ::= email | email "(" email-safe ")" | email-address ::= email | email "(" email-safe ")" |
email-safe "<" email ">" email-safe "<" email ">"
email ::= ;defined in RFC822 email ::= ;defined in RFC822
uri::= ;defined in RFC1630 uri::= ;defined in RFC1630
phone-number ::= phone | phone "(" email-safe ")" | phone-number ::= phone | phone "(" email-safe ")" |
email-safe "<" phone ">" email-safe "<" phone ">"
phone ::= "+" POS-DIGIT (space | "-" | DIGIT)+ phone ::= "+" POS-DIGIT 1*(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
addr ::= unicast-address addr ::= unicast-address
skipping to change at page 30, line 27 skipping to change at page 32, line 23
unicast-address ::= IP4-address | IP6-address unicast-address ::= IP4-address | IP6-address
IP4-address ::= b1 "." decimal_uchar "." decimal_uchar "." b4 IP4-address ::= b1 "." decimal_uchar "." decimal_uchar "." b4
b1 ::= decimal_uchar b1 ::= decimal_uchar
;less than "224"; not "0" or "127" ;less than "224"; not "0" or "127"
b4 ::= decimal_uchar b4 ::= decimal_uchar
;not "0" ;not "0"
IP6-address ::= ;to be defined IP6-address ::= ;to be defined
text ::= (printable-iso8859-1)+ | (unicode-1-1-utf-7)+ text ::= 1*(printable-iso8859-1) | 1*(iso10646-utf-8)
;unicode requires a "a=charset:unicode-1-1-utf-7" ;ISO 8859-1 requires a "a=charset:iso8859-1"
;attribute to be used ;session-level attribute to be used
printable-iso8859-1 ::= ;8 bit ascii character printable-iso8859-1 ::= ;8 bit ascii character
;decimal 9 (TAB), 32-126 and 161-255 ;decimal 9 (TAB), 32-126 and 161-255
unicode-1-1-utf-7 ::= unicode-safe iso10646-1-1-utf-8 ::= ;defined in RFC 2044
;defined in RFC 1642 ;CR and LF are disallowed in this context they are
;an SDP field terminator
decimal_uchar ::= DIGIT decimal_uchar ::= DIGIT
| POS-DIGIT DIGIT | POS-DIGIT DIGIT
| (1 2*DIGIT) | (1 2*(DIGIT))
| (2 (0|1|2|3|4) DIGIT) | (2 (0|1|2|3|4) DIGIT)
| (2 5 (0|1|2|3|4|5)) | (2 5 (0|1|2|3|4|5))
integer ::= POS-DIGIT (DIGIT)* integer ::= POS-DIGIT *(DIGIT)
alpha-numeric ::= ALPHA | DIGIT alpha-numeric ::= ALPHA | DIGIT
printable-ascii ::= unicode-safe | "~" | " printable-ascii ::= unicode-safe | "~" | "
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 ::= email-safe | "(" | ")" | "<" | ">"
;although unicode allows newline and carriage
;return, we don't here.
email-safe ::= safe | space | tab email-safe ::= safe | space | tab
safe ::= alpha-numeric | safe ::= alpha-numeric |
"'" | "'" | "-" | "." | "/" | ":" | "?" | """ | "'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
"#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" | "#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" |
"]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" | "]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
"~" | " "~" | "
space ::= ;ascii code 32 space ::= ;ascii code 32
tab ::= ;ascii code 9 tab ::= ;ascii code 9
newline ::= ;ascii code 10 CRLF ::= ;ascii code 13 followed by 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
Cambridge, MA 02139 Cambridge, MA 02139
United States United States
electronic mail: mjh@isi.edu electronic mail: mjh@isi.edu
skipping to change at page 33, line 10 skipping to change at page 35, line 10
Technical Report #4, The Unicode Standard, Version 1.1" (available from Technical Report #4, The Unicode Standard, Version 1.1" (available from
The Unicode Consortium, and soon to be published by Addison- Wesley). 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(E) Information Technology--Universal Multiple-
octet Coded Character Set (UCS). octet Coded Character Set (UCS).
[8] D. Goldsmith, M. Davis, ``Using Unicode with MIME'', RFC1641, July [8] D. Goldsmith, M. Davis, ``Using Unicode with MIME'', RFC1641, July
1994 1994
[9] D. Goldsmith, M. Davis, ``UTF-7 - A Mail-Safe Transformation Format [9] F. Yergeau, ``UTF-8, a transformation format of Unicode and ISO
of Unicode'', RFC1642, July 1994 10646'', RFC 2044, Oct 30th 1996
[10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for Receiv-
ing Internet-based H.323 Conferences".
[11] M. Handley, E. Schooler, H. Schulzrinne, ``Session Initiation Pro-
tocol (SIP)'' Internet Draft, July 1997.
[12] H. Schulzrinne, A. Rao, R. Lanphier, ``Real Time Streaming Protocol
(RTSP)'' Internet Draft, July 1997.
 End of changes. 

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