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Network Working Group M. Handley
Request for Comments: 4566 UCL
Obsoletes: 2327, 3266 V. Jacobson
Category: Standards Track Packet Design
C. Perkins
University of Glasgow
July 2006
SDP: Session Description Protocol
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo defines the Session Description Protocol (SDP). SDP is
intended for describing multimedia sessions for the purposes of
session announcement, session invitation, and other forms of
multimedia session initiation.
Table of Contents
1. Introduction ....................................................3
2. Glossary of Terms ...............................................3
3. Examples of SDP Usage ...........................................4
3.1. Session Initiation .........................................4
3.2. Streaming Media ............................................4
3.3. Email and the World Wide Web ...............................4
3.4. Multicast Session Announcement .............................4
4. Requirements and Recommendations ................................5
4.1. Media and Transport Information ............................6
4.2. Timing Information .........................................6
4.3. Private Sessions ...........................................7
4.4. Obtaining Further Information about a Session ..............7
4.5. Categorisation .............................................7
4.6. Internationalisation .......................................7
Handley, et al. Standards Track [Page 1]
RFC 4566 SDP July 2006
5. SDP Specification ...............................................7
5.1. Protocol Version ("v=") ...................................10
5.2. Origin ("o=") .............................................11
5.3. Session Name ("s=") .......................................12
5.4. Session Information ("i=") ................................12
5.5. URI ("u=") ................................................13
5.6. Email Address and Phone Number ("e=" and "p=") ............13
5.7. Connection Data ("c=") ....................................14
5.8. Bandwidth ("b=") ..........................................16
5.9. Timing ("t=") .............................................17
5.10. Repeat Times ("r=") ......................................18
5.11. Time Zones ("z=") ........................................19
5.12. Encryption Keys ("k=") ...................................19
5.13. Attributes ("a=") ........................................21
5.14. Media Descriptions ("m=") ................................22
6. SDP Attributes .................................................24
7. Security Considerations ........................................31
8. IANA Considerations ............................................33
8.1. The "application/sdp" Media Type ..........................33
8.2. Registration of Parameters ................................34
8.2.1. Media Types ("media") ..............................34
8.2.2. Transport Protocols ("proto") ......................34
8.2.3. Media Formats ("fmt") ..............................35
8.2.4. Attribute Names ("att-field") ......................36
8.2.5. Bandwidth Specifiers ("bwtype") ....................37
8.2.6. Network Types ("nettype") ..........................37
8.2.7. Address Types ("addrtype") .........................38
8.2.8. Registration Procedure .............................38
8.3. Encryption Key Access Methods .............................39
9. SDP Grammar ....................................................39
10. Summary of Changes from RFC 2327 ..............................44
11. Acknowledgements ..............................................45
12. References ....................................................45
12.1. Normative References .....................................45
12.2. Informative References ...................................46
Handley, et al. Standards Track [Page 2]
RFC 4566 SDP July 2006
1. Introduction
When initiating multimedia teleconferences, voice-over-IP calls,
streaming video, or other sessions, there is a requirement to convey
media details, transport addresses, and other session description
metadata to the participants.
SDP provides a standard representation for such information,
irrespective of how that information is transported. SDP is purely a
format for session description -- it does not incorporate a transport
protocol, and it is intended to use different transport protocols as
appropriate, including the Session Announcement Protocol [14],
Session Initiation Protocol [15], Real Time Streaming Protocol [16],
electronic mail using the MIME extensions, and the Hypertext
Transport Protocol.
SDP is intended to be general purpose so that it can be used in a
wide range of network environments and applications. However, it is
not intended to support negotiation of session content or media
encodings: this is viewed as outside the scope of session
description.
This memo obsoletes RFC 2327 [6] and RFC 3266 [10]. Section 10
outlines the changes introduced in this memo.
2. Glossary of Terms
The following terms are used in this document and have specific
meaning within the context of this document.
Conference: A multimedia conference is a set of two or more
communicating users along with the software they are using to
communicate.
Session: A multimedia session is a set of multimedia senders and
receivers and the data streams flowing from senders to receivers.
A multimedia conference is an example of a multimedia session.
Session Description: A well-defined format for conveying sufficient
information to discover and participate in a multimedia session.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3].
Handley, et al. Standards Track [Page 3]
RFC 4566 SDP July 2006
3. Examples of SDP Usage
3.1. Session Initiation
The Session Initiation Protocol (SIP) [15] is an application-layer
control protocol for creating, modifying, and terminating sessions
such as Internet multimedia conferences, Internet telephone calls,
and multimedia distribution. The SIP messages used to create
sessions carry session descriptions that allow participants to agree
on a set of compatible media types. These session descriptions are
commonly formatted using SDP. When used with SIP, the offer/answer
model [17] provides a limited framework for negotiation using SDP.
3.2. Streaming Media
The Real Time Streaming Protocol (RTSP) [16], is an application-level
protocol for control over the delivery of data with real-time
properties. RTSP provides an extensible framework to enable
controlled, on-demand delivery of real-time data, such as audio and
video. An RTSP client and server negotiate an appropriate set of
parameters for media delivery, partially using SDP syntax to describe
those parameters.
3.3. Email and the World Wide Web
Alternative means of conveying session descriptions include
electronic mail and the World Wide Web (WWW). For both email and WWW
distribution, the media type "application/sdp" is used. This enables
the automatic launching of applications for participation in the
session from the WWW client or mail reader in a standard manner.
Note that announcements of multicast sessions made only via email or
the WWW do not have the property that the receiver of a session
announcement can necessarily receive the session because the
multicast sessions may be restricted in scope, and access to the WWW
server or reception of email is possible outside this scope.
3.4. Multicast Session Announcement
In order to assist the advertisement of multicast multimedia
conferences and other multicast sessions, and to communicate the
relevant session setup information to prospective participants, a
distributed session directory may be used. An instance of such a
session directory periodically sends packets containing a description
of the session to a well-known multicast group. These advertisements
are received by other session directories such that potential remote
participants can use the session description to start the tools
required to participate in the session.
Handley, et al. Standards Track [Page 4]
RFC 4566 SDP July 2006
One protocol used to implement such a distributed directory is the
Session Announcement Protocol (SAP) [14]. SDP provides the
recommended session description format for such session
announcements.
4. Requirements and Recommendations
The purpose of SDP is to convey information about media streams in
multimedia sessions to allow the recipients of a session description
to participate in the session. SDP is primarily intended for use in
an internetwork, although it is sufficiently general that it can
describe conferences in other network environments. Media streams
can be many-to-many. Sessions need not be continually active.
Thus far, multicast-based sessions on the Internet have differed from
many other forms of conferencing in that anyone receiving the traffic
can join the session (unless the session traffic is encrypted). In
such an environment, SDP serves two primary purposes. It is a means
to communicate the existence of a session, and it is a means to
convey sufficient information to enable joining and participating in
the session. In a unicast environment, only the latter purpose is
likely to be relevant.
An SDP session description includes the following:
o Session name and purpose
o Time(s) the session is active
o The media comprising the session
o Information needed to receive those media (addresses, ports,
formats, etc.)
As resources necessary to participate in a session may be limited,
some additional information may also be desirable:
o Information about the bandwidth to be used by the session
o Contact information for the person responsible for the session
In general, SDP must convey sufficient information to enable
applications to join a session (with the possible exception of
encryption keys) and to announce the resources to be used to any
non-participants that may need to know. (This latter feature is
primarily useful when SDP is used with a multicast session
announcement protocol.)
Handley, et al. Standards Track [Page 5]
RFC 4566 SDP July 2006
4.1. Media and Transport Information
An SDP session description includes the following media information:
o The type of media (video, audio, etc.)
o The transport protocol (RTP/UDP/IP, H.320, etc.)
o The format of the media (H.261 video, MPEG video, etc.)
In addition to media format and transport protocol, SDP conveys
address and port details. For an IP multicast session, these
comprise:
o The multicast group address for media
o The transport port for media
This address and port are the destination address and destination
port of the multicast stream, whether being sent, received, or both.
For unicast IP sessions, the following are conveyed:
o The remote address for media
o The remote transport port for media
The semantics of this address and port depend on the media and
transport protocol defined. By default, this SHOULD be the remote
address and remote port to which data is sent. Some media types may
redefine this behaviour, but this is NOT RECOMMENDED since it
complicates implementations (including middleboxes that must parse
the addresses to open Network Address Translation (NAT) or firewall
pinholes).
4.2. Timing Information
Sessions may be either bounded or unbounded in time. Whether or not
they are bounded, they may be only active at specific times. SDP can
convey:
o An arbitrary list of start and stop times bounding the session
o For each bound, repeat times such as "every Wednesday at 10am for
one hour"
This timing information is globally consistent, irrespective of local
time zone or daylight saving time (see Section 5.9).
Handley, et al. Standards Track [Page 6]
RFC 4566 SDP July 2006
4.3. Private Sessions
It is possible to create both public sessions and private sessions.
SDP itself does not distinguish between these; private sessions are
typically conveyed by encrypting the session description during
distribution. The details of how encryption is performed are
dependent on the mechanism used to convey SDP; mechanisms are
currently defined for SDP transported using SAP [14] and SIP [15],
and others may be defined in the future.
If a session announcement is private, it is possible to use that
private announcement to convey encryption keys necessary to decode
each of the media in a conference, including enough information to
know which encryption scheme is used for each media.
4.4. Obtaining Further Information about a Session
A session description should convey enough information to decide
whether or not to participate in a session. SDP may include
additional pointers in the form of Uniform Resource Identifiers
(URIs) for more information about the session.
4.5. Categorisation
When many session descriptions are being distributed by SAP, or any
other advertisement mechanism, it may be desirable to filter session
announcements that are of interest from those that are not. SDP
supports a categorisation mechanism for sessions that is capable of
being automated (the "a=cat:" attribute; see Section 6).
4.6. Internationalisation
The SDP specification recommends the use of the ISO 10646 character
sets in the UTF-8 encoding [5] to allow many different languages to
be represented. However, to assist in compact representations, SDP
also allows other character sets such as ISO 8859-1 to be used when
desired. Internationalisation only applies to free-text fields
(session name and background information), and not to SDP as a whole.
5. SDP Specification
An SDP session description is denoted by the media type
"application/sdp" (See Section 8).
An SDP session description is entirely textual using the ISO 10646
character set in UTF-8 encoding. SDP field names and attribute names
use only the US-ASCII subset of UTF-8, but textual fields and
attribute values MAY use the full ISO 10646 character set. Field and
Handley, et al. Standards Track [Page 7]
RFC 4566 SDP July 2006
attribute values that use the full UTF-8 character set are never
directly compared, hence there is no requirement for UTF-8
normalisation. The textual form, as opposed to a binary encoding
such as ASN.1 or XDR, was chosen to enhance portability, to enable a
variety of transports to be used, and to allow flexible, text-based
toolkits to be used to generate and process session descriptions.
However, since SDP may be used in environments where the maximum
permissible size of a session description is limited, the encoding is
deliberately compact. Also, since announcements may be transported
via very unreliable means or damaged by an intermediate caching
server, the encoding was designed with strict order and formatting
rules so that most errors would result in malformed session
announcements that could be detected easily and discarded. This also
allows rapid discarding of encrypted session announcements for which
a receiver does not have the correct key.
An SDP session description consists of a number of lines of text of
the form:
<type>=<value>
where <type> MUST be exactly one case-significant character and
<value> is structured text whose format depends on <type>. In
general, <value> is either a number of fields delimited by a single
space character or a free format string, and is case-significant
unless a specific field defines otherwise. Whitespace MUST NOT be
used on either side of the "=" sign.
An SDP session description consists of a session-level section
followed by zero or more media-level sections. The session-level
part starts with a "v=" line and continues to the first media-level
section. Each media-level section starts with an "m=" line and
continues to the next media-level section or end of the whole session
description. In general, session-level values are the default for
all media unless overridden by an equivalent media-level value.
Some lines in each description are REQUIRED and some are OPTIONAL,
but all MUST appear in exactly the order given here (the fixed order
greatly enhances error detection and allows for a simple parser).
OPTIONAL items are marked with a "*".
Handley, et al. Standards Track [Page 8]
RFC 4566 SDP July 2006
Session description
v= (protocol version)
o= (originator and session identifier)
s= (session name)
i=* (session information)
u=* (URI of description)
e=* (email address)
p=* (phone number)
c=* (connection information -- not required if included in
all media)
b=* (zero or more bandwidth information lines)
One or more time descriptions ("t=" and "r=" lines; see below)
z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)
Zero or more media descriptions
Time description
t= (time the session is active)
r=* (zero or more repeat times)
Media description, if present
m= (media name and transport address)
i=* (media title)
c=* (connection information -- optional if included at
session level)
b=* (zero or more bandwidth information lines)
k=* (encryption key)
a=* (zero or more media attribute lines)
The set of type letters is deliberately small and not intended to be
extensible -- an SDP parser MUST completely ignore any session
description that contains a type letter that it does not understand.
The attribute mechanism ("a=" described below) is the primary means
for extending SDP and tailoring it to particular applications or
media. Some attributes (the ones listed in Section 6 of this memo)
have a defined meaning, but others may be added on an application-,
media-, or session-specific basis. An SDP parser MUST ignore any
attribute it doesn't understand.
An SDP session description may contain URIs that reference external
content in the "u=", "k=", and "a=" lines. These URIs may be
dereferenced in some cases, making the session description non-self-
contained.
Handley, et al. Standards Track [Page 9]
RFC 4566 SDP July 2006
The connection ("c=") and attribute ("a=") information in the
session-level section applies to all the media of that session unless
overridden by connection information or an attribute of the same name
in the media description. For instance, in the example below, each
media behaves as if it were given a "recvonly" attribute.
An example SDP description is:
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
[email protected] (Jane Doe)
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 99
a=rtpmap:99 h263-1998/90000
Text fields such as the session name and information are octet
strings that may contain any octet with the exceptions of 0x00 (Nul),
0x0a (ASCII newline), and 0x0d (ASCII carriage return). The sequence
CRLF (0x0d0a) is used to end a record, although parsers SHOULD be
tolerant and also accept records terminated with a single newline
character. If the "a=charset" attribute is not present, these octet
strings MUST be interpreted as containing ISO-10646 characters in
UTF-8 encoding (the presence of the "a=charset" attribute may force
some fields to be interpreted differently).
A session description can contain domain names in the "o=", "u=",
"e=", "c=", and "a=" lines. Any domain name used in SDP MUST comply
with [1], [2]. Internationalised domain names (IDNs) MUST be
represented using the ASCII Compatible Encoding (ACE) form defined in
[11] and MUST NOT be directly represented in UTF-8 or any other
encoding (this requirement is for compatibility with RFC 2327 and
other SDP-related standards, which predate the development of
internationalised domain names).
5.1. Protocol Version ("v=")
v=0
The "v=" field gives the version of the Session Description Protocol.
This memo defines version 0. There is no minor version number.
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5.2. Origin ("o=")
o=<username> <sess-id> <sess-version> <nettype> <addrtype>
<unicast-address>
The "o=" field gives the originator of the session (her username and
the address of the user's host) plus a session identifier and version
number:
<username> is the user's login on the originating host, or it is "-"
if the originating host does not support the concept of user IDs.
The <username> MUST NOT contain spaces.
<sess-id> is a numeric string such that the tuple of <username>,
<sess-id>, <nettype>, <addrtype>, and <unicast-address> forms a
globally unique identifier for the session. The method of
<sess-id> allocation is up to the creating tool, but it has been
suggested that a Network Time Protocol (NTP) format timestamp be
used to ensure uniqueness [13].
<sess-version> is a version number for this session description. Its
usage is up to the creating tool, so long as <sess-version> is
increased when a modification is made to the session data. Again,
it is RECOMMENDED that an NTP format timestamp is used.
<nettype> is a text string giving the type of network. Initially
"IN" is defined to have the meaning "Internet", but other values
MAY be registered in the future (see Section 8).
<addrtype> is a text string giving the type of the address that
follows. Initially "IP4" and "IP6" are defined, but other values
MAY be registered in the future (see Section 8).
<unicast-address> is the address of the machine from which the
session was created. For an address type of IP4, this is either
the fully qualified domain name of the machine or the dotted-
decimal representation of the IP version 4 address of the machine.
For an address type of IP6, this is either the fully qualified
domain name of the machine or the compressed textual
representation of the IP version 6 address of the machine. For
both IP4 and IP6, the fully qualified domain name is the form that
SHOULD be given unless this is unavailable, in which case the
globally unique address MAY be substituted. A local IP address
MUST NOT be used in any context where the SDP description might
leave the scope in which the address is meaningful (for example, a
local address MUST NOT be included in an application-level
referral that might leave the scope).
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In general, the "o=" field serves as a globally unique identifier for
this version of this session description, and the subfields excepting
the version taken together identify the session irrespective of any
modifications.
For privacy reasons, it is sometimes desirable to obfuscate the
username and IP address of the session originator. If this is a
concern, an arbitrary <username> and private <unicast-address> MAY be
chosen to populate the "o=" field, provided that these are selected
in a manner that does not affect the global uniqueness of the field.
5.3. Session Name ("s=")
s=<session name>
The "s=" field is the textual session name. There MUST be one and
only one "s=" field per session description. The "s=" field MUST NOT
be empty and SHOULD contain ISO 10646 characters (but see also the
"a=charset" attribute). If a session has no meaningful name, the
value "s= " SHOULD be used (i.e., a single space as the session
name).
5.4. Session Information ("i=")
i=<session description>
The "i=" field provides textual information about the session. There
MUST be at most one session-level "i=" field per session description,
and at most one "i=" field per media. If the "a=charset" attribute
is present, it specifies the character set used in the "i=" field.
If the "a=charset" attribute is not present, the "i=" field MUST
contain ISO 10646 characters in UTF-8 encoding.
A single "i=" field MAY also be used for each media definition. In
media definitions, "i=" fields are primarily intended for labelling
media streams. As such, they are most likely to be useful when a
single session has more than one distinct media stream of the same
media type. An example would be two different whiteboards, one for
slides and one for feedback and questions.
The "i=" field is intended to provide a free-form human-readable
description of the session or the purpose of a media stream. It is
not suitable for parsing by automata.
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5.5. URI ("u=")
u=<uri>
A URI is a Uniform Resource Identifier as used by WWW clients [7].
The URI should be a pointer to additional information about the
session. This field is OPTIONAL, but if it is present it MUST be
specified before the first media field. No more than one URI field
is allowed per session description.
5.6. Email Address and Phone Number ("e=" and "p=")
e=<email-address>
p=<phone-number>
The "e=" and "p=" lines specify contact information for the person
responsible for the conference. This is not necessarily the same
person that created the conference announcement.
Inclusion of an email address or phone number is OPTIONAL. Note that
the previous version of SDP specified that either an email field or a
phone field MUST be specified, but this was widely ignored. The
change brings the specification into line with common usage.
If an email address or phone number is present, it MUST be specified
before the first media field. More than one email or phone field can
be given for a session description.
Phone numbers SHOULD be given in the form of an international public
telecommunication number (see ITU-T Recommendation E.164) preceded by
a "+". Spaces and hyphens may be used to split up a phone field to
aid readability if desired. For example:
p=+1 617 555-6011
Both email addresses and phone numbers can have an OPTIONAL free text
string associated with them, normally giving the name of the person
who may be contacted. This MUST be enclosed in parentheses if it is
present. For example:
[email protected] (Jane Doe)
The alternative RFC 2822 [29] name quoting convention is also allowed
for both email addresses and phone numbers. For example:
e=Jane Doe <[email protected]>
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The free text string SHOULD be in the ISO-10646 character set with
UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if
the appropriate session-level "a=charset" attribute is set.
5.7. Connection Data ("c=")
c=<nettype> <addrtype> <connection-address>
The "c=" field contains connection data.
A session description MUST contain either at least one "c=" field in
each media description or a single "c=" field at the session level.
It MAY contain a single session-level "c=" field and additional "c="
field(s) per media description, in which case the per-media values
override the session-level settings for the respective media.
The first sub-field ("<nettype>") is the network type, which is a
text string giving the type of network. Initially, "IN" is defined
to have the meaning "Internet", but other values MAY be registered in
the future (see Section 8).
The second sub-field ("<addrtype>") is the address type. This allows
SDP to be used for sessions that are not IP based. This memo only
defines IP4 and IP6, but other values MAY be registered in the future
(see Section 8).
The third sub-field ("<connection-address>") is the connection
address. OPTIONAL sub-fields MAY be added after the connection
address depending on the value of the <addrtype> field.
When the <addrtype> is IP4 and IP6, the connection address is defined
as follows:
o If the session is multicast, the connection address will be an IP
multicast group address. If the session is not multicast, then
the connection address contains the unicast IP address of the
expected data source or data relay or data sink as determined by
additional attribute fields. It is not expected that unicast
addresses will be given in a session description that is
communicated by a multicast announcement, though this is not
prohibited.
o Sessions using an IPv4 multicast connection address MUST also have
a time to live (TTL) value present in addition to the multicast
address. The TTL and the address together define the scope with
which multicast packets sent in this conference will be sent. TTL
values MUST be in the range 0-255. Although the TTL MUST be
specified, its use to scope multicast traffic is deprecated;
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RFC 4566 SDP July 2006
applications SHOULD use an administratively scoped address
instead.
The TTL for the session is appended to the address using a slash as a
separator. An example is:
c=IN IP4 224.2.36.42/127
IPv6 multicast does not use TTL scoping, and hence the TTL value MUST
NOT be present for IPv6 multicast. It is expected that IPv6 scoped
addresses will be used to limit the scope of conferences.
Hierarchical or layered encoding schemes are data streams where the
encoding from a single media source is split into a number of layers.
The receiver can choose the desired quality (and hence bandwidth) by
only subscribing to a subset of these layers. Such layered encodings
are normally transmitted in multiple multicast groups to allow
multicast pruning. This technique keeps unwanted traffic from sites
only requiring certain levels of the hierarchy. For applications
requiring multiple multicast groups, we allow the following notation
to be used for the connection address:
<base multicast address>[/<ttl>]/<number of addresses>
If the number of addresses is not given, it is assumed to be one.
Multicast addresses so assigned are contiguously allocated above the
base address, so that, for example:
c=IN IP4 224.2.1.1/127/3
would state that addresses 224.2.1.1, 224.2.1.2, and 224.2.1.3 are to
be used at a TTL of 127. This is semantically identical to including
multiple "c=" lines in a media description:
c=IN IP4 224.2.1.1/127
c=IN IP4 224.2.1.2/127
c=IN IP4 224.2.1.3/127
Similarly, an IPv6 example would be:
c=IN IP6 FF15::101/3
which is semantically equivalent to:
c=IN IP6 FF15::101
c=IN IP6 FF15::102
c=IN IP6 FF15::103
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(remembering that the TTL field is not present in IPv6 multicast).
Multiple addresses or "c=" lines MAY be specified on a per-media
basis only if they provide multicast addresses for different layers
in a hierarchical or layered encoding scheme. They MUST NOT be
specified for a session-level "c=" field.
The slash notation for multiple addresses described above MUST NOT be
used for IP unicast addresses.
5.8. Bandwidth ("b=")
b=<bwtype>:<bandwidth>
This OPTIONAL field denotes the proposed bandwidth to be used by the
session or media. The <bwtype> is an alphanumeric modifier giving
the meaning of the <bandwidth> figure. Two values are defined in
this specification, but other values MAY be registered in the future
(see Section 8 and [21], [25]):
CT If the bandwidth of a session or media in a session is different
from the bandwidth implicit from the scope, a "b=CT:..." line
SHOULD be supplied for the session giving the proposed upper limit
to the bandwidth used (the "conference total" bandwidth). The
primary purpose of this is to give an approximate idea as to
whether two or more sessions can coexist simultaneously. When
using the CT modifier with RTP, if several RTP sessions are part
of the conference, the conference total refers to total bandwidth
of all RTP sessions.
AS The bandwidth is interpreted to be application specific (it will
be the application's concept of maximum bandwidth). Normally,
this will coincide with what is set on the application's "maximum
bandwidth" control if applicable. For RTP-based applications, AS
gives the RTP "session bandwidth" as defined in Section 6.2 of
[19].
Note that CT gives a total bandwidth figure for all the media at all
sites. AS gives a bandwidth figure for a single media at a single
site, although there may be many sites sending simultaneously.
A prefix "X-" is defined for <bwtype> names. This is intended for
experimental purposes only. For example:
b=X-YZ:128
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RFC 4566 SDP July 2006
Use of the "X-" prefix is NOT RECOMMENDED: instead new modifiers
SHOULD be registered with IANA in the standard namespace. SDP
parsers MUST ignore bandwidth fields with unknown modifiers.
Modifiers MUST be alphanumeric and, although no length limit is
given, it is recommended that they be short.
The <bandwidth> is interpreted as kilobits per second by default.
The definition of a new <bwtype> modifier MAY specify that the
bandwidth is to be interpreted in some alternative unit (the "CT" and
"AS" modifiers defined in this memo use the default units).
5.9. Timing ("t=")
t=<start-time> <stop-time>
The "t=" lines specify the start and stop times for a session.
Multiple "t=" lines MAY be used if a session is active at multiple
irregularly spaced times; each additional "t=" line specifies an
additional period of time for which the session will be active. If
the session is active at regular times, an "r=" line (see below)
should be used in addition to, and following, a "t=" line -- in which
case the "t=" line specifies the start and stop times of the repeat
sequence.
The first and second sub-fields give the start and stop times,
respectively, for the session. These values are the decimal
representation of Network Time Protocol (NTP) time values in seconds
since 1900 [13]. To convert these values to UNIX time, subtract
decimal 2208988800.
NTP timestamps are elsewhere represented by 64-bit values, which wrap
sometime in the year 2036. Since SDP uses an arbitrary length
decimal representation, this should not cause an issue (SDP
timestamps MUST continue counting seconds since 1900, NTP will use
the value modulo the 64-bit limit).
If the <stop-time> is set to zero, then the session is not bounded,
though it will not become active until after the <start-time>. If
the <start-time> is also zero, the session is regarded as permanent.
User interfaces SHOULD strongly discourage the creation of unbounded
and permanent sessions as they give no information about when the
session is actually going to terminate, and so make scheduling
difficult.
The general assumption may be made, when displaying unbounded
sessions that have not timed out to the user, that an unbounded
session will only be active until half an hour from the current time
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RFC 4566 SDP July 2006
or the session start time, whichever is the later. If behaviour
other than this is required, an end-time SHOULD be given and modified
as appropriate when new information becomes available about when the
session should really end.
Permanent sessions may be shown to the user as never being active
unless there are associated repeat times that state precisely when
the session will be active.
5.10. Repeat Times ("r=")
r=<repeat interval> <active duration> <offsets from start-time>
"r=" fields specify repeat times for a session. For example, if a
session is active at 10am on Monday and 11am on Tuesday for one hour
each week for three months, then the <start-time> in the
corresponding "t=" field would be the NTP representation of 10am on
the first Monday, the <repeat interval> would be 1 week, the <active
duration> would be 1 hour, and the offsets would be zero and 25
hours. The corresponding "t=" field stop time would be the NTP
representation of the end of the last session three months later. By
default, all fields are in seconds, so the "r=" and "t=" fields might
be the following:
t=3034423619 3042462419
r=604800 3600 0 90000
To make description more compact, times may also be given in units of
days, hours, or minutes. The syntax for these is a number
immediately followed by a single case-sensitive character.
Fractional units are not allowed -- a smaller unit should be used
instead. The following unit specification characters are allowed:
d - days (86400 seconds)
h - hours (3600 seconds)
m - minutes (60 seconds)
s - seconds (allowed for completeness)
Thus, the above session announcement could also have been written:
r=7d 1h 0 25h