RFC2327 - SDP: Session Description Protocol

Network Working Group M. Handley
Request for Comments: 2327 V. Jacobson
Category: Standards Track ISI/LBNL
April 1998
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 (1998). All Rights Reserved.
Abstract
This document 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.
This document is a prodUCt of the Multiparty Multimedia Session
Control (MMUSIC) working group of the Internet Engineering Task
Force. Comments are solicited and should be addressed to the working
group"s mailing list at confctrl@isi.edu and/or the authors.
1. Introduction
On the Internet multicast backbone (Mbone), a session Directory tool
is used to advertise multimedia conferences and communicate the
conference addresses and conference tool-specific information
necessary for participation. This document defines a session
description protocol for this purpose, and for general real-time
multimedia session description purposes. This memo does not describe
multicast address allocation or the distribution of SDP messages in
detail. These are described in accompanying memos. SDP is not
intended for negotiation of media encodings.
2. Background
The Mbone is the part of the internet that supports IP multicast, and
thus permits efficient many-to-many communication. It is used
extensively for multimedia conferencing. Such conferences usually
have the property that tight coordination of conference membership is
not necessary; 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 conference data streams.
Session directories assist the advertisement of conference sessions
and communicate the relevant conference setup information to
prospective participants. SDP is designed to convey such information
to recipients. SDP is purely a format for session description - it
does not incorporate a transport protocol, and is intended to use
different transport protocols as appropriate including the Session
Announcement Protocol [4], Session Initiation Protocol [11], Real-
Time Streaming Protocol [12], electronic mail using the MIME
extensions, and the Hypertext Transport Protocol.
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 session directories. However, it is not intended to
support negotiation of session content or media encodings - this is
viewed as outside the scope of session description.
3. Glossary of Terms
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 Advertisement
See session announcement.
Session Announcement
A session announcement is a mechanism by which a session
description is conveyed to users in a proactive fashion, i.e., the
session description was not eXPlicitly requested by the user.
Session Description
A well defined format for conveying sufficient information to
discover and participate in a multimedia session.
3.1. Terminology
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 RFC2119.
4. SDP Usage
4.1. Multicast Announcements
SDP is a session description protocol for multimedia sessions. A
common mode of usage is for a client to announce a conference session
by periodically multicasting an announcement packet to a well known
multicast address and port using the Session Announcement Protocol
(SAP).
SAP packets are UDP packets with the following format:
--------------------
SAP header
--------------------
text payload
//////////
The header is the Session Announcement Protocol header. SAP is
described in more detail in a companion memo [4]
The text payload is an SDP session description, as described in this
memo. The text payload should be no greater than 1 Kbyte in length.
If announced by SAP, only one session announcement is permitted in a
single packet.
4.2. Email and WWW Announcements
Alternative means of conveying session descriptions include
electronic mail and the World Wide Web. For both email and WWW
distribution, the use of the MIME content type "application/sdp"
should be used. This enables the automatic launching of applications
for participation in the session from the WWW client or mail reader
in a standard manner.
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 session announcement can necessarily receive the session because
the multicast sessions may be restricted in scope, and Access to the
WWW server or reception of email is possible outside this scope. SAP
announcements do not suffer from this mismatch.
5. Requirements and Recommendations
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.
A multimedia session, for these purposes, is defined as a set of
media streams that exist for some duration of time. Media streams
can be many-to-many. The times during which the session is active
need not be continuous.
Thus far, multicast based sessions on the Internet have differed from
many other forms of conferencing in that anyone receiving the traffic
can join the session (unless the session traffic is encrypted). In
such an environment, SDP serves two primary purposes. It is a means
to communicate the existence of a session, and is a means to convey
sufficient information to enable joining and participating in the
session. In a unicast environment, only the latter purpose is likely
to be relevant.
Thus SDP includes:
o Session name and purpose
o Time(s) the session is active
o The media comprising the session
o Information to receive those media (addresses, ports, formats and
so on)
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 conference
o Contact information for the person responsible for the session
In general, SDP must convey sufficient information to be able to join
a session (with the possible exception of encryption keys) and to
announce the resources to be used to non-participants that may need
to know.
5.1. Media Information
SDP includes:
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)
For an IP multicast session, the following are also conveyed:
o Multicast address for media
o Transport Port for media
This address and port are the destination address and destination
port of the multicast stream, whether being sent, received, or both.
For an IP unicast session, the following are conveyed:
o Remote address for media
o Transport port for contact address
The semantics of this address and port depend on the media and
transport protocol defined. By default, this is the remote address
and remote port to which data is sent, and the remote address and
local port on which to receive data. However, some media may define
to use these to establish a control channel for the actual media
flow.
5.2. Timing Information
Sessions may either be 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.
5.3. Private Sessions
It is possible to create both public sessions and private sessions.
Private sessions will typically be conveyed by encrypting the session
description to distribute it. The details of how encryption is
performed are dependent on the mechanism used to convey SDP - see [4]
for how this is done for session announcements.
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.
5.4. 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 Universal Resources Identifiers
(URIs) for more information about the session.
5.5. Categorisation
When many session descriptions are being distributed by SAP or any
other advertisement mechanism, it may be desirable to filter
announcements that are of interest from those that are not. SDP
supports a categorisation mechanism for sessions that is capable of
being automated.
5.6. Internationalization
The SDP specification recommends the use of the ISO 10646 character
sets in the UTF-8 encoding (RFC2044) 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. Internationalization only applies to
free-text fields (session name and background information), and not
to SDP as a whole.
6. SDP Specification
SDP session descriptions are entirely textual using the ISO 10646
character set in UTF-8 encoding. SDP field names and attributes names
use only the US-ASCII subset of UTF-8, but textual fields and
attribute values may use the full ISO 10646 character set. 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 (e.g, session 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 descriptions. However, since the
total bandwidth allocated to all SAP announcements is strictly
limited, the encoding is deliberately compact. Also, since
announcements may be transported via very unreliable means (e.g.,
email) or damaged by an intermediate caching server, the encoding was
designed with strict order and formatting rules so that most errors
would result in malformed announcements which could be detected
easily and discarded. This also allows rapid discarding of encrypted
announcements for which a receiver does not have the correct key.
An SDP session description consists of a number of lines of text of
the form <type>=<value> <type> is always exactly one character and is
case-significant. <value> is a structured text string whose format
depends on <type>. It also will be case-significant unless a
specific field defines otherwise. Whitespace is not permitted either
side of the `=" sign. In general <value> is either a number of fields
delimited by a single space character or a free format string.
A session description consists of a session-level description
(details that apply to the whole session and all media streams) and
optionally several media-level descriptions (details that apply onto
to a single media stream).
An announcement consists of a session-level section followed by zero
or more media-level sections. The session-level part starts with a
`v=" line and continues to the first media-level section. The media
description starts with an `m=" line and continues to the next media
description or end of the whole session description. In general,
session-level values are the default for all media unless overridden
by an equivalent media-level value.
When SDP is conveyed by SAP, only one session description is allowed
per packet. When SDP is conveyed by other means, many SDP session
descriptions may be concatenated together (the `v=" line indicating
the start of a session description terminates the previous
description). 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 `*".
Session description
v= (protocol version)
o= (owner/creator and session identifier).
s= (session name)
i=* (session information)
u=* (URI of description)
e=* (email address)
p=* (phone number)
c=* (connection information - not required if included in all media)
b=* (bandwidth information)
One or more time descriptions (see below)
z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)
Zero or more media descriptions (see below)
Time description
t= (time the session is active)
r=* (zero or more repeat times)
Media description
m= (media name and transport address)
i=* (media title)
c=* (connection information - optional if included at session-level)
b=* (bandwidth information)
k=* (encryption key)
a=* (zero or more media attribute lines)
The set of `type" letters is deliberately small and not intended to
be extensible -- SDP parsers must completely ignore any announcement
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 this document) have a defined
meaning but others may be added on an application-, media- or
session-specific basis. A session directory must ignore any
attribute it doesn"t understand.
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=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
e=mjh@isi.edu (Mark Handley)
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 31
m=application 32416 udp wb
a=orient:portrait
Text records such as the session name and information are bytes
strings which may contain any byte 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. By default these byte strings contain ISO-10646
characters in UTF-8 encoding, but this default may be changed using
the `charset" attribute.
Protocol Version
v=0
The "v=" field gives the version of the Session Description Protocol.
There is no minor version number.
Origin
o=<username> <session id> <version> <network type> <address type>
<address>
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 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.
<username> must not contain spaces. <session id> is a numeric string
such that the tuple of <username>, <session id>, <network type>,
<address type> and <address> form a globally unique identifier for
the session.
The method of <session id> allocation is up to the creating tool, but
it has been suggested that a Network Time Protocol (NTP) timestamp be
used to ensure uniqueness [1].
<version> is a version number for this announcement. It is needed
for proxy announcements to detect which of several announcements for
the same session is the most recent. Again its usage is up to the
creating tool, so long as <version> is increased when a modification
is made to the session data. Again, it is recommended (but not
mandatory) that an NTP timestamp is used.
<network type> is a text string giving the type of network.
Initially "IN" is defined to have the meaning "Internet". <address
type> is a text string giving the type of the address that follows.
Initially "IP4" and "IP6" are defined. <address> is the globally
unique address of the machine from which the session was created.
For an address type of IP4, this is 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.
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
s=<session name>
The "s=" field is the session name. There must be one and only one
"s=" field per session description, and it must contain ISO 10646
characters (but see also the `charset" attribute below).
Session and Media Information
i=<session description>
The "i=" field is information about the session. There may be at
most one session-level "i=" field per session description, and at
most one "i=" field per media. Although it may be omitted, this is
discouraged for session announcements, and user interfaces for
composing sessions should require text to be entered. If it is
present it must contain ISO 10646 characters (but see also the
`charset" attribute below).
A single "i=" field can also be used for each media definition. In
media definitions, "i=" fields are primarily intended for labeling
media streams. As such, they are most likely to be useful when a
single session has more than one distinct media stream of the same
media type. An example would be two different whiteboards, one for
slides and one for feedback and questions.
URI
u=<URI>
o A URI is a Universal Resource Identifier as used by WWW clients
o The URI should be a pointer to additional information about the
conference
o This field is optional, but if it is present it should be specified
before the first media field
o No more than one URI field is allowed per session description
Email Address and Phone Number
e=<email address>
p=<phone number>
o These specify contact information for the person responsible for
the conference. This is not necessarily the same person that
created the conference announcement.
o Either an email field or a phone field must be specified.
Additional email and phone fields are allowed.
o If these are present, they should be specified before the first
media field.
o More than one email or phone field can be given for a session
description.
o Phone numbers should be given in the conventional international
format - preceded by a "+ and the international country code.
There must be a space or a hyphen ("-") between the country code
and the rest of the phone number. Spaces and hyphens may be used
to split up a phone field to aid readability if desired. For
example:
p=+44-171-380-7777 or p=+1 617 253 6011
o Both email addresses and phone numbers can have an optional free
text string associated with them, normally giving the name of the
person who may be contacted. This should be enclosed in
parenthesis if it is present. For example:
e=mjh@isi.edu (Mark Handley)
The alternative RFC822 name quoting convention is also allowed for
both email addresses and phone numbers. For example,
e=Mark Handley <mjh@isi.edu>
The 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 charset session-level attribute is set.
Connection Data
c=<network type> <address type> <connection address>
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 type of network. Initially "IN" is defined to have the
meaning "Internet".
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.
The third sub-field is the connection address. Optional extra
subfields may be added after the connection address depending on the
value of the <address type> field.
For IP4 addresses, the connection address is defined as follows:
o Typically the connection address will be a class-D IP multicast
group address. If the session is not multicast, then the
connection address contains the fully-qualified domain name or 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 fully-qualified domain names or unicast addresses
will be given in a session description that is communicated by a
multicast announcement, though this is not prohibited. If a
unicast data stream is to pass through a network address
translator, the use of a fully-qualified domain name rather than an
unicast IP address is RECOMMENDED. In other cases, the use of an
IP address to specify a particular interface on a multi-homed host
might be required. Thus this specification leaves the decision as
to which to use up to the individual application, but all
applications MUST be able to cope with receiving both formats.
o Conferences using an IP 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.
The TTL for the session is appended to the address using a slash as
a separator. An example is:
c=IN IP4 224.2.1.1/127
Hierarchical or layered encoding schemes are data streams where the
encoding from a single media source is split into a number of
layers. The receiver can choose the desired quality (and hence
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
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
IP unicast addresses.
Bandwidth
b=<modifier>:<bandwidth-value>
o This specifies the proposed bandwidth to be used by the session or
media, and is optional.
o <bandwidth-value> is in kilobits per second
o <modifier> is a single alphanumeric word giving the meaning of the
bandwidth figure.
o Two modifiers are initially defined:
CT Conference Total: An implicit maximum bandwidth is associated with
each TTL on the Mbone or within a particular multicast
administrative scope region (the Mbone bandwidth vs. TTL limits are
given in the MBone FAQ). 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 primary purpose of
this is to give an approximate idea as to whether two or more
conferences can co-exist simultaneously.
AS Application-Specific Maximum: The bandwidth is interpreted to be
application-specific, i.e., 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.
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.
o Extension Mechanism: Tool writers can define experimental bandwidth
modifiers by prefixing their modifier with "X-". For example:
b=X-YZ:128
SDP parsers should ignore bandwidth fields with unknown modifiers.
Modifiers should be alpha-numeric and, although no length limit is
given, they are recommended to be short.
Times, Repeat Times and Time Zones
t=<start time> <stop time>
o "t=" fields specify the start and stop times for a conference
session. Multiple "t=" fields may be used if a session is active
at multiple irregularly spaced times; each additional "t=" field
specifies an additional period of time for which the session will
be active. If the session is active at regular times, an "r="
field (see below) should be used in addition to and following a
"t=" field - in which case the "t=" field specifies the start and
stop times of the repeat sequence.
o The first and second sub-fields give the start and stop times for
the conference respectively. These values are the decimal
representation of Network Time Protocol (NTP) time values in
seconds [1]. To convert these values to UNIX time, subtract
decimal 2208988800.
o If the stop-time is set to zero, then the session is not bounded,
though it will not become active until after the start-time. If
the start-time is also zero, the session is regarded as permanent.
User interfaces should strongly discourage the creation of
unbounded and permanent sessions as they give no information about
when the session is actually going to terminate, and so make
scheduling difficult.
The general assumption may be made, when displaying unbounded
sessions that have not timed out to the user, that an unbounded
session will only be active until half an hour from the current
time or the session start time, whichever is the later. If
behaviour other than this is required, an end-time should be given
and modified as appropriate when new information becomes available
about when the session should really end.
Permanent sessions may be shown to the user as never being active
unless there are associated repeat times which state precisely when
the session will be active. In general, permanent sessions should
not be created for any session expected to have a duration of less
than 2 months, and should be discouraged for sessions expected to
have a duration of less than 6 months.
r=<repeat interval> <active duration> <list of offsets from start-
time>
o "r=" fields specify repeat times for a session. For example, if
a session is active at 10am on Monday and 11am on Tuesday for one
hour each week for three months, then the <start time> in the
corresponding "t=" field would be the NTP representation of 10am on
the first Monday, the <repeat interval> would be 1 week, the
<active duration> would be 1 hour, and the offsets would be zero
and 25 hours. The corresponding "t=" field stop time would be the
NTP representation of the end of the last session three months
later. By default all fields are in seconds, so the "r=" and "t="
fields might be:
t=3034423619 3042462419
r=604800 3600 0 90000
To make announcements more compact, times may also be given in units
of days, hours or minutes. The syntax for these is a number
immediately followed by a single case-sensitive character.
Fractional units are not allowed - a smaller unit should be used
instead. The following unit specification characters are allowed:
d - days (86400 seconds)
h - minutes (3600 seconds)
m - minutes (60 seconds)
s - seconds (allowed for completeness but not recommended)
Thus, the above announcement could also have been written:
r=7d 1h 0 25h
Monthly and yearly repeats cannot currently be directly specified
with a single SDP repeat time - instead separate "t" fields should
be used to explicitly list the session times.
z=<adjustment time> <offset> <adjustment time> <offset> ....
o To schedule a repeated session which spans a change from daylight-
saving time to standard time or vice-versa, it is necessary to
specify offsets from the base repeat times. This is required
because different time zones change time at different times of day,
different countries change to or from daylight time on different
dates, and some countries do not have daylight saving time at all.
Thus in order to schedule a session that is at the same time winter
and summer, it must be possible to specify unambiguously by whose
time zone a session is scheduled. To simplify this task for
receivers, we allow the sender to specify the NTP time that a time
zone adjustment happens and the offset from the time when the
session was first scheduled. The "z" field allows the sender to
specify a list of these adjustment times and offsets from the base
time.
An example might be:
z=2882844526 -1h 2898848070 0
This specifies that at time 2882844526 the time base by which the
session"s repeat times are calculated is shifted back by 1 hour,
and that at time 2898848070 the session"s original time base is
restored. Adjustments are always relative to the specified start
time - they are not cumulative.
o If a session is likely to last several years, it is expected
that
the session announcement will be modified periodically rather than
transmit several years worth of adjustments in one announcement.
Encryption Keys
k=<method>
k=<method>:<encryption key>
o The session description protocol may be used to convey encryption
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
media entry as required.
o The format of keys and their usage is outside the scope of this
document, but see [3].
o The method indicates the mechanism to be used to obtain a usable
key by external means, or from the encoded encryption key given.
The following methods are defined:
k=clear:<encryption key>
The encryption key (as described in [3] for RTP media streams
under the AV profile) is included untransformed in this key
field.
k=base64:<encoded encryption key>
The encryption key (as described in [3] for RTP media streams
under the AV profile) is included in this key field but has been
base64 encoded because it includes characters that are
prohibited in SDP.
k=uri:<URI to obtain key>
A Universal Resource Identifier as used by WWW clients is
included in this key field. The URI refers to the data
containing the key, and may require additional authentication
before the key can be returned. When a request is made to the
given URI, the MIME content-type of the reply specifies the
encoding for the key in the reply. The key should not be
obtained until the user wishes to join the session to reduce
synchronisation of requests to the WWW server(s).
k=prompt
No key is included in this SDP description, but the session or
media stream referred to by this key field is encrypted. The
user should be prompted for the key when attempting to join the
session, and this user-supplied key should then be used to
decrypt the media streams.
Attributes
a=<attribute>
a=<attribute>:<value>
Attributes are the primary means for extending SDP. Attributes may
be defined to be used as "session-level" attributes, "media-level"
attributes, or both.
A media description may have any number of attributes ("a=" fields)
which are media specific. These are referred to as "media-level"
attributes and add information about the media stream. Attribute
fields can also be added before the first media field; these
"session-level" attributes convey additional information that applies
to the conference as a whole rather than to individual media; an
example might be the conference"s floor control policy.
Attribute fields may be of two forms:
o property attributes. A property attribute is simply of the form
"a=<flag>". These are binary attributes, and the presence of the
attribute conveys that the attribute is a property of the session.
An example might be "a=recvonly".
o value attributes. A value attribute is of the form
"a=<attribute>:<value>". An example might be that a whiteboard
could have the value attribute "a=orient:landscape"
Attribute interpretation depends on the media tool being invoked.
Thus receivers of session descriptions should be configurable in
their interpretation of announcements in general and of attributes in
particular.
Attribute names must be in the US-ASCII subset of ISO-10646/UTF-8.
Attribute values are byte strings, and MAY use any byte value except
0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values
are to be interpreted as in ISO-10646 character set with UTF-8
encoding. Unlike other text fields, attribute values are NOT
normally affected by the `charset" attribute as this would make
comparisons against known values problematic. However, when an
attribute is defined, it can be defined to be charset-dependent, in
which case it"s value should be interpreted in the session charset
rather than in ISO-10646.
Attributes that will be commonly used can be registered with IANA
(see Appendix B). Unregistered attributes should begin with "X-" to
prevent inadvertent collision with registered attributes. In either
case, if an attribute is received that is not understood, it should
simply be ignored by the receiver.
Media Announcements
m=<media> <port> <transport> <fmt list>
A session description may contain a number of media descriptions.
Each media description starts with an "m=" field, and is terminated
by either the next "m=" field or by the end of the session
description. A media field also has several sub-fields:
o The first sub-field is the media type. Currently defined media are
"audio", "video", "application", "data" and "control", though this
list may be extended as new communication modalities emerge (e.g.,
telepresense). 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 multicasting of program
executables which will not typically be displayed to the user.
"control" is used to specify an additional conference control
channel for the session.
o The second sub-field is the transport port to which the media
stream will be sent. The meaning of the transport port depends on
the network being used as specified in the relevant "c" field and
on the transport protocol defined in the third sub-field. Other
ports used by the media application (such as the RTCP port, see
[2]) should be derived algorithmically from the base media port.
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
number.
For applications where hierarchically encoded streams are being
sent to a unicast address, it may be necessary to specify multiple
transport ports. This is done using a similar notation to that
used for IP multicast addresses in the "c=" field:
m=<media> <port>/<number of ports> <transport> <fmt list>
In such a case, the ports used depend on the transport protocol.
For RTP, only the even ports are used for data and the
corresponding one-higher odd port is used for RTCP. For example:
m=video 49170/2 RTP/AVP 31
would specify that ports 49170 and 49171 form one RTP/RTCP pair and
49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the
transport protocol and 31 is the format (see below).
It is illegal for both multiple addresses to be specified in the
"c=" field and for multiple ports to be specified in the "m=" field
in the same session description.
o The third sub-field is the transport protocol. The transport
protocol values are dependent on the address-type field in the "c="
fields. Thus a "c=" field of IP4 defines that the transport
protocol runs over IP4. For IP4, it is normally expected that most
media traffic will be carried as RTP over UDP. The following
transport protocols are preliminarily defined, but may be extended
through registration of new protocols with IANA:
- RTP/AVP - the IETF"s Realtime Transport Protocol using the
Audio/Video profile carried over UDP.
- udp - User Datagram Protocol
If an application uses a single combined proprietary media format
and transport protocol over UDP, then simply specifying the
transport protocol as udp and using the format field to distinguish
the combined protocol is recommended. If a transport protocol is
used over UDP to carry several distinct media types that need to be
distinguished by a session directory, then specifying the transport
protocol and media format separately is necessary. RTP is an
example of a transport-protocol that carries multiple payload
formats that must be distinguished by the session directory for it
to know how to start appropriate tools, relays, mixers or
recorders.
The main reason to specify the transport-protocol in addition to
the media format is that the same standard media formats may be
carried over different transport protocols even when the network
protocol is the same - a historical example is vat PCM audio and
RTP PCM audio. In addition, relays and monitoring tools that are
transport-protocol-specific but format-independent are possible.
For RTP media streams operating under the RTP Audio/Video Profile
[3], the protocol field is "RTP/AVP". Should other RTP profiles be
defined in the future, their profiles will be specified in the same
way. For example, the protocol field "RTP/XYZ" would specify RTP
operating under a profile whose short name is "XYZ".
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 the RTP Audio/Video Profile.
When a list of payload formats is given, this implies that all of
these formats may be used in the session, but the first of these
formats is the default format for the session.
For media whose transport protocol is not RTP or UDP the format
field is protocol specific. Such formats should be defined in an
additional specification document.
For media whose transport protocol is RTP, SDP can be used to
provide a dynamic binding of media encoding to RTP payload type.
The encoding names in the RTP AV Profile do not specify unique
audio encodings (in terms of clock rate and number of audio
channels), and so they are not used directly in SDP format fields.
Instead, the payload type number should be used to specify the
format for static payload types and the payload type number along
with additional encoding information should be used for dynamically
allocated payload types.
An example of a static payload type is u-law PCM coded single
channel audio sampled at 8KHz. This is completely defined in the
RTP Audio/Video profile as payload type 0, so the media field for
such a stream sent to UDP port 49232 is:
m=video 49232 RTP/AVP 0
An example of a dynamic payload type is 16 bit linear encoded
stereo audio sampled at 16KHz. If we wish to use dynamic RTP/AVP
payload type 98 for such a stream, additional information is
required to decode it:
m=video 49232 RTP/AVP 98
a=rtpmap:98 L16/16000/2
The general form of an rtpmap attribute is:
a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
parameters>]
For audio streams, <encoding parameters> may specify the number of
audio channels. This parameter may be omitted if the number of
channels is one provided no additional parameters are needed. For
video streams, no encoding parameters are currently specified.
Additional parameters may be defined in the future, but
codecspecific parameters should not be added. Parameters added to
an rtpmap attribute should only be those required for a session
directory to make the choice of appropriate media too to
participate in a session. Codec-specific parameters should be
added in other attributes.
Up to one rtpmap attribute can be defined for each media format
specified. Thus we might have:
m=audio 49230 RTP/AVP 96 97 98
a=rtpmap:96 L8/8000
a=rtpmap:97 L16/8000
a=rtpmap:98 L16/11025/2
RTP profiles that specify the use of dynamic payload types must
define the set of valid encoding names and/or a means to register
encoding names if that profile is to be used with SDP.
Experimental encoding formats can also be specified using rtpmap.
RTP formats that are not registered as standard format names must
be preceded by "X-". Thus a new experimental redundant audio
stream called GSMLPC using dynamic payload type 99 could be
specified as:
m=video 49232 RTP/AVP 99
a=rtpmap:99 X-GSMLPC/8000
Such an experimental encoding requires that any site wishing to
receive the media stream has relevant configured state in its
session directory to know which tools are appropriate.
Note that RTP audio formats typically do not include information
about the number of samples per packet. If a non-default (as
defined in the RTP Audio/Video Profile) packetisation is required,
the "ptime" attribute is used as given below.
For more details on RTP audio and video formats, see [3].
o Formats for non-RTP media should be registered as MIME content
types as described in Appendix B. For example, the LBL whiteboard
application might be registered as MIME content-type application/wb
with encoding considerations specifying that it operates over UDP,
with no appropriate file format. In SDP this would then be
expressed using a combination of the "media" field and the "fmt"
field, as follows:
m=application 32416 udp wb
Suggested Attributes
The following attributes are suggested. Since application writers
may add new attributes as they are required, this list is not
exhaustive.
a=cat:<category>
This attribute gives the dot-separated hierarchical category of
the session. This is to enable a receiver to filter unwanted
sessions by category. It would probably have been a compulsory
separate field, except for its experimental nature at this time.
It is a session-level attribute, and is not dependent on charset.
a=keywds:<keywords>
Like the cat attribute, this is to assist identifying wanted
sessions at the receiver. This allows a receiver to select
interesting session based on keywords describing the purpose of
the session. It is a session-level attribute. It is a charset
dependent attribute, meaning that its value should be interpreted
in the charset specified for the session description if one is
specified, or by default in ISO 10646/UTF-8.
a=tool:<name and version of tool>
This gives the name and version number of the tool used to create
the session description. It is a session-level attribute, and is
not dependent on charset.
a=ptime:<packet time>
This gives the length of time in milliseconds represented by the
media in a packet. This is probably only meaningful for audio
data. It should not be necessary to know ptime to decode RTP or
vat audio, and it is intended as a recommendation for the
encoding/packetisation of audio. It is a media attribute, and is
not dependent on charset.
a=recvonly
This specifies that the tools should be started in receive-only
mode where applicable. It can be either a session or media
attribute, and is not dependent on charset.
a=sendrecv
This specifies that the tools should be started in send and
receive mode. This is necessary for interactive conferences with
tools such as wb which defaults to receive only mode. It can be
either a session or media attribute, and is not dependent on
charset.
a=sendonly
This specifies that the tools should be started in send-only
mode. An example may be where a different unicast address is to
be used for a traffic destination than for a traffic source. In
such a case, two media descriptions may be use, one sendonly and
one recvonly. It can be either a session or media attribute, but
would normally only be used as a media attribute, and is not
dependent on charset.
a=orient:<whiteboard orientation>
Normally this is only used in a whiteboard media specification.
It specifies the orientation of a the whiteboard on the screen.
It is a media attribute. Permitted values are `portrait",
`landscape" and `seascape" (upside down landscape). It is not
dependent on charset
a=type:<conference type>
This specifies the type of the conference. Suggested values are
`broadcast", `meeting", `moderated", `test" and `H332".
`recvonly" should be the default for `type:broadcast" sessions,
`type:meeting" should imply `sendrecv" and `type:moderated"
should indicate the use of a floor control tool and that the
media tools are started so as to "mute" new sites joining the
conference.
Specifying the attribute type:H332 indicates that this loosely
coupled session is part of a H.332 session as defined in the ITU
H.332 specification [10]. Media tools should be started
`recvonly".
Specifying the attribute type:test is suggested as a hint that,
unless explicitly requested otherwise, receivers can safely avoid
displaying this session description to users.
The type attribute is a session-level attribute, and is not
dependent on charset.
a=charset:<character set>
This specifies the character set to be used to display the
session name and information data. By default, the ISO-10646
character set in UTF-8 encoding is used. If a more compact
representation is required, other character sets may be used such
as ISO-8859-1 for Northern European languages. In particular,
the ISO 8859-1 is specified with the following SDP attribute:
a=charset:ISO-8859-1
This is a session-level attribute; if this attribute is present,
it must be before the first media field. The charset specified
MUST be one of those registered with IANA, such as ISO-8859-1.
The character set identifier is a US-ASCII string and MUST be
compared against the IANA identifiers using a case-insensitive
comparison. If the identifier is not recognised or not
supported, all strings that are affected by it SHOULD be regarded
as byte strings.
Note that a character set specified MUST still prohibit the use
of bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets
requiring the use of these characters MUST define a quoting
mechanism that prevents these bytes appearing within text fields.
a=sdplang:<language tag>
This can be a session level attribute or a media level attribute.
As a session level attribute, it specifies the language for the
session description. As a media level attribute, it specifies
the language for any media-level SDP information field associated
with that media. Multiple sdplang attributes can be provided
either at session or media level if multiple languages in the
session description or media use multiple languages, in which
case the order of the attributes indicates the order of
importance of the various languages in the session or media from
most important to least important.
In general, sending session descriptions consisting of multiple
languages should be discouraged. Instead, multiple descriptions
should be sent describing the session, one in each language.
However this is not possible with all transport mechanisms, and
so multiple sdplang attributes are allowed although not
recommended.
The sdplang attribute value must be a single RFC1766 language
tag in US-ASCII. It is not dependent on the charset attribute.
An sdplang attribute SHOULD be specified when a session is of
sufficient scope to cross geographic boundaries where the
language of recipients cannot be assumed, or where the session is
in a different language from the locally assumed norm.
a=lang:<language tag>
This can be a session level attribute or a media level attribute.
As a session level attribute, it specifies the default language
for the session being described. As a media level attribute, it
specifies the language for that media, overriding any session-
level language specified. Multiple lang attributes can be
provided either at session or media level if multiple languages
if the session description or media use multiple languages, in
which case the order of the attributes indicates the order of
importance of the various languages in the session or media from
most important to least important.
The lang attribute value must be a single RFC1766 language tag
in US-ASCII. It is not dependent on the charset attribute. A
lang attribute SHOULD be specified when a session is of
sufficient scope to cross geographic boundaries where the
language of recipients cannot be assumed, or where the session is
in a different language from the locally assumed norm.
a=framerate:<frame rate>
This gives the maximum video frame rate in frames/sec. It is
intended as a recommendation for the encoding of video data.
Decimal representations of fractional values using the notation
"<integer>.<fraction>" are allowed. It is a media attribute, is
only defined for video media, and is not dependent on charset.
a=quality:<quality>
This gives a suggestion for the quality of the encoding as an
integer value.
The intention of the quality attribute for video is to specify a
non-default trade-off between frame-rate and still-image quality.
For video, the value in the range 0 to 10, with the following
suggested meaning:
10 - the best still-image quality the compression scheme can
give.
5 - the default behaviour given no quality suggestion.
0 - the worst still-image quality the codec designer thinks is
still usable.
It is a media attribute, and is not dependent on charset.
a=fmtp:<format> <format specific parameters>
This attribute allows parameters that are specific to a
particular format to be conveyed in a way that SDP doesn"t have
to understand them. The format must be one of the formats
specified for the media. Format-specific parameters may be any
set of parameters required to be conveyed by SDP and given
unchanged to the media tool that will use this format.
It is a media attribute, and is not dependent on charset.
6.1. Communicating Conference Control Policy
There is some debate over the way conference control policy should be
communicated. In general, the authors believe that an implicit
declarative style of specifying conference control is desirable where
possible.
A simple declarative style uses a single conference attribute field
before the first media field, possibly supplemented by properties
such as `recvonly" for some of the media tools. This conference
attribute conveys the conference control policy. An example might be:
a=type:moderated
In some cases, however, it is possible that this may be insufficient
to communicate the details of an unusual conference control policy.
If this is the case, then a conference attribute specifying external
control might be set, and then one or more "media" fields might be
used to specify the conference control tools and configuration data
for those tools. An example is an ITU H.332 session:
c=IN IP4 224.5.6.7
a=type:H332
m=audio 49230 RTP/AVP 0
m=video 49232 RTP/AVP 31
m=application 12349 udp wb
m=control 49234 H323 mc
c=IN IP4 134.134.157.81
In this example, a general conference attribute (type:H332) is
specified stating that conference control will be provided by an
external H.332 tool, and a contact addresses for the H.323 session
multipoint controller is given.
In this document, only the declarative style of conference control
declaration is specified. Other forms of conference control should
specify an appropriate type attribute, and should define the
implications this has for control media.
7. Security Considerations
SDP is a session description format that describes multimedia
sessions. A session description should not be trusted unless it has
been obtained by an authenticated transport protocol from a trusted
source. Many different transport protocols may be used to distribute
session description, and the nature of the authentication will differ
from transport to transport.
One transport that will frequently be used to distribute session
descriptions is the Session Announcement Protocol (SAP). SAP
provides both encryption and authentication mechanisms but due to the
nature of session announcements it is likely that there are many
occasions where the originator of a session announcement cannot be
authenticated because they are previously unknown to the receiver of
the announcement and because no common public key infrastructure is
available.
On receiving a session description over an unauthenticated transport
mechanism or from an untrusted party, software parsing the session
should take a few precautions. Session description contain
information required to start software on the receivers system.
Software that parses a session description MUST not be able to start
other software except that which is specifically configured as
appropriate software to participate in multimedia sessions. It is
normally considered INAPPROPRIATE for software parsing a session
description to start, on a user"s system, software that is
appropriate to participate in multimedia sessions, without the user
first being informed that such software will be started and giving
their consent. Thus a session description arriving by session
announcement, email, 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
circumstances it might be appropriate to define such attributes. If
this is done an application parsing a session description containing
such attributes SHOULD either ignore them, or inform the user that
joining this session will result in the automatic transmission of
multimedia data. The default behaviour for an unknown attribute is
to ignore it.
Session descriptions may be parsed at intermediate systems such as
firewalls for the purposes of opening a hole in the firewall to allow
the participation in multimedia sessions. It is considered
INAPPROPRIATE for a firewall to open such holes for unicast data
streams unless the session description comes in a request from inside
the firewall.
For multicast sessions, it is likely that local administrators will
apply their own policies, but the exclusive use of "local" or "site-
local" administrative scope within the firewall and the refusal of
the firewall to open a hole for such scopes will provide separation
of global multicast sessions from local ones.
Appendix A: SDP Grammar
This appendix provides an Augmented BNF grammar for SDP. ABNF is
defined in RFC2234.
announcement = proto-version
origin-field
session-name-field
information-field
uri-field
email-fields
phone-fields
connection-field
bandwidth-fields
time-fields
key-field
attribute-fields
media-descriptions
proto-version = "v=" 1*DIGIT CRLF
;this memo describes version 0
origin-field = "o=" username space
sess-id space sess-version space
nettype space addrtype space
addr CRLF
session-name-field = "s=" text CRLF
information-field = ["i=" text CRLF]
uri-field = ["u=" uri CRLF]
email-fields = *("e=" email-address CRLF)
phone-fields = *("p=" phone-number CRLF)
connection-field = ["c=" nettype space addrtype space
connection-address CRLF]
;a connection field must be present
;in every media description or at the
;session-level
bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF)
time-fields = 1*( "t=" start-time space stop-time
*(CRLF repeat-fields) CRLF)
[zone-adjustments CRLF]
repeat-fields = "r=" repeat-interval space typed-time
1*(space typed-time)
zone-adjustments = time space ["-"] typed-time
*(space time space ["-"] typed-time)
key-field = ["k=" key-type CRLF]
key-type = "prompt"
"clear:" key-data
"base64:" key-data
"uri:" uri
key-data = email-safe "~" "
attribute-fields = *("a=" attribute CRLF)
media-descriptions = *( media-field
information-field
*(connection-field)
bandwidth-fields
key-field
attribute-fields )
media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF
media = 1*(alpha-numeric)
;typically "audio", "video", "application"
;or "data"
fmt = 1*(alpha-numeric)
;typically an RTP payload type for audio
;and video media
proto = 1*(alpha-numeric)
;typically "RTP/AVP" or "udp" for IP4
port = 1*(DIGIT)
;should in the range "1024" to "65535" inclusive
;for UDP based media
attribute = (att-field ":" att-value) att-field
att-field = 1*(alpha-numeric)
att-value = byte-string
sess-id = 1*(DIGIT)
;should be unique for this originating username/host
sess-version = 1*(DIGIT)
;0 is a new session
connection-address = multicast-address
addr
multicast-address = 3*(decimal-uchar ".") decim