| Rfc | 4566 |
|---|
| Title | SDP: Session Description Protocol |
|---|
| Author | M. Handley, V. Jacobson, C.
Perkins |
|---|
| Date | July 2006 |
|---|
| Format: | TXT, HTML |
|---|
| Obsoletes | RFC2327, RFC3266 |
|---|
| Obsoleted by | RFC8866 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
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
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
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].
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.
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.)
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).
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
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 "*".
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.
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
e=j.doe@example.com (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.
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).
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.
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:
e=j.doe@example.com (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 <j.doe@example.com>
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;
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
(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
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
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
Monthly and yearly repeats cannot be directly specified with a single
SDP repeat time; instead, separate "t=" fields should be used to
explicitly list the session times.
5.11. Time Zones ("z=")
z=<adjustment time> <offset> <adjustment time> <offset> ....
To schedule a repeated session that spans a change from daylight
saving time to standard time or vice versa, it is necessary to
specify offsets from the base time. This is required because
different time zones change time at different times of day, different
countries change to or from daylight saving 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 the following:
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. Adjustments apply to all "t=" and
"r=" lines in a session description.
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 session
announcement.
5.12. Encryption Keys ("k=")
k=<method>
k=<method>:<encryption key>
If transported over a secure and trusted channel, the Session
Description Protocol MAY be used to convey encryption keys. A simple
mechanism for key exchange is provided by the key field ("k="),
although this is primarily supported for compatibility with older
implementations and its use is NOT RECOMMENDED. Work is in progress
to define new key exchange mechanisms for use with SDP [27] [28], and
it is expected that new applications will use those mechanisms.
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. The format of keys and their usage are outside the scope
of this document, and the key field provides no way to indicate the
encryption algorithm to be used, key type, or other information about
the key: this is assumed to be provided by the higher-level protocol
using SDP. If there is a need to convey this information within SDP,
the extensions mentioned previously SHOULD be used. Many security
protocols require two keys: one for confidentiality, another for
integrity. This specification does not support transfer of two keys.
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 is included untransformed in this key field.
This method MUST NOT be used unless it can be guaranteed that
the SDP is conveyed over a secure channel. The encryption key
is interpreted as text according to the charset attribute; use
the "k=base64:" method to convey characters that are otherwise
prohibited in SDP.
k=base64:<encoded encryption key>
The encryption key is included in this key field but has been
base64 encoded [12] because it includes characters that are
prohibited in SDP. This method MUST NOT be used unless it can
be guaranteed that the SDP is conveyed over a secure channel.
k=uri:<URI to obtain key>
A Uniform Resource Identifier is included in the 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 reply should specify
the encoding for the key. The URI is often an Secure Socket
Layer/Transport Layer Security (SSL/TLS)-protected HTTP URI
("https:"), although this is not required.
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. The use of user-specified keys is
NOT RECOMMENDED, since such keys tend to have weak security
properties.
The key field MUST NOT be used unless it can be guaranteed that the
SDP is conveyed over a secure and trusted channel. An example of
such a channel might be SDP embedded inside an S/MIME message or a
TLS-protected HTTP session. It is important to ensure that the
secure channel is with the party that is authorised to join the
session, not an intermediary: if a caching proxy server is used, it
is important to ensure that the proxy is either trusted or unable to
access the SDP.
5.13. Attributes ("a=")
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)
that 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.
Attribute fields may be of two forms:
o 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 A value attribute is of the form "a=<attribute>:<value>". For
example, 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 session descriptions in general and of
attributes in particular.
Attribute names MUST use the US-ASCII subset of ISO-10646/UTF-8.
Attribute values are octet strings, and MAY use any octet 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 its value should be interpreted in the session charset
rather than in ISO-10646.
Attributes MUST be registered with IANA (see Section 8). If an
attribute is received that is not understood, it MUST be ignored by
the receiver.
5.14. Media Descriptions ("m=")
m=<media> <port> <proto> <fmt> ...
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 has several sub-fields:
<media> is the media type. Currently defined media are "audio",
"video", "text", "application", and "message", although this list
may be extended in the future (see Section 8).
<port> is the transport port to which the media stream is 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 <proto> sub-field of the media field.
Other ports used by the media application (such as the RTP Control
Protocol (RTCP) port [19]) MAY be derived algorithmically from the
base media port or MAY be specified in a separate attribute (for
example, "a=rtcp:" as defined in [22]).
If non-contiguous ports are used or if they don't follow the
parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:"
attribute MUST be used. Applications that are requested to send
media to a <port> that is odd and where the "a=rtcp:" is present
MUST NOT subtract 1 from the RTP port: that is, they MUST send the
RTP to the port indicated in <port> and send the RTCP to the port
indicated in the "a=rtcp" attribute.
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> <proto> <fmt> ...
In such a case, the ports used depend on the transport protocol.
For RTP, the default is that only the even-numbered ports are used
for data with the corresponding one-higher odd ports used for the
RTCP belonging to the RTP session, and the <number of ports>
denoting the number of RTP sessions. 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). If non-
contiguous ports are required, they must be signalled using a
separate attribute (for example, "a=rtcp:" as defined in [22]).
If multiple addresses are specified in the "c=" field and multiple
ports are specified in the "m=" field, a one-to-one mapping from
port to the corresponding address is implied. For example:
c=IN IP4 224.2.1.1/127/2
m=video 49170/2 RTP/AVP 31
would imply that address 224.2.1.1 is used with ports 49170 and
49171, and address 224.2.1.2 is used with ports 49172 and 49173.
The semantics of multiple "m=" lines using the same transport
address are undefined. This implies that, unlike limited past
practice, there is no implicit grouping defined by such means and
an explicit grouping framework (for example, [18]) should instead
be used to express the intended semantics.
<proto> is the transport protocol. The meaning of the transport
protocol is dependent on the address type field in the relevant
"c=" field. Thus a "c=" field of IP4 indicates that the transport
protocol runs over IP4. The following transport protocols are
defined, but may be extended through registration of new protocols
with IANA (see Section 8):
* udp: denotes an unspecified protocol running over UDP.
* RTP/AVP: denotes RTP [19] used under the RTP Profile for Audio
and Video Conferences with Minimal Control [20] running over
UDP.
* RTP/SAVP: denotes the Secure Real-time Transport Protocol [23]
running over UDP.
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 Pulse Code
Modulation (PCM) audio and RTP PCM audio; another might be TCP/RTP
PCM audio. In addition, relays and monitoring tools that are
transport-protocol-specific but format-independent are possible.
<fmt> is a media format description. The fourth and any subsequent
sub-fields describe the format of the media. The interpretation
of the media format depends on the value of the <proto> sub-field.
If the <proto> sub-field is "RTP/AVP" or "RTP/SAVP" the <fmt>
sub-fields contain RTP payload type numbers. When a list of
payload type numbers is given, this implies that all of these
payload formats MAY be used in the session, but the first of these
formats SHOULD be used as the default format for the session. For
dynamic payload type assignments the "a=rtpmap:" attribute (see
Section 6) SHOULD be used to map from an RTP payload type number
to a media encoding name that identifies the payload format. The
"a=fmtp:" attribute MAY be used to specify format parameters (see
Section 6).
If the <proto> sub-field is "udp" the <fmt> sub-fields MUST
reference a media type describing the format under the "audio",
"video", "text", "application", or "message" top-level media
types. The media type registration SHOULD define the packet
format for use with UDP transport.
For media using other transport protocols, the <fmt> field is
protocol specific. Rules for interpretation of the <fmt> sub-
field MUST be defined when registering new protocols (see Section
8.2.2).
6. SDP Attributes
The following attributes are defined. Since application writers may
add new attributes as they are required, this list is not exhaustive.
Registration procedures for new attributes are defined in Section
8.2.4.
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. There is no central registry of
categories. It is a session-level attribute, and it 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; there is no central registry of keywords. 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 it 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, but may be used with other media types if it makes
sense. 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-level
attribute, and it is not dependent on charset.
a=maxptime:<maximum packet time>
This gives the maximum amount of media that can be encapsulated
in each packet, expressed as time in milliseconds. The time
SHALL be calculated as the sum of the time the media present in
the packet represents. For frame-based codecs, the time SHOULD
be an integer multiple of the frame size. This attribute is
probably only meaningful for audio data, but may be used with
other media types if it makes sense. It is a media-level
attribute, and it is not dependent on charset. Note that this
attribute was introduced after RFC 2327, and non-updated
implementations will ignore this attribute.
a=rtpmap:<payload type> <encoding name>/<clock rate> [/<encoding
parameters>]
This attribute maps from an RTP payload type number (as used in
an "m=" line) to an encoding name denoting the payload format
to be used. It also provides information on the clock rate and
encoding parameters. It is a media-level attribute that is not
dependent on charset.
Although an RTP profile may make static assignments of payload
type numbers to payload formats, it is more common for that
assignment to be done dynamically using "a=rtpmap:" attributes.
As an example of a static payload type, consider u-law PCM
coded single-channel audio sampled at 8 kHz. This is
completely defined in the RTP Audio/Video profile as payload
type 0, so there is no need for an "a=rtpmap:" attribute, and
the media for such a stream sent to UDP port 49232 can be
specified as:
m=audio 49232 RTP/AVP 0
An example of a dynamic payload type is 16-bit linear encoded
stereo audio sampled at 16 kHz. If we wish to use the dynamic
RTP/AVP payload type 98 for this stream, additional information
is required to decode it:
m=audio 49232 RTP/AVP 98
a=rtpmap:98 L16/16000/2
Up to one rtpmap attribute can be defined for each media format
specified. Thus, we might have the following:
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.
The "RTP/AVP" and "RTP/SAVP" profiles use media subtypes for
encoding names, under the top-level media type denoted in the
"m=" line. In the example above, the media types are
"audio/l8" and "audio/l16".
For audio streams, <encoding parameters> indicates the number
of audio channels. This parameter is OPTIONAL and may be
omitted if the number of channels is one, provided that no
additional parameters are needed.
For video streams, no encoding parameters are currently
specified.
Additional encoding parameters MAY be defined in the future,
but codec-specific parameters SHOULD NOT be added. Parameters
added to an "a=rtpmap:" attribute SHOULD only be those required
for a session directory to make the choice of appropriate media
to participate in a session. Codec-specific parameters should
be added in other attributes (for example, "a=fmtp:").
Note: 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 above.
a=recvonly
This specifies that the tools should be started in receive-only
mode where applicable. It can be either a session- or media-
level attribute, and it is not dependent on charset. Note that
recvonly applies to the media only, not to any associated
control protocol (e.g., an RTP-based system in recvonly mode
SHOULD still send RTCP packets).
a=sendrecv
This specifies that the tools should be started in send and
receive mode. This is necessary for interactive conferences
with tools that default to receive-only mode. It can be either
a session or media-level attribute, and it is not dependent on
charset.
If none of the attributes "sendonly", "recvonly", "inactive",
and "sendrecv" is present, "sendrecv" SHOULD be assumed as the
default for sessions that are not of the conference type
"broadcast" or "H332" (see below).
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 used, one
sendonly and one recvonly. It can be either a session- or
media-level attribute, but would normally only be used as a
media attribute. It is not dependent on charset. Note that
sendonly applies only to the media, and any associated control
protocol (e.g., RTCP) SHOULD still be received and processed as
normal.
a=inactive
This specifies that the tools should be started in inactive
mode. This is necessary for interactive conferences where
users can put other users on hold. No media is sent over an
inactive media stream. Note that an RTP-based system SHOULD
still send RTCP, even if started inactive. It can be either a
session or media-level attribute, and it is not dependent on
charset.
a=orient:<orientation>
Normally this is only used for a whiteboard or presentation
tool. It specifies the orientation of a the workspace on the
screen. It is a media-level 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 an H.332 session as defined
in the ITU H.332 specification [26]. 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 it 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.
For example, the ISO 8859-1 is specified with the following SDP
attribute:
a=charset:ISO-8859-1
This is a session-level attribute and is not dependent on
charset. 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 octet 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 from 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 is 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 RFC 3066
language tag in US-ASCII [9]. 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
the session description or media use multiple languages, in
which case the order of the attributes indicates the order of
importance of the various languages in the session or media
from most important to least important.
The "lang" attribute value must be a single RFC 3066 language
tag in US-ASCII [9]. 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-level
attribute, defined only for video media, and it 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 is 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-level attribute, and it 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 does not
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. At most
one instance of this attribute is allowed for each format.
It is a media-level attribute, and it is not dependent on
charset.
7. Security Considerations
SDP is frequently used with the Session Initiation Protocol [15]
using the offer/answer model [17] to agree on parameters for unicast
sessions. When used in this manner, the security considerations of
those protocols apply.
SDP is a session description format that describes multimedia
sessions. Entities receiving and acting upon an SDP message SHOULD
be aware that a session description cannot be trusted unless it has
been obtained by an authenticated transport protocol from a known and
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. For some transports,
security features are often not deployed. In case a session
description has not been obtained in a trusted manner, the endpoint
SHOULD exercise care because, among other attacks, the media sessions
received may not be the intended ones, the destination where media is
sent to may not be the expected one, any of the parameters of the
session may be incorrect, or the media security may be compromised.
It is up to the endpoint to make a sensible decision taking into
account the security risks of the application and the user
preferences and may decide to ask the user whether or not to accept
the session.
One transport that can 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 the originator is 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 descriptions contain
information required to start software on the receiver's 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
the user's consent. Thus, a session description arriving by session
announcement, email, session invitation, or WWW page MUST NOT deliver
the user into an interactive multimedia session unless the user has
explicitly pre-authorised such action. As it is not always simple to
tell whether or not a session is interactive, applications that are
unsure should assume sessions are interactive.
In this specification, there are no attributes that 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.
In certain environments, it has become common for intermediary
systems to intercept and analyse session descriptions contained
within other signalling protocols. This is done for a range of
purposes, including but not limited to opening holes in firewalls to
allow media streams to pass, or to mark, prioritize, or block traffic
selectively. In some cases, such intermediary systems may modify the
session description, for example, to have the contents of the session
description match NAT bindings dynamically created. These behaviours
are NOT RECOMMENDED unless the session description is conveyed in
such a manner that allows the intermediary system to conduct proper
checks to establish the authenticity of the session description, and
the authority of its source to establish such communication sessions.
SDP by itself does not include sufficient information to enable these
checks: they depend on the encapsulating protocol (e.g., SIP or
RTSP).
Use of the "k=" field poses a significant security risk, since it
conveys session encryption keys in the clear. SDP MUST NOT be used
to convey key material, unless it can be guaranteed that the channel
over which the SDP is delivered is both private and authenticated.
Moreover, the "k=" line provides no way to indicate or negotiate
cryptographic key algorithms. As it provides for only a single
symmetric key, rather than separate keys for confidentiality and
integrity, its utility is severely limited. The use of the "k=" line
is NOT RECOMMENDED, as discussed in Section 5.12.
8. IANA Considerations
8.1. The "application/sdp" Media Type
One media type registration from RFC 2327 is to be updated, as
defined below.
To: ietf-types@iana.org
Subject: Registration of media type "application/sdp"
Type name: application
Subtype name: sdp
Required parameters: None.
Optional parameters: None.
Encoding considerations:
SDP files are primarily UTF-8 format text. The "a=charset:"
attribute may be used to signal the presence of other
character sets in certain parts of an SDP file (see
Section 6 of RFC 4566). Arbitrary binary content cannot
be directly represented in SDP.
Security considerations:
See Section 7 of RFC 4566
Interoperability considerations:
See RFC 4566
Published specification:
See RFC 4566
Applications which use this media type:
Voice over IP, video teleconferencing, streaming media, instant
messaging, among others. See also Section 3 of RFC 4566.
Additional information:
Magic number(s): None.
File extension(s): The extension ".sdp" is commonly used.
Macintosh File Type Code(s): "sdp "
Person & email address to contact for further information:
Mark Handley <M.Handley@cs.ucl.ac.uk>
Colin Perkins <csp@csperkins.org>
IETF MMUSIC working group <mmusic@ietf.org>
Intended usage: COMMON
Author/Change controller:
Authors of RFC 4566
IETF MMUSIC working group delegated from the IESG
8.2. Registration of Parameters
There are seven field names that may be registered with IANA. Using
the terminology in the SDP specification Backus-Naur Form (BNF), they
are "media", "proto", "fmt", "att-field", "bwtype", "nettype", and
"addrtype".
8.2.1. Media Types ("media")
The set of media types is intended to be small and SHOULD NOT be
extended except under rare circumstances. The same rules should
apply for media names as for top-level media content types, and where
possible the same name should be registered for SDP as for MIME. For
media other than existing top-level media content types, a Standards
Track RFC MUST be produced for a new top-level content type to be
registered, and the registration MUST provide good justification why
no existing media name is appropriate (the "Standards Action" policy
of RFC 2434 [8].
This memo registers the media types "audio", "video", "text",
"application", and "message".
Note: The media types "control" and "data" were listed as valid in
the previous version of this specification [6]; however, their
semantics were never fully specified and they are not widely used.
These media types have been removed in this specification, although
they still remain valid media type capabilities for a SIP user agent
as defined in RFC 3840 [24]. If these media types are considered
useful in the future, a Standards Track RFC MUST be produced to
document their use. Until that is done, applications SHOULD NOT use
these types and SHOULD NOT declare support for them in SIP
capabilities declarations (even though they exist in the registry
created by RFC 3840).
8.2.2. Transport Protocols ("proto")
The "proto" field describes the transport protocol used. This SHOULD
reference a standards-track protocol RFC. This memo registers three
values: "RTP/AVP" is a reference to RTP [19] used under the RTP
Profile for Audio and Video Conferences with Minimal Control [20]
running over UDP/IP, "RTP/SAVP" is a reference to the Secure Real-
time Transport Protocol [23], and "udp" indicates an unspecified
protocol over UDP.
If other RTP profiles are defined in the future, their "proto" name
SHOULD be specified in the same manner. For example, an RTP profile
whose short name is "XYZ" would be denoted by a "proto" field of
"RTP/XYZ".
New transport protocols SHOULD be registered with IANA.
Registrations MUST reference an RFC describing the protocol. Such an
RFC MAY be Experimental or Informational, although it is preferable
that it be Standards Track. Registrations MUST also define the rules
by which their "fmt" namespace is managed (see below).
8.2.3. Media Formats ("fmt")
Each transport protocol, defined by the "proto" field, has an
associated "fmt" namespace that describes the media formats that may
be conveyed by that protocol. Formats cover all the possible
encodings that might want to be transported in a multimedia session.
RTP payload formats under the "RTP/AVP" and "RTP/SAVP" profiles MUST
use the payload type number as their "fmt" value. If the payload
type number is dynamically assigned by this session description, an
additional "rtpmap" attribute MUST be included to specify the format
name and parameters as defined by the media type registration for the
payload format. It is RECOMMENDED that other RTP profiles that are
registered (in combination with RTP) as SDP transport protocols
specify the same rules for the "fmt" namespace.
For the "udp" protocol, new formats SHOULD be registered. Use of an
existing media subtype for the format is encouraged. If no media
subtype exists, it is RECOMMENDED that a suitable one be registered
through the IETF process [31] by production of, or reference to, a
standards-track RFC that defines the transport protocol for the
format.
For other protocols, formats MAY be registered according to the rules
of the associated "proto" specification.
Registrations of new formats MUST specify which transport protocols
they apply to.
8.2.4. Attribute Names ("att-field")
Attribute field names ("att-field") MUST be registered with IANA and
documented, because of noticeable issues due to conflicting
attributes under the same name. Unknown attributes in SDP are simply
ignored, but conflicting ones that fragment the protocol are a
serious problem.
New attribute registrations are accepted according to the
"Specification Required" policy of RFC 2434, provided that the
specification includes the following information:
o contact name, email address, and telephone number
o attribute name (as it will appear in SDP)
o long-form attribute name in English
o type of attribute (session level, media level, or both)
o whether the attribute value is subject to the charset attribute
o a one-paragraph explanation of the purpose of the attribute
o a specification of appropriate attribute values for this attribute
The above is the minimum that IANA will accept. Attributes that are
expected to see widespread use and interoperability SHOULD be
documented with a standards-track RFC that specifies the attribute
more precisely.
Submitters of registrations should ensure that the specification is
in the spirit of SDP attributes, most notably that the attribute is
platform independent in the sense that it makes no implicit
assumptions about operating systems and does not name specific pieces
of software in a manner that might inhibit interoperability.
IANA has registered the following initial set of attribute names
("att-field" values), with definitions as in Section 6 of this memo
(these definitions update those in RFC 2327):
Name | Session or Media level? | Dependent on charset?
----------+-------------------------+----------------------
cat | Session | No
keywds | Session | Yes
tool | Session | No
ptime | Media | No
maxptime | Media | No
rtpmap | Media | No
recvonly | Either | No
sendrecv | Either | No
sendonly | Either | No
inactive | Either | No
orient | Media | No
type | Session | No
charset | Session | No
sdplang | Either | No
lang | Either | No
framerate | Media | No
quality | Media | No
fmtp | Media | No
8.2.5. Bandwidth Specifiers ("bwtype")
A proliferation of bandwidth specifiers is strongly discouraged.
New bandwidth specifiers ("bwtype" fields) MUST be registered with
IANA. The submission MUST reference a standards-track RFC specifying
the semantics of the bandwidth specifier precisely, and indicating
when it should be used, and why the existing registered bandwidth
specifiers do not suffice.
IANA has registered the bandwidth specifiers "CT" and "AS" with
definitions as in Section 5.8 of this memo (these definitions update
those in RFC 2327).
8.2.6. Network Types ("nettype")
New network types (the "nettype" field) may be registered with IANA
if SDP needs to be used in the context of non-Internet environments.
Although these are not normally the preserve of IANA, there may be
circumstances when an Internet application needs to interoperate with
a non-Internet application, such as when gatewaying an Internet
telephone call into the Public Switched Telephone Network (PSTN).
The number of network types should be small and should be rarely
extended. A new network type cannot be registered without
registering at least one address type to be used with that network
type. A new network type registration MUST reference an RFC that
gives details of the network type and address type and specifies how
and when they would be used.
IANA has registered the network type "IN" to represent the Internet,
with definition as in Sections 5.2 and 5.7 of this memo (these
definitions update those in RFC 2327).
8.2.7. Address Types ("addrtype")
New address types ("addrtype") may be registered with IANA. An
address type is only meaningful in the context of a network type, and
any registration of an address type MUST specify a registered network
type or be submitted along with a network type registration. A new
address type registration MUST reference an RFC giving details of the
syntax of the address type. Address types are not expected to be
registered frequently.
IANA has registered the address types "IP4" and "IP6" with
definitions as in Sections 5.2 and 5.7 of this memo (these
definitions update those in RFC 2327).
8.2.8. Registration Procedure
In the RFC documentation that registers SDP "media", "proto", "fmt",
"bwtype", "nettype", and "addrtype" fields, the authors MUST include
the following information for IANA to place in the appropriate
registry:
o contact name, email address, and telephone number
o name being registered (as it will appear in SDP)
o long-form name in English
o type of name ("media", "proto", "fmt", "bwtype", "nettype", or
"addrtype")
o a one-paragraph explanation of the purpose of the registered name
o a reference to the specification for the registered name (this
will typically be an RFC number)
IANA may refer any registration to the IESG for review, and may
request revisions to be made before a registration will be made.
8.3. Encryption Key Access Methods
The IANA previously maintained a table of SDP encryption key access
method ("enckey") names. This table is obsolete, since the "k=" line
is not extensible. New registrations MUST NOT be accepted.
9. SDP Grammar
This section provides an Augmented BNF grammar for SDP. ABNF is
defined in [4].
; SDP Syntax
session-description = 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 = %x76 "=" 1*DIGIT CRLF
;this memo describes version 0
origin-field = %x6f "=" username SP sess-id SP sess-version SP
nettype SP addrtype SP unicast-address CRLF
session-name-field = %x73 "=" text CRLF
information-field = [%x69 "=" text CRLF]
uri-field = [%x75 "=" uri CRLF]
email-fields = *(%x65 "=" email-address CRLF)
phone-fields = *(%x70 "=" phone-number CRLF)
connection-field = [%x63 "=" nettype SP addrtype SP
connection-address CRLF]
;a connection field must be present
;in every media description or at the
;session-level
bandwidth-fields = *(%x62 "=" bwtype ":" bandwidth CRLF)
time-fields = 1*( %x74 "=" start-time SP stop-time
*(CRLF repeat-fields) CRLF)
[zone-adjustments CRLF]
repeat-fields = %x72 "=" repeat-interval SP typed-time
1*(SP typed-time)
zone-adjustments = %x7a "=" time SP ["-"] typed-time
*(SP time SP ["-"] typed-time)
key-field = [%x6b "=" key-type CRLF]
attribute-fields = *(%x61 "=" attribute CRLF)
media-descriptions = *( media-field
information-field
*connection-field
bandwidth-fields
key-field
attribute-fields )
media-field = %x6d "=" media SP port ["/" integer]
SP proto 1*(SP fmt) CRLF
; sub-rules of 'o='
username = non-ws-string
;pretty wide definition, but doesn't
;include space
sess-id = 1*DIGIT
;should be unique for this username/host
sess-version = 1*DIGIT
nettype = token
;typically "IN"
addrtype = token
;typically "IP4" or "IP6"
; sub-rules of 'u='
uri = URI-reference
; see RFC 3986
; sub-rules of 'e=', see RFC 2822 for definitions
email-address = address-and-comment / dispname-and-address
/ addr-spec
address-and-comment = addr-spec 1*SP "(" 1*email-safe ")"
dispname-and-address = 1*email-safe 1*SP "<" addr-spec ">"
; sub-rules of 'p='
phone-number = phone *SP "(" 1*email-safe ")" /
1*email-safe "<" phone ">" /
phone
phone = ["+"] DIGIT 1*(SP / "-" / DIGIT)
; sub-rules of 'c='
connection-address = multicast-address / unicast-address
; sub-rules of 'b='
bwtype = token
bandwidth = 1*DIGIT
; sub-rules of 't='
start-time = time / "0"
stop-time = time / "0"
time = POS-DIGIT 9*DIGIT
; Decimal representation of NTP time in
; seconds since 1900. The representation
; of NTP time is an unbounded length field
; containing at least 10 digits. Unlike the
; 64-bit representation used elsewhere, time
; in SDP does not wrap in the year 2036.
; sub-rules of 'r=' and 'z='
repeat-interval = POS-DIGIT *DIGIT [fixed-len-time-unit]
typed-time = 1*DIGIT [fixed-len-time-unit]
fixed-len-time-unit = %x64 / %x68 / %x6d / %x73
; sub-rules of 'k='
key-type = %x70 %x72 %x6f %x6d %x70 %x74 / ; "prompt"
%x63 %x6c %x65 %x61 %x72 ":" text / ; "clear:"
%x62 %x61 %x73 %x65 "64:" base64 / ; "base64:"
%x75 %x72 %x69 ":" uri ; "uri:"
base64 = *base64-unit [base64-pad]
base64-unit = 4base64-char
base64-pad = 2base64-char "==" / 3base64-char "="
base64-char = ALPHA / DIGIT / "+" / "/"
; sub-rules of 'a='
attribute = (att-field ":" att-value) / att-field
att-field = token
att-value = byte-string
; sub-rules of 'm='
media = token
;typically "audio", "video", "text", or
;"application"
fmt = token
;typically an RTP payload type for audio
;and video media
proto = token *("/" token)
;typically "RTP/AVP" or "udp"
port = 1*DIGIT
; generic sub-rules: addressing
unicast-address = IP4-address / IP6-address / FQDN / extn-addr
multicast-address = IP4-multicast / IP6-multicast / FQDN
/ extn-addr
IP4-multicast = m1 3( "." decimal-uchar )
"/" ttl [ "/" integer ]
; IPv4 multicast addresses may be in the
; range 224.0.0.0 to 239.255.255.255
m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) /
("23" DIGIT )
IP6-multicast = hexpart [ "/" integer ]
; IPv6 address starting with FF
ttl = (POS-DIGIT *2DIGIT) / "0"
FQDN = 4*(alpha-numeric / "-" / ".")
; fully qualified domain name as specified
; in RFC 1035 (and updates)
IP4-address = b1 3("." decimal-uchar)
b1 = decimal-uchar
; less than "224"
; The following is consistent with RFC 2373 [30], Appendix B.
IP6-address = hexpart [ ":" IP4-address ]
hexpart = hexseq / hexseq "::" [ hexseq ] /
"::" [ hexseq ]
hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG
; Generic for other address families
extn-addr = non-ws-string
; generic sub-rules: datatypes
text = byte-string
;default is to interpret this as UTF8 text.
;ISO 8859-1 requires "a=charset:ISO-8859-1"
;session-level attribute to be used
byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF)
;any byte except NUL, CR, or LF
non-ws-string = 1*(VCHAR/%x80-FF)
;string of visible characters
token-char = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39
/ %x41-5A / %x5E-7E
token = 1*(token-char)
email-safe = %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF
;any byte except NUL, CR, LF, or the quoting
;characters ()<>
integer = POS-DIGIT *DIGIT
; generic sub-rules: primitives
alpha-numeric = ALPHA / DIGIT
POS-DIGIT = %x31-39 ; 1 - 9
decimal-uchar = DIGIT
/ POS-DIGIT DIGIT
/ ("1" 2*(DIGIT))
/ ("2" ("0"/"1"/"2"/"3"/"4") DIGIT)
/ ("2" "5" ("0"/"1"/"2"/"3"/"4"/"5"))
; external references:
; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 4234
; URI-reference: from RFC 3986
; addr-spec: from RFC 2822
10. Summary of Changes from RFC 2327
The memo has been significantly restructured, incorporating a large
number of clarifications to the specification in light of use. With
the exception of those items noted below, the changes to the memo are
intended to be backward-compatible clarifications. However, due to
inconsistencies and unclear definitions in RFC 2327 it is likely that
some implementations interpreted that memo in ways that differ from
this version of SDP.
The ABNF grammar in Section 9 has been extensively revised and
updated, correcting a number of mistakes and incorporating the RFC
3266 IPv6 extensions. Known inconsistencies between the grammar and
the specification text have been resolved.
A media type registration for SDP is included. Requirements for the
registration of attributes and other parameters with IANA have been
clarified and tightened (Section 8). It is noted that "text" and
"message" are valid media types for use with SDP, but that "control"
and "data" are under-specified and deprecated.
RFC 2119 terms are now used throughout to specify requirements
levels. Certain of those requirements, in particular in relation to
parameter registration, are stricter than those in RFC 2327.
The "RTP/SAVP" RTP profile and its "fmt" namespace are registered.
The attributes "a=inactive" and "a=maxptime" have been added.
RFC 2327 mandated that either "e=" or "p=" was required. Both are
now optional, to reflect actual usage.
The significant limitations of the "k=" field are noted, and its use
is deprecated.
Most uses of the "x-" prefix notation for experimental parameters are
disallowed and the other uses are deprecated.
11. Acknowledgements
Many people in the IETF Multiparty Multimedia Session Control
(MMUSIC) working group have made comments and suggestions
contributing to this document. In particular, we would like to thank
Eve Schooler, Steve Casner, Bill Fenner, Allison Mankin, Ross
Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Steve Hanna,
Jonathan Lennox, Keith Drage, Sean Olson, Bernie Hoeneisen, Jonathan
Rosenberg, John Elwell, Flemming Andreasen, Jon Peterson, and Spencer
Dawkins.
12. References
12.1. Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[5] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD
63, RFC 3629, November 2003.
[6] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[7] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[8] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[9] Alvestrand, H., "Tags for the Identification of Languages", BCP
47, RFC 3066, January 2001.
[10] Olson, S., Camarillo, G., and A. Roach, "Support for IPv6 in
Session Description Protocol (SDP)", RFC 3266, June 2002.
[11] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)", RFC
3490, March 2003.
[12] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
12.2. Informative References
[13] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC 1305, March 1992.
[14] Handley, M., Perkins, C., and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[15] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[16] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[17] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[18] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
"Grouping of Media Lines in the Session Description Protocol
(SDP)", RFC 3388, December 2002.
[19] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[20] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[21] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
July 2003.
[22] Huitema, C., "Real Time Control Protocol (RTCP) attribute in
Session Description Protocol (SDP)", RFC 3605, October 2003.
[23] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
[24] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[25] Westerlund, M., "A Transport Independent Bandwidth Modifier for
the Session Description Protocol (SDP)", RFC 3890, September
2004.
[26] International Telecommunication Union, "H.323 extended for
loosely coupled conferences", ITU Recommendation H.332,
September 1998.
[27] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP)", RFC
4567, July 2006.
[28] Andreasen, F., Baugher, M., and D. Wing, "Session Description
Protocol (SDP) Security Descriptions for Media Streams", RFC
4568, July 2006.
[29] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[30] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[31] Freed, N. and J. Klensin, "Media Type Specifications and
Registration Procedures", BCP 13, RFC 4288, December 2005.
Authors' Addresses
Mark Handley
University College London
Department of Computer Science
Gower Street
London WC1E 6BT
UK
EMail: M.Handley@cs.ucl.ac.uk
Van Jacobson
Packet Design
2465 Latham Street
Mountain View, CA 94040
USA
EMail: van@packetdesign.com
Colin Perkins
University of Glasgow
Department of Computing Science
17 Lilybank Gardens
Glasgow G12 8QQ
UK
EMail: csp@csperkins.org
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