One document matched: draft-ietf-mmusic-sdp-01.txt
Differences from draft-ietf-mmusic-sdp-00.txt
Internet Engineering Task Force MMUSIC WG
INTERNET-DRAFT Mark Handley/Van Jacobson
draft-ietf-mmusic-sdp-01.ps UCL/LBL
22nd Nov 1995
Expires: 22nd May 1995
SDP: Session Description Protocol
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its areas, and its
working groups. Note that other groups may also distribute working
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ftp.isi.edu (US West Coast).
Distribution of this document is unlimited.
Abstract
The sd session directory tool has been in use for some time on
the Mbone for announcing multicast sessions. This document
describes an enhanced version of the sd protocol (SDP v2), and
explains the extensions to the protocol that have become
desirable.
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
The LBL session directory tool (sd) has been in use on the Mbone for
some time to advertise multimedia conferences and communicate the
conference addresses and conference tool specific information necessary
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for participation. This document defines an extended version of the
session directory protocol and some extensions to the protocol that have
become desirable. In the body of the paper, we describe a proposed Ses-
sion Description Protocol (SDP v2). In Appendix A, we describe how this
differs from the protocol currently used by sd (SDP v1). This draft
does not describe multicast address allocation or the distribution of
SDP messages in detail - these are left to accompanying drafts.
2. Background
The Multicast Backbone (Mbone) is an experimental overlay network on the
internet that permits efficient many to many communication. For the
past few years it has been used extensively for multimedia conferencing.
Such multimedia conferences usually have the property that tight coordi-
nation of conference membership is not necessary; in order to receive a
conference, a user at an Mbone site has to know only the correct multi-
cast group address for the conference and the UDP ports the conferencing
applications will use to receive the conference data streams.
In order to assist the advertisement of conference sessions and to com-
municate the relevant conference setup information to prospective parti-
cipants, the session directory (sd) tool was written. Sd has now been
in wide scale use for close to 2 years, during which time Mbone usage
has greatly increased and diversified. The Mbone has now reached the
stage where assistance with coordination of resource usage is required,
and where compatible session announcement tools are starting to emerge.
This document is an attempt to prevent diversification of the sd proto-
col as tool writers each add their own modifications. It is also an
attempt to provide guidelines to the writers of such announcement tools
in order to protect the Mbone from misuse and to preserve the inherent
scalability of the original sd program whilst enhancing its functional-
ity.
In defining SDP v2, we also aim to enhance to generality of SDP so that
it can be used for a wider range of network environments and applica-
tions.
3. The Use of SDP (background)
3.1. Multicast Announcement
SDP is a session description protocol for multimedia sessions. It is
normally used by an SDP client which announces a conference session by
periodically multicasting an announcement packet on a well known multi-
cast address and port. With the advent of administrative scoping in the
Mbone, it is likely that sd clients will need to be able to listen for
such announcements on multiple addresses. The Session Directory
Announcement Protocol is described in more detail in a companion draft.
Sd packets are UDP packets of the following format:
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0 31
|____________________|
| SDAP header |
|____________________|
| text payload |
|/\/\/\/\/\/\/\/\/\/\|
The first four bytes are Session Directory Announcement Protocol (SDAP)
header.
The text payload is an SDP session description, as described in this
draft. The text payload should be no greater than 1 Kbyte in length.
If announced by SDAP, only one session annoucement is permitted in a
single packet.
3.2. SDP announcement by email and WWW
It should be noted that announcements of multicast sessions made only
via email or the World Wide Web (WWW) do not have property that the
receiver of a session announcement can receive the session, nor do they
provide Mbone booking feedback or allow scalable dynamic multicast
address allocation, and so should normally be used to supplement
periodic multicast announcements.
For both email and WWW distribution, the use of the MIME content type
``application/x-sd'' is suggested. This enables the automatic launching
of applications from the WWW client or mail reader in a standard manner.
4. Requirements
The purpose of SDP is to convey information about media streams in mul-
timedia 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 a duration of time. Media streams can be many-
to-many. The times during which the session is active need not be con-
tinuous.
Multicast based sessions on the internet differ 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 - as a means to communicate the existence of
a session, and as 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.
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Thus the information SDP must convey includes:
o Name and purpose of session
o Time(s) the session is active
o The media comprising the session
o Information to receive those media
As resources (such as bandwidth) necessary to participate in a session
may be limited, some additional information is also desirable:
o Contact information for the person responsible for the session
o Information about the bandwidth to be used by the conference
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.
4.1. Media Information
The information that must be conveyed is:
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 an IP multicast session, the following must also be conveyed:
o Multicast address for media
o Transport Port for media
In an IP unicast session, the following must be conveyed:
o Contact address for media
o Transport port for contact address
This may or may not be be the source and destination of the media
stream.
Sessions being conveyed over other networks will have their own specific
requirements - SDP must be extensible for these.
4.2. Timing Information
Sessions may either be bounded in time, or they may be unbounded.
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Whether or not they are bounded, they may be only active at specific
times.
SDP must be able to 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"
o These times must be globally consistent, irrespective of local
time zone or daylight saving time
4.3. Private Sessions
It should be possible to create both public sessions and private ses-
sions. However, private sessions on the existing Mbone infrastructure
rapidly use up the available bandwidth. It should be possible to create
private sessions along with contact information if those sessions become
a problem.
If a session announcement is private (encrypted) it should be 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. Further Information
SDP should convey enough information to decide whether a session is the
session a user wishes to participate in. It should also convey where to
go to find more information about the session. This extra information
should be in the form of Universal Resources Identifiers (URIs).
4.5. Categorisation
When many session descriptions are being conveyed by SDAP or any other
advertisement mechanism, it is important to be able to filter session
announcements that are of interest from those that are not. SDP should
support a categorisation mechanism for sessions that can be automated.
4.6. Internationalization
The SDP specification recommends the use of 8 bit ISO 8859-1 character
sets to allow the extended ASCII characters used by many western and
northern European languages to be represented. However, there are many
languages that cannot be represented in an ISO 8859-1 character set.
SDP should also allow extensions to allow other font types to be used
when required.
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5. SDP Specification
SDP session descriptions are entirely textual. The textual form, as
opposed to a binary encoding such as ASN/1 or XDR, was chosen to enhance
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 process session
descriptions. However, since the total bandwidth allocated to all SDAP
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 likely
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 takes the form of a number of lines of text
of the form
<type>=<value>
<type> is always exactly one character and case is significant. <value>
is a structured text string whose format depends on <type>. 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 free
format string.
Each announcement consists of a session description section followed by
zero or more `media' description sections. The session description
starts with an `v=' line and continues to the first media description or
the next session description. The media description starts with an `m='
line and continues to the next media description or session description.
When SDP is conveyed by SDAP, only one session description is allowed in
each packet. When SDP is conveyed by other means, many SDP session
descriptions may be carried together. 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 `*'.)
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Session description
v= (protocol version)
o= (owner/creator and session identifier).
s= (session name)
i=* (session information)
u=* (URL of description)
e=* (email address)
p=* (phone number)
c= (connection information)
b=* (bandwidth information)
t=* (zero or more times)
k=* (encryption key)
a=* (zero or more session attribute lines)
Media description
m= (media name and transport address)
i=* (media title)
c=* (connection information)
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 (described below) is the primary means for extending sd 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
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. In the example below, each media behaves as if it were
given a `recvonly' attribute.
An example SDP v2 description is:
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=Sd Seminar
i=A Seminar on the session description protocol
u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.01.ps
e=M.Handley@cs.ucl.ac.uk (Mark Handley)
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 3456 VAT PCMU
m=video 2232 RTP H261
m=whiteboard 32416 UDP WB
a=orient:portrait
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Text records such as the session name and information may contain any
printable 8 bit ISO 8859-1 character with the exceptions of 0x0a (new-
line) and 0x0d (carriage return). Carriage Return is prohibited, and
Newline is used to end a record.
SDP version 1 is not compatible with SDP version 2 described below,
although it is similar. The differences between SDP v1 and SDP v2 are
described in Appendix A.
Protocol Version
v=0
The ``v'' field gives the version of the Session Description Protocol.
As SDP v1 had no version number, we begin numbering with SDP v2 as pro-
tocol version 0. 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 their 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. <session
id> is a numeric string such that the triple of <username>, <session id>
and <address> form a globally unique identifier for the session. Its
method of allocation is up to the creating tool, but it has been sug-
gested that a Network Time Protocol (NTP, [1]) timestamp be used to
ensure uniqueness. <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 has been suggested
(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 that the
session was created from. For an address type of IP4, this is the
dotted-decimal representation of the IP version 4 address of the
machine.
Session Name
s=<session name>
The ``s'' field is the session name. There must be one and only one
``s'' field per announcement, and it must contain printable ISO 8859-1
characters (but see also the `charset' attribute below).
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Information
i=<session description>
The ``i'' field is information about the session. There must be no more
than one ``i'' field per session announcement. Although it may be omit-
ted, this is discouraged, and user interfaces for composing sessions
should require text to be entered. If it is present it must contain
printable ISO 8859-1 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. Addi-
tional 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.
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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
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=M.Handley@cs.ucl.ac.uk (Mark Handley)
The alternative RFC822 name quoting convention is also allowed for
both email addresses and phone numbers. For example,
e=Mark Handley <M.Handley@cs.ucl.ac.uk>
The free text string should be in an IS0-8859-1 character set, or
alternatively in unicode UTF-7 encoding if the appropriate charset
conference attribute is set.
Connection Data
c=<network type> <address type> <connection address>
The ``c'' field contains connection data.
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 sub-
fields 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 conference is not multicast, then the connec-
tion address contains the unicast IP address of the expected data
source or data relay or data sink as determined by additional attri-
bute fields. It is not expected that unicast addresses will be
given in a session description that is communicated by a multicast
announcement.
Conferences using a IP multicast connection address must also have a
TTL (time to live) value present in addition to the multicast
address. The TTL defines the scope with which multicast packets
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sent in this conference should be sent. TTL values must be in the
range 0-255. The Mbone usage guidelines (currently available at
ftp://ftp.isi.edu/mbone/faq.txt) define several standard settings
for ttl:
local net: 1
site: 15
region: 63
world: 127
Other settings may have local meaning (e.g., 47 for all sites within
an organization).
The TTL to be used for the session must be appended to the address
using a slash as a separator. An example is:
c=IN IP4 224.2.1.1/127
The RSVP WG of the IETF has defined that hierarchical encoding
schemes should be transmitted in multiple multicast groups to allow
multicast pruning to keep unwanted traffic from sites only requiring
some levels of the hierarchy. For applications which require multi-
ple 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.
It is illegal for the slash notation described above for either ttl
or number of addresses to be used for IP unicast addresses.
A session announcement must contain at least one ``c'' field. It
may contain one additional ``c'' field per media field (see below),
in which case the per-media values override the conference-wide set-
tings for the relevant media.
Bandwidth
b=<modifier>:<bandwidth-value>
o This specifies the proposed bandwidth to be used by the session or
media, and is optional.
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o bandwidth is in kilobits per second
o modifier is an 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 administra-
tive scope region (the Mbone bandwidth vs. ttl limits are given in
the mbone faq). If the bandwidth of a session or media in a ses-
sion is different from the bandwidth implicit in the ttl, A
`b=CT:...' line should be supplied for the session giving the pro-
posed 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: The bandwidth is interpreted to be applica-
tion specific, i.e., will be the application's concept of maximum
bandwidth. Normally this will coincide with what is set on the
applications ``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
___________________________________________________________________________
| It is unclear who (if anyone) should be the registry for bandwidth |
|_________________________________________________________________________|
Times, Repeat Times and Time Zones
t=<start time> <stop time>
o ``t'' fields specify the start and stop times for a conference ses-
sion. Multiple ``t'' fields may be used if a session is active at
multiple irregularly spaced times; each additional ``t'' field
specifies an addition period of time that the session will be active
for. If the session is active at regular times, an ``r'' field
should be used in addition to 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 represen-
tation of Network Time Protocol (NTP, [1]) time values in seconds.
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To convert these values to UNIX time, subtract decimal 2208988800.
If these values are both set to zero, then the conference is not
bounded. User interfaces should prohibit or strongly discourage the
creation of unbounded conferences as they give no information about
when the session is actually going to be active. It is prohibited
for the start time to be after the stop time.
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'' field would be:
r=604800 3600 0 90000
To make announcements more compact, times may also be given in units
of days, hours or minutes. To allow yearly or monthly announcements
(same day each year or month), units of years and months are also
allowed. 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 specifica-
tion characters are allowed:
Y - years (same day of same month each repeated year)
M - months (same day of the month each repeated month)
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 have been written:
r=1d 1h 0 25h
z=<adjustment time> <offset> <adjustment time> <offset> ....
Should it be necessary to schedule a repeated session which spans a
change from daylight time to standard time or vice-versa, it is
necessary to specify offsets from the base repeat times. This is
necessary because different time zones change time at different
times of day, because different countries change to or from day-
light time on different dates, and because some countries to 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
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zone adjustment happens and the offset from the time when the ses-
sion 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
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=<encryption key>
o In countries where encrypted sessions are not prohibited by law,
the session description protocol may be use to convey encryption
keys.
o A key field is permitted before the first media entry, or for each
media entry as required.
o The format of keys and their usage is outside the scope of this
document, but see [4]
Attributes
a=<flag>
a=<attribute>:<value>
A media field may also have any number of attributes (``a'' fields)
which are media specific. Attribute fields may be of two forms:
o flag attributes. A flag attribute is simply of the form
``a=<flag>''. These are binary attributes, and the presence of the
attribute conveys that the attribute is ``true''.
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 sd session descriptions should be configurable in their
interpretation of announcements in general and of attributes in particu-
lar.
Attribute fields (``a'' fields) can also be added before the first media
field. These attributes would convey additional information that
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applies to the conference as a whole rather than to individual media.
An example might be the conference's floor control policy.
Media Announcements
m=<media> <port> <transport> <fmt>
A session announcement may contain a number of media announcements.
Each media announcement starts with an ``m'' field, and is terminated by
either the next ``m'' field or by the end of the session announcement.
A media field also has several sub-fields:
o The first sub-field is the media type. Currently defined media are
``audio'', ``video'', ``whiteboard'' and ``text'', though this list
may be extended as new communication modalities emerge (e.g.,
telepresense or conference control).
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.
For transports based on UDP, the value should be in the range 1024
to 65535 inclusive. For RTPv2 compliance it should be an even
number. If the port is allocated randomly by the creating applica-
tion, it is recommended that ports above 5000 are chosen as, on Unix
systems, ports below 5000 may be allocated automatically by the
operating system.
For applications where hierarchically encoded streams are being send
to a unicast address, it may be necessary to specify multiple tran-
sport ports. This is done using a similar notation to that used for
IP multicast addresses in the ``c'' field:
m=<media> <port>/<number of ports> <transport> <fmt>
In such a case, the ports used depend on the transport protocol.
For RTPv2, only the even ports are used for data and the correspond-
ing one-higher odd port is used for RTCP. For example:
m=video 3456/2 RTP H261
would specify that ports 3456 and 3457 form one RTP/RTCP pair and
3458 and 3459 form the second RTP/RTCP pair.
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 announcement.
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o The third sub-field is the transport protocol. The transport pro-
tocol values are dependent on the address-type field in the ``c''
fields. Thus a ``c'' field of IP4 defines that the transport proto-
col runs over IP4. For IP4, it is normally expected that most
media traffic will be carried as RTP over UDP. However, some com-
monly used applications such as vat [5] do not use RTP. Thus the
following transport protocols are defined:
- RTP - the IETF's Realtime Transport Protocol carried over UDP.
- VAT - LBL's Visual Audio Tool packet format carried over UDP.
- UDP - User Datagram Protocol
If an application uses a propriety media format and transport proto-
col over UDP, then simply specifying the transport protocol as UDP
is recommended.
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 - for example vat PCM audio and RTP PCM audio.
o The fourth sub-field is the media format. For audio and video,
this will normally be a media format string as defined in the RTP
Audio/Video Profile.
Predefined formats are as below. For more details on audio and video
formats, see [3].
o Audio Formats:
PCMU: 8-bit mu-law encoded 8kHz PCM
PCMA: 8-bit A-law encoded 8kHz PCM
IDVI: Intel DVI ADPCM
GSM: GSM (Group Speciale Mobile)
LPC: An experimental Linear Predictive Coder written by Ron
Frederick
1016: CELP encoding as specified in FED-STD 1016
G721: ITU recommendation G721
G723: ITU recommendation G723
L8: 8 bit linear audio
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L16: 16 bit linear audio
MPA: MPEG audio
VDVI: A variable rate version of IDVI
TSP0: TrueSpeech proprietary encoding
VSC: Vocaltec Software Compression proprietary encoding.
o Video Formats:
JPEG: Motion JPEG
MPV: MPEG encoding
MP2T: MPEG II transport stream
CelB: Sun Cell-B encoding
H261: CCITT/ITU-T recommendation H.261
nv: Xerox Parc Network Video
CPV: Compressed Packet Video (proprietary encoding)
HDCC: HDCC proprietary encoding from Silicon Graphics
CUSM: CU-SeeMe video encoding
PicW: PictureWindow encoding from BBN
RGB: 8 bit encoding of RGB values
o Whiteboard Formats:
WB: LBL Whiteboard (transport: UDP)
o Text Formats:
NT: UCL Network Text Editor (transport: UDP)
MMBL: mumble text chat tool (transport: UDP)
o Note that audio formats do not include packetisation information.
If a non-default (as defined in the RTP Audio/Video Profile)
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packetisation is required, the ``ptime'' attribute is used as given
below.
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 attribute.
a=keywds:<keywords>
Like the cat attribute, this is to assist identifying wanted ses-
sions at the receiver. It is a session attribute.
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
- it is intended as a recommendation for the encoding/packetisation
of audio. It is a media attribute.
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.
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 ses-
sion or media attribute.
a=sendonly
This specifies that the tools should be started in send-only mode.
Typically this may be used where a different unicast address is to
be used for a traffic destination that for a traffic source. It can
be either a session or media attribute, but would normally only be
used as a media attribute.
a=orient:<whiteboard orientation>
Normally only used in a whiteboard media specification, this speci-
fies the orientation of a the whiteboard on the screen. It is a
media attribute. Permitted values are `portrait', `landscape' and
`seascape' (upside down landscape).
a=type:<conference type>
This specifies the type of the conference. Suggested values are
`broadcast', `meeting', and `moderated'. `recvonly' should be the
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default for `type:broadcast' sessions, `type:meeting' should imply
`sendrecv' and `type:moderated' should imply the use of a floor con-
trol tool and that the media tools are started so as to ``mute'' new
sites joining the conference. It is a session attribute.
a=charset:<character set>
This specifies the character set to be used to display the session
name and information data. By default, an ISO 8859-1 character set
is used. If an ISO 8859-1 character set is not suitable, the use of
unicode (ISO 10646, [6],[7]), as specified in RFC1641 [8] is sug-
gested. In particular, the UTF-7 (RFC1642, [9]) encoding is sug-
gested with the following SDP attribute:
a=charset:unicode-1-1-utf-7
This is a session attribute; if this attribute is present, it must
be before the first media field.
5.1. 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 declara-
tive 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 flags such as
`recvonly' for some of the media tools. This conference attribute con-
veys 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 con-
trol might be set, and then one or more ``media'' fields might be used
to specify the conference control tools and configuration data for those
tools. A fictional example might be:
...
a=type:external-control
m=audio 12345 VAT PCMU
m=video 12347 RTP H261
m=whiteboard 12349 UDP WB
m=control 12341 UDP CCCP
a=mode:chaired
a=chair:128.16.64.2
a=video:follows-audio
a=audio:on-demand
a=audio:chair-mutes-mike
a=whiteboard:chaired
In this fictional example (i.e., this is not implemented anywhere), a
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general conference attribute is specified stating that conference con-
trol will be provided by an external tool, and specific attributes are
given to specify the control policy that tool should use.
In this document, only the former style of conference control declara-
tion is specified, though we recognise that some variant on the latter
may also be used eventually.
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Appendix A: SDP Grammar
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=" (DIGIT)+
;this draft describes version 0
origin-field ::= "o=" username space
sess-id space sess-version space
nettype space addrtype space
addr newline
session-name-field ::= "s=" text
information-field ::= ["i=" text newline]
uri-field ::= ["u=" uri newline]
email-fields ::= ("e=" email-address newline)*
phone-fields ::= ("p=" phone-number newline)*
connection-field ::= "c=" nettype space addrtype space
connection-address newline
bandwidth-fields ::= ("b=" bwtype ":" bandwidth newline)*
time-fields ::= ( "t=" start-time space stop-time
(newline repeat-fields)* newline)*
[zone-adjustments newline]
repeat-fields ::= repeat-interval space typed-time (space typed-time)+
zone-adjustments ::= time space [``-''] typed-time
(space time space [``-''] typed-time)*
key-field ::= ["k=" (printable-ascii)+ newline]
attribute-fields ::= ("a=" attribute newline)*
Handley/Jacobson [Page 21]
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media-descriptions ::= ( media-field
information-field
connection-field
bandwidth-fields
key-field
attribute-fields )*
media-field ::= "m=" media space port ["/" integer]
space proto space fmt newline
media ::= (alpha-numeric)+
;typically "audio", "video", "whiteboard"
;or "text"
fmt ::= (alpha-numeric)+
;typically an RTP media type for audio
;and video media
proto ::= (alpha-numeric)+
;typically "RTP", "VAT", or "UDP" for IP4
port ::= (DIGIT)+
;should in the range "1024" to "65535" inclusive
;for UDP based media ;random allocation should
;only assign above UDP port "5000".
attribute ::= att-field ":" att-value | att-field
att-field ::= (ALPHA)+
att-value ::= (att-char)+
att-char ::= alpha-numeric | "-"
;is this too tight a restriction
sess-id ::= (DIGIT)+
;should be unique for this originating username/host
sess-version ::= (DIGIT)+
;0 is a new session
connection-address ::= multicast-conf-address | multicast-scoped-address
| unicast-address
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multicast-conf-address ::=
"224.2." decimal_uchar "." decimal_uchar "/" ttl
[ "/" integer ]
;multicast addresses may be in a larger range
;but only these should be assigned by an sdp tool
multicast-scoped-address ::=
"239." decimal_uchar "." decimal_uchar "."
decimal_uchar "/" ttl [ "/" integer ]
ttl ::= decimal_uchar
start-time ::= time | "0"
stop-time ::= time | "0"
time ::= POS-DIGIT 9*DIGIT
;sufficient for 2 more centuries
repeat-interval ::= typed-time | interval-time
typed-time ::= (DIGIT)+ [fixed-len-time-unit]
interval-time ::= (DIGIT)+ variable-len-time-unit
fixed-len-time-unit ::= ``d'' | ``h'' | ``m'' | ``s''
variable-len-time-unit ::= ``Y'' | ``M''
bwtype ::= (alpha-numeric)+
bandwidth ::= (DIGIT)+
username ::= ;not defined here
email-address ::= email | email "(" text ")" | text "<" email ">"
email ::= ;defined in RFC822
uri::= ;defined in RFC1630
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phone-number ::= phone | phone "(" text ")" |
text "<" phone ">"
phone ::= "+" POS-DIGIT (space | "-" | DIGIT)+
;there must be a space or hyphen between the
;international code and the rest of the number.
nettype ::= "IN"
;list to be extended
addrtype ::= "IP4" | "IP6"
;list to be extended
addr ::= unicast-address
unicast-address ::= IP4-address | IP6-address
IP4-address ::= b1 "." decimal_uchar "." decimal_uchar "." b4
b1 ::= decimal_uchar
;less than "224"; not "0" or "127"
b4 ::= decimal_uchar
;not "0"
IP6-address ::= ;to be defined
text ::= (printable-iso8859-1)+ | (unicode-1-1-utf-7)+
;unicode requires a "a=charset:unicode-1-1-utf-7"
;attribute to be used
printable-iso8859-1 ::= ;8 bit ascii character
;decimal 9 (TAB), 32-126 and 161-255
unicode-1-1-utf-7 ::= unicode-safe
;defined in RFC 1642
decimal_uchar ::= DIGIT
| POS-DIGIT DIGIT
| (1 2*DIGIT)
| (2 (0|1|2|3|4) DIGIT)
| (2 5 (0|1|2|3|4|5))
integer ::= POS-DIGIT (DIGIT)*
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alpha-numeric ::= ALPHA | DIGIT
printable-ascii ::= unicode-safe | "~" | "\"
DIGIT ::= 0 | POS-DIGIT
POS-DIGIT ::= 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
ALPHA ::= a | b | c | d | e | f | g | h | i | j | k |
l | m | n | o | p | q | r | s | t | u | v |
w | x | y | z | A | B | C | D | E | F | G |
H | I | J | K | L | M | N | O | P | Q | R |
S | T | U | V | W | X | Y | Z
unicode-safe ::= alpha-numeric |
"'" | "(" | ")" | "'" | "-" | "." | "/" | ":" |
"?" | """ | "#" | "$" | "&" | "*" | ";" | "<" |
"=" | ">" | "@" | "[" | "]" | "^" | "_" | "`" |
"{" | "|" | "}" | "+" | space | tab
;although unicode allows newline and carriage
;return, we don't here.
space ::= ;ascii code 32
tab ::= ;ascii code 9
newline ::= ;ascii code 10
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Appendix B: Summary of differences between SDPv1 and SDPv2
For this purpose SDPv1 is defined as the protocol in use by version 1.14
of the sd session description tool. SDPv2 is the proposed protocol
described in the document.
SDPv1 allows the use of only the following SDP fields:
s= session name - no change
i= session information - SDP v2 allows ``i'' fields to be used in
the media descriptions as media labels.
o= originator - in SDP v1, originator is of the form
username@hostname, and the sd packet headers contain a binary
representation of the IP 4 address of the originating host.
This essentially contains duplicate information and cannot cope
with non-IP4 addresses. Thus in SDP v2 we add an address type
field and put the address of the originating machine in the ori-
ginator field. In SDP v2 we also add two more sub-fields - a
session id and a session version - to the origin field. This
means that all the information to identify a session and whether
that session has changed is in one field. To make this field
easier to find, we move to to the beginning of the announcement
after the new protocol version field.
c= conference data - only one conference data field is allowed
before the first media field in SDPv1. In SDP v1 the conference
start and stop times are the third and fourth fields of the
conference data field. In SDP v2 they have moved to the t= time
field. To convert from SDPv1 time-stamps to UNIX time, subtract
decimal 2085978496. To convert from SDPv2 time-stamps to UNIX
time, subtract decimal 2208988800 (SDPv1 uses NTP time-stamps
incorrectly).
In SDPv1 no network type or address type subfields are present,
and the ttl is a separate subfield following the multicast
address. Multiple multicast addresses are not allowed in SDPv1.
m= media - In SDP v1, the third subfield is the RTP v1 ID. This is
now obsolete. In SDP v1, the media format allowed a default
format if none was specific, but non-default values were speci-
fied using the fmt attribute. In SDP v2, allow default attri-
butes and not allowed, and the fourth subfield in a media field
gives the media format. In SDPv1, there is no way to distin-
guish between the same media format carried by different tran-
sport protocols. In SDP v2 the third subfield in a media field
gives the transport protocol.
In SDPv1 multiple ports are not allowed for hierarchical encod-
ings on a unicast address.
a= attributes - attributes are allowed only after the first media
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field in SDPv1.
SDPv2 additionally defines the following fields which are defined above:
v= Protocol Version
b= Bandwidth
e= Email Address
p= Phone Number
u= URL
t= Time
r= Repeat Time
z= Time Zone info
k= Encryption Key
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Appendix C: Authors' Addresses
Mark Handley
Department of Computer Science
University College London
London WC1E 6BT
United Kingdom
electronic mail: M.Handley@cs.ucl.ac.uk
Van Jacobson
MS 46a-1121
Lawrence Berkeley Laboratory
Berkeley, CA 94720
United States
electronic mail: van@ee.lbl.gov
References
[1] D. Mills, ``Network Time Protocol version 2 specification and imple-
mentation", RFC1119, 1st Sept 1989.
[2] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, ``RTP: A Tran-
sport Protocol for Real-Time Applications'', INTERNET-DRAFT, draft-
ietf-avt-rtp-07.txt, 21st March 1995.
[3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with
Minimal Control'', INTERNET-DRAFT, draft-ietf-avt-profile-04.txt, 21st
March 1995.
[4] M. Handley (editor), ``The Use of Plain Text Keys for Multimedia
Conferences'', Research Note RN-95-19, Department of Computer Science,
University College London, Feb 1995.
http://www.cs.ucl.ac.uk/rns/RN9519.ps
[5] V. Jacobson, S. McCanne, ``vat - X11-based audio teleconferencing
tool'' vat manual page, Lawrence Berkeley Laboratory, 1994.
[6] ``The Unicode Standard, Version 1.1'': Version 1.0, Volume 1 (ISBN
0-201-56788-1), Version 1.0, Volume 2 (ISBN 0-201-60845-6), and "Unicode
Technical Report #4, The Unicode Standard, Version 1.1" (available from
The Unicode Consortium, and soon to be published by Addison- Wesley).
[7] ISO/IEC 10646-1:1993(E) Information Technology--Universal Multiple-
octet Coded Character Set (UCS).
[8] D. Goldsmith, M. Davis, ``Using Unicode with MIME'', RFC1641, July
1994
[9] D. Goldsmith, M. Davis, ``UTF-7 - A Mail-Safe Transformation Format
of Unicode'', RFC1642, July 1994
Handley/Jacobson [Page 28]
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