Internet Engineering Task Force MMUSIC WG
INTERNET-DRAFT Mark Handley/Van Jacobson
draft-ietf-mmusic-sdp-05.ps ISI/LBNL
21st Nov 1997
Expires: 21st May 1997
SDP: Session Description Protocol
Status of this Memo
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Distribution of this document is unlimited.
Abstract
This document defines the Session Description Protocol, SDP.
SDP is intended for describing multimedia sessions for the
purposes of session announcement, session invitation, and
other forms of multimedia session initiation.
This document is a product of the Multiparty Multimedia Session Control
(MMUSIC) working group of the Internet Engineering Task Force. Comments
are solicited and should be addressed to the working group's mailing
list at confctrl@isi.edu and/or the authors.
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1. Introduction
On the Internet multicast backbone (Mbone), a session directory tool is
used to advertise multimedia conferences and communicate the conference
addresses and conference tool-specific information necessary for parti-
cipation. This document defines a session description protocol for this
purpose, and for general real-time multimedia session description pur-
poses. This draft does not describe multicast address allocation or the
distribution of SDP messages in detail. These are described in accom-
panying drafts. SDP is not intended for negotitation of media encod-
ings.
2. Background
The Mbone is the part of the internet that supports IP multicast, and
thus permits efficient many-to-many communication. It is used exten-
sively for multimedia conferencing. Such conferences usually have the
property that tight coordination of conference membership is not neces-
sary; to receive a conference, a user at an Mbone site only has to know
the conference's multicast group address and the UDP ports for the
conference data streams.
Session directories assist the advertisement of conference sessions and
communicate the relevant conference setup information to prospective
participants. SDP is designed to convey such information to recipients.
SDP is purely a format for session description - it does not incorpor-
tate a transport protocol, and is intended to use different transport
protocols as appropriate including the Session Announcement Protocol
[4], Session Inititation Protocol [11], Real-Time Streaming Protocol
[12], electronic mail using the MIME extensions, and the Hypertext Tran-
sport Protocol.
SDP is intended to be general purpose so that it can be used for a wider
range of network environments and applications than just multicast ses-
sion directories. However, it is not intended to support negotiation of
session content or media encodings - this is viewed as outside the scope
of session description.
3. Glossary of Terms
The following terms are used in this document, and have specific meaning
within the context of this document.
Conference
A multimedia conference is a set of two or more communicating users
along with the software they are using to communicate.
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Session
A multimedia session is a set of multimedia senders and receivers
and the data streams flowing from senders to receivers. A mul-
timedia conference is an example of a multimedia session.
Session Advertisement
See session announcement.
Session Announcement
A session announcement is a mechanism by which a session description
is conveyed to users in a pro-active fashion, i.e., the session
description was not explicitly requested by the user.
Session Description
A well defined format for conveying sufficient information to dis-
cover and participate in a multimedia session.
4. SDP Usage
4.1. Multicast Announcements
SDP is a session description protocol for multimedia sessions. A common
mode of usage is for a client to announce a conference session by
periodically multicasting an announcement packet to a well known multi-
cast address and port using the Session Announcement Protocol (SAP).
SAP packets are UDP packets with the following format:
0 31
|--------------------|
| SAP header |
|--------------------|
| text payload |
|//////////
The header is the Session Announcement Protocol header. SAP is
described in more detail in a companion draft [4]
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 SAP, only one session announcement is permitted in a
single packet.
4.2. Email and WWW Announcements
Alternative means of conveying session descriptions include electronic
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mail and the World Wide Web. For both email and WWW distribution, the
use of the MIME content type ``application/sdp'' should be used. This
enables the automatic launching of applications for participation in the
session from the WWW client or mail reader in a standard manner.
Note that announcements of multicast sessions made only via email or the
World Wide Web (WWW) do not have the property that the receiver of a
session announcement can necessarily receive the session because the
multicast sessions may be restricted in scope, and access to the WWW
server or reception of email is possible outside this scope. SAP
announcements do not suffer from this mismatch.
5. Requirements and Recommendations
The purpose of SDP is to convey information about media streams in 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 some duration of time. Media streams can be
many-to-many. The times during which the session is active need not be
continuous.
Thus far, multicast based sessions on the Internet have differed from
many other forms of conferencing in that anyone receiving the traffic
can join the session (unless the session traffic is encrypted). In such
an environment, SDP serves two primary purposes. It is a means to com-
municate the existence of a session, and is a means to convey sufficient
information to enable joining and participating in the session. In a
unicast environment, only the latter purpose is likely to be relevant.
Thus SDP includes:
o Session name and purpose
o Time(s) the session is active
o The media comprising the session
o Information to receive those media (addresses, ports, formats and
so on)
As resources necessary to participate in a session may be limited, some
additional information may also be desirable:
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o Information about the bandwidth to be used by the conference
o Contact information for the person responsible for the session
In general, SDP must convey sufficient information to be able to join a
session (with the possible exception of encryption keys) and to announce
the resources to be used to non-participants that may need to know.
5.1. Media Information
SDP includes:
o The type of media (video, audio, etc)
o The transport protocol (RTP/UDP/IP, H.320, etc)
o The format of the media (H.261 video, MPEG video, etc)
For an IP multicast session, the following are also conveyed:
o Multicast address for media
o Transport Port for media
This address and port are the destination address and destination port
of the multicast stream, whether being sent, received, or both.
For an IP unicast session, the following are conveyed:
o Remote address for media
o Transport port for contact address
The semantics of this address and port depend on the media and transport
protocol defined. By default, this is the remote address and remote
port to which data is sent, and the remote address and local port on
which to receive data. However, some media may define to use these to
establish a control channel for the actual media flow.
5.2. Timing Information
Sessions may either be bounded or unbounded in time. Whether or not
they are bounded, they may be only active at specific times.
SDP can convey:
o An arbitrary list of start and stop times bounding the session
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o For each bound, repeat times such as "every Wednesday at 10am for
one hour"
This timing information is globally consistent, irrespective of local
time zone or daylight saving time.
5.3. Private Sessions
It is possible to create both public sessions and private sessions.
Private sessions will typically be conveyed by encrypting the session
description to distribute it. The details of how encryption is per-
formed are dependent on the mechanism used to convey SDP - see [4] for
how this is done for session announcements.
If a session announcement is private it is possible to use that private
announcement to convey encryption keys necessary to decode each of the
media in a conference, including enough information to know which
encryption scheme is used for each media.
5.4. Obtaining Further Information about a Session
A session description should convey enough information to decide whether
or not to participate in a session. SDP may include additional pointers
in the form of Universal Resources Identifiers (URIs) for more informa-
tion about the session.
5.5. Categorisation
When many session descriptions are being distributed by SAP or any other
advertisement mechanism, it may be desirable to filter announcements
that are of interest from those that are not. SDP supports a categori-
sation mechanism for sessions that is capable of being automated.
5.6. Internationalization
The SDP specification recommends the use of the ISO 10646 character sets
in the UTF-8 encoding (RFC 2044) to allow many different languages to be
represented. However, to assist in compact representations, SDP also
allows other character sets such as ISO 8859-1 to be used when desired.
Internationalization only applies to free-text fields (session name and
background information), and not to SDP as a whole.
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6. SDP Specification
SDP session descriptions are entirely textual using the ISO 10646 char-
acter set in UTF-8 encoding. SDP field names and attributes names use
only the US-ASCII subset of UTF-8, but textual fields and attribute
values may use the full ISO 10646 character set. The textual form, as
opposed to a binary encoding such as ASN/1 or XDR, was chosen to enhance
portability, to enable a variety of transports to be used (e.g, session
description in a MIME email message) and to allow flexible, text-based
toolkits (e.g., Tcl/Tk ) to be used to generate and to process session
descriptions. However, since the total bandwidth allocated to all SAP
announcements is strictly limited, the encoding is deliberately compact.
Also, since announcements may be transported via very unreliable means
(e.g., email) or damaged by an intermediate caching server, the encoding
was designed with strict order and formatting rules so that most errors
would result in malformed announcements which could be detected easily
and discarded. This also allows rapid discarding of encrypted announce-
ments for which a receiver does not have the correct key.
An SDP session description consists of a number of lines of text of the
form
<type>=<value>
<type> is always exactly one character and is case-significant. <value>
is a structured text string whose format depends on <type>. It also
will be case-significant unless a specific field defines otherwise.
Whitespace is not permitted either side of the `=' sign. In general
<value> is either a number of fields delimited by a single space charac-
ter or a free format string.
A session description consists of a session-level description (details
that apply to the whole session and all media streams) and optionally
several media-level descriptions (details that apply onto to a single
media stream).
An announcement consists of a session-level section followed by zero or
more media-level sections. The session-level part starts with a `v='
line and continues to the first media-level section. The media descrip-
tion starts with an `m=' line and continues to the next media descrip-
tion or end of the whole session description. In general, session-level
values are the default for all media unless overridden by an equivalent
media-level value.
When SDP is conveyed by SAP, only one session description is allowed per
packet. When SDP is conveyed by other means, many SDP session descrip-
tions may be concatenated together (the `v=' line indicating the start
of a session description terminates the previous description). Some
lines in each description are required and some are optional but all
must appear in exactly the order given here (the fixed order greatly
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enhances error detection and allows for a simple parser). Optional
items are marked with a `*'.
Session description
v= (protocol version)
o= (owner/creator and session identifier).
s= (session name)
i=* (session information)
u=* (URI of description)
e=* (email address)
p=* (phone number)
c=* (connection information - not required if included in all media)
b=* (bandwidth information)
One or more time descriptions (see below)
z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)
Zero or more media descriptions (see below)
Time description
t= (time the session is active)
r=* (zero or more repeat times)
Media description
m= (media name and transport address)
i=* (media title)
c=* (connection information - optional if included at session-level)
b=* (bandwidth information)
k=* (encryption key)
a=* (zero or more media attribute lines)
The set of `type' letters is deliberately small and not intended to be
extensible -- SDP parsers must completely ignore any announcement that
contains a `type' letter that it does not understand. The `attribute'
mechanism ("a=" described below) is the primary means for extending SDP
and tailoring it to particular applications or media. Some attributes
(the ones listed in this document) have a defined meaning but others may
be added on an application-, media- or session-specific basis. A ses-
sion directory must ignore any attribute it doesn't understand.
The connection (`c=') and attribute (`a=') information in the session-
level section applies to all the media of that session unless overridden
by connection information or an attribute of the same name in the media
description. For instance, in the example below, each media behaves as
if it were given a `recvonly' attribute.
An example SDP description is:
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v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
e=mjh@isi.edu (Mark Handley)
c=IN IP4 224.2.17.12/127
t=2873397496 2873404696
a=recvonly
m=audio 3456 RTP/AVP 0
m=video 2232 RTP/AVP 31
m=application 32416 udp wb
a=orient:portrait
Text records such as the session name and information are bytes strings
which may contain any byte with the exceptions of 0x00 (Nul), 0x0a
(ASCII newline) and 0x0d (ASCII carriage return). The sequence CRLF
(0x0d0a) is used to end a record, although parsers should be tolerant
and also accept records terminated with a single newline character. By
default these byte strings contain ISO-10646 characters in UTF-8 encod-
ing, but this default may be changed using the `charset' attribute.
Protocol Version
v=0
The ``v='' field gives the version of the Session Description Protocol.
There is no minor version number.
Origin
o=<username> <session id> <version> <network type> <address type>
<address>
The ``o='' field gives the originator of the session (their username and
the address of the user's host) plus a session id and session version
number. <username> is the user's login on the originating host, or it
is ``-'' if the originating host does not support the concept of user
ids. <username> must not contain spaces. <session id> is a numeric
string such that the tuple of <username>, <session id>, <network type>,
<address type> and <address> form a globally unique identifier for the
session. The method of session id allocation is up to the creating
tool, but it has been suggested that a Network Time Protocol (NTP)
timestamp be used to ensure uniqueness [1]. <version> is a version
number for this announcement. It is needed for proxy announcements to
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detect which of several announcements for the same session is the most
recent. Again its usage is up to the creating tool, so long as <ver-
sion> is increased when a modification is made to the session data.
Again, it is recommended (but not mandatory) that an NTP timestamp is
used. <network type> is a text string giving the type of network. Ini-
tially ``IN'' is defined to have the meaning ``Internet''. <address
type> is a text string giving the type of the address that follows.
Initially ``IP4'' and ``IP6'' are defined. <address> is the globally
unique address of the machine from which the session was created. For
an address type of IP4, this is the dotted-decimal representation of the
IP version 4 address of the machine. For an address type of IP6, this
is the compressed textual representation of the IP version 6 address of
the machine.
In general, the ``o='' field serves as a globally unique identifier for
this version of this session description, and the subfields excepting
the version taken together identify the session irrespective of any
modifications.
Session Name
s=<session name>
The ``s='' field is the session name. There must be one and only one
``s='' field per session description, and it must contain ISO 10646
characters (but see also the `charset' attribute below).
Session and Media Information
i=<session description>
The ``i='' field is information about the session. There may be at most
one session-level ``i='' field per session description, and at most one
``i='' field per media. Although it may be omitted, this is discouraged
for session announcements, and user interfaces for composing sessions
should require text to be entered. If it is present it must contain ISO
10646 characters (but see also the `charset' attribute below).
A single ``i='' field can also be used for each media definition. In
media definitions, ``i='' fields are primarily intended for labeling
media streams. As such, they are most likely to be useful when a single
session has more than one distinct media stream of the same media type.
An example would be two different whiteboards, one for slides and one
for feedback and questions.
URI
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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.
o Phone numbers should be given in the conventional international
format - preceded by a ``+'' and the international country code.
There must be a space or a hyphen (``-'') between the country code
and the rest of the phone number. Spaces and hyphens may be used to
split up a phone field to aid readability if desired. For example:
p=+44-171-380-7777 or p=+1 617 253 6011
o Both email addresses and phone numbers can have an optional free
text string associated with them, normally giving the name of the
person who may be contacted. This should be enclosed in parenthesis
if it is present. For example:
e=mjh@isi.edu (Mark Handley)
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The alternative RFC822 name quoting convention is also allowed for
both email addresses and phone numbers. For example,
e=Mark Handley <mjh@isi.edu>
The free text string should be in the ISO-10646 character set with
UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if
the appropriate charset session-level attribute is set.
Connection Data
c=<network type> <address type> <connection address>
The ``c='' field contains connection data.
A session announcement must contain one ``c='' field in each media
description (see below) or a ``c='' field at the session-level. It may
contain a session-level ``c='' field and one additional ``c='' field per
media description, in which case the per-media values override the
session-level settings for the relevant media.
The first sub-field is the network type, which is a text string giving
the type of network. Initially ``IN'' is defined to have the meaning
``Internet''.
The second sub-field is the address type. This allows SDP to be used
for sessions that are not IP based. Currently only IP4 is defined.
The third sub-field is the connection address. Optional extra 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, though this is not prohibited.
o Conferences using an IP multicast connection address must also have
a time to live (TTL) value present in addition to the multicast
address. The TTL and the address together define the scope with
which multicast packets sent in this conference will be sent. TTL
values must be in the range 0-255.
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The TTL for the session is appended to the address using a slash as
a separator. An example is:
c=IN IP4 224.2.1.1/127
Hierarchical or layered encoding schemes are data streams where the
encoding from a single media source is split into a number of
layers. The receiver can choose the desired quality (and hence
bandwidth) by only subscribing to a subset of these layers. Such
layered encodings are normally transmitted in multiple multicast
groups to allow multicast pruning. This technique keeps unwanted
traffic from sites only requiring certain levels of the hierarchy.
For applications requiring multiple multicast groups, we allow the
following notation to be used for the connection address:
<base multicast address>/<ttl>/<number of addresses>
If the number of addresses is not given it is assumed to be one.
Multicast addresses so assigned are contiguously allocated above the
base address, so that, for example:
c=IN IP4 224.2.1.1/127/3
would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are to
be used at a ttl of 127. This is semantically identical to includ-
ing multiple ``c='' lines in a media description:
c=IN IP4 224.2.1.1/127
c=IN IP4 224.2.1.2/127
c=IN IP4 224.2.1.3/127
Multiple addresses or ``c='' lines can only be specified on a per-
media basis, and not for a session-level ``c='' field.
It is illegal for the slash notation described above to be used for
IP unicast addresses.
Bandwidth
b=<modifier>:<bandwidth-value>
o This specifies the proposed bandwidth to be used by the session or
media, and is optional.
o <bandwidth-value> is in kilobits per second
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o <modifier> is a single alphanumeric word giving the meaning of the
bandwidth figure.
o Two modifiers are initially defined:
CT Conference Total: An implicit maximum bandwidth is associated with
each TTL on the Mbone or within a particular multicast 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 from the scope, 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 Maximum: The bandwidth is interpreted to be
application-specific, i.e., will be the application's concept of
maximum bandwidth. Normally this will coincide with what is set
on the application's ``maximum bandwidth'' control if applicable.
Note that CT gives a total bandwidth figure for all the media at all
sites. AS gives a bandwidth figure for a single media at a single
site, although there may be many sites sending simultaneously.
o Extension Mechanism: Tool writers can define experimental bandwidth
modifiers by prefixing their modifier with ``X-''. For example:
b=X-YZ:128
SDP parsers should ignore bandwidth fields with unknown modifiers.
Modifiers should be alpha-numeric and, although no length limit is
given, they are recommended to be short.
Times, Repeat Times and Time Zones
t=<start time> <stop time>
o ``t='' fields specify the start and stop times for a conference
session. Multiple ``t='' fields may be used if a session is active
at multiple irregularly spaced times; each additional ``t='' field
specifies an additional period of time for which the session will be
active. If the session is active at regular times, an ``r='' field
(see below) should be used in addition to and following a ``t=''
field - in which case the ``t='' field specifies the start and stop
times of the repeat sequence.
o The first and second sub-fields give the start and stop times for
the conference respectively. These values are the decimal
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representation of Network Time Protocol (NTP) time values in seconds
[1]. To convert these values to UNIX time, subtract decimal
2208988800.
o If the stop-time is set to zero, then the session is not bounded,
though it will not become active until after the start-time. If the
start-time is also zero, the session is regarded as permanent.
User interfaces should strongly discourage the creation of unbounded
and permanent sessions as they give no information about when the
session is actually going to terminate, and so make scheduling dif-
ficult.
The general assumption may be made, when displaying unbounded ses-
sions that have not timed out to the user, that an unbounded session
will only be active until half an hour from the current time or the
session start time, whichever is the later. If behaviour other than
this is required, an end-time should be given and modified as
appropriate when new information becomes available about when the
session should really end.
Permanent sessions may be shown to the user as never being active
unless there are associated repeat times which state precisely when
the session will be active. In general, permanent sessions should
not be created for any session expected to have a duration of less
than 2 months, and should be discouraged for sessions expected to
have a duration of less than 6 months.
r=<repeat interval> <active duration> <list of offsets from start-time>
o ``r='' fields specify repeat times for a session. For example, if
a session is active at 10am on Monday and 11am on Tuesday for one
hour each week for three months, then the <start time> in the
corresponding ``t='' field would be the NTP representation of 10am
on the first Monday, the <repeat interval> would be 1 week, the
<active duration> would be 1 hour, and the offsets would be zero and
25 hours. The corresponding ``t='' field stop time would be the NTP
representation of the end of the last session three months later. By
default all fields are in seconds, so the ``r='' and ``t='' fields
might be:
t=3034423619 3042462419
r=604800 3600 0 90000
To make announcements more compact, times may also be given in
units of days, hours or minutes. The syntax for these is a number
immediately followed by a single case-sensitive character.
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Fractional units are not allowed - a smaller unit should be used
instead. The following unit specification characters are allowed:
d - days (86400 seconds)
h - minutes (3600 seconds)
m - minutes (60 seconds)
s - seconds (allowed for completeness but not recommended)
Thus, the above announcement could also have been written:
r=7d 1h 0 25h
Monthly and yearly repeats cannot currently be directly specified
with a single SDP repeat time - instead separate "t" fields should
be used to explicitly list the session times.
z=<adjustment time> <offset> <adjustment time> <offset> ....
o To schedule a repeated session which spans a change from daylight-
saving time to standard time or vice-versa, it is necessary to
specify offsets from the base repeat times. This is required because
different time zones change time at different times of day, dif-
ferent countries change to or from daylight time on different dates,
and some countries do not have daylight saving time at all.
Thus in order to schedule a session that is at the same time winter
and summer, it must be possible to specify unambiguously by whose
time zone a session is scheduled. To simplify this task for
receivers, we allow the sender to specify the NTP time that a time
zone adjustment happens and the offset from the time when the 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
This specifies that at time 2882844526 the time base by which the
session's repeat times are calculated is shifted back by 1 hour, and
that at time 2898848070 the session's original time base is
restored. Adjustments are always relative to the specified start
time - they are not cumulative.
o If a session is likely to last several years, it is expected that
the session announcement will be modified periodically rather than
transmit several years worth of adjustments in one announcement.
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Encryption Keys
k=<method>
k=<method>:<encryption key>
o The session description protocol may be used to convey encryption
keys. A key field is permitted before the first media entry (in
which case it applies to all media in the session), or for each
media entry as required.
o The format of keys and their usage is outside the scope of this
document, but see [3].
o The method indicates the mechanism to be used to obtain a usable
key by external means, or from the encoded encryption key given.
The following methods are defined:
k=clear:<encryption key>
The encryption key (as described in [3] for RTP media streams
under the AV profile) is included untransformed in this key
field.
k=base64:<encoded encryption key>
The encryption key (as described in [3] for RTP media streams
under the AV profile) is included in this key field but has been
base64 encoded because it includes characters that are prohi-
bited in SDP.
k=uri:<URI to obtain key>
A Universal Resource Identifier as used by WWW clients is
included in this key field. The URI refers to the data contain-
ing the key, and may require additional authentication before
the key can be returned. When a request is made to the given
URI, the MIME content-type of the reply specifies the encoding
for the key in the reply. The key should not be obtained until
the user wishes to join the session to reduce synchronisation of
requests to the WWW server(s).
k=prompt
No key is included in this SDP description, but the session or
media stream referred to by this key field is encrypted. The
user should be prompted for the key when attempting to join the
session, and this user-supplied key should then be used to
decrypt the media streams.
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Attributes
a=<attribute>
a=<attribute>:<value>
A media field may also have any number of attributes (``a='' fields)
which are media specific. Attribute fields can also be added before the
first media field. These attributes would convey additional information
that applies to the conference as a whole rather than to individual
media; an example might be the conference's floor control policy.
Attribute fields may be of two forms:
o property attributes. A property attribute is simply of the form
``a=<flag>''. These are binary attributes, and the presence of the
attribute conveys that the attribute is a property of the session.
An example might be ``a=recvonly''.
o value attributes. A value attribute is of the form
``a=<attribute>:<value>''. An example might be that a whiteboard
could have the value attribute ``a=orient:landscape''
Attribute interpretation depends on the media tool being invoked. Thus
receivers of session descriptions should be configurable in their
interpretation of announcements in general and of attributes in particu-
lar.
Attribute names must be in the US-ASCII subset of ISO-10646/UTF-8.
Attribute values are byte strings, and MAY use any byte value except
0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values are
to be interpreted as in ISO-10646 character set with UTF-8 encoding.
Unlike other text fields, attribute values are NOT normally affected by
the `charset' attribute as this would make comparisons against known
values problematic. However, when an attribute is defined, it can be
defined to be charset-dependant, in which case it's value should be
interpreted in the session charset rather than in ISO-10646.
Media Announcements
m=<media> <port> <transport> <fmt list>
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 announce-
ment. A media field also has several sub-fields:
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o The first sub-field is the media type. Currently defined media are
``audio'', ``video'', ``application'', ``data'' and ``control'',
though this list may be extended as new communication modalities
emerge (e.g., telepresence). The difference between ``application''
and ``data'' is that the former is a media flow such as whiteboard
information, and the latter is bulk-data transfer such as multicast-
ing of program executables which will not typically be displayed to
the user. ``control'' is used to specify an additional conference
control channel for the session.
o The second sub-field is the transport port to which the media
stream will be sent. The meaning of the transport port depends on
the network being used as specified in the relevant ``c'' field and
on the transport protocol defined in the third sub-field. Other
ports used by the media application (such as the RTCP port, see [2])
should be derived algorithmically from the base media port.
Note: For transports based on UDP, the value should be in the range
1024 to 65535 inclusive. For RTP compliance it should be an even
number.
For applications where hierarchically encoded streams are being sent
to a unicast address, it may be necessary to specify multiple 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 list>
In such a case, the ports used depend on the transport protocol.
For RTP, only the even ports are used for data and the corresponding
one-higher odd port is used for RTCP. For example:
m=video 3456/2 RTP/AVP 31
would specify that ports 3456 and 3457 form one RTP/RTCP pair and
3458 and 3459 form the second RTP/RTCP pair. RTP/AVP is the tran-
sport protocol and 31 is the format (see below).
It is illegal for both multiple addresses to be specified in the
``c='' field and for multiple ports to be specified in the ``m=''
field in the same session announcement.
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 pro-
tocol runs over IP4. For IP4, it is normally expected that most
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media traffic will be carried as RTP over UDP. The following tran-
sport protocols are preliminarily defined, but may be extended
through registration of new protocols with IANA:
- RTP/AVP - the IETF's Realtime Transport Protocol using the
Audio/Video profile carried over UDP.
- udp - User Datagram Protocol
If an application uses a single combined proprietary media format
and transport protocol over UDP, then simply specifying the tran-
sport protocol as udp and using the format field to distinguish the
combined protocol is recommended. If a transport protocol is used
over UDP to carry several distinct media types that need to be dis-
tinguished by a session directory, then specifying the transport
protocol and media format separately is necessary. RTP is an exam-
ple of a transport-protocol that carries multiple payload formats
that must be distinguished by the session directory for it to know
how to start appropriate tools, relays, mixers or recorders.
The main reason to specify the transport-protocol in addition to the
media format is that the same standard media formats may be carried
over different transport protocols even when the network protocol is
the same - a historical example is vat PCM audio and RTP PCM audio.
In addition, relays and monitoring tools that are transport-
protocol-specific but format-independent are possible.
For RTP media streams operating under the RTP Audio/Video Profile
[3], the protocol field is ``RTP/AVP''. Should other RTP profiles
be defined in the future, their profiles will be specified in the
same way. For example, the protocol field ``RTP/XYZ'' would specify
RTP operating under a profile whose short name is ``XYZ''.
o The fourth and subsequent sub-fields are media formats. For audio
and video, these will normally be a media payload type as defined in
the RTP Audio/Video Profile.
When a list of payload formats is given, this implies that all of
these formats may be used in the session, but the first of these
formats is the default format for the session.
For media whose transport protocol is not RTP or UDP the format
field is protocol specific. Such formats should be defined in an
additional specification document.
For media whose transport protocol is RTP, SDP can be used to
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provide a dynamic binding of media encoding to RTP payload type.
The payload names in the RTP AV Profile do not specify unique audio
encodings (in terms of clock rate and number of audio channels), and
so they are not used directly in SDP format fields. Instead, the
payload type number should be used to specify the format for static
payload types and the payload type number along with additional
encoding information should be used for dynamically allocated pay-
load types.
An example of a static payload type is u-law PCM coded single chan-
nel audio sampled at 8KHz. This is completely defined in the RTP
Audio/Video profile as payload type 0, so the media field for such a
stream sent to UDP port 3456 is:
m=video 3456 RTP/AVP 0
An example of a dynamic payload type is 16 bit linear encoded stereo
audio sampled at 16KHz. If we wish to use dynamic RTP/AVP payload
type 98 for such a stream, additional information is required to
decode it:
m=video 3456 RTP/AVP 98
a=rtpmap:98 L16/16000/2
The general form of an rtpmap attribute is:
a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding parameters>]
For audio streams, <encoding parameters> may specify the number of
audio channels. This parameter may be omitted if the number of
channels is one provided no additional parameters are needed.
For video streams, no encoding parameters are currently specified.
Additional parameters may be defined in the future, but codec-
specific parameters should not be added. Parameters added to an
rtpmap attribute should only be those required for a session direc-
tory to make the choice of appropriate media too to participate in a
session. Codec-specific parameters should be added in other attri-
butes.
Up to one rtpmap attribute can be define for each media format
specified. Thus we might have:
m=audio 12345 RTP/AVP 96 97 98
a=rtpmap:96 L8/8000
a=rtpmap:97 L16/8000
a=rtpmap:98 L16/11025/2
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Experimental encoding formats can also be specified in this way.
RTP formats that are not registered with IANA as standard format
names must be preceded by ``X-''. Thus a new experimental redundant
audio stream called GSMLPC using dynamic payload type 99 could be
specified as:
m=video 3456 RTP/AVP 99
a=rtpmap:99 X-GSMLPC/8000
Such an experimental encoding requires that any site wishing to
receive the media stream has relevant configured state in its ses-
sion directory to know which tools are appropriate.
Note that RTP audio formats typically do not include information
about the number of samples per packet. If a non-default (as
defined in the RTP Audio/Video Profile) packetisation is required,
the``ptime'' attribute is used as given below.
For more details on RTP audio and video formats, see [3].
o Predefined formats for UDP protocol non-RTP media are as below.
Application Formats:
wb: LBL Whiteboard (transport: udp)
nt: UCL Network Text Editor (transport: udp)
Suggested Attributes
The following attributes are suggested. Since application writers may
add new attributes as they are required, this list is not exhaustive.
a=cat:<category>
This attribute gives the dot-separated hierarchical category of the
session. This is to enable a receiver to filter unwanted sessions
by category. It would probably have been a compulsory separate
field, except for its experimental nature at this time. It is a
session-level attribute, and is not dependent on charset.
a=keywds:<keywords>
Like the cat attribute, this is to assist identifying wanted ses-
sions at the receiver. This allows a receiver to select interesting
session based on keywords describing the purpose of the session. It
is a session-level attribute. It is a charset dependent attribute,
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meaning that its value should be interpreted in the charset speci-
fied for the session description if one is specified, or by default
in ISO 10646/UTF-8.
a=tool:<name and version of tool>
This gives the name and version number of the tool used to create
the session description. It is a session-level attribute, and is
not dependent on charset.
a=ptime:<packet time>
This gives the length of time in milliseconds represented by the
media in a packet. This is probably only meaningful for audio data.
It should not be necessary to know ptime to decode RTP or vat audio,
and it is intended as a recommendation for the
encoding/packetisation of audio. It is a media attribute, and is
not dependent on charset.
a=recvonly
This specifies that the tools should be started in receive-only mode
where applicable. It can be either a session or media attribute, and
is not dependent on charset.
a=sendrecv
This specifies that the tools should be started in send and receive
mode. This is necessary for interactive conferences with tools such
as wb which defaults to receive only mode. It can be either a ses-
sion or media attribute, and is not dependent on charset.
a=sendonly
This specifies that the tools should be started in send-only mode.
An example may be where a different unicast address is to be used
for a traffic destination than for a traffic source. In such a case,
two media descriptions may be use, one sendonly and one recvonly. It
can be either a session or media attribute, but would normally only
be used as a media attribute, and is not dependent on charset.
a=orient:<whiteboard orientation>
Normally this is only used in a whiteboard media specification. It
specifies the orientation of a the whiteboard on the screen. It is
a media attribute. Permitted values are `portrait', `landscape' and
`seascape' (upside down landscape). It is not dependent on charset
a=type:<conference type>
This specifies the type of the conference. Suggested values are
`broadcast', `meeting', `moderated', `test' and `H332'. `recvonly'
should be the default for `type:broadcast' sessions, `type:meeting'
should imply `sendrecv' and `type:moderated' should indicate the use
of a floor control tool and that the media tools are started so as
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to ``mute'' new sites joining the conference.
Specifying the attribute type:H332 indicates that this loosely cou-
pled session is part of a H.332 session as defined in the ITU H.332
specification [10]. Media tools should be started `recvonly'.
Specifying the attribute type:test is suggested as a hint that,
unless explicitly requested otherwise, receivers can safely avoid
displaying this session description to users.
The type attribute is a session-level attribute, and is not depen-
dent on charset.
a=charset:<character set>
This specifies the character set to be used to display the session
name and information data. By default, the ISO-10646 character set
in UTF-8 encoding is used. If a more compact representation is
required, other character sets may be used such as ISO-8859-1 for
Northern European languages. In particular, the ISO 8859-1 is
specified with the following SDP attribute:
a=charset:ISO-8859-1
This is a session-level attribute; if this attribute is present, it
must be before the first media field. The charset specified MUST be
one of those registered with IANA, such as ISO-8859-1. The charac-
ter set identifier is a US-ASCII string and MUST be compared against
the IANA identifiers using a case-insensitive comparison. If the
indentifier is not recognised or not supported, all strings that are
affected by it SHOULD be regarded as byte strings.
Note that a character set specified MUST still prohibit the use of
bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets requiring
the use of these characters MUST define a quoting mechanism that
prevents these bytes appearing within text fields.
a=sdplang:<language tag>
This can be a session level attribute or a media level attribute.
As a session level attribute, it specifies the language for the ses-
sion 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 if the session descrip-
tion 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
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important.
The sdplang attribute value must be a single RFC 1766 language tag
in US-ASCII. It is not dependent on the charset attribute. An
sdplang attribute SHOULD be specified when a session is of suffi-
cient 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 speci-
fies the language for that media, overriding any session-level
language specified. Multiple lang attributes can be provided either
at session or media level if multiple languages if the session
description or media use multiple languages, in which case the order
of the attributes indicates the order of importance of the various
languages in the session or media from most important to least
important.
The lang attribute value must be a single RFC 1766 language tag in
US-ASCII. It is not dependent on the charset attribute. A lang
attribute SHOULD be specified when a session is of sufficient scope
to cross geographic boundaries where the language of recipients can-
not be assumed, or where the session is in a different language from
the locally assumed norm.
a=framerate:<frame rate>
This gives the maximum video frame rate in frames/sec. It is
intended as a recommendation for the encoding of video data.
Decimal representations of fractional values using the notation
"<integer>.<fraction>" are allowed. It is a media attribute, is
only defined for video media, and is not dependent on charset.
a=quality:<quality>
This gives a suggestion for the quality of the encoding as an
integer value.
The intention of the quality attribute for video is to specify a
non-default trade-off between frame-rate and still-image quality.
For video, the value in the range 0 to 10, with the following sug-
gested meaning:
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10 - the best still-image quality the compression scheme can give.
5 - the default behaviour given no quality suggestion.
0 - the worst still-image quality the codec designer thinks is
still usable.
It is a media attribute, and is not dependent on charset.
a=fmtp:<format> <format specific parameters>
This attribute allows parameters that are specific to a particular
format to be conveyed in a way that SDP doesn't have to understand
them. The format must be one of the formats specified for the
media. Format-specific parameters may be any set of parameters
required to be conveyed by SDP and given unchanged to the media tool
that will use this format.
It is a media attribute, and is not dependent on charset.
6.1. Communicating Conference Control Policy
There is some debate over the way conference control policy should be
communicated. In general, the authors believe that an implicit 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 properties such
as `recvonly' for some of the media tools. This conference attribute
conveys the conference control policy. An example might be:
a=type:moderated
In some cases, however, it is possible that this may be insufficient to
communicate the details of an unusual conference control policy. If
this is the case, then a conference attribute specifying external 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. An example is an ITU H.332 session:
...
c=IN IP4 224.5.6.7
a=type:H332
m=audio 12345 RTP/AVP 0
m=video 12347 RTP/AVP 31
m=application 12349 udp wb
m=control 12341 H323 mc
c=IN IP4 134.134.157.81
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In this example, a general conference attribute (type:H332) is specified
stating that conference control will be provided by an external H.332
tool, and a contact addresses for the H.323 session multipoint con-
troller is given.
In this document, only the declaritive style of conference control
declaration is specified. Other forms of conference control should
specify an appropriate type attribute, and should define the implica-
tions this has for control media.
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7. Security Considerations
SDP is a session description format that describes multimedia sessions.
A session description should not be trusted unless it has been obtained
by an authenticated transport protocol from a trusted source. Many dif-
ferent transport protocols may be used to distribute session descrip-
tion, and the nature of the authentication will differ from transport to
transport.
One transport that will frequently be used to distribute session
descriptions is the Session Announcement Protocol (SAP). SAP provides
both encryption and authentication mechanisms but due to the nature of
session announcements it is likely that there are many occasions where
the originator of a session announcement cannot be authenticated because
they are previously unknown to the receiver of the announcement and
because no common public key infrastructure is available.
On receiving a session description over an unauthenticated transport
mechanism or from an untrusted party, software parsing the session
should take a few precautions. Session description contain information
required to start software on the receivers system. Software that
parses a session description MUST not be able to start other software
except that which is specifically configured as appropriate software to
participate in multimedia sessions. It is normally considered INAP-
PROPRIATE for software parsing a session description to start, on a
user's system, software that is appropriate to participate in multimedia
sessions, without the user first being informed that such software will
be started and giving their consent. Thus a session description arriv-
ing by session announcement, email, session invitation, or WWW page
SHOULD not deliver the user into an {it interactive} multimedia session
without the user being aware that this will happen. As it is not always
simple to tell whether a session is interactive or not, applications
that are unsure should assume sessions are interactive.
In this specification, there are no attributes which would allow the
recipient of a session description to be informed to start multimedia
tools in a mode where they default to transmitting. Under some cir-
cumstances it might be appropriate to define such attributes. If this
is done an application parsing a session description containing such
attributes SHOULD either ignore them, or inform the user that joining
this session will result in the automatic transmission of multimedia
data. The default behaviour for an unknown attribute is to ignore it.
Session descriptions may be parsed at intermediate systems such as
firewalls for the purposes of opening a hole in the firewall to allow
the participation in multimedia sessions. It is considered INAPPROPRI-
ATE for a firewall to open such holes for unicast data streams unless
the session description comes in a request from inside the firewall.
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For multicast sessions, it is likely that local administrators will
apply their own policies, but the exclusive use of "local" or "site-
local" administrative scope within the firewall and the refusal of the
firewall to open a hole for such scopes will provide separation of glo-
bal multicast sessions from local ones.
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Appendix A: SDP Grammar
This appendix provides an Augmented BNF grammar for SDP.
announcement ::= proto-version
origin-field
session-name-field
information-field
uri-field
email-fields
phone-fields
connection-field
bandwidth-fields
time-fields
key-field
attribute-fields
media-descriptions
proto-version ::= "v=" 1*(DIGIT) CRLF
;this draft describes version 0
origin-field ::= "o=" username space
sess-id space sess-version space
nettype space addrtype space
addr CRLF
session-name-field ::= "s=" text CRLF
information-field ::= ["i=" text CRLF]
uri-field ::= ["u=" uri CRLF]
email-fields ::= *("e=" email-address CRLF)
phone-fields ::= *("p=" phone-number CRLF)
connection-field ::= ["c=" nettype space addrtype space
connection-address CRLF]
;a connection field must be present
;in every media description or at the
;session-level
bandwidth-fields ::= *("b=" bwtype ":" bandwidth CRLF)
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time-fields ::= 1*( "t=" start-time space stop-time
*(CRLF repeat-fields) CRLF)
[zone-adjustments CRLF]
repeat-fields ::= "r=" repeat-interval space typed-time
1*(space typed-time)
zone-adjustments ::= time space [``-''] typed-time
*(space time space [``-''] typed-time)
key-field ::= ["k=" key-type CRLF]
key-type ::= "prompt" |
"clear:" key-data |
"base64:" key-data |
"uri:" uri
key-data ::= email-safe | "~" | "
attribute-fields ::= *("a=" attribute CRLF)
media-descriptions ::= *( media-field
information-field
*(connection-field)
bandwidth-fields
key-field
attribute-fields )
media-field ::= "m=" media space port ["/" integer]
space proto (space fmt)+ CRLF
media ::= 1*(alpha-numeric)
;typically "audio", "video", "application"
;or "data"
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fmt ::= 1*(alpha-numeric)
;typically an RTP payload type for audio
;and video media
proto ::= 1*(alpha-numeric)
;typically "RTP/AVP" or "udp" for IP4
port ::= 1*(DIGIT)
;should in the range "1024" to "65535" inclusive
;for UDP based media
attribute ::= (att-field ":" att-value) | att-field
att-field ::= 1*(alpha-numeric)
att-value ::= byte-string
sess-id ::= 1*(DIGIT)
;should be unique for this originating username/host
sess-version ::= 1*(DIGIT)
;0 is a new session
connection-address ::= multicast-address
| unicast-address
multicast-address ::=
3*(decimal_uchar ".") decimal_uchar "/" ttl
[ "/" integer ]
;multicast addresses may be in the range
;224.0.0.0 to 239.255.255.255
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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
typed-time ::= 1*(DIGIT) [fixed-len-time-unit]
fixed-len-time-unit ::= ``d'' | ``h'' | ``m'' | ``s''
bwtype ::= 1*(alpha-numeric)
bandwidth ::= 1*(DIGIT)
username ::= safe
;pretty wide definition, but doesn't include space
email-address ::= email | email "(" email-safe ")" |
email-safe "<" email ">"
email ::= ;defined in RFC822
uri::= ;defined in RFC1630
phone-number ::= phone | phone "(" email-safe ")" |
email-safe "<" phone ">"
phone ::= "+" POS-DIGIT 1*(space | "-" | DIGIT)
;there must be a space or hyphen between the
;international code and the rest of the number.
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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 ::= byte-string
;default is to interpret this as IS0-10646 UTF8
;ISO 8859-1 requires a "a=charset:ISO-8859-1"
;session-level attribute to be used
byte-string ::= 1*(0x01..0x09|0x0b|0x0c|0x0e..0xff)
;any byte except NUL, CR or LF
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)
alpha-numeric ::= ALPHA | DIGIT
DIGIT ::= 0 | POS-DIGIT
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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
email-safe ::= safe | space | tab
safe ::= alpha-numeric |
"'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
"#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" |
"]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
"~" | "
space ::= ;ascii code 32
tab ::= ;ascii code 9
CRLF ::= ;ascii code 13 followed by ascii code 10
Handley/Jacobson [Page 35]
INTERNET-DRAFT 21st Nov 1997
Appendix C: Authors' Addresses
Mark Handley
Information Sciences Institute
c/o MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139
United States
electronic mail: mjh@isi.edu
Van Jacobson
MS 46a-1121
Lawrence Berkeley Laboratory
Berkeley, CA 94720
United States
electronic mail: van@ee.lbl.gov
Acknowledgments
Many people in the IETF 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 and Steve Hanna.
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'', RFC 1889
[3] H. Schulzrinne, ``RTP Profile for Audio and Video Conferences with
Minimal Control'', RFC 1890
[4] M. Handley, ``SAP - Session Announcement Protocol'', INTERNET-DRAFT,
November 25th 1996.
[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-
Handley/Jacobson [Page 36]
INTERNET-DRAFT 21st Nov 1997
octet Coded Character Set (UCS).
[8] D. Goldsmith, M. Davis, ``Using Unicode with MIME'', RFC1641, July
1994
[9] F. Yergeau, ``UTF-8, a transformation format of Unicode and ISO
10646'', RFC 2044, Oct 30th 1996
[10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for Receiv-
ing Internet-based H.323 Conferences".
[11] M. Handley, E. Schooler, H. Schulzrinne, ``Session Initiation Pro-
tocol (SIP)'' Internet Draft, July 1997.
[12] H. Schulzrinne, A. Rao, R. Lanphier, ``Real Time Streaming Protocol
(RTSP)'' Internet Draft, July 1997.
Handley/Jacobson [Page 37]