SDP: Session Description Protocol
Network Working Group M. Handley
Request for Comments: 2327 V. Jacobson
Category: Standards Track ISI/LBNL
April 1998
SDP: Session Description Protocol
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document defines the Session Description Protocol, SDP. SDP is
intended for describing multimedia sessions for the purposes of
session announcement, session invitation, and other forms of
multimedia session initiation.
This document is a product of the Multiparty Multimedia Session
Control (MMUSIC) working group of the Internet Engineering Task
Force. Comments are solicited and should be addressed to the working
group's mailing list at confctrl@isi.edu and/or the authors.
1. Introduction
On the Internet multicast backbone (Mbone), a session directory tool
is used to advertise multimedia conferences and communicate the
conference addresses and conference tool-specific information
necessary for participation. This document defines a session
description protocol for this purpose, and for general real-time
multimedia session description purposes. This memo does not describe
multicast address allocation or the distribution of SDP messages in
detail. These are described in accompanying memos. SDP is not
intended for negotiation of media encodings.
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2. Background
The Mbone is the part of the internet that supports IP multicast, and
thus permits efficient many-to-many communication. It is used
extensively for multimedia conferencing. Such conferences usually
have the property that tight coordination of conference membership is
not necessary; to receive a conference, a user at an Mbone site only
has to know the conference's multicast group address and the UDP
ports for the conference data streams.
Session directories assist the advertisement of conference sessions
and communicate the relevant conference setup information to
prospective participants. SDP is designed to convey such information
to recipients. SDP is purely a format for session description - it
does not incorporate a transport protocol, and is intended to use
different transport protocols as appropriate including the Session
Announcement Protocol [4], Session Initiation Protocol [11], Real-
Time Streaming Protocol [12], electronic mail using the MIME
extensions, and the Hypertext Transport Protocol.
SDP is intended to be general purpose so that it can be used for a
wider range of network environments and applications than just
multicast session directories. However, it is not intended to
support negotiation of session content or media encodings - this is
viewed as outside the scope of session description.
3. Glossary of Terms
The following terms are used in this document, and have specific
meaning within the context of this document.
Conference
A multimedia conference is a set of two or more communicating users
along with the software they are using to communicate.
Session
A multimedia session is a set of multimedia senders and receivers
and the data streams flowing from senders to receivers. A
multimedia conference is an example of a multimedia session.
Session Advertisement
See session announcement.
Session Announcement
A session announcement is a mechanism by which a session
description is conveyed to users in a proactive fashion, i.e., the
session description was not explicitly requested by the user.
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Session Description
A well defined format for conveying sufficient information to
discover and participate in a multimedia session.
3.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
4. SDP Usage
4.1. Multicast Announcements
SDP is a session description protocol for multimedia sessions. A
common mode of usage is for a client to announce a conference session
by periodically multicasting an announcement packet to a well known
multicast address and port using the Session Announcement Protocol
(SAP).
SAP packets are UDP packets with the following format:
|--------------------|
| SAP header |
|--------------------|
| text payload |
|//////////
The header is the Session Announcement Protocol header. SAP is
described in more detail in a companion memo [4]
The text payload is an SDP session description, as described in this
memo. The text payload should be no greater than 1 Kbyte in length.
If announced by SAP, only one session announcement is permitted in a
single packet.
4.2. Email and WWW Announcements
Alternative means of conveying session descriptions include
electronic mail and the World Wide Web. For both email and WWW
distribution, the use of the MIME content type "application/sdp"
should be used. This enables the automatic launching of applications
for participation in the session from the WWW client or mail reader
in a standard manner.
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Note that announcements of multicast sessions made only via email or
the World Wide Web (WWW) do not have the property that the receiver
of a session announcement can necessarily receive the session because
the multicast sessions may be restricted in scope, and access to the
WWW server or reception of email is possible outside this scope. SAP
announcements do not suffer from this mismatch.
5. Requirements and Recommendations
The purpose of SDP is to convey information about media streams in
multimedia sessions to allow the recipients of a session description
to participate in the session. SDP is primarily intended for use in
an internetwork, although it is sufficiently general that it can
describe conferences in other network environments.
A multimedia session, for these purposes, is defined as a set of
media streams that exist for some duration of time. Media streams
can be many-to-many. The times during which the session is active
need not be continuous.
Thus far, multicast based sessions on the Internet have differed from
many other forms of conferencing in that anyone receiving the traffic
can join the session (unless the session traffic is encrypted). In
such an environment, SDP serves two primary purposes. It is a means
to communicate the existence of a session, and is a means to convey
sufficient information to enable joining and participating in the
session. In a unicast environment, only the latter purpose is likely
to be relevant.
Thus SDP includes:
o Session name and purpose
o Time(s) the session is active
o The media comprising the session
o Information to receive those media (addresses, ports, formats and
so on)
As resources necessary to participate in a session may be limited,
some additional information may also be desirable:
o Information about the bandwidth to be used by the conference
o Contact information for the person responsible for the session
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In general, SDP must convey sufficient information to be able to join
a session (with the possible exception of encryption keys) and to
announce the resources to be used to non-participants that may need
to know.
5.1. Media Information
SDP includes:
o The type of media (video, audio, etc)
o The transport protocol (RTP/UDP/IP, H.320, etc)
o The format of the media (H.261 video, MPEG video, etc)
For an IP multicast session, the following are also conveyed:
o Multicast address for media
o Transport Port for media
This address and port are the destination address and destination
port of the multicast stream, whether being sent, received, or both.
For an IP unicast session, the following are conveyed:
o Remote address for media
o Transport port for contact address
The semantics of this address and port depend on the media and
transport protocol defined. By default, this is the remote address
and remote port to which data is sent, and the remote address and
local port on which to receive data. However, some media may define
to use these to establish a control channel for the actual media
flow.
5.2. Timing Information
Sessions may either be bounded or unbounded in time. Whether or not
they are bounded, they may be only active at specific times.
SDP can convey:
o An arbitrary list of start and stop times bounding the session
o For each bound, repeat times such as "every Wednesday at 10am for
one hour"
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This timing information is globally consistent, irrespective of local
time zone or daylight saving time.
5.3. Private Sessions
It is possible to create both public sessions and private sessions.
Private sessions will typically be conveyed by encrypting the session
description to distribute it. The details of how encryption is
performed are dependent on the mechanism used to convey SDP - see [4]
for how this is done for session announcements.
If a session announcement is private it is possible to use that
private announcement to convey encryption keys necessary to decode
each of the media in a conference, including enough information to
know which encryption scheme is used for each media.
5.4. Obtaining Further Information about a Session
A session description should convey enough information to decide
whether or not to participate in a session. SDP may include
additional pointers in the form of Universal Resources Identifiers
(URIs) for more information about the session.
5.5. Categorisation
When many session descriptions are being distributed by SAP or any
other advertisement mechanism, it may be desirable to filter
announcements that are of interest from those that are not. SDP
supports a categorisation mechanism for sessions that is capable of
being automated.
5.6. Internationalization
The SDP specification recommends the use of the ISO 10646 character
sets in the UTF-8 encoding (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.
6. SDP Specification
SDP session descriptions are entirely textual using the ISO 10646
character set in UTF-8 encoding. SDP field names and attributes names
use only the US-ASCII subset of UTF-8, but textual fields and
attribute values may use the full ISO 10646 character set. The
textual form, as opposed to a binary encoding such as ASN/1 or XDR,
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was chosen to enhance portability, to enable a variety of transports
to be used (e.g, session description in a MIME email message) and to
allow flexible, text-based toolkits (e.g., Tcl/Tk ) to be used to
generate and to process session descriptions. However, since the
total bandwidth allocated to all SAP announcements is strictly
limited, the encoding is deliberately compact. Also, since
announcements may be transported via very unreliable means (e.g.,
email) or damaged by an intermediate caching server, the encoding was
designed with strict order and formatting rules so that most errors
would result in malformed announcements which could be detected
easily and discarded. This also allows rapid discarding of encrypted
announcements for which a receiver does not have the correct key.
An SDP session description consists of a number of lines of text of
the form = is always exactly one character and is
case-significant. is a structured text string whose format
depends on . It also will be case-significant unless a
specific field defines otherwise. Whitespace is not permitted either
side of the `=' sign. In general is either a number of fields
delimited by a single space character or a free format string.
A session description consists of a session-level description
(details that apply to the whole session and all media streams) and
optionally several media-level descriptions (details that apply onto
to a single media stream).
An announcement consists of a session-level section followed by zero
or more media-level sections. The session-level part starts with a
`v=' line and continues to the first media-level section. The media
description starts with an `m=' line and continues to the next media
description or end of the whole session description. In general,
session-level values are the default for all media unless overridden
by an equivalent media-level value.
When SDP is conveyed by SAP, only one session description is allowed
per packet. When SDP is conveyed by other means, many SDP session
descriptions may be concatenated together (the `v=' line indicating
the start of a session description terminates the previous
description). Some lines in each description are required and some
are optional but all must appear in exactly the order given here (the
fixed order greatly enhances error detection and allows for a simple
parser). Optional items are marked with a `*'.
Session description
v= (protocol version)
o= (owner/creator and session identifier).
s= (session name)
i=* (session information)
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u=* (URI of description)
e=* (email address)
p=* (phone number)
c=* (connection information - not required if included in all media)
b=* (bandwidth information)
One or more time descriptions (see below)
z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)
Zero or more media descriptions (see below)
Time description
t= (time the session is active)
r=* (zero or more repeat times)
Media description
m= (media name and transport address)
i=* (media title)
c=* (connection information - optional if included at session-level)
b=* (bandwidth information)
k=* (encryption key)
a=* (zero or more media attribute lines)
The set of `type' letters is deliberately small and not intended to
be extensible -- SDP parsers must completely ignore any announcement
that contains a `type' letter that it does not understand. The
`attribute' mechanism ("a=" described below) is the primary means for
extending SDP and tailoring it to particular applications or media.
Some attributes (the ones listed in this document) have a defined
meaning but others may be added on an application-, media- or
session-specific basis. A session directory must ignore any
attribute it doesn't understand.
The connection (`c=') and attribute (`a=') information in the
session-level section applies to all the media of that session unless
overridden by connection information or an attribute of the same name
in the media description. For instance, in the example below, each
media behaves as if it were given a `recvonly' attribute.
An example SDP description is:
v=0
o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
e=mjh@isi.edu (Mark Handley)
c=IN IP4 224.2.17.12/127
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t=2873397496 2873404696
a=recvonly
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 31
m=application 32416 udp wb
a=orient:portrait
Text records such as the session name and information are bytes
strings which may contain any byte with the exceptions of 0x00 (Nul),
0x0a (ASCII newline) and 0x0d (ASCII carriage return). The sequence
CRLF (0x0d0a) is used to end a record, although parsers should be
tolerant and also accept records terminated with a single newline
character. By default these byte strings contain ISO-10646
characters in UTF-8 encoding, but this default may be changed using
the `charset' attribute.
Protocol Version
v=0
The "v=" field gives the version of the Session Description Protocol.
There is no minor version number.
Origin
o=
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. is the user's login on the originating
host, or it is "-" if the originating host does not support the
concept of user ids. must not contain spaces. is a numeric string
such that the tuple of , , ,
and
form a globally unique identifier for the session. The
method of 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]. 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 Handley & Jacobson Standards Track [Page 9]
RFC 2327 SDP April 1998 creating tool, so long as is increased when
a modification is made to the session data. Again, it is
recommended (but not mandatory) that an NTP timestamp is used. is a
text string giving the type of network. Initially "IN" is defined
to have the meaning "Internet".
is a text string giving the type of the address
that follows. Initially "IP4" and "IP6" are defined.
is the globally unique address of the machine from which
the session was created. For an address type of IP4, this is either
the fully-qualified domain name of the machine, or the
dotted-decimal representation of the IP version 4 address of the
machine. For an address type of IP6, this is either the
fully-qualified domain name of the machine, or the compressed
textual representation of the IP version 6 address of the machine.
For both IP4 and IP6, the fully-qualified domain name is the form
that SHOULD be given unless this is unavailable, in which case the
globally unique address may be substituted. A local IP address MUST
NOT be used in any context where the SDP description might leave
the scope in which the address is meaningful. In general, the "o="
field serves as a globally unique identifier for this version of
this session description, and the subfields excepting the version
taken together identify the session irrespective of any
modifications. Session Name s= 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= 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 Handley & Jacobson Standards
Track [Page 10] RFC 2327 SDP April 1998 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= 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= p= o These
specify contact information for the person responsible for the
conference. This is not necessarily the same person that created
the conference announcement. o Either an email field or a phone
field must be specified. Additional email and phone fields are
allowed. o If these are present, they should be specified before
the first media field. o More than one email or phone field can be
given for a session description. o Phone numbers should be given in
the conventional international format - preceded by a "+ and the
international country code. There must be a space or a hyphen ("-")
between the country code and the rest of the phone number. Spaces
and hyphens may be used to split up a phone field to aid
readability if desired. For example: p=+44-171-380-7777 or p=+1 617
253 6011 Handley & Jacobson Standards Track [Page 11] RFC 2327
SDP April 1998 o Both email addresses and phone numbers can have an
optional free text string associated with them, normally giving the
name of the person who may be contacted. This should be enclosed in
parenthesis if it is present. For example: e=mjh@isi.edu (Mark
Handley) The alternative RFC822 name quoting convention is also
allowed for both email addresses and phone numbers. For example,
e=Mark Handley 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=
The "c=" field contains connection data. A session
announcement must contain one "c=" field in each media description
(see below) or a "c=" field at the session-level. It may contain a
session-level "c=" field and one additional "c=" field per media
description, in which case the per-media values override the
session-level settings for the relevant media. The first sub-field
is the network type, which is a text string giving the type of
network. Initially "IN" is defined to have the meaning "Internet".
The second sub-field is the address type. This allows SDP to be
used for sessions that are not IP based. Currently only IP4 is
defined. The third sub-field is the connection address. Optional
extra subfields may be added after the connection address depending
on the value of the
field. For IP4 addresses, the connection address
is defined as follows: o Typically the connection address will be a
class-D IP multicast group address. If the session is not
multicast, then the connection address contains the fully-qualified
domain name or the unicast IP address of the expected data source
or data relay or data sink as determined by additional attribute
fields. It is not expected that fully-qualified domain names or
unicast addresses Handley & Jacobson Standards Track [Page 12]
RFC 2327 SDP April 1998 will be given in a session description that
is communicated by a multicast announcement, though this is not
prohibited. If a unicast data stream is to pass through a network
address translator, the use of a fully-qualified domain name rather
than an unicast IP address is RECOMMENDED. In other cases, the use
of an IP address to specify a particular interface on a multi-homed
host might be required. Thus this specification leaves the decision
as to which to use up to the individual application, but all
applications MUST be able to cope with receiving both formats. o
Conferences using an IP multicast connection address must also have
a time to live (TTL) value present in addition to the multicast
address. The TTL and the address together define the scope with
which multicast packets sent in this conference will be sent. TTL
values must be in the range 0-255. The TTL for the session is
appended to the address using a slash as a separator. An example
is: c=IN IP4 224.2.1.1/127 Hierarchical or layered encoding schemes
are data streams where the encoding from a single media source is
split into a number of layers. The receiver can choose the desired
quality (and hence bandwidth) by only subscribing to a subset of
these layers. Such layered encodings are normally transmitted in
multiple multicast groups to allow multicast pruning. This
technique keeps unwanted traffic from sites only requiring certain
levels of the hierarchy. For applications requiring multiple
multicast groups, we allow the following notation to be used for
the connection address: // If the number of addresses is not given
it is assumed to be one. Multicast addresses so assigned are
contiguously allocated above the base address, so that, for
example: c=IN IP4 224.2.1.1/127/3 would state that addresses
224.2.1.1, 224.2.1.2 and 224.2.1.3 are to be used at a ttl of 127.
This is semantically identical to including multiple "c=" lines in
a media description: c=IN IP4 224.2.1.1/127 c=IN IP4 224.2.1.2/127
c=IN IP4 224.2.1.3/127 Handley & Jacobson Standards Track [Page
13] RFC 2327 SDP April 1998 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=:
o This specifies the proposed bandwidth to be used by the session
or media, and is optional. o is in kilobits per second o is a
single alphanumeric word giving the meaning of the bandwidth
figure. o Two modifiers are initially defined: CT Conference Total:
An implicit maximum bandwidth is associated with each TTL on the
Mbone or within a particular multicast administrative scope region
(the Mbone bandwidth vs. TTL limits are given in the MBone FAQ). If
the bandwidth of a session or media in a session is different from
the bandwidth implicit from the scope, a `b=CT:...' line should be
supplied for the session giving the proposed upper limit to the
bandwidth used. The primary purpose of this is to give an
approximate idea as to whether two or more conferences can co-exist
simultaneously. AS Application-Specific Maximum: The bandwidth is
interpreted to be application-specific, i.e., will be the
application's concept of maximum bandwidth. Normally this will
coincide with what is set on the application's "maximum bandwidth"
control if applicable. Note that CT gives a total bandwidth figure
for all the media at all sites. AS gives a bandwidth figure for a
single media at a single site, although there may be many sites
sending simultaneously. o Extension Mechanism: Tool writers can
define experimental bandwidth modifiers by prefixing their modifier
with "X-". For example: b=X-YZ:128 SDP parsers should ignore
bandwidth fields with unknown modifiers. Modifiers should be
alpha-numeric and, although no length limit is given, they are
recommended to be short. Handley & Jacobson Standards Track
[Page 14] RFC 2327 SDP April 1998 Times, Repeat Times and Time
Zones t= o "t=" fields specify the start and stop times for a
conference session. Multiple "t=" fields may be used if a session
is active at multiple irregularly spaced times; each additional
"t=" field specifies an additional period of time for which the
session will be active. If the session is active at regular times,
an "r=" field (see below) should be used in addition to and
following a "t=" field - in which case the "t=" field specifies the
start and stop times of the repeat sequence. o The first and second
sub-fields give the start and stop times for the conference
respectively. These values are the decimal representation of
Network Time Protocol (NTP) time values in seconds [1]. To convert
these values to UNIX time, subtract decimal 2208988800. o If the
stop-time is set to zero, then the session is not bounded, though
it will not become active until after the start-time. If the
start-time is also zero, the session is regarded as permanent. User
interfaces should strongly discourage the creation of unbounded and
permanent sessions as they give no information about when the
session is actually going to terminate, and so make scheduling
difficult. The general assumption may be made, when displaying
unbounded sessions that have not timed out to the user, that an
unbounded session will only be active until half an hour from the
current time or the session start time, whichever is the later. If
behaviour other than this is required, an end-time should be given
and modified as appropriate when new information becomes available
about when the session should really end. Permanent sessions may be
shown to the user as never being active unless there are associated
repeat times which state precisely when the session will be active.
In general, permanent sessions should not be created for any
session expected to have a duration of less than 2 months, and
should be discouraged for sessions expected to have a duration of
less than 6 months. r= 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 Handley & Jacobson Standards Track
[Page 15] RFC 2327 SDP April 1998 hour each week for three months,
then the in the corresponding "t=" field would be the NTP
representation of 10am on the first Monday, the would be 1 week,
the would be 1 hour, and the offsets would be zero and 25 hours.
The corresponding "t=" field stop time would be the NTP
representation of the end of the last session three months later.
By default all fields are in seconds, so the "r=" and "t=" fields
might be: t=3034423619 3042462419 r=604800 3600 0 90000 To make
announcements more compact, times may also be given in units of
days, hours or minutes. The syntax for these is a number
immediately followed by a single case-sensitive character.
Fractional units are not allowed - a smaller unit should be used
instead. The following unit specification characters are allowed: d
- days (86400 seconds) h - minutes (3600 seconds) m - minutes (60
seconds) s - seconds (allowed for completeness but not recommended)
Thus, the above announcement could also have been written: r=7d 1h
0 25h Monthly and yearly repeats cannot currently be directly
specified with a single SDP repeat time - instead separate "t"
fields should be used to explicitly list the session times. z= ....
o To schedule a repeated session which spans a change from
daylight- saving time to standard time or vice-versa, it is
necessary to specify offsets from the base repeat times. This is
required because different time zones change time at different
times of day, different countries change to or from daylight time
on different dates, and some countries do not have daylight saving
time at all. Thus in order to schedule a session that is at the
same time winter and summer, it must be possible to specify
unambiguously by whose time zone a session is scheduled. To
simplify this task for receivers, we allow the sender to specify
the NTP time that a time zone adjustment happens and the offset
from the time when the session was first scheduled. The "z" field
allows the sender to specify a list of these adjustment times and
offsets from the base time. Handley & Jacobson Standards Track
[Page 16] RFC 2327 SDP April 1998 An example might be: z=2882844526
-1h 2898848070 0 This specifies that at time 2882844526 the time
base by which the session's repeat times are calculated is shifted
back by 1 hour, and that at time 2898848070 the session's original
time base is restored. Adjustments are always relative to the
specified start time - they are not cumulative. o If a session is
likely to last several years, it is expected that the session
announcement will be modified periodically rather than transmit
several years worth of adjustments in one announcement. Encryption
Keys k= k=: 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: The encryption key (as
described in [3] for RTP media streams under the AV profile) is
included untransformed in this key field. k=base64: The encryption
key (as described in [3] for RTP media streams under the AV
profile) is included in this key field but has been base64 encoded
because it includes characters that are prohibited in SDP. k=uri: A
Universal Resource Identifier as used by WWW clients is included in
this key field. The URI refers to the data containing the key, and
may require additional authentication Handley & Jacobson
Standards Track [Page 17] RFC 2327 SDP April 1998 before the key
can be returned. When a request is made to the given URI, the MIME
content-type of the reply specifies the encoding for the key in the
reply. The key should not be obtained until the user wishes to join
the session to reduce synchronisation of requests to the WWW
server(s). k=prompt No key is included in this SDP description, but
the session or media stream referred to by this key field is
encrypted. The user should be prompted for the key when attempting
to join the session, and this user-supplied key should then be used
to decrypt the media streams. Attributes a= a=: Attributes are the
primary means for extending SDP. Attributes may be defined to be
used as "session-level" attributes, "media-level" attributes, or
both. A media description may have any number of attributes ("a="
fields) which are media specific. These are referred to as
"media-level" attributes and add information about the media
stream. Attribute fields can also be added before the first media
field; these "session-level" attributes convey additional
information that applies to the conference as a whole rather than
to individual media; an example might be the conference's floor
control policy. Attribute fields may be of two forms: o property
attributes. A property attribute is simply of the form "a=". 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=:". An example might be that a whiteboard could have the
value attribute "a=orient:landscape" Attribute interpretation
depends on the media tool being invoked. Thus receivers of session
descriptions should be configurable in their interpretation of
announcements in general and of attributes in particular. Attribute
names must be in the US-ASCII subset of ISO-10646/UTF-8. Handley
& Jacobson Standards Track [Page 18] RFC 2327 SDP April 1998
Attribute values are byte strings, and MAY use any byte value
except 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute
values are to be interpreted as in ISO-10646 character set with
UTF-8 encoding. Unlike other text fields, attribute values are NOT
normally affected by the `charset' attribute as this would make
comparisons against known values problematic. However, when an
attribute is defined, it can be defined to be charset-dependent, in
which case it's value should be interpreted in the session charset
rather than in ISO-10646. Attributes that will be commonly used can
be registered with IANA (see Appendix B). Unregistered attributes
should begin with "X-" to prevent inadvertent collision with
registered attributes. In either case, if an attribute is received
that is not understood, it should simply be ignored by the
receiver. Media Announcements m= A session description may contain
a number of media descriptions. Each media description starts with
an "m=" field, and is terminated by either the next "m=" field or
by the end of the session description. A media field also has
several sub-fields: o The first sub-field is the media type.
Currently defined media are "audio", "video", "application", "data"
and "control", though this list may be extended as new
communication modalities emerge (e.g., telepresense). The
difference between "application" and "data" is that the former is a
media flow such as whiteboard information, and the latter is
bulk-data transfer such as multicasting of program executables
which will not typically be displayed to the user. "control" is
used to specify an additional conference control channel for the
session. o The second sub-field is the transport port to which the
media stream will be sent. The meaning of the transport port
depends on the network being used as specified in the relevant "c"
field and on the transport protocol defined in the third sub-field.
Other ports used by the media application (such as the RTCP port,
see [2]) should be derived algorithmically from the base media
port. Note: For transports based on UDP, the value should be in the
range 1024 to 65535 inclusive. For RTP compliance it should be an
even number. Handley & Jacobson Standards Track [Page 19] RFC 2327 SDP April 1998 For applications where hierarchically encoded
streams are being sent to a unicast address, it may be necessary to
specify multiple transport ports. This is done using a similar
notation to that used for IP multicast addresses in the "c=" field:
m= / In such a case, the ports used depend on the transport
protocol. For RTP, only the even ports are used for data and the
corresponding one-higher odd port is used for RTCP. For example:
m=video 49170/2 RTP/AVP 31 would specify that ports 49170 and 49171
form one RTP/RTCP pair and 49172 and 49173 form the second RTP/RTCP
pair. RTP/AVP is the transport protocol and 31 is the format (see
below). It is illegal for both multiple addresses to be specified
in the "c=" field and for multiple ports to be specified in the
"m=" field in the same session description. o The third sub-field
is the transport protocol. The transport protocol values are
dependent on the address-type field in the "c=" fields. Thus a "c="
field of IP4 defines that the transport protocol runs over IP4. For
IP4, it is normally expected that most media traffic will be
carried as RTP over UDP. The following transport protocols are
preliminarily defined, but may be extended through registration of
new protocols with IANA: - RTP/AVP - the IETF's Realtime Transport
Protocol using the Audio/Video profile carried over UDP. - udp -
User Datagram Protocol If an application uses a single combined
proprietary media format and transport protocol over UDP, then
simply specifying the transport protocol as udp and using the
format field to distinguish the combined protocol is recommended.
If a transport protocol is used over UDP to carry several distinct
media types that need to be distinguished by a session directory,
then specifying the transport protocol and media format separately
is necessary. RTP is an example of a transport-protocol that
carries multiple payload formats that must be distinguished by the
session directory for it to know how to start appropriate tools,
relays, mixers or recorders. Handley & Jacobson Standards Track
[Page 20] RFC 2327 SDP April 1998 The main reason to specify the
transport-protocol in addition to the media format is that the same
standard media formats may be carried over different transport
protocols even when the network protocol is the same - a historical
example is vat PCM audio and RTP PCM audio. In addition, relays and
monitoring tools that are transport-protocol-specific but
format-independent are possible. For RTP media streams operating
under the RTP Audio/Video Profile [3], the protocol field is
"RTP/AVP". Should other RTP profiles be defined in the future,
their profiles will be specified in the same way. For example, the
protocol field "RTP/XYZ" would specify RTP operating under a
profile whose short name is "XYZ". o The fourth and subsequent
sub-fields are media formats. For audio and video, these will
normally be a media payload type as defined in the RTP Audio/Video
Profile. When a list of payload formats is given, this implies that
all of these formats may be used in the session, but the first of
these formats is the default format for the session. For media
whose transport protocol is not RTP or UDP the format field is
protocol specific. Such formats should be defined in an additional
specification document. For media whose transport protocol is RTP,
SDP can be used to provide a dynamic binding of media encoding to
RTP payload type. The encoding names in the RTP AV Profile do not
specify unique audio encodings (in terms of clock rate and number
of audio channels), and so they are not used directly in SDP format
fields. Instead, the payload type number should be used to specify
the format for static payload types and the payload type number
along with additional encoding information should be used for
dynamically allocated payload types. An example of a static payload
type is u-law PCM coded single channel audio sampled at 8KHz. This
is completely defined in the RTP Audio/Video profile as payload
type 0, so the media field for such a stream sent to UDP port 49232
is: m=video 49232 RTP/AVP 0 An example of a dynamic payload type is
16 bit linear encoded stereo audio sampled at 16KHz. If we wish to
use dynamic RTP/AVP payload type 98 for such a stream, additional
information is required to decode it: m=video 49232 RTP/AVP 98
Handley & Jacobson Standards Track [Page 21] RFC 2327 SDP April
1998 a=rtpmap:98 L16/16000/2 The general form of an rtpmap
attribute is: a=rtpmap: /[/] For audio streams, may specify the
number of audio channels. This parameter may be omitted if the
number of channels is one provided no additional parameters are
needed. For video streams, no encoding parameters are currently
specified. Additional parameters may be defined in the future, but
codecspecific parameters should not be added. Parameters added to
an rtpmap attribute should only be those required for a session
directory to make the choice of appropriate media too to
participate in a session. Codec-specific parameters should be added
in other attributes. Up to one rtpmap attribute can be defined for
each media format specified. Thus we might have: m=audio 49230
RTP/AVP 96 97 98 a=rtpmap:96 L8/8000 a=rtpmap:97 L16/8000
a=rtpmap:98 L16/11025/2 RTP profiles that specify the use of
dynamic payload types must define the set of valid encoding names
and/or a means to register encoding names if that profile is to be
used with SDP. Experimental encoding formats can also be specified
using rtpmap. RTP formats that are not registered as standard
format names must be preceded by "X-". Thus a new experimental
redundant audio stream called GSMLPC using dynamic payload type 99
could be specified as: m=video 49232 RTP/AVP 99 a=rtpmap:99
X-GSMLPC/8000 Such an experimental encoding requires that any site
wishing to receive the media stream has relevant configured state
in its session directory to know which tools are appropriate. Note
that RTP audio formats typically do not include information about
the number of samples per packet. If a non-default (as defined in
the RTP Audio/Video Profile) packetisation is required, the "ptime"
attribute is used as given below. Handley & Jacobson Standards
Track [Page 22] RFC 2327 SDP April 1998 For more details on RTP
audio and video formats, see [3]. o Formats for non-RTP media
should be registered as MIME content types as described in Appendix
B. For example, the LBL whiteboard application might be registered
as MIME content-type application/wb with encoding considerations
specifying that it operates over UDP, with no appropriate file
format. In SDP this would then be expressed using a combination of
the "media" field and the "fmt" field, as follows: m=application
32416 udp wb Suggested Attributes The following attributes are
suggested. Since application writers may add new attributes as they
are required, this list is not exhaustive. a=cat: 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: Like the cat
attribute, this is to assist identifying wanted sessions at the
receiver. This allows a receiver to select interesting session
based on keywords describing the purpose of the session. It is a
session-level attribute. It is a charset dependent attribute,
meaning that its value should be interpreted in the charset
specified for the session description if one is specified, or by
default in ISO 10646/UTF-8. a=tool: 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:
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. Handley & Jacobson Standards Track
[Page 23] RFC 2327 SDP April 1998 a=recvonly This specifies that
the tools should be started in receive-only mode where applicable.
It can be either a session or media attribute, and is not dependent
on charset. a=sendrecv This specifies that the tools should be
started in send and receive mode. This is necessary for interactive
conferences with tools such as wb which defaults to receive only
mode. It can be either a session or media attribute, and is not
dependent on charset. a=sendonly This specifies that the tools
should be started in send-only mode. An example may be where a
different unicast address is to be used for a traffic destination
than for a traffic source. In such a case, two media descriptions
may be use, one sendonly and one recvonly. It can be either a
session or media attribute, but would normally only be used as a
media attribute, and is not dependent on charset. a=orient:
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: This specifies the type of the conference. Suggested values
are `broadcast', `meeting', `moderated', `test' and `H332'.
`recvonly' should be the default for `type:broadcast' sessions,
`type:meeting' should imply `sendrecv' and `type:moderated' should
indicate the use of a floor control tool and that the media tools
are started so as to "mute" new sites joining the conference.
Specifying the attribute type:H332 indicates that this loosely
coupled session is part of a H.332 session as defined in the ITU
H.332 specification [10]. Media tools should be started `recvonly'.
Specifying the attribute type:test is suggested as a hint that,
unless explicitly requested otherwise, receivers can safely avoid
displaying this session description to users. The type attribute is
a session-level attribute, and is not dependent on charset. Handley
& Jacobson Standards Track [Page 24] RFC 2327 SDP April 1998
a=charset: This specifies the character set to be used to display
the session name and information data. By default, the ISO-10646
character set in UTF-8 encoding is used. If a more compact
representation is required, other character sets may be used such
as ISO-8859-1 for Northern European languages. In particular, the
ISO 8859-1 is specified with the following SDP attribute:
a=charset:ISO-8859-1 This is a session-level attribute; if this
attribute is present, it must be before the first media field. The
charset specified MUST be one of those registered with IANA, such
as ISO-8859-1. The character set identifier is a US-ASCII string
and MUST be compared against the IANA identifiers using a
case-insensitive comparison. If the identifier is not recognised or
not supported, all strings that are affected by it SHOULD be
regarded as byte strings. Note that a character set specified MUST
still prohibit the use of bytes 0x00 (Nul), 0x0A (LF) and 0x0d
(CR). Character sets requiring the use of these characters MUST
define a quoting mechanism that prevents these bytes appearing
within text fields. a=sdplang: This can be a session level
attribute or a media level attribute. As a session level attribute,
it specifies the language for the session description. As a media
level attribute, it specifies the language for any media-level SDP
information field associated with that media. Multiple sdplang
attributes can be provided either at session or media level if
multiple languages in the session description or media use multiple
languages, in which case the order of the attributes indicates the
order of importance of the various languages in the session or
media from most important to least important. In general, sending
session descriptions consisting of multiple languages should be
discouraged. Instead, multiple descriptions should be sent
describing the session, one in each language. However this is not
possible with all transport mechanisms, and so multiple sdplang
attributes are allowed although not recommended. The sdplang
attribute value must be a single 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 Handley & Jacobson
Standards Track [Page 25] RFC 2327 SDP April 1998 sufficient scope
to cross geographic boundaries where the language of recipients
cannot be assumed, or where the session is in a different language
from the locally assumed norm. a=lang: This can be a session level
attribute or a media level attribute. As a session level attribute,
it specifies the default language for the session being described.
As a media level attribute, it specifies the language for that
media, overriding any session- level language specified. Multiple
lang attributes can be provided either at session or media level if
multiple languages if the session description or media use multiple
languages, in which case the order of the attributes indicates the
order of importance of the various languages in the session or
media from most important to least important. The lang attribute
value must be a single 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 cannot be assumed, or
where the session is in a different language from the locally
assumed norm. a=framerate:
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
"." are allowed. It is a media attribute, is
only defined for video media, and is not dependent on charset.
a=quality:
This gives a suggestion for the quality of the encoding as an
integer value.
The intention of the quality attribute for video is to specify a
non-default trade-off between frame-rate and still-image quality.
For video, the value in the range 0 to 10, with the following
suggested meaning:
10 - the best still-image quality the compression scheme can
give.
5 - the default behaviour given no quality suggestion.
0 - the worst still-image quality the codec designer thinks is
still usable.
It is a media attribute, and is not dependent on charset.
Handley & Jacobson Standards Track [Page 26]
RFC 2327 SDP April 1998
a=fmtp:
This attribute allows parameters that are specific to a
particular format to be conveyed in a way that SDP doesn't have
to understand them. The format must be one of the formats
specified for the media. Format-specific parameters may be any
set of parameters required to be conveyed by SDP and given
unchanged to the media tool that will use this format.
It is a media attribute, and is not dependent on charset.
6.1. Communicating Conference Control Policy
There is some debate over the way conference control policy should be
communicated. In general, the authors believe that an implicit
declarative style of specifying conference control is desirable where
possible.
A simple declarative style uses a single conference attribute field
before the first media field, possibly supplemented by properties
such as `recvonly' for some of the media tools. This conference
attribute conveys the conference control policy. An example might be:
a=type:moderated
In some cases, however, it is possible that this may be insufficient
to communicate the details of an unusual conference control policy.
If this is the case, then a conference attribute specifying external
control might be set, and then one or more "media" fields might be
used to specify the conference control tools and configuration data
for those tools. An example is an ITU H.332 session:
c=IN IP4 224.5.6.7
a=type:H332
m=audio 49230 RTP/AVP 0
m=video 49232 RTP/AVP 31
m=application 12349 udp wb
m=control 49234 H323 mc
c=IN IP4 134.134.157.81
In this example, a general conference attribute (type:H332) is
specified stating that conference control will be provided by an
external H.332 tool, and a contact addresses for the H.323 session
multipoint controller is given.
In this document, only the declarative style of conference control
declaration is specified. Other forms of conference control should
specify an appropriate type attribute, and should define the
implications this has for control media.
Handley & Jacobson Standards Track [Page 27]
RFC 2327 SDP April 1998
7. Security Considerations
SDP is a session description format that describes multimedia
sessions. A session description should not be trusted unless it has
been obtained by an authenticated transport protocol from a trusted
source. Many different transport protocols may be used to distribute
session description, and the nature of the authentication will differ
from transport to transport.
One transport that will frequently be used to distribute session
descriptions is the Session Announcement Protocol (SAP). SAP
provides both encryption and authentication mechanisms but due to the
nature of session announcements it is likely that there are many
occasions where the originator of a session announcement cannot be
authenticated because they are previously unknown to the receiver of
the announcement and because no common public key infrastructure is
available.
On receiving a session description over an unauthenticated transport
mechanism or from an untrusted party, software parsing the session
should take a few precautions. Session description contain
information required to start software on the receivers system.
Software that parses a session description MUST not be able to start
other software except that which is specifically configured as
appropriate software to participate in multimedia sessions. It is
normally considered INAPPROPRIATE for software parsing a session
description to start, on a user's system, software that is
appropriate to participate in multimedia sessions, without the user
first being informed that such software will be started and giving
their consent. Thus a session description arriving by session
announcement, email, session invitation, or WWW page SHOULD not
deliver the user into an {it interactive} multimedia session without
the user being aware that this will happen. As it is not always
simple to tell whether a session is interactive or not, applications
that are unsure should assume sessions are interactive.
In this specification, there are no attributes which would allow the
recipient of a session description to be informed to start multimedia
tools in a mode where they default to transmitting. Under some
circumstances it might be appropriate to define such attributes. If
this is done an application parsing a session description containing
such attributes SHOULD either ignore them, or inform the user that
joining this session will result in the automatic transmission of
multimedia data. The default behaviour for an unknown attribute is
to ignore it.
Handley & Jacobson Standards Track [Page 28]
RFC 2327 SDP April 1998
Session descriptions may be parsed at intermediate systems such as
firewalls for the purposes of opening a hole in the firewall to allow
the participation in multimedia sessions. It is considered
INAPPROPRIATE for a firewall to open such holes for unicast data
streams unless the session description comes in a request from inside
the firewall.
For multicast sessions, it is likely that local administrators will
apply their own policies, but the exclusive use of "local" or "site-
local" administrative scope within the firewall and the refusal of
the firewall to open a hole for such scopes will provide separation
of global multicast sessions from local ones.
Handley & Jacobson Standards Track [Page 29]
RFC 2327 SDP April 1998
Appendix A: SDP Grammar
This appendix provides an Augmented BNF grammar for SDP. ABNF is
defined in RFC 2234.
announcement = proto-version
origin-field
session-name-field
information-field
uri-field
email-fields
phone-fields
connection-field
bandwidth-fields
time-fields
key-field
attribute-fields
media-descriptions
proto-version = "v=" 1*DIGIT CRLF
;this memo describes version 0
origin-field = "o=" username space
sess-id space sess-version space
nettype space addrtype space
addr CRLF
session-name-field = "s=" text CRLF
information-field = ["i=" text CRLF]
uri-field = ["u=" uri CRLF]
email-fields = *("e=" email-address CRLF)
phone-fields = *("p=" phone-number CRLF)
connection-field = ["c=" nettype space addrtype space
connection-address CRLF]
;a connection field must be present
;in every media description or at the
;session-level
bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF)
Handley & Jacobson Standards Track [Page 30]
RFC 2327 SDP April 1998
time-fields = 1*( "t=" start-time space stop-time
*(CRLF repeat-fields) CRLF)
[zone-adjustments CRLF]
repeat-fields = "r=" repeat-interval space typed-time
1*(space typed-time)
zone-adjustments = time space ["-"] typed-time
*(space time space ["-"] typed-time)
key-field = ["k=" key-type CRLF]
key-type = "prompt" |
"clear:" key-data |
"base64:" key-data |
"uri:" uri
key-data = email-safe | "~" | "
attribute-fields = *("a=" attribute CRLF)
media-descriptions = *( media-field
information-field
*(connection-field)
bandwidth-fields
key-field
attribute-fields )
media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF
media = 1*(alpha-numeric)
;typically "audio", "video", "application"
;or "data"
fmt = 1*(alpha-numeric)
;typically an RTP payload type for audio
;and video media
Handley & Jacobson Standards Track [Page 31]
RFC 2327 SDP April 1998
proto = 1*(alpha-numeric)
;typically "RTP/AVP" or "udp" for IP4
port = 1*(DIGIT)
;should in the range "1024" to "65535" inclusive
;for UDP based media
attribute = (att-field ":" att-value) | att-field
att-field = 1*(alpha-numeric)
att-value = byte-string
sess-id = 1*(DIGIT)
;should be unique for this originating username/host
sess-version = 1*(DIGIT)
;0 is a new session
connection-address = multicast-address
| addr
multicast-address = 3*(decimal-uchar ".") decimal-uchar "/" ttl
[ "/" integer ]
;multicast addresses may be in the range
;224.0.0.0 to 239.255.255.255
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
Handley & Jacobson Standards Track [Page 32]
RFC 2327 SDP April 1998
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.
nettype = "IN"
;list to be extended
addrtype = "IP4" | "IP6"
;list to be extended
addr = FQDN | unicast-address
FQDN = 4*(alpha-numeric|"-"|".")
;fully qualified domain name as specified in RFC1035
Handley & Jacobson Standards Track [Page 33]
RFC 2327 SDP April 1998
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
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"
Handley & Jacobson Standards Track [Page 34]
RFC 2327 SDP April 1998
email-safe = safe | space | tab
safe = alpha-numeric |
"'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
"#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" |
"]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
"~" | "
space = %d32
tab = %d9
CRLF = %d13.10
Handley & Jacobson Standards Track [Page 35]
RFC 2327 SDP April 1998
Appendix B: Guidelines for registering SDP names with IANA
There are seven field names that may be registered with IANA. Using
the terminology in the SDP specification BNF, they are "media",
"proto", "fmt", "att-field", "bwtype", "nettype" and "addrtype".
"media" (eg, audio, video, application, data).
Packetized media types, such as those used by RTP, share the
namespace used by media types registry [RFC 2048] (i.e. "MIME
types"). The list of valid media names is the set of top-level
MIME content types. The set of media is intended to be small and
not to be extended except under rare circumstances. (The MIME
subtype corresponds to the "fmt" parameter below).
"proto"
In general this should be an IETF standards-track transport
protocol identifier such as RTP/AVP (rfc 1889 under the rfc 1890
profile).
However, people will want to invent their own proprietary
transport protocols. Some of these should be registered as a
"fmt" using "udp" as the protocol and some of which probably
can't be.
Where the protocol and the application are intimately linked,
such as with the LBL whiteboard wb which used a proprietary and
special purpose protocol over UDP, the protocol name should be
"udp" and the format name that should be registered is "wb". The
rules for formats (see below) apply to such registrations.
Where the proprietary transport protocol really carries many
different data formats, it is possible to register a new protocol
name with IANA. In such a case, an RFC MUST be produced
describing the protocol and referenced in the registration. Such
an RFC MAY be informational, although it is preferable if it is
standards-track.
"fmt"
The format namespace is dependent on the context of the "proto"
field, so a format cannot be registered without specifying one or
more transport protocols that it applies to.
Formats cover all the possible encodings that might want to be
transported in a multimedia session.
Handley & Jacobson Standards Track [Page 36]
RFC 2327 SDP April 1998
For RTP formats that have been assigned static payload types, the
payload type number is used. For RTP formats using a dynamic
payload type number, the dynamic payload type number is given as
the format and an additional "rtpmap" attribute specifies the
format and parameters.
For non-RTP formats, any unregistered format name may be
registered through the MIME-type registration process [RFC 2048].
The type given here is the MIME subtype only (the top-level MIME
content type is specified by the media parameter). The MIME type
registration SHOULD reference a standards-track RFC which
describes the transport protocol for this media type. If there
is an existing MIME type for this format, the MIME registration
should be augmented to reference the transport specification for
this media type. If there is not an existing MIME type for this
format, and there exists no appropriate file format, this should
be noted in the encoding considerations as "no appropriate file
format".
"att-field" (Attribute names)
Attribute field names MAY be registered with IANA, although this
is not compulsory, and unknown attributes are simply ignored.
When an attribute is registered, it must be accompanied by a
brief specification stating the following:
o contact name, email address and telephone number
o attribute-name (as it will appear in SDP)
o long-form attribute name in English
o type of attribute (session level, media level, or both)
o whether the attribute value is subject to the charset
attribute.
o a one paragraph explanation of the purpose of the attribute.
o a specification of appropriate attribute values for this
attribute.
IANA will not sanity check such attribute registrations except to
ensure that they do not clash with existing registrations.
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Although the above is the minimum that IANA will accept, if the
attribute is expected to see widespread use and interoperability
is an issue, authors are encouraged to produce a standards-track
RFC that specifies the attribute more precisely.
Submitters of registrations should ensure that the specification
is in the spirit of SDP attributes, most notably that the
attribute is platform independent in the sense that it makes no
implicit assumptions about operating systems and does not name
specific pieces of software in a manner that might inhibit
interoperability.
"bwtype" (bandwidth specifiers)
A proliferation of bandwidth specifiers is strongly discouraged.
New bandwidth specifiers may be registered with IANA. The
submission MUST reference a standards-track RFC specifying the
semantics of the bandwidth specifier precisely, and indicating
when it should be used, and why the existing registered bandwidth
specifiers do not suffice.
"nettype" (Network Type)
New network types may be registered with IANA if SDP needs to be
used in the context of non-internet environments. Whilst these
are not normally the preserve of IANA, there may be circumstances
when an Internet application needs to interoperate with a non-
internet application, such as when gatewaying an internet
telephony call into the PSTN. The number of network types should
be small and should be rarely extended. A new network type
cannot be registered without registering at least one address
type to be used with that network type. A new network type
registration MUST reference an RFC which gives details of the
network type and address type and specifies how and when they
would be used. Such an RFC MAY be Informational.
"addrtype" (Address Type)
New address types may be registered with IANA. An address type
is only meaningful in the context of a network type, and any
registration of an address type MUST specify a registered network
type, or be submitted along with a network type registration. A
new address type registration MUST reference an RFC giving
details of the syntax of the address type. Such an RFC MAY be
Informational. Address types are not expected to be registered
frequently.
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Registration Procedure
To register a name the above guidelines should be followed regarding
the required level of documentation that is required. The
registration itself should be sent to IANA. Attribute registrations
should include the information given above. Other registrations
should include the following additional information:
o contact name, email address and telephone number
o name being registered (as it will appear in SDP)
o long-form name in English
o type of name ("media", "proto", "fmt", "bwtype", "nettype", or
"addrtype")
o a one paragraph explanation of the purpose of the registered name.
o a reference to the specification (eg RFC number) of the registered
name.
IANA may refer any registration to the IESG or to any appropriate
IETF working group for review, and may request revisions to be made
before a registration will be made.
Handley & Jacobson Standards Track [Page 39]
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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, Rob
Lanphier and Steve Hanna.
References
[1] Mills, D., "Network Time Protocol (version 3) specification and
implementation", RFC 1305, March 1992.
[2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
A Transport Protocol for Real-Time Applications", RFC 1889, January
1996.
[3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
with Minimal Control", RFC 1890, January 1996
[4] Handley, M., "SAP - Session Announcement Protocol", Work in
Progress.
[5] V. Jacobson, S. McCanne, "vat - X11-based audio teleconferencing
tool" vat manual page, Lawrence Berkeley Laboratory, 1994.
[6] The Unicode Consortium, "The Unicode Standard -- Version 2.0",
Addison-Wesley, 1996.
Handley & Jacobson Standards Track [Page 40]
RFC 2327 SDP April 1998
[7] ISO/IEC 10646-1:1993. International Standard -- Information
technol- ogy -- Universal Multiple-Octet Coded Character Set (UCS) --
Part 1: Architecture and Basic Multilingual Plane. Five amendments
and a techn- ical corrigendum have been published up to now. UTF-8
is described in Annex R, published as Amendment 2.
[8] Goldsmith, D., and M. Davis, "Using Unicode with MIME", RFC 1641,
July 1994.
[9] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
10646", RFC 2044, October 1996.
[10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for
Receiving Internet-based H.323 Conferences", ITU, Geneva.
[11] Handley, M., Schooler, E., and H. Schulzrinne, "Session
Initiation Protocol (SIP)", Work in Progress.
[12] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
Handley & Jacobson Standards Track [Page 41]
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Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Handley & Jacobson Standards Track [Page 42]