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RFC2327 - SDP: Session Description Protocol

王朝other·作者佚名  2008-05-31
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Network Working Group M. Handley

Request for Comments: 2327 V. Jacobson

Category: Standards Track ISI/LBNL

April 1998

SDP: Session Description Protocol

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This document defines the Session Description Protocol, SDP. SDP is

intended for describing multimedia sessions for the purposes of

session announcement, session invitation, and other forms of

multimedia session initiation.

This document is a prodUCt of the Multiparty Multimedia Session

Control (MMUSIC) working group of the Internet Engineering Task

Force. Comments are solicited and should be addressed to the working

group's mailing list at confctrl@isi.edu and/or the authors.

1. Introduction

On the Internet multicast backbone (Mbone), a session Directory tool

is used to advertise multimedia conferences and communicate the

conference addresses and conference tool-specific information

necessary for participation. This document defines a session

description protocol for this purpose, and for general real-time

multimedia session description purposes. This memo does not describe

multicast address allocation or the distribution of SDP messages in

detail. These are described in accompanying memos. SDP is not

intended for negotiation of media encodings.

2. Background

The Mbone is the part of the internet that supports IP multicast, and

thus permits efficient many-to-many communication. It is used

extensively for multimedia conferencing. Such conferences usually

have the property that tight coordination of conference membership is

not necessary; to receive a conference, a user at an Mbone site only

has to know the conference's multicast group address and the UDP

ports for the conference data streams.

Session directories assist the advertisement of conference sessions

and communicate the relevant conference setup information to

prospective participants. SDP is designed to convey such information

to recipients. SDP is purely a format for session description - it

does not incorporate a transport protocol, and is intended to use

different transport protocols as appropriate including the Session

Announcement Protocol [4], Session Initiation Protocol [11], Real-

Time Streaming Protocol [12], electronic mail using the MIME

extensions, and the Hypertext Transport Protocol.

SDP is intended to be general purpose so that it can be used for a

wider range of network environments and applications than just

multicast session directories. However, it is not intended to

support negotiation of session content or media encodings - this is

viewed as outside the scope of session description.

3. Glossary of Terms

The following terms are used in this document, and have specific

meaning within the context of this document.

Conference

A multimedia conference is a set of two or more communicating users

along with the software they are using to communicate.

Session

A multimedia session is a set of multimedia senders and receivers

and the data streams flowing from senders to receivers. A

multimedia conference is an example of a multimedia session.

Session Advertisement

See session announcement.

Session Announcement

A session announcement is a mechanism by which a session

description is conveyed to users in a proactive fashion, i.e., the

session description was not eXPlicitly requested by the user.

Session Description

A well defined format for conveying sufficient information to

discover and participate in a multimedia session.

3.1. Terminology

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in RFC2119.

4. SDP Usage

4.1. Multicast Announcements

SDP is a session description protocol for multimedia sessions. A

common mode of usage is for a client to announce a conference session

by periodically multicasting an announcement packet to a well known

multicast address and port using the Session Announcement Protocol

(SAP).

SAP packets are UDP packets with the following format:

--------------------

SAP header

--------------------

text payload

//////////

The header is the Session Announcement Protocol header. SAP is

described in more detail in a companion memo [4]

The text payload is an SDP session description, as described in this

memo. The text payload should be no greater than 1 Kbyte in length.

If announced by SAP, only one session announcement is permitted in a

single packet.

4.2. Email and WWW Announcements

Alternative means of conveying session descriptions include

electronic mail and the World Wide Web. For both email and WWW

distribution, the use of the MIME content type "application/sdp"

should be used. This enables the automatic launching of applications

for participation in the session from the WWW client or mail reader

in a standard manner.

Note that announcements of multicast sessions made only via email or

the World Wide Web (WWW) do not have the property that the receiver

of a session announcement can necessarily receive the session because

the multicast sessions may be restricted in scope, and Access to the

WWW server or reception of email is possible outside this scope. SAP

announcements do not suffer from this mismatch.

5. Requirements and Recommendations

The purpose of SDP is to convey information about media streams in

multimedia sessions to allow the recipients of a session description

to participate in the session. SDP is primarily intended for use in

an internetwork, although it is sufficiently general that it can

describe conferences in other network environments.

A multimedia session, for these purposes, is defined as a set of

media streams that exist for some duration of time. Media streams

can be many-to-many. The times during which the session is active

need not be continuous.

Thus far, multicast based sessions on the Internet have differed from

many other forms of conferencing in that anyone receiving the traffic

can join the session (unless the session traffic is encrypted). In

such an environment, SDP serves two primary purposes. It is a means

to communicate the existence of a session, and is a means to convey

sufficient information to enable joining and participating in the

session. In a unicast environment, only the latter purpose is likely

to be relevant.

Thus SDP includes:

o Session name and purpose

o Time(s) the session is active

o The media comprising the session

o Information to receive those media (addresses, ports, formats and

so on)

As resources necessary to participate in a session may be limited,

some additional information may also be desirable:

o Information about the bandwidth to be used by the conference

o Contact information for the person responsible for the session

In general, SDP must convey sufficient information to be able to join

a session (with the possible exception of encryption keys) and to

announce the resources to be used to non-participants that may need

to know.

5.1. Media Information

SDP includes:

o The type of media (video, audio, etc)

o The transport protocol (RTP/UDP/IP, H.320, etc)

o The format of the media (H.261 video, MPEG video, etc)

For an IP multicast session, the following are also conveyed:

o Multicast address for media

o Transport Port for media

This address and port are the destination address and destination

port of the multicast stream, whether being sent, received, or both.

For an IP unicast session, the following are conveyed:

o Remote address for media

o Transport port for contact address

The semantics of this address and port depend on the media and

transport protocol defined. By default, this is the remote address

and remote port to which data is sent, and the remote address and

local port on which to receive data. However, some media may define

to use these to establish a control channel for the actual media

flow.

5.2. Timing Information

Sessions may either be bounded or unbounded in time. Whether or not

they are bounded, they may be only active at specific times.

SDP can convey:

o An arbitrary list of start and stop times bounding the session

o For each bound, repeat times such as "every Wednesday at 10am for

one hour"

This timing information is globally consistent, irrespective of local

time zone or daylight saving time.

5.3. Private Sessions

It is possible to create both public sessions and private sessions.

Private sessions will typically be conveyed by encrypting the session

description to distribute it. The details of how encryption is

performed are dependent on the mechanism used to convey SDP - see [4]

for how this is done for session announcements.

If a session announcement is private it is possible to use that

private announcement to convey encryption keys necessary to decode

each of the media in a conference, including enough information to

know which encryption scheme is used for each media.

5.4. OBTaining Further Information about a Session

A session description should convey enough information to decide

whether or not to participate in a session. SDP may include

additional pointers in the form of Universal Resources Identifiers

(URIs) for more information about the session.

5.5. Categorisation

When many session descriptions are being distributed by SAP or any

other advertisement mechanism, it may be desirable to filter

announcements that are of interest from those that are not. SDP

supports a categorisation mechanism for sessions that is capable of

being automated.

5.6. Internationalization

The SDP specification recommends the use of the ISO 10646 character

sets in the UTF-8 encoding (RFC2044) to allow many different

languages to be represented. However, to assist in compact

representations, SDP also allows other character sets such as ISO

8859-1 to be used when desired. Internationalization only applies to

free-text fields (session name and background information), and not

to SDP as a whole.

6. SDP Specification

SDP session descriptions are entirely textual using the ISO 10646

character set in UTF-8 encoding. SDP field names and attributes names

use only the US-ASCII subset of UTF-8, but textual fields and

attribute values may use the full ISO 10646 character set. The

textual form, as opposed to a binary encoding such as ASN/1 or XDR,

was chosen to enhance portability, to enable a variety of transports

to be used (e.g, session description in a MIME email message) and to

allow flexible, text-based toolkits (e.g., Tcl/Tk ) to be used to

generate and to process session descriptions. However, since the

total bandwidth allocated to all SAP announcements is strictly

limited, the encoding is deliberately compact. Also, since

announcements may be transported via very unreliable means (e.g.,

email) or damaged by an intermediate caching server, the encoding was

designed with strict order and formatting rules so that most errors

would result in malformed announcements which could be detected

easily and discarded. This also allows rapid discarding of encrypted

announcements for which a receiver does not have the correct key.

An SDP session description consists of a number of lines of text of

the form <type>=<value> <type> is always exactly one character and is

case-significant. <value> is a structured text string whose format

depends on <type>. It also will be case-significant unless a

specific field defines otherwise. Whitespace is not permitted either

side of the `=' sign. In general <value> is either a number of fields

delimited by a single space character or a free format string.

A session description consists of a session-level description

(details that apply to the whole session and all media streams) and

optionally several media-level descriptions (details that apply onto

to a single media stream).

An announcement consists of a session-level section followed by zero

or more media-level sections. The session-level part starts with a

`v=' line and continues to the first media-level section. The media

description starts with an `m=' line and continues to the next media

description or end of the whole session description. In general,

session-level values are the default for all media unless overridden

by an equivalent media-level value.

When SDP is conveyed by SAP, only one session description is allowed

per packet. When SDP is conveyed by other means, many SDP session

descriptions may be concatenated together (the `v=' line indicating

the start of a session description terminates the previous

description). Some lines in each description are required and some

are optional but all must appear in exactly the order given here (the

fixed order greatly enhances error detection and allows for a simple

parser). Optional items are marked with a `*'.

Session description

v= (protocol version)

o= (owner/creator and session identifier).

s= (session name)

i=* (session information)

u=* (URI of description)

e=* (email address)

p=* (phone number)

c=* (connection information - not required if included in all media)

b=* (bandwidth information)

One or more time descriptions (see below)

z=* (time zone adjustments)

k=* (encryption key)

a=* (zero or more session attribute lines)

Zero or more media descriptions (see below)

Time description

t= (time the session is active)

r=* (zero or more repeat times)

Media description

m= (media name and transport address)

i=* (media title)

c=* (connection information - optional if included at session-level)

b=* (bandwidth information)

k=* (encryption key)

a=* (zero or more media attribute lines)

The set of `type' letters is deliberately small and not intended to

be extensible -- SDP parsers must completely ignore any announcement

that contains a `type' letter that it does not understand. The

`attribute' mechanism ("a=" described below) is the primary means for

extending SDP and tailoring it to particular applications or media.

Some attributes (the ones listed in this document) have a defined

meaning but others may be added on an application-, media- or

session-specific basis. A session directory must ignore any

attribute it doesn't understand.

The connection (`c=') and attribute (`a=') information in the

session-level section applies to all the media of that session unless

overridden by connection information or an attribute of the same name

in the media description. For instance, in the example below, each

media behaves as if it were given a `recvonly' attribute.

An example SDP description is:

v=0

o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4

s=SDP Seminar

i=A Seminar on the session description protocol

u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps

e=mjh@isi.edu (Mark Handley)

c=IN IP4 224.2.17.12/127

t=2873397496 2873404696

a=recvonly

m=audio 49170 RTP/AVP 0

m=video 51372 RTP/AVP 31

m=application 32416 udp wb

a=orient:portrait

Text records such as the session name and information are bytes

strings which may contain any byte with the exceptions of 0x00 (Nul),

0x0a (ASCII newline) and 0x0d (ASCII carriage return). The sequence

CRLF (0x0d0a) is used to end a record, although parsers should be

tolerant and also accept records terminated with a single newline

character. By default these byte strings contain ISO-10646

characters in UTF-8 encoding, but this default may be changed using

the `charset' attribute.

Protocol Version

v=0

The "v=" field gives the version of the Session Description Protocol.

There is no minor version number.

Origin

o=<username> <session id> <version> <network type> <address type>

<address>

The "o=" field gives the originator of the session (their username

and the address of the user's host) plus a session id and session

version number.

<username> is the user's login on the originating host, or it is "-"

if the originating host does not support the concept of user ids.

<username> must not contain spaces. <session id> is a numeric string

such that the tuple of <username>, <session id>, <network type>,

<address type> and <address> form a globally unique identifier for

the session.

The method of <session id> allocation is up to the creating tool, but

it has been suggested that a Network Time Protocol (NTP) timestamp be

used to ensure uniqueness [1].

<version> is a version number for this announcement. It is needed

for proxy announcements to detect which of several announcements for

the same session is the most recent. Again its usage is up to the

creating tool, so long as <version> is increased when a modification

is made to the session data. Again, it is recommended (but not

mandatory) that an NTP timestamp is used.

<network type> is a text string giving the type of network.

Initially "IN" is defined to have the meaning "Internet". <address

type> is a text string giving the type of the address that follows.

Initially "IP4" and "IP6" are defined. <address> is the globally

unique address of the machine from which the session was created.

For an address type of IP4, this is either the fully-qualified domain

name of the machine, or the dotted-decimal representation of the IP

version 4 address of the machine. For an address type of IP6, this

is either the fully-qualified domain name of the machine, or the

compressed textual representation of the IP version 6 address of the

machine. For both IP4 and IP6, the fully-qualified domain name is

the form that SHOULD be given unless this is unavailable, in which

case the globally unique address may be substituted. A local IP

address MUST NOT be used in any context where the SDP description

might leave the scope in which the address is meaningful.

In general, the "o=" field serves as a globally unique identifier for

this version of this session description, and the subfields excepting

the version taken together identify the session irrespective of any

modifications.

Session Name

s=<session name>

The "s=" field is the session name. There must be one and only one

"s=" field per session description, and it must contain ISO 10646

characters (but see also the `charset' attribute below).

Session and Media Information

i=<session description>

The "i=" field is information about the session. There may be at

most one session-level "i=" field per session description, and at

most one "i=" field per media. Although it may be omitted, this is

discouraged for session announcements, and user interfaces for

composing sessions should require text to be entered. If it is

present it must contain ISO 10646 characters (but see also the

`charset' attribute below).

A single "i=" field can also be used for each media definition. In

media definitions, "i=" fields are primarily intended for labeling

media streams. As such, they are most likely to be useful when a

single session has more than one distinct media stream of the same

media type. An example would be two different whiteboards, one for

slides and one for feedback and questions.

URI

u=<URI>

o A URI is a Universal Resource Identifier as used by WWW clients

o The URI should be a pointer to additional information about the

conference

o This field is optional, but if it is present it should be specified

before the first media field

o No more than one URI field is allowed per session description

Email Address and Phone Number

e=<email address>

p=<phone number>

o These specify contact information for the person responsible for

the conference. This is not necessarily the same person that

created the conference announcement.

o Either an email field or a phone field must be specified.

Additional email and phone fields are allowed.

o If these are present, they should be specified before the first

media field.

o More than one email or phone field can be given for a session

description.

o Phone numbers should be given in the conventional international

format - preceded by a "+ and the international country code.

There must be a space or a hyphen ("-") between the country code

and the rest of the phone number. Spaces and hyphens may be used

to split up a phone field to aid readability if desired. For

example:

p=+44-171-380-7777 or p=+1 617 253 6011

o Both email addresses and phone numbers can have an optional free

text string associated with them, normally giving the name of the

person who may be contacted. This should be enclosed in

parenthesis if it is present. For example:

e=mjh@isi.edu (Mark Handley)

The alternative RFC822 name quoting convention is also allowed for

both email addresses and phone numbers. For example,

e=Mark Handley <mjh@isi.edu>

The free text string should be in the ISO-10646 character set with

UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings

if the appropriate charset session-level attribute is set.

Connection Data

c=<network type> <address type> <connection address>

The "c=" field contains connection data.

A session announcement must contain one "c=" field in each media

description (see below) or a "c=" field at the session-level. It may

contain a session-level "c=" field and one additional "c=" field per

media description, in which case the per-media values override the

session-level settings for the relevant media.

The first sub-field is the network type, which is a text string

giving the type of network. Initially "IN" is defined to have the

meaning "Internet".

The second sub-field is the address type. This allows SDP to be used

for sessions that are not IP based. Currently only IP4 is defined.

The third sub-field is the connection address. Optional extra

subfields may be added after the connection address depending on the

value of the <address type> field.

For IP4 addresses, the connection address is defined as follows:

o Typically the connection address will be a class-D IP multicast

group address. If the session is not multicast, then the

connection address contains the fully-qualified domain name or the

unicast IP address of the expected data source or data relay or

data sink as determined by additional attribute fields. It is not

expected that fully-qualified domain names or unicast addresses

will be given in a session description that is communicated by a

multicast announcement, though this is not prohibited. If a

unicast data stream is to pass through a network address

translator, the use of a fully-qualified domain name rather than an

unicast IP address is RECOMMENDED. In other cases, the use of an

IP address to specify a particular interface on a multi-homed host

might be required. Thus this specification leaves the decision as

to which to use up to the individual application, but all

applications MUST be able to cope with receiving both formats.

o Conferences using an IP multicast connection address must also have

a time to live (TTL) value present in addition to the multicast

address. The TTL and the address together define the scope with

which multicast packets sent in this conference will be sent. TTL

values must be in the range 0-255.

The TTL for the session is appended to the address using a slash as

a separator. An example is:

c=IN IP4 224.2.1.1/127

Hierarchical or layered encoding schemes are data streams where the

encoding from a single media source is split into a number of

layers. The receiver can choose the desired quality (and hence

bandwidth) by only subscribing to a subset of these layers. Such

layered encodings are normally transmitted in multiple multicast

groups to allow multicast pruning. This technique keeps unwanted

traffic from sites only requiring certain levels of the hierarchy.

For applications requiring multiple multicast groups, we allow the

following notation to be used for the connection address:

<base multicast address>/<ttl>/<number of addresses>

If the number of addresses is not given it is assumed to be one.

Multicast addresses so assigned are contiguously allocated above

the base address, so that, for example:

c=IN IP4 224.2.1.1/127/3

would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are

to be used at a ttl of 127. This is semantically identical to

including multiple "c=" lines in a media description:

c=IN IP4 224.2.1.1/127

c=IN IP4 224.2.1.2/127

c=IN IP4 224.2.1.3/127

Multiple addresses or "c=" lines can only be specified on a per-

media basis, and not for a session-level "c=" field.

It is illegal for the slash notation described above to be used for

IP unicast addresses.

Bandwidth

b=<modifier>:<bandwidth-value>

o This specifies the proposed bandwidth to be used by the session or

media, and is optional.

o <bandwidth-value> is in kilobits per second

o <modifier> is a single alphanumeric word giving the meaning of the

bandwidth figure.

o Two modifiers are initially defined:

CT Conference Total: An implicit maximum bandwidth is associated with

each TTL on the Mbone or within a particular multicast

administrative scope region (the Mbone bandwidth vs. TTL limits are

given in the MBone FAQ). If the bandwidth of a session or media in

a session is different from the bandwidth implicit from the scope,

a `b=CT:...' line should be supplied for the session giving the

proposed upper limit to the bandwidth used. The primary purpose of

this is to give an approximate idea as to whether two or more

conferences can co-exist simultaneously.

AS Application-Specific Maximum: The bandwidth is interpreted to be

application-specific, i.e., will be the application's concept of

maximum bandwidth. Normally this will coincide with what is set on

the application's "maximum bandwidth" control if applicable.

Note that CT gives a total bandwidth figure for all the media at

all sites. AS gives a bandwidth figure for a single media at a

single site, although there may be many sites sending

simultaneously.

o Extension Mechanism: Tool writers can define experimental bandwidth

modifiers by prefixing their modifier with "X-". For example:

b=X-YZ:128

SDP parsers should ignore bandwidth fields with unknown modifiers.

Modifiers should be alpha-numeric and, although no length limit is

given, they are recommended to be short.

Times, Repeat Times and Time Zones

t=<start time> <stop time>

o "t=" fields specify the start and stop times for a conference

session. Multiple "t=" fields may be used if a session is active

at multiple irregularly spaced times; each additional "t=" field

specifies an additional period of time for which the session will

be active. If the session is active at regular times, an "r="

field (see below) should be used in addition to and following a

"t=" field - in which case the "t=" field specifies the start and

stop times of the repeat sequence.

o The first and second sub-fields give the start and stop times for

the conference respectively. These values are the decimal

representation of Network Time Protocol (NTP) time values in

seconds [1]. To convert these values to UNIX time, subtract

decimal 2208988800.

o If the stop-time is set to zero, then the session is not bounded,

though it will not become active until after the start-time. If

the start-time is also zero, the session is regarded as permanent.

User interfaces should strongly discourage the creation of

unbounded and permanent sessions as they give no information about

when the session is actually going to terminate, and so make

scheduling difficult.

The general assumption may be made, when displaying unbounded

sessions that have not timed out to the user, that an unbounded

session will only be active until half an hour from the current

time or the session start time, whichever is the later. If

behaviour other than this is required, an end-time should be given

and modified as appropriate when new information becomes available

about when the session should really end.

Permanent sessions may be shown to the user as never being active

unless there are associated repeat times which state precisely when

the session will be active. In general, permanent sessions should

not be created for any session expected to have a duration of less

than 2 months, and should be discouraged for sessions expected to

have a duration of less than 6 months.

r=<repeat interval> <active duration> <list of offsets from start-

time>

o "r=" fields specify repeat times for a session. For example, if

a session is active at 10am on Monday and 11am on Tuesday for one

hour each week for three months, then the <start time> in the

corresponding "t=" field would be the NTP representation of 10am on

the first Monday, the <repeat interval> would be 1 week, the

<active duration> would be 1 hour, and the offsets would be zero

and 25 hours. The corresponding "t=" field stop time would be the

NTP representation of the end of the last session three months

later. By default all fields are in seconds, so the "r=" and "t="

fields might be:

t=3034423619 3042462419

r=604800 3600 0 90000

To make announcements more compact, times may also be given in units

of days, hours or minutes. The syntax for these is a number

immediately followed by a single case-sensitive character.

Fractional units are not allowed - a smaller unit should be used

instead. The following unit specification characters are allowed:

d - days (86400 seconds)

h - minutes (3600 seconds)

m - minutes (60 seconds)

s - seconds (allowed for completeness but not recommended)

Thus, the above announcement could also have been written:

r=7d 1h 0 25h

Monthly and yearly repeats cannot currently be directly specified

with a single SDP repeat time - instead separate "t" fields should

be used to explicitly list the session times.

z=<adjustment time> <offset> <adjustment time> <offset> ....

o To schedule a repeated session which spans a change from daylight-

saving time to standard time or vice-versa, it is necessary to

specify offsets from the base repeat times. This is required

because different time zones change time at different times of day,

different countries change to or from daylight time on different

dates, and some countries do not have daylight saving time at all.

Thus in order to schedule a session that is at the same time winter

and summer, it must be possible to specify unambiguously by whose

time zone a session is scheduled. To simplify this task for

receivers, we allow the sender to specify the NTP time that a time

zone adjustment happens and the offset from the time when the

session was first scheduled. The "z" field allows the sender to

specify a list of these adjustment times and offsets from the base

time.

An example might be:

z=2882844526 -1h 2898848070 0

This specifies that at time 2882844526 the time base by which the

session's repeat times are calculated is shifted back by 1 hour,

and that at time 2898848070 the session's original time base is

restored. Adjustments are always relative to the specified start

time - they are not cumulative.

o If a session is likely to last several years, it is expected

that

the session announcement will be modified periodically rather than

transmit several years worth of adjustments in one announcement.

Encryption Keys

k=<method>

k=<method>:<encryption key>

o The session description protocol may be used to convey encryption

keys. A key field is permitted before the first media entry (in

which case it applies to all media in the session), or for each

media entry as required.

o The format of keys and their usage is outside the scope of this

document, but see [3].

o The method indicates the mechanism to be used to obtain a usable

key by external means, or from the encoded encryption key given.

The following methods are defined:

k=clear:<encryption key>

The encryption key (as described in [3] for RTP media streams

under the AV profile) is included untransformed in this key

field.

k=base64:<encoded encryption key>

The encryption key (as described in [3] for RTP media streams

under the AV profile) is included in this key field but has been

base64 encoded because it includes characters that are

prohibited in SDP.

k=uri:<URI to obtain key>

A Universal Resource Identifier as used by WWW clients is

included in this key field. The URI refers to the data

containing the key, and may require additional authentication

before the key can be returned. When a request is made to the

given URI, the MIME content-type of the reply specifies the

encoding for the key in the reply. The key should not be

obtained until the user wishes to join the session to reduce

synchronisation of requests to the WWW server(s).

k=prompt

No key is included in this SDP description, but the session or

media stream referred to by this key field is encrypted. The

user should be prompted for the key when attempting to join the

session, and this user-supplied key should then be used to

decrypt the media streams.

Attributes

a=<attribute>

a=<attribute>:<value>

Attributes are the primary means for extending SDP. Attributes may

be defined to be used as "session-level" attributes, "media-level"

attributes, or both.

A media description may have any number of attributes ("a=" fields)

which are media specific. These are referred to as "media-level"

attributes and add information about the media stream. Attribute

fields can also be added before the first media field; these

"session-level" attributes convey additional information that applies

to the conference as a whole rather than to individual media; an

example might be the conference's floor control policy.

Attribute fields may be of two forms:

o property attributes. A property attribute is simply of the form

"a=<flag>". These are binary attributes, and the presence of the

attribute conveys that the attribute is a property of the session.

An example might be "a=recvonly".

o value attributes. A value attribute is of the form

"a=<attribute>:<value>". An example might be that a whiteboard

could have the value attribute "a=orient:landscape"

Attribute interpretation depends on the media tool being invoked.

Thus receivers of session descriptions should be configurable in

their interpretation of announcements in general and of attributes in

particular.

Attribute names must be in the US-ASCII subset of ISO-10646/UTF-8.

Attribute values are byte strings, and MAY use any byte value except

0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values

are to be interpreted as in ISO-10646 character set with UTF-8

encoding. Unlike other text fields, attribute values are NOT

normally affected by the `charset' attribute as this would make

comparisons against known values problematic. However, when an

attribute is defined, it can be defined to be charset-dependent, in

which case it's value should be interpreted in the session charset

rather than in ISO-10646.

Attributes that will be commonly used can be registered with IANA

(see Appendix B). Unregistered attributes should begin with "X-" to

prevent inadvertent collision with registered attributes. In either

case, if an attribute is received that is not understood, it should

simply be ignored by the receiver.

Media Announcements

m=<media> <port> <transport> <fmt list>

A session description may contain a number of media descriptions.

Each media description starts with an "m=" field, and is terminated

by either the next "m=" field or by the end of the session

description. A media field also has several sub-fields:

o The first sub-field is the media type. Currently defined media are

"audio", "video", "application", "data" and "control", though this

list may be extended as new communication modalities emerge (e.g.,

telepresense). The difference between "application" and "data" is

that the former is a media flow such as whiteboard information, and

the latter is bulk-data transfer such as multicasting of program

executables which will not typically be displayed to the user.

"control" is used to specify an additional conference control

channel for the session.

o The second sub-field is the transport port to which the media

stream will be sent. The meaning of the transport port depends on

the network being used as specified in the relevant "c" field and

on the transport protocol defined in the third sub-field. Other

ports used by the media application (such as the RTCP port, see

[2]) should be derived algorithmically from the base media port.

Note: For transports based on UDP, the value should be in the range

1024 to 65535 inclusive. For RTP compliance it should be an even

number.

For applications where hierarchically encoded streams are being

sent to a unicast address, it may be necessary to specify multiple

transport ports. This is done using a similar notation to that

used for IP multicast addresses in the "c=" field:

m=<media> <port>/<number of ports> <transport> <fmt list>

In such a case, the ports used depend on the transport protocol.

For RTP, only the even ports are used for data and the

corresponding one-higher odd port is used for RTCP. For example:

m=video 49170/2 RTP/AVP 31

would specify that ports 49170 and 49171 form one RTP/RTCP pair and

49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the

transport protocol and 31 is the format (see below).

It is illegal for both multiple addresses to be specified in the

"c=" field and for multiple ports to be specified in the "m=" field

in the same session description.

o The third sub-field is the transport protocol. The transport

protocol values are dependent on the address-type field in the "c="

fields. Thus a "c=" field of IP4 defines that the transport

protocol runs over IP4. For IP4, it is normally expected that most

media traffic will be carried as RTP over UDP. The following

transport protocols are preliminarily defined, but may be extended

through registration of new protocols with IANA:

- RTP/AVP - the IETF's Realtime Transport Protocol using the

Audio/Video profile carried over UDP.

- udp - User Datagram Protocol

If an application uses a single combined proprietary media format

and transport protocol over UDP, then simply specifying the

transport protocol as udp and using the format field to distinguish

the combined protocol is recommended. If a transport protocol is

used over UDP to carry several distinct media types that need to be

distinguished by a session directory, then specifying the transport

protocol and media format separately is necessary. RTP is an

example of a transport-protocol that carries multiple payload

formats that must be distinguished by the session directory for it

to know how to start appropriate tools, relays, mixers or

recorders.

The main reason to specify the transport-protocol in addition to

the media format is that the same standard media formats may be

carried over different transport protocols even when the network

protocol is the same - a historical example is vat PCM audio and

RTP PCM audio. In addition, relays and monitoring tools that are

transport-protocol-specific but format-independent are possible.

For RTP media streams operating under the RTP Audio/Video Profile

[3], the protocol field is "RTP/AVP". Should other RTP profiles be

defined in the future, their profiles will be specified in the same

way. For example, the protocol field "RTP/XYZ" would specify RTP

operating under a profile whose short name is "XYZ".

o The fourth and subsequent sub-fields are media formats. For audio

and video, these will normally be a media payload type as defined

in the RTP Audio/Video Profile.

When a list of payload formats is given, this implies that all of

these formats may be used in the session, but the first of these

formats is the default format for the session.

For media whose transport protocol is not RTP or UDP the format

field is protocol specific. Such formats should be defined in an

additional specification document.

For media whose transport protocol is RTP, SDP can be used to

provide a dynamic binding of media encoding to RTP payload type.

The encoding names in the RTP AV Profile do not specify unique

audio encodings (in terms of clock rate and number of audio

channels), and so they are not used directly in SDP format fields.

Instead, the payload type number should be used to specify the

format for static payload types and the payload type number along

with additional encoding information should be used for dynamically

allocated payload types.

An example of a static payload type is u-law PCM coded single

channel audio sampled at 8KHz. This is completely defined in the

RTP Audio/Video profile as payload type 0, so the media field for

such a stream sent to UDP port 49232 is:

m=video 49232 RTP/AVP 0

An example of a dynamic payload type is 16 bit linear encoded

stereo audio sampled at 16KHz. If we wish to use dynamic RTP/AVP

payload type 98 for such a stream, additional information is

required to decode it:

m=video 49232 RTP/AVP 98

a=rtpmap:98 L16/16000/2

The general form of an rtpmap attribute is:

a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding

parameters>]

For audio streams, <encoding parameters> may specify the number of

audio channels. This parameter may be omitted if the number of

channels is one provided no additional parameters are needed. For

video streams, no encoding parameters are currently specified.

Additional parameters may be defined in the future, but

codecspecific parameters should not be added. Parameters added to

an rtpmap attribute should only be those required for a session

directory to make the choice of appropriate media too to

participate in a session. Codec-specific parameters should be

added in other attributes.

Up to one rtpmap attribute can be defined for each media format

specified. Thus we might have:

m=audio 49230 RTP/AVP 96 97 98

a=rtpmap:96 L8/8000

a=rtpmap:97 L16/8000

a=rtpmap:98 L16/11025/2

RTP profiles that specify the use of dynamic payload types must

define the set of valid encoding names and/or a means to register

encoding names if that profile is to be used with SDP.

Experimental encoding formats can also be specified using rtpmap.

RTP formats that are not registered as standard format names must

be preceded by "X-". Thus a new experimental redundant audio

stream called GSMLPC using dynamic payload type 99 could be

specified as:

m=video 49232 RTP/AVP 99

a=rtpmap:99 X-GSMLPC/8000

Such an experimental encoding requires that any site wishing to

receive the media stream has relevant configured state in its

session directory to know which tools are appropriate.

Note that RTP audio formats typically do not include information

about the number of samples per packet. If a non-default (as

defined in the RTP Audio/Video Profile) packetisation is required,

the "ptime" attribute is used as given below.

For more details on RTP audio and video formats, see [3].

o Formats for non-RTP media should be registered as MIME content

types as described in Appendix B. For example, the LBL whiteboard

application might be registered as MIME content-type application/wb

with encoding considerations specifying that it operates over UDP,

with no appropriate file format. In SDP this would then be

expressed using a combination of the "media" field and the "fmt"

field, as follows:

m=application 32416 udp wb

Suggested Attributes

The following attributes are suggested. Since application writers

may add new attributes as they are required, this list is not

exhaustive.

a=cat:<category>

This attribute gives the dot-separated hierarchical category of

the session. This is to enable a receiver to filter unwanted

sessions by category. It would probably have been a compulsory

separate field, except for its experimental nature at this time.

It is a session-level attribute, and is not dependent on charset.

a=keywds:<keywords>

Like the cat attribute, this is to assist identifying wanted

sessions at the receiver. This allows a receiver to select

interesting session based on keywords describing the purpose of

the session. It is a session-level attribute. It is a charset

dependent attribute, meaning that its value should be interpreted

in the charset specified for the session description if one is

specified, or by default in ISO 10646/UTF-8.

a=tool:<name and version of tool>

This gives the name and version number of the tool used to create

the session description. It is a session-level attribute, and is

not dependent on charset.

a=ptime:<packet time>

This gives the length of time in milliseconds represented by the

media in a packet. This is probably only meaningful for audio

data. It should not be necessary to know ptime to decode RTP or

vat audio, and it is intended as a recommendation for the

encoding/packetisation of audio. It is a media attribute, and is

not dependent on charset.

a=recvonly

This specifies that the tools should be started in receive-only

mode where applicable. It can be either a session or media

attribute, and is not dependent on charset.

a=sendrecv

This specifies that the tools should be started in send and

receive mode. This is necessary for interactive conferences with

tools such as wb which defaults to receive only mode. It can be

either a session or media attribute, and is not dependent on

charset.

a=sendonly

This specifies that the tools should be started in send-only

mode. An example may be where a different unicast address is to

be used for a traffic destination than for a traffic source. In

such a case, two media descriptions may be use, one sendonly and

one recvonly. It can be either a session or media attribute, but

would normally only be used as a media attribute, and is not

dependent on charset.

a=orient:<whiteboard orientation>

Normally this is only used in a whiteboard media specification.

It specifies the orientation of a the whiteboard on the screen.

It is a media attribute. Permitted values are `portrait',

`landscape' and `seascape' (upside down landscape). It is not

dependent on charset

a=type:<conference type>

This specifies the type of the conference. Suggested values are

`broadcast', `meeting', `moderated', `test' and `H332'.

`recvonly' should be the default for `type:broadcast' sessions,

`type:meeting' should imply `sendrecv' and `type:moderated'

should indicate the use of a floor control tool and that the

media tools are started so as to "mute" new sites joining the

conference.

Specifying the attribute type:H332 indicates that this loosely

coupled session is part of a H.332 session as defined in the ITU

H.332 specification [10]. Media tools should be started

`recvonly'.

Specifying the attribute type:test is suggested as a hint that,

unless explicitly requested otherwise, receivers can safely avoid

displaying this session description to users.

The type attribute is a session-level attribute, and is not

dependent on charset.

a=charset:<character set>

This specifies the character set to be used to display the

session name and information data. By default, the ISO-10646

character set in UTF-8 encoding is used. If a more compact

representation is required, other character sets may be used such

as ISO-8859-1 for Northern European languages. In particular,

the ISO 8859-1 is specified with the following SDP attribute:

a=charset:ISO-8859-1

This is a session-level attribute; if this attribute is present,

it must be before the first media field. The charset specified

MUST be one of those registered with IANA, such as ISO-8859-1.

The character set identifier is a US-ASCII string and MUST be

compared against the IANA identifiers using a case-insensitive

comparison. If the identifier is not recognised or not

supported, all strings that are affected by it SHOULD be regarded

as byte strings.

Note that a character set specified MUST still prohibit the use

of bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets

requiring the use of these characters MUST define a quoting

mechanism that prevents these bytes appearing within text fields.

a=sdplang:<language tag>

This can be a session level attribute or a media level attribute.

As a session level attribute, it specifies the language for the

session description. As a media level attribute, it specifies

the language for any media-level SDP information field associated

with that media. Multiple sdplang attributes can be provided

either at session or media level if multiple languages in the

session description or media use multiple languages, in which

case the order of the attributes indicates the order of

importance of the various languages in the session or media from

most important to least important.

In general, sending session descriptions consisting of multiple

languages should be discouraged. Instead, multiple descriptions

should be sent describing the session, one in each language.

However this is not possible with all transport mechanisms, and

so multiple sdplang attributes are allowed although not

recommended.

The sdplang attribute value must be a single RFC1766 language

tag in US-ASCII. It is not dependent on the charset attribute.

An sdplang attribute SHOULD be specified when a session is of

sufficient scope to cross geographic boundaries where the

language of recipients cannot be assumed, or where the session is

in a different language from the locally assumed norm.

a=lang:<language tag>

This can be a session level attribute or a media level attribute.

As a session level attribute, it specifies the default language

for the session being described. As a media level attribute, it

specifies the language for that media, overriding any session-

level language specified. Multiple lang attributes can be

provided either at session or media level if multiple languages

if the session description or media use multiple languages, in

which case the order of the attributes indicates the order of

importance of the various languages in the session or media from

most important to least important.

The lang attribute value must be a single RFC1766 language tag

in US-ASCII. It is not dependent on the charset attribute. A

lang attribute SHOULD be specified when a session is of

sufficient scope to cross geographic boundaries where the

language of recipients cannot be assumed, or where the session is

in a different language from the locally assumed norm.

a=framerate:<frame rate>

This gives the maximum video frame rate in frames/sec. It is

intended as a recommendation for the encoding of video data.

Decimal representations of fractional values using the notation

"<integer>.<fraction>" are allowed. It is a media attribute, is

only defined for video media, and is not dependent on charset.

a=quality:<quality>

This gives a suggestion for the quality of the encoding as an

integer value.

The intention of the quality attribute for video is to specify a

non-default trade-off between frame-rate and still-image quality.

For video, the value in the range 0 to 10, with the following

suggested meaning:

10 - the best still-image quality the compression scheme can

give.

5 - the default behaviour given no quality suggestion.

0 - the worst still-image quality the codec designer thinks is

still usable.

It is a media attribute, and is not dependent on charset.

a=fmtp:<format> <format specific parameters>

This attribute allows parameters that are specific to a

particular format to be conveyed in a way that SDP doesn't have

to understand them. The format must be one of the formats

specified for the media. Format-specific parameters may be any

set of parameters required to be conveyed by SDP and given

unchanged to the media tool that will use this format.

It is a media attribute, and is not dependent on charset.

6.1. Communicating Conference Control Policy

There is some debate over the way conference control policy should be

communicated. In general, the authors believe that an implicit

declarative style of specifying conference control is desirable where

possible.

A simple declarative style uses a single conference attribute field

before the first media field, possibly supplemented by properties

such as `recvonly' for some of the media tools. This conference

attribute conveys the conference control policy. An example might be:

a=type:moderated

In some cases, however, it is possible that this may be insufficient

to communicate the details of an unusual conference control policy.

If this is the case, then a conference attribute specifying external

control might be set, and then one or more "media" fields might be

used to specify the conference control tools and configuration data

for those tools. An example is an ITU H.332 session:

c=IN IP4 224.5.6.7

a=type:H332

m=audio 49230 RTP/AVP 0

m=video 49232 RTP/AVP 31

m=application 12349 udp wb

m=control 49234 H323 mc

c=IN IP4 134.134.157.81

In this example, a general conference attribute (type:H332) is

specified stating that conference control will be provided by an

external H.332 tool, and a contact addresses for the H.323 session

multipoint controller is given.

In this document, only the declarative style of conference control

declaration is specified. Other forms of conference control should

specify an appropriate type attribute, and should define the

implications this has for control media.

7. Security Considerations

SDP is a session description format that describes multimedia

sessions. A session description should not be trusted unless it has

been obtained by an authenticated transport protocol from a trusted

source. Many different transport protocols may be used to distribute

session description, and the nature of the authentication will differ

from transport to transport.

One transport that will frequently be used to distribute session

descriptions is the Session Announcement Protocol (SAP). SAP

provides both encryption and authentication mechanisms but due to the

nature of session announcements it is likely that there are many

occasions where the originator of a session announcement cannot be

authenticated because they are previously unknown to the receiver of

the announcement and because no common public key infrastructure is

available.

On receiving a session description over an unauthenticated transport

mechanism or from an untrusted party, software parsing the session

should take a few precautions. Session description contain

information required to start software on the receivers system.

Software that parses a session description MUST not be able to start

other software except that which is specifically configured as

appropriate software to participate in multimedia sessions. It is

normally considered INAPPROPRIATE for software parsing a session

description to start, on a user's system, software that is

appropriate to participate in multimedia sessions, without the user

first being informed that such software will be started and giving

their consent. Thus a session description arriving by session

announcement, email, session invitation, or WWW page SHOULD not

deliver the user into an {it interactive} multimedia session without

the user being aware that this will happen. As it is not always

simple to tell whether a session is interactive or not, applications

that are unsure should assume sessions are interactive.

In this specification, there are no attributes which would allow the

recipient of a session description to be informed to start multimedia

tools in a mode where they default to transmitting. Under some

circumstances it might be appropriate to define such attributes. If

this is done an application parsing a session description containing

such attributes SHOULD either ignore them, or inform the user that

joining this session will result in the automatic transmission of

multimedia data. The default behaviour for an unknown attribute is

to ignore it.

Session descriptions may be parsed at intermediate systems such as

firewalls for the purposes of opening a hole in the firewall to allow

the participation in multimedia sessions. It is considered

INAPPROPRIATE for a firewall to open such holes for unicast data

streams unless the session description comes in a request from inside

the firewall.

For multicast sessions, it is likely that local administrators will

apply their own policies, but the exclusive use of "local" or "site-

local" administrative scope within the firewall and the refusal of

the firewall to open a hole for such scopes will provide separation

of global multicast sessions from local ones.

Appendix A: SDP Grammar

This appendix provides an Augmented BNF grammar for SDP. ABNF is

defined in RFC2234.

announcement = proto-version

origin-field

session-name-field

information-field

uri-field

email-fields

phone-fields

connection-field

bandwidth-fields

time-fields

key-field

attribute-fields

media-descriptions

proto-version = "v=" 1*DIGIT CRLF

;this memo describes version 0

origin-field = "o=" username space

sess-id space sess-version space

nettype space addrtype space

addr CRLF

session-name-field = "s=" text CRLF

information-field = ["i=" text CRLF]

uri-field = ["u=" uri CRLF]

email-fields = *("e=" email-address CRLF)

phone-fields = *("p=" phone-number CRLF)

connection-field = ["c=" nettype space addrtype space

connection-address CRLF]

;a connection field must be present

;in every media description or at the

;session-level

bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF)

time-fields = 1*( "t=" start-time space stop-time

*(CRLF repeat-fields) CRLF)

[zone-adjustments CRLF]

repeat-fields = "r=" repeat-interval space typed-time

1*(space typed-time)

zone-adjustments = time space ["-"] typed-time

*(space time space ["-"] typed-time)

key-field = ["k=" key-type CRLF]

key-type = "prompt"

"clear:" key-data

"base64:" key-data

"uri:" uri

key-data = email-safe "~" "

attribute-fields = *("a=" attribute CRLF)

media-descriptions = *( media-field

information-field

*(connection-field)

bandwidth-fields

key-field

attribute-fields )

media-field = "m=" media space port ["/" integer]

space proto 1*(space fmt) CRLF

media = 1*(alpha-numeric)

;typically "audio", "video", "application"

;or "data"

fmt = 1*(alpha-numeric)

;typically an RTP payload type for audio

;and video media

proto = 1*(alpha-numeric)

;typically "RTP/AVP" or "udp" for IP4

port = 1*(DIGIT)

;should in the range "1024" to "65535" inclusive

;for UDP based media

attribute = (att-field ":" att-value) att-field

att-field = 1*(alpha-numeric)

att-value = byte-string

sess-id = 1*(DIGIT)

;should be unique for this originating username/host

sess-version = 1*(DIGIT)

;0 is a new session

connection-address = multicast-address

addr

multicast-address = 3*(decimal-uchar ".") 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

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

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..0x090x0b0x0c0x0e..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"

email-safe = safe space tab

safe = alpha-numeric

"'" "'" "-" "." "/" ":" "?" """

"#" "$" "&" "*" ";" "=" "@" "["

"]" "^" "_" "`" "{" "" "}" "+"

"~" "

space = %d32

tab = %d9

CRLF = %d13.10

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 [RFC2048] (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 RFCMUST be produced

describing the protocol and referenced in the registration. Such

an RFCMAY 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.

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 [RFC2048].

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 RFCwhich

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.

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

RFCthat 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 RFCspecifying 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 RFCwhich gives details of the

network type and address type and specifies how and when they

would be used. Such an RFCMAY 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 RFCgiving

details of the syntax of the address type. Such an RFCMAY be

Informational. Address types are not expected to be registered

frequently.

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 RFCnumber) 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.

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", RFC1305, March 1992.

[2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:

A Transport Protocol for Real-Time Applications", RFC1889, January

1996.

[3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences

with Minimal Control", RFC1890, 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.

[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", RFC1641,

July 1994.

[9] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO

10646", RFC2044, 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)", RFC2326, April 1998.

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

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

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.

 
 
 
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