Network Working Group M. Handley
Internet-Draft UCL
Obsoletes: 4566 (if approved) V. Jacobson
Intended status: Standards Track
Expires: December 31, 2017 C. Perkins
University of Glasgow
A. Begen
Networked Media
June 29, 2017
SDP: Session Description Protocol
draft-ietf-mmusic-rfc4566bis-21
Abstract
This memo defines the Session Description Protocol (SDP). SDP is
intended for describing multimedia sessions for the purposes of
session announcement, session invitation, and other forms of
multimedia session initiation. This document obsoletes RFC 4566.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . 4
3. Examples of SDP Usage . . . . . . . . . . . . . . . . . . . . 4
3.1. Session Initiation . . . . . . . . . . . . . . . . . . . 4
3.2. Streaming Media . . . . . . . . . . . . . . . . . . . . . 5
3.3. Email and the World Wide Web . . . . . . . . . . . . . . 5
3.4. Multicast Session Announcement . . . . . . . . . . . . . 5
4. Requirements and Recommendations . . . . . . . . . . . . . . 5
4.1. Media and Transport Information . . . . . . . . . . . . . 6
4.2. Timing Information . . . . . . . . . . . . . . . . . . . 7
4.3. Obtaining Further Information about a Session . . . . . . 7
4.4. Categorisation . . . . . . . . . . . . . . . . . . . . . 8
4.5. Internationalisation . . . . . . . . . . . . . . . . . . 8
5. SDP Specification . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Protocol Version ("v=") . . . . . . . . . . . . . . . . . 11
5.2. Origin ("o=") . . . . . . . . . . . . . . . . . . . . . . 12
5.3. Session Name ("s=") . . . . . . . . . . . . . . . . . . . 13
5.4. Session Information ("i=") . . . . . . . . . . . . . . . 13
5.5. URI ("u=") . . . . . . . . . . . . . . . . . . . . . . . 14
5.6. Email Address and Phone Number ("e=" and "p=") . . . . . 14
5.7. Connection Data ("c=") . . . . . . . . . . . . . . . . . 15
5.8. Bandwidth ("b=") . . . . . . . . . . . . . . . . . . . . 17
5.9. Timing ("t=") . . . . . . . . . . . . . . . . . . . . . . 18
5.10. Repeat Times ("r=") . . . . . . . . . . . . . . . . . . . 19
5.11. Time Zones ("z=") . . . . . . . . . . . . . . . . . . . . 19
5.12. Encryption Keys ("k=") . . . . . . . . . . . . . . . . . 20
5.13. Attributes ("a=") . . . . . . . . . . . . . . . . . . . . 22
5.14. Media Descriptions ("m=") . . . . . . . . . . . . . . . . 23
6. SDP Attributes . . . . . . . . . . . . . . . . . . . . . . . 25
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6.1. cat (category) . . . . . . . . . . . . . . . . . . . . . 26
6.2. keywds (keywords) . . . . . . . . . . . . . . . . . . . . 26
6.3. tool . . . . . . . . . . . . . . . . . . . . . . . . . . 27
6.4. ptime (packet time) . . . . . . . . . . . . . . . . . . . 27
6.5. maxptime (maximum packet time) . . . . . . . . . . . . . 28
6.6. rtpmap . . . . . . . . . . . . . . . . . . . . . . . . . 28
6.7. Media Direction Attributes . . . . . . . . . . . . . . . 30
6.7.1. recvonly (receive-only) . . . . . . . . . . . . . . . 31
6.7.2. sendrecv (send-receive) . . . . . . . . . . . . . . . 31
6.7.3. sendonly (send-only) . . . . . . . . . . . . . . . . 32
6.7.4. inactive . . . . . . . . . . . . . . . . . . . . . . 32
6.8. orient (orientation) . . . . . . . . . . . . . . . . . . 33
6.9. type (conference type) . . . . . . . . . . . . . . . . . 33
6.10. charset (character set) . . . . . . . . . . . . . . . . . 34
6.11. sdplang (SDP language) . . . . . . . . . . . . . . . . . 35
6.12. lang (language) . . . . . . . . . . . . . . . . . . . . . 36
6.13. framerate (frame rate) . . . . . . . . . . . . . . . . . 37
6.14. quality . . . . . . . . . . . . . . . . . . . . . . . . . 38
6.15. fmtp (format parameters) . . . . . . . . . . . . . . . . 38
7. Security Considerations . . . . . . . . . . . . . . . . . . . 39
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
8.1. The "application/sdp" Media Type . . . . . . . . . . . . 41
8.2. Registration of Parameters . . . . . . . . . . . . . . . 43
8.2.1. Media Types ("media") . . . . . . . . . . . . . . . . 43
8.2.2. Transport Protocols ("proto") . . . . . . . . . . . . 43
8.2.3. Media Formats ("fmt") . . . . . . . . . . . . . . . . 44
8.2.4. Attribute Names ("att-field") . . . . . . . . . . . . 44
8.2.5. Bandwidth Specifiers ("bwtype") . . . . . . . . . . . 47
8.2.6. Network Types ("nettype") . . . . . . . . . . . . . . 47
8.2.7. Address Types ("addrtype") . . . . . . . . . . . . . 48
8.2.8. Registration Procedure . . . . . . . . . . . . . . . 48
8.3. Encryption Key Access Methods . . . . . . . . . . . . . . 49
8.4. Reorganization of the nettype Registry . . . . . . . . . 49
8.5. Reorganization of the att-field Registries . . . . . . . 49
9. SDP Grammar . . . . . . . . . . . . . . . . . . . . . . . . . 50
10. Summary of Changes from RFC 4566 . . . . . . . . . . . . . . 55
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 56
12.1. Normative References . . . . . . . . . . . . . . . . . . 56
12.2. Informative References . . . . . . . . . . . . . . . . . 57
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 60
1. Introduction
When initiating multimedia teleconferences, voice-over-IP calls,
streaming video, or other sessions, there is a requirement to convey
media details, transport addresses, and other session description
metadata to the participants.
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SDP provides a standard representation for such information,
irrespective of how that information is transported. SDP is purely a
format for session description -- it does not incorporate a transport
protocol, and it is intended to use different transport protocols as
appropriate, including the Session Announcement Protocol (SAP)
[RFC2974], Session Initiation Protocol (SIP) [RFC3261], Real Time
Streaming Protocol (RTSP) [RFC7826], electronic mail using the MIME
extensions, and the Hypertext Transport Protocol (HTTP).
SDP is intended to be general purpose so that it can be used in a
wide range of network environments and applications. However, it is
not intended to support negotiation of session content or media
encodings: this is viewed as outside the scope of session
description.
This memo obsoletes [RFC4566]. The changes relative to [RFC4566] are
limited to essential corrections, and are outlined in Section 10 of
this memo.
2. Glossary of Terms
The following term is used in this document and has specific meaning
within the context of this document.
Session Description: A well-defined format for conveying sufficient
information to discover and participate in a multimedia session.
The terms "multimedia conference" and "multimedia session" are used
in this document as defined in [RFC7656]. The terms "session" and
"multimedia session" are used interchangeably in this document.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. Examples of SDP Usage
3.1. Session Initiation
The Session Initiation Protocol (SIP) [RFC3261] is an application-
layer control protocol for creating, modifying, and terminating
sessions such as Internet multimedia conferences, Internet telephone
calls, and multimedia distribution. The SIP messages used to create
sessions carry session descriptions that allow participants to agree
on a set of compatible media types. These session descriptions are
commonly formatted using SDP. When used with SIP, the offer/answer
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model [RFC3264] provides a limited framework for negotiation using
SDP.
3.2. Streaming Media
The Real Time Streaming Protocol (RTSP) [RFC7826], is an application-
level protocol for control over the delivery of data with real-time
properties. RTSP provides an extensible framework to enable
controlled, on-demand delivery of real-time data, such as audio and
video. An RTSP client and server negotiate an appropriate set of
parameters for media delivery, partially using SDP syntax to describe
those parameters.
3.3. Email and the World Wide Web
Alternative means of conveying session descriptions include
electronic mail and the World Wide Web (WWW). For both email and WWW
distribution, the media type "application/sdp" is used. This enables
the automatic launching of applications for participation in the
session from the WWW client or mail reader in a standard manner.
Note that announcements of multicast sessions made only via email or
the WWW do not have the property that the receiver of a session
announcement can necessarily receive the session because the
multicast sessions may be restricted in scope, and access to the WWW
server or reception of email is possible outside this scope.
3.4. Multicast Session Announcement
In order to assist the advertisement of multicast multimedia
conferences and other multicast sessions, and to communicate the
relevant session setup information to prospective participants, a
distributed session directory may be used. An instance of such a
session directory periodically sends packets containing a description
of the session to a well-known multicast group. These advertisements
are received by other session directories such that potential remote
participants can use the session description to start the tools
required to participate in the session.
One protocol used to implement such a distributed directory is the
SAP [RFC2974]. SDP provides the recommended session description
format for such session announcements.
4. Requirements and Recommendations
The purpose of SDP is to convey information about media streams in
multimedia sessions to allow the recipients of a session description
to participate in the session. SDP is primarily intended for use in
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an internetwork, although it is sufficiently general that it can
describe multimedia conferences in other network environments. Media
streams can be many-to-many. Sessions need not be continually
active.
Thus far, multicast-based sessions on the Internet have differed from
many other forms of conferencing in that anyone receiving the traffic
can join the session (unless the session traffic is encrypted). In
such an environment, SDP serves two primary purposes. It is a means
to communicate the existence of a session, and it is a means to
convey sufficient information to enable joining and participating in
the session. In a unicast environment, only the latter purpose is
likely to be relevant.
An SDP description includes the following:
o Session name and purpose
o Time(s) the session is active
o The media comprising the session
o Information needed to receive those media (addresses, ports,
formats, etc.)
As resources necessary to participate in a session may be limited,
some additional information may also be desirable:
o Information about the bandwidth to be used by the session
o Contact information for the person responsible for the session
In general, SDP must convey sufficient information to enable
applications to join a session (with the possible exception of
encryption keys) and to announce the resources to be used to any non-
participants that may need to know. (This latter feature is
primarily useful when SDP is used with a multicast session
announcement protocol.)
4.1. Media and Transport Information
An SDP description includes the following media information:
o The type of media (video, audio, etc.)
o The media transport protocol (RTP/UDP/IP, H.320, etc.)
o The format of the media (H.261 video, MPEG video, etc.)
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In addition to media format and transport protocol, SDP conveys
address and port details. For an IP multicast session, these
comprise:
o The multicast group address for media
o The transport port for media
This address and port are the destination address and destination
port of the multicast stream, whether being sent, received, or both.
For unicast IP sessions, the following are conveyed:
o The remote address for media
o The remote transport port for media
The semantics of this address and port depend on the media and
transport protocol defined. By default, this SHOULD be the remote
address and remote port to which data is sent. Some media types may
redefine this behaviour, but this is NOT RECOMMENDED since it
complicates implementations (including middleboxes that must parse
the addresses to open Network Address Translation (NAT) or firewall
pinholes).
4.2. Timing Information
Sessions may be either bounded or unbounded in time. Whether or not
they are bounded, they may be only active at specific times. SDP can
convey:
o An arbitrary list of start and stop times bounding the session
o For each bound, repeat times such as "every Wednesday at 10am for
one hour"
This timing information is globally consistent, irrespective of local
time zone or daylight saving time (see Section 5.9).
4.3. Obtaining Further Information about a Session
A session description could convey enough information to decide
whether or not to participate in a session. SDP may include
additional pointers in the form of Uniform Resource Identifiers
(URIs) for more information about the session.
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4.4. Categorisation
When many session descriptions are being distributed by SAP, or any
other advertisement mechanism, it may be desirable to filter session
announcements that are of interest from those that are not. SDP
supports a categorisation mechanism for sessions that is capable of
being automated (the "a=cat:" attribute; see Section 6).
4.5. Internationalisation
The SDP specification recommends the use of the ISO 10646 character
set in the UTF-8 encoding [RFC3629] to allow many different languages
to be represented. However, to assist in compact representations,
SDP also allows other character sets such as ISO 8859-1 to be used
when desired. Internationalisation only applies to free-text fields
(session name and background information), and not to SDP as a whole.
5. SDP Specification
An SDP description is denoted by the media type "application/sdp"
(See Section 8).
An SDP description is entirely textual. SDP field names and
attribute names use only the US-ASCII subset of UTF-8, but textual
fields and attribute values MAY use the full ISO 10646 character set
in UTF-8 encoding, or some other character set defined by the
"a=charset:" attribute. Field and attribute values that use the full
UTF-8 character set are never directly compared, hence there is no
requirement for UTF-8 normalisation. The textual form, as opposed to
a binary encoding such as ASN.1 or XDR, was chosen to enhance
portability, to enable a variety of transports to be used, and to
allow flexible, text-based toolkits to be used to generate and
process session descriptions. However, since SDP may be used in
environments where the maximum permissible size of a session
description is limited, the encoding is deliberately compact. Also,
since announcements may be transported via very unreliable means or
damaged by an intermediate caching server, the encoding was designed
with strict order and formatting rules so that most errors would
result in malformed session announcements that could be detected
easily and discarded. This also allows rapid discarding of encrypted
session announcements for which a receiver does not have the correct
key.
An SDP description consists of a number of lines of text of the form:
<type>=<value>
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where <type> MUST be exactly one case-significant character and
<value> is structured text whose format depends on <type>. In
general, <value> is either a number of fields delimited by a single
space character or a free format string, and is case-significant
unless a specific field defines otherwise. Whitespace separators
MUST NOT be used on either side of the "=" sign, however, if the
value can contain a leading whitespace as part of its syntax, i.e.,
that whitespace is part of the value.
An SDP description consists of a session-level section followed by
zero or more media-level sections. The session-level part starts
with a "v=" line and continues to the first media-level section (or
the end of the whole description, whichever comes first). Each
media-level section starts with an "m=" line and continues to the
next media-level section or the end of the whole session description
- whichever comes first. In general, session-level values are the
default for all media unless overridden by an equivalent media-level
value.
Some lines in each description are REQUIRED and some are OPTIONAL,
but all MUST appear in exactly the order given here (the fixed order
greatly enhances error detection and allows for a simple parser).
OPTIONAL items are marked with a "*".
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Session description
v= (protocol version)
o= (originator and session identifier)
s= (session name)
i=* (session information)
u=* (URI of description)
e=* (email address)
p=* (phone number)
c=* (connection information -- not required if included in
all media descriptions)
b=* (zero or more bandwidth information lines)
One or more time descriptions ("t=" and "r=" lines; see below)
z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)
Zero or more media descriptions
Time description
t= (time the session is active)
r=* (zero or more repeat times)
Media description, if present
m= (media name and transport address)
i=* (media title)
c=* (connection information -- optional if included at
session level)
b=* (zero or more bandwidth information lines)
k=* (encryption key)
a=* (zero or more media attribute lines)
The set of type letters is deliberately small and not intended to be
extensible -- an SDP parser MUST completely ignore any session
description that contains a type letter that it does not understand.
The attribute mechanism ("a=" described below) is the primary means
for extending SDP and tailoring it to particular applications or
media. Some attributes (the ones listed in Section 6 of this memo)
have a defined meaning, but others may be added on an application-,
media-, or session-specific basis. An SDP parser MUST ignore any
attribute it doesn't understand.
An SDP description may contain URIs that reference external content
in the "u=", "k=", and "a=" lines. These URIs may be dereferenced in
some cases, making the session description non-self- contained.
The connection ("c=") information in the session-level section
applies to all the media of that session unless overridden by
connection information in the media description. For instance, in
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the example below, each audio media description behaves as if it were
given a "c=IN IP4 233.252.0.2".
An example SDP description is:
v=0
o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe)
c=IN IP4 233.252.0.2
t=2873397496 2873404696
a=recvonly
m=audio 49170 RTP/AVP 0
m=audio 49180 RTP/AVP 0
m=video 51372 RTP/AVP 99
c=IN IP4 233.252.0.1/127
a=rtpmap:99 h263-1998/90000
Text fields such as the session name and information are octet
strings that may contain any octet with the exceptions of 0x00 (Nul),
0x0a (ASCII newline), and 0x0d (ASCII carriage return). The sequence
CRLF (0x0d0a) is used to end a record, although parsers SHOULD be
tolerant and also accept records terminated with a single newline
character. If the "a=charset" attribute is not present, these octet
strings MUST be interpreted as containing ISO-10646 characters in
UTF-8 encoding (the presence of the "a=charset" attribute may force
some fields to be interpreted differently).
A session description can contain domain names in the "o=", "u=",
"e=", "c=", and "a=" lines. Any domain name used in SDP MUST comply
with [RFC1034], [RFC1035]. Internationalised domain names (IDNs)
MUST be represented using the ASCII Compatible Encoding (ACE) form
defined in [RFC5890] and MUST NOT be directly represented in UTF-8 or
any other encoding (this requirement is for compatibility with
[RFC2327] and other early SDP-related standards, which predate the
development of internationalised domain names).
5.1. Protocol Version ("v=")
v=0
The "v=" line gives the version of the Session Description Protocol.
This memo defines version 0. There is no minor version number.
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5.2. Origin ("o=")
o=<username> <sess-id> <sess-version> <nettype> <addrtype>
<unicast-address>
The "o=" line gives the originator of the session (her username and
the address of the user's host) plus a session identifier and version
number:
<username> is the user's login on the originating host, or it is "-"
if the originating host does not support the concept of user IDs.
The <username> MUST NOT contain spaces.
<sess-id> is a numeric string such that the tuple of <username>,
<sess-id>, <nettype>, <addrtype>, and <unicast-address> forms a
globally unique identifier for the session. The method of <sess-
id> allocation is up to the creating tool, but it has been
suggested that a Network Time Protocol (NTP) format timestamp be
used to ensure uniqueness [RFC5905].
<sess-version> is a version number for this session description.
Its usage is up to the creating tool, so long as <sess-version> is
increased when a modification is made to the session data. Again,
it is RECOMMENDED that an NTP format timestamp is used.
<nettype> is a text string giving the type of network. Initially
"IN" is defined to have the meaning "Internet", but other values
MAY be registered in the future (see Section 8).
<addrtype> is a text string giving the type of the address that
follows. Initially "IP4" and "IP6" are defined, but other values
MAY be registered in the future (see Section 8).
<unicast-address> is an address of the machine from which the
session was created. For an address type of IP4, this is either a
fully qualified domain name of the machine or the dotted-decimal
representation of an IP version 4 address of the machine. For an
address type of IP6, this is either a fully qualified domain name
of the machine or the compressed textual representation of an 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 a globally unique address MAY be
substituted. Unless an SDP extension for NAT traversal is used
(e.g., ICE [RFC5245], ICE TCP [RFC6544]), a local IP address MUST
NOT be used in any context where the SDP description might leave
the scope in which the address is meaningful (for example, a local
address MUST NOT be included in an application-level referral that
might leave the scope).
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In general, the "o=" line serves as a globally unique identifier for
this version of this session description, and the subfields excepting
the version taken together identify the session irrespective of any
modifications.
For privacy reasons, it is sometimes desirable to obfuscate the
username and IP address of the session originator. If this is a
concern, an arbitrary <username> and private <unicast-address> MAY be
chosen to populate the "o=" line, provided that these are selected in
a manner that does not affect the global uniqueness of the field.
5.3. Session Name ("s=")
s=<session name>
The "s=" line is the textual session name. There MUST be one and
only one "s=" line per session description. The "s=" line MUST NOT
be empty and SHOULD contain ISO 10646 characters (but see also the
"a=charset" attribute). If a session has no meaningful name, the
value "s= " SHOULD be used (i.e., a single space as the session
name).
5.4. Session Information ("i=")
i=<session description>
The "i=" line provides textual information about the session. There
MUST be at most one session-level "i=" line per session description,
and at most one "i=" line per media description/definition. Unless a
media level "i="" line is used, the session-level "i="" line applies
to that media description. If the "a=charset" attribute is present,
it specifies the character set used in the "i=" line. If the
"a=charset" attribute is not present, the "i=" line MUST contain ISO
10646 characters in UTF-8 encoding.
A single "i=" line can be used for each media definition. In media
definitions, "i=" lines are primarily intended for labelling media
streams. As such, they are most likely to be useful when a single
session has more than one distinct media stream of the same media
type. An example would be two different whiteboards, one for slides
and one for feedback and questions.
The "i=" line is intended to provide a free-form human-readable
description of the session or the purpose of a media stream. It is
not suitable for parsing by automata.
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5.5. URI ("u=")
u=<uri>
A URI is a Uniform Resource Identifier as used by WWW clients
[RFC3986]. The URI should be a pointer to additional information
about the session. This line is OPTIONAL. No more than one URI line
is allowed per session description.
5.6. Email Address and Phone Number ("e=" and "p=")
e=<email-address>
p=<phone-number>
The "e=" and "p=" lines specify contact information for the person
responsible for the session. This is not necessarily the same person
that created the session description.
Inclusion of an email address or phone number is OPTIONAL.
If an email address or phone number is present, it MUST be specified
before the first media field. More than one email or phone line can
be given for a session description.
Phone numbers SHOULD be given in the form of an international public
telecommunication number (see ITU-T Recommendation E.164) preceded by
a "+". Spaces and hyphens may be used to split up a phone field to
aid readability if desired. For example:
p=+1 617 555-6011
Both email addresses and phone numbers can have an OPTIONAL free text
string associated with them, normally giving the name of the person
who may be contacted. This MUST be enclosed in parentheses if it is
present. For example:
e=j.doe@example.com (Jane Doe)
The alternative [RFC5322] name quoting convention is also allowed for
both email addresses and phone numbers. For example:
e=Jane Doe <j.doe@example.com>
The free text string SHOULD be in the ISO-10646 character set with
UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings if
the appropriate session-level "a=charset" attribute is set.
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5.7. Connection Data ("c=")
c=<nettype> <addrtype> <connection-address>
The "c=" line contains connection data.
A session description MUST contain either at least one "c=" line in
each media description or a single "c=" line at the session level.
It MAY contain a single session-level "c=" line and additional "c="
line(s) per media description, in which case the per-media values
override the session-level settings for the respective media.
The first sub-field ("<nettype>") is the network type, which is a
text string giving the type of network. Initially, "IN" is defined
to have the meaning "Internet", but other values MAY be registered in
the future (see Section 8).
The second sub-field ("<addrtype>") is the address type. This allows
SDP to be used for sessions that are not IP based. This memo only
defines IP4 and IP6, but other values MAY be registered in the future
(see Section 8).
The third sub-field ("<connection-address>") is the connection
address. OPTIONAL sub-fields MAY be added after the connection
address depending on the value of the <addrtype> field.
When the <addrtype> is IP4 and IP6, the connection address is defined
as follows:
o If the session is multicast, the connection address will be an IP
multicast group address. If the session is not multicast, then
the connection address contains the unicast IP address of the
expected data source or data relay or data sink as determined by
additional attribute fields. It is not expected that unicast
addresses will be given in a session description that is
communicated by a multicast announcement, though this is not
prohibited.
o Sessions using an IP4 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 session will be sent. TTL
values MUST be in the range 0-255. Although the TTL MUST be
specified, its use to scope multicast traffic is deprecated;
applications SHOULD use an administratively scoped address
instead.
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The TTL for the session is appended to the address using a slash as a
separator. An example is:
c=IN IP4 233.252.0.1/127
IP6 multicast does not use TTL scoping, and hence the TTL value MUST
NOT be present for IP6 multicast. It is expected that IP6 scoped
addresses will be used to limit the scope of multimedia conferences.
Hierarchical or layered encoding schemes are data streams where the
encoding from a single media source is split into a number of layers.
The receiver can choose the desired quality (and hence bandwidth) by
only subscribing to a subset of these layers. Such layered encodings
are normally transmitted in multiple multicast groups to allow
multicast pruning. This technique keeps unwanted traffic from sites
only requiring certain levels of the hierarchy. For applications
requiring multiple multicast groups, we allow the following notation
to be used for the connection address:
<base multicast address>[/<ttl>]/<number of addresses>
If the number of addresses is not given, it is assumed to be one.
Multicast addresses so assigned are contiguously allocated above the
base address, so that, for example:
c=IN IP4 233.252.0.1/127/3
would state that addresses 233.252.0.1, 233.252.0.2, and 233.252.0.3
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 233.252.0.1/127
c=IN IP4 233.252.0.2/127
c=IN IP4 233.252.0.3/127
Similarly, an IP6 example would be:
c=IN IP6 FF15::101/3
which is semantically equivalent to:
c=IN IP6 FF15::101
c=IN IP6 FF15::102
c=IN IP6 FF15::103
(remembering that the TTL field is not present in IP6 multicast).
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Multiple addresses or "c=" lines MAY be specified on a per-media
basis only if they provide multicast addresses for different layers
in a hierarchical or layered encoding scheme. They MUST NOT be
specified for a session-level "c=" line.
The slash notation for multiple addresses described above MUST NOT be
used for IP unicast addresses.
5.8. Bandwidth ("b=")
b=<bwtype>:<bandwidth>
This OPTIONAL line denotes the proposed bandwidth to be used by the
session or media. The <bwtype> is an alphanumeric modifier giving
the meaning of the <bandwidth> figure. Two values are defined in
this specification, but other values MAY be registered in the future
(see Section 8 and [RFC3556], [RFC3890]):
CT If the bandwidth of a session is different from the bandwidth
implicit from the scope, a "b=CT:..." line SHOULD be supplied for
the session giving the proposed upper limit to the bandwidth used
(the "conference total" bandwidth). Similarly, if the bandwidth
of bundled media streams in an m line is different from the
implicit value from the scope, a "b=CT:..." line SHOULD be
supplied in the media level. The primary purpose of this is to
give an approximate idea as to whether two or more sessions (or
bundled media streams) can coexist simultaneously. Note that CT
gives a total bandwidth figure for all the media at all endpoints.
AS The bandwidth is interpreted to be application specific (it will
be the application's concept of maximum bandwidth). Normally,
this will coincide with what is set on the application's "maximum
bandwidth" control if applicable. For RTP-based applications, AS
gives the RTP "session bandwidth" as defined in Section 6.2 of
[RFC3550]. Note that AS gives a bandwidth figure for a single
media at a single endpoint, although there may be many endpoints
sending simultaneously.
A prefix "X-" is defined for <bwtype> names. This is intended for
experimental purposes only. For example:
b=X-YZ:128
Use of the "X-" prefix is NOT RECOMMENDED: instead new modifiers
SHOULD be registered with IANA in the standard namespace. SDP
parsers MUST ignore bandwidth fields with unknown modifiers.
Modifiers MUST be alphanumeric and, although no length limit is
given, it is recommended that they be short.
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The <bandwidth> is interpreted as kilobits per second by default.
The definition of a new <bwtype> modifier MAY specify that the
bandwidth is to be interpreted in some alternative unit (the "CT" and
"AS" modifiers defined in this memo use the default units).
5.9. Timing ("t=")
t=<start-time> <stop-time>
The "t=" lines specify the start and stop times for a session.
Multiple "t=" lines MAY be used if a session is active at multiple
irregularly spaced times; each additional "t=" line specifies an
additional period of time for which the session will be active. If
the session is active at regular times, an "r=" line (see below)
should be used in addition to, and following, a "t=" line -- in which
case the "t=" line specifies the start and stop times of the repeat
sequence.
The first and second sub-fields give the start and stop times,
respectively, for the session. These values are the decimal
representation of Network Time Protocol (NTP) time values in seconds
since 1900 [RFC5905]. To convert these values to UNIX time, subtract
decimal 2208988800.
NTP timestamps are elsewhere represented by 64-bit values, which wrap
sometime in the year 2036. Since SDP uses an arbitrary length
decimal representation, this should not cause an issue (SDP
timestamps MUST continue counting seconds since 1900, NTP will use
the value modulo the 64-bit limit).
If the <stop-time> is set to zero, then the session is not bounded,
though it will not become active until after the <start-time>. If
the <start-time> is also zero, the session is regarded as permanent.
User interfaces SHOULD strongly discourage the creation of unbounded
and permanent sessions as they give no information about when the
session is actually going to terminate, and so make scheduling
difficult.
The general assumption may be made, when displaying unbounded
sessions that have not timed out to the user, that an unbounded
session will only be active until half an hour from the current time
or the session start time, whichever is the later. If behaviour
other than this is required, an end-time SHOULD be given and modified
as appropriate when new information becomes available about when the
session should really end.
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Permanent sessions may be shown to the user as never being active
unless there are associated repeat times that state precisely when
the session will be active.
5.10. Repeat Times ("r=")
r=<repeat interval> <active duration> <offsets from start-time>
"r=" line specifies 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=" line 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=" line 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=" lines might
be the following:
t=3034423619 3042462419
r=604800 3600 0 90000
To make the description more compact, times may also be given in
units of days, hours, or minutes. The syntax for these is a number
immediately followed by a single case-sensitive character.
Fractional units are not allowed -- a smaller unit should be used
instead. The following unit specification characters are allowed:
d - days (86400 seconds)
h - hours (3600 seconds)
m - minutes (60 seconds)
s - seconds (allowed for completeness)
Thus, the above session announcement could also have been written:
r=7d 1h 0 25h
Monthly and yearly repeats cannot be directly specified with a single
SDP repeat time; instead, separate "t=" lines should be used to
explicitly list the session times.
5.11. Time Zones ("z=")
z=<adjustment time> <offset> <adjustment time> <offset> ....
To schedule a repeated session that spans a change from daylight
saving time to standard time or vice versa, it is necessary to
specify offsets from the base time. This is required because
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different time zones change time at different times of day, different
countries change to or from daylight saving time on different dates,
and some countries do not have daylight saving time at all.
Thus, in order to schedule a session that is at the same time winter
and summer, it must be possible to specify unambiguously by whose
time zone a session is scheduled. To simplify this task for
receivers, we allow the sender to specify the NTP time that a time
zone adjustment happens and the offset from the time when the session
was first scheduled. The "z=" line allows the sender to specify a
list of these adjustment times and offsets from the base time.
An example might be the following:
z=2882844526 -1h 2898848070 0
This specifies that at time 2882844526, the time base by which the
session's repeat times are calculated is shifted back by 1 hour, and
that at time 2898848070, the session's original time base is
restored. Adjustments are always relative to the specified start
time -- they are not cumulative. Adjustments apply to all "t=" and
"r=" lines in a session description.
If a session is likely to last several years, it is expected that the
session description will be modified periodically rather than
transmit several years' worth of adjustments in one session
description.
5.12. Encryption Keys ("k=")
k=<method>
k=<method>:<encryption key>
If transported over a secure and trusted channel, the Session
Description Protocol MAY be used to convey encryption keys. A simple
mechanism for key exchange is provided by the key line ("k="),
although this is primarily supported for compatibility with older
implementations and its use is NOT RECOMMENDED. Work is in progress
to define new key exchange mechanisms for use with SDP [RFC4567]
[RFC4568], and it is expected that new applications will use those
mechanisms.
A key line is permitted before the first media entry (in which case
it applies to all media in the session), or for each media entry as
required. The format of keys and their usage are outside the scope
of this document, and the key field provides no way to indicate the
encryption algorithm to be used, key type, or other information about
the key: this is assumed to be provided by the higher-level protocol
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using SDP. If there is a need to convey this information within SDP,
the extensions mentioned previously SHOULD be used. Many security
protocols require two keys: one for confidentiality, another for
integrity. This specification does not support transfer of two keys.
The method indicates the mechanism to be used to obtain a usable key
by external means, or from the encoded encryption key given. The
following methods are defined:
k=clear:<encryption key>
The encryption key is included untransformed in this key line.
This method MUST NOT be used unless it can be guaranteed that
the SDP is conveyed over a secure channel. The encryption key
is interpreted as text according to the charset attribute; use
the "k=base64:" method to convey characters that are otherwise
prohibited in SDP.
k=base64:<encoded encryption key>
The encryption key is included in this key line but has been
base64 encoded [RFC4648] because it includes characters that
are prohibited in SDP. This method MUST NOT be used unless it
can be guaranteed that the SDP is conveyed over a secure
channel.
k=uri:<URI to obtain key>
A Uniform Resource Identifier is included in the key line. The
URI refers to the data containing the key, and may require
additional authentication before the key can be returned. When
a request is made to the given URI, the reply should specify
the encoding for the key. The URI is often an Secure Socket
Layer/Transport Layer Security (SSL/TLS)-protected HTTP URI
("https:"), although this is not required.
k=prompt
No key is included in this SDP description, but the session or
media stream referred to by this key line is encrypted. The
user should be prompted for the key when attempting to join the
session, and this user-supplied key should then be used to
decrypt the media streams. The use of user-specified keys is
NOT RECOMMENDED, since such keys tend to have weak security
properties.
The key line MUST NOT be used unless it can be guaranteed that the
SDP is conveyed over a secure and trusted channel. An example of
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such a channel might be SDP embedded inside an S/MIME message or a
TLS-protected HTTP session. It is important to ensure that the
secure channel is with the party that is authorised to join the
session, not an intermediary: if a caching proxy server is used, it
is important to ensure that the proxy is either trusted or unable to
access the SDP.
5.13. Attributes ("a=")
a=<attribute>
a=<attribute>:<value>
Attributes are the primary means for extending SDP. Attributes may
be defined to be used as "session-level" attributes, "media-level"
attributes, or both.
A media description may have any number of attributes ("a=" lines)
that are media specific. These are referred to as "media-level"
attributes and add information about the media stream. Attribute
lines can also be added before the first media field; these "session-
level" attributes convey additional information that applies to the
session as a whole rather than to individual media.
Attribute lines may be of two forms:
o A property attribute is simply of the form "a=<flag>". These are
binary attributes, and the presence of the attribute conveys that
the attribute is a property of the session. An example might be
"a=recvonly".
o A value attribute is of the form "a=<attribute>:<value>". For
example, a whiteboard could have the value attribute
"a=orient:landscape"
Attribute interpretation depends on the media tool being invoked.
Thus receivers of session descriptions should be configurable in
their interpretation of session descriptions in general and of
attributes in particular.
Attribute names MUST use the US-ASCII subset of ISO-10646/UTF-8.
Attribute values are octet strings, and MAY use any octet value
except 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute
values are to be interpreted as in ISO-10646 character set with UTF-8
encoding. Unlike other text fields, attribute values are NOT
normally affected by the "charset" attribute as this would make
comparisons against known values problematic. However, when an
attribute is defined, it can be defined to be charset dependent, in
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which case its value should be interpreted in the session charset
rather than in ISO-10646.
Attributes MUST be registered with IANA (see Section 8). If an
attribute is received that is not understood, it MUST be ignored by
the receiver.
5.14. Media Descriptions ("m=")
m=<media> <port> <proto> <fmt> ...
A session description may contain a number of media descriptions.
Each media description starts with an "m=" line and is terminated by
either the next "m=" line or by the end of the session description.
A media field has several sub-fields:
<media> is the media type. This document defines the values
"audio", "video", "text", "application", and "message". This list
is extended and may be further extended by other memos registering
media types in the future (see Section 8).
<port> is the transport port to which the media stream is sent. The
meaning of the transport port depends on the network being used as
specified in the relevant "c=" line, and on the transport protocol
defined in the <proto> sub-field of the media field. Other ports
used by the media application (such as the RTP Control Protocol
(RTCP) port [RFC3550]) MAY be derived algorithmically from the
base media port or MAY be specified in a separate attribute (for
example, "a=rtcp:" as defined in [RFC3605]).
If non-contiguous ports are used or if they don't follow the
parity rule of even RTP ports and odd RTCP ports, the "a=rtcp:"
attribute MUST be used. Applications that are requested to send
media to a <port> that is odd and where the "a=rtcp:" is present
MUST NOT subtract 1 from the RTP port: that is, they MUST send the
RTP to the port indicated in <port> and send the RTCP to the port
indicated in the "a=rtcp" attribute.
For applications where hierarchically encoded streams are being
sent to a unicast address, it may be necessary to specify multiple
transport ports. This is done using a similar notation to that
used for IP multicast addresses in the "c=" line:
m=<media> <port>/<number of ports> <proto> <fmt> ...
In such a case, the ports used depend on the transport protocol.
For RTP, the default is that only the even-numbered ports are used
for data with the corresponding one-higher odd ports used for the
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RTCP belonging to the RTP session, and the <number of ports>
denoting the number of RTP sessions. For example:
m=video 49170/2 RTP/AVP 31
would specify that ports 49170 and 49171 form one RTP/RTCP pair
and 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the
transport protocol and 31 is the format (see below). If non-
contiguous ports are required, they must be signalled using a
separate attribute (for example, "a=rtcp:" as defined in
[RFC3605]).
If multiple addresses are specified in the "c=" line and multiple
ports are specified in the "m=" line, a one-to-one mapping from
port to the corresponding address is implied. For example:
c=IN IP4 233.252.0.1/127/2
m=video 49170/2 RTP/AVP 31
would imply that address 233.252.0.1 is used with ports 49170 and
49171, and address 233.252.0.2 is used with ports 49172 and 49173.
The semantics of multiple "m=" lines using the same transport
address are undefined. This implies that, unlike limited past
practice, there is no implicit grouping defined by such means and
an explicit grouping framework (for example, [RFC5888]) should
instead be used to express the intended semantics.
<proto> is the transport protocol. The meaning of the transport
protocol is dependent on the address type field in the relevant
"c=" line. Thus a "c=" field of IP4 indicates that the transport
protocol runs over IP4. The following transport protocols are
defined, but may be extended through registration of new protocols
with IANA (see Section 8):
* udp: denotes an unspecified protocol running over UDP.
* RTP/AVP: denotes RTP [RFC3550] used under the RTP Profile for
Audio and Video Conferences with Minimal Control [RFC3551]
running over UDP.
* RTP/SAVP: denotes the Secure Real-time Transport Protocol
[RFC3711] running over UDP.
The main reason to specify the transport protocol in addition to
the media format is that the same standard media formats may be
carried over different transport protocols even when the network
protocol is the same -- a historical example is vat Pulse Code
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Modulation (PCM) audio and RTP PCM audio; another might be TCP/RTP
PCM audio. In addition, relays and monitoring tools that are
transport-protocol-specific but format-independent are possible.
<fmt> is a media format description. The fourth and any subsequent
sub-fields describe the format of the media. The interpretation
of the media format depends on the value of the <proto> sub-field.
If the <proto> sub-field is "RTP/AVP" or "RTP/SAVP" the <fmt> sub-
fields contain RTP payload type numbers. When a list of payload
type numbers is given, this implies that all of these payload
formats MAY be used in the session, but the first of these formats
SHOULD be used as the default format for the session. For dynamic
payload type assignments the "a=rtpmap:" attribute (see Section 6)
SHOULD be used to map from an RTP payload type number to a media
encoding name that identifies the payload format. The "a=fmtp:"
attribute MAY be used to specify format parameters (see
Section 6).
If the <proto> sub-field is "udp" the <fmt> sub-fields MUST
reference a media type describing the format under the "audio",
"video", "text", "application", or "message" top-level media
types. The media type registration SHOULD define the packet
format for use with UDP transport.
For media using other transport protocols, the <fmt> field is
protocol specific. Rules for interpretation of the <fmt> sub-
field MUST be defined when registering new protocols (see
Section 8.2.2).
Section 3 of [RFC4855] states that the payload format (encoding)
names defined in the RTP Profile are commonly shown in upper case,
while media subtype names are commonly shown in lower case. It
also states that both of these names are case-insensitive in both
places, similar to parameter names which are case-insensitive both
in media type strings and in the default mapping to the SDP a=fmtp
attribute.
6. SDP Attributes
The following attributes are defined. Since application writers may
add new attributes as they are required, this list is not exhaustive.
Registration procedures for new attributes are defined in
Section 8.2.4.
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6.1. cat (category)
Name: cat
Value: cat-value
Usage Level: session
Charset Dependent: no
Syntax:
cat-value = category
category = non-ws-string
Example:
a=cat:foo.bar
This attribute gives the dot-separated hierarchical category of the
session. This is to enable a receiver to filter unwanted sessions by
category. There is no central registry of categories. This
attribute is obsoleted.
6.2. keywds (keywords)
Name: keywds
Value: keywds-value
Usage Level: session
Charset Dependent: yes
Syntax:
keywds-value = keywords
keywords = text
Example:
a=keywds:SDP session description protocol
Like the cat attribute, this is to assist identifying wanted sessions
at the receiver. This allows a receiver to select interesting
session based on keywords describing the purpose of the session;
there is no central registry of keywords. Its value should be
interpreted in the charset specified for the session description if
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one is specified, or by default in ISO 10646/UTF-8. This attribute
is obsoleted.
6.3. tool
Name: tool
Value: tool-value
Usage Level: session
Charset Dependent: no
Syntax:
tool-value = tool-name-and-version
tool-name-and-version = text
Example:
a=tool:foobar V3.2
This gives the name and version number of the tool used to create the
session description.
6.4. ptime (packet time)
Name: ptime
Value: ptime-value
Usage Level: media
Charset Dependent: no
Syntax:
ptime-value = non-zero-int-or-real
Example:
a=ptime:20
This gives the length of time in milliseconds represented by the
media in a packet. This is probably only meaningful for audio data,
but may be used with other media types if it makes sense. It should
not be necessary to know ptime to decode RTP or vat audio, and it is
intended as a recommendation for the encoding/packetisation of audio.
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6.5. maxptime (maximum packet time)
Name: maxptime
Value: maxptime-value
Usage Level: media
Charset Dependent: no
Syntax:
maxptime-value = non-zero-int-or-real
Example:
a=maxptime:20
This gives the maximum amount of media that can be encapsulated in
each packet, expressed as time in milliseconds. The time SHALL be
calculated as the sum of the time the media present in the packet
represents. For frame-based codecs, the time SHOULD be an integer
multiple of the frame size. This attribute is probably only
meaningful for audio data, but may be used with other media types if
it makes sense. Note that this attribute was introduced after
[RFC2327], and non-updated implementations will ignore this
attribute.
6.6. rtpmap
Name: rtpmap
Value: rtpmap-value
Usage Level: media
Charset Dependent: no
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Syntax:
rtpmap-value = payload-type SP encoding-name
"/" clock-rate [ "/" encoding-params ]
payload-type = zero-based-integer
encoding-name = token
clock-rate = integer
; Editor's note: Do we want to define a limited range for this?
encoding-params = channels
; Editor's note: 4566 is vague about what this can be. RFC4855 seems to be
; the authoritative source, and only allows the
; value of the media subtype "channels" parameter - the
; number of audio channels.
; Does anyone think this can be used for something else?
; (The implication that multiple parameters might be included
; seems a misdirection - additional parameters are
; to go into a=fmtp.)
; Does anyone have an example of other parameters
; using this field? If none, it will stay as it is.
channels = integer
; Editor's note: Is there any reason to make this less restrictive?
This attribute maps from an RTP payload type number (as used in an
"m=" line) to an encoding name denoting the payload format to be
used. It also provides information on the clock rate and encoding
parameters. Note that the payload type number is indicated in a
7-bit field, limiting the values to incusively between 0 and 127.
Although an RTP profile can make static assignments of payload type
numbers to payload formats, it is more common for that assignment to
be done dynamically using "a=rtpmap:" attributes. As an example of a
static payload type, consider u-law PCM coded single-channel audio
sampled at 8 kHz. This is completely defined in the RTP Audio/Video
profile as payload type 0, so there is no need for an "a=rtpmap:"
attribute, and the media for such a stream sent to UDP port 49232 can
be specified as:
m=audio 49232 RTP/AVP 0
An example of a dynamic payload type is 16-bit linear encoded stereo
audio sampled at 16 kHz. If we wish to use the dynamic RTP/AVP
payload type 98 for this stream, additional information is required
to decode it:
m=audio 49232 RTP/AVP 98
a=rtpmap:98 L16/16000/2
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Up to one rtpmap attribute can be defined for each media format
specified. Thus, we might have the following:
m=audio 49230 RTP/AVP 96 97 98
a=rtpmap:96 L8/8000
a=rtpmap:97 L16/8000
a=rtpmap:98 L16/11025/2
RTP profiles that specify the use of dynamic payload types MUST
define the set of valid encoding names and/or a means to register
encoding names if that profile is to be used with SDP. The "RTP/AVP"
and "RTP/SAVP" profiles use media subtypes for encoding names, under
the top-level media type denoted in the "m=" line. In the example
above, the media types are "audio/l8" and "audio/l16".
For audio streams, <encoding parameters> indicates the number of
audio channels. This parameter is OPTIONAL and may be omitted if the
number of channels is one, provided that no additional parameters are
needed.
For video streams, no encoding parameters are currently specified.
Additional encoding parameters MAY be defined in the future, but
codec-specific parameters SHOULD NOT be added. Parameters added to
an "a=rtpmap:" attribute SHOULD only be those required for a session
directory to make the choice of appropriate media to participate in a
session. Codec-specific parameters should be added in other
attributes (for example, "a=fmtp:").
Note: RTP audio formats typically do not include information about
the number of samples per packet. If a non-default (as defined in
the RTP Audio/Video Profile) packetisation is required, the "ptime"
attribute is used as given above.
6.7. Media Direction Attributes
At most one of recvonly/sendrecv/sendonly/inactive MAY appear at
session level, and at most one MAY appear in each media section.
If any one of these appears in a media section then it applies for
that media section. If none appear in a media section then the one
from session level, if any, applies to that media section.
If none of the media direction attributes is present at either
session level or media level, "sendrecv" SHOULD be assumed as the
default for sessions that are not of the multimedia conference type
"broadcast" or "H332" (see below).
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Within the following SDP example, the "inactive" attribute applies to
audio media and the "recvonly" attribute applies to video media.
v=0
o=jdoe 2890844526 2890842807 IN IP4 198.51.100.1
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe)
c=IN IP4 233.252.0.1/127
t=2873397496 2873404696
a=inactive
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 99
a=rtpmap:99 h263-1998/90000
a=recvonly
6.7.1. recvonly (receive-only)
Name: recvonly
Value:
Usage Level: session, media
Charset Dependent: no
Example:
a=recvonly
This specifies that the tools should be started in receive-only mode
where applicable. Note that recvonly applies to the media only, not
to any associated control protocol (e.g., an RTP-based system in
recvonly mode SHOULD still send RTCP packets).
6.7.2. sendrecv (send-receive)
Name: sendrecv
Value:
Usage Level: session, media
Charset Dependent: no
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Example:
a=sendrecv
This specifies that the tools should be started in send and receive
mode. This is necessary for interactive multimedia conferences with
tools that default to receive-only mode.
6.7.3. sendonly (send-only)
Name: sendonly
Value:
Usage Level: session, media
Charset Dependent: no
Example:
a=sendonly
This specifies that the tools should be started in send-only mode.
An example may be where a different unicast address is to be used for
a traffic destination than for a traffic source. In such a case, two
media descriptions may be used, one sendonly and one recvonly. Note
that sendonly applies only to the media, and any associated control
protocol (e.g., RTCP) SHOULD still be received and processed as
normal.
6.7.4. inactive
Name: inactive
Value:
Usage Level: session, media
Charset Dependent: no
Example:
a=inactive
This specifies that the tools should be started in inactive mode.
This is necessary for interactive multimedia conferences where users
can put other users on hold. No media is sent over an inactive media
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stream. Note that an RTP-based system MUST still send RTCP (if RTCP
is used), even if started inactive.
6.8. orient (orientation)
Name: orient
Value: orient-value
Usage Level: media
Charset Dependent: no
Syntax:
orient-value = portrait / landscape / seascape
portrait = %s"portrait"
landscape = %s"landscape"
seascape = %s"seascape"
; NOTE: These names are case-sensitive.
Example:
a=orient:portrait
Normally this is only used for a whiteboard or presentation tool. It
specifies the orientation of the workspace on the screen. Permitted
values are "portrait", "landscape", and "seascape" (upside-down
landscape).
6.9. type (conference type)
Name: type
Value: type-value
Usage Level: session
Charset Dependent: no
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Syntax:
type-value = conference-type
conference-type = broadcast / meeting / moderated / test /
H332
broadcast = %s"broadcast"
meeting = %s"meeting"
moderated = %s"moderated"
test = %s"test"
H332 = %s"H332"
; NOTE: These names are case-sensitive.
Example:
a=type:moderated
This specifies the type of the multimedia 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 multimedia conference.
Specifying the attribute "type:H332" indicates that this loosely
coupled session is part of an H.332 session as defined in the ITU
H.332 specification [ITU.H332.1998]. 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.
6.10. charset (character set)
Name: charset
Value: charset-value
Usage Level: session
Charset Dependent: no
Syntax:
charset-value = mime-charset
(as defined in [RFC 2978])
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This specifies the character set to be used to display the session
name and information data. By default, the ISO-10646 character set
in UTF-8 encoding is used. If a more compact representation is
required, other character sets may be used. For example, the ISO
8859-1 is specified with the following SDP attribute:
a=charset:ISO-8859-1
The charset specified MUST be one of those registered in the IANA
Character Sets registry (http://www.iana.org/assignments/character-
sets), such as ISO-8859-1. The character set identifier is a US-
ASCII string and MUST be compared against identifiers from the "Name"
or "Preferred MIME Name" field of the registry using a case-
insensitive comparison. If the identifier is not recognised or not
supported, all strings that are affected by it SHOULD be regarded as
octet strings.
Note that a character set specified MUST still prohibit the use of
bytes 0x00 (Nul), 0x0A (LF), and 0x0d (CR). Character sets requiring
the use of these characters MUST define a quoting mechanism that
prevents these bytes from appearing within text fields.
6.11. sdplang (SDP language)
Name: sdplang
Value: sdplang-value
Usage Level: session, media
Charset Dependent: no
Syntax:
sdplang-value = Language-Tag
; Language-Tag defined in RFC5646
Example:
a=sdplang:fr
Multiple sdplang attributes can be provided either at session or
media level if the session description or media use multiple
languages.
As a session-level attribute, it specifies the language for the
session description (not the language of the media). As a media-
level attribute, it specifies the language for any media-level SDP
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information field associated with that media (again not the language
of the media), overriding any sdplang attributes specified at
session-level.
In general, sending session descriptions consisting of multiple
languages is discouraged. Instead, multiple descriptions SHOULD be
sent describing the session, one in each language. However, this is
not possible with all transport mechanisms, and so multiple sdplang
attributes are allowed although NOT RECOMMENDED.
The "sdplang" attribute value must be a single [RFC5646] language tag
in US-ASCII. An "sdplang" attribute SHOULD be specified when a
session is distributed with 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.
6.12. lang (language)
Name: lang
Value: lang-value
Usage Level: session, media
Charset Dependent: no
Syntax:
lang-value = Language-Tag
; Language-Tag defined in RFC5646
Example:
a=lang:de
Multiple lang attributes can be provided either at session or media
level if the session or media has capabilities in more than one
language, in which case the order of the attributes indicates the
order of preference of the various languages in the session or media,
from most preferred to least preferred.
As a session-level attribute, lang specifies a language capability
for the session being described. As a media-level attribute, it
specifies a language capability for that media, overriding any
session-level language(s) specified.
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The "lang" attribute value must be a single [RFC5646] language tag in
US-ASCII. A "lang" attribute SHOULD be specified when a session is
of sufficient scope to cross geographic boundaries where the language
of participants cannot be assumed, or where the session has
capabilities in languages different from the locally assumed norm.
The "lang" attribute is supposed to be used for settling the initial
language(s) used in the session. Events during the session may
influence which language(s) are used, and the participants are not
strictly bound to only use the declared languages.
Most real-time use cases start with just one language used, while
other cases involve a range of languages, e.g. an interpreted or
subtitled session. When more than one 'lang' attribute is specified,
the "lang" attribute itself does not provide any information about if
multiple languages are intended to be used during the session, or if
the intention is to only select one language. Other attributes or
the semantics in which the "lang" attributes are used might indicate
such conditions. Without such indications of usage intent, it is
assumed that for a negotiated session one of the declared languages
will be selected and used.
6.13. framerate (frame rate)
Name: framerate
Value: framerate-value
Usage Level: media
Charset Dependent: no
Syntax:
framerate-value = non-zero-int-or-real
Example:
a=framerate:60
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 are allowed. It is defined only
for video media.
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6.14. quality
Name: quality
Value: quality-value
Usage Level: media
Charset Dependent: no
Syntax:
quality-value = zero-based-integer
Example:
a=quality:10
This gives a suggestion for the quality of the encoding as an integer
value. The intention of the quality attribute for video is to
specify a non-default trade-off between frame-rate and still-image
quality. For video, the value is in the range 0 to 10, with the
following suggested meaning:
10 - the best still-image quality the compression scheme
can give.
5 - the default behaviour given no quality suggestion.
0 - the worst still-image quality the codec designer
thinks is still usable.
Editor's note: The usage should be checked with the SIP Forum to see
whether anybody is using this. Otherwise, the recommendation will be
not to use this attribute and the receiver ignores it upon reception.
6.15. fmtp (format parameters)
Name: fmtp
Value: fmtp-value
Usage Level: media
Charset Dependent: no
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Syntax:
fmtp-value = fmt SP format-specific-params
format-specific-params = byte-string
; Notes:
; - The format parameters are media type parameters and
need to reflect their syntax.
Example:
a=fmtp:96 profile-level-id=42e016;max-mbps=108000;max-fs=3600
This attribute allows parameters that are specific to a particular
format to be conveyed in a way that SDP does not have to understand
them. The format must be one of the formats specified for the media.
Format-specific parameters may be any set of parameters required to
be conveyed by SDP and given unchanged to the media tool that will
use this format. At most one instance of this attribute is allowed
for each format.
7. Security Considerations
SDP is frequently used with the Session Initiation Protocol [RFC3261]
using the offer/answer model [RFC3264] to agree on parameters for
unicast sessions. When used in this manner, the security
considerations of those protocols apply.
SDP is a session description format that describes multimedia
sessions. Entities receiving and acting upon an SDP message SHOULD
be aware that a session description cannot be trusted unless it has
been obtained by an authenticated transport protocol from a known and
trusted source. Many different transport protocols may be used to
distribute session descriptions, and the nature of the authentication
will differ from transport to transport. For some transports,
security features are often not deployed. In case a session
description has not been obtained in a trusted manner, the endpoint
SHOULD exercise care because, among other attacks, the media sessions
received may not be the intended ones, the destination where media is
sent to may not be the expected one, any of the parameters of the
session may be incorrect, or the media security may be compromised.
It is up to the endpoint to make a sensible decision taking into
account the security risks of the application and the user
preferences and may decide to ask the user whether or not to accept
the session.
One transport that can be used to distribute session descriptions is
the SAP. SAP provides both encryption and authentication mechanisms,
but due to the nature of session announcements it is likely that
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there are many occasions where the originator of a session
announcement cannot be authenticated because the originator is
previously unknown to the receiver of the announcement and because no
common public key infrastructure is available.
On receiving a session description over an unauthenticated transport
mechanism or from an untrusted party, software parsing the session
should take a few precautions. Session descriptions contain
information required to start software on the receiver's system.
Software that parses a session description MUST NOT be able to start
other software except that which is specifically configured as
appropriate software to participate in multimedia sessions. It is
normally considered inappropriate for software parsing a session
description to start, on a user's system, software that is
appropriate to participate in multimedia sessions, without the user
first being informed that such software will be started and giving
the user's consent. Thus, a session description arriving by session
announcement, email, session invitation, or WWW page MUST NOT deliver
the user into an interactive multimedia session unless the user has
explicitly pre-authorised such action. As it is not always simple to
tell whether or not a session is interactive, applications that are
unsure should assume sessions are interactive.
In this specification, there are no attributes that would allow the
recipient of a session description to be informed to start multimedia
tools in a mode where they default to transmitting. Under some
circumstances it might be appropriate to define such attributes. If
this is done, an application parsing a session description containing
such attributes SHOULD either ignore them or inform the user that
joining this session will result in the automatic transmission of
multimedia data. The default behaviour for an unknown attribute is
to ignore it.
In certain environments, it has become common for intermediary
systems to intercept and analyse session descriptions contained
within other signalling protocols. This is done for a range of
purposes, including but not limited to opening holes in firewalls to
allow media streams to pass, or to mark, prioritize, or block traffic
selectively. In some cases, such intermediary systems may modify the
session description, for example, to have the contents of the session
description match NAT bindings dynamically created. These behaviours
are NOT RECOMMENDED unless the session description is conveyed in
such a manner that allows the intermediary system to conduct proper
checks to establish the authenticity of the session description, and
the authority of its source to establish such communication sessions.
SDP by itself does not include sufficient information to enable these
checks: they depend on the encapsulating protocol (e.g., SIP or
RTSP).
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Use of the "k=" line poses a significant security risk, since it
conveys session encryption keys in the clear. SDP MUST NOT be used
to convey key material, unless it can be guaranteed that the channel
over which the SDP is delivered is both private and authenticated.
Moreover, the "k=" line provides no way to indicate or negotiate
cryptographic key algorithms. As it provides for only a single
symmetric key, rather than separate keys for confidentiality and
integrity, its utility is severely limited. The use of the "k=" line
is NOT RECOMMENDED, as discussed in Section 5.12.
8. IANA Considerations
8.1. The "application/sdp" Media Type
One media type registration from [RFC4566] is to be updated, as
defined below.
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To: ietf-types@iana.org
Subject: Registration of media type "application/sdp"
Type name: application
Subtype name: sdp
Required parameters: None.
Optional parameters: None.
Encoding considerations:
SDP files are primarily UTF-8 format text. The "a=charset:"
attribute may be used to signal the presence of other character
sets in certain parts of an SDP file (see Section 6 of RFC
XXXX). Arbitrary binary content cannot be directly
represented in SDP.
Security considerations:
See Section 7 of RFC XXXX.
Interoperability considerations:
See RFC XXXX.
Published specification:
See RFC XXXX.
Applications which use this media type:
Voice over IP, video teleconferencing, streaming media, instant
messaging, among others. See also Section 3 of RFC XXXX.
Additional information:
Magic number(s): None.
File extension(s): The extension ".sdp" is commonly used.
Macintosh File Type Code(s): "sdp "
Person & email address to contact for further information:
IETF MMUSIC working group <mmusic@ietf.org>
Intended usage: COMMON
Author/Change controller:
Authors of RFC XXXX
IETF MMUSIC working group delegated from the IESG
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8.2. Registration of Parameters
There are seven field names that are registered with IANA. Using the
terminology in the SDP specification Backus-Naur Form (BNF), they are
"media", "proto", "fmt", "att-field", "bwtype", "nettype", and
"addrtype".
The contact address for all parameters registered below is:
IETF MMUSIC working group <mmusic@ietf.org>
8.2.1. Media Types ("media")
The set of media types is intended to be small and SHOULD NOT be
extended except under rare circumstances. The same rules should
apply for media names as for top-level media types, and where
possible the same name should be registered for SDP as for MIME. For
media other than existing top-level media types, a Standards Track
RFC MUST be produced for a new top-level media type to be registered,
and the registration MUST provide good justification why no existing
media name is appropriate (the "Standards Action" policy of
[RFC5226].
This memo registers the media types "audio", "video", "text",
"application", and "message".
Note: The media types "control" and "data" were listed as valid in an
early version of this specification (RFC 2327); however, their
semantics were never fully specified and they are not widely used.
These media types have been removed in this specification, although
they still remain valid media type capabilities for a SIP user agent
as defined in [RFC3840]. If these media types are considered useful
in the future, a Standards Track RFC MUST be produced to document
their use. Until that is done, applications SHOULD NOT use these
types and SHOULD NOT declare support for them in SIP capabilities
declarations (even though they exist in the registry created by
[RFC3840]).
8.2.2. Transport Protocols ("proto")
The "proto" field describes the transport protocol used. This SHOULD
reference a standards-track protocol RFC. This memo registers three
values: "RTP/AVP" is a reference to [RFC3550] used under the RTP
Profile for Audio and Video Conferences with Minimal Control
[RFC3551] running over UDP/IP, "RTP/SAVP" is a reference to the
Secure Real-time Transport Protocol [RFC3711], and "udp" indicates an
unspecified protocol over UDP.
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If other RTP profiles are defined in the future, their "proto" name
SHOULD be specified in the same manner. For example, an RTP profile
whose short name is "XYZ" would be denoted by a "proto" field of
"RTP/XYZ".
New transport protocols SHOULD be registered with IANA.
Registrations MUST reference an RFC describing the protocol. Such an
RFC MAY be Experimental or Informational, although it is preferable
that it be Standards Track. Registrations MUST also define the rules
by which their "fmt" namespace is managed (see below).
8.2.3. Media Formats ("fmt")
Each transport protocol, defined by the "proto" field, has an
associated "fmt" namespace that describes the media formats that may
be conveyed by that protocol. Formats cover all the possible
encodings that could be transported in a multimedia session.
RTP payload formats under the "RTP/AVP" and "RTP/SAVP" profiles MUST
use the payload type number as their "fmt" value. If the payload
type number is dynamically assigned by this session description, an
additional "rtpmap" attribute MUST be included to specify the format
name and parameters as defined by the media type registration for the
payload format. It is RECOMMENDED that other RTP profiles that are
registered (in combination with RTP) as SDP transport protocols
specify the same rules for the "fmt" namespace.
For the "udp" protocol, new formats SHOULD be registered. Use of an
existing media subtype for the format is encouraged. If no media
subtype exists, it is RECOMMENDED that a suitable one be registered
through the IETF process [RFC6838] by production of, or reference to,
a standards-track RFC that defines the transport protocol for the
format.
For other protocols, formats MAY be registered according to the rules
of the associated "proto" specification.
Registrations of new formats MUST specify which transport protocols
they apply to.
8.2.4. Attribute Names ("att-field")
8.2.4.1. New Attributes
Attribute field names ("att-field") MUST be registered with IANA and
documented, because of noticeable issues due to conflicting
attributes under the same name. Unknown attributes in SDP are simply
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ignored, but conflicting ones that fragment the protocol are a
serious problem.
New attribute registrations are accepted according to the
"Specification Required" policy of [RFC5226], provided that the
specification includes the following information:
o Contact Name.
o Contact Email Address.
o Attribute Name: The name of the attribute that will appear in
SDP). This MUST conform to the definition of <att-field>.
o Attribute Syntax: For a value attribute (see clause 5.13), an ABNF
definition of the attribute value <att-value> syntax (See
Section Section 9) MUST be provided. The syntax MUST follow the
rule form as per Section 2.2 of [RFC5234]. This SHALL define the
allowable values that the attribute might take. It MAY also
define an extension method for the addition of future values. For
a property attribute, the ABNF definition is omitted as the
property attribute takes no values.
o Attribute Semantics: For a value attribute, a semantic description
of the values that the attribute might take MUST be provided. The
usage of a property attribute is described under purpose below.
o Attribute Value: The name of an ABNF syntax rule defining the
syntax of the value. Absence of a rule name indicates that the
attribute takes no values. Enclosing the rule name in "[" and "]"
indicates that a value is optional.
o Usage Level: Usage level(s) of the attribute. One or more of:
session, media, source, dcsa, dcsa(subprotocol). For a definition
of source level attributes, see [RFC5576]. For a definition of
dcsa attributes see: [I-D.ietf-mmusic-data-channel-sdpneg].
o Charset Dependent: Whether the attribute value is subject to the
charset attribute or not (Yes/No).
o Purpose: An explanation of the purpose and usage of the attribute.
o O/A Procedures: Offer/Answer procedures as explained in [RFC3264].
o Mux Category: Indication of which multiplexing "category"
[I-D.ietf-mmusic-sdp-mux-attributes] an attribute is associated
with.
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o Reference: A reference to the specification defining the
attribute.
The above is the minimum that IANA will accept. Attributes that are
expected to see widespread use and interoperability SHOULD be
documented with a standards-track RFC that specifies the attribute
more precisely.
Submitters of registrations should ensure that the specification is
in the spirit of SDP attributes, most notably that the attribute is
platform independent in the sense that it makes no implicit
assumptions about operating systems and does not name specific pieces
of software in a manner that might inhibit interoperability.
Submitters of registrations should also carefully choose the
attribute usage level. They should not choose only "session" when
the attribute can have different values when media is disaggregated,
i.e., when each m= section has its own IP address on a different
endpoint. In that case the attribute type chosen should be "session,
media". The default rule is that for all new SDP attributes that can
occur both in session and media level, the media level overrides the
session level. When this is not the case for a new SDP attribute, it
MUST be explicitly stated.
IANA has registered the initial set of attribute names ("att-field"
values), with definitions as in Section 6 of this memo (these
definitions replace those in [RFC4566]).
8.2.4.2. Updates to Existing Attributes
Updated attribute registrations are accepted according to the
"Specification Required" policy of [RFC5226], provided that the
specification updating the attribute (for example, by adding a new
value) considers the registration information items from
Section Section 8.2.4.1 according to the following bullets:
o Contact Name: A name MUST be provided.
o Contact Email Address: An email address MUST be provided.
o Attribute Name: MUST be provided and MUST NOT be changed.
Otherwise it is a new attribute.
o Attribute Syntax: The existing rule syntax with the syntax
extensions MUST be provided if there is a change to the syntax. A
revision to an existing attribute usage MAY extend the syntax of
an attribute, but MUST be backward compatible.
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o Attribute Semantics: A semantic description of new additional
attributes values or a semantic extension of existing values.
Existing attribute values semantics MUST only be extended in a
backward compatible manner.
o Usage Level: Updates MAY only add additional levels.
o Charset Dependent: MUST NOT be changed.
o Purpose: MAY be extended according to the updated usage.
o O/A Procedures: MAY be updated in a backward compatible manner
and/or it applies to a new usage level only.
o Mux Category: No change unless from TBD to another value. It MAY
also change if 'media' level is being added to the definition of
an attribute that previously did not include it.
o Reference: A new reference MUST be provided.
Items SHOULD be omitted if there is no impact to them as a result of
the attribute update.
8.2.5. Bandwidth Specifiers ("bwtype")
A proliferation of bandwidth specifiers is strongly discouraged.
New bandwidth specifiers ("bwtype" fields) MUST be registered with
IANA. The submission MUST reference a standards-track RFC specifying
the semantics of the bandwidth specifier precisely, and indicating
when it should be used, and why the existing registered bandwidth
specifiers do not suffice.
IANA has registered the bandwidth specifiers "CT" and "AS" with
definitions as in Section 5.8 of this memo (these definitions update
those in [RFC4566]).
8.2.6. Network Types ("nettype")
New network types (the "nettype" field) MUST be registered with IANA
if SDP needs to be used in the context of non-Internet environments.
The registration is subject to the RFC Required - RFC publication
policy of [RFC5226]. Although these are not normally the preserve of
IANA, there may be circumstances when an Internet application needs
to interoperate with a non-Internet application, such as when
gatewaying an Internet telephone call into the Public Switched
Telephone Network (PSTN). The number of network types should be
small and should be rarely extended. A new network type cannot be
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registered without registering at least one address type to be used
with that network type. A new network type registration MUST
reference an RFC that gives details of the network type and address
type(s) and specifies how and when they would be used.
IANA has registered the network type "IN" to represent the Internet,
with definition as in Sections 5.2 and 5.7 of this memo (these
definitions update those in [RFC4566]).
8.2.7. Address Types ("addrtype")
New address types ("addrtype") MUST be registered with IANA. The
registration is subject to the RFC Required - RFC publication policy
of [RFC5226]. An address type is only meaningful in the context of a
network type, and any registration of an address type MUST specify a
registered network type or be submitted along with a network type
registration. A new address type registration MUST reference an RFC
giving details of the syntax of the address type. Address types are
not expected to be registered frequently.
IANA has registered the address types "IP4" and "IP6" with
definitions as in Sections 5.2 and 5.7 of this memo (these
definitions update those in [RFC4566]).
8.2.8. Registration Procedure
In the RFC documentation that registers SDP "media", "proto", "fmt",
"bwtype", "nettype", and "addrtype" fields, the authors MUST include
the following information for IANA to place in the appropriate
registry:
o contact name, email address, and telephone number
o name being registered (as it will appear in SDP)
o long-form name in English
o type of name ("media", "proto", "fmt", "bwtype", "nettype", or
"addrtype")
o a one-paragraph explanation of the purpose of the registered name
o a reference to the specification for the registered name (this
will typically be an RFC number)
In the case of a new addrtype registration, the author has to check
whether the new address type is usable with the existing network
types. If yes, the "nettype" registry MUST be updated accordingly.
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In the case of a new nettype registration, the author MUST specify
the usable address type(s).
IANA may refer any registration to the IESG for review, and may
request revisions to be made before a registration will be made.
8.3. Encryption Key Access Methods
The IANA previously maintained a table of SDP encryption key access
method ("enckey") names. This table is obsolete, since the "k=" line
is not extensible. New registrations MUST NOT be accepted.
8.4. Reorganization of the nettype Registry
This document adds a new column in the "nettype" registry with the
title "Usable addrtype Values" and updates the "nettype" registry as
follows:
--------------------------------------------------------------------
|Type | SDP Name | Usable addrtype Values | Reference |
--------------------------------------------------------------------
|nettype | IN | IP4, IP6 | [RFC4566] |
|nettype | TN | RFC2543 | [RFC2848] |
|nettype | ATM | NSAP, GWID, E164 | [RFC3108] |
|nettype | PSTN | E164 | [RFC7195] |
--------------------------------------------------------------------
Note that both [RFC7195] and [RFC3108] registered "E164" as an
address type, although [RFC7195] mentions that the "E164" address
type has a different context for ATM and PSTN networks.
8.5. Reorganization of the att-field Registries
This document combines all of the (currently) five "att-field"
registries into one registry called "att-field" registry, and updates
the columns to reflect the name, usage level(s), charset dependency
and reference. As such IANA is requested to create a new combined
registry using the following columns:
Name | Usage Level | Dependent on Charset? | Mux Category | Reference
The "Name" column reflects the attribute name (as it will appear in
the SDP). The "Usage Level" column MUST indicate one or more of the
following: session, media, source, dcsa and dcsa(subprotocol). The
"Dependent on Charset?" column MUST indicate "Yes" or "No" depending
on whether the attribute value is subject to the charset attribute.
The "Mux Category" column MUST indicate one of the following
categories: NORMAL, NOT RECOMMENDED, IDENTICAL, SUM, TRANSPORT,
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INHERIT, IDENTICAL-PER-PT, SPECIAL or TBD as defined by
[I-D.ietf-mmusic-sdp-mux-attributes]. Finally, the "Reference"
column indicates the specification(s) where the attribute is defined.
For example, the attribute "setup" which is defined for both session
and media level, will be listed in the new registry as follows:
Name | Usage Level | Dependent on Charset?|Mux Category| Reference |
setup | session,media, | No |IDENTICAL | [RFC4145] |
| dcsa,dcsa(msrp)| | | [RFC6135] |
| | | | [I-D.mmusic
| | | |-msrp-usage-
| | | |data-channel
| | | |] |
9. SDP Grammar
This section provides an Augmented BNF grammar for SDP. ABNF is
defined in [RFC5234] and [RFC7405].
; SDP Syntax
session-description = proto-version
origin-field
session-name-field
[information-field]
[uri-field]
*email-field
*phone-field
[connection-field]
*bandwidth-field
1*time-field
[key-field]
*attribute-field
*media-description
proto-version = %s"v" "=" 1*DIGIT CRLF
;this memo describes version 0
origin-field = %s"o" "=" username SP sess-id SP sess-version SP
nettype SP addrtype SP unicast-address CRLF
session-name-field = %s"s" "=" text CRLF
information-field = %s"i" "=" text CRLF
uri-field = %s"u" "=" uri CRLF
email-field = %s"e" "=" email-address CRLF
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phone-field = %s"p" "=" phone-number CRLF
connection-field = %s"c" "=" nettype SP addrtype SP
connection-address CRLF
;a connection field must be present
;in every media description or at the
;session-level
bandwidth-field = %s"b" "=" bwtype ":" bandwidth CRLF
time-field = %s"t" "=" start-time SP stop-time
*(CRLF repeat-fields) CRLF
[zone-adjustments CRLF]
repeat-fields = %s"r" "=" repeat-interval SP typed-time
1*(SP typed-time)
zone-adjustments = %s"z" "=" time SP ["-"] typed-time
*(SP time SP ["-"] typed-time)
key-field = %s"k" "=" key-type CRLF
attribute-field = %s"a" "=" attribute CRLF
media-description = media-field
[information-field]
*connection-field
*bandwidth-fields
[key-field]
*attribute-field
media-field = %s"m" "=" media SP port ["/" integer]
SP proto 1*(SP fmt) CRLF
; sub-rules of 'o='
username = non-ws-string
;pretty wide definition, but doesn't
;include space
sess-id = 1*DIGIT
;should be unique for this username/host
sess-version = 1*DIGIT
nettype = token
;typically "IN"
addrtype = token
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;typically "IP4" or "IP6"
; sub-rules of 'u='
uri = URI-reference
; see RFC 3986
; sub-rules of 'e=', see RFC 5322 for definitions
email-address = address-and-comment / dispname-and-address
/ addr-spec
address-and-comment = addr-spec 1*SP "(" 1*email-safe ")"
dispname-and-address = 1*email-safe 1*SP "<" addr-spec ">"
; sub-rules of 'p='
phone-number = phone *SP "(" 1*email-safe ")" /
1*email-safe "<" phone ">" /
phone
phone = ["+"] DIGIT 1*(SP / "-" / DIGIT)
; sub-rules of 'c='
connection-address = multicast-address / unicast-address
; sub-rules of 'b='
bwtype = token
bandwidth = 1*DIGIT
; sub-rules of 't='
start-time = time / "0"
stop-time = time / "0"
time = POS-DIGIT 9*DIGIT
; Decimal representation of NTP time in
; seconds since 1900. The representation
; of NTP time is an unbounded length field
; containing at least 10 digits. Unlike the
; 64-bit representation used elsewhere, time
; in SDP does not wrap in the year 2036.
; sub-rules of 'r=' and 'z='
repeat-interval = POS-DIGIT *DIGIT [fixed-len-time-unit]
typed-time = 1*DIGIT [fixed-len-time-unit]
fixed-len-time-unit = %s"d" / %s"h" / %s"m" / %s"s"
; NOTE: These units are case-sensitive.
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; sub-rules of 'k='
key-type = %s"prompt"
%s"clear:"
%s"base64:"
%s"uri:"
; NOTE: These names are case-sensitive.
base64 = *base64-unit [base64-pad]
base64-unit = 4base64-char
base64-pad = 2base64-char "==" / 3base64-char "="
base64-char = ALPHA / DIGIT / "+" / "/"
; sub-rules of 'a='
attribute = (att-field ":" att-value) / att-field
att-field = token
att-value = byte-string
; sub-rules of 'm='
media = token
;typically "audio", "video", "text", "image"
;or "application"
fmt = token
;typically an RTP payload type for audio
;and video media
proto = token *("/" token)
;typically "RTP/AVP" or "udp"
port = 1*DIGIT
; generic sub-rules: addressing
unicast-address = IP4-address / IP6-address / FQDN / extn-addr
multicast-address = IP4-multicast / IP6-multicast / FQDN
/ extn-addr
IP4-multicast = m1 3( "." decimal-uchar )
"/" ttl [ "/" numaddr ]
; IP4 multicast addresses may be in the
; range 224.0.0.0 to 239.255.255.255
m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) /
("23" DIGIT )
IP6-multicast = IP6-address [ "/" numaddr ]
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; IP6 address starting with FF
numaddr = integer
ttl = (POS-DIGIT *2DIGIT) / "0"
FQDN = 4*(alpha-numeric / "-" / ".")
; fully qualified domain name as specified
; in RFC 1035 (and updates)
IP4-address = b1 3("." decimal-uchar)
b1 = decimal-uchar
; less than "224"
IP6-address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
h16 = 1*4HEXDIG
ls32 = ( h16 ":" h16 ) / IP4-address
; Generic for other address families
extn-addr = non-ws-string
; generic sub-rules: datatypes
text = byte-string
;default is to interpret this as UTF8 text.
;ISO 8859-1 requires "a=charset:ISO-8859-1"
;session-level attribute to be used
byte-string = 1*(%x01-09/%x0B-0C/%x0E-FF)
;any byte except NUL, CR, or LF
non-ws-string = 1*(VCHAR/%x80-FF)
;string of visible characters
token-char = ALPHA / DIGIT
/ "!" / "#" / "$" / "%" / "&"
/ "'" ; (single quote)
/ "*" / "+" / "-" / "." / "^" / "_"
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/ "`" ; (Grave accent)
/ "{" / "|" / "}" / "~"
token = 1*(token-char)
email-safe = %x01-09/%x0B-0C/%x0E-27/%x2A-3B/%x3D/%x3F-FF
;any byte except NUL, CR, LF, or the quoting
;characters ()<>
integer = POS-DIGIT *DIGIT
zero-based-integer = "0" / integer
non-zero-int-or-real = integer / non-zero-real
non-zero-real = zero-based-integer "." *DIGIT POS-DIGIT
; generic sub-rules: primitives
alpha-numeric = ALPHA / DIGIT
POS-DIGIT = %x31-39 ; 1 - 9
decimal-uchar = DIGIT
/ POS-DIGIT DIGIT
/ ("1" 2*(DIGIT))
/ ("2" ("0"/"1"/"2"/"3"/"4") DIGIT)
/ ("2" "5" ("0"/"1"/"2"/"3"/"4"/"5"))
; external references:
; ALPHA, DIGIT, CRLF, SP, VCHAR: from RFC 5234
; URI-reference: from RFC 3986
; addr-spec: from RFC 5322
10. Summary of Changes from RFC 4566
The ABNF rule for IP6-address has been corrected. As a result, the
ABNF rule for IP6-multicast has changed, and the (now unused) rules
for hexpart, hexseq, and hex4 have been removed.
IP4 unicast and multicast addresses in the example SDP descriptions
have been revised per RFCs 5735 and 5771.
Text in Section 5.2 has been revised to clarify the use of local
addresses in case of ICE-like SDP extensions.
Normative and informative references have been updated.
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The text regarding the session vs. media-level attribute usage has
been clarified.
The case-insensitivity rules from RFC 4855 have been included in this
document.
11. Acknowledgements
Many people in the IETF Multiparty Multimedia Session Control
(MMUSIC) working group have made comments and suggestions
contributing to this document.
12. References
12.1. Normative References
[I-D.ietf-mmusic-data-channel-sdpneg]
Drage, K., Makaraju, M., Stoetzer-Bradler, J., Ejzak, R.,
and J. Marcon, "SDP-based Data Channel Negotiation",
draft-ietf-mmusic-data-channel-sdpneg-12 (work in
progress), March 2017.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
(work in progress), December 2016.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
October 2000, <http://www.rfc-editor.org/info/rfc2978>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, DOI 10.17487/RFC5576, June 2009,
<http://www.rfc-editor.org/info/rfc5576>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <http://www.rfc-editor.org/info/rfc5646>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
12.2. Informative References
[ITU.H332.1998]
International Telecommunication Union, "H.323 extended for
loosely coupled conferences", ITU Recommendation H.332,
September 1998.
[RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, DOI 10.17487/RFC2327, April 1998,
<http://www.rfc-editor.org/info/rfc2327>.
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[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, DOI 10.17487/RFC2974,
October 2000, <http://www.rfc-editor.org/info/rfc2974>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <http://www.rfc-editor.org/info/rfc3550>.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
<http://www.rfc-editor.org/info/rfc3551>.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth",
RFC 3556, DOI 10.17487/RFC3556, July 2003,
<http://www.rfc-editor.org/info/rfc3556>.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
DOI 10.17487/RFC3605, October 2003,
<http://www.rfc-editor.org/info/rfc3605>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<http://www.rfc-editor.org/info/rfc3711>.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840,
DOI 10.17487/RFC3840, August 2004,
<http://www.rfc-editor.org/info/rfc3840>.
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[RFC3890] Westerlund, M., "A Transport Independent Bandwidth
Modifier for the Session Description Protocol (SDP)",
RFC 3890, DOI 10.17487/RFC3890, September 2004,
<http://www.rfc-editor.org/info/rfc3890>.
[RFC4567] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session
Description Protocol (SDP) and Real Time Streaming
Protocol (RTSP)", RFC 4567, DOI 10.17487/RFC4567, July
2006, <http://www.rfc-editor.org/info/rfc4567>.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
<http://www.rfc-editor.org/info/rfc4568>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<http://www.rfc-editor.org/info/rfc4855>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888,
DOI 10.17487/RFC5888, June 2010,
<http://www.rfc-editor.org/info/rfc5888>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<http://www.rfc-editor.org/info/rfc5905>.
[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
March 2012, <http://www.rfc-editor.org/info/rfc6544>.
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[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<http://www.rfc-editor.org/info/rfc6838>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<http://www.rfc-editor.org/info/rfc7405>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
[RFC7826] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
and M. Stiemerling, Ed., "Real-Time Streaming Protocol
Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December
2016, <http://www.rfc-editor.org/info/rfc7826>.
Authors' Addresses
Mark Handley
University College London
Department of Computer Science
London WC1E 6BT
UK
EMail: M.Handley@cs.ucl.ac.uk
Van Jacobson
USA
EMail: van@parc.com
Colin Perkins
University of Glasgow
School of Computing Science
University of Glasgow
Glasgow G12 8QQ
UK
EMail: csp@csperkins.org
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Ali Begen
Networked Media
Konya
Turkey
EMail: ali.begen@networked.media
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