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+
+Network Working Group M. Handley
+Request for Comments: 2327 V. Jacobson
+Category: Standards Track ISI/LBNL
+ April 1998
+
+
+ SDP: Session Description Protocol
+
+Status of this Memo
+
+ This document specifies an Internet standards track protocol for the
+ Internet community, and requests discussion and suggestions for
+ improvements. Please refer to the current edition of the "Internet
+ Official Protocol Standards" (STD 1) for the standardization state
+ and status of this protocol. Distribution of this memo is unlimited.
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (1998). All Rights Reserved.
+
+Abstract
+
+ This document defines the Session Description Protocol, SDP. SDP is
+ intended for describing multimedia sessions for the purposes of
+ session announcement, session invitation, and other forms of
+ multimedia session initiation.
+
+ This document is a product of the Multiparty Multimedia Session
+ Control (MMUSIC) working group of the Internet Engineering Task
+ Force. Comments are solicited and should be addressed to the working
+ group's mailing list at [email protected] and/or the authors.
+
+1. Introduction
+
+ On the Internet multicast backbone (Mbone), a session directory tool
+ is used to advertise multimedia conferences and communicate the
+ conference addresses and conference tool-specific information
+ necessary for participation. This document defines a session
+ description protocol for this purpose, and for general real-time
+ multimedia session description purposes. This memo does not describe
+ multicast address allocation or the distribution of SDP messages in
+ detail. These are described in accompanying memos. SDP is not
+ intended for negotiation of media encodings.
+
+
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 1]
+
+RFC 2327 SDP April 1998
+
+
+2. Background
+
+ The Mbone is the part of the internet that supports IP multicast, and
+ thus permits efficient many-to-many communication. It is used
+ extensively for multimedia conferencing. Such conferences usually
+ have the property that tight coordination of conference membership is
+ not necessary; to receive a conference, a user at an Mbone site only
+ has to know the conference's multicast group address and the UDP
+ ports for the conference data streams.
+
+ Session directories assist the advertisement of conference sessions
+ and communicate the relevant conference setup information to
+ prospective participants. SDP is designed to convey such information
+ to recipients. SDP is purely a format for session description - it
+ does not incorporate a transport protocol, and is intended to use
+ different transport protocols as appropriate including the Session
+ Announcement Protocol [4], Session Initiation Protocol [11], Real-
+ Time Streaming Protocol [12], electronic mail using the MIME
+ extensions, and the Hypertext Transport Protocol.
+
+ SDP is intended to be general purpose so that it can be used for a
+ wider range of network environments and applications than just
+ multicast session directories. However, it is not intended to
+ support negotiation of session content or media encodings - this is
+ viewed as outside the scope of session description.
+
+3. Glossary of Terms
+
+ The following terms are used in this document, and have specific
+ meaning within the context of this document.
+
+ Conference
+ A multimedia conference is a set of two or more communicating users
+ along with the software they are using to communicate.
+
+ Session
+ A multimedia session is a set of multimedia senders and receivers
+ and the data streams flowing from senders to receivers. A
+ multimedia conference is an example of a multimedia session.
+
+ Session Advertisement
+ See session announcement.
+
+ Session Announcement
+ A session announcement is a mechanism by which a session
+ description is conveyed to users in a proactive fashion, i.e., the
+ session description was not explicitly requested by the user.
+
+
+
+
+Handley & Jacobson Standards Track [Page 2]
+
+RFC 2327 SDP April 1998
+
+
+ Session Description
+ A well defined format for conveying sufficient information to
+ discover and participate in a multimedia session.
+
+3.1. Terminology
+
+ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+ "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+ document are to be interpreted as described in RFC 2119.
+
+4. SDP Usage
+
+4.1. Multicast Announcements
+
+ SDP is a session description protocol for multimedia sessions. A
+ common mode of usage is for a client to announce a conference session
+ by periodically multicasting an announcement packet to a well known
+ multicast address and port using the Session Announcement Protocol
+ (SAP).
+
+ SAP packets are UDP packets with the following format:
+
+ |--------------------|
+ | SAP header |
+ |--------------------|
+ | text payload |
+ |//////////
+
+
+ The header is the Session Announcement Protocol header. SAP is
+ described in more detail in a companion memo [4]
+
+ The text payload is an SDP session description, as described in this
+ memo. The text payload should be no greater than 1 Kbyte in length.
+ If announced by SAP, only one session announcement is permitted in a
+ single packet.
+
+4.2. Email and WWW Announcements
+
+ Alternative means of conveying session descriptions include
+ electronic mail and the World Wide Web. For both email and WWW
+ distribution, the use of the MIME content type "application/sdp"
+ should be used. This enables the automatic launching of applications
+ for participation in the session from the WWW client or mail reader
+ in a standard manner.
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 3]
+
+RFC 2327 SDP April 1998
+
+
+ Note that announcements of multicast sessions made only via email or
+ the World Wide Web (WWW) do not have the property that the receiver
+ of a session announcement can necessarily receive the session because
+ the multicast sessions may be restricted in scope, and access to the
+ WWW server or reception of email is possible outside this scope. SAP
+ announcements do not suffer from this mismatch.
+
+5. Requirements and Recommendations
+
+ The purpose of SDP is to convey information about media streams in
+ multimedia sessions to allow the recipients of a session description
+ to participate in the session. SDP is primarily intended for use in
+ an internetwork, although it is sufficiently general that it can
+ describe conferences in other network environments.
+
+ A multimedia session, for these purposes, is defined as a set of
+ media streams that exist for some duration of time. Media streams
+ can be many-to-many. The times during which the session is active
+ need not be continuous.
+
+ Thus far, multicast based sessions on the Internet have differed from
+ many other forms of conferencing in that anyone receiving the traffic
+ can join the session (unless the session traffic is encrypted). In
+ such an environment, SDP serves two primary purposes. It is a means
+ to communicate the existence of a session, and is a means to convey
+ sufficient information to enable joining and participating in the
+ session. In a unicast environment, only the latter purpose is likely
+ to be relevant.
+
+ Thus SDP includes:
+
+ o Session name and purpose
+
+ o Time(s) the session is active
+
+ o The media comprising the session
+
+ o Information to receive those media (addresses, ports, formats and
+ so on)
+
+ As resources necessary to participate in a session may be limited,
+ some additional information may also be desirable:
+
+ o Information about the bandwidth to be used by the conference
+
+ o Contact information for the person responsible for the session
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 4]
+
+RFC 2327 SDP April 1998
+
+
+ In general, SDP must convey sufficient information to be able to join
+ a session (with the possible exception of encryption keys) and to
+ announce the resources to be used to non-participants that may need
+ to know.
+
+5.1. Media Information
+
+ SDP includes:
+
+ o The type of media (video, audio, etc)
+
+ o The transport protocol (RTP/UDP/IP, H.320, etc)
+
+ o The format of the media (H.261 video, MPEG video, etc)
+
+ For an IP multicast session, the following are also conveyed:
+
+ o Multicast address for media
+
+ o Transport Port for media
+
+ This address and port are the destination address and destination
+ port of the multicast stream, whether being sent, received, or both.
+
+ For an IP unicast session, the following are conveyed:
+
+ o Remote address for media
+
+ o Transport port for contact address
+
+ The semantics of this address and port depend on the media and
+ transport protocol defined. By default, this is the remote address
+ and remote port to which data is sent, and the remote address and
+ local port on which to receive data. However, some media may define
+ to use these to establish a control channel for the actual media
+ flow.
+
+5.2. Timing Information
+
+ Sessions may either be bounded or unbounded in time. Whether or not
+ they are bounded, they may be only active at specific times.
+
+ SDP can convey:
+
+ o An arbitrary list of start and stop times bounding the session
+
+ o For each bound, repeat times such as "every Wednesday at 10am for
+ one hour"
+
+
+
+Handley & Jacobson Standards Track [Page 5]
+
+RFC 2327 SDP April 1998
+
+
+ This timing information is globally consistent, irrespective of local
+ time zone or daylight saving time.
+
+5.3. Private Sessions
+
+ It is possible to create both public sessions and private sessions.
+ Private sessions will typically be conveyed by encrypting the session
+ description to distribute it. The details of how encryption is
+ performed are dependent on the mechanism used to convey SDP - see [4]
+ for how this is done for session announcements.
+
+ If a session announcement is private it is possible to use that
+ private announcement to convey encryption keys necessary to decode
+ each of the media in a conference, including enough information to
+ know which encryption scheme is used for each media.
+
+5.4. Obtaining Further Information about a Session
+
+ A session description should convey enough information to decide
+ whether or not to participate in a session. SDP may include
+ additional pointers in the form of Universal Resources Identifiers
+ (URIs) for more information about the session.
+
+5.5. Categorisation
+
+ When many session descriptions are being distributed by SAP or any
+ other advertisement mechanism, it may be desirable to filter
+ announcements that are of interest from those that are not. SDP
+ supports a categorisation mechanism for sessions that is capable of
+ being automated.
+
+5.6. Internationalization
+
+ The SDP specification recommends the use of the ISO 10646 character
+ sets in the UTF-8 encoding (RFC 2044) to allow many different
+ languages to be represented. However, to assist in compact
+ representations, SDP also allows other character sets such as ISO
+ 8859-1 to be used when desired. Internationalization only applies to
+ free-text fields (session name and background information), and not
+ to SDP as a whole.
+
+6. SDP Specification
+
+ SDP session descriptions are entirely textual using the ISO 10646
+ character set in UTF-8 encoding. SDP field names and attributes names
+ use only the US-ASCII subset of UTF-8, but textual fields and
+ attribute values may use the full ISO 10646 character set. The
+ textual form, as opposed to a binary encoding such as ASN/1 or XDR,
+
+
+
+Handley & Jacobson Standards Track [Page 6]
+
+RFC 2327 SDP April 1998
+
+
+ was chosen to enhance portability, to enable a variety of transports
+ to be used (e.g, session description in a MIME email message) and to
+ allow flexible, text-based toolkits (e.g., Tcl/Tk ) to be used to
+ generate and to process session descriptions. However, since the
+ total bandwidth allocated to all SAP announcements is strictly
+ limited, the encoding is deliberately compact. Also, since
+ announcements may be transported via very unreliable means (e.g.,
+ email) or damaged by an intermediate caching server, the encoding was
+ designed with strict order and formatting rules so that most errors
+ would result in malformed announcements which could be detected
+ easily and discarded. This also allows rapid discarding of encrypted
+ announcements for which a receiver does not have the correct key.
+
+ An SDP session description consists of a number of lines of text of
+ the form <type>=<value> <type> is always exactly one character and is
+ case-significant. <value> is a structured text string whose format
+ depends on <type>. It also will be case-significant unless a
+ specific field defines otherwise. Whitespace is not permitted either
+ side of the `=' sign. In general <value> is either a number of fields
+ delimited by a single space character or a free format string.
+
+ A session description consists of a session-level description
+ (details that apply to the whole session and all media streams) and
+ optionally several media-level descriptions (details that apply onto
+ to a single media stream).
+
+ An announcement consists of a session-level section followed by zero
+ or more media-level sections. The session-level part starts with a
+ `v=' line and continues to the first media-level section. The media
+ description starts with an `m=' line and continues to the next media
+ description or end of the whole session description. In general,
+ session-level values are the default for all media unless overridden
+ by an equivalent media-level value.
+
+ When SDP is conveyed by SAP, only one session description is allowed
+ per packet. When SDP is conveyed by other means, many SDP session
+ descriptions may be concatenated together (the `v=' line indicating
+ the start of a session description terminates the previous
+ description). Some lines in each description are required and some
+ are optional but all must appear in exactly the order given here (the
+ fixed order greatly enhances error detection and allows for a simple
+ parser). Optional items are marked with a `*'.
+
+Session description
+ v= (protocol version)
+ o= (owner/creator and session identifier).
+ s= (session name)
+ i=* (session information)
+
+
+
+Handley & Jacobson Standards Track [Page 7]
+
+RFC 2327 SDP April 1998
+
+
+ u=* (URI of description)
+ e=* (email address)
+ p=* (phone number)
+ c=* (connection information - not required if included in all media)
+ b=* (bandwidth information)
+ One or more time descriptions (see below)
+ z=* (time zone adjustments)
+ k=* (encryption key)
+ a=* (zero or more session attribute lines)
+ Zero or more media descriptions (see below)
+
+Time description
+ t= (time the session is active)
+ r=* (zero or more repeat times)
+
+Media description
+ m= (media name and transport address)
+ i=* (media title)
+ c=* (connection information - optional if included at session-level)
+ b=* (bandwidth information)
+ k=* (encryption key)
+ a=* (zero or more media attribute lines)
+
+ The set of `type' letters is deliberately small and not intended to
+ be extensible -- SDP parsers must completely ignore any announcement
+ that contains a `type' letter that it does not understand. The
+ `attribute' mechanism ("a=" described below) is the primary means for
+ extending SDP and tailoring it to particular applications or media.
+ Some attributes (the ones listed in this document) have a defined
+ meaning but others may be added on an application-, media- or
+ session-specific basis. A session directory must ignore any
+ attribute it doesn't understand.
+
+ The connection (`c=') and attribute (`a=') information in the
+ session-level section applies to all the media of that session unless
+ overridden by connection information or an attribute of the same name
+ in the media description. For instance, in the example below, each
+ media behaves as if it were given a `recvonly' attribute.
+
+ An example SDP description is:
+
+ v=0
+ o=mhandley 2890844526 2890842807 IN IP4 126.16.64.4
+ s=SDP Seminar
+ i=A Seminar on the session description protocol
+ u=http://www.cs.ucl.ac.uk/staff/M.Handley/sdp.03.ps
+ [email protected] (Mark Handley)
+ c=IN IP4 224.2.17.12/127
+
+
+
+Handley & Jacobson Standards Track [Page 8]
+
+RFC 2327 SDP April 1998
+
+
+ t=2873397496 2873404696
+ a=recvonly
+ m=audio 49170 RTP/AVP 0
+ m=video 51372 RTP/AVP 31
+ m=application 32416 udp wb
+ a=orient:portrait
+
+ Text records such as the session name and information are bytes
+ strings which may contain any byte with the exceptions of 0x00 (Nul),
+ 0x0a (ASCII newline) and 0x0d (ASCII carriage return). The sequence
+ CRLF (0x0d0a) is used to end a record, although parsers should be
+ tolerant and also accept records terminated with a single newline
+ character. By default these byte strings contain ISO-10646
+ characters in UTF-8 encoding, but this default may be changed using
+ the `charset' attribute.
+
+ Protocol Version
+
+ v=0
+
+ The "v=" field gives the version of the Session Description Protocol.
+ There is no minor version number.
+
+ Origin
+
+ o=<username> <session id> <version> <network type> <address type>
+ <address>
+
+ The "o=" field gives the originator of the session (their username
+ and the address of the user's host) plus a session id and session
+ version number.
+
+ <username> is the user's login on the originating host, or it is "-"
+ if the originating host does not support the concept of user ids.
+ <username> must not contain spaces. <session id> is a numeric string
+ such that the tuple of <username>, <session id>, <network type>,
+ <address type> and <address> form a globally unique identifier for
+ the session.
+
+ The method of <session id> allocation is up to the creating tool, but
+ it has been suggested that a Network Time Protocol (NTP) timestamp be
+ used to ensure uniqueness [1].
+
+ <version> is a version number for this announcement. It is needed
+ for proxy announcements to detect which of several announcements for
+ the same session is the most recent. Again its usage is up to the
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 9]
+
+RFC 2327 SDP April 1998
+
+
+ creating tool, so long as <version> is increased when a modification
+ is made to the session data. Again, it is recommended (but not
+ mandatory) that an NTP timestamp is used.
+
+ <network type> is a text string giving the type of network.
+ Initially "IN" is defined to have the meaning "Internet". <address
+ type> is a text string giving the type of the address that follows.
+ Initially "IP4" and "IP6" are defined. <address> is the globally
+ unique address of the machine from which the session was created.
+ For an address type of IP4, this is either the fully-qualified domain
+ name of the machine, or the dotted-decimal representation of the IP
+ version 4 address of the machine. For an address type of IP6, this
+ is either the fully-qualified domain name of the machine, or the
+ compressed textual representation of the IP version 6 address of the
+ machine. For both IP4 and IP6, the fully-qualified domain name is
+ the form that SHOULD be given unless this is unavailable, in which
+ case the globally unique address may be substituted. A local IP
+ address MUST NOT be used in any context where the SDP description
+ might leave the scope in which the address is meaningful.
+
+ In general, the "o=" field serves as a globally unique identifier for
+ this version of this session description, and the subfields excepting
+ the version taken together identify the session irrespective of any
+ modifications.
+
+ Session Name
+
+ s=<session name>
+
+ The "s=" field is the session name. There must be one and only one
+ "s=" field per session description, and it must contain ISO 10646
+ characters (but see also the `charset' attribute below).
+
+ Session and Media Information
+
+ i=<session description>
+
+ The "i=" field is information about the session. There may be at
+ most one session-level "i=" field per session description, and at
+ most one "i=" field per media. Although it may be omitted, this is
+ discouraged for session announcements, and user interfaces for
+ composing sessions should require text to be entered. If it is
+ present it must contain ISO 10646 characters (but see also the
+ `charset' attribute below).
+
+ A single "i=" field can also be used for each media definition. In
+ media definitions, "i=" fields are primarily intended for labeling
+ media streams. As such, they are most likely to be useful when a
+
+
+
+Handley & Jacobson Standards Track [Page 10]
+
+RFC 2327 SDP April 1998
+
+
+ single session has more than one distinct media stream of the same
+ media type. An example would be two different whiteboards, one for
+ slides and one for feedback and questions.
+
+ URI
+
+ u=<URI>
+
+ o A URI is a Universal Resource Identifier as used by WWW clients
+
+ o The URI should be a pointer to additional information about the
+ conference
+
+ o This field is optional, but if it is present it should be specified
+ before the first media field
+
+ o No more than one URI field is allowed per session description
+
+
+ Email Address and Phone Number
+
+ e=<email address>
+ p=<phone number>
+
+ o These specify contact information for the person responsible for
+ the conference. This is not necessarily the same person that
+ created the conference announcement.
+
+ o Either an email field or a phone field must be specified.
+ Additional email and phone fields are allowed.
+
+ o If these are present, they should be specified before the first
+ media field.
+
+ o More than one email or phone field can be given for a session
+ description.
+
+ o Phone numbers should be given in the conventional international
+
+ format - preceded by a "+ and the international country code.
+ There must be a space or a hyphen ("-") between the country code
+ and the rest of the phone number. Spaces and hyphens may be used
+ to split up a phone field to aid readability if desired. For
+ example:
+
+ p=+44-171-380-7777 or p=+1 617 253 6011
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 11]
+
+RFC 2327 SDP April 1998
+
+
+ o Both email addresses and phone numbers can have an optional free
+ text string associated with them, normally giving the name of the
+ person who may be contacted. This should be enclosed in
+ parenthesis if it is present. For example:
+
+ [email protected] (Mark Handley)
+
+ The alternative RFC822 name quoting convention is also allowed for
+ both email addresses and phone numbers. For example,
+
+ e=Mark Handley <[email protected]>
+
+ The free text string should be in the ISO-10646 character set with
+ UTF-8 encoding, or alternatively in ISO-8859-1 or other encodings
+ if the appropriate charset session-level attribute is set.
+
+ Connection Data
+
+ c=<network type> <address type> <connection address>
+
+ The "c=" field contains connection data.
+
+ A session announcement must contain one "c=" field in each media
+ description (see below) or a "c=" field at the session-level. It may
+ contain a session-level "c=" field and one additional "c=" field per
+ media description, in which case the per-media values override the
+ session-level settings for the relevant media.
+
+ The first sub-field is the network type, which is a text string
+ giving the type of network. Initially "IN" is defined to have the
+ meaning "Internet".
+
+ The second sub-field is the address type. This allows SDP to be used
+ for sessions that are not IP based. Currently only IP4 is defined.
+
+ The third sub-field is the connection address. Optional extra
+ subfields may be added after the connection address depending on the
+ value of the <address type> field.
+
+ For IP4 addresses, the connection address is defined as follows:
+
+ o Typically the connection address will be a class-D IP multicast
+
+ group address. If the session is not multicast, then the
+ connection address contains the fully-qualified domain name or the
+ unicast IP address of the expected data source or data relay or
+ data sink as determined by additional attribute fields. It is not
+ expected that fully-qualified domain names or unicast addresses
+
+
+
+Handley & Jacobson Standards Track [Page 12]
+
+RFC 2327 SDP April 1998
+
+
+ will be given in a session description that is communicated by a
+ multicast announcement, though this is not prohibited. If a
+ unicast data stream is to pass through a network address
+ translator, the use of a fully-qualified domain name rather than an
+ unicast IP address is RECOMMENDED. In other cases, the use of an
+ IP address to specify a particular interface on a multi-homed host
+ might be required. Thus this specification leaves the decision as
+ to which to use up to the individual application, but all
+ applications MUST be able to cope with receiving both formats.
+
+ o Conferences using an IP multicast connection address must also have
+ a time to live (TTL) value present in addition to the multicast
+ address. The TTL and the address together define the scope with
+ which multicast packets sent in this conference will be sent. TTL
+ values must be in the range 0-255.
+
+ The TTL for the session is appended to the address using a slash as
+ a separator. An example is:
+
+ c=IN IP4 224.2.1.1/127
+
+ Hierarchical or layered encoding schemes are data streams where the
+ encoding from a single media source is split into a number of
+ layers. The receiver can choose the desired quality (and hence
+ bandwidth) by only subscribing to a subset of these layers. Such
+ layered encodings are normally transmitted in multiple multicast
+ groups to allow multicast pruning. This technique keeps unwanted
+ traffic from sites only requiring certain levels of the hierarchy.
+ For applications requiring multiple multicast groups, we allow the
+ following notation to be used for the connection address:
+
+ <base multicast address>/<ttl>/<number of addresses>
+
+ If the number of addresses is not given it is assumed to be one.
+ Multicast addresses so assigned are contiguously allocated above
+ the base address, so that, for example:
+
+ c=IN IP4 224.2.1.1/127/3
+
+ would state that addresses 224.2.1.1, 224.2.1.2 and 224.2.1.3 are
+ to be used at a ttl of 127. This is semantically identical to
+ including multiple "c=" lines in a media description:
+
+ c=IN IP4 224.2.1.1/127
+ c=IN IP4 224.2.1.2/127
+ c=IN IP4 224.2.1.3/127
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 13]
+
+RFC 2327 SDP April 1998
+
+
+ Multiple addresses or "c=" lines can only be specified on a per-
+ media basis, and not for a session-level "c=" field.
+
+ It is illegal for the slash notation described above to be used for
+ IP unicast addresses.
+
+ Bandwidth
+
+ b=<modifier>:<bandwidth-value>
+
+ o This specifies the proposed bandwidth to be used by the session or
+ media, and is optional.
+
+ o <bandwidth-value> is in kilobits per second
+
+ o <modifier> is a single alphanumeric word giving the meaning of the
+ bandwidth figure.
+
+ o Two modifiers are initially defined:
+
+ CT Conference Total: An implicit maximum bandwidth is associated with
+ each TTL on the Mbone or within a particular multicast
+ administrative scope region (the Mbone bandwidth vs. TTL limits are
+ given in the MBone FAQ). If the bandwidth of a session or media in
+ a session is different from the bandwidth implicit from the scope,
+ a `b=CT:...' line should be supplied for the session giving the
+ proposed upper limit to the bandwidth used. The primary purpose of
+ this is to give an approximate idea as to whether two or more
+ conferences can co-exist simultaneously.
+
+ AS Application-Specific Maximum: The bandwidth is interpreted to be
+ application-specific, i.e., will be the application's concept of
+ maximum bandwidth. Normally this will coincide with what is set on
+ the application's "maximum bandwidth" control if applicable.
+
+ Note that CT gives a total bandwidth figure for all the media at
+ all sites. AS gives a bandwidth figure for a single media at a
+ single site, although there may be many sites sending
+ simultaneously.
+
+ o Extension Mechanism: Tool writers can define experimental bandwidth
+ modifiers by prefixing their modifier with "X-". For example:
+
+ b=X-YZ:128
+
+ SDP parsers should ignore bandwidth fields with unknown modifiers.
+ Modifiers should be alpha-numeric and, although no length limit is
+ given, they are recommended to be short.
+
+
+
+Handley & Jacobson Standards Track [Page 14]
+
+RFC 2327 SDP April 1998
+
+
+ Times, Repeat Times and Time Zones
+
+ t=<start time> <stop time>
+
+ o "t=" fields specify the start and stop times for a conference
+ session. Multiple "t=" fields may be used if a session is active
+ at multiple irregularly spaced times; each additional "t=" field
+ specifies an additional period of time for which the session will
+ be active. If the session is active at regular times, an "r="
+ field (see below) should be used in addition to and following a
+ "t=" field - in which case the "t=" field specifies the start and
+ stop times of the repeat sequence.
+
+ o The first and second sub-fields give the start and stop times for
+ the conference respectively. These values are the decimal
+ representation of Network Time Protocol (NTP) time values in
+ seconds [1]. To convert these values to UNIX time, subtract
+ decimal 2208988800.
+
+ o If the stop-time is set to zero, then the session is not bounded,
+ though it will not become active until after the start-time. If
+ the start-time is also zero, the session is regarded as permanent.
+
+ User interfaces should strongly discourage the creation of
+ unbounded and permanent sessions as they give no information about
+ when the session is actually going to terminate, and so make
+ scheduling difficult.
+
+ The general assumption may be made, when displaying unbounded
+ sessions that have not timed out to the user, that an unbounded
+ session will only be active until half an hour from the current
+ time or the session start time, whichever is the later. If
+ behaviour other than this is required, an end-time should be given
+ and modified as appropriate when new information becomes available
+ about when the session should really end.
+
+ Permanent sessions may be shown to the user as never being active
+ unless there are associated repeat times which state precisely when
+ the session will be active. In general, permanent sessions should
+ not be created for any session expected to have a duration of less
+ than 2 months, and should be discouraged for sessions expected to
+ have a duration of less than 6 months.
+
+ r=<repeat interval> <active duration> <list of offsets from start-
+ time>
+
+ o "r=" fields specify repeat times for a session. For example, if
+ a session is active at 10am on Monday and 11am on Tuesday for one
+
+
+
+Handley & Jacobson Standards Track [Page 15]
+
+RFC 2327 SDP April 1998
+
+
+ hour each week for three months, then the <start time> in the
+ corresponding "t=" field would be the NTP representation of 10am on
+ the first Monday, the <repeat interval> would be 1 week, the
+ <active duration> would be 1 hour, and the offsets would be zero
+ and 25 hours. The corresponding "t=" field stop time would be the
+ NTP representation of the end of the last session three months
+ later. By default all fields are in seconds, so the "r=" and "t="
+ fields might be:
+
+ t=3034423619 3042462419
+ r=604800 3600 0 90000
+
+ To make announcements more compact, times may also be given in units
+ of days, hours or minutes. The syntax for these is a number
+ immediately followed by a single case-sensitive character.
+ Fractional units are not allowed - a smaller unit should be used
+ instead. The following unit specification characters are allowed:
+
+ d - days (86400 seconds)
+ h - minutes (3600 seconds)
+ m - minutes (60 seconds)
+ s - seconds (allowed for completeness but not recommended)
+
+ Thus, the above announcement could also have been written:
+
+ r=7d 1h 0 25h
+
+ Monthly and yearly repeats cannot currently be directly specified
+ with a single SDP repeat time - instead separate "t" fields should
+ be used to explicitly list the session times.
+
+ z=<adjustment time> <offset> <adjustment time> <offset> ....
+
+ o To schedule a repeated session which spans a change from daylight-
+ saving time to standard time or vice-versa, it is necessary to
+ specify offsets from the base repeat times. This is required
+ because different time zones change time at different times of day,
+ different countries change to or from daylight time on different
+ dates, and some countries do not have daylight saving time at all.
+
+ Thus in order to schedule a session that is at the same time winter
+ and summer, it must be possible to specify unambiguously by whose
+ time zone a session is scheduled. To simplify this task for
+ receivers, we allow the sender to specify the NTP time that a time
+ zone adjustment happens and the offset from the time when the
+ session was first scheduled. The "z" field allows the sender to
+ specify a list of these adjustment times and offsets from the base
+ time.
+
+
+
+Handley & Jacobson Standards Track [Page 16]
+
+RFC 2327 SDP April 1998
+
+
+ An example might be:
+
+ z=2882844526 -1h 2898848070 0
+
+ This specifies that at time 2882844526 the time base by which the
+ session's repeat times are calculated is shifted back by 1 hour,
+ and that at time 2898848070 the session's original time base is
+ restored. Adjustments are always relative to the specified start
+ time - they are not cumulative.
+
+ o If a session is likely to last several years, it is expected
+ that
+ the session announcement will be modified periodically rather than
+ transmit several years worth of adjustments in one announcement.
+
+ Encryption Keys
+
+ k=<method>
+ k=<method>:<encryption key>
+
+ o The session description protocol may be used to convey encryption
+ keys. A key field is permitted before the first media entry (in
+ which case it applies to all media in the session), or for each
+ media entry as required.
+
+ o The format of keys and their usage is outside the scope of this
+ document, but see [3].
+
+ o The method indicates the mechanism to be used to obtain a usable
+ key by external means, or from the encoded encryption key given.
+
+ The following methods are defined:
+
+ k=clear:<encryption key>
+ The encryption key (as described in [3] for RTP media streams
+ under the AV profile) is included untransformed in this key
+ field.
+
+ k=base64:<encoded encryption key>
+ The encryption key (as described in [3] for RTP media streams
+ under the AV profile) is included in this key field but has been
+ base64 encoded because it includes characters that are
+ prohibited in SDP.
+
+ k=uri:<URI to obtain key>
+ A Universal Resource Identifier as used by WWW clients is
+ included in this key field. The URI refers to the data
+ containing the key, and may require additional authentication
+
+
+
+Handley & Jacobson Standards Track [Page 17]
+
+RFC 2327 SDP April 1998
+
+
+ before the key can be returned. When a request is made to the
+ given URI, the MIME content-type of the reply specifies the
+ encoding for the key in the reply. The key should not be
+ obtained until the user wishes to join the session to reduce
+ synchronisation of requests to the WWW server(s).
+
+ k=prompt
+ No key is included in this SDP description, but the session or
+ media stream referred to by this key field is encrypted. The
+ user should be prompted for the key when attempting to join the
+ session, and this user-supplied key should then be used to
+ decrypt the media streams.
+
+ Attributes
+
+ a=<attribute>
+ a=<attribute>:<value>
+
+ Attributes are the primary means for extending SDP. Attributes may
+ be defined to be used as "session-level" attributes, "media-level"
+ attributes, or both.
+
+ A media description may have any number of attributes ("a=" fields)
+ which are media specific. These are referred to as "media-level"
+ attributes and add information about the media stream. Attribute
+ fields can also be added before the first media field; these
+ "session-level" attributes convey additional information that applies
+ to the conference as a whole rather than to individual media; an
+ example might be the conference's floor control policy.
+
+ Attribute fields may be of two forms:
+
+ o property attributes. A property attribute is simply of the form
+ "a=<flag>". These are binary attributes, and the presence of the
+ attribute conveys that the attribute is a property of the session.
+ An example might be "a=recvonly".
+
+ o value attributes. A value attribute is of the form
+ "a=<attribute>:<value>". An example might be that a whiteboard
+ could have the value attribute "a=orient:landscape"
+
+ Attribute interpretation depends on the media tool being invoked.
+ Thus receivers of session descriptions should be configurable in
+ their interpretation of announcements in general and of attributes in
+ particular.
+
+ Attribute names must be in the US-ASCII subset of ISO-10646/UTF-8.
+
+
+
+
+Handley & Jacobson Standards Track [Page 18]
+
+RFC 2327 SDP April 1998
+
+
+ Attribute values are byte strings, and MAY use any byte value except
+ 0x00 (Nul), 0x0A (LF), and 0x0D (CR). By default, attribute values
+ are to be interpreted as in ISO-10646 character set with UTF-8
+ encoding. Unlike other text fields, attribute values are NOT
+ normally affected by the `charset' attribute as this would make
+ comparisons against known values problematic. However, when an
+ attribute is defined, it can be defined to be charset-dependent, in
+ which case it's value should be interpreted in the session charset
+ rather than in ISO-10646.
+
+ Attributes that will be commonly used can be registered with IANA
+ (see Appendix B). Unregistered attributes should begin with "X-" to
+ prevent inadvertent collision with registered attributes. In either
+ case, if an attribute is received that is not understood, it should
+ simply be ignored by the receiver.
+
+ Media Announcements
+
+ m=<media> <port> <transport> <fmt list>
+
+ A session description may contain a number of media descriptions.
+ Each media description starts with an "m=" field, and is terminated
+ by either the next "m=" field or by the end of the session
+ description. A media field also has several sub-fields:
+
+ o The first sub-field is the media type. Currently defined media are
+ "audio", "video", "application", "data" and "control", though this
+ list may be extended as new communication modalities emerge (e.g.,
+ telepresense). The difference between "application" and "data" is
+ that the former is a media flow such as whiteboard information, and
+ the latter is bulk-data transfer such as multicasting of program
+ executables which will not typically be displayed to the user.
+ "control" is used to specify an additional conference control
+ channel for the session.
+
+ o The second sub-field is the transport port to which the media
+ stream will be sent. The meaning of the transport port depends on
+ the network being used as specified in the relevant "c" field and
+ on the transport protocol defined in the third sub-field. Other
+ ports used by the media application (such as the RTCP port, see
+ [2]) should be derived algorithmically from the base media port.
+
+ Note: For transports based on UDP, the value should be in the range
+ 1024 to 65535 inclusive. For RTP compliance it should be an even
+ number.
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 19]
+
+RFC 2327 SDP April 1998
+
+
+ For applications where hierarchically encoded streams are being
+ sent to a unicast address, it may be necessary to specify multiple
+ transport ports. This is done using a similar notation to that
+ used for IP multicast addresses in the "c=" field:
+
+ m=<media> <port>/<number of ports> <transport> <fmt list>
+
+ In such a case, the ports used depend on the transport protocol.
+ For RTP, only the even ports are used for data and the
+ corresponding one-higher odd port is used for RTCP. For example:
+
+ m=video 49170/2 RTP/AVP 31
+
+ would specify that ports 49170 and 49171 form one RTP/RTCP pair and
+ 49172 and 49173 form the second RTP/RTCP pair. RTP/AVP is the
+ transport protocol and 31 is the format (see below).
+
+ It is illegal for both multiple addresses to be specified in the
+ "c=" field and for multiple ports to be specified in the "m=" field
+ in the same session description.
+
+ o The third sub-field is the transport protocol. The transport
+ protocol values are dependent on the address-type field in the "c="
+ fields. Thus a "c=" field of IP4 defines that the transport
+ protocol runs over IP4. For IP4, it is normally expected that most
+ media traffic will be carried as RTP over UDP. The following
+ transport protocols are preliminarily defined, but may be extended
+ through registration of new protocols with IANA:
+
+ - RTP/AVP - the IETF's Realtime Transport Protocol using the
+ Audio/Video profile carried over UDP.
+
+ - udp - User Datagram Protocol
+
+ If an application uses a single combined proprietary media format
+ and transport protocol over UDP, then simply specifying the
+ transport protocol as udp and using the format field to distinguish
+ the combined protocol is recommended. If a transport protocol is
+ used over UDP to carry several distinct media types that need to be
+ distinguished by a session directory, then specifying the transport
+ protocol and media format separately is necessary. RTP is an
+ example of a transport-protocol that carries multiple payload
+ formats that must be distinguished by the session directory for it
+ to know how to start appropriate tools, relays, mixers or
+ recorders.
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 20]
+
+RFC 2327 SDP April 1998
+
+
+ The main reason to specify the transport-protocol in addition to
+ the media format is that the same standard media formats may be
+ carried over different transport protocols even when the network
+ protocol is the same - a historical example is vat PCM audio and
+ RTP PCM audio. In addition, relays and monitoring tools that are
+ transport-protocol-specific but format-independent are possible.
+
+ For RTP media streams operating under the RTP Audio/Video Profile
+ [3], the protocol field is "RTP/AVP". Should other RTP profiles be
+ defined in the future, their profiles will be specified in the same
+ way. For example, the protocol field "RTP/XYZ" would specify RTP
+ operating under a profile whose short name is "XYZ".
+
+ o The fourth and subsequent sub-fields are media formats. For audio
+ and video, these will normally be a media payload type as defined
+ in the RTP Audio/Video Profile.
+
+ When a list of payload formats is given, this implies that all of
+ these formats may be used in the session, but the first of these
+ formats is the default format for the session.
+
+ For media whose transport protocol is not RTP or UDP the format
+ field is protocol specific. Such formats should be defined in an
+ additional specification document.
+
+ For media whose transport protocol is RTP, SDP can be used to
+ provide a dynamic binding of media encoding to RTP payload type.
+ The encoding names in the RTP AV Profile do not specify unique
+ audio encodings (in terms of clock rate and number of audio
+ channels), and so they are not used directly in SDP format fields.
+ Instead, the payload type number should be used to specify the
+ format for static payload types and the payload type number along
+ with additional encoding information should be used for dynamically
+ allocated payload types.
+
+ An example of a static payload type is u-law PCM coded single
+ channel audio sampled at 8KHz. This is completely defined in the
+ RTP Audio/Video profile as payload type 0, so the media field for
+ such a stream sent to UDP port 49232 is:
+
+ m=video 49232 RTP/AVP 0
+
+ An example of a dynamic payload type is 16 bit linear encoded
+ stereo audio sampled at 16KHz. If we wish to use dynamic RTP/AVP
+ payload type 98 for such a stream, additional information is
+ required to decode it:
+
+ m=video 49232 RTP/AVP 98
+
+
+
+Handley & Jacobson Standards Track [Page 21]
+
+RFC 2327 SDP April 1998
+
+
+ a=rtpmap:98 L16/16000/2
+
+ The general form of an rtpmap attribute is:
+
+ a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
+ parameters>]
+
+ For audio streams, <encoding parameters> may specify the number of
+ audio channels. This parameter may be omitted if the number of
+ channels is one provided no additional parameters are needed. For
+ video streams, no encoding parameters are currently specified.
+
+ Additional parameters may be defined in the future, but
+ codecspecific parameters should not be added. Parameters added to
+ an rtpmap attribute should only be those required for a session
+ directory to make the choice of appropriate media too to
+ participate in a session. Codec-specific parameters should be
+ added in other attributes.
+
+ Up to one rtpmap attribute can be defined for each media format
+ specified. Thus we might have:
+
+ m=audio 49230 RTP/AVP 96 97 98
+ a=rtpmap:96 L8/8000
+ a=rtpmap:97 L16/8000
+ a=rtpmap:98 L16/11025/2
+
+ RTP profiles that specify the use of dynamic payload types must
+ define the set of valid encoding names and/or a means to register
+ encoding names if that profile is to be used with SDP.
+
+ Experimental encoding formats can also be specified using rtpmap.
+ RTP formats that are not registered as standard format names must
+ be preceded by "X-". Thus a new experimental redundant audio
+ stream called GSMLPC using dynamic payload type 99 could be
+ specified as:
+
+ m=video 49232 RTP/AVP 99
+ a=rtpmap:99 X-GSMLPC/8000
+
+ Such an experimental encoding requires that any site wishing to
+ receive the media stream has relevant configured state in its
+ session directory to know which tools are appropriate.
+
+ Note that RTP audio formats typically do not include information
+ about the number of samples per packet. If a non-default (as
+ defined in the RTP Audio/Video Profile) packetisation is required,
+ the "ptime" attribute is used as given below.
+
+
+
+Handley & Jacobson Standards Track [Page 22]
+
+RFC 2327 SDP April 1998
+
+
+ For more details on RTP audio and video formats, see [3].
+
+ o Formats for non-RTP media should be registered as MIME content
+ types as described in Appendix B. For example, the LBL whiteboard
+ application might be registered as MIME content-type application/wb
+ with encoding considerations specifying that it operates over UDP,
+ with no appropriate file format. In SDP this would then be
+ expressed using a combination of the "media" field and the "fmt"
+ field, as follows:
+
+ m=application 32416 udp wb
+
+ Suggested Attributes
+
+ The following attributes are suggested. Since application writers
+ may add new attributes as they are required, this list is not
+ exhaustive.
+
+ a=cat:<category>
+ This attribute gives the dot-separated hierarchical category of
+ the session. This is to enable a receiver to filter unwanted
+ sessions by category. It would probably have been a compulsory
+ separate field, except for its experimental nature at this time.
+ It is a session-level attribute, and is not dependent on charset.
+
+ a=keywds:<keywords>
+ Like the cat attribute, this is to assist identifying wanted
+ sessions at the receiver. This allows a receiver to select
+ interesting session based on keywords describing the purpose of
+ the session. It is a session-level attribute. It is a charset
+ dependent attribute, meaning that its value should be interpreted
+ in the charset specified for the session description if one is
+ specified, or by default in ISO 10646/UTF-8.
+
+ a=tool:<name and version of tool>
+ This gives the name and version number of the tool used to create
+ the session description. It is a session-level attribute, and is
+ not dependent on charset.
+
+ a=ptime:<packet time>
+ This gives the length of time in milliseconds represented by the
+ media in a packet. This is probably only meaningful for audio
+ data. It should not be necessary to know ptime to decode RTP or
+ vat audio, and it is intended as a recommendation for the
+ encoding/packetisation of audio. It is a media attribute, and is
+ not dependent on charset.
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 23]
+
+RFC 2327 SDP April 1998
+
+
+ a=recvonly
+ This specifies that the tools should be started in receive-only
+ mode where applicable. It can be either a session or media
+ attribute, and is not dependent on charset.
+
+ a=sendrecv
+ This specifies that the tools should be started in send and
+ receive mode. This is necessary for interactive conferences with
+ tools such as wb which defaults to receive only mode. It can be
+ either a session or media attribute, and is not dependent on
+ charset.
+
+ a=sendonly
+ This specifies that the tools should be started in send-only
+ mode. An example may be where a different unicast address is to
+ be used for a traffic destination than for a traffic source. In
+ such a case, two media descriptions may be use, one sendonly and
+ one recvonly. It can be either a session or media attribute, but
+ would normally only be used as a media attribute, and is not
+ dependent on charset.
+
+ a=orient:<whiteboard orientation>
+ Normally this is only used in a whiteboard media specification.
+ It specifies the orientation of a the whiteboard on the screen.
+ It is a media attribute. Permitted values are `portrait',
+ `landscape' and `seascape' (upside down landscape). It is not
+ dependent on charset
+
+ a=type:<conference type>
+ This specifies the type of the conference. Suggested values are
+ `broadcast', `meeting', `moderated', `test' and `H332'.
+ `recvonly' should be the default for `type:broadcast' sessions,
+ `type:meeting' should imply `sendrecv' and `type:moderated'
+ should indicate the use of a floor control tool and that the
+ media tools are started so as to "mute" new sites joining the
+ conference.
+
+ Specifying the attribute type:H332 indicates that this loosely
+ coupled session is part of a H.332 session as defined in the ITU
+ H.332 specification [10]. Media tools should be started
+ `recvonly'.
+
+ Specifying the attribute type:test is suggested as a hint that,
+ unless explicitly requested otherwise, receivers can safely avoid
+ displaying this session description to users.
+
+ The type attribute is a session-level attribute, and is not
+ dependent on charset.
+
+
+
+Handley & Jacobson Standards Track [Page 24]
+
+RFC 2327 SDP April 1998
+
+
+ a=charset:<character set>
+ This specifies the character set to be used to display the
+ session name and information data. By default, the ISO-10646
+ character set in UTF-8 encoding is used. If a more compact
+ representation is required, other character sets may be used such
+ as ISO-8859-1 for Northern European languages. In particular,
+ the ISO 8859-1 is specified with the following SDP attribute:
+
+ a=charset:ISO-8859-1
+
+ This is a session-level attribute; if this attribute is present,
+ it must be before the first media field. The charset specified
+ MUST be one of those registered with IANA, such as ISO-8859-1.
+ The character set identifier is a US-ASCII string and MUST be
+ compared against the IANA identifiers using a case-insensitive
+ comparison. If the identifier is not recognised or not
+ supported, all strings that are affected by it SHOULD be regarded
+ as byte strings.
+
+ Note that a character set specified MUST still prohibit the use
+ of bytes 0x00 (Nul), 0x0A (LF) and 0x0d (CR). Character sets
+ requiring the use of these characters MUST define a quoting
+ mechanism that prevents these bytes appearing within text fields.
+
+ a=sdplang:<language tag>
+ This can be a session level attribute or a media level attribute.
+ As a session level attribute, it specifies the language for the
+ session description. As a media level attribute, it specifies
+ the language for any media-level SDP information field associated
+ with that media. Multiple sdplang attributes can be provided
+ either at session or media level if multiple languages in the
+ session description or media use multiple languages, in which
+ case the order of the attributes indicates the order of
+ importance of the various languages in the session or media from
+ most important to least important.
+
+ In general, sending session descriptions consisting of multiple
+ languages should be discouraged. Instead, multiple descriptions
+ should be sent describing the session, one in each language.
+ However this is not possible with all transport mechanisms, and
+ so multiple sdplang attributes are allowed although not
+ recommended.
+
+ The sdplang attribute value must be a single RFC 1766 language
+ tag in US-ASCII. It is not dependent on the charset attribute.
+ An sdplang attribute SHOULD be specified when a session is of
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 25]
+
+RFC 2327 SDP April 1998
+
+
+ sufficient scope to cross geographic boundaries where the
+ language of recipients cannot be assumed, or where the session is
+ in a different language from the locally assumed norm.
+
+ a=lang:<language tag>
+ This can be a session level attribute or a media level attribute.
+ As a session level attribute, it specifies the default language
+ for the session being described. As a media level attribute, it
+ specifies the language for that media, overriding any session-
+ level language specified. Multiple lang attributes can be
+ provided either at session or media level if multiple languages
+ if the session description or media use multiple languages, in
+ which case the order of the attributes indicates the order of
+ importance of the various languages in the session or media from
+ most important to least important.
+
+ The lang attribute value must be a single RFC 1766 language tag
+ in US-ASCII. It is not dependent on the charset attribute. A
+ lang attribute SHOULD be specified when a session is of
+ sufficient scope to cross geographic boundaries where the
+ language of recipients cannot be assumed, or where the session is
+ in a different language from the locally assumed norm.
+
+ a=framerate:<frame rate>
+ This gives the maximum video frame rate in frames/sec. It is
+ intended as a recommendation for the encoding of video data.
+ Decimal representations of fractional values using the notation
+ "<integer>.<fraction>" are allowed. It is a media attribute, is
+ only defined for video media, and is not dependent on charset.
+
+ a=quality:<quality>
+ This gives a suggestion for the quality of the encoding as an
+ integer value.
+
+ The intention of the quality attribute for video is to specify a
+ non-default trade-off between frame-rate and still-image quality.
+ For video, the value in the range 0 to 10, with the following
+ suggested meaning:
+
+ 10 - the best still-image quality the compression scheme can
+ give.
+
+ 5 - the default behaviour given no quality suggestion.
+
+ 0 - the worst still-image quality the codec designer thinks is
+ still usable.
+
+ It is a media attribute, and is not dependent on charset.
+
+
+
+Handley & Jacobson Standards Track [Page 26]
+
+RFC 2327 SDP April 1998
+
+
+ a=fmtp:<format> <format specific parameters>
+ This attribute allows parameters that are specific to a
+ particular format to be conveyed in a way that SDP doesn't have
+ to understand them. The format must be one of the formats
+ specified for the media. Format-specific parameters may be any
+ set of parameters required to be conveyed by SDP and given
+ unchanged to the media tool that will use this format.
+
+ It is a media attribute, and is not dependent on charset.
+
+6.1. Communicating Conference Control Policy
+
+ There is some debate over the way conference control policy should be
+ communicated. In general, the authors believe that an implicit
+ declarative style of specifying conference control is desirable where
+ possible.
+
+ A simple declarative style uses a single conference attribute field
+ before the first media field, possibly supplemented by properties
+ such as `recvonly' for some of the media tools. This conference
+ attribute conveys the conference control policy. An example might be:
+
+ a=type:moderated
+
+ In some cases, however, it is possible that this may be insufficient
+ to communicate the details of an unusual conference control policy.
+ If this is the case, then a conference attribute specifying external
+ control might be set, and then one or more "media" fields might be
+ used to specify the conference control tools and configuration data
+ for those tools. An example is an ITU H.332 session:
+
+ c=IN IP4 224.5.6.7
+ a=type:H332
+ m=audio 49230 RTP/AVP 0
+ m=video 49232 RTP/AVP 31
+ m=application 12349 udp wb
+ m=control 49234 H323 mc
+ c=IN IP4 134.134.157.81
+
+ In this example, a general conference attribute (type:H332) is
+ specified stating that conference control will be provided by an
+ external H.332 tool, and a contact addresses for the H.323 session
+ multipoint controller is given.
+
+ In this document, only the declarative style of conference control
+ declaration is specified. Other forms of conference control should
+ specify an appropriate type attribute, and should define the
+ implications this has for control media.
+
+
+
+Handley & Jacobson Standards Track [Page 27]
+
+RFC 2327 SDP April 1998
+
+
+7. Security Considerations
+
+ SDP is a session description format that describes multimedia
+ sessions. A session description should not be trusted unless it has
+ been obtained by an authenticated transport protocol from a trusted
+ source. Many different transport protocols may be used to distribute
+ session description, and the nature of the authentication will differ
+ from transport to transport.
+
+ One transport that will frequently be used to distribute session
+ descriptions is the Session Announcement Protocol (SAP). SAP
+ provides both encryption and authentication mechanisms but due to the
+ nature of session announcements it is likely that there are many
+ occasions where the originator of a session announcement cannot be
+ authenticated because they are previously unknown to the receiver of
+ the announcement and because no common public key infrastructure is
+ available.
+
+ On receiving a session description over an unauthenticated transport
+ mechanism or from an untrusted party, software parsing the session
+ should take a few precautions. Session description contain
+ information required to start software on the receivers system.
+ Software that parses a session description MUST not be able to start
+ other software except that which is specifically configured as
+ appropriate software to participate in multimedia sessions. It is
+ normally considered INAPPROPRIATE for software parsing a session
+ description to start, on a user's system, software that is
+ appropriate to participate in multimedia sessions, without the user
+ first being informed that such software will be started and giving
+ their consent. Thus a session description arriving by session
+ announcement, email, session invitation, or WWW page SHOULD not
+ deliver the user into an {it interactive} multimedia session without
+ the user being aware that this will happen. As it is not always
+ simple to tell whether a session is interactive or not, applications
+ that are unsure should assume sessions are interactive.
+
+ In this specification, there are no attributes which would allow the
+ recipient of a session description to be informed to start multimedia
+ tools in a mode where they default to transmitting. Under some
+ circumstances it might be appropriate to define such attributes. If
+ this is done an application parsing a session description containing
+ such attributes SHOULD either ignore them, or inform the user that
+ joining this session will result in the automatic transmission of
+ multimedia data. The default behaviour for an unknown attribute is
+ to ignore it.
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 28]
+
+RFC 2327 SDP April 1998
+
+
+ Session descriptions may be parsed at intermediate systems such as
+ firewalls for the purposes of opening a hole in the firewall to allow
+ the participation in multimedia sessions. It is considered
+ INAPPROPRIATE for a firewall to open such holes for unicast data
+ streams unless the session description comes in a request from inside
+ the firewall.
+
+ For multicast sessions, it is likely that local administrators will
+ apply their own policies, but the exclusive use of "local" or "site-
+ local" administrative scope within the firewall and the refusal of
+ the firewall to open a hole for such scopes will provide separation
+ of global multicast sessions from local ones.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 29]
+
+RFC 2327 SDP April 1998
+
+
+Appendix A: SDP Grammar
+
+ This appendix provides an Augmented BNF grammar for SDP. ABNF is
+ defined in RFC 2234.
+
+
+ announcement = proto-version
+ origin-field
+ session-name-field
+ information-field
+ uri-field
+ email-fields
+ phone-fields
+ connection-field
+ bandwidth-fields
+ time-fields
+ key-field
+ attribute-fields
+ media-descriptions
+
+ proto-version = "v=" 1*DIGIT CRLF
+ ;this memo describes version 0
+
+ origin-field = "o=" username space
+ sess-id space sess-version space
+ nettype space addrtype space
+ addr CRLF
+
+ session-name-field = "s=" text CRLF
+
+ information-field = ["i=" text CRLF]
+
+ uri-field = ["u=" uri CRLF]
+
+ email-fields = *("e=" email-address CRLF)
+
+ phone-fields = *("p=" phone-number CRLF)
+
+
+ connection-field = ["c=" nettype space addrtype space
+ connection-address CRLF]
+ ;a connection field must be present
+ ;in every media description or at the
+ ;session-level
+
+
+ bandwidth-fields = *("b=" bwtype ":" bandwidth CRLF)
+
+
+
+
+Handley & Jacobson Standards Track [Page 30]
+
+RFC 2327 SDP April 1998
+
+
+ time-fields = 1*( "t=" start-time space stop-time
+ *(CRLF repeat-fields) CRLF)
+ [zone-adjustments CRLF]
+
+
+ repeat-fields = "r=" repeat-interval space typed-time
+ 1*(space typed-time)
+
+
+ zone-adjustments = time space ["-"] typed-time
+ *(space time space ["-"] typed-time)
+
+
+ key-field = ["k=" key-type CRLF]
+
+
+ key-type = "prompt" |
+ "clear:" key-data |
+ "base64:" key-data |
+ "uri:" uri
+
+
+ key-data = email-safe | "~" | "
+
+
+ attribute-fields = *("a=" attribute CRLF)
+
+
+ media-descriptions = *( media-field
+ information-field
+ *(connection-field)
+ bandwidth-fields
+ key-field
+ attribute-fields )
+
+
+ media-field = "m=" media space port ["/" integer]
+ space proto 1*(space fmt) CRLF
+
+
+ media = 1*(alpha-numeric)
+ ;typically "audio", "video", "application"
+ ;or "data"
+
+ fmt = 1*(alpha-numeric)
+ ;typically an RTP payload type for audio
+ ;and video media
+
+
+
+
+Handley & Jacobson Standards Track [Page 31]
+
+RFC 2327 SDP April 1998
+
+
+ proto = 1*(alpha-numeric)
+ ;typically "RTP/AVP" or "udp" for IP4
+
+
+ port = 1*(DIGIT)
+ ;should in the range "1024" to "65535" inclusive
+ ;for UDP based media
+
+
+ attribute = (att-field ":" att-value) | att-field
+
+
+ att-field = 1*(alpha-numeric)
+
+
+ att-value = byte-string
+
+
+ sess-id = 1*(DIGIT)
+ ;should be unique for this originating username/host
+
+
+ sess-version = 1*(DIGIT)
+ ;0 is a new session
+
+
+ connection-address = multicast-address
+ | addr
+
+
+ multicast-address = 3*(decimal-uchar ".") decimal-uchar "/" ttl
+ [ "/" integer ]
+ ;multicast addresses may be in the range
+ ;224.0.0.0 to 239.255.255.255
+
+ ttl = decimal-uchar
+
+ start-time = time | "0"
+
+ stop-time = time | "0"
+
+ time = POS-DIGIT 9*(DIGIT)
+ ;sufficient for 2 more centuries
+
+
+ repeat-interval = typed-time
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 32]
+
+RFC 2327 SDP April 1998
+
+
+ typed-time = 1*(DIGIT) [fixed-len-time-unit]
+
+
+ fixed-len-time-unit = "d" | "h" | "m" | "s"
+
+
+ bwtype = 1*(alpha-numeric)
+
+ bandwidth = 1*(DIGIT)
+
+
+ username = safe
+ ;pretty wide definition, but doesn't include space
+
+
+ email-address = email | email "(" email-safe ")" |
+ email-safe "<" email ">"
+
+
+ email = ;defined in RFC822
+
+
+ uri= ;defined in RFC1630
+
+
+ phone-number = phone | phone "(" email-safe ")" |
+ email-safe "<" phone ">"
+
+
+ phone = "+" POS-DIGIT 1*(space | "-" | DIGIT)
+ ;there must be a space or hyphen between the
+ ;international code and the rest of the number.
+
+
+ nettype = "IN"
+ ;list to be extended
+
+
+ addrtype = "IP4" | "IP6"
+ ;list to be extended
+
+
+ addr = FQDN | unicast-address
+
+
+ FQDN = 4*(alpha-numeric|"-"|".")
+ ;fully qualified domain name as specified in RFC1035
+
+
+
+
+Handley & Jacobson Standards Track [Page 33]
+
+RFC 2327 SDP April 1998
+
+
+ unicast-address = IP4-address | IP6-address
+
+
+ IP4-address = b1 "." decimal-uchar "." decimal-uchar "." b4
+ b1 = decimal-uchar
+ ;less than "224"; not "0" or "127"
+ b4 = decimal-uchar
+ ;not "0"
+
+ IP6-address = ;to be defined
+
+
+ text = byte-string
+ ;default is to interpret this as IS0-10646 UTF8
+ ;ISO 8859-1 requires a "a=charset:ISO-8859-1"
+ ;session-level attribute to be used
+
+
+ byte-string = 1*(0x01..0x09|0x0b|0x0c|0x0e..0xff)
+ ;any byte except NUL, CR or LF
+
+
+ decimal-uchar = DIGIT
+ | POS-DIGIT DIGIT
+ | ("1" 2*(DIGIT))
+ | ("2" ("0"|"1"|"2"|"3"|"4") DIGIT)
+ | ("2" "5" ("0"|"1"|"2"|"3"|"4"|"5"))
+
+
+ integer = POS-DIGIT *(DIGIT)
+
+
+ alpha-numeric = ALPHA | DIGIT
+
+
+ DIGIT = "0" | POS-DIGIT
+
+
+ POS-DIGIT = "1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"
+
+
+ ALPHA = "a"|"b"|"c"|"d"|"e"|"f"|"g"|"h"|"i"|"j"|"k"|
+ "l"|"m"|"n"|"o "|"p"|"q"|"r"|"s"|"t"|"u"|"v"|
+ "w"|"x"|"y"|"z"|"A"|"B"|"C "|"D"|"E"|"F"|"G"|
+ "H"|"I"|"J"|"K"|"L"|"M"|"N"|"O"|"P"|" Q"|"R"|
+ "S"|"T"|"U"|"V"|"W"|"X"|"Y"|"Z"
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 34]
+
+RFC 2327 SDP April 1998
+
+
+ email-safe = safe | space | tab
+
+
+ safe = alpha-numeric |
+ "'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
+ "#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" |
+ "]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
+ "~" | "
+
+
+ space = %d32
+ tab = %d9
+ CRLF = %d13.10
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 35]
+
+RFC 2327 SDP April 1998
+
+
+Appendix B: Guidelines for registering SDP names with IANA
+
+ There are seven field names that may be registered with IANA. Using
+ the terminology in the SDP specification BNF, they are "media",
+ "proto", "fmt", "att-field", "bwtype", "nettype" and "addrtype".
+
+ "media" (eg, audio, video, application, data).
+
+ Packetized media types, such as those used by RTP, share the
+ namespace used by media types registry [RFC 2048] (i.e. "MIME
+ types"). The list of valid media names is the set of top-level
+ MIME content types. The set of media is intended to be small and
+ not to be extended except under rare circumstances. (The MIME
+ subtype corresponds to the "fmt" parameter below).
+
+ "proto"
+
+ In general this should be an IETF standards-track transport
+ protocol identifier such as RTP/AVP (rfc 1889 under the rfc 1890
+ profile).
+
+ However, people will want to invent their own proprietary
+ transport protocols. Some of these should be registered as a
+ "fmt" using "udp" as the protocol and some of which probably
+ can't be.
+
+ Where the protocol and the application are intimately linked,
+ such as with the LBL whiteboard wb which used a proprietary and
+ special purpose protocol over UDP, the protocol name should be
+ "udp" and the format name that should be registered is "wb". The
+ rules for formats (see below) apply to such registrations.
+
+ Where the proprietary transport protocol really carries many
+ different data formats, it is possible to register a new protocol
+ name with IANA. In such a case, an RFC MUST be produced
+ describing the protocol and referenced in the registration. Such
+ an RFC MAY be informational, although it is preferable if it is
+ standards-track.
+
+ "fmt"
+
+ The format namespace is dependent on the context of the "proto"
+ field, so a format cannot be registered without specifying one or
+ more transport protocols that it applies to.
+
+ Formats cover all the possible encodings that might want to be
+ transported in a multimedia session.
+
+
+
+
+Handley & Jacobson Standards Track [Page 36]
+
+RFC 2327 SDP April 1998
+
+
+ For RTP formats that have been assigned static payload types, the
+ payload type number is used. For RTP formats using a dynamic
+ payload type number, the dynamic payload type number is given as
+ the format and an additional "rtpmap" attribute specifies the
+ format and parameters.
+
+ For non-RTP formats, any unregistered format name may be
+ registered through the MIME-type registration process [RFC 2048].
+ The type given here is the MIME subtype only (the top-level MIME
+ content type is specified by the media parameter). The MIME type
+ registration SHOULD reference a standards-track RFC which
+ describes the transport protocol for this media type. If there
+ is an existing MIME type for this format, the MIME registration
+ should be augmented to reference the transport specification for
+ this media type. If there is not an existing MIME type for this
+ format, and there exists no appropriate file format, this should
+ be noted in the encoding considerations as "no appropriate file
+ format".
+
+ "att-field" (Attribute names)
+
+ Attribute field names MAY be registered with IANA, although this
+ is not compulsory, and unknown attributes are simply ignored.
+
+ When an attribute is registered, it must be accompanied by a
+ brief specification stating the following:
+
+ o contact name, email address and telephone number
+
+ o attribute-name (as it will appear in SDP)
+
+ o long-form attribute name in English
+
+ o type of attribute (session level, media level, or both)
+
+ o whether the attribute value is subject to the charset
+ attribute.
+
+ o a one paragraph explanation of the purpose of the attribute.
+
+ o a specification of appropriate attribute values for this
+ attribute.
+
+ IANA will not sanity check such attribute registrations except to
+ ensure that they do not clash with existing registrations.
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 37]
+
+RFC 2327 SDP April 1998
+
+
+ Although the above is the minimum that IANA will accept, if the
+ attribute is expected to see widespread use and interoperability
+ is an issue, authors are encouraged to produce a standards-track
+ RFC that specifies the attribute more precisely.
+
+ Submitters of registrations should ensure that the specification
+ is in the spirit of SDP attributes, most notably that the
+ attribute is platform independent in the sense that it makes no
+ implicit assumptions about operating systems and does not name
+ specific pieces of software in a manner that might inhibit
+ interoperability.
+
+ "bwtype" (bandwidth specifiers)
+
+ A proliferation of bandwidth specifiers is strongly discouraged.
+
+ New bandwidth specifiers may be registered with IANA. The
+ submission MUST reference a standards-track RFC specifying the
+ semantics of the bandwidth specifier precisely, and indicating
+ when it should be used, and why the existing registered bandwidth
+ specifiers do not suffice.
+
+ "nettype" (Network Type)
+
+ New network types may be registered with IANA if SDP needs to be
+ used in the context of non-internet environments. Whilst these
+ are not normally the preserve of IANA, there may be circumstances
+ when an Internet application needs to interoperate with a non-
+ internet application, such as when gatewaying an internet
+ telephony call into the PSTN. The number of network types should
+ be small and should be rarely extended. A new network type
+ cannot be registered without registering at least one address
+ type to be used with that network type. A new network type
+ registration MUST reference an RFC which gives details of the
+ network type and address type and specifies how and when they
+ would be used. Such an RFC MAY be Informational.
+
+ "addrtype" (Address Type)
+
+ New address types may be registered with IANA. An address type
+ is only meaningful in the context of a network type, and any
+ registration of an address type MUST specify a registered network
+ type, or be submitted along with a network type registration. A
+ new address type registration MUST reference an RFC giving
+ details of the syntax of the address type. Such an RFC MAY be
+ Informational. Address types are not expected to be registered
+ frequently.
+
+
+
+
+Handley & Jacobson Standards Track [Page 38]
+
+RFC 2327 SDP April 1998
+
+
+ Registration Procedure
+
+ To register a name the above guidelines should be followed regarding
+ the required level of documentation that is required. The
+ registration itself should be sent to IANA. Attribute registrations
+ should include the information given above. Other registrations
+ should include the following additional information:
+
+ o contact name, email address and telephone number
+
+ o name being registered (as it will appear in SDP)
+
+ o long-form name in English
+
+ o type of name ("media", "proto", "fmt", "bwtype", "nettype", or
+ "addrtype")
+
+ o a one paragraph explanation of the purpose of the registered name.
+
+ o a reference to the specification (eg RFC number) of the registered
+ name.
+
+ IANA may refer any registration to the IESG or to any appropriate
+ IETF working group for review, and may request revisions to be made
+ before a registration will be made.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 39]
+
+RFC 2327 SDP April 1998
+
+
+Appendix C: Authors' Addresses
+
+ Mark Handley
+ Information Sciences Institute
+ c/o MIT Laboratory for Computer Science
+ 545 Technology Square
+ Cambridge, MA 02139
+ United States
+ electronic mail: [email protected]
+
+ Van Jacobson
+ MS 46a-1121
+ Lawrence Berkeley Laboratory
+ Berkeley, CA 94720
+ United States
+ electronic mail: [email protected]
+
+Acknowledgments
+
+ Many people in the IETF MMUSIC working group have made comments and
+ suggestions contributing to this document. In particular, we would
+ like to thank Eve Schooler, Steve Casner, Bill Fenner, Allison
+ Mankin, Ross Finlayson, Peter Parnes, Joerg Ott, Carsten Bormann, Rob
+ Lanphier and Steve Hanna.
+
+References
+
+ [1] Mills, D., "Network Time Protocol (version 3) specification and
+ implementation", RFC 1305, March 1992.
+
+ [2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
+ A Transport Protocol for Real-Time Applications", RFC 1889, January
+ 1996.
+
+ [3] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
+ with Minimal Control", RFC 1890, January 1996
+
+ [4] Handley, M., "SAP - Session Announcement Protocol", Work in
+ Progress.
+
+ [5] V. Jacobson, S. McCanne, "vat - X11-based audio teleconferencing
+ tool" vat manual page, Lawrence Berkeley Laboratory, 1994.
+
+ [6] The Unicode Consortium, "The Unicode Standard -- Version 2.0",
+ Addison-Wesley, 1996.
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 40]
+
+RFC 2327 SDP April 1998
+
+
+ [7] ISO/IEC 10646-1:1993. International Standard -- Information
+ technol- ogy -- Universal Multiple-Octet Coded Character Set (UCS) --
+ Part 1: Architecture and Basic Multilingual Plane. Five amendments
+ and a techn- ical corrigendum have been published up to now. UTF-8
+ is described in Annex R, published as Amendment 2.
+
+ [8] Goldsmith, D., and M. Davis, "Using Unicode with MIME", RFC 1641,
+ July 1994.
+
+ [9] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
+ 10646", RFC 2044, October 1996.
+
+ [10] ITU-T Recommendation H.332 (1998): "Multimedia Terminal for
+ Receiving Internet-based H.323 Conferences", ITU, Geneva.
+
+ [11] Handley, M., Schooler, E., and H. Schulzrinne, "Session
+ Initiation Protocol (SIP)", Work in Progress.
+
+ [12] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
+ Protocol (RTSP)", RFC 2326, April 1998.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Handley & Jacobson Standards Track [Page 41]
+
+RFC 2327 SDP April 1998
+
+
+Full Copyright Statement
+
+ Copyright (C) The Internet Society (1998). All Rights Reserved.
+
+ This document and translations of it may be copied and furnished to
+ others, and derivative works that comment on or otherwise explain it
+ or assist in its implementation may be prepared, copied, published
+ and distributed, in whole or in part, without restriction of any
+ kind, provided that the above copyright notice and this paragraph are
+ included on all such copies and derivative works. However, this
+ document itself may not be modified in any way, such as by removing
+ the copyright notice or references to the Internet Society or other
+ Internet organizations, except as needed for the purpose of
+ developing Internet standards in which case the procedures for
+ copyrights defined in the Internet Standards process must be
+ followed, or as required to translate it into languages other than
+ English.
+
+ The limited permissions granted above are perpetual and will not be
+ revoked by the Internet Society or its successors or assigns.
+
+ This document and the information contained herein is provided on an
+ "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+ TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+ BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+ HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+ MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+
+
+
+
+
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+
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+
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+Handley & Jacobson Standards Track [Page 42]
+