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| author | Erlang/OTP <[email protected]> | 2009-11-20 14:54:40 +0000 |
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| committer | Erlang/OTP <[email protected]> | 2009-11-20 14:54:40 +0000 |
| commit | 84adefa331c4159d432d22840663c38f155cd4c1 (patch) | |
| tree | bff9a9c66adda4df2106dfd0e5c053ab182a12bd /lib/megaco/doc/standard/rfc2327.txt | |
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The R13B03 release.OTP_R13B03
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diff --git a/lib/megaco/doc/standard/rfc2327.txt b/lib/megaco/doc/standard/rfc2327.txt new file mode 100644 index 0000000000..ce77de6128 --- /dev/null +++ b/lib/megaco/doc/standard/rfc2327.txt @@ -0,0 +1,2355 @@ + + + + + + +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. + + + + + + + + + + + + + + + + + + + + + + + + +Handley & Jacobson Standards Track [Page 42] + |