Editing Session Description Protocol (section)

==Session description==
The Session Description Protocol describes a session as a group of fields in a text-based format, one field per line.<ref group="note">Lines are terminated by a [[carriage return]] and a [[line feed]] character, but implementations may relax this by omitting the carriage return.</ref> The form of each field is as follows.
<pre><character>=<value><CR><LF></pre>
Where <code><nowiki><character></nowiki></code> is a single [[case-sensitive]] character and <code><nowiki><value></nowiki></code> is structured text in a format that depends on the character. Values are typically [[UTF-8]] encoded.<ref group="note">The ''session information'' and ''session name'' values are subject to the encoding specified in any ''charset'' attribute of the section.</ref> [[Whitespace character|Whitespace]] is not allowed immediately to either side of the equal sign.<ref name="RFC 4566"/>{{rp|Section 5}}

Session descriptions consist of three sections: session, timing, and media descriptions. Each description may contain multiple timing and media descriptions. Names are only unique within the associated syntactic construct.<ref>[http://www.konnetic.com/Documents/KonneticSDPTechnicalOverview.pdf An In-Depth Overview of SDP] {{webarchive|url=https://web.archive.org/web/20110713164318/http://www.konnetic.com/Documents/KonneticSDPTechnicalOverview.pdf |date=2011-07-13 }}</ref>

Fields must appear in the order shown; optional fields are marked with an asterisk:
* ''Session description''
     v=  (protocol version number, currently only 0)
     o=  (originator and session identifier : username, id, version number, network address)
     s=  (session name : mandatory with at least one UTF-8-encoded character)
     i=* (session title or short information)
     u=* (URI of description)
     e=* (zero or more email address with optional name of contacts)
     p=* (zero or more phone number with optional name of contacts)
     c=* (connection information—not required if included in all media)
     b=* (zero or more bandwidth information lines)
     ''One or more '''time descriptions''' ("t=" and "r=" lines; see [[#Time description|below]])''
     z=* (time zone adjustments)
     k=* (encryption key)
     a=* (zero or more session attribute lines)
     ''Zero or more '''Media descriptions''' (each one starting by an "m=" line; see [[#Media description|below]])''

* ''{{vanchor|Time description}}'' (mandatory)
     t=  (time the session is active)
     r=* (zero or more repeat times)

* ''{{vanchor|Media description}}'' (optional)
     m=  (media name and transport address)
     i=* (media title or information field)
     c=* (connection information — optional if included at session level)
     b=* (zero or more bandwidth information lines)
     k=* (encryption key)
     a=* (zero or more media attribute lines — overriding the Session attribute lines)

Below is a sample session description from RFC 4566. This session is originated by the user "jdoe", at IPv4 address 10.47.16.5. Its name is "SDP Seminar" and extended session information ("A Seminar on the session description protocol") is included along with a link for additional information and an email address to contact the responsible party, Jane Doe. This session is specified to last for two hours using NTP timestamps, with a connection address (which indicates the address clients must connect to or — when a multicast address is provided, as it is here — subscribe to) specified as IPv4 224.2.17.12 with a [[Time to live|TTL]] of 127. Recipients of this session description are instructed to only receive media. Two media descriptions are provided, both using RTP Audio Video Profile. The first is an audio stream on port 49170 using RTP/AVP payload type 0 (defined by RFC 3551 as [[PCMU]]), and the second is a video stream on port 51372 using RTP/AVP payload type 99 (defined as "dynamic"). Finally, an attribute is included which maps RTP/AVP payload type 99 to format h263-1998 with a 90&nbsp;kHz clock rate. [[RTCP]] ports for the audio and video streams of 49171 and 51373, respectively, are implied.
<div class="plainlinks">
     v=0
     o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
     s=SDP Seminar
     i=A Seminar on the session description protocol
     u=http://www.example.com/seminars/sdp.pdf
     e=j.doe@example.com (Jane Doe)
     c=IN IP4 224.2.17.12/127
     t=2873397496 2873404696
     a=recvonly
     m=audio 49170 RTP/AVP 0
     m=video 51372 RTP/AVP 99
     a=rtpmap:99 h263-1998/90000
</div>
The SDP specification is purely a format for session description. It is intended to be distributed over different transport protocols as necessary, including [[Session Announcement Protocol|SAP]], [[Session Initiation Protocol|SIP]], and [[Real Time Streaming Protocol|RTSP]]. SDP could even be transmitted by email or as an HTTP payload.