When a call is initiated between two endpoints it is important to have information of calling and called party IP address both for signaling and media, codec they are going to use and several other important information. In a SIP based call (call where SIP protocol is used) we use SDP to advertise media information like transport address, transport protocol, codec, ports and other session description information. SDP works at application layer. SDP is defined in RFC 2327, 4566 (newer). Normally we see SDP in Invite, 200 OK or ACK depending on the call type which we will discuss soon.
SDP protocol is divided into three parts:
Session Description- Indicates Session Details
Time Description- Advertises Timing Details
Media Description- Advertises detail about media
Below is the syntax of the three:
v = (protocol version)
o = (owner/creator and session identifier)
s = (session name)
i =* (session information)
u =* (URI of description)
e =* (email address)
p =* (phone number)
c =* (connection information - not required if included in all media)
b =* (bandwidth information)
z =* (time zone adjustments)
k =* (encryption key)
a =* (zero or more session attribute lines)
t =* (session active time)
r =* (zero or more repeat times)
m =* (media names and transport address)
i =* (media title/information field)
c =* (connection information - not required if included at session)
b =* (bandwidth information lines)
k =* (encryption key)
a =* (media attribute lines)
Here is a sample SDP from a call:
v = 0 ---------> SDP version (current version is 0)
o = jammy 2890844526 2890842807 IN IP4 188.8.131.52 -----> contains information about originator.
"o" = <username><session-id><version><network-type><address-type>
s = SDP Virtual Lab-----> name of the session
i = A Seminar on the session description protocol ---->Human Readable description of session
u = http://www.ciscovoicevirtuallab.blogspot.com /Santosh Agrawal ----->Specifies a URI for more info of session
e = email@example.com (Santosh Agrawal) -----> email of the host of the session
c = IN IP4 10.10.0.1/127 -----> Contains info about the media connection, IP is of host who is sending media.
t = 287335269 287346324 -----> start and stop time of session
m = audio 49170 RTP/AVP 0 8 18 -----> media type - audio, port 49170 on which this party will receive audio, protocol is RTP, media format means codec
a=rtpmap: 0 PCMU/8000/1 -----> In the above m line 0 stands for G711 u law codec sampled at the rate of 8000 samples per second.
a=rtpmap: 8 PCMA/8000/1 ----->in above m line 8 stands for G711 a law codec sampled at the rate of 8000 samples per second.
a=rtpmap: 18 G729/8000/1 -----> in above m line 0 stands for G729 codec sampled at the rate of 8000 samples per second.
m = video 51372 RTP/AVP 31 -------> its same as above but media type is video and codecs would be different.
For more details on SDP, you can refer RFC 4566 https://tools.ietf.org/html/rfc4566
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