Session Description Protocol
SIP uses SDP (defined in RFC 2327), which defines a syntax to describe the media sessions. The SDP is carried as an application body (Content-Type: application/SDP) in the SIP messages. SDP consists of text messages using the ISO 10646 character set in UTF-8 encoding. A SDP 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 to a single media stream). Table 5-2 describes the session-level SDP parameters as defined by RFC 2327.
|
Field Type |
Mandatory/ Optional |
Description |
Example |
|
v= |
Mandatory |
Protocol version |
v=0 |
|
o= |
Mandatory |
Owner/creator or session identifier |
o=Sam 154954610 0 IN IP4 10.10.10.26 |
|
s= |
Mandatory |
Session name |
s=conference call |
|
i= |
Optional |
Session information |
i=conference call to Cisco Unified MeetingPlace Express |
continues continues
|
Field Type |
Mandatory/ Optional |
Description |
Example |
|
u= |
Optional |
URI of description |
u=http://www.cisco.com/sdp |
|
e= |
Optional |
E-mail address |
|
|
p= |
Optional |
Phone number |
p=+91-44-510623456 |
|
c= |
Optional |
Connection information; not required if it is included in the media level |
c=IN IP4 10.10.10.22 |
|
b= |
Optional |
Bandwidth information; not required if it is included in the session level |
b=CT:128 |
|
k= |
Optional |
Encryption key; not required if it is included in the media level |
k=base64:7658339339 |
|
t= |
Optional |
Time the session is active |
t=7776543 987656 |
Table 5-3 shows the syntax and examples of media-level SDP parameters.
|
Field Type |
Mandatory/ Optional |
Description |
Example |
|
m= |
Mandatory |
Media name and transport address |
m=video RTP/AVP 31 |
|
i= |
Optional |
Media title |
i=conference call to Cisco Unified MeetingPlace Express |
|
c= |
Mandatory |
Connection information; optional if it is included in the session level |
c=IN IP4 10.10.10.22 |
|
b= |
Mandatory |
Bandwidth information; optional if it is included in the session level |
b=CT:128 |
|
k= |
Optional |
Encryption key |
k=base64:7658339339 |
|
a= |
Optional |
Attribute lines |
a=rtpmap:0 PCMU/8000 |
Real-time Transport Protocol RTP specifies how media streams should be packetized. The headers of RTP packets include a payload type (ptype) that defines the type of data packet. RTP payload types can be static or dynamic. Static payload types are fully defined in the audio/video profile.
Payload numbers 0 through 96 are static payload types. In the following example, payload number 0 is a static payload type for PCMU (G.711 |>law):
m=audio 49000 RTP/AVP 0 a=rtpmap:0 PCMU/8000
Dynamic payload types are defined during the session setup and are identified using payload numbers above 96. In the following example, the originator assigns 121 to identify codec G.722.1 during the SDP offer:
m=audio 49000 RTP/AVP 121 a=rtpmap:121 G7221/16000
Continue reading here: Early and Delayed Offer
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