Session Description Protocol (SDP)¶

Overview¶

Session Description Protocol (SDP) is used to describe multimedia sessions for session announcement, invitation, and other forms of session initiation.

SDP Structure¶

SDP messages consist of lines of text in the format:

<type>=<value>

Session-level Description¶

v= Protocol version
o= Origin/creator and session ID
s= Session name
c= Connection information
t= Time session is active

Media-level Description¶

m= Media type, port, protocol
a= Attributes
b= Bandwidth information

Example SDP Message¶

v=0
o=alice 2890844526 2890844526 IN IP4 host.atlanta.com
s=Session Description
c=IN IP4 host.atlanta.com
t=0 0
m=audio 49170 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/AVP 31
a=rtpmap:31 H261/90000

SDP in SIP¶

Offer/Answer Model¶

Offer - Initial SDP in INVITE
Answer - Response SDP in 200 OK
Negotiation - Codec and parameter agreement

Common Attributes¶

Media Attributes¶

sendrecv - Send and receive media
sendonly - Only send media
recvonly - Only receive media
inactive - No media flow

Codec Attributes¶

rtpmap - RTP payload mapping
fmtp - Format parameters
ptime - Packetization time

Advanced Features¶

ICE Candidates¶

a=candidate:1 1 UDP 2130706431 192.168.1.100 54400 typ host

DTLS Fingerprint¶

a=fingerprint:SHA-256 12:34:56:78:90:AB:CD:EF...

Best Practices¶

Validate SDP syntax
Handle multiple media streams
Support codec negotiation
Implement proper error handling
Consider bandwidth constraints