Simple definition
Session Description Protocol specifies media formats, connection information, and ports during session negotiation.
Purpose and use
SDP, Session Description Protocol, describes multimedia-session parameters rather than carrying audio or video itself.
How does it work?
A description includes proposed addresses, ports, media types, codecs and attributes. SIP's offer-answer model negotiates the selected parameters.
Example and verification
For calls without audio, compare advertised addresses and ports with actual traffic observed on interfaces.
Points to watch
A private address advertised in the wrong context can make media unreachable. Compatible descriptions guarantee neither routing nor firewall permission.
In short
SDP describes where and how media should flow; verify traffic along its actual path afterward.