What is the purpose of SDP in a SIP session?

• A. Describes media capabilities so endpoints can agree on how to exchange audio and video.
• B. Establishes the SIP signaling method between peers.
• C. Encrypts the media stream to protect privacy.
• D. Determines the destination address of the SIP server.

Answer: (A)

SDP’s job in a SIP session is to describe what media the call will carry and how to transport it, so both ends can agree on a compatible way to exchange audio and video. In a SIP exchange, the caller places an SDP offer in the INVITE that lists the media types (audio, video), the codecs and payload formats it supports, the transport (usually RTP), and the ports and IP addresses to use. The callee replies with an SDP answer selecting a common set of parameters (codec, clock rate, payload type, and media ports), establishing the exact plan for the media streams. This negotiation happens before any actual media flows and is separate from how SIP signaling itself is carried, from any encryption of the media (which is handled by other mechanisms like SRTP), and from routing to the SIP server.