SDP refers to "Session Description Protocol". It is a companion protocol to the SIP protocol in the IMS network. It exists within the SIP message body to describe the session parameters.

SDP describes the characteristics of the multimedia session, such as the type of media (e.g., audio, video, text), the codec(s) of the media session, the transport protocol(s) (e.g., RTP, UDP), and the network parameters (e.g., IP address, port numbers). This information helps the session participants to negotiate and establish the session.

In addition, SDP can modify an ongoing session, such as adding or removing media types, changing the codec settings, or updating the network parameters.