When a call is initiated between two endpoints it is important to have information of calling and called party IP address both for signaling and media, codec they are going to use and several other important information. In a SIP based call (call where SIP protocol is used) we use SDP to advertise media information like transport address, transport protocol, codec, ports and other session description information. SDP works at application layer. SDP is defined in RFC 2327, 4566 (newer). Normally we see SDP in Invite, 200 OK or ACK depending on the call type which we will discuss soon.