Session Description Protocol
The Session Description Protocol (SDP) is described in RFC 4566. It is used to negotiate session parameters between the user agents. Media details, transport addresses, and other media-related information is exchanged between the user agents using the SDP protocol. Normally, the INVITE message contains the SDP offer message, while the 200 OK contains the answer message. These messages are shown in the following figures. You can observe that the GSM codec is offered, but the other phone does not support it. Then it answers with the supported codecs, in this case, G.711 u-law (PCMU) and G.729. The rtpmap:101 session is the DTMF relay described in RFC 2833.
INVITE (SDP Offer)
200 OK (SDP Answer)
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