The Session Description Protocol (SDP)

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3cx.com/blog/voip-howto/sdp-voip2/

There are detailed information, very useful in order to debug a SIP call.

When a SIP based VoIP call is established, the audio or video sent between two SIP entities or more is streamed. Since many different codecs are supported by different devices or software, and each individual SIP entity taking part in the call does not know the IP address of the other SIP entity or to which port the stream should be sent to, SDP is used to advertise such details about the media stream during the VoIP call initialization process.