sip-media-negotiation

SDP offer/answer negotiation exchanges session descriptions between SIP endpoints following RFC 3264 semantics. One endpoint sends an offer describing its media capabilities; the other responds with an answer confirming accepted formats, codecs, and transport parameters to establish the RTP session.

Codec negotiation is the process where SIP endpoints agree on compression formats, sample rates, and parameters for audio or video streams. Both sides exchange supported codecs in SDP; the answer selects compatible ones to ensure real-time media can flow without transcoding delays.

Parse SDP by extracting media descriptions, attributes, bandwidth, and session origin data per RFC 4566. Generate SDP by structuring session-level and media-level lines with codec formats, RTP parameters, and connection details, then exchange via SIP INVITE or response for endpoint capability alignment.

Yes. SDP offer/answer applies to any SIP-based real-time communication: video calls, screensharing, instant messaging over RTP, and conferencing. The negotiation process handles multiple media types and their transport parameters identically.

You need understanding of SIP signaling flow, RTP protocol basics, and media codecs (G.711, Opus, VP9, etc.). Familiarity with SDP syntax, RFC 3264 offer/answer rules, and your VoIP stack's media layer is required to correctly parse capabilities and align endpoints.

If endpoints fail to negotiate a common codec during SDP exchange, no mutually supported format exists for RTP payload encoding. The call cannot proceed without either transcoding (costly latency) or fallback codecs, making successful negotiation critical for call setup.