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Collaboration, Voice & AV · Intermediate

Offer/Answer Model

The RFC 3264 negotiation in which one side offers SDP media options and the other answers with the chosen subset.

Definition

The offerer lists supported codecs and parameters; the answerer must select from those offered and return its own address and port. Offers can be in the INVITE (early offer) or in the 200 OK when the INVITE has no body (delayed offer), with the answer in the ACK. Re-INVITEs or UPDATE messages carry new offers to change an existing session.

Example

CUCM sends an offer with G.722, G.711 and G.729, and the carrier answers with G.711 only.

Exam tip

Know where the offer and answer appear in early versus delayed offer flows.

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