# Offer, answer, and SDP

Study notes. Not vendor documentation and not legal advice.

- RFC 3264 is titled An Offer/Answer Model with the Session Description Protocol (SDP).
- RFC 8866 defines SDP for describing multimedia sessions for announcement, invitation, and other initiation. The page says this document obsoletes RFC 4566.
- Wikipedia's SIP article says media type and parameter negotiation and media setup are performed with SDP.
- RFC 3966 specifies the tel URI for telephone numbers.
- RFC 4733 is the RTP payload for DTMF digits, telephony tones, and telephony signals, and it obsoletes RFC 2833.

A phone menu needs digits. Write RFC 4733 next to the digit event, RFC 8866 next to the media description, and RFC 3966 next to the telephone-number URI. Do not put the digit event inside the SIP request line.