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T2 · T2-R05 · SIP, WebRTC, RTP

Offer, answer, and SDP

Offer and answer are how two ends agree on media. SDP is the description. The tel URI is a way to write a telephone number. DTMF digits, when carried in RTP, have their own payload RFC. Four documents, four jobs.

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Counts on this page

4boxes
5opened sources
1confirm rows
5study lines

Opened sources

Confirm rows, no URL

Learn it in 5 minutes

  • 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.

Visual

Offer, answer, and SDP SDP. Teaching sketch. Confirm a live system. Offer SDP tel URI DTMF Names come from pages opened in 2026-10. A missing name stays confirm.

Count the boxes: 4. Each label is a name from a page or a study label marked on the drawing.

Flow

Offer, answer, and SDP Caller SIP Called session setup session setup RTP, not required to hairpin Teaching sketch from RFC 3261 and RFC 3550. Not a packet capture.

What is going on

Offer and answer are how two ends agree on media. SDP is the description. The tel URI is a way to write a telephone number. DTMF digits, when carried in RTP, have their own payload RFC. Four documents, four jobs.

RFC 4566 is the document RFC 8866 says it obsoletes. If a note still cites only 4566, write that 8866 is the SDP page that opened in this session, and that the older number is the one it replaced.

Worked example

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.

Quiz

1. RFC 8866 says it obsoletes RFC 4566.

2. RFC 4733 is the SIP registration specification.

Ecosystem

Sister pieces sit on the ecosystem page: the glossary, the mind map, the integration map, and the bridges.

  • Offer/answer — The model in RFC 3264 for using SDP to agree on a session.
  • tel URI — The URI RFC 3966 specifies for telephone numbers.

Config. Write which RFC owns the object: description, number URI, or digit payload.

Troubleshooting. If tones are garbled and signaling is fine, look at the telephone-event payload, not at a new SIP method.

Bridge. IVR rooms collect digits. This room only names the payload RFC. The menu logic is later.

Mind map, sister technologies, and future hooks live on the ecosystem page.

Sources and go deeper

Opened 2026-10. A row marked confirm has no link, because that page was not verified.

sources.json Markdown