ZTA LabFree University

T6 · T6-R17 · SIP and media

Offer, answer, and SDP

RFC 3261 says the invitation carries session descriptions so the parties can agree on media types. RFC 3264 is the model for that agreement. Read the offer and the answer as a pair. An answer that invents a codec the offer never listed is not the model those RFCs describe.

ExportSVGFlowSheet

Learn it in 5 minutes

  • RFC 3264 defines an offer/answer model that uses SDP.
  • RFC 4566's page says that RFC is now obsolete and points to RFC 8866.
  • RFC 8866 opened as SDP: Session Description Protocol.
  • The offer is where codecs are proposed. The answer picks from that offer.
  • This lab does not claim a vendor's default codec.

Visual

Offer, answer, and SDP SDP. Teaching sketch. Confirm a live system. Offer Answer RFC 3264 RFC 8866 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

RFC 3261 says the invitation carries session descriptions so the parties can agree on media types. RFC 3264 is the model for that agreement. Read the offer and the answer as a pair. An answer that invents a codec the offer never listed is not the model those RFCs describe.

SDP's current number, on the page that opened, is RFC 8866. Quoting only RFC 4566 without the obsolete notice will date your note. Cite both: 4566 for the history the editor still hosts, 8866 for the RFC the editor marks current.

Direction attributes and m= lines are how you see send and receive. This session did not extract a worked SDP sample from the RFC, so a sample in class should be marked as a teaching sketch, not a capture.

Worked example

Write offer, then answer, on two cards. Under answer, write picks from the offer. Under both, cite RFC 3264. Add RFC 8866 as the SDP citation. Leave the codec name blank.

Quiz

1. The RFC 4566 page says that RFC is obsolete.

2. This lab verified Zoom's default audio codec.

Ecosystem

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

  • Offer/answer — The RFC 3264 model for agreeing media with SDP.
  • RFC 8866 — The SDP RFC that the RFC editor presented as the current document.

Config. Store the offer and the answer together. Note which message carried each one only after you see them.

Troubleshooting. One-way audio can be a direction attribute. Look there before you replace a headset. Confirm the attribute in the capture you hold.

Bridge. SRTP, RFC 3711, is the secure profile of RTP. Encryption is not the same step as the offer.

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