What early media is
Early media is media exchanged before a final successful response establishes the answered session. Carrier announcements and network-generated ringback are common examples. A provisional response such as 183 may carry SDP, but the response code alone neither guarantees audible media nor proves billing has begun. A 180 Ringing response often leads the caller to generate local ringback. Real behavior depends on SDP, reliable provisional response support and the endpoint's policy. Capture the actual media rather than treating 180 and 183 as universal audio instructions.
Where PRACK fits
The 100rel extension makes certain provisional responses reliable. A reliable provisional response includes sequencing information, and PRACK acknowledges it using RAck. PRACK itself is a normal request that receives a response; it is different from ACK for an INVITE final response. When 100rel is required but unsupported, setup can fail before answer. Compare Supported and Require headers, the reliable response's RSeq and the PRACK's RAck. Do not simply remove a required extension at a border without understanding how that changes the offer/answer exchange.
Diagnose silence before answer
Check whether an offer and answer actually authorized a usable media path. Then examine advertised addresses, RTP arrival and codec interpretation. In a forked call, different early dialogs can produce different media; identify the To tag before combining their signaling. Reproduce with one known destination that provides early announcements and another that answers normally. Separate early-media silence from post-answer one-way audio. If the final answer changes media parameters, confirm the endpoint moved to the new destination and did not keep using the early stream.
Apply the protocol model to Asterisk PJSIP
The Asterisk PJSIP guide maps 100rel, inband_progress, 183 filtering, fork handling and SDP updates onto the generic early-media and PRACK exchange.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- IETF RFC 3261 ↗
- IETF RFC 3262 ↗
- IETF RFC 3264 ↗
- Asterisk 22 res_pjsip endpoint reference ↗
- Asterisk PJSIP troubleshooting guide ↗
- Asterisk early-media and Progress guidance ↗
- Asterisk 22 Progress application ↗
- RFC 3262: reliable provisional responses and PRACK ↗
- RFC 6337: SIP offer/answer usage ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.