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Progress

RFC 3960 and 3261

Early media

183 with SDP is progress media. It is not answer, and it is not a confirmed dialog.

183 Session ProgressCallerCalleeINVITE183 + SDPRTP200

  1. INVITE — Early offer, so the callee can send media before answer.
  2. 183 + SDP — Early media. Ringback or an announcement can be RTP, not a local tone.
  3. RTP — One direction may start before 200.
  4. 200 — Answer. Cut-through must be checked again.

If you treat 183 as answer, billing and recording start too early. If you ignore 183, the caller hears silence while the network is already playing an announcement.

Sources

Study card carried forward

SIP · RFC 3960

Early media

Early media is audio, or sometimes video, that flows before the called party answers. The usual signal is 183 Session Progress with an SDP answer.

Ladder: INVITE, 100, 183 Session Progress with SDP, one-way early RTP, then 200 OK, ACK, and two-way RTP.
The gateway both answers the offer early and sends the one-way RTP.

On a phone, swipe a wide ladder sideways. The steps below repeat the same sequence.

Sequence

  1. INVITE — Alice to Gateway. SDP offer
  2. 100 Trying — Gateway to Alice. Call is progressing
  3. 183 Session Progress — Gateway to Alice. SDP answer for early media
  4. Early RTP — Gateway to Alice. Announcement or remote ringback
  5. 200 OK — Gateway to Alice. Called party answered
  6. ACK — Alice to Gateway. Dialog confirmed
  7. RTP both ways — Alice to Gateway. Early media gives way to the call

Ringback is a policy choice

If Alice receives 180 Ringing and that response has no SDP, her phone generates local ringback. If she receives 183 Session Progress with SDP, she plays the RTP that arrives instead. That stream might be a recorded announcement, a carrier tone, or an interactive voice prompt. RFC 3960 describes this split.

Playing local ringback on top of early RTP produces the doubled audio people report as "ringing over the announcement." Once a 183 with SDP has been accepted, the phone should treat the network as the source of tone.

Early media is often one-way, from the gateway toward the caller. Two-way RTP starts at answer, or earlier if both sides negotiated it. The 200 OK still matters: it confirms the dialog and means a person or a final application accepted the call. Early media is not an answer.

An unreliable 183 can be lost. A later 183 can also collide with an offer/answer that already finished. When the announcement must be heard, carry the SDP in a reliable provisional response.

Sources

  • RFC 3960, early media and ringtone generation
  • RFC 3261, provisional responses
  • RFC 3262, when that 183 must be reliable

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