Add late SDP answer support for inbound calls. by alexfish8 · Pull Request #820 · livekit/sip

alexfish8 · 2026-08-29T01:19:37Z

Whenever we don't receive an offer with the initial invite, we respond with a 200 and an SDP offer, and we expect an ACK to eventually arrive with an answer
- In the event that we are waiting for a late answer, do not proceed with the call until we receive an ACK (previously, this logic was solely gated on some experimental flag being set)
- Note: this is is gated by a feature-flag currently
Small refactor of acceptCall function variable (defined in handleInvite)
- Move logic out of function variable into three top-level functions: acceptCallAndWaitForMedia, acceptCall, and waitForMedia
Reinvite handling for late offers
- Reply with the negotiated SDP (rather than the offer we sent in response to the offerless INVITE)
Tests

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devin-ai-integration · 2026-08-29T01:21:25Z

-		// Encryption is picked here, before the room is selected.
-		answerData, err = c.negotiateMedia(rawSDP)
+	var sdpResponseBody []byte
+	expectingLateAnswer := len(rawSDP) == 0


🟡 Non-SDP invitations miss late negotiation

When an offerless invitation carries another body type, expectingLateAnswer treats it as an SDP offer. Negotiation rejects the valid call.

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alexlivekit

Nice. Mostly a bunch of nits, but one concern to map out.

alexlivekit · 2026-09-02T22:17:39Z

+	if c.media.NegotiatedAudio() != nil {
+		return nil
+	}


Highly unexpected, no?
Can this happen on a re-invite?

Yeah, this seems pretty unlikely and would involve a re-invite. We'd need to see a sequence like:

INVITE comes in without an offer. handleInvite sends an offer and waits for an ACK

A second INVITE comes with both an offer and a higher cseq than the first invite, and so updateRemoteFromSDP is invoked, which sets up media.

ACK comes in for the first invite, negotiateMediaForLateAnswer is eventually invoked, and this branch is hit.

Per spec, I think it's illegal for a client to send a second, new invite while the first invite's transaction hasn't closed, but, this is still probably something we should guard against. This would also affect initial INVITEs with offers, too. Given that this is a pre-existing thing, maybe we add guards against this in a follow-up PR?

alexfish8

thanks for the review!

alexfish8 · 2026-09-03T01:00:51Z

+	if c.media.NegotiatedAudio() != nil {
+		return nil
+	}


Yeah, this seems pretty unlikely and would involve a re-invite. We'd need to see a sequence like:

INVITE comes in without an offer. handleInvite sends an offer and waits for an ACK

A second INVITE comes with both an offer and a higher cseq than the first invite, and so updateRemoteFromSDP is invoked, which sets up media.

ACK comes in for the first invite, negotiateMediaForLateAnswer is eventually invoked, and this branch is hit.

Per spec, I think it's illegal for a client to send a second, new invite while the first invite's transaction hasn't closed, but, this is still probably something we should guard against. This would also affect initial INVITEs with offers, too. Given that this is a pre-existing thing, maybe we add guards against this in a follow-up PR?

alexfish8 · 2026-09-03T01:01:56Z

Still need to write tests and handle the ACK issue.

alexfish8 · 2026-09-05T01:45:11Z

PR updates:

Add tests
Gate late offer support on a feature flag (there's additional codec changes that will be deployed to cloud-sip, and so as to minimize the risk of breaking anything by deploying yet more changes, we'll feature gate this for now)
Make code more robust to reinvites (ignore reinvites requests if we're waiting for a late answer in an ACK for a previous invite)
Ensure that, if a reinvite request comes in, we reply with the negotiated SDP (rather than the offer we sent in response to the offerless invite).

alexfish8 requested a review from a team as a code owner August 29, 2026 01:19

devin-ai-integration Bot reviewed Aug 29, 2026

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Base automatically changed from tel-886/media-port to main August 31, 2026 15:54

devin-ai-integration Bot reviewed Aug 31, 2026

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Comment thread pkg/sip/inbound.go

Comment thread pkg/sip/outbound.go Outdated

alexfish8 added 2 commits September 2, 2026 08:09

Small refactor of accepting inbound calls.

3611df9

Support late offer for inbound calls.

dde2f94

alexlivekit force-pushed the afish/late-offer-inbound branch from 109daae to dde2f94 Compare September 2, 2026 15:12

devin-ai-integration Bot reviewed Sep 2, 2026

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Comment thread pkg/sip/inbound.go

alexlivekit reviewed Sep 2, 2026

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Respond to PR feedback.

e7c3bd5

alexfish8 commented Sep 3, 2026

View reviewed changes

alexfish8 added 6 commits September 4, 2026 17:08

Add tests.

c40048e

More robust ack handling.

19515aa

Add feature gate.

bae0e6a

Reply with generated answer for re-invites.

6730947

Small style tweaks.

6df9da7

Record SDP offer size sent in response to offerless invites.

c36461b

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