SIP 200 OK: meaning, causes and fixes

What SIP 200 means

200 OK is the success response. To an INVITE it means the call was answered, and it carries the SDP answer that fixes the codec and the media addresses. To REGISTER it means the binding was stored, and to OPTIONS it means the server is up and willing to talk.

A 200 to an INVITE must be confirmed with an ACK, sent end to end by the caller. Until the ACK arrives, the answering side keeps retransmitting its 200; if no ACK comes, it gives up after about 32 seconds and ends the call with a BYE. A 200 to OPTIONS often lists what the server supports in its Allow, Accept and Supported headers.

Example response

SIP/2.0 200 OK
Via: SIP/2.0/UDP 198.51.100.13:5060;branch=z9hG4bK-200-e5;rport=5060
From: <sip:[email protected]>;tag=0c61
To: <sip:sbc.example.net>;tag=sbc-77a0
Call-ID: [email protected]
CSeq: 1 OPTIONS
Allow: INVITE, ACK, CANCEL, BYE, OPTIONS, INFO
Accept: application/sdp
Supported: replaces, timer
Content-Length: 0

Common causes of SIP 200

The call was answered but has no audio in one direction, because the SDP in the 200 advertises an address the other side can't reach.
The ACK never reaches the answering side, so it keeps resending the 200 and drops the call after about 32 seconds.
The 200 selected a codec that was offered but that some hop on the media path can't carry.
An SBC answers OPTIONS with 200 while the trunk or PBX behind it is down, so the probe looks healthy and calls still fail.

How to fix SIP 200

For calls that drop at about 32 seconds, follow the ACK: check Record-Route, Contact and NAT so it reaches the device that sent the 200.
For one-way audio after the 200, compare the connection address and port in its SDP with where RTP actually arrives.
Check the codec chosen in the 200 against the offer and against what every hop on the media path supports.
Don't treat a 200 to OPTIONS as proof that calls work; place real test calls through the same trunk.

SIP 200 as an answer to OPTIONS

A 200 to OPTIONS is the healthy answer: the server is up, processing SIP and willing to take requests. It proves the SBC, not the trunk or PBX behind it, because many SBCs answer OPTIONS themselves. A PingVo Route counts it as reachable, and if you set the Route's expected codes to 200, every other response counts as a failure.

See which code your SBC returns. Run a free SIP OPTIONS check

Questions about SIP 200

Does a 200 OK to OPTIONS mean calls will work?

No. It means the server you probed is up. The trunk, the dial plan and the destination behind it can still fail, and only a real call shows that.

Why does my call drop after about 32 seconds?

The ACK for the 200 isn't arriving. The answering side keeps resending its 200, then gives up and hangs up. Fix the path the ACK takes, which is usually a NAT or Record-Route problem.

Can a 200 OK have no SDP?

Yes. If offer and answer were already exchanged in a reliable provisional response, the 200 to the INVITE needs no SDP. A 200 to OPTIONS or REGISTER usually has no body at all.

SIP 200 OK