SIP 408 Request Timeout
The server could not produce a response in time, often while waiting on a downstream hop.
4xx Client failure · RFC 3261 · Answer to OPTIONS: Up, but timed out waiting on something behind it.
What SIP 408 means
A 408 says the server couldn't produce a response within a suitable time, for example because it couldn't find the user, or the device it forked the call to never answered. It is a real response from a working server, which makes it different from silence: something behind that server took too long.
Clients also report 408 internally when their own transaction timer expires with no response at all. For an INVITE over UDP that takes about 32 seconds. So a 408 in a call record can mean two different things, and only a packet capture shows whether a server sent it. RFC 3398 maps Q.850 cause 18, no user responding, to 408.
Example response
SIP/2.0 408 Request Timeout
Via: SIP/2.0/UDP 198.51.100.22:5060;branch=z9hG4bK-408-p4
From: <sip:+[email protected]>;tag=5b70
To: <sip:+[email protected]>;tag=to-93
Call-ID: [email protected]
CSeq: 9 INVITE
Reason: Q.850;cause=18;text="No user responding"
Content-Length: 0Common causes of SIP 408
- The proxy forked the call to registered devices, and none of them answered with anything before its timer ran out.
- A device is still registered but no longer reachable, so the INVITE sent to it goes unanswered.
- The next hop is slow or overloaded, and the server gives up waiting on it.
- The phone network reported no user responding, and the gateway mapped Q.850 cause 18 to 408.
- No response came back at all, and the client's own timer produced the 408 locally.
How to fix SIP 408
- Check in a capture whether the 408 arrived on the wire or was generated by your own system after a timeout.
- If a server sent it, find the hop it was waiting on; registration state and the forked branches are the first places to look.
- Shorten the registration expiry for devices that drop off the network, so stale bindings go away sooner.
- If nothing came back at all, treat it as a reachability problem: a firewall, a route, or a server that is down.
SIP 408 as an answer to OPTIONS
A 408 to OPTIONS means the server is up but timed out waiting on something behind it. That is different from no response, which the free check reports separately after 2 seconds of silence. A PingVo Route counts a 408 as reachable by default, because the SBC itself answered.
Questions about SIP 408
What is the difference between SIP 408 and 504?
408 means the server couldn't find or reach the user in time. 504 means a gateway or proxy didn't get a timely answer from a server it relies on, such as the next hop or a lookup service.
Why do my logs show 408 when nothing came back?
Your own system wrote it. A SIP client reports a timed-out transaction as if it had received a 408, so the log shows a code no server sent.