SIP 408 Request Timeout means the UAC sent a request but received no response within the timer window. Unlike a 503 (server busy) or 404 (not found), a 408 means the server never replied at all. Here's how to find out why.
SIP 408 is defined in RFC 3261 Section 21.4.9. It means the server could not produce a response within a suitable time. The key word is no response — the request reached the server (or a proxy) but nothing came back before the transaction timer expired.
Timer B (INVITE transactions) and Timer F (non-INVITE) both default to 32 seconds (64 × T1 at 500ms). If no final response arrives within this window, the UAC generates a 408 locally or receives one from a proxy that gave up waiting.
408 is always a connectivity or timing issue. If you get a 408, the problem is between the sender and the server, not with the user or the server's logic.
Step 1 — Capture a trace on both sides. If you only have the sender-side trace, you don't know if the packets arrived. Capture on the receiver side too. If the INVITE appears on the receiver side, the problem is the response path. If it doesn't appear, the problem is the request path.
Step 2 — Check for retransmissions. With UDP, the sender retransmits the INVITE at T1, 2×T1, 4×T1 intervals. If you see multiple identical INVITEs with no response, confirm the retransmits are reaching the destination.
Step 3 — Test with OPTIONS. Send a SIP OPTIONS ping to the destination. If you get a 200 OK, basic connectivity is fine and the problem is specific to the call flow. If OPTIONS also times out, it's a pure connectivity issue.
Step 4 — Check the network path. Traceroute to the destination. Look for high latency or packet loss at any hop. Use ping -i 0.2 destination to generate rapid pings and measure loss percentage.
Firewall: Verify UDP port 5060 (or TCP 5060/TLS 5061) is open bidirectionally. Disable SIP ALG. Check that stateful firewall timeout for UDP is at least 60 seconds.
Network loss: Implement QoS DSCP EF marking to prioritize SIP signaling. Check for duplex mismatches on network interfaces. Consider switching from UDP to TCP transport for more reliable delivery on lossy networks.
DNS: Pre-resolve SIP server hostnames and cache them. Verify DNS SRV records resolve within 1-2 seconds. Use IP addresses directly in SIP configuration if DNS is unreliable.
Server load: Monitor server CPU and active connection counts. Implement call admission control to reject with 503 before hitting capacity rather than letting requests time out.
SIP 408 means no response was received within the transaction timer window (32 seconds by default). The request was sent but the server never replied, either because packets were lost, a firewall dropped them, or the server was too busy to respond in time.
SIP 408 means no response at all — a network or timing issue. SIP 480 Temporarily Unavailable means the server responded but the user is not available. If you get a 408, check network connectivity. If you get a 480, the network is fine but the user's device is offline or not answering.
To fix SIP 408: check that UDP port 5060 is open bidirectionally, disable SIP ALG on your router, verify DNS resolves the SIP server hostname, and test with a SIP OPTIONS ping to confirm basic connectivity. If the OPTIONS also times out, it is a pure network path issue.
Paste your SIP trace into SIPSymposium. The analyzer identifies whether the request is reaching the server, checks for retransmission patterns, and pinpoints the network hop where packets are being lost.