Socket.IO server pingTimeout shorter than client reconnectionDelayMax causes premature disconnects

Answer: The server enforces pingTimeout independently of the client’s reconnectionDelayMax.

Confirmed facts

• The server’s pingTimeout is evaluated solely on whether a pong arrives within that interval; it does not consult the client’s back‑off schedule.
• If pingTimeout expires, the server closes the socket even if the client is still waiting for its next reconnection attempt.
• The client’s actual reconnection delay is: baseDelay = Math.min(reconnectionDelay * 2^attempt, reconnectionDelayMax) plus jitter jitter = Math.round(Math.random() * baseDelay * randomizationFactor).
• Lowering randomizationFactor reduces the spread of jitter, making retry timing more predictable, but setting it to 0 removes jitter entirely and can cause a thundering herd.

Likely explanation / practical steps

1. Measure or estimate the average round‑trip time (RTT) between client and server (e.g., via a simple ping or by logging socket latency). If you cannot provide this value, use a conservative safety margin of 200 ms.
2. Choose a reconnectionDelayMax that reflects the longest back‑off you are willing to tolerate (e.g., 30000 ms).
3. Set the server’s pingTimeout to at least twice that value plus the RTT safety margin:

   pingTimeout ≥ 2 * reconnectionDelayMax + RTT_safety

   For example, with reconnectionDelayMax = 30000 ms and an estimated RTT of 100 ms, use pingTimeout ≥ 60200 ms.
4. Configure pingInterval to be roughly half of pingTimeout (the default Engine.IO recommendation):

   pingInterval = pingTimeout / 2

5. Keep randomizationFactor above zero to retain jitter, but lower it to reduce collision probability. A value of 0.2–0.3 often provides a good trade‑off.
6. Apply the settings on both server and client (if you control the client) and verify with logs that the server does not close the socket before the client’s first reconnection attempt after a simulated drop.

Missing diagnostic detail If you can provide the observed average RTT, the safety margin in step 3 can be tightened; otherwise the 200 ms conservative margin is recommended.