tcp

Ordering and retransmission are what TCP adds, and both of them cost time. Naming something that deliberately gives them up, and saying why, is what turns a textbook answer into a real one.

A staple of every networking round, and the follow-up is always about TLS termination. Knowing which layer sees what decides most of the answer.

The oldest question in systems interviewing and still the best, because it is unbounded: every layer you can describe is another layer you could debug.

The classic ephemeral port exhaustion question. TIME_WAIT is a familiar phrase and the useful facts are narrower: which side accumulates it, and why the popular fix is dangerous.

A growing backlog of sockets waiting for the application to close its side usually points at lifecycle handling. Here is the hunt from ss output to the pool timeout that caused it.

The signature is unmistakable once seen: handshakes complete, then the connection freezes at the same byte count every time. Here is the probe sequence and the fixes, ranked.

Latency fine, loss low, link idle, yet a single copy takes hours. The arithmetic of bandwidth-delay product settles this question before any tool opens.

A reset at a suspiciously round number is a fingerprint, not a bug. Matching client keepalives to the lowest timer in the path is the fix; here is the full ladder of knobs.

Loss-based versus model-based congestion avoidance decides whether your WAN transfers fill the pipe or die on the first stray drop. Here is the operator's decision, not the RFC summary.

Both can be right, which is the whole point. Retransmissions mean a segment was not acknowledged, and the network is only one of the places that can happen.