A capability stays maintainable only when the team can explain every state, retry, and piece of residual data—not merely show one successful run. SCTP and lower layers fragment messages, but VPNs and firewalls may drop fragments or ICMP, leaving heartbeats healthy and large chunks retransmitting.
Network diagnostics separates observed facts from inference. Candidate type, selected route, and stage timing describe connectivity; complete IP addresses or one probe are unnecessary and insufficient evidence.
Questions the design must answer
List non-negotiable invariants before selecting performance knobs. Tuning can roll out gradually; identity, permission, and terminal-state rules cannot drift at runtime.
- Keep conservative chunks and gain throughput through windows, observing RTT, retransmission, drain, and chunk ACK; reset tuning after path change.
- Separate protocol facts, user intent, and automatic recovery; automation may restore facts but never overturn an explicit choice.
- Ship conservative defaults, server-side ceilings, and a rollout switch instead of trusting browser-provided numbers as resource budgets.
The delivery standard for MTU and Fragmentation When Small Messages Work but Files Stall is a usable normal path, convergent failures, bounded resources, and a state users can understand. The result is a production capability that can be explained, degraded safely, and rolled back—not a demo that works once.
Edge cases are part of the feature
Prioritize faults that silently preserve false facts: the interface looks recovered while a queue, permission, or counter has diverged. The defect often appears only on the next action.
- A 1 MB chunk succeeding on one path does not avoid message head-of-line elsewhere, and ping does not validate DTLS payload behavior.
- Fixing only the UI leaves queues, locks, or expired credentials for the next operation to inherit and fail again.
- Without backpressure or quota, a slow consumer raises memory, queue depth, and tail latency until unrelated users are affected.
Prove that it works with evidence
Do not stop verification when the final action succeeds. Count side effects, measure wait time, inspect privacy, and prove the next run begins from a clean baseline.
- Transfer varied chunks through 1280/1400/1500 MTUs with dropped ICMP or fragments; record goodput, repair, control latency, and fallback.
- Disconnect, change networks, and recover mid-operation; reconcile endpoint state, persistence, and resource counts.
- Cover direct, relayed, weak-network, background-tab, and mobile paths; do not rely on averages or one successful screenshot.
The release bar is clear: users understand the current state, failures stop or recover, resources stay bounded, and operators can identify the phase from minimum necessary evidence.