Network & Infrastructure / IPv6, TCP & Transport Engineering
Separate TCP reordering from true loss in packet analysis
Duplicate acknowledgments and retransmissions increase after an ECMP change. Some apparent loss events may originate from packet reordering rather than packets disappearing from the network.
Environment and inputs: Dual-stack routed networks and transport endpoints; validate the complete path and the active operating-system version. Diagnostic/design example; not tested on production equipment.
Collect the evidence
Compare sequence and acknowledgment progression, selective acknowledgments, path changes and captures near both endpoints. Account for capture drops and NIC offload before labeling missing segments as network loss.
Explain the result
TCP infers delivery conditions from acknowledgments; reordered segments can trigger recovery behavior even when all packets eventually arrive. A single capture point may not establish where disorder begins.
Plan the fix
Restore stable per-flow forwarding or correct the device introducing reordering. Avoid disabling congestion or loss recovery as a substitute for repairing an unstable forwarding path.
Check the result and rollback
Measure reorder depth, retransmission rate and application latency under representative parallel flows. Revert the hash or path change if disorder persists or one path experiences a genuine loss increase.