Packet Reordering, Duplication, and Their Impact
Learn about packet reordering, duplication, and their impact
Packet loss and jitter dominate VoIP troubleshooting discussions, but there are two less common impairments that can be equally confusing: reordering and duplication. They're rare enough that engineers forget to check for them, but when they occur, they produce baffling symptoms.
Packet Reordering
What Happens
Packets sent in order 1, 2, 3, 4, 5 arrive as 1, 2, 4, 3, 5. Packet 3 arrives after packet 4. In a jitter buffer, this can play out two ways:
- Mild reordering (within buffer depth): Packet 4 arrives, packet 3 arrives before packet 3's playout time. The jitter buffer sorts them correctly. No audible impact.
- Severe reordering (exceeds buffer depth): Packet 4 arrives, packet 3's playout time passes (the buffer plays silence or PLC), then packet 3 arrives too late. It's treated as lost. From the audio perspective, this is identical to packet loss.
Why Reordering Happens
ECMP (Equal-Cost Multi-Path) Routing: When multiple equal-cost paths exist to a destination, routers distribute packets across them. If the hash function assigns consecutive packets to different paths with different latencies, they arrive out of order.
Most ECMP implementations hash on the 5-tuple (src IP, dst IP, src port, dst port, protocol) to keep a single flow on a single path. But some implementations hash per-packet or use hash collisions that split a flow.
Link Aggregation (LAG): Similar to ECMP — multiple physical links bonded together. If the hashing sends consecutive packets over different member links with different queue depths, reordering occurs.
Route Changes: A BGP route change mid-stream can cause packets already in the old path's pipeline to arrive after packets sent on the new path.
Parallel Processing: Some network devices (load balancers, DPI systems) use multiple processing cores. If consecutive packets are handled by different cores with different processing times, they exit out of order.
💡 NOC Tip: you see reordering affecting calls through a specific network path, check if that path uses ECMP or LAG. The fix is usually ensuring the hash function keeps RTP flows on a single path — which should happen naturally with 5-tuple hashing, but can fail if a load balancer or intermediate device changes the source port.
Detecting Reordering
In Wireshark's RTP stream analysis, reordered packets appear as:
- A sequence number gap (looks like loss) followed by the "missing" packet arriving later
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