Duplicate Packets
Count of RTP packets received more than once — indicates routing loops, misconfigured network paths, or overzealous retransmission mechanisms.
Duplicate Packets
| Property | Value |
|---|---|
| Key | duplicate_packets |
| Unit | Packets |
| Type | Counter |
| Direction | Receive |
| RFC | RFC 3550 |
What It Measures
Duplicate Packets counts the number of times an RTP packet with an already-seen sequence number arrives at the endpoint. In normal operation, each packet has a unique sequence number and arrives exactly once. Receiving the same sequence number twice means the packet took two different paths to the receiver — or was retransmitted when it did not need to be.
Think of it like receiving two copies of the same letter in the mail. It is not harmful (you just ignore the extra copy), but it tells you something unusual is happening with the delivery route.
Why It Matters
Duplicate packets are not directly harmful — the receiver simply discards the extra copy. However, they are a diagnostic signal:
- Routing loops — The most common cause. A misconfigured router sends packets around a loop, with one copy escaping to the destination at each pass.
- Redundant paths — Some network configurations intentionally send duplicates for resilience (RFC 2198 or similar), but this should be expected and configured.
- Wasted bandwidth — Every duplicate packet consumes bandwidth without delivering new media data. At scale, this wastes capacity.
- Network misconfiguration indicator — Duplicates in a network that should not produce them point to a routing or switching problem that may also cause other issues.
How CallMeter Measures It
CallMeter tracks RTP sequence numbers for each incoming stream. When a packet arrives with a sequence number that has already been received and processed, it is counted as a duplicate. The counter increments throughout the call.
Thresholds
| Level | Value | Meaning |
|---|---|---|
| Normal | 0 | Expected in a healthy network |
| Warning | Above 2 | Investigate network path |
| Critical | Above 10 | Routing loop or significant misconfiguration |
What Causes Duplicate Packets
- Routing loops — Misconfigured routing tables cause packets to circle through the same devices, with copies leaking out at each pass.
- Multipath routing — ECMP (Equal-Cost Multi-Path) or similar load balancing that sends copies down multiple paths.
- Network redundancy mechanisms — Some carrier networks intentionally duplicate RTP for resilience, though this is uncommon.
- Switch or bridge loops — Layer 2 loops in switched networks can produce packet duplication.
- Retransmission at the wrong layer — Some middleboxes implement UDP "retransmission" by resending packets they suspect were lost, even though RTP does not use retransmission.
How to Fix It
- Check for routing loops. Use traceroute from the sender to the receiver and look for repeated hops or circular paths.
- Verify STP/RSTP. On switched networks, ensure Spanning Tree Protocol is properly configured to prevent Layer 2 loops.
- Review ECMP configuration. If using multipath routing, verify it is not duplicating flows across paths.
- Check middleboxes. Some Session Border Controllers or media proxies may duplicate packets in error. Test with and without the middlebox.
- Monitor the trend. A low, steady duplicate count may be benign. A rapidly increasing count indicates an active loop.
Related Metrics
- Packets Received — Total includes duplicates; compare against expected count
- Out-of-Order Packets — Often co-occurs with duplicates in multipath scenarios
- Bytes Received — Duplicates inflate the byte count
Receive Bitrate
Inbound bitrate in bits per second — detect bandwidth throttling, congestion, and asymmetric network conditions.
Out-of-Order Packets
Count of RTP packets arriving in the wrong sequence — indicates multipath routing, network instability, or QoS issues that cause audio glitches and video artifacts.