Packets Lost
Absolute count of RTP packets lost during transmission — the raw loss number before computing a percentage rate.
Packets Lost
| Property | Value |
|---|---|
| Key | packets_lost |
| Unit | Packets |
| Type | Gauge |
| Direction | Send and Receive |
| RFC | RFC 3550 Section 6.4.1 |
What It Measures
Packets Lost counts the total number of RTP packets that were sent but never arrived at the receiver. It is the raw, absolute number — not a percentage or rate.
If 10,000 packets were sent and 9,950 arrived, the packets lost count is 50. This gives you the magnitude of loss in concrete terms rather than as a ratio.
The count is derived from RTCP Receiver Reports as defined in RFC 3550. The receiver tracks the highest sequence number received and the total number of packets expected versus received. The difference is reported as cumulative packets lost.
Why It Matters
While the Packet Loss Rate (percentage) is more commonly used for threshold evaluation, the absolute count provides context that percentages hide:
- Scale matters. 1% loss on a 30-second call is 75 packets (at 50pps). 1% loss on a 30-minute call is 4,500 packets. The rate is the same, but the user experience impact accumulates.
- Burst detection. A sudden spike in the packets lost counter, even if the overall rate stays low, may indicate a burst loss event that caused a brief but noticeable glitch.
- Capacity planning. Absolute counts help you estimate the volume of retransmissions or recovery mechanisms needed.
How CallMeter Measures It
CallMeter tracks packets lost by processing RTCP Receiver Reports from the remote endpoint. The receiver calculates loss by comparing the expected number of packets (based on RTP sequence numbers) against the count actually received. This value is updated at each RTCP reporting interval and reported per second.
Thresholds
This metric does not have fixed thresholds because acceptable loss depends on call duration and packet rate. Use Packet Loss Rate for threshold-based alerting.
Packets Lost vs Packet Loss Rate
Use packets_lost to understand the magnitude of loss. Use fraction_lost (Packet Loss Rate) to understand the severity relative to total traffic. Both are important — a high absolute count with a low rate means a long call with minor issues, while a low count with a high rate means a short call with serious problems.
What Causes Packet Loss
- Network congestion — Routers drop packets when queues overflow during traffic spikes.
- Firewall or security device drops — Stateful firewalls may drop UDP packets that exceed connection tracking limits.
- Rate limiting — ISP or network-level rate limiters may throttle UDP traffic.
- MTU issues — Oversized packets that require fragmentation are more likely to be partially lost.
- Wireless interference — WiFi packet loss due to radio contention, interference, or weak signal.
How to Fix It
- Compare with Packet Loss Rate. If the absolute count is high but the rate is low, the loss may be spread across a long call and less impactful than it appears.
- Check the timeline. Use CallMeter's per-second charts to see if loss is steady (chronic) or burst (acute).
- Investigate network path. See the remediation steps in Packet Loss Rate.
Related Metrics
- Packet Loss Rate — The percentage equivalent of this metric
- MOS Score — Packet loss is a primary driver of MOS degradation
- Packets Sent — The denominator for loss ratio calculation
- Packets Received — What actually arrived
- Sequence Gaps — Shows the size of individual burst loss events