# Kb62

## Emulab FAQ: Troubleshooting: I set a non-zero packet-loss (or delay) but 'ping' shows a different packet-loss (or delay)

Short answer: Ping is round trip, PLR and delay are "one way".

Long Answer: If you're not seeing any traffic shaping at all (100Mbps, 0ms, 0plr), see this Troubleshooting entry. If you are seeing shaping, but something different than you expected, it is probably because link characteristics are one way, and you're measuring them over the round trip.

For instance, if you asked for a link that was 100Mbps, 30ms, with 5% (0.05) packet loss rate (plr), you may expect ping to show 30ms ping times and 5% loss rate. But what you should see is 60ms latency for the round trip, and a loss rate of 9.75%. Latencies can be added, therefore 30ms + 30ms gives 60ms. However, loss rates are probabilities, and must be multiplied. The chance of a packet making it across a 5% lossy link is 95%, so with a 95% chance of arriving at the destination, and a 95% chance of returning if it made it there, and the total chance of making a round trip is .95 * .95 = .9025 or 90.25%, or a round trip loss rate of 9.75% on a 5% lossy link.