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I've run a traceroute. (see image). The "Avg" column shows that on hop 7, the ping actually decreases between ping 6 and 8. This can't be due to an anomaly because this data is averaged over 24 samples, hence the range/error bars in the diagram. If not an anomaly, then what causes the ping to decrease on hop seven?

At first I thought it could be due to inter-AS policy, but seeing as the destination is always the same, surely this cannot be the cause.

The traceroute runs from London, UK to Australia

Traceroute using PingPlotter software

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The later routers are faster at responding to pings than the earlier routers. Routers are heavily optimized for forwarding traffic and often do that much faster than they can respond to requests.

Another possibility is that the return path is better for the later packet. The earlier packets may take an inferior return path that makes the ping reply take longer to get back to you. (This is unlikely in this particular case since both routers are part of the same provider's network.)

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ah, a QoS thing, that makes complete sense! Your point about the return path is also quite interesting. Thanks! –  Odyssey Apr 21 '13 at 21:07
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While the destination is always the same, the return path potentially changes with each hop, even on the same network. Traceroute, in its default configuration, bursts of 3 packets, with a Time To Live of 1. The first router they come to, decrements the TTL, and upon a TTL of 0, generates an ICMP time exceeded in-transit message, which is sent back to the originating host, and the original packet is discarded. The traceroute program notes the IP of the router that expired the packet, and the time it took round-trip to complete that process. If for some reason, the router can't get a packet back to the tracerouting system... perhaps it has private IP address space; perhaps it doesn't have a route back to you (it doesn't need one; just a route to the destination, though this is unlikely), traceroute will display * * * after a timeout period has expired. There may be asymmetrical congestion that adds a few milliseconds to the path there, or the path back, but not both, which can decrease an interim RTT.

It's normal to see traceroutes like the one you provided, and there are a myriad of reasons for such.

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