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Intermediate nodes’ higher reported rtt figure in the midst of traceroute

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Weaver:
Imagine we are doing a traceroute to example.com. There are six hops, say. Now our traceroute reports an RTT to node 4 which is higher than that to node 5. I am assuming that these RTTs are just that, they are not differences, that is they are for the whole round trip from the start to node n, and are not just for the last hop only, ie from node n-1 to node n and back.

Now why would an intermediate number be higher? Indeed how could it? I am ruling out one-off errors, so say we take a series average. The only explanation I can think of is that there is a weird delay at the intermediate node in returning the ping, but this delay is absent for node n+1. Perhaps because node n is very busy.

What do you think?

johnson:
I have often wondered about this. My hand waving answer is that intermediary nodes may not prioritise ICMP requests, quite a lot do not even respond to them.

Weaver:
They ought to prioritise ICMPs. Do you think they get stuck in egress queues? Anyway that would make sense of it, would it not?

johnson:
I don't know, but figure hops deep in links between datacenters have little reason to have ICMP responses enabled.

What purpose would it serve, other than to generate more traffic and expose another attack mechanism?

banger:
Johnson's theory is my understanding of intermediate routers as well.

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