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I am trying to measure latency to a server that I don't control. This is in a colocated environment, so the latency is on the order of 500 us (.5 ms).

I understand that Cisco gear frequently deprioritizes ICMP traffic, making ping times unreliable. Is there a way for me to tell if this is the case on the gear I am traversing?

Can I use TCP acknowledgements to determine the minimum latency to the remote server? To do this, I would somehow need to force the remote server to send a TCP ack immediately on receiving my data.

+1  A: 

You could try using arping, which does a ping using ARPs.

caf
I'm seeing the times reported from arping are ~100 us higher than those reported by ping. Would you expect that? Does it indicate that ICMP pings are not being deprioritized?
Ted Graham
Possibly that's just indicative of the extra processing an ARP reply causes in the local kernel. Anyway it certainly appears that ICMP pings are not being deprioritized.
caf