3

By using the ping,

  1. I calculate the RTT between A and B.
  2. I calculate the RTT between A and C.
  3. I calculate the RTT between C and B.

    • Is it possible to compare the RTT of topological link A->B with the RTT of the link A->B as RTT(A->B) = RTT(A->C)+ RTT(C->B)?
    • Can we say that passing by C is more optimal than the direct link if we have:

                  RTT(A->B) > RTT(A->C)+ RTT(C->B)
      

In other words, are the RTTs additives?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Dec 25 '18 at 8:14
3

That depends on the topology. With A - C - B connected sequentially it's likely true, but connected in a triangle it's not.

Asymmetric routing changes the whole game (even though A routes B via C, C might have a shorter route back to A).

PS: missed the ping reference on top: as Ron has pointed out, ping might not be reliable for measuring RTT. Pinging network devices can result in much larger values than real RTT since ICMP echo requests are usually low priority and they're processed by a device's control backplane (often software) while normal network operations use the much faster data backplane (=hardware).

2

Ping uses an ICMP echo request and an ICMP echo reply. The RTTs you see are only for ICMP (does not reflect how other protocols, e.g. TCP, will perform), and that includes the time for the target to process the ICMP request and create an ICMP reply. ICMP message generation is normally a low-priority process, so if a machine is busy with something else, the ICMP reply can be delayed. If the middle machine is very busy doing something else, it can take longer to reply than for the end machine.

  • Also, the path from C to B may be different than B to C. – Ron Trunk Apr 1 '18 at 20:53

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