[e2e] What's wrong with this picture?

Jeroen Massar jeroen at unfix.org
Mon Sep 7 03:46:44 PDT 2009

Ihsan Ayyub Qazi wrote:
>     TCP != ICMP ;)
> Sure but the large delay (and variation) is probably due to large
> buffers getting filled up by TCP traffic; ping just measured the delay
> rather than causing it.

That is not what I meant. Just think of the case where there is a nice
ISP involved, or for that matter several, who are doing "QoS" and
prioritizes ICMP a lot lower than TCP... you do know that all bittorrent
is evil and that everything over port 80 is 'good' don't you? :)

Also think of that little fact that traceroute is unidirectional, you
only see the return path from your vantage point, not the path that the
packets are taking from those points, you know that the RTT is <x> but
you don't know if that is a symmetric path or not, let alone where the
delay is happening. This is why there was this feature called
source-routing which allowed one to partially make that happen, but
unfortunately that feature has a lot of abuse added to it.

David: a cooler question would involve some 6to4 hosts, then one really
does not know where the packets are truly going ;)

As long as one does not own/control/operate the complete path it will be
really quite hard to tell what happens to packets outside ones control.


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