? how cisco router handle the out-of-order ICMP echo-reply packets

Mikael Abrahamsson swmike at swm.pp.se
Tue Jan 6 13:26:44 UTC 2009

On Tue, 6 Jan 2009, Scott Morris wrote:

> There aren't sequence numbers with ICMP.  And the timeout value is
> watched/triggered before the next ICMP is sent, so there shouldn't really be
> any ordering problem/interpretation anyway.

Linux ping command does sequencing (so that part of your statement isn't 
accurate), and you can get out of order packets. It'll say a sequence 
number and ping time, and there really isn't any "timeout", an ICMP packet 
can come back 60 seconds later and it'll be counted, even though there 
were 59 other packets send and returned in the meantime.

$ ping localhost
PING localhost ( 56(84) bytes of data.
64 bytes from localhost ( icmp_seq=1 ttl=64 time=0.023 ms
64 bytes from localhost ( icmp_seq=2 ttl=64 time=0.020 ms

In IOS, my interpretation anyway, is that the timeout value (2 seconds) 
mean that it really considers this packet as dropped, so no, in IOS you 
cannot get out of order packets, at least not that the CLI will show. If 
the ICMP response packet comes back after timeout value has triggered, 
it's considered lost.

Mikael Abrahamsson    email: swmike at swm.pp.se

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