check host alive

Andreas Ericsson ae at op5.se
Fri Jan 19 15:27:35 CET 2007


Andy Shellam (Mailing Lists) wrote:
> "Time to live exceeded" is an error response from a router/intermediate 
> host during a Ping/traceroute - I think it usually means that a packet 
> has exceeded its "max hops" count, so if you're getting this error, 
> chances are it's encountered a loop somewhere as I'm sure the default 
> TTL is 128 hops (I could be wrong though.)
> 

Most systems have 64 as default TTL, otherwise you're right. There are two
"variants" of "Time to live exceeded". One is where the there are more hops
to the target than the TTL set in the packet. This is known as "Time to live
exceeded in transit".

The other is if you're sending packets larger than the networks MTU and one
part of the packet gets lost on the way. In this case you get a "Packet
reassembly time exceeded". Both usually show up as "Time to live exceeded"
by most system pings, as that's the "icmp type" (while the "icmp code", a 
different field in the icmp packet header, shows the variant).

-- 
Andreas Ericsson                   andreas.ericsson at op5.se
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

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