Re: hidden routers

["Alex Nedelcu" <alexpheno () gmail com>]

It's unlikely for a RFC compliant router to behave as you described,
there are cases though where it's useful or even necessary to
implement such a thing.

I've seen cases where a service provider enforces the use of a single
terminal (computer or other equipment with no routing logic)  for
certain internet services, this is done by setting the TTL value on
the provider's edge router to 1, thus if the client wants to
redistribute that internet to other computers (NAT), during the
routing process the TTL would reach 0,   disallowing the natted hosts
to see the returning traffic from the internet. This is a situation
where keeping a value of 1 or resetting that field would be necessary.

I know that it's probably not your case but routers that process
traffic traversing Layer 2 and layer 3 VPNs also do not decrease TTL
of the encapsulated packet, for the endpoints participating in the
communication the only routers that would be visible with a traceroute
would be only the ones that were taversed prior to the local
encapsulation and after decapsulation at the remote location.

The routers you speak of could also be set in bridging mode, in this
situation they would not be reachable at layer 3.

The last scenario that comes in mind is that all the routers do
decrement the TTL but not all icmp time exceeded packets reach you due
to filtering or network congestion.