hidden routers

["Kristian Hermansen" <kristian.hermansen () gmail com>]

How common is it that a router does not decrement the TTL of packets,
such that it is unable to be identified using traceroute?  Choosing
not to decrement the TTL causes the next router to appear as the hop,
but the current router to remain hidden.  How does one commonly
identify such hidden routers in an automated fashion?  And is it
policy for any organizations to actually do this, or only with certain
packet types?
--
Kristian Hermansen