Re: draft-bonica-tunneltrace-02

[Eric Rosen <erosen@cisco.com>]

Shahram> 1) If a router  is not GTTP upgraded, it will  drop the TTL expired
Shahram>    GTTP messages.  Consequently the host will not receive any reply
Shahram>    from that router,  which translates to a break  in the tunnel at
Shahram>    that point.

For GTTP to be useful at all, the tunnel head ends must support it. 

If a particular  node within a tunnel does not support  GTTP, but some nodes
beyond  it do,  we won't  get complete  trace info,  but should  be  able to
continue tracing beyond the non-supporting node.

But the  basic point is valid,  that in order  to use GTTP to  trace through
your network,  your routers must  support it.  I  guess I don't see  this as
much of a problem.  I  wouldn't call it an interoperability problem, because
no existing mechanisms are broken by the use of GTTP. 

Shahram> 2) TTL expired user packets will now be forwarded to UDP module
Shahram>    instead of being dropped. Which could overload the UDP module in
Shahram>    certain situations.  

TTL-expired user packets are not simply dropped today; they are forwarded to
the ICMP module  to cause the generation of an  ICMP message.  Usually there
is some sort of limit placed on the number of packets that can be queued for
ICMP processing, or the  number of such packets that can be  seen in a given
amount of  time, etc.  The  marginal overhead to  see whether a  packet that
makes it through to ICMP processing  is a GTTP packet doesn't seem like that
big a deal.