Questions about tunneltrace

[Eric Rosen <erosen@cisco.com>]

From draft-bonica-tunneltrace-01:

      9) When the tunneling technology isolates the user-plane from the
      control-plane, do not rely upon the control plane to discover the
      path.

From draft-bonica-tunproto-00:

   D2 receives the TraceProbe, exercises access control, and determines
   that it is the hop's head end device. D2 consults its FIB and
   determines that an IP-in-IP tunnel supports H1. 

I think this is saying that  D2 must consult its control structures in order
to determine whether the "hop" H1  is a tunnel or not.  Doesn't this violate
the requirement above?  In the  event of failures, D2's "determination" that
data packets are supposed to be traveling through a tunnel doesn't necessar-
ily mean that data packets would actually travel through that tunnel.

I am also  puzzled more generally about the phrase "D2  consults its FIB and
determines  that a  ...  tunnel  supports H1."   I don't  understand  how in
general  D2  can  make  this  determination from  the  information  that  is
available in  the traceprobe packet.  If  we're trying to  trace nested MPLS
tunnels, then it would  seem that we at least need to  know the label that a
data packet would have  when it enters D2, but I don't  see any place to put
that.  More generally,  there doesn't seem to be  any provision for handling
the case  in which  the routing  of a packet  is not  based entirely  on the
packet's destination address.

Similarly, I don't see  how a hop can be identified by  a "hop object" which
has only the IP addresses of the  hop endpoints.  There can be any number of
tunnels between the same  two endpoints, each with different characteristics
and  even with  different routes.   The  only data  you seem  to provide  of
choosing one of these tunnels over the others is the destination IP address,
and this isn't necessarily sufficient.

On a  more trivial  matter, the  protocol draft will  need to  say something
about timing out remembered traceprobes,  in case the trace response doesn't
come back.