[e2e] Question on XCP

Jon Crowcroft Jon.Crowcroft at cl.cam.ac.uk
Fri Feb 11 00:00:05 PST 2005

In missive <20050210212933.GE520 at isi.edu>, Aaron Falk typed:

 >>The simplest, albeit imperfect, approach is to add a field in the
 >>packet header which each XCP-capable router would decrement.  If the
 >>change in this field was less from the change in TTL, the endpoints
 >>would be aware that some routers were not practicing XCP.  This is
 >>imperfect since tunnels would hid hops which might be practicing XCP
 >>and non-router queues might not be detectectable at all using this

if you start the xcp-router-capable-counter field at the same as ttl, then all you have to
do is see if it == ttl at the receiver (or any intermediate node) to tell if you traversed an
xcp-agnostic box.

not sure about the TTL - lots of boxes might be layer 2 with some layer 3 function - 
we might if XCP tool off, consider implementing the intermediate node functionality in switches, 
which dont have to do ttl-- 

otherwise the receiver needs to know what the initial value of ttl was, which is non triv. (or reflect it)

 >>Strictly speaking, though, for XCP only the bottleneck router needs to
 >>be running the algorithm.  Non-XCP queues which never congest need not
 >>participate.  Of course, the trick is knowing which is which.  :)

If XCP has a function in the bottleneck router, 
then if you run XCP you can infer if that function is applied - 
you can see if there's packet loss without XCP operating, or ECN without

If you could choose to deploy XCP in bottlenecks, and only in bottlenecks, 
you would probably have a good claim to being the Internet God.

 >>You can find more on our XCP work at http://www.isi.edu/isi-xcp .
 >>Arjuna Sathiaseelan wrote:
 >>> Dear All,
 >>>   I would be very much obliged if anyone could let me know how a XCP
 >>> sender determines whether all the routers in its path is XCP
 >>> enabled?  The XCP sender works only if all the routers in its path
 >>> are enabled with XCP.
 >>> I have a mechanism similar to XCP which requires all the routers in
 >>> its path to be enabled with the mechanism proposed by me. Thanks.
 >>> Regds,
 >>> Arjuna



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