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Re: shim6 @ NANOG (forwarded note from John Payne)


El 02/03/2006, a las 3:56, Erik Nordmark escribió:


Geoff Huston wrote:

Reposted on behalf of Iljitsch van Beijnum




Push back by transit ASes is harder. This is what I mean by that:
     A --- B
   /         \
X             Y
   \         /
     C --- D
C's link to D may be low capacity or expensive, so D would prefer it
if X would send traffic to Y over another route if possible. C can
make this happen in BGP by prepending its AS one or more times so X
will see the following AS paths:
A B Y
C C C D Y
All else being equal, X will choose the path over A to reach Y.
The simple answer here is that if the multihomed site receives a BGP
feed just like today (except that it's a read only feed) and thus
makes outgoing path selection decisions just like today, transit ASes
have exactly the same tools as they have today. But presumably, if
shim6 takes off many smaller sites that aren't comfortable with BGP
will multihome also, so this push back won't work as well anymore.
Creating a new way to accomplish this result is probably possible,
but not entirely non-trivial, and probably something we wouldn't want
to deliver on day one.


Wouldn't the hosts in X and Y have two IPv6 addresses?
Thus BGP itself at site X wouldn't know that B:Y and D:Y is the same host, hence BGP wouldn't compare them to find the best. 



yes, but i think could work anyway
I mean, suppose that we are performing source address rewriting by the multihomed site exit routers Now suppose that intra site routing is performed based solely in the destination address (just as it is in the current paradigm) Assume that the multihomed site obtains BGP feed through both of its providers. Now, based on the BGP information obtained, the intra site routing will preffer one or other exit to reach a given destiantion. So, when a host within the multihomed site sends a packet to that destination the exit route will be determined by this BGP information hence the policy information pushed by the transit ISPs can affect the intra site route selection Since the site exit routers can rewrite the source address, they will select the appropriate source prefix 

makes sense?

regards, marcelo



   Erik