Problem B:

Consider the $4-$router network shown below, where packet forwarding is controlled by flow tables $($e$.$g$.,$ configured via OpenFlow in an SDN controller$),$ rather than by a forwarding table computed by a routing algorithm. The addresses of networks attached to each of the router is also shown. The interfaces at each of the routers are also as indicated.

Suppose we want the following forwarding behavior of packets to be implemented:

Packets coming from the source network attached to $51$ and destined to the network attached to $s3$ should be forwarded along the path: $s1-s2-s3.$

Complete the match$-$plus$-$action tables in each of the routers, $s1,52$ and $s3,$ that implement these forwarding behaviors. Your rules should be as strict as possible $($should only allow these behaviors, and no other forwarding behaviors$).$ You can assume that any packet arriving at a router that does not match a rule in that table will be dropped.

Write the three rules in $$1,$$$2$ and $s3$ that implement this behavior. $[1$ point for each

rule$]$