Answer the following problems.

problem 1) There are both network-layer addresses (IP-addresses) and link-layer addresses (i.e. LAN addresses), there is a need to translate between them. For the Internet, this is the job of address resolution protocol (ARP). The command arp permits you to view ARP table which contains the mapping of IP addresses to LAN addresses on the local network.

Run arp on your machine. Is there an entry in the ARP table for every host on the same LAN? Determine how entries enter and leave ARP table and give a brief explanation. How can you use this information to find out the Ethernet address of a host on the same LAN that is currently not present in the ARP table of your machine?

problem 2) Suppose a hacker obtains control of all the BGP-speaking routers in several different Autonomous Systems (ASes). Our hacker has each AS \hijack" several IP blocks. That is, each AS under his or her control announces via BGP that it owns IP blocks for which it does not. For ex, our hacker has AS (CMU) announce a one-hop path to the IP block 18.0.0.0/8 (MIT).

i) Suppose that the AS graph still converges to a stable state, can this attack cause routing loops to form? describe why or why not.

ii) Assume the ASes under attack are identified. Can other ASes change their routing policies to ensure that their traffic  still reaches the hijacked IP blocks? describe.

iii) In response to this attack, suppose all ASes agree to check a central registry for IP block ownership before a path is considered valid. That is, whenever an AS receives a route to a prefix P, it checks that the last AS in the route actually owns P. Such as, upon receiving a path to 18.0.0.0/8 (MIT), an AS will check that last AS in the route is 3 (MIT). Can a hacker still hijack IP address blocks belonging to ASes he or she does not control? (i.e., can he or she cause traffic destined to those IP blocks to be routed to the ASes he controls?) Describe.

iv) Assume a solution was devised where IANA hosted a server on the Internet that was able to validate all AS paths. Assume that this server is always trustworthy and paths are valid if and only if the server says so.

True or false: With this solution an AS can always check the validity of a BGP path advertisement it receives.