1. Identify the main differences between interior and exterior gateway protocols. Is there any protocol that could be used as both? Explain.
   • An IGP is used within an AS; an EGP is used between ASs.
   • For an IGP fast convergence needed to keep up with rapid changes; an EGP has only rare changes of ASs
   • IGP is used between nodes all overseen by same organisation; EGP is between nodes run by different people with different priorities
   • There is some trust between IGP nodes, less between EGP nodes, and there is danger from spoofing and denial of service from hostile ASs
   • Details of the structure of a network might be safely passed around and used in an IGP, but such details would not want to be passed outside the organisation
   • In an EGP policy routing is most important, including costs of transits

   No single current routing protocol is suitable: an EGP could be used as an IGP, but this would be very inefficient. IGPs are not suitable as EGPs as they don't support the required features.

2. Discover what routing protocols are being used in your local networks and find the policy decisions that lie behind them.

   Experimental. This involves talking to people! Some amount of sniffing network traffic using tcpdump might reveal routing packets going past, but this is unlikely for end-user machines on a switched network.

3. Implement Dijkstra's algorithm and use it to determine some shortest paths within example networks.

   Many implementations exist, but it is much more educational to implement this kind of thing yourself.

   An animation.

4. Look at the Bellman-Ford algorithm for finding shortest paths. Compare it to Dijkstra's algorithm.

   Links. Bellman-Ford is more general than Dijkstra—it will work on networks with negative edge costs—but such kinds of costs are unlikely in real networks. As Dijkstra is faster when it applies, it is usually preferred.

5. Identify the AS your network lives within. Find the ASs of several other networks and determine the routes between them.

   The whois tool sometimes gives the AS for an IP address. If not, try whois -h riswhois.ripe.net <address>

   The allocation table of AS numbers tells you which regional registry owns which AS

   • AfriNIC
   • APNIC
   • ARIN
   • LACNIC
   • RIPE

   Combining this with traceroute will allow you to map transits between ASs.

6. Identify the security problems that an arise from using an unauthenticated routing protocol.

   There are many. The simplest being that a router insert routes into the Network that direct traffic to come via itself, thus making it readable as it goes past. Other kinds of attack include denial of service, where a router attracts traffic and then refuses to pass it on.

7. Multicast routing (that is, routing of multicast traffic) is very different from unicast routing. Explain why this is so and investigate the various routing solutions.

   Multicast routing involves building trees across routers with the root at the source and the leaves at the destinations. These trees change rapidly as hosts join and leave groups. Care must be taken not to create loops. Protocol Independent Routing (PIM) employs normal unicast routing, but in reverse. A multicast packet has a unicast source address: unicast routing tells us where this packet came from, so PIM can forward the packet to the other interfaces.

   PIM can run in

   • dense mode: assume most routers downstream want the multicast and a router should send a message back upstream if it doesn't RFC3973
   • sparse mode: assume only a few routers downstream want the multicast and there is a mechanism to send join messages upstream RFC2362

   or a combination of the two. Other dense mode protocols are RFC1075 (an extension to RIP) and RFC1584 (an extension to OSPF), but these do not scale as well.

   In source specific routing the host's request to join a group contains the source's unicast address and routers build a reverse path as the request passes upstream RFC3569

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