1. 18-WAN Technologies and Dynamic routing
Dr. John P. Abraham
Professor
UTPA

2. Traditional WAN architecture
Packet switches (specialized computers) were used to direct packets. See fig 18.1
WAN existed before LAN. When LAN was introduced the packet switching was divided into local and outside. Layer two was assigned LAN and layer 3 WAN.
Many WANs used leased circuits.
WANs were formed by interconnecting packet switches

3. Store and forward Paradigm
Packet switch buffers packets in memory to compensate for the speed of the routes.
The store operation occurs as the packet arrive
The forward operation occurs once a packet has arrived and waiting in memory. The processor examines the packet, determines its destination and sends the packet over the appropriate interface.
Structure of a router: 4 components
Input ports, output ports, routing processor and switching fabric. The routing processor has other components, such as the buffer, boot ROM, nvram, etc.

4. Addressing
Hierarchical addressing has two parts, site and the computer at that site. The first part of the address actually identifies a router (not exactly, we will discuss this later). IP addresses are not permanent.
Once the packet arrives at the router, then it queries all computers to find out which machine has a particular IP address. The machine with that address responds with its MAC address and the packet converted to frame is sent to that address. MAC addresses are permanent to a machine.

5. Next-Hop routing
When a packet arrives, the routing processor must choose an outgoing path. If it is destined for a local computer, the router sends to that computer using its MAC address. Otherwise, it is forwarded to one of the output ports on the router that leads to the destination.
To do this the router examines the destination address and extracts the network portion of the address. If the extracted address matches its own the packets are kept otherwise forwarded.
The router does not need to know the complete routing information, rather just next-hop. Think of baggage transport in the airlines.
Forwarding has nothing to do with the source (source independence)

6. Dynamic routing updates in wan
Each router must have a forwarding table. The table must guarantee:
Universal communication – a valid next-hop route for each possible destination.
Optimal routes. The route must be shortest path, either in hops, time, or using other factors.
If two paths exist, one become unavailable (hardware failure, routes should be tested), forwarding table should be changed. So the table must be dynamic.
See illustration of wan and how it could be viewed as a graph, on page 314. fig 18.6.

7. Explanation of fig 18.6 and 18.7
Nodes in the graph are given a label (address of the router). Graph can be used to discover next hop, and shortest path.
To reach node 4 from node 1, the next hops are given 1,3, and 3,4. Alternatively, we can discover next hop as 1,3, 3,2 and 2,4. We can actually prepare a table of next hop for each router to reach all the others.

8. Default routes
In case of router one, next hop is 1,3 regardless where you are going. So instead of listing 1,3 three times, why don’t we have one default entry as 1,3.
So, we can have next hops for the ones we know, and all the repeated ones should have a default route. Default route is optional.

9. Forwarding table
Static routing. All routes are entered into the router and it does not change. The routes are stored therefore, upon reboot it will retain these routes. In case of a down line, the router can’t send any more packets to that destination.
Dynamic routing. Initial routing table is keyed in. Then in learns all the other routes. Most routers today use dynamic table.

10. Shortest Path computation in a graph
Dijkstra’s agorithm
Next hop table is constructed during the computation of shortest path
In this algorithm, the edges in a graph can assume any non-negative value. i.e. Weight.

11. Routing problems Link State Routing (shortest path first)
– routers periodically send messages across the network that carry status of the link (up, down), these messages can be used to build graphs. If packets are lost then two routers can disagree about the shortest path.
Distance Vector routing
Each link is assigned a weight. The distance the destination is the total of all weights. It also sends periodic messages, but adds weight like “I can reach destination x, and its current distance from me is Y”. DVR can create a loop, by one saying it can’t reach, and the neighbor saying it can reach through the one just reported it can’t reach. DVR employs split horizon to prevent this loop (router does not send information back to the one reported it can’t reach).