1. CECS 474 Computer Network Interoperability WAN Technologies & Routing
CHAPTER 18
WAN Technologies & Routing
Tracy Bradley Maples, Ph.D.
Computer Engineering & Computer Science
California State University, Long Beach
Notes for Douglas E. Comer, Computer Networks and Internets (5th Edition)

2. Large Networks and Wide Areas
LANs
Usually span a single building.
Limited by size (unless connected by satellite bridges)
=> Limited scalability
WANs
Can span large geographic distances.
Can cross public right-of-ways (streets, buildings, railroads).
Deliver high (or reasonable) performance for many users.
KEY ADVANTAGE => Scalability
A WAN must be able to grow as needed to connect many sites spread across large geographic distances.
A technology is not classified as a WAN unless it can deliver reasonable performance for a large scale network.
WANs are constructed out of:
Point-to-point long-distance connections
Packet Switches

3. Packet Switches Packet switches are: Hardware devices
Packet switches are:
Hardware devices
They connect to other packet switches or computers
They forward packets
They use destination IP addresses for forwarding
Packet switches are special-purpose
computer systems consisting of:
CPU
Memory
I/O interfaces
Firmware
Two types of I/O devices are used:
High speed -- to connect to other packet switches
Lower speed -- to connect to individual computers

4. Modern WAN Architecture

5. Forming a WAN
WANs are formed by placing one or more packet switches at each site.
Switches are interconnected using:
LAN technology for local connections.
Leased digital circuits for long-distance connections.
The switches can be connected asymmetrically.
The number and type of interconnections depend on:
The estimated traffic.
The reliability needed.
The cost of the link.

6. Physical Addressing in a WAN
Store and Forward
Store and forward is the basic paradigm used in packet switched networks.
Each Packet:
Is sent from the source computer.
Travels from switch-to-switch across the network.
Is delivered to the destination.
At Each Switch:
Packets are “stored” in memory.
The packet’s destination address is examined.
The packet is forwarded toward the destination.
Physical Addressing in a WAN
WANs require:
A unique address for each computer
An efficient forwarding scheme.
A two-part address is used with:
A packet switch number.
Specific computer on that switch.

7. Illustration of WAN Addressing
The figure shows the two-part addresses.
In practice, the addresses are encoded as a single binary value with:
The high-order bits for the switch number.
The low-order bits for the computer number.
Users and application programs can treat the address as a single integer, they do not need to know that addresses are assigned hierarchically.

8. Next-Hop Forwarding
Next-hop forwarding is
Performed by packet switches
Uses a table of routes (Called a Forwarding Table and created by a routing algorithm)
The table gives the next hop

9. Source Independence and Hierarchical Routing
Defn: Source independence means that Next-hop forwarding does not depend on the packet’s original source or on any paths the packet has taken before it arrives at a particular switch.
Source independence allows the forwarding on switches to be compact and efficient.
Defn: A forwarding table is the table of next-hop information stored in a switch.
When forwarding a packet to another switch, a switch examines only the first part of the hierarchical address.
A switch needs to look at the second part of the hierarchical address only if the destination machine is attached to that switch.
Condensing the entries improves efficiency.

10. Routing in a WAN There are two sources of routing table information:
There are two sources of routing table information:
1. Manual (seldom used)
The table is created by hand
Useful in small networks
Useful if routes never change
2. Automatic routing
Software creates/updates the tables
Needed in large networks
Software changes routes when failures occur

11. Routing and Graph Theory
Model a network as a graph with:
Each switch as a node
Each connection as an edge

12. Default Routes A large WAN may contain hundreds of duplicate entries.
A large WAN may contain hundreds of duplicate entries.
A default route is a mechanism that allows a single entry in a forwarding table to replace a long list of entries that have the same next-hop value.
only one default entry is allowed in a forwarding table
the default route has lower priority than other entries
If the forwarding mechanism does not find an explicit entry for a given destination it uses the default.

13. Shortest Path Computation
Shortest paths through the network can be computed using:
Algorithms from graph theory
Distributed computation (no central authority)
A switch:
Must learn the route to each destination
Only communicates directly with attached neighbors
For the above graph, the label on each edge represents the “distance” metric
Geographic distance
Economic cost
Inverse of capacity

14. Algorithms for Computing the Shortest Paths
Both of these approaches are used in practice to compute the shortest paths:
Distance Vector (DV)
Switches exchange information with their neighbors and then use the Distance Vector algorithm. Use the Bellman-Ford Algorithm.
Link-State
Switches exchange link status information and then use Dijkstra’s algorithm. Broadcasting information.
The Distance Vector Algorithm
Periodically, exchange data between neighboring switches giving your information about the paths of the network.
During the exchange, the switches send:
List of pairs giving the (destination, distance) of connections
The receiving switch:
--Compares each item in the switch to local routes
--Changes the routes if a better path exists

15. Distance Vector Intuition
Let
N be the neighbor that sent the routing message
V be the destination in the pair
D be the distance in the pair
C be D plus the cost to reach the sender
If no local route to V exists, or if the local route has a greater cost than C, install a route with next hop N and cost C.
Else ignore the pair.
Example for Distance Vector Routing
Note: In DV Routing, the information that is passed from one node to another, takes time to propagate through the entire network.
“Good news travels quickly”
but
“Bad news travels slowly”
Consider transmission of one DV message:
Node 2 sends to 3, 5 and 6
Node 6 installs cost 8 for route 2
Later 3 sends update to 6
6 changes route to make 3 the next-hop for destination 2

16. Dijkstra’s Shortest Path Algorithm
Link-State Routing
Overcomes some of the instabilities of Distance Vector Routing.
Pairs of switches periodically:
Test the link between them
Broadcast a link status message
Each switch:
Receives the status message
Computes new routes
Uses Dijkstra’s algorithm
Dijkstra’s Shortest Path Algorithm
Input: • Graph with weighted edges
Node, n
Output: • Set of shortest paths from n to each node
Cost of each path
Called the Shortest Path First (SPF) algorithm.

17. Dijskra’s Algorithm Intuition
Start with self as the source node
Move outward
At each step:
Find node u such that it
Has not been considered
Is “closest” to the source
Compute
Distance from u to each neighbor v
If the distance is shorter than what is currently recorded, make a path from u go through v
Example: Routes from node 6:
To node 3: next hop = 3, cost = 2
To node 2: next hop = 3, cost = 5
To node 7: next hop = 7, cost = 5
To node 4: next hop = 7, cost = 8
To node 5: next hop = 3, cost = 11
To node 1: next hop = 3, cost = 20