CHAPTE R 18 WAN Technologies & Routing

1. CECS 474 Computer Network Interoperability CHAPTER18 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 • 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. 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: • 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 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: • DistanceVector (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 withnext 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 thenext-hop for destination 2

16. 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