CHAPTER 18WAN Technologies

& Routing

CECS 474 Computer Network Interoperability

Notes for Douglas E. Comer, Computer Networks and Internets (5th Edition)

Tracy Bradley Maples, Ph.D.Computer Engineering & Computer ScienceCalifornia State University, Long Beach

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

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-purposecomputer systems consisting of:

• CPU• Memory• I/O interfaces• Firmware

 Two types of I/O devices are used:High speed -- to connect to other packet switchesLower speed -- to connect to individual computers

Modern WAN Architecture

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.

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.

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.

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

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.

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

Routing and Graph Theory

Model a network as a graph with:• Each switch as a node• Each connection as an edge

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.

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

Algorithms for Computing the Shortest Paths Both of these approaches are used in practice to compute the shortest paths:

1. Distance Vector (DV)Switches exchange information with their neighbors and then use the Distance Vector algorithm. Use the Bellman-Ford Algorithm.

2. Link-StateSwitches 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

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 RoutingConsider 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

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”

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.

Dijskra’s Algorithm Intuition 1. Start with self as the source node2. Move outward3. 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 = 2To node 2: next hop = 3, cost = 5To node 7: next hop = 7, cost = 5To node 4: next hop = 7, cost = 8To node 5: next hop = 3, cost = 11To node 1: next hop = 3, cost = 20