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WAN technologies and routing

Packet switches and store and forward

Hierarchical addresses, routing and routing

tables

Routing table computation

Example WAN technologies

Categories of network technology

Local Area Network (LAN)

Metropolitan Area Network (MAN)

Wide Area network (WAN)

Key distinguishing feature is scale:

geographic distance AND

number of connected computers

Packet Switches

In order to grow, WANs use many switches

Basic component is the packet switch that

can connect to local computers and to

other packet switches

WAN topology

Chosen to accommodate expected traffic

and to provide redundancy

Store and forward

Each switch receives packets, queues

them in its memory and then sends them

out when possible (i.e., when the

destination is available)

Many computers can send packets

simultaneously

Physical addressing in a WAN

A WAN defines a frame format and assigns

physical addresses to its computers

Hierarchical addressing, e.g.,

first part identifies packet switch

second part identifies computer on this switch

switch 1 switch 2

A

B

C

D

[1,2]

[1,5]

[2,2]

[2,6]address

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Next hop forwarding

A packet switch determines the destination

for each packet from the destination address

local computer or

another packet switch

Only has information about how to reach the

next switch - next hop

This is held in a routing table

Example routing table

A

B

C

D

E F

S1

S2

S3

Routing table for S2

Destination Next Hop

[1,2] interface 1

[1,5] interface 1

[3,2] interface 4

[3,5] interface 4

[2,1] computer E

[2,6] computer F

[1,2]

[1,5]

[2,1] [2,6]

[3,2]

[3,5]

Source independence

The next hop depends upon the destination

address but not on the source address

Hierarchical addresses and routing

Routing is the process of forwarding a

packet to its next hop

Hierarchical addresses simplify routing

smaller routing tables

more efficient look up

Routing in a WAN

Large WANs use interior switches and exterior

switches

Their routing tables must guarantee that

Universal routing - there must be a next hop for

every possible destination

Optimal routes- the next hop must point to the

shortest path to the destination

Default routes

A routing table may be simplified by including

(at most) one default route

Switch 1

Destn next hop

1 -

* interface 3

Switch 2

Destn next hop

2 -

4 interface 4

* interface 3

Switch 3

Destn next hop

3 -

interface 2

interface 3

4 interface 4

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Routing table computation

Manual computation is not feasible for large

networks

Static routing- program computes and installs

routes when a switch boots; these routes do

not change.

Dynamic routing- program builds an initial

table when the switch boots; this alters as

conditions change.

WANs and graphs A Wan can be modelled as a graph Nodes are switches: 1, 2, 3, 4

Edges are connections between switches: (1,3) (2,3) (2,4) (3,4) Weights are distance along a connection

Computing the shortest path

Dijkstras algorithm - find the distance from

a source node to each other node in a

graph

Run this for each switch and create next-

hop routing table as part of the process

Distance represented by weights on

edges

Example of the shortest path

Dijkstras algorithm

S = set of nodes, labelled with current distancefrom source, but for which minimumdistanceis not yet known

Initialise S to everything but the source

Iterate:

select the node, u, from S that has smallest currentdistance from source

examine each neighbour of u and if distance tothe neighbour from source is less via u than iscurrently recorded in S then update

Implementation

Data structure to store information about thegraph (nodes and edges)

Integer labels for the nodes [1..n]

Three data structures:

current distance to each node - array - D[1..n]

next hop destination for each node array - R[1..n]

set S of remaining nodes - linked list - S

weight(i, j) function (infinity if no edge)

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Repeat forever {

wait for the next routing message to arrive over the network;

let the sender be switch N

for each entry in the message {

let V be the destination in the entry and D the distance

compute C as D plus the weight of the link over whichthe message arrived

examine and update the local routing table {

if ( no route exists to V ) {

add an entry to the local table for V

with next hop N and distance C

} else if ( a route exists with next hop N ) {

replace existing distance with C

} else if (route exists with distance > C )

change next hop to N and distance to C

}

}

}

Link state routing

Each switch periodically broadcasts state of

specific links to other switches

Switches collect these messages and then

apply Dijkstras algorithm to their version of

the state of the network