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
C [2,2]
[1,2] A
address
[1,5] B
D [2,6]
switch 1
switch 2
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
[3,2]
C
[1,2]
A
S3
S1
B
[1,5]
D
[3,5]
S2
E
[2,1]
F
[2,6]
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
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
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
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•
•
•
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
• Dijkstra’s algorithm - find the distance from
a source node to each other node in a
graph
• Run this for each switch and create nexthop routing table as part of the process
• “Distance” represented by weights on
edges
Example of the shortest path
Dijkstra’s algorithm
• S = set of nodes, labelled with current distance
from source, but for which minimum distance
is not yet known
• Initialise S to everything but the source
• Iterate:
– select the node, u, from S that has smallest current
distance from source
– examine each neighbour of u and if distance to
the neighbour from source is less via u than is
currently recorded in S then update
Implementation
• Data structure to store information about the
graph (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)
Given:
a graph with nonnegative weight assigned to each edge
and a designated source node
Compute:
the shortest distance from the source node to
each other node and a next-hop routing table
Method:
Initialise the set S to contain all nodes except
the source
Initialise D so that D[u] = weight(source, u)
Initialise R so that R[v] = v if there is an edge
from the source to v and zero otherwise
while ( set S is not empty ) {
choose a node u from S such that D[u] is minimum
if ( D[u] is infinity ) {
error: no path exists to nodes in S; quit
}
delete u from set S
for each node v such that (u, v) is an edge {
if v still in S {
c = D[u] + weight (u,v);
if c < D[v] {
R[v] = R[u]
D[v] = c;
}
}
}
}
Example
S = { 1,
1
2,
Distance D
2 3 4 5 6
3, 4,
7
5,
6, 7 }
Next hop route R
1 2 3 4 5 6 7
Example
S = source = 6
S = { 1,
2,
1
Distance D
2 3 4 5 6
∞
8X
5
2 ∞
X
13
∞
-
3, 4,
5,
6, 7 }
7
Next hop route R
1 2 3 4 5 6 7
5
0
2
3
0
0
-
7
0
3
3
3
0
-
7
Distributed route computation
• Dijkstra’s algorithm requires each switch to
hold a complete description of the network
• In distributed route computation
computation, each switch periodically
computes a new local table and sends it to
its neighbours
• After a while, the switches learn shortest
routes or adapt to changes in the network
Given:
a local routing table, a weight for each link that connects
to another switch and an incoming routing message
Compute:
an updated routing table
Method:
maintain a distance field in each routing table entry
initialise routing table with a single entry that has the
destination equal to the local packet switch, the next hop
unused and the distance set to sero
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 which
the 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 Dijkstra’s algorithm to their version of
the state of the network