2. Routing
Routing is the process of selecting
paths in a network along which to send
network traffic
Process of relaying packets among
multiple interconnected networks
3. Least Cost Routing
Based on efficiency
Which of the available paths is cheapest
A value is assigned to each link; length
of the particular route is equal to the
total no of the values of the component
links.
Shortest means the route requiring the
smallest no of relays
4. Non adaptive and Adaptive routing
Non Adaptive
Once the pathway to destination has
been selected, the router sends all
packets for that destination along that
route
Adaptive
Router may select a new route for each
packet
5. Routing Algorithms
Shortest Path Routing
Flooding
Flow Based Routing
Distance Vector Routing
Link State Routing
Hierarchical Routing
Broadcast Routing
Multicast routing
6. Shortest Path Routing
A graph of subnet is built.
Each node represents a router
Each arc of graph represents a
communication link.
To choose a route algorithm just finds
the shortest path between them on the
graph
7. Flooding
Every incoming packet is sent out on every outgoing
line except the one it arrived on.
So it generates vast no. of duplicate packets.
A hop counter is maintained in header of each packet
Hop counter is decremented at each hop
Packet is discarded when the counter reaches zero
Ideally hop counter is initialized to the length of the
path from source to destination
Selective flooding- incoming packets are sent on lines
that are going approximately in the right direction
8. Flow Based Routing
Used when data flow between each pair of nodes is relatively
stable and predictable
Considers the flow in the network; mean consider the amount
of traffic in the network before deciding on which outgoing line
to place the packet.
The key here is to be able to characterize the nature of the
traffic flows over time.
The basic idea behind the algorithm, is that any given line, if
the capacity and average flow are known, it is possible to
compute the mean packet delay on that line from queuing
theory.
The routing problem thus reduces to finding the routing
algorithm that produces the minimum average delay for the
subnet.
9. Distance Vector Routing
Each router maintains a table giving the
best known distance to each destination
and which line to use to get there
These tables are updated by
exchanging info from neighbors.
10. Distance Vector Routing-
Mechanism
Each router maintains a table indexed by and
containing one entry for each router in the subnet.
The entry has two parts
Preferred outgoing line
Estimate of time or distance to that destination
The router is assumed to know the “distance” to
each of its neighbors.
if metrics is hops , distance is one hops
If metrics is queue length, the route simply examines each
queue
If metrics is delay, the router can measure it directly with
the special echo packets that the receiver just timestamps
and send back as fast it can
11. TO A I H K Line
A 0 24 20 21 8 A
B 12 36 31 28 20 A New
estimated
C 25 18 19 36 28 I delay from J
D 40 27 8 24 20 H
E 14 7 30 22 17 I
A B C D F 23 20 19 40 30 I
G 18 31 6 31 18 H
H 17 20 0 19 12 H
I 21 0 14 22 10 I
F G
E H J 9 11 7 10 0 -
K 24 22 22 0 6 K
L 29 33 9 9 15 K
L
K
I J J computes its new route J to H=12 msec
to router G H to G =6 msec
JA Delay=8 J to A=8 msec Total=18 msec
JI Delay=10 A to g=18 msec
J to K =6 msec
Total=26 msec K To G=31 msec
JH Delay=12
Total=37 msec
JK Delay=6 J to I=10 msec
I to G=31 msec Best : 18 i.e via H
Total=41 msec
12. Link State Routing
Each router must
Discover its neighbour and learn their network
address by sending Hello packet
Measure the delay or cost to each of the
neighbours by sending echo packet
Construct a packet telling all it has learned
Send this packet to all other routers
Compute the shortest path to every other router.
In effect, the complete topology and all delays are
experimentally measured and distributed to every
router.
13. Hierarchical Routing
As network grow, routing tables increase in size
consuming router memory
It is not feasible for every router to have an entry for
every other router
Routing is done hierarchically
Routers are divided into regions
Each router knows all details of router within its own
region.
In very large networks
regions -> clusters
Clusters->zones
Zones->groups
15. Routing for mobile hosts
When a new user enters an area it must register
itself with the foreign agent here
Each foreign agent broadcasts a packet announcing
its existence and address
A newly arrived mobile host broadcasts a packet
inquiring any foreign agents
Mobile hosts registers with the foreign agent giving
its home address, DL layer Address and security info
Foreign agent contacts mobile host’s home address
Home agent checks info sent by foreign agent, if ok,
informs foreign agent to proceed
Foreign agent the makes an entry in tables and
informs mobile hosts that it is registered
16. Broadcast Routing
Send messages to many or all hosts simultaneously
Each packet contains a list of destinations
Router examines this list to determine set of output
lines that will be needed
It generates a new copy of packet for each output
line to be used and include in each packet only those
destination that are to use the line
Destination set is partitioned amount output lines
Each packet will carry only one destination
17. Multicast Routing
Sending messages to well defined network
groups
With a multicast design, applications can send
one copy of each packet and address it to the
group of computers that want to receive it.
This technique addresses packets to a group
of receivers rather than to a single receiver
It depends on the network to forward the
packets to only the networks that need to
receive them.
