Network layer Delivery, Forwarding and

thoughtlessskytopNetworking and Communications

Oct 29, 2013 (3 years and 11 months ago)

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1

Chapter 22

Network layer

Delivery, Forwarding and

Routing

(part3)

Three
-
Node Instability

Routing Information Protocol (RIP)

RIP

is based on
Distance vector routing:

RIP implement Distance vector routing with some considerations:



Destination in the routing table is a network (first column defines
network address)


Metric(distance) is
Hop count

: is the number of networks that a packet
encounters to reach its final destination



Infinity is defined as value of 16 .


Therefore, the Max limited of hops is
15



suitable for small networks (local area environments)


Router sends update message to neighbors every 30 sec.



If router does not receive update message from neighbor X within this
limit, it assumes the link to X has failed and sets the corresponding
minimum cost to 16 (infinity).

3

4

In distance vector routing, each node shares its
routing table with its

immediate neighbors periodically and when there is a
change.

Note

Routing Information Protocol (RIP)

Routing table



Has one entry for each
destination network
of which the router is aware.



Each entry has destination network address, the shortest distance to reach
the destination in
hop count,
and next router to which the packet should be
delivered to reach its final destination.



5

Destination

Hop

Count

Next

Router

Other information

163.5.0.0

7

172.6.23.4

197.5.13.0

5

176.3.6.17

189.45.0.0

4

200.5.1.6

115.0.0.0

6

131.4.7.19

6

Example Internetwork


When a router is added to a network, it initializes a
routing table for itself, using its configuration file.


The table consists only the directly attached networks and
the hop counts, which are initialized to 1.


The next
-
hop field, which identifies the next router, is
empty.

7

Initial routing tables in a small autonomous system

8

Updating Routing Table for Router A


Each routing table is updated upon receipt of
RIP
messages

using the RIP updating algorithm.

9

Final routing tables

10

Example of updating a routing table

11

Example of a domain using RIP

Link State Routing


Each node in the domain has the entire topology of the domain.


Analogous to a city map.


Each node uses the same topology to create a routing table.


but the routing table for each node is unique because the calculations are
based on different interpretations of the topology.


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13

Building Routing table

1.

Creation link state packet (LSP)

2.
LSP is a tabular representation of the topology of
the internet

3.
Every router in the same area has the same LSP

4.
After a node has prepared an LSP , it must be
d
isseminated to all other nodes . The process is
called
Flooding

5.
Formation of shortest path tree for each node
(router) using
Dijkstra

algorithm

6.
Calculation of routing table.

14

Building Routing table


Creation
link state packet (LSP
):


LSP can carry a large amount of information such as : the node
identity, list of links, sequence number,…etc.


LSP are generated on two occasions:



When there is a
change in topology
of the domain. Triggering
of LSP dissemination to inform any node in the domain to
update its topology


On a periodic basis.
The period in this case is much longer
compared to distance vector routing
. The timer set for
periodically dissemination is in the range
1
or
2
h.


It done to ensure that the old LSP is removed


A longer period ensures that flooding
doesn’t create much
traffic on the network









15

Flooding

Building Routing table


The

creating

node

sends

a

copy

of

the

LSP

out

of

each

interface



A

node

receives

an

LSP

compares

it

with

the

copy

it

has
.

If

the

newly

arrived

is

older

than

one

it

has

,

it

discards


If

it

is

newer

the

node

do

the

following

:



It

discards

the

old

LSP

and

keep

the

new

one


It

sends

a

copy

of

it

out

of

each

interface

except

the

one

from

which

the

packet

arrived
.



Calculates the shortest path between two points on a network, using a
graph made up of nodes and edges.



Algorithm divides the nodes into two sets:
tentative and permanent
.
It chooses nodes, makes them tentative, examines them, and if they
pass the criteria, makes them permanent.

16

Dijkstra

Algorithm

Examples of formation of shortest path tree
using
Dijkstra algorithm

17

Example 1:

Examples of formation of shortest path tree
using
Dijkstra algorithm

18

Routing table for node A

Example 1:

Open Shortest Path First :
OSPF


OSPF Based on Link state Routing


OSPF divides an autonomous system into
areas.

19

Special routers called
autonomous system boundary routers

are
responsible for dissipating information about other autonomous
systems into the current system



Area is a collection of networks, hosts, and routers all
contained within an autonomous system.



Routers inside an area flood the area with routing
information.



Area border routers: Summarize the information about the
area and send it to other routers.



Backbone area
[Primary area]: All the areas inside an
autonomous system must be connected to the backbone.
Routers in this area are called as backbone routers. This area
identification number is
0
.



If, due to some problem, the connectivity between a
backbone and an area is broken,
a virtual link between routers
must

be created by the administration to allow continuity of
the functions of the backbone as the primary area.

20

Areas in an Autonomous System

OSPF



Each router should have the exact topology of the internet at
every moment.


From this topology, a router can calculate the shortest path between
itself and each network using
Dijkstra algorithm


Types of Links


21

1. Point
-
to
-
point


Connects two routers without any other router
or host in between.


Directly connected routers using serial line.


Only one neighbour.


No need to assign a network address to this
type of link


The metrics are the same at the two ends


22

Types of Links

2
. Transient link


A network with several routers attached to it.


Each router has many neighbours
.

23

Types of Links

3
. Stub

A network that is connected to only one router. The data
packets enter the network through this single router and
leave the network through this same router
.

4
. Virtual

When the link between two routers is broken, the administration
may create a virtual link between them, using a longer path that
probably goes through several routers.

24

Example of an internet & Graphical representation



Point to point
: between A_D and B
-
E




Stub
: N
5
, N
4
and N
2




Transient

: N
1
and N
3

25

Shortest
-
path calculation using
Dijkstra

Example
2
:

26

Shortest
-
path calculation using Dijkstra

Example 2:

27

Shortest
-
path calculation using Dijkstra

Example 2:

28

92
permanent

66
permanent

Routing Table for Router A

Shortest
-
path calculation using Dijkstra

Example 2:

29

Shortest
-
path calculation using
Dijkstra

Example
3
:

30

Shortest
-
path calculation

31

Shortest
-
path calculation

Network

Cost

Next
Router

N1

5

----

N2


7

C

N3

10

D

N4

11

B

N5

15

D

OSPF

NOTE
:



OSPF differs from RIP in that each router knows the exact
topology of the network




OSPF reduces
routing bandwidth
usage


OSPF is faster than RIP
.

32

Reaction to Failure

If a link fails,



Router sets link distance to infinity & floods the network

with an update packet




All routers immediately update their link database

(LSP)&

recalculate their shortest paths




Recovery very quick