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How Internet Works
EMC 165 Computer and
Communication Networks
Feb 3, 2004
Outline
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How Internet Instrastructure Works
How Routers Work
How TCP/IP networks work
How Routing Algorithms Work
How NAT works
What is the Internet?

It is a global collection of networks, both big and
small.

Recall in Lecture 2, we mentioned that one of the
greatest things about the Internet is that nobody
really owns it.

These networks connect together in many different
ways to form the single entity that we know as the
Internet. In fact, the very name comes from this idea
of interconnected networks.
The Internet Concept
The Internet Concept (Cont’d)
Internet: Network of Networks

Every computer that is connected to the Internet is
part of a network, even the one in your home.

For example, you may use a modem and dial a local
number to connect to an Internet Service Provider
(ISP).

At school/work, you may be part of a local area
network (LAN), but you most likely still connect to
the Internet using an ISP that your school/company
has contracted with.
Internet: Network of Networks (Cont’d)
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When you connect to your ISP, you become part of
their network.

The ISP may then connect to a larger network and
become part of their network.
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The Internet is simply a network of networks.
Connecting Network of Networks
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The amazing thing here is that there is no overall
controlling network.
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Instead, there are several high-level networks
connecting to each other through Network Access
Points or NAPs.

All the networks that make up the Internet rely on
NAPs, backbones and routers to talk to each other.
History of Internet
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1962, Paul Baran of the RAND Corporation was commissioned by the
US Air Force to do a study on how it could maintain its command and
control over its missiles and bombers, after a nuclear attack.
Baran’s final proposal was a packet switched network.
1968, Advanced Research Project Agency (ARPA) awarded the
APRPANET contract to BBN. The physical network was constructed in
1969, linking 4 nodes: UCLA, SRI (Stanford), UCSB, University of Utah
via 50 Kbps circuits.
The 1st email program was created by Ray Tomlison of BBN in 1972.
ARPA was later renamed the Defense Advanced Research Projects
Agency (DARPA) in 1972.
In 1973, developments began on the protocol later to be called TCP/IP
by a group headed by Vinton Cerf from Stanford and Bob Kahn from
DARPA.
The term Internet was coined by Vint Cerf and Bob Kahn in their paper
on TCP in 1974
History of Internet - contd
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Dr R. Metcalfe developed Ethernet in 1976,
which allowed coaxiable cable to move data
extremely fast.
Dept of Defense begain experimenting with
the TCP/IP protocol in 1976 and soon
decided to require it for use on ARPANET.
Total number of hosts on the backbones in
1976: 111+
History of Internet - contd
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National Science Foundation (NSF) created the 1st highspeed backbone in 1987 called the NSFNET.
NSFNET is a T1 line that connected 170 smaller networks
together and operated at 1.544 Mbps.
IBM, MCI, and Merit worked with NSF to create the backbone
and developed a T3 (45 Mbps) backbone the following year
Total number of hosts in the Internet: 56,000 in 1988. In 1990,
this number has jumped up to 313,000
In 1992, World-Wide Web was released by CERN. NSFNET
backbone completely upgraded to T3.
Total number of hosts in the Internet in 1992 – 1.136 millions
History of Internet - contd
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In 1994, ATM (145 Mbps) backbone is
installed on NSFNET.
Total number of hosts has increased to 3.864
millions in 1994
Most Internet traffic is carried by backbones
of independent ISPs including MCI, AT&T,
Sprint, UUNet, BBN planet etc.
The total number of hosts in 1999 was
around 15 millions and growing rapidly.
Backbones
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Backbones are typically fiber optic trunk lines
The trunk line has multiple fiber optic cables
combined together to increase the capacity.
Fiber optic cables are designated OC for
optical carrier such as OC-3, OC-12 or OC48.
An OC-3 line is capable of transmitting 155
Mbps while an OC-48 can transmit 2,488
Mbps (2.488 Gbps).
Logical addresses
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Every piece of equipment that connects to a network
has a physical address. This is an address unique to
the piece of equipment.
The physical address is also called the Medium
Access Control (MAC) address. It has 2 parts each
3 bytes long. The 1st 3 bytes identify the company
that made the Network Interface Card (NIC), and the
2nd 3 bytes are the serial number of the NIC itself.
The interesting thing to note is a computer can have
several logical addresses at the same time.
Logical addresses like IP address are assigned
statically or dynamically.
Internet Protocol: IP addresses
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Every machine on the Internet has a unique underlying number, called
an IP address.
The IP stands for Internet Protocol which is the language that
computers use to communicate over the Internet.
A protocol is a pre-defined way that someone who wants to use a
service talks with that service. That someone could be a person, but
more often it is a computer program like a Web-browser.
A typical IP address looks like this
216.27.61.137
The four numbers in an IP address are called octets, because they each have
eight bits.
Each octet can contain any value between zero and 255.
So, combining 4 octets give us 232 possible unique values.
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Certain values are restricted from use as typical IP addresses e.g.
0.0.0.0 is reserved for the default network and 255.255.255.255 is
reserved for broadcasts
How TCP/IP network works.
IPv4 Header
0
31
Version HLength
Type of Service
Identification
Time-to-Live
Total Length
Flags
(Next) Protocol
Fragment Offset
Header Checksum
Source Address
Destination Address
IP Options
Data
Payload up to 65,535 bytes
IP Addresses - Motivation
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Key aspect of a virtual network is a single, uniform
address format
Can't use hardware addresses because different
technologies have different address formats
 Format must be independent of any particular
hardware address format
Sending host puts destination internet address in
packet
Destination address can be interpreted by any
intermediate router
 Routers examine address and forward packet on
to the destination
Classfull Addresses
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Properties
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32-bit number
globally unique (with a few exceptions!)
hierarchical: network + host
Classes of addresses for specific types of networks
Classfull Addresses
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Generally assigned by authorities
except from:
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A-class net: 10.0.0.0
B-class net 172.16.0.0
C-class net 192.16.8.0
Some college have a B-class net e.g.134.226.0.0

Can arrange for Dept. of Comp. Science. to have a
number of subnets in this domain e.g. 134.226.32.0,
134.226.51.0
Summary
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Virtual network needs uniform addressing
scheme, independent of hardware
IP address is a 32-bit address
IP address is composed of a network address
and a host address
Network addresses are divided into classes e.g.
A, B and C
Dotted decimal notation is a standard format for
Internet addresses: 134.226.32.57
IP Address & Ethernet Address
Computer B
IP Address
Computer A
172.16.1.1
00-08-74-32-24-89
MAC Address
Computer X
Computer Y
Address Resolution Protocol
(ARP)
for
Computer B
Computer A
Computer B
172.16.1.2
MAC ADDRESS???
172.16.1.1
00-08-74-32-24-89
Computer X
Computer Y
ARP’s “Who has…?” Packet
Who has
172.16.1.2
Computer B
Computer A
172.16.1.1
00-08-74-32-24-89
Computer X
Computer Y
ARP’s Reply Packet172.16.1.2
00-08-7421-20-D7
Computer A
Computer B
172.16.1.2
00-08-74-21-20-D7
172.16.1.1
00-08-74-32-24-89
Computer X
Computer Y
Routed (Sub-)Networks
Router
Packet Size Matters!!!
7000
Packet Size=
7000 bytes
Network-specific MTU*
*Maximum Transfer Unit
Fragmentation
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One technique - limit datagram size to smallest MTU
of any network
However: This approach requires knowledge about
all networks involved in communication
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IP uses fragmentation - datagrams can be split into
pieces to fit in network with small MTU
Router detects datagram larger than network MTU
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Splits into pieces
Each piece smaller than outbound network MTU
Fragmentation (details)
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Each fragment is an independent datagram
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Includes all header fields
Bit in header indicates datagram is a fragment
Other fields have information for reconstructing original datagram
FRAGMENT OFFSET gives original location of fragment
Router uses local MTU to compute size of each fragment
Puts part of data from original datagram in each fragment
Puts other information into header
Fields for Fragmentation
0
31
Version HLength
Type of Service
Identification
Time-to-Live
Total Length
Flags
(Next) Protocol
Fragment Offset
Header Checksum
Source Address
Destination Address
IP Options
Data
Payload up to 65,535 bytes
Ethernet to Tokenring
Tokenring to Ethernet
Tokenring to Ethernet
Fragmentation & Reassembly
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Each network has a Maximum Transmission Unit (MTU)
IP datagrams can be larger than most hardware MTUs
IP: 216 - 1
 Ethernet: 1500
 Token ring: 2048 or 4096
Strategy
 fragment when necessary (Datagram > MTU)
 try to avoid fragmentation at source host
 re-fragmentation is possible
 fragments are self-contained datagrams
 delay reassembly until destination host
 do not recover from lost fragments
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Fragment Loss
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IP may drop fragment
What happens to original datagram?
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How does destination identify lost fragment?
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Destination drops entire original datagram
Sets timer with each fragment
If timer expires before all fragments arrive, fragment
assumed lost
Datagram dropped
Source (application layer protocol) assumed to
retransmit
Best Effort Delivery
Internet Protocol: Domain Name System
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If there are only a few hosts, then working with IP
addresses is fine but with more and more hosts that
came online, it becomes unwieldly.
The first solution is a simple text file maintained by
the Network Information Center that mapped names
to IP addresses.
But soon this text file became so large that it was
too cumbersome to manage.
So, in 1983, University of Wisconsin created the
Domain Name System which maps a hostname to
an IP address automatically.
Uniform Resource Locators
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When you use the Web or send an email message, you use a
domain name to do it. For example, the URL
http://www.howstuffworks.com contains the domain name
howstuffworks.com. So does the email address:
[email protected].
Everytime we use a domain name, we use the Internet’s DNS
servers to translate the human-readable domain name into the
machine-readable IP address.
Top-level domain names include .com, .org, .net, .edu, .gov.
Within every top-level domain, there is a huge list of 2nd-level
domains. For example, in the .com 1st-level domain, there is
 Yahoo
 Microsoft
 Amazon
Every name in the .com top-level domain must be unique.
Internet Naming Hierarchy
The silent dot at the
end of all addresses
.com
.net
.ie
.tcd
www
.uk
.ac
.co
How to find www.cse.lehigh.edu?
Domains
edu
Name server
in Berkeley, CA
lehigh
1. Ask top-level server
for edu-server
cse
www
2. Ask .edu server
for lehigh-server
3. Ask .lehigh server
for cse-server
DNS
server
4. Ask .cse server
for “www” machine
134.226.32.57
DNS
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DNS servers accept requests from programs, and
other name servers, to convert domain names into
IP addresses. When a request comes in, the DNS
server can do one of the 4 things with it:
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It can answer the request with an IP address because it
already knows the IP address for the requested domain.
It can contact another DNS server and try to find the IP
address for the name requested. It may have to do this
multiple times
It can say, “I don’t know the IP address for the domain but
here’s the IP address for a DNS server that knows more
than I do”
It can return an error message because the requested
domain name is invalid or does not exist.
Name Server Architecture
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Name agent (Resolver)
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Interface with the local user programs
Identifies objects based on symbolic names
Name server
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Converts symbolic names to addresses
Queries other name servers if the name is
unknown
Recursive Name Server
Name server
Name server
Name server
Name server
Name agent
Iterative Name Server
Name server
Name server
Name server
Name server
Name agent
Transitive Name Server
Name server
Name server
Name server
Name server
Name agent
Domain Name System (DNS)
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Name server
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Serves a hierarchical name space
Maps names to addresses
Stores auxiliary information
Authoritative name server
 Mail exchanger
 Round robin (load balancing)
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Putting
it
together
Computer B in Berkeley, CS wants to
uses a DNS query to
find out what IP address
find a web page at
www.cse.lehigh.edu has
“www.cse.lehigh.edu”
Computer B
knows that
knows about has an
has an
the Dept. of agreement agreement 134.226.0.0
www
Comp. Sc. with Lehigh with AT&T is routed into
direction of
east coast
cse.lehigh.edu
Router
in CSE
Router in Router
at AT&T
Lehigh
knows that 134.226.32.57 is on
the local ethernet and uses ARP
to get its ethernet address
Router
Router at
in New
Berkeley
York
replies with
134.226.36.57
Berkeley DNS
Best-Effort Delivery
D1
D2
• Transfer of datagrams D1 & D2
• Possible deliveries:
D2 D1
D1 D2
D1
D2
nothing
How Routers Work
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Assume that there is a small company with 10 employees, each
with a computer. 4 of the employees are animators, while the rest
are in sales, accounting and management.
The animators send many very large files back and forth to one
another. To do this, they will need a network.
When one animator sends a file to another, every one sees the
traffic if the network used is Ethernet. Each computer checks to
see if the packet is meant for its address. But since the file is big,
this makes the network run very slowly for other users.
So, to keep the animators’ work from interfering with others, the
company sets up 2 separate networks, one for the animators and
one for the rest of the company. A router links the two networks
and connects both networks to the Internet.
How Routers Work - contd
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Router is the only device that sees every message sent by any
computer.
When the animator sends a huge file to another animator, the
router looks at the recipient’s address and keeps the traffic on the
animators’ network.
When the animator sends a message to the bookeeper, the
router sees the recipient’s address and forwards the message
between the two networks.
One of the tools a router uses to decide where to forward a
packet is a configuration table. Such a table contains the
following information
 Information on which connections lead to particular groups of
addresses
 Priorities for connections to be used
 Rules for handling both routine and special cases of traffic.
How Routers Work – contd
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A router has 2 separate but related tasks
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It ensures that information does not go where it is not
needed.
It makes sure that information does make it to the intended
destination.
As the number of networks attached to one another
grows, the configuration table for handling traffic
among them grows, and the processing power of
the router is increased.
Recall that the Internet is a packet-switched network
which means each packet may take a different route
to reach its destination. Each packet contains a
header that tells its source and destination address.
Routing packets: An example
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Consider a medium-sized router in a company’s office network
with 50 computers and devices and the Internet.
The office network connects to the router through an Ethernet
connection (e.g. 100 base-T connection meaning 100 Mbps).
There are 2 connections between the router and the ISP. One is
a T1 connection (1.5 Mbps) and the other is an ISDN line (128
Kbps).
The configuration table tells it that all out-bound packets are to
use the T1 line, unless it is not available. If T1 is not available,
then the ISDN line will be used.
The router also has rules limiting how computers from outside
the network can connect to computers inside the network and
how the office network appears to the outside world, and other
security functions.
Routing packets: An example
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One of the crucial tasks for any router is knowing when a packet
of information stays on the local network.
For this, a router uses a mechanism called a subnet mask.
The subnet mask looks like an IP address but usually reads
“255.255.255.0”. This tells the router all the messages with the
sender and receiver having an address sharing the 1st 3 groups
of numbers are on the same network, and shouldn’t be sent to
another network.
Here is an example: The computer at address 15.57.31.40 sends
a request to the computer at 15.57.31.52. The router, which sees
all packets, matches the 1st 3 groups in the address of both
sender and receiver (15.57.31), and keeps the packet on the
local network.
Service Model
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Connectionless (datagram-based)
Best-effort delivery (unreliable service)
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packets may be lost
packets may be delivered out of order
duplicate copies of a packet may be delivered
packets can be delayed for a long time
Datagram format
0
4
Version
8
HLen
16
TOS
31
Length
Ident
TTL
19
Flags
Protocol
Offset
Checksum
SourceAddr
DestinationAddr
Options (variable)
Data
Pad
(variable)
Basics of Routing Algorithms
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Routers use routing algorithms to find the best route to
the destination.
What does it mean by best route?
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Based on some metrics e.g. the number of hops, time
delay and communication cost of packet transmission
Two categories of routing algorithms
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Global routing algorithms
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Every router has complete info about all other routers in the
network and the traffic status of the network
Sometimes known as link state (LS) algorithms.
Decentralized routing algorithms
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Every router has information about the routers it is directly
connected to.
Sometimes known as distance vector (DV) algorithms.
LS Algorithms
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Every router follows the following steps

Identify the routers that are physically connected to them and get their IP
addresses. When a router starts working, it first sends a “hello” packet over
network. Each router that receives the packet replies with a message that
contains its IP address

Measure the delay time for neighbor routers. In order to do that, routers send
echo packets over the network. Every router that receives these packets replies
with an echo reply packet. By dividing round trip time by 2, routers can count the
delay time. This time includes both transmission and processing times – the time
it takes the packets to reach the destination and the time it takes the receiver to
process it and reply.

Broadcast its information over the network for other routers and receive the other
routers’ information.

Using an appropriate algorithm, identify the best router between 2 nodes of the
network. A well known algorithm is called the Dijkstra shortest path algorithm. In
this algorithm, a router based on the information that has been collected from
other routers, builds a graph of the network. This graph shows the location of
routers in the network and their links to each other. Every link is labeled with a
number called the weight or cost. This number is a function of delay time,
average traffic, and sometimes simply the number of hops between nodes. The
router chooses the link with the lowest weight.
Example: Dijkstra Algorithm
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Let’s try to find the best route between A and E.
There are 6 possible routes between A and E
(ABE,ACE,ABDE,ACDE,ABDCE,ACDBE).
Let start from A.
Example: Dijkstra Algorithm
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A know that it is directly linked to (B,C). Since B has less
weight, it has been chosen as the next hop.
The status record set of tentative nodes that have a
direct link to B is (D,E). Since D has less weight, it has
been chosen as the next hop.
Example: Dijkstra Algorithm

At D, there is only one node E but it is also the destination. So, we find the route ABDE.

In general, each router builds a graph of the network and identifies the source and destination
nodes.
Then, it builds a matrix, called the adjacency matrix. In this matrix, a coordinate indicates weight.
For example, [I,j] is the weight of a link between Vi and Vj. If there is no direct link between Vi and
Vj, then the weight is set to infinity
The router builds a status record set for every node on the network. The record contains 3 fields:
predecessor, length, label field. The length shows the sum of the weights from the source to that
node. The label shows the status of node – whether it is permanent or tentative.
The router initializes the parameters of the status record set (for all nodes) and sets their length to
infinity and label to tentative.
If a node is picked as the next hop,its label is changed to permanent and update its length.
The router updates the status record set for all tentative nodes that are directly linked to the next
hop once this next hop node is identified and repeat the procedure until the destination node is
reached.

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Distance Vector (DV) algorithms
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DV algorithms are also known as Bellman-Ford routing
algorithms and Ford-Fulkerson routing algorithms.
Every router has a routing table that shows the best
route for any destination. A typical graph and routing
table for router J is shown in the next slide.
DV algorithms
Destination
Weight
Line
A
8
A
B
20
A
C
28
I
D
20
H
E
17
I
F
30
I
G
18
H
H
12
H
I
10
I
J
0
---
K
6
K
L
15
K
Routing Table for
Router J
DV algorithms – contd

In DV algorithms, each router has to follow
these steps



It counts the weight of the links directly connected
to it and saves the information to its table
In a specific period of time, it sends its table to its
neighbor routers and receive the routing table of
each of its neighbors
Based on the info in its neighbors’ routing tables,
it updates its own.
Counting to Infinity Problem



A
B
C
D
A
0,-
1,A
2,B
3,C
B
1,B
0,-
2,C
3,D
C
2,B
1,C
0,-
1,C
D
3,B
2,C
1,D
0,-
Imagine that the link between A and B is cut.
B corrects its table. After a specific amount of time, routers
exchange their tables, and so B receives C’s routing table.
Since C doesn’t know what has happened to the link between A
and B, it says that it has a link to A with the weight of 2. B thinks
that there is a separate link between C and A, so it corrects its
table and changes infinity to 3.
Counting to Infinity Problem – contd

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
Once again the routers exchange their tables.
When C receives B’s routing table, it sees that B has changed the
weight of its link to A from 1 to 3, so C updates its table and change
the weight of the link to A to 4.
This process loops until all nodes find out that the weight of link to A
is infinity. This situation shows that DV algorithms have a slow
convergence rate.
One way to solve this problem is for routers to send information only
to neighbors that are not exclusive links to the destination. For
example, C shouldn’t send any information to B about A because B
is the only way to A.
Hierarchical Routing

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

When the network size grows, the number of
routers in the network increases.
The size of routing tables increases as well,
and routers can’t handle network traffic
efficiently.
Thus, hierarchical routing is used to
overcome this scaling problem.
Let us see an example
Hierarchical Routing Example

If we use DV algorithms to find best routes between
nodes, each node has to save a routing table with 17
records.
Hierarchical Routing Example
Destination
Line
Weight
A
---
---
B
B
1
C
C
1
D
B
2
E
B
3
F
B
3
G
B
4
H
B
5
I
C
5
J
C
6
K
C
5
L
C
4
M
C
4
N
C
3
O
C
4
P
C
2
Q
C
3
Node A’s routing
table
Hierarchical Routing Example



In hierarchical routing, routes are classified in groups known as
regions. Each router has only the information about the routers in its
own region and has no information about routers in other regions.
So routers just save one record in their table for every other region.
In the example before, we have 5 regions.
Hierarchical Routing
Hierarchical Routing
Destination
Line
Weight
A
---
---
B
B
1
C
C
1
Region 2
B
2
Region 3
C
2
Region 4
C
3
Region 5
C
4
Routing Table
At Node A
References



How Internet Infrastructure works at
www.howstuffwork.com
How routing algorithms work at
www.howstuffwork.com
How routers work at www.howstuffwork.com