4. Discuss the following with respect to Internetworking: o Internet ...

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4. Discuss the following with respect to Internetworking:

o

Internet architecture

o

Internet layering model

Internet services



Internet architecture :


The
Internet
is by definition a meta
-
network, a constantly changing collection of thousan
ds of
individual networks intercommunicating with a common protocol.

The Internet's architecture is described in its name, a short from of the compound word "inter
-
networking". This architecture is based in the very specification of the standard
TCP/IP
protocol,
designed to connect any two networks which may be very different in internal hardware, software,
and technical design. Once two networks are interconnected, communication with TCP/IP is
enabled

end
-
to
-
end
, so that any node on the Internet has the near magical ability to
communicate with any other no matter where they are. This openness of design has enabled the
Internet architecture to grow to a global scale.

In practic
e, the Internet technical architecture looks a bit like a multi
-
dimensional river system,
with small

tributaries

feeding medium
-
sized streams feeding large rivers. For example, an
individual'
s access to the Internet is often from home over a

modem
to a local Internet service
provider who connects to a regional network connected to a national network. At the office, a
desktop compu
ter might be connected to a local area network with a company connection to a
corporate Intranet connected to several national Internet service providers. In general, small local
Internet service providers connect to medium
-
sized regional networks which co
nnect to large
national networks, which then connect to very large bandwidth networks on the
Internet

backbone
. Most Internet service providers have several redundant network cross
-
connections to other providers in order to ensure continuo
us availability.

The companies running the Internet backbone operate very high bandwidth networks relied on by
governments, corporations, large organizations, and other Internet service providers. Their
technical infrastructure often includes global connec
tions through underwater cables
and

satellite

links to enable communication between countries and continents. As always, a
larger scale introduces new phenomena: the number of packets flowing thro
ugh the switches on
the backbone is so large that it exhibits the kind of complex non
-
linear patterns usually found in
natural, analog systems like the flow of water or development of the rings of Saturn (
RFC 3439
,
S2.2).

Each communication

packet

goes up the hierarchy of Internet networks as far as necessary to get
to its destination network where local

routing

takes over to deliver it to the addressee. In th
e
same way, each level in the hierarchy pays the next level for the bandwidth they use, and then
the large backbone companies settle up with each other. Bandwidth is priced by large Internet
service providers by several methods, such as at a fixed rate for

constant availability of a certain
number of megabits per second, or by a variety of use methods that amount to a cost per
gigabyte. Due to economies of scale and efficiencies in management, bandwidth cost drops
dramatically at the higher levels of the ar
chitecture.








Internet layering mode

Internet uses the TCP/IP reference model. The model is also called as Internet layering model or

internet reference model.

TCP/IP protocols are built on a simplified five
-
layer model (cf. Figure 1.1
) which was ada
pted to the
internetworking concept and architectural model.

The

physical layer

is the hardware connection between hosts on a given network.

The basic unit of communication is the

bit
.

The

network interface layer

puts the network hardware frames together,
and map IP addresses to
hardware addresses. It handles communications between machines on the same physical network. It is of
course hardware specific, and its characteristics and abilities differ widely between different implementations.

The basic unit of

communication is the

frame
.

The

internet layer

is the only internet specific layer. It insures communications between machines on
different physical networks, thus providing the layers above with a virtual network connecting all the hosts
which are actual
ly on the internet.

So the internet layer is the one handling all the complex routing decisions required by an internet.

The service provided is actually an unreliable, best effort, connectionless packet delivery system. Unreliable
because packet delivery
is not guaranteed. Connectionless because each packet is treated independently
from the others. Best effort since unreliability appears only because of exhaustion of resources or failure of
underlying networks. As we said before, the internet layer present
s the layers above the image of a packet
switching network, while dealing underneath with an internetwork.

The basic unit of communication is the
IP datagram
.

The

transport layer

provides end
-
to
-
end communication, i.e. communication between two application

programs. The characteristics of this layer depends much on the protocol used. The TCP/IP protocol suite
proposes two protocols with very different abilities at this level, TCP and UDP.

UDP provides unreliable, best effort, connectionless packet delivery
service. TCP provides reliable stream
delivery.

The basic unit of communication is the

UDP datagram

or the

TCP stream
.

The

application layer

consists of the application programs used to access services across the Internet (so
user's programs are usually on
e level above the application layer).

The basic unit of communication is a

message

or a

stream
.


Fig. 1.1: TCP/IP internet layering model


Internet Services :

Here we discuss the services that focus on standards and protocols. Protocols like TCP and IP
p
rovide the syntactic and semantic rules for communication. They contain the details of message
formats, describe how a computer responds when a message arrives, and also specify how a
computer handles errors or handles abnormal conditions. Here we discuss
few application
-
level
services as well as those define network level services.

Application level services


Internet appears to consist of a set of application programs that use the underlying network to
carryout useful communication tasks. Most users
access the internet by simple running application
programs without understanding the types of computers being accessed, the TCP/IP technology, even
the structure of the internet or even the path the data travels to its destination. The most popular and
wi
despread application services include

1.

World wide web:

The web allows users to view doucments that contain not only texts but graphics too. It allows
users to follow hypermedia links from one document to another. It has proved to be the largest
source of t
raffic on the global internet . Some service provider's estimates web accounts 80%
of the internet traffic.

2.

Electronic Mail :

Electronic mail
, often abbreviated as

e
-
mail

or

email
, is a method of exchanging digital messages, designed
primarily for human u
se. A message at least consists of its content, an author address and one or more recipient
addresses. The foundation for today's global Internet email service was created in the early
Arpanet

and was
codified as a standard for

encoding of messages, as

RFC 733
. An email sent in the early 1970s looked very
similar to one sent on the Internet today. Conversion from Arpanet to Internet in the early 1980s produced the
modern details
of the current, core service, with transport provided by the

Simple Mail Transfer
Protocol

(
SMTP
), first published as

Internet Standard

1
0 (
RFC 821
) in 1982, and a revision of

RFC
733

to be Internet Standard 11 (
RFC 822
). Multi
-
media content a
ttachments were standardized in 1996
with

RFC 2045

through

RFC 2049
, collectively called,

Multi
purpose Internet Mail
Extensions

(
MIME
).
Email systems that operate over a network (rather than being limited to a single, shared
machine) are based on a

store
-
and
-
forward

model in which em
ail computer server systems accept, forward,
deliver or store messages on behalf of users, who only need to connect to the email infrastructure with their
personal computer or other network
-
enabled device for the duration of message submission to, or retri
eval from,
their designated server. Rarely is email transmitted directly from one user's device to another's.

3.

File transfer

is a generic term for the act of

transmitting

files

over a

computer network

or the

Internet
.
There are numerous ways and

protocols

to transfer files over a network. Computers which

provide a file transfer
service are often called

file servers
. Depending on the

client
's perspective the data transfer is called

uplo
ading
or downloading
.


4.

Remote Login:

Remote login allows users sitting on one machine to connect remote machine and establish an
interactive login session. When remote login session terminates, the application returns the
user to the local system.

Networ
k level Services

Programmers who write network application programs need to view a TCP/IP internet as a network
and need to understand some of the network technology. At the network level, an internet provides two
broad types of service that all applicati
on program use.

1.

Connection less packet delivery service

This is the service that most packet switching networks offer. Here each packet is routed
separately. It does not guarantee reliable, in
-
order delivery.

2.

Reliable stream transport service:

Most appli
cations need much more than just packet delivery. Packets delivered should be free
from transmission errors. Recover from lost packets or failure of intermediate links/switches
along the path between the source and destination. This service allows an appli
cation on one
computer to establish a connection with an application on another computer and transfer large
volume of data across the connection as if it was permanent, hardware connection.


5. Discuss the following:

Subnet address extension


Variable len
gth subnets



Variable Length Subnetting
One of the original uses for subnetting was to subdivide a class
-
based network ID into a series of equal
-
sized subnets. For example, a 4
-
bit subnetting of a class B network ID produced 16 equal
-
sized subnets
(usin
g the all
-
ones and all
-
zeros subnets). However, subnetting is a general method of utilizing host bits
to express subnets and does not require equal
-
sized subnets.

Subnets of different size can exist within a class
-
based network ID. This is well
-
suited to r
eal
-
world environments,
where networks of an organization contain different numbers of hosts, and different
-
sized subnets are needed to
minimize the wasting of IP addresses. The creation and deployment of various
-
sized subnets of a network ID is
known as

v
ariable length subnetting

and uses variable length subnet masks (VLSM).

Variable length subnetting is a technique of allocating subnetted network IDs that use subnet masks of different
sizes. However, all subnetted network IDs are unique and can be disting
uished from each other by their
corresponding subnet mask.

The mechanics of variable length subnetting are essentially that of performing subnetting on a previously subnetted
network ID. When subnetting, the network ID bits are fixed and a certain number o
f host bits are chosen to express
subnets. With variable length subnetting, the network ID being subnetted has already been subnetted.

For example, given the class
-
based network ID of 135.41.0.0/16, a required configuration is one subnet with up to
32,000
hosts, 15 subnets with up to 2,000 hosts, and eight subnets with up to 250 hosts.

One Subnet with up to 32,000 Hosts

To achieve a requirement of one subnet with approximately 32,000 hosts, a 1
-
bit subnetting of the class
-
based
network ID of 135.41.0.0 is d
one, producing 2 subnets, 135.41.0.0/17 and 135.41.128.0/17. This subnetting
allows up to 32,766 hosts per subnet. 135.41.0.0/17 is chosen as the network ID, which fulfills the requirement.

Table 1.23 shows one subnet with up to 32,766 hosts per subnet.

Ta
ble 1.23 One Subnet with up to 32,766 Hosts

Subnet Number

Network ID (Dotted Decimal)

Network ID (Network Prefix)

1

135.41.0.0, 255.255.128.0

135.41.0.0/17


Fifteen Subnets with up to 2,000 Hosts

To achieve a requirement of 15 subnets with approximately
2,000 hosts, a 4
-
bit subnetting of the subnetted
network ID of 135.41.128.0/17 is done. This produces 16 subnets (135.41.128.0/21, 135.41.136.0/21 . . .
135.41.240.0/21, 135.41.248.0/21), allowing up to 2,046 hosts per subnet. The first 15 subnetted networ
k IDs
(135.41.128.0/21 to 135.41.240.0/21) are chosen as the network IDs, which fulfills the requirement.
6. Discuss the architecture and applications of E
-
mail.