Network

slateobservantNetworking and Communications

Oct 26, 2013 (3 years and 8 months ago)

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(
4.4
) Internet Protocols


Layered approach to Internet Software

1

Internet Protocols


In

this

section

we

investigate

how

messages

are

transferred

over

the

Internet
.


2

4
-
3

Figure
4.12
Package
-
shipping example

4
-
4

Internet Software Layers


Application
:

Constructs

message

with

address


Transport
:

Chops

message

into

packets


Network
:

Handles

routing

through

the

Internet


Link
:

Handles

actual

transmission

of

packets


4
-
5

Figure
4.13
The Internet

software layers

Application layer:


The application layer consists of those
software units such as clients and servers
that use Internet Communication to carry
out there tasks.


The application layer uses transport layer to
sends and receives messages over the
Internet .


The responsibility of application layer is to
provide an address that is compatible with
the transport layer.

6

Application layer:


Application layer use the services of the
name servers within the Internet to
translate mnemonic addresses used by
humans into Internet
-
compatible.


7

Transport layer:


The major task of transport layer is to
accept messages from the application
layer and to ensure that the messages are
properly formatted for transmission over
the Internet.


Transport layer divides long messages into
small segments, which are transmitted
over the Internet as individual units.



8

Transport layer:


This division is necessary because a single
long message can obstruct the flow of other
messages at the points on the Internet
where numerous messages must cross paths.


Small segments are interweave at these
points, whereas a long message forces others
to wait while it passes.


Transport layer adds sequence numbers to
the small segments it produces so that, the
segments can be reassembled at the
message’s destination.



9

Transport layer:


Then it attaches the destination address
to each segment and hands these address
segments, known as
packets
, to the
network layer.


10

Network layer:


Has the task for forwarding the packets it
receives from one network within the
Internet to another until they reach their
final destinations.


Thus, network layer that must deal with
the Internet topology.



11

Network layer:


The decision is made a follows:


If the final destination of the packet is
within the current network, the network
layer will send the packet there.


Otherwise, the network layer will send
the packet to a router in the current
network through which the packet can be
transferred into an adjacent network.


12

Network layer:


The network layer appends this address
to the packet as an intermediate address
and hands the packet to the link layer.

13

Link layer:


The link layer has a responsibility of transferring the
packet to the intermediate address that was
determined by the network layer.


Thus the link layer must deal with the communication
details particular to the individual network in which
the computer resides.


If that network is a token ring, the link layer must be
wait for possession of the token before transmitting.



if the network uses CSMA/CD, the link layer must
listen for a silent bus before transmitting.

14

Note :


Only the link layer and network layer are
involved at intermediate stops.

15

In the opposite side :


When the packet transmitted, it is
received by the link layer at the computer
designated by the local address attached
to the message.


Then hand it to its network layer where
the packet final destination is compared
to the current location.


Then it hands to its transport layer.


16

In the opposite side :


As transport layer receives packets from the
network layer, it extracts the underlying
message segments and reconstructs the
original message according to the sequence
numbers that were provided by the
transport layer at the message’s origin.


Ones the message is assembled, transport
layer hands it to the appropriate unit within
the application layer.

17

Port number:


The units within the application layer
should receive an incoming message is an
important task of the transport layer.


This is handle by assigning unique port
numbers to the various units and
requiring that the appropriate port
number be appended to a message’s
address before starting the transmission.

18

Port number:


For example:


If web browser asked to retrieve the
document whose URL is
http://www.zoo.org/animals/frog.html


The browser assume that it should
contact the HTTP server at
www.zoo.org

via port number
80
.


But FTP client should communicate with
the FTP server through port number
20
and
21

19

4
-
20

Figure
4.14
Following a message
through the Internet

4
-
21

TCP/IP Protocol Suite


Is a collection of protocols used by the
Internet to implement the four level
communication

hierarchy implemented in
the Internet.


TCP/IP

is for Transmission Control
Protocol/ Internet Protocol

4
-
22

Where TCP/IP and UDP


Transport Layer


TCP


UDP (User Datagram Protocol)


Network Layer IP:


(IPv
4
) use
32
bits


(IPv
6
) use
128
bits

4
-
23

The differences between TCP and
UDP


The first one:



before sending the message as requested by the
application layer, a transport layer based on TCP
sends its own message to the transport layer at
the destination telling it that a message is about to
be sent.



It then be waits for this message to be
acknowledged before starting to sent the
application layer’s message.

4
-
24

The differences between TCP and
UDP


For this manner,


TCP transport layer is said to establish a
connection before sending a message.


But a transport layer based on UDB does not
establish such a connection prior to sending a
message. Just send a message to the address and
forget about it.


UDB is called a connectionless protocol.

4
-
25

The differences between TCP and
UDP


The second:


TCP transport layers at the origin and destination
work together by means of acknowledgments and
packet retransmissions to confirm that all
segments of a message are
successfully transferred
to the destination.


TCP called a reliable protocol.


But UDP is unreliable protocol. Because it does
not offer such retransmission services.

4
-
26

Figure
4.15
Choosing between TCP
and UDP

4
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27

Security


Attacks


Malware (viruses, worms, Trojan horses, spyware,
phishing software)


Denial of service


Spam


Protection


Firewalls


Spam filters


Proxy Servers


Antivirus software

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-
28

Encryption


FTPS, HTTPS, SSL


Public
-
key Encryption


Public key: Used to encrypt messages


Private key: Used to decrypt messages


Certificates and Digital Signatures

4
-
29

Figure
4.16
Public
-
key encryption