(4.4) Internet Protocols Layered approach to Internet Software
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Internet Protocols In this section we investigate how messages
are transferred over the Internet. 2
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4-3 Figure 4.12 Package-shipping example
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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
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4-5 Figure 4.13 The Internet software layers
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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
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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
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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
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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 messages destination. 9
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Transport layer: Then it attaches the destination address to
each segment and hands these address segments, known as packets, to
the network layer. 10
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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
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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
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Network layer: The network layer appends this address to the
packet as an intermediate address and hands the packet to the link
layer. 13
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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
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Note : Only the link layer and network layer are involved at
intermediate stops. 15
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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
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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 messages origin.
Ones the message is assembled, transport layer hands it to the
appropriate unit within the application layer. 17
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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 messages address before starting the transmission.
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Port number: For example: If web browser asked to retrieve the
document whose URL is http://www.zoo.org/animals/frog.html
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.www.zoo.org But FTP client should communicate with the FTP
server through port number 20 and 21 19
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4-20 Figure 4.14 Following a message through the Internet
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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
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4-22 Where TCP/IP and UDP Transport Layer TCP UDP (User
Datagram Protocol) Network Layer IP: (IPv4) use 32 bits (IPv6) use
128 bits
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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 layers message.
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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.
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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.
4-28 Encryption FTPS, HTTPS, SSL Public-key Encryption Public
key: Used to encrypt messages Private key: Used to decrypt messages
Certificates and Digital Signatures