Download Computer Network Unit-V

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UNIT-V
Computer Network
Transport Layer
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Transport Layer
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 Next Layer in OSI Model is recognized as Transport Layer
(Layer-4).
 This layer communicates with its peer Transport layer of
the remote host.
 Transport layer offers peer-to-peer and end-to-end
connection between two processes on remote hosts.
 Transport layer takes data from upper layer (i.e.
Application layer) and then breaks it into smaller size
segments, numbers each byte, and hands over to lower
layer (Network Layer) for delivery.
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Design Issues of Transport Layer
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 Addressing
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Every layer needs a mechanism to identify senders and
receivers.
 Error control
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It is an important issue because physical communication
circuits are not perfect.
 Flow control
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This property leads to mechanisms for disassembling,
transmitting and then reassembling messages.
 Multiplexing
 Demultiplexing
 Routing
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When there are multiple paths netween source and
destination a route must be chosen.
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User Datagram Protocol
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 UDP stands for “user datagram protocol” and it is
another protocol built on top of IP, just like TCP.
 With UDP we can send a packet to a destination IP
address (eg. 112.140.20.10) and port (say 52423), and it
will get passed from computer to computer until it arrives
at the destination computer or is lost along the way.
 On the receiver side, we just sit there listening on a
specific port (eg. 52423) and when a packet arrives from
any computer (remember there are no connections!),
we get notified of the address and port of the computer
that sent the packet, the size of the packet, and can
read the packet data.
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User Datagram Protocol (contd)
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 UDP is an unreliable protocol.
 In practice, most packets that are sent will get
through, but you’ll usually have around 1-5%
packet loss.
 There is also no guarantee of ordering of
packets.
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UDP vs TCP
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TCP
UDP
Connection based
No concept of connection, you have
to code this yourself
Guaranteed reliable and ordered
No guarantee of reliability or ordering
of packets, they may arrive out of
order, be duplicated, or not arrive at
all!
Automatically breaks up your data
into packets for you
You have to manually break your
data up into packets and send them
Makes sure it doesn’t send data too
fast for the internet connection to
handle (flow control)
You have to make sure you don’t
send data too fast for your internet
connection to handle
Easy to use, you just read and write
data like its a file
If a packet is lost, you need to devise
some way to detect this, and resend
that data if necessary
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UDP : Header Format
Size
Field Name
Description
(bytes)
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Source Port
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Source Port: The 16-bit port number of the process that originated the UDP
message on the source device.
Destination
Port
2
Destination Port: The 16-bit port number of the process that is the ultimate
intended recipient of the message on the destination device.
Length
2
Length: The length of the entire UDP datagram, including both header and
Data fields.
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Checksum: An optional 16-bit checksum computed over the entire UDP
datagram plus a special “pseudo header” of fields. See below for more
information.
Checksum
Data
Variable Data: The encapsulated higher-layer message to be sent.
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Transmission Control Protocol
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 TCP is a connection-oriented protocol.
 TCP (Transmission Control Protocol) is a standard that defines
how to establish and maintain a network conversation via
which application programs can exchange data.
 TCP works with the Internet Protocol (IP), which defines how
computers send packets of data to each other.
 It determines how to break application data into packets that
networks can deliver, sends packets to and accepts packets
from the network layer, manages flow control, and—because
it is meant to provide error-free data transmission—handles
retransmission of dropped or garbled packets as well as
acknowledgement of all packets that arrive.
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TCP Header Format
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Session Layer
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 In the Open Systems Interconnection (OSI) communications
model, the Session layer (sometimes called the "port layer")
manages the setting up and taking down of the association
between two communicating end points that is called a
connection.
 A connection is maintained while the two end points are
communicating back and forth in a conversation or session of
some duration.
 For Internet applications, each session is related to a
particular port.
 For example, the HTTP program or daemon always has port
number 80.
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Presentation Layer
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 The presentation layer is layer 6 of the 7-layer Open Systems
Interconnection (OSI) model.
 It is used to present data to the application layer (layer 7) in
an accurate, well-defined and standardized format.
 The presentation layer is sometimes called the syntax layer.
 The presentation layer is responsible for the following:
 Data encryption/decryption
 Character/string conversion
 Data compression
 Graphic handling
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Application Layer
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 The application layer is a layer in the Open Systems
Interconnection (OSI) seven-layer model and in the TCP/IP
protocol suite.
 It consists of protocols that focus on process-to-process
communication across an IP network and provides a firm
communication interface and end-user services.
 The application layer is the seventh layer of the OSI model
and the only one that directly interacts with the end user.
 The application layer provides full end-user access to a
variety of shared network services for efficient OSI model data
flow.
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Application Layer (contd)
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 The application layer provides many services,
including:
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
Simple Mail Transfer Protocol
File transfer
Web surfing
Web chat
Email clients
Network data sharing
Virtual terminals
Various file and data operations
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World Wide Web
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 The World Wide Web (www, W3) is an information
system of interlinked hypertext documents that are
accessed via the Internet.
 Individual document pages on the World Wide Web
are called web pages and are accessed with a
software application running on the user's
computer, commonly called a web browser.
 Web pages may contain text, images, videos, and
other multimedia components, as well as web
navigation features consisting of hyperlinks.
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HTTP
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 The Hypertext Transfer Protocol (HTTP) is an application
protocol for distributed, collaborative, hypermedia
information systems.
 Hypertext is structured text that uses logical links (hyperlinks)
between nodes containing text.
 HTTP is the protocol to exchange or transfer hypertext.
 HTTP functions as a request-response protocol in the client-
server computing model.
 A web browser, for example, may be the client and an
application running on a computer hosting a web site may be
the server.
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FTP
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 The File Transfer Protocol (FTP) is a standard network
protocol used to transfer computer files from one
host to another host over a TCP-based network,
such as the Internet.
 FTP is built on a client-server architecture and uses
separate control and data connections between
the client and the server.
 For secure transmission that protects the username
and password, and encrypts the content, FTP is
often secured with SSL/TLS (FTPS).
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SMTP
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 Simple Mail Transfer Protocol (SMTP) is an Internet standard for
electronic mail (e-mail) transmission.
 SMTP by default uses TCP port 25.
 SMTP connections secured by SSL, known as SMTPS, default to
port 465 (nonstandard, but sometimes used for legacy
reasons).
 Although electronic mail servers and other mail transfer
agents use SMTP to send and receive mail messages, userlevel client mail applications typically use SMTP only for
sending messages to a mail server for relaying. For receiving
messages, client applications usually use either POP3 or IMAP.
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SNMP
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 Simple Network Management Protocol (SNMP) is an
"Internet-standard protocol for managing devices
on IP networks".
 Devices that typically support SNMP include routers,
switches, servers, workstations, printers, modem
racks and more.
 SNMP is used mostly in network management
systems to monitor network-attached devices for
conditions that warrant administrative attention.
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