Net 13 14 Transport Layer Protocol

7/31/2019 Net 13 14 Transport Layer Protocol

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Ashis Talukder, Assistant Professor,MIS, DU.

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Lecture On

Transport LayerProtocols


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2

Last Class


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IP Address & IP Protocol:

Introduction: IP address

Subnet Mask

Classes of IP Address: A, B, C, D, E

IP Header format

ICPM


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Todays Class


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Transport Layer Protocols

TCP header

3-way handshaking

PAR

Denial of service attack (DoS)

Sequence Number

Windowing

Multiple Conversation

Socket

UDP


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Transport Layer

encapsulation

Segmentation andreassembly

connection control

Establish

Maintain

terminate

ordered delivery

flow control error control

Detection

recovery


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Transport Layer

Addressing

Port Address

Handle Multiple

Conversation Reliable transmission

Sendsacknowledgements

Also provideconnectionless streamdata transmission(UDP)


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Transport Layer Protocols

Transmission Control Protocol (TCP)

User Datagram Protocol (UDP)


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Transmission Control Protocol(TCP)


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Ashis Talukder, Assistant Professor,MIS, DU. 10

TCP Header


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Three Way handshaking

For a connection to beestablished, the two endstations must synchronize oneach other's initial TCPsequence numbers

Sequence numbers are used totrack the order of packets andto ensure that no packets arelost in transmission.

The initial sequence number isthe starting number used whena TCP connection isestablished.

Exchanging beginningsequence numbers during theconnection sequence ensuresthat lost data can berecovered.


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3wayHandshaking: 1st Step

In step one, the initiating host (client) sendssynchronization (SYN flag set) packet toinitiate a connection.

This indicates that a packet has a valid initial

Sequence Number value in this segment for thissession of x.

The SYN bit set in the header indicates aconnection request.

The SYN bit is single bit in the code field of theTCP segment header.

The Sequence Number is a 32 bit field TCPsegment header.


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3wayHandshaking: 2nd Step

In step two, the other host receivesthe packet, records the SequenceNumber of x from the client, andreplies with an acknowledgment (ACKflag set).

The ACK control bit set indicates thatthe Acknowledgment Number fieldcontains a valid acknowledgmentvalue.

The ACK flag is a single bit in the codefield of the TCP segment header and

the Acknowledgment Number is a 32bit field TCP segment header.

Once a connection is established, theACK flag is set for all segments duringthe session.


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3wayHandshaking: 2nd Step

The Acknowledgment Number fieldcontains the next sequencenumber that this host is expectingto receive (x + 1).

The Acknowledgment Number of x+ 1 means the host has receivedall bytes up to and including x,and expects to next receive byte x+ 1.

The host also initiates a return

session. This includes a TCPsegment with its own initialSequence Number value of y andwith the SYN flag set.


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3wayHandshaking: 3rd Step

In step three, theinitiating host respondswith a simpleAcknowledgment

Number value of y + 1,which is the SequenceNumber value of Host B+ 1.

This indicates that it

received the previousacknowledgment andfinalizes the connectionprocess for this session.


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Denial of Service Attack (DoS)

Denial of service (DoS) attacks is designed todeny services to legitimate hosts that attemptto establish connections.

DoS attacks are commonly used by hackers tohalt system responses.

One example is SYN flooding, which occursduring the three-way handshake process.


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Denial of Service Attack

As a packet with the SYN bitset is sent, it includes its IPaddress and the destinationIP address.

This information is then usedby the destination host tosend the SYN/ACK packetback.

In the DoS attack, the

hacker initiatessynchronization but spoofsthe source IP address.


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Denial of Service Attack The destination device responds

to a non-existent, unreachableIP address and is placed in awaiting state.

This waiting state is placed in aholding area that uses memory.

Hackers flood the host withthese false SYN requests todeplete all the connection andmemory resources of the host.

To defend against theseattacks:

system administrators maydecrease the connectiontimeout period and

increase the connectionqueue size.


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Windowing & Window Size

TCP segments the upperlayer Data

Once the data is segmented,it must be transmitted to the

destination device. One of the services provided

by TCP is flow control, whichregulates how much data issent during a given

transmission period. The process of flow control is

known as windowing.


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Windowing & Window Size Window size determines the

amount of data that can betransmitted at one time beforethe destination responds withan acknowledgment.

After a host transmits thewindow-sized number of bytes,the host must receive anacknowledgment that the datahas been received before it cansend any more data.

For example, if the window sizeis 1, each byte must beacknowledged before the nextbyte is sent.


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Windowing & Window Size TCP utilizes windowing to

dynamically determinetransmission size.

Devices negotiate a windowsize to allow a specificnumber of bytes to betransmitted before anacknowledgment.

This process of dynamicallyvarying the window size

increases reliability. The window size can be

varied based uponacknowledgments.


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Windowing & Window Size

TCP breaks data into segments.

After the synchronization processoccurs and the window size hasbeen established, the data

segments are transported fromthe sender to the receiver.

The data segments must bereassembled after all the data isreceived.

There is no guarantee that thedata will arrive in the order itwas transmitted.


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Windowing & Window Size TCP applies sequence numbers

to the data segments that aretransmitted so that the receivercan reassemble the bytes intheir original order.

This way, if TCP segments arriveout of order, the segments willstill be assembled correctly.

These sequencing numbers alsoact as reference numbers so

that the receiver will know if ithas received all of the data.

They also identify the missingdata pieces to the sender so itcan retransmit the missing data.


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Windowing & Window Size This offers increased efficiency

since the sender only needs toresend the missing segmentsinstead of the entire set of data.

Each TCP segment is numbered

before transmission. The sequence number portion

comes after the destination portin the segment format.

At the receiving station, TCPuses the sequence numbers to

reassemble the segments into acomplete message. If a sequence number is missing

in the series, that segment isretransmitted.


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UDP


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UDP UDP provides connectionless,

unreliable transmission of packetsat Layer 4 of the OSI model.

Both TCP and UDP use IP as theirLayer 3 protocol.

In addition, TCP and UDP are usedby various application layerprotocols.

TCP provides services forapplications such as FTP, HTTP,

SMTP, and DNS. UDP is the transport layer

protocol used by DNS, TFTP,SNMP, and DHCP.


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UDP TCP must be used

when applications need toguarantee that a packet arrivesintact, in sequence, andunduplicated.

The overhead necessary to ensuredelivery of a packet is sometimesa problem with TCP.

Not all applications need to guaranteedelivery of the data packet, so theyuse the faster, connectionless delivery

mechanism afforded by UDP.

UDP does not use windowing or ACKsso application layer protocols mustprovide error detection.


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Port Number

Port numbers are used to keep track ofdifferent conversations that cross the networkat the same time.

Port numbers are needed when a hostcommunicates with a server that uses multipleservices.

Both TCP and UDP use port numbers to pass

information to the upper layers. Software developers use the well-known port

numbers defined in RFC1700.


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Port Number

Conversations that do not involve applicationswith well-known port numbers are assignedport numbers that have been randomlyselected from a specific range.

Port numbers have the following ranges:

The Well Known Ports: from 0 through 1023

The Registered Ports: from 1024 through 49151 The Dynamic and/or Private Ports: from 49152 through 65535


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Multiple Conversation At any given moment, thousands of packets destined for

hundreds of different services travel through a network.

Servers provide services for a multitude of differentrequests. This causes unique problems for addressing ofpackets.

For example, if a server uses SMTP and WWW services, aclient cannot construct a packet that is destined for justthe IP address of the server with TCP because both SMTPand WWW use TCP as their transport layer protocol.

A port number must be associated with the conversation

to ensure that the packet reaches the appropriateservice.

Port numbers are used to keep track of differentconversations that cross the network at the same time.


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Multiple Conversation A port number must be associated with the conversation

to ensure that the packet reaches the appropriateservice.

The source and destination port numbers combinewith the network address to form a socket.

A pair of sockets, one on each host, forms a uniqueconnection. For instance, a host might have a Telnet connection

through port 23 and an Internet connection through port80.

The IP and the MAC addresses would be the same

because the packets are received from the same host. Therefore, each conversation on the source side needs

its own port number, and each service requested needsits own port number.


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Next Class


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IP Shortage & Subnetting

IP Shortage

Remedy of IP shortage

Subnetting

VLSM CIDR

Private & Public IP

NAT, PAT

Concept of Subnet Subnet Calculation


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Ashis Talukder, Assistant Professor,MIS DU

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Thank You!!!

Documents

Net 13 14 Transport Layer Protocol