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Ethical hacking simplified (System fundamentals) OSI reference model continuation

 

This article is continuation.... For Application layer to session layer click here 

Transport layer: I know you have start guessing what this layer will be like! Since any thing transport involves movement of “something” from one form to another. Transport layer ensures data is been transported successfully between source and the destination. To complete it task it needs to check errors, data dropping etc.
Transport layer uses TCP (transmission control protocol) and UDP (user datagram protocol). The first is connection oriented and the later is connectionless.

TCP/IP (connection oriented): This makes sure data reached to its final destination without dropping. It uses flow control, connection oriented communication windowing and acknowledgment to make sure communication is successful.

Flow control: This is process of controlling the flow of data between the sending end and the receiving end. It prevents the sender to send too much data that the receiver cannot handle. For example, compare rain and tap water, when you want to get water from the tap you can off it when your bucket is full thus no water will drop to the ground due to overload. What of the rain, when you cant control the water flow from the rain thus making must of the water goes down after your bucket is full! You can agree with me the amount of water coming from the sky is just too much but large percentage of gone! Imagine flow control exist there wow!!! no water scarcity forever!!! but we cant collect the water all because it just come down in its full force. Tap water is limited but we can utilize it using flow control, thus reducing the water lose to its minimal. To make connection oriented communication successful data are broken down into segments(smaller pieces) to ease transportation. One of the main purpose of flaw control is to control how the receiver receives data without overloading. The below rules are applied for successful data communication:
  • The receiver sends an acknowledgment upon the receiving of data.
  • If the sender didn't receive acknowledgment of a packet it resend that segments.
  • Upon arrival at the receiving end, all segments are regrouped accordingly.
  • A reasonable data flow is maintained to avoid overloading which will leads to data type.

Connection oriented communication: A device that wants to transmit data setup a connection with the receiving end by creating a session. It established a peer system called three way handshake. After transmission is successful the session is then tear down.
Three way handshake.
Sender synchronize
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

sender and the receiver negotiates connection
<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>

receiver synchronize
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

|connection established|

Data transfer begins
<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>

In the three way handshake, example you want to call your sweetheart on phone. The phone is ringing dindindin (sender synchronize), when he or she pick the call(sender and receiver negotiates connection), He or She will say in a lovely voice ”hello darling” (receiver synchronize) and (connection is already established) then data transfer begins “....My love I couldn’t sleep because I'm missing U.... blablabla” lol! It seems flawless right? Problems do arise sometimes, a machine can receives many connections at the same time. What it does it keep other requests in a buffer while handling the other(so nice). What will happen if the buffer is full? (no place keep it request? I guess it will drop the excess) if that is it, then what is the essence of calling it reliable connection? Big grin! A system exist that will handle such conditions, tongue out. Its called flood control system. This system sends out not ready signal the sender when the buffer is full! Thus the sender has to be on red light, it will only send data after it sees the green light. Interesting........

Characteristics of connection oriented service
  • Virtual circuit eg. Three way handshake
  • Sequencing
  • Acknowledgment
  • Flow control

Windowing: This sound simple and clear. So in TCP/IP window exist? Windowing is act of creating window! As simple as that! Laughing in my native language! Is not true! Windowing means contrary to what you think! Free your mind! Data movement Rx and Tx (Reception and transmission) happens too fast! But don't forget connection oriented(reliable connection) must receive acknowledgment before sending another segment, as you can see there is an existing idle time, which the system has to wait for between acknowledgment! With windowing the system will send data between that waiting time! The amount of data(measured in byte) that the sending machine is allowed to send in that waiting time without waiting for acknowledgment is known as window! Look alike with what you thought? NO!!! tongue out!!!
window size: this defines the amount of data the transmission machine is allowed to send without waiting for acknowledgment! If for example you configure window size of 3, the transmission machine can send 3bytes of data without waiting for acknowledgment, after that it has to receive acknowledgment before it can send again.

Sending (transmission) Machine Receiving (reception) Machine

window size one

send 1 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> receive 1

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Acknowledge 2

send 2 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Receive 2

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<Acknowledge

Window size 3

send 1 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Send 2 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Send 3 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Acknowledge 4

send 4 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Send 5 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Send 6 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Acknowledge 7


As you can if the window size is one, it send one segment of data and wait for acknowledgment before resending, if the size is three it send three segments of data and wait for acknowledgment before resending. So its not act of creating window! Its rather act of creating a means of sending segment of data when the receiving machine is processing acknowledgment, in other to maximize speed and save time.

Acknowledgment: This is the positive response a system gets when transmitting data segments. When connection is established and data flow is going on, the transmission machine creates a timer in which it will receive acknowledgment from the receiving end, if the timer expires without acknowledgment, that means the transmission wasn't successful, the same segment of data is resend. If it receives acknowledgment the next segment (not the sent one) is transmitted. This really help successful data transmission.

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Next is network layer.. to be continued....
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