 Welcome to the session on TCP timers. In that first we will discuss in this video about the first TCP timer that is a retransmission timer. Learning outcome at the end of the session students will be able to describe the concept of TCP timers and they are also able to find the value of RTOs about content about TCP timer types of TCP timers. TCP timers, these topics will come under TCP protocol. In this there are different timers are there, we will study about the different timers. First to perform its operation smoothly, most TCP implementation uses at least four different types of timers. First one is retransmission timer, second one is a persistent timer, third one is a keep alive timer and fourth one is a time and wait timer. These are the different timers are there. In this video we will discuss about the first TCP timer that is retransmission timer. Retransmission timer, to retransmit loss segment in TCP, TCP employs one retransmission timer that handles the retransmission timeout. In upcoming slides we will use the RTO. What is RTO? It means retransmission timeout. The waiting time for an acknowledgement of a segment is an RTO. To perform this one we have to use some different rules that are first one is a when TCP sends a segment in front of the sending queue it starts the timer, when the second rule is when the timer expires TCP resends the first segment in front of the queue and restart the timer. When a segments are cumulatively acknowledged the segments are purged from the queue. Last one is if the queue is emptying the TCP start stop the timer otherwise it can TCP restart the timer. These are the different rules how to use in a retransmission of the timer. In that round trip time that is RTT, RTT is a round trip time to calculate the RTO that is a retransmission timeout we first need to calculate the RTT that is round trip time. However the calculating RTT in TCP is an involved process that we explain step by step with the help of some examples. Now, next point is a major RTT, RTT is a retransmission round trip time in that RTT is a round trip time RTTM indicates a major round trip time. In TCP there can be only one RTT measurement in progress at any time. If we need to find how long it takes to send a segment and receive an acknowledgement for it this is the measured RTT what is mean by measured RTT it means how long it can takes to send a segment and receive an acknowledgement from the receiver side that we can call it as a measured RTT, measured RTT the measured round trip time for a segment is the time required for a segment to reach the destination can be acknowledged in TCP only one RTT measured can be in progress at any time. This means that if an RTT measures is started no other measurement starts until the value of this RTT is finalized ok. We can use notations RTTM to stand to stand for measured RTT ok in the measured RTT how we how to send the data and how much time it will takes to reach the data to the destination if the user has started sending the data they how to wait until to get the acknowledgement ok. Smooth RTT that is the RTTS is a smooth round trip time. The measured RTT RTTM is likely to change for each round trip ok. The function is too high in today's internet that a single measurement alone cannot be used for retransmission time out purpose ok. This is simple example how we how to find out the smooth RTT initially we do not have any values that is no values and after first transmission that is RTTS is equals to RTTM. RTTS is a round trip time or smooth round trip time is equals to measured round trip time after first measurement both are equal and then after each measurement then we have to use this formula that is RTTS is a smooth round trip time is equals to 1 minus alpha into RTTS plus alpha into RTTM ok. What is mean by alpha in this one that is value of alpha is implementation dependent that is depends upon the type of implementation we are using the value of alpha we have to use ok. Most implementation uses smooth RTT called RTTS which is a weighted average of RTTM and previous RTTS ok here. If we want to find out the RTT in upcoming slides we will find out the value of RTTS RTTM and RTOS RTT deviation. The most implementation do not just use RTTS they also calculate the RTT deviation called RTT ok. First we have to find out the RTTO and RTTS and then we can go to the RTT ok RTT D that is deviation RTT deviation ok. Retransmission timeout we have to find out the value of retransmission timeout the value of RTO is based on the smooth round trip time and its deviation. How we have to find out this one it means originally or initially we do not have any values after that after any measurement then RTO is equals to RTTS into plus 4 into RTT ok. This formula we have to use to find the value of RTO that is RTO is equals to RTTS that is smooth round trip time plus 4 into RTT round trip time ok. Most implementation uses the following formula to find out to calculate the value of RTO. Now we can go to the think and write now it can contain one question that is when a synchronization plus acknowledgement segment arrives RTTM is measured and is equal to 1.5 seconds. Find the value of RTTM, RTTS, RTD and RTO ok by using this one just you have to find out the value of RTTM, RTTS, RTD and RTO. Now pass the video and find the value of this one you can go to the answer. Now first solution is already the how given the synchronization plus acknowledgement segment error is in 1.5 seconds that is RTTM, RTTM that is measured round trip time is 1.5 is equals to RTTS though after first measurement both RTTM as well as RTTS as equals. So RTTM is in 1.5 as well as RTTS is also 1.5 RTTD that is deviation we have to find out they have given the RTTD that is 1.5 divided by 2 is equals to 0.75. So RTO is equals to RTTS plus 4 into RTD then we have to use this formula that is the RTO value is 4.5. This formulas we have already discussed in the previous PPT that is in the RTO as well RTTD, RTTES and RTTM ok in this one the applications of TCP timers the TCP timers can be used while transmitting the data from different users before transmitting any data the user how to check out the TCP timers how they how to transfer the data and how they how to wait for the acknowledgement in the main point in the TCP protocol how we how to find out the values and how we how to make the changes before transmitting any data then if the user is transmitting one data then they how to wait for the acknowledgement then after transmitting first data the client how to wait until to get the acknowledgement ok how they can send the acknowledgement and all those things it is they have given the applications of TCP timers there are different applications are there first one is time wait timer ok time wait timer it means first they will send the data then they can wait for the acknowledgement if the user is transmitting 2 or more than 2 segments then first they will transfer the first segment then they how to wait then while sending the first packet they how to start the timer then how much delay time they will mention then the current user how to wait until whatever the delay time they have mentioned then after reaching the data to the destination site then once again the user can or client can resend the next packet that is time wait timer it means first they how to wait after passing the first data that is used to use during TCP connection termination ok if the user want to terminate the connection first they will check out the start timer of the TCP then they will find out the time how much time they have taken ok second one is the timer starts after sending the last acknowledgement for second termination and closing the connection then how we how to start it means first they will start the timer then they can send the data ok after TCP connection is closed it is possible for datagram that are still making their ways through the network to attempt to access the closed port ok here how we how to send the data TCP connection is closed it means if any one user do not want to communicate with each other it means they can go to the termination phase this the termination phase we have already discussed in the TCP types types of TCP connections in that they we have to use this FIN command to terminate the connection in TCP ok this is the reference I use to prepare the above PPT. Thank you.