TCP protocol

Introduction:

 First, TCP provides connections between clients and servers. A TCP client establishes a connection with a given server, exchanges data with that server across the connection, and then terminates the connection.

TCP also provides reliability. When TCP sends data to the other end, it requires an acknowledgment in return. If an acknowledgment is not received, TCP automatically retransmits the data and waits a longer amount of time. After some number of retransmissions, TCP will give up, with the total amount of time spent trying to send data typically between 4 and 10 minutes (depending on the implementation).

TCP Connection establishment:

TCP has a three way hand shake for connection establishment .

1) a client will send connection request to server. this is called (synchronize) SYN segment

2) server responds to SYN segment with Acknowledgement . this is called SYN + ACK segment.

3) when client receives ACK from server, it establishes connection.

to implement tcp 3 way handshake 

1) a server should bind to the port and should  listen for incoming connections. this is done by using socket,bind,listen calls from socket library

2) client calls connect()n which sends SYN segment to server.which tells the server the client's initial sequence number for the data that the client will send on the connection

3)The server must acknowledge (ACK) the client's SYN and the server must also send its own SYN containing the initial sequence number for the data that the server will send on the connection. The server sends its SYN and the ACK of the client's SYN in a single segment.

4)The client must acknowledge the server's SYN.

Picture of 3 way handshake: click to enlarge.

Note that TCP does not guarantee that the data will be received by the other endpoint, as this is impossible. It delivers data to the other endpoint if possible, and notifies the user (by giving up on retransmissions and breaking the connection) if it is not possible. Therefore, TCP cannot be described as a 100% reliable protocol; it provides reliable delivery of data or reliable notification of failure.

TCP contains algorithms to estimate the round-trip time (RTT) between a client and server dynamically so that it knows how long to wait for an acknowledgment. For example, the RTT on a LAN can be milliseconds while across a WAN, it can be seconds. Furthermore, TCP continuously estimates the RTT of a given connection, because the RTT is affected by variations in the network traffic.

TCP also sequences the data by associating a sequence number with every byte that it sends. For example, assume an application writes 2,048 bytes to a TCP socket, causing TCP to send two segments, the first containing the data with sequence numbers 1–1,024 and the second containing the data with sequence numbers 1,025–2,048. (A segment is the unit of data that TCP passes to IP.) If the segments arrive out of order, the receiving TCP will reorder the two segments based on their sequence numbers before passing the data to the receiving application. If TCP receives duplicate data from its peer (say the peer thought a segment was lost and retransmitted it, when it wasn't really lost, the network was just overloaded), it can detect that the data has been duplicated (from the sequence numbers), and discard the duplicate. 

TCP provides flow control. TCP always tells its peer exactly how many bytes of data it is willing to accept from the peer at any one time. This is called the advertised window. At any time, the window is the amount of room currently available in the receive buffer, guaranteeing that the sender cannot overflow the receive buffer. The window changes dynamically over time: As data is received from the sender, the window size decreases, but as the receiving application reads data from the buffer, the window size increases. It is possible for the window to reach 0: when TCP's receive buffer for a socket is full and it must wait for the application to read data from the buffer before it can take any more data from the peer.

TCP Connection termination:
when it comes for termination. TCP has 4 steps .
step 1) when application calls close(), the tcp peer will send the FIN segment. it means it has finished sending data .this is called active close
step 2) The other end that receives the FIN performs the passive close. The received FIN is acknowledged by TCP. The receipt of the FIN is also passed to the application as an end-of-file (after any data that may have already been queued for the application to receive), since the receipt of the FIN means the application will not receive any additional data on the connection.
step 3)sometime later, the application that received the end-of-file will close its socket. This causes its TCP to send a FIN.
step 4) The TCP on the system that receives this final FIN (the end that did the active close) acknowledges the FIN.