#What role "TCP" plays in the TCP/IP?

Firstly, TCP/IP uses Internet protocol for discovering a target computer on a network for establishing communication. Once the target computer is discovered on the network, role of TCP begins.

#Role of TCP is to establish communication between at least two computers on a network.

Few points to ponder:

• TCP works on Transport layer, i.e., Layer 4 of OSI model.
• TCP ensures how two computers communicate with each other on a network. For example, what language they are going to use, how fast will be the communication speed will be, etc.
• TCP breaks a transmission message into multiple small packets, which are sent over the network (instead of the original message as a whole). A packet may only have few bytes of information at max.

Below is an exemplary process describing how TCP facilitates communication between two computers:

1. For communication, router of first computer (using TCP) sends one packet (i.e., Packet 1) to IP address of the second computer and waits for acknowledgment from the second computer. If the second computer replies that Packet 1 is received, process proceeds to next step. Otherwise, the first computer retries.
2. When second computer confirms reception of Packet 1, the first computer sends double the amount of packets sent earlier (i.e., packet 2 and packet 3, ) to the second computer and waits for confirmation. The second computer acknowledges only the last packet, if received. So, if second computer acknowledges reception of 3rd packet then the process proceeds to step 3. Otherwise, if the second computer acknowledges reception of 2nd packet, then the first computer understands that there has been some communication issue, and the first computer begins again by sending just one packet (not double the number of packets) i.e., packet 3 (as packet 2 was already received by the second computer) and the process goes on in the same flow.
3. When second computer confirms reception of Packet 3, the first computer sends double the amount of packets sent earlier (i.e., packet 4,5,6, and packet 7) to the second computer and waits for confirmation. The second computer acknowledges only the last packet, if received. So, if second computer acknowledges reception of 7th packet then the process proceeds to step 4. Otherwise, if the second computer acknowledges reception of any packet other that packet 7, then the first computer understands that there has been some communication issue, and the first computer begins again by sending just one packet (not double the number of packets) and the process goes on in flow.
4. The first computer sends double the amount of packets sent earlier (i.e., packet 8,9,10,11,12,13,14 and packet 15) to the second computer and waits for confirmation for packet 15. The process keeps on going as described above. This is the only reason we used to notice that file transfer process is slow initially, progress successively up to a saturation level (which is dependent on bandwidth, latency, etc.),suddenly slows down in between (if a packet loss occurred), and then regains its momentum. This was prominently noticeable with 56k or slower modems and not much noticeable with a broadband or higher speed connections.

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