Transmission Control is an essential aspect of network activity and governs the behavior of many services we take for granted. When sending your emails or just browsing the web you are relying on TCP to send and receive your packets in a reliable fashion. Thanks to two DARPA scientists, Vinton Cerf and Bob Kahn who developed TCP/IP in 1970, we have a specific set of rules that define how we communicate over a network. When Vinton and Bob first conceptualized TCP/IP, they set up a basic network topology and a device that can interface between two other hosts.

In the Figure 1 we have two networks connected by a single gateway. The gateway plays an essential role in the development of any network and bares the responsibility of routing data properly between these two networks.

Since the gateway must understand the addresses of each host on the network, it is necessary to have a standard format in every packet that arrives. Vince and Bob called this the internetwork header prefixed to the packet by the source host.

The source and destination entries, along with the IP address, uniquely identify every host on the network so that the gateway can accurately forward packets.

The sequence number and byte count identifies each packet sent from the source, and accounts for all of the text within the segment. The receiver can use this to determine if it has already seen the packet and discard if necessary.

The check sum is used to validate each packet being sent to ensure error free transmission. This checksum uses a false header and encapsulates the data of the original TCP header, such as source/destination entries , header length and byte count .