1.1.4. Transmission Control Protocol (TCP)

We saw that the Internet Protocol is an unreliable service. We therefore present you next the Transmission Control Protocol which is responsible for the transport of the datagrams and provides reliability.

TCP in the Protocol Hierarchy

IP versus TCP

TCP is a protocol used along with the Internet Protocol (IP) to send data in the form of packets between computers over the Internet. Whereas IP handles the actual delivery of the data, TCP keeps track of the packets that a message is divided into for efficient routing through the Internet. (Wireless Telcorp)

For example, when a web page is downloaded from a Web server, the TCP program layer in that server divides the file into packets, numbers the packets, and then forwards them individually to the IP program layer. Although each packet has the same destination IP address, it may get routed differently through the network. At the other end, TCP reassembles the individual packets and waits until they have all arrived to forward them as a single file. (Wireless Telcorp)

Transferring the textfile using IP and TCP

The Transmission Control Protocol guarantees reliable and in-order delivery of sender to receiver data. To understand better the reliability of TCP have a look at the next graphic.

Reliability of TCP

The header of a TCP looks as following. Move mouse over the red text to get information about the function of the component. The function of "ports" will be explained in the next chapter:

In this lesson we discuss only the main components of the TCP header. If you are interested in detailed information of all components, have a look at (HALSALL 2005, p. 447-450)

As you can see in the graphic Protocol Hierarchy, the "brother" of TCP is UDP (User Datagram Protocol). We won't go into it's details. The following statements should be enough to understand it's principles:

The protocol UDP is is used to transfer streams of audio and / or video data. For example when you listen to the radio over Internet, the data is transferred using the User Datagram Protocol. As IP it is a unreliable service.