Transport layer

The transport layer is layer 4 of the OSI model. It falls right below the session layer and right above the network layer. As previously discussed, the session layer is responsible for creating, maintaining, and tearing down connections between applications on two systems. The transport layer performs a similar functionality, but on a larger level. Where the session layer works with two applications, the transport layer manages connections between two computers. When two computers need to communicate, they look to the transport layer to establish the connection.

There are two types of protocols that function at this level; connection-oriented protocols and connectionless protocols. Connection-oriented protocols are concerned with maintaining a persistent connection between the two systems. These protocols will undergo a process known as a handshake to establish parameters that guide the connection. During the handshake, the communicating computers will agree on how much information will be sent at a time, how integrity will be checked once the data is received, and how to detect if a packet was lost along the way. Connection-oriented protocols provide error-checking and are able to retransmit packets that were lost or damaged. Well known connection-oriented protocols include Transmission Control Protocol (TCP) and Secure Sockets Layer (SSL).

Connectionless protocols do not concern themselves with error-checking or agreeing on how much or at what rate to transfer data. These protocols will simply send the data without first trying to contact the recipient. The result is that the data may or may not reach its destination and the sending system has no way of confirming this. This sounds like a big deal, but there is much data that gets sent that is not crucial, and if a few packets occasionally do not make it to their destination it is not a big issue. Developers will often use a connectionless protocol because it carries significantly less overhead since there is no need to create and maintain a connection or provide error-checking. The most well known connectionless protocol is User Datagram Protocol (UDP).