Network Basics: OSI Transport Layer
The Open Systems Interconnection (OSI) Transport layer is the layer where you’ll find the most well-known networking protocols: TCP (normally paired with IP) and SPX (normally paired with IPX). As its name implies, the Transport layer is concerned with the transportation of information from one computer to another.
The main purpose of the Transport layer is to ensure that packets are transported reliably and without errors. The Transport layer does this task by establishing connections between network devices, acknowledging the receipt of packets, and resending packets that aren’t received or are corrupted when they arrive.
In many cases, the Transport layer protocol divides large messages into smaller packets that can be sent over the network efficiently. The Transport layer protocol reassembles the message on the receiving end, making sure that all the packets that comprise a single transmission are received so that no data is lost.
For some applications, speed and efficiency are more important than reliability. In such cases, a connectionless protocol can be used. A connectionless protocol doesn’t go to the trouble of establishing a connection before sending a packet. Instead, it simply sends the packet. TCP is a connection-oriented Transport layer protocol. The connectionless protocol that works alongside TCP is called UDP.
In Windows XP or Vista, you can view information about the status of TCP and UDP connections by running the Netstat command from a command window. In the figure, you can see that several TCP connections are established.
In fact, you can use the command Netstat /N to see the numeric network addresses instead of the names. With the /N switch, the output would look like this:
Active Connections Proto Local Address Foreign Address State TCP 127.0.0.1:2869 127.0.0.1:54170 ESTABLISHED TCP 127.0.0.1:5357 127.0.0.1:54172 TIME_WAIT TCP 127.0.0.1:27015 127.0.0.1:49301 ESTABLISHED TCP 127.0.0.1:49301 127.0.0.1:27015 ESTABLISHED TCP 127.0.0.1:54170 127.0.0.1:2869 ESTABLISHED TCP 192.168.1.100:49300 192.168.1.101:445 ESTABLISHED
TCP is a connection-oriented Transport layer protocol. UDP is a connectionless Transport layer protocol.