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.

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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.