A header is placed in front of a PDU


As traffic passes through the network, it is handled by different protocols. These include Ethernet, IP, TCP, and UDP. These protocols work at different layers of the network.

The protocols that work in layers 2 to 4 will usually divide information into manageable chunks, generically called PDUs, or Protocol Data Units. Each protocol has a specific name for these PDUs. For example, IP uses the term ‘packet’.

A ‘header’ is attached to the front of each PDU. This is extra information that tells network devices about the information in the PDU. Specifically, how it needs to be handled.

For example, depending on the protocol, the header might include things like the source and destination address. This is so the PDU can be delivered to the right place.

It might also include information about the priority of the traffic. This allows technologies like QoS to make sure the high-priority PDUs are handled quickly and efficiently.

Some protocols, such as Ethernet, will also add a trailer, which is a small piece of information that is placed at the end of the frame. This is used for error checking.