CST311 Week 7

The seventh week of CST 311 focused on the link layer.

The link layer is concerned with how data moves across a single link. There are many different link layer protocols, all providing a unique set of services to the network layer. An important service provided by some link layer protocols is error detection, (and possibly correction). Error detection can be found throughout the internet protocol stack, though the link layer's error detection has some key advantages.

A significant portion of the link layer is implemented in hardware. This allows adapters to perform complex error detection and correction on link layer frames; operations that are much too complex to be done by software in the upper layers. Additionally, performing error correction at the link layer provides throughput and latency improvements when portions of the route are unreliable. For example, imagine that during a link layer frame’s last hop, one of its bits is flipped. The frame is simply dropped without error correction at the link layer. Subsequently, it would be an upper protocol’s responsibility to detect and handle this packet loss – which ultimately entails resending the packet. When error correction is performed at the link layer it is possible to recover an otherwise damaged packet in-flight, saving the time it would have taken for an upper protocol to retransmit and repropagate the packet to the current node.