QoS

What this chapter covers

Quality of service tools decide how a limited link is shared between the hosts that use it.

This chapter covers the simple queue, which limits a host or a network. It then covers PCQ, which lets one queue share the link fairly among all hosts without a queue for each one.

Both topics use the same menu and the same upload and download notation, so the second page builds directly on the first.

How the topics fit together

The simple queue is the building block, and PCQ is a queue type that you attach to it.

A simple queue has a target and a pair of limits. The ceiling is the maximum, the guarantee is the minimum, and the burst settings allow a short spurt of extra speed.

PCQ changes what happens inside the queue. Instead of one pipe shared on a first come basis, traffic is split by address into sub-streams. Each sub-stream gets an equal part, or a fixed part if you set a rate.

The points to carry across both pages are these:

• Values are written as upload first and download second.
• The first matching queue in the list handles the packet.
• Download is classified by destination address, and upload by source address.
• Fast tracked connections bypass simple queues.

How to use these pages

Each topic page has the same shape, so you can skim for what you need.

• What it is: the idea in plain words.
• How it works: the mechanism, with diagrams and console examples.
• What to remember for the exam: the facts to memorise.
• Common mistakes: the classic traps.
• Check yourself: three questions to test your understanding.

A lab with two hosts and a router is enough. Download a large file on both hosts at once, then watch the queue statistics to see how the limit is divided.