ScreenOS Cookbook

Chapter 14. Traffic Shaping

14.0. Introduction

Quality of Service (QoS) deals with the order of packets in transmission queues, also called outgoing interface buffers, by controlling the order in which packets are dispatched from those queues. The simplest form is called FIFO (first in, first out) queuing, whereby an interface has a single outgoing buffer in which packets are dispatched into the link in the order they arrive in the queue. More sophisticated forms use multiple queues and complex distribution schemes, and can even manipulate higher-protocol-layer traffic control mechanisms. QoS is an important part of today’s shared public backbone networks. Although QoS is essential for providers to deliver service-level agreements (SLAs) to their subscribers, it never gained the same significance in enterprise networks.

On ScreenOS devices, QoS was developed to provide better utilization for narrow band access links, in particular those with a speed of 1.5 Mbps or lower. QoS can help to soften the spikes in bursty traffic flows, therefore increasing overall bandwidth utilization, but it cannot increase bandwidth over those provided by the network, as it is not a compression technology. It is also possible to prioritize one traffic flow over another, causing degradation of the lower-priority traffic flow, and it can even block the lower-priority traffic flow altogether. QoS is therefore not a magic bullet for solving bandwidth problems. Increasing network bandwidth would be the solution ...