Quality of Service – FortiBalancer

Chapter 12 Quality of Service (QoS)

12.1 Overview

This chapter introduces how to setup the QoS (Quality of Service) function on the FortiBalancer appliance. We setup the QoS functionality to provide administrators with the control over network bandwidth and allow them to manage the network from the business perspective, rather than the technical perspective.

12.2 Understanding QoS

QoS for networks is an industry-wide set of standards and mechanisms for ensuring high-quality performance for critical applications. By using QoS mechanisms, network administrators can use existing resources efficiently and ensure the required service level without reactively expanding or over-provisioning their networks.

QoS provides network administrators with the capacity of TCP, UDP and ICMP flow management by using queuing mechanism and packet filtering policies. By using queuing mechanism and filter rules, QoS supports both bandwidth management and priority control.

12.2.1 Queuing Mechanism

The FortiBalancer appliance has developed a queue-based QoS. Queue means a queue of network packet buffers. After the packet at the beginning of the queue has been processed, a new packet to be processed will be put at the end of the queue.

Each queue is bound with a particular network interface and controls either incoming or outgoing network traffic of that interface. QoS queues are organized in tree-like structures. On the top of a tree, a root queue is defined for either incoming or outgoing traffic of a network interface. Under the root queue, there can be multiple sub-queues. Sub-queues can also have their sub-queues. For each interface, at most two queue trees can be configured: one for the incoming traffic, and the other for the outgoing data.

Each queue is configured with bandwidth limit and priority for packet processing.

12.2.2 Packet Filter Rule

A QoS filter is a rule which associates particular network traffic with a QoS queue.

In filter rule, the network traffic is specified by five parameters: source IP subnet, source port, destination IP subnet, destination port and protocol. By this association, administrators can deploy either application-oriented or link-oriented QoS control. Normally, application-oriented filter rules have TCP or UDP ports defined while link-oriented filter rules focus on source or destination IP addresses.

12.2.3 Bandwidth Management

Bandwidth management is realized by a set of QoS filter rules which bind particular network traffic to pre-defined QoS queues with limited bandwidth settings. The QoS filter rules help FortiBalancer appliance servers to allocate appropriate bandwidth to satisfy the needs from various applications and links.

For more flexible bandwidth control, “BORROW/UNBORROW” strategy is applied to QoS queues in a tree-like structure. When a queue’s “BORROW” flag is turned on, its bandwidth can be expanded by borrowing from its parent queue. If the parent queue does not have extra bandwidth to share, it can also fall back on its parent, until the parent queue is the root queue.

12.2.4 Priority Control

Priority Control is accomplished by QoS queues in different priorities. All packets from different applications or links are firstly classified by QoS filter rules and then distributed to predefined queues enjoying the pre-configured priorities.

This priority mechanism works well especially when the network become crowded. If the traffic reaches a peak, packet loss will arise when the number of packets waiting for processing exceeds the maximum queuing buffers. Under such circumstance, the packets belonging to the queues with the highest priority will be processed in the first place, while other packets with lower priorities may be dropped. In this way, the mission-critical applications will be assigned with the highest priority, therefore the functionality of the most important transactions is guaranteed.


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