No discussion of traffic shaping would be complete without mentioning the different types of queues that can be utilized. The most basic queuing policy is a FIFO queue, sometimes also referred to as first come, first served (FCFS) queuing. With FIFO queuing, no packet is given priority; nor are there different classes of traffic. All packets are treated equally within the FIFO queue and packets are sent out in the order that they arrive.

Obviously, this makes for a very easy to implement policy. But when implemented, it can result in some users and applications consuming all the bandwidth. Even when they are not consuming bandwidth constantly, their usage may spike at inopportune times, delaying time-sensitive and important traffic. Worse yet, important traffic may be dropped because there is less important traffic in the queue.

Nonetheless, if you have congestion-free internet connection on which users and applications have as much bandwidth as they need, FIFO queuing may be appropriate. It is easy to implement and also guarantees that your access to bandwidth is delay-free. If this is not the case, however, it is time to consider one of the many alternatives to FIFO queuing.

Fair queuing represented the first improvement over FIFO queuing. In it, each program process is given its own FIFO queue. This prevents a badly behaving process from monopolizing the bandwidth. A further refinement of fair queuing is weighted fair queuing (WFQ), which provides a form of priority management. Packets are ultimately classified into high-bandwidth traffic and low-bandwidth traffic, and low-bandwidth traffic is given priority over high-bandwidth traffic. In turn, high-bandwidth traffic shares the connection proportionally based on assigned weights. This ensures that low-bandwidth streams, which represent the bulk of network traffic, are transmitted in a timely manner. One of the drawbacks of WFQ is that it relies on examining the packets, so it does not work on encrypted connections.

There are three queuing disciplines supported by the current version of pfSense: priority queuing (PRIQ), class-based queuing (CBQ), and Hierarchical Fair Service Curve (HFSC). Each of these policies has advantages and disadvantages, as we will soon see.