Weighted Fair Queueing (WFQ)

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For situations in which it is desirable to provide consistent response time to heavy and light network users alike without adding excessive bandwidth, the solution is WFQ. WFQ is one of Cisco's premier queuing techniques. It is a flow-based queuing algorithm that does two things simultaneously: It schedules interactive traffic to the front of the queue to reduce response time, and it fairly shares the remaining bandwidth between high bandwidth flows.

WFQ ensures that queues do not starve for bandwidth, and that traffic gets predictable service. Low-volume traffic streams--which comprise the majority of traffic--receive preferential service, transmitting their entire offered loads in a timely fashion. High-volume traffic streams share the remaining capacity proportionally between them, as shown in Figure .

WFQ is designed to minimize configuration effort and adapts automatically to changing network traffic conditions. In fact, WFQ does such a good job for most applications that it has been made the default queuing mode on most serial interfaces configured to run at or below E1 speeds (2.048 Mbps).

WFQ is efficient in that it will use whatever bandwidth is available to forward traffic from lower priority flows if no traffic from higher priority flows is present. This is different from Time Division Multiplexing (TDM) which simply carves up the bandwidth and lets it go unused if no traffic is present for a particular traffic type. WFQ works with both of Cisco's primary QoS signaling techniques, IP Precedence and RSVP, described later in this white paper, to help provide differentiated QoS as well as guaranteed QoS services.

The WFQ algorithm also addresses the problem of round-trip delay variability. If multiple high-volume conversations are active, their transfer rates and interarrival periods are made much more predictable. WFQ greatly enhances algorithms such as the SNA Logical Link Control (LLC) and the Transmission Control Protocol (TCP) congestion control and slow-start features.The result is more predictable throughput and response time for each active flow

Cooperation between WFQ and QoS Signaling Technologies

WFQ is IP Precedence-aware, that is, it is able to detect higher priority packets marked with precedence by the IP Forwarder and can schedule them faster, providing superior response time for this traffic. The IP Precedence field has values between 0 (the default) and 7. As the precedence value increases, the algorithm allocates more bandwidth to that conversation to make sure that it gets served more quickly when congestion occurs. WFQ assigns a weight to each flow, which determines the transmit order for queued packets. In this scheme, lower weights are served first. IP Precedence serves as a divisor to this weighting factor. For instance, traffic with an IP Precedence field value of 7 gets a lower weight than traffic with an IP Precedence field value of 3, and thus has priority in the transmit order.

An example: If you have one flow at each precedence level on an interface, each flow will get precedence+1 parts of the link, as follows:

and the flows will get 8/36, 7/36, 6/36, and 5/36 of the link, and so on. However, if you have 18 precedence-1 flows and one of each of the others, the formula looks like this:

and the flows will get 8/70, 7/70, 6/70, 5/70, 4/70, 3/70, 2/70, and 1/70 of the link, and 18 of the flows will get approximately 2/70 of the link.

WFQ is also RSVP aware; RSVP (see "RSVP Guarantees QoS" on page 21) uses WFQ to allocate buffer space and schedule packets, and guarantees bandwidth for reserved flows.

Additionally, in a Frame Relay network, the presence of congestion is flagged by the forward explicit congestion notification (FECN) and backward explicit congestion notification (BECN) bits. WFQ weights are affected by Frame Relay discard eligible (DE), FECN, and BECN bits when the traffic is switched by the Frame Relay switching module. Once congestion is flagged, the weights used by the algorithm are altered so that the conversation encountering the congestion transmits less frequently.