Weighted fair queuing (WFQ)
provides equal amounts of bandwidth to each conversation that
traverses the interface. WFQ uses a process that refers to the timestamp found on the last bit of
a packet as it enters the queue.
Assigning Priorities
WFQ assigns a high priority to all low-volume traffic. Figure 30.3 demonstrates how the timing
mechanism for priority assignment occurs. The algorithm determines which frames belong to either
a high-volume or low-volume conversation and forwards the low-volume packets from the queue
first. Through this timing convention, remaining packets can be assigned an exiting priority.
In Figure 30.3, packets are labeled A through F. As depicted in this figure, Packet A will
be forwarded first because it’s part of a low-volume conversation, even though the last bit
of session B will arrive before the last bit of the packets associated with Packet A did. The
remaining packets are divided between the two high-traffic conversations, with their timestamps
determining the order in which they will exit the queue.
FIGURE 3 0 . 3
Priority assignment using WFQ
Assigning Conversations
We’ve discussed how priority is assigned to a packet or conversation, but it’s also important to
understand the type of information that the processor needs to associate a group of packets with
an established conversation.
The most common elements used to establish a conversation are as follows:
Source and destination IP addresses
MAC addresses
Port numbers
Type of service
DLCI number assigned to an interface
Say a router has two active conversations, one a large FTP transfer and the other an HTTP
session. The router, using some or all of the factors just listed to determine which conversation
a packet belongs to, allocates equal amounts of bandwidth to each conversation. Each of the
two conversations receives half of the available bandwidth.