Queuing Tools

Queuing Tools 249

FIFO Queuing

NOTE This section covers FIFO Queuing, which like TX Queues, is not currently covered on the DQOS 9E0-601 exam. This section does cover some useful information for all people interested in QoS. Therefore, for you DQOS exam takers, you might choose to just focus on the concepts about FIFO, and not worry about memorization or configuration. And as always, check the websites listed in the Introduction for any news about changes to the exams.

The first reason that a router or switch needs output queues is to hold a packet while waiting for the interface to become available for sending the packet. Whereas the other queuing tools in this chapter also perform other functions, like reordering packets, FIFO Queuing just provides a means to hold packets while they are waiting to exit an interface.

FIFO Queuing does not need the two most interesting features of the other queuing tools, namely classification and scheduling. FIFO Queuing uses a single queue for the interface. Because there is only one queue, there is no need for classification to decide the queue into which the packet should be placed. Also there is no need for scheduling logic to pick which queue from which to take the next packet. The only really interesting part of FIFO Queuing is the queue length, which is configurable, and how the queue length affects delay and loss.

FIFO Queuing uses tail drop to decide when to drop or enqueue packets. If you configure a longer FIFO queue, more packets can be in the queue, which means that the queue will be less likely to fill. If the queue is less likely to fill, fewer packets will be dropped. However, with a longer queue, packets may experience more delay and jitter. With a shorter queue, less delay occurs, but the single FIFO queue fills more quickly, which in turn causes more tail drops of new packets. These facts are true for any queuing method, including FIFO.

Figure 4-8 outlines simple FIFO Queuing. R1 uses FIFO Queuing on the interface connected to R2. The only decision required when configuring FIFO Queuing is whether to change the length of the queue.

Figure 4-8 Simple FIFO Queuing

R1

Queuing Tools 251

Example 4-2 FIFO Queuing Configuration (Continued)

R3#conf t

Enter configuration commands, one per line. End with CNTL/Z. R3(config)#int s 0/0

R3(config-if)#hold-queue 50 out

R3(config-if)#^Z

!

R3#sh int s 0/0

Serial0/0 is up, line protocol is up Hardware is PowerQUICC Serial

!Lines omitted for brevity Queueing strategy: fifo Output queue :0/50 (size/max)

!Line omitted for brevity

Example 4-2 shows FIFO Queuing being configured by removing the default WFQ configuration with the no fair-queue command. The show interface command lists the fact that FIFO Queuing is used, and the output queue has 40 entries maximum. After configuring the output queue to hold 50 packets with the hold-queue 50 out command, the show interface output still lists FIFO Queuing, but now with a maximum queue size of 50.

FIFO Queuing is pretty basic, but it does provide a useful function: It provides the basic queuing function of holding packets until the interface is no longer busy.

Priority Queuing

Priority Queuing’s most distinctive feature is its scheduler. PQ schedules traffic such that the higher-priority queues always get serviced, with the side effect of starving the lower-priority queues. With a maximum of four queues, called High, Medium, Normal, and Low, the complete logic of the scheduler can be easily represented, as is shown in Figure 4-9.

As seen in Figure 4-9, if the High queue always has a packet waiting, the scheduler will always take the packets in the High queue. If the High queue does not have a packet waiting, but the Medium queue does, one packet is taken from the Medium queue—and then the process always starts over at the High queue. The Low queue only gets serviced if the High, Medium, and Normal queues do not have any packets waiting.

The PQ scheduler has some obvious benefits and drawbacks. Packets in the High queue can claim 100 percent of the link bandwidth, with minimal delay, and minimal jitter. The lower queues suffer, however. In fact, when congested, packets in the lower queues take significantly longer to be serviced than under lighter loads. In fact, when the link is congested, user applications may stop working if their packets are placed into lower-priority queues.

1PQ can classify packets using access-control lists (ACLs) for most Layer 3 protocols, matching anything allowed by any of the types of ACLs. PQ can also directly match, without using an ACL, the incoming interface, packet length, and TCP and UDP port numbers.

2Tail drop is the only available drop policy.

3Four queues maximum

4Maximum queue length can be set to zero, which means the queue has theoretically infinite length. Defaults are 20, 40, 60, and 80 packets for High, Medium, Normal, and Low queues, respectively.

5Inside a queue, PQ uses FIFO logic.

6When scheduling among the queues, PQ always services the highest-priority queue.

Independent of the process leading up to placing the packet into the queue, the scheduler continually reacts when the TX Ring empties a packet out the interface, implying that there is now more room in the TX Ring/TX Queue for another packet. When the TX Ring frees space, the PQ scheduler then performs the logic described in Figure 4-9, taking a packet from the highestpriority queue that has a packet waiting. The PQ scheduler moves the packet to the TX Ring, for later transmission on the interface.