R3303-HP HSR6800 Routers ACL and QoS Configuration Guide

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FIFO
Figure 16 FIFO queuing
As shown in Figure 16, the first in first out (FIFO) uses a single queue and does not classify traffic or
schedule queues. FIFO delivers packets depending on their arrival order, with the one arriving earlier
scheduled first. The only concern of FIFO is queue length, which affects delay and packet loss rate. On
a device, resources are assigned for packets depending on their arrival order and load status of the
device. The best-effort service model uses FIFO queuing.
FIFO does not address congestion problems. If only one FIFO output/input queue exists on a port, you
can hardly ensure timely delivery of mission-critical or delay-sensitive traffic or smooth traffic jitter. The
situation is worsened if malicious traffic is present to occupy bandwidth aggressively. To control
congestion and prioritize forwarding of critical traffic, use other queue scheduling mechanisms, where
multiple queues can be configured. Within each queue, however, FIFO is still used.
By default, FIFO queuing is used on interfaces.
PQ
Figure 17 Priority queuing (PQ)
Priority queuing is designed for mission-critical applications. The key feature of mission-critical
applications is they require preferential service to reduce the response delay when congestion occurs.
Priority queuing can flexibly determine the order of forwarding packets by network protocol (for example,
IP and IPX), incoming interface, packet length, source/destination address, and so on. Priority queuing
classifies packets into four queues: top, middle, normal, and bottom, in descending priority order. By
default, packets are assigned to the normal queue. Each of the four queues is a FIFO queue.