User`s guide

220 XSR User’s Guide
Mechanisms to Provide QoS Chapter 10
Configuring Quality of Service
After a short delay, all sessions try to ramp up using slow-start in a process
called Global Synchronization. The queue grows, congestion and packet
drops recur, and undesirable global synchronization repeats. The end result is
a distinctive “peak and trough” traffic pattern where the outgoing queue is
full just before packets are dropped, delay throughout the network is high
and varies by large margins.
RED attempts to avoid congestion by proactively dropping packets randomly
at an early sign of congestion (when the queue grows above a certain
threshold). Because packets are dropped randomly, all TCP/IP sessions will
be affected eventually and the treatment made fair to all sessions.
By dropping packets early - before it reaches its queue limit - RED starts to
“throttle” the traffic source before the queue grows too large. It helps limit
delay, which is proportional to the number of packets in the queue, and avoid
queue overflow and global TCP synchronization.
The
random-detect command includes three parameters to configure RED
for a queue: minimum threshold (MinThres), maximum threshold (MaxThres)
and maximum drop probability (MaxProb). The drop probability of a packet is
based on the average queue size and the three parameters mentioned earlier.
The calculation of the drop probability is pictured below.
Figure 37 RED Drop Probability Calculation
Drop Probability
Average Queue Size
MaxP
0
MinTh
MaxTh
1