wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

19-12 Industrial Communication Systems
Tail drop [25] is the router default queuing response to congestion. When the output queue is full and
tail drop running, all packets trying to enter the queue are dropped until the congestion is eliminated
and the queue is no longer full.
Tail drop treats all traffic equally and does not differentiate between classes of service. The tail drop
mechanism is not a good scheme in environments with a large number of TCP flows or in any environment
in which selective dropping is wanted.
Some of the worst consequences of simple tail drop mechanism are
• With congestion in the network, dropping affects most of the TCP sessions, which simultaneously
back off and then restart again, provoking inefficient link use at the congestion point.
• TCP starvation, with buffers temporarily seized by aggressive flows, and normal TCP flows
experience buffer starvation.
• It is possible that buffering introduce delay and jitter because packets are stopped, waiting in
queues.
• Premium traffic is also dropped because of the inexistence of differentiated drop mechanisms.
One of the approaches for avoiding these problems is the random early detection (RED) feature. RED
[25] does not take into account the precedence or CoS, it simply uses one of the single threshold when
that threshold value for the buffer fills. RED start to drop packets randomly (not all packets, as with tail
drop) until the maximum threshold is got, and then all the packets are dropped. The idea is that the
probability of dropping a packet rises linearly with the increase of buffer filling above the threshold.
RED is very efficient only with TCP-based traffic, because it takes advantage of the windowing mechanism
used by TCP to manage congestion. As a consequence, it is not very useful when the traffic type
is not TCP-based.
Other useful mechanism is Weighted RED (WRED, Figure 19.7) [25]. It is very similar to RED in that
both define some threshold, and that threshold starts to randomly drop packets; but WRED is CoSaware,
meaning that a CoS value is added to each of the thresholds.
When the threshold is exceeded, WRED randomly drops packets with the CoS assigned. For example,
with two thresholds in the queue
• CoS 0 and 1 are assigned to threshold 1, and the threshold set to 40% of buffer filling.
• CoS 2 and 3 are assigned to threshold 2, and the threshold set to 80% of buffer filling.
At the moment that buffer exceeds 40% of use, packets with CoS 0 and 1 will start to be randomly
dropped. More packets will be dropped while the buffer use grows. If 80% is reached, packets with CoS
Calculate average
queue size
Current
queue
size
IP packet
IP precedence
or
DSCP
WRED
Queue
full ?
Yes
No
FIFO queue
Select
WRED
profile
Minimum threshold
Maximum threshold
Maximum probability
denominator
Random
drop
Tail drop
FIGURE 19.7
Weighted random early detection mechanism.
© 2011 by Taylor and Francis Group, LLC