NO.239 Which QoS mechanism will prevent a decrease in TCP performance?
Explanation
Weighted Random Early Detection (WRED) is just a congestion avoidance mechanism. WRED
drops packets selectively based on IP precedence. Edge routers assign IP precedences to packets
as they enter the network. When a packet arrives, the following events occur:
The average queue size is calculated.
2. If the average is less than the minimum queue threshold, the arriving packet is queued.
3. If the average is between the minimum queue threshold for that type of traffic and the
maximum threshold for the interface, the packet is either dropped or queued, depending on the
packet drop probability for that type of traffic.
4. If the average queue size is greater than the maximum threshold, the packet is dropped.
WRED reduces the chances of tail drop (when the queue is full, the packet is dropped) by
selectively dropping packets when the output interface begins to show signs of congestion (thus it
can mitigate congestion by preventing the queue from filling up). By dropping some packets early
rather than waiting until the queue is full, WRED avoids dropping large numbers of packets at
once and minimizes the chances of global synchronization. Thus, WRED allows the transmission
line to be used fully at all times.
WRED generally drops packets selectively based on IP precedence. Packets with a higher IP
precedence are less likely to be dropped than packets with a lower precedence. Thus, the higher
the priority of a packet, the higher the probability that the packet will be delivered.
Reference:
mt/qos-conavd-15-mt-book/qos-conavd-cfg-wred.html
WRED is only useful when the bulk of the traffic is TCP/IP traffic. With TCP, dropped packets
indicate congestion, so the packet source will reduce its transmission rate. With other protocols,
packet sources may not respond or may resend dropped packets at the same rate. Thus, dropping
packets does not decrease congestion.
16/qos-conavd-xe-16-book/qos-conavd-oview.html
Note: Global synchronization occurs when multiple TCP hosts reduce their transmission rates in
response to congestion. But when congestion is reduced, TCP hosts try to increase their
transmission rates again simultaneously (known as slow-start algorithm), which causes another
congestion. Global synchronization produces this graph: