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DES-7200 Configuration Guide Chapter 4 WRED Configuration
4-1
4 WRED Configuration
4.1 Understanding WRED
The working process of the window mechanism for TCP(Transmission Control
Protocol) is described as follows: the sender sends a message segment in the
size of one window; if it is successful for the sender to receive the response
from the receiver, the sender will send the message segments in the size of 2
windows; if it succeeds, the sender will send the message segments in the size
of 4 windows; the window size increases exponentially. Howerver, if the
message segment loses, TCP flow will start slowly, and the window size is
reduced to 1. The TCP flow window size grows in an exponential manner till it
reaches half of the congestion size. Then the TCP flow window size grows
linearly. TCP slow start is relevant to QoS, for the reason that when the export
queue on an interface is full, all packets newly-arrived are dropped, which is “tail
dropped”. All TCP flows are in the TCP slow start simultaneously.
Global synchronization refers to multiple TCP flows are in the state of TCP slow
start at the same time. When the TCP synchronization occurs, the connection
bandwidth can not be fully utilized, resulting in a waste of bandwidth.
To avoid that, Weighted Random Early Detection (WRED), the packet dropping
policy, could be adopted. WRED provides a mechanism for randomly dropping
packets, greatly reducing the synchronization of the speed of sending multiple
TCP connections and preventing the TCP global synchronization. When the
packets of one TCP connection are dropped, other TCP connections still send
the packets at a high speed. In this way, the TCP connections always send the
packets rapidly, improving the utilization of the line bandwidth.
The user can set the minimum and maximum threshold for the queue with
WRED configured. When the queue length is less than the minimum threshold,
the packets are not dropped; when the queue length is between the minimum
and maximum thresholds, WRED starts to drop ttge packets randomly(the
longer the queue length is, the higher the probability of the packet drop is);
when the queue length is more than the maximum threshold, all packets are
dropped.
The difference of RED and WRED lies in that WRED introduces the IP
precedence to differentiate the drop policy. RED is a special condition of WRED.
When all CoS-to-Min_threshold and CoS-to-Max_threshold are the same on the
interface, the enabled WRED becomes the RED.