Weighted random early detection (WRED) is a queueing discipline for a
network scheduler
A network scheduler, also called packet scheduler, queueing discipline (qdisc) or queueing algorithm, is an arbiter on a node in a packet switching communication network. It manages the sequence of network packets in the transmit and receive q ...
suited for
congestion avoidance
Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...
. It is an extension to
random early detection
Random early detection (RED), also known as random early discard or random early drop is a queuing discipline for a network scheduler suited for congestion avoidance.
In the conventional tail drop algorithm, a router or other network component b ...
(RED) where a single queue may have several different sets of queue thresholds. Each threshold set is associated to a particular
traffic class.
For example, a queue may have lower thresholds for lower priority packet. A queue buildup will cause the lower priority packets to be dropped, hence protecting the higher priority
packets in the same queue. In this way
quality of service
Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...
prioritization is made possible for important packets from a pool of packets using the same buffer.
It is more likely that standard traffic will be dropped instead of higher prioritized traffic.
Restrictions
On Cisco switches WRED is restricted to
*
TCP/IP
The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...
traffic. Only this kind of traffic indicates congestion to the sender to enable a reduction of the transmission rate.
Non-IP traffic will be dropped more often than TCP/IP traffic because it is treated with the lowest possible precedence.
Functional Description
WRED proceeds in this order when a packet arrives:
* Calculation of the average queue size.
* The arriving packet is queued immediately if the average queue size is below the minimum queue threshold.
* Depending on the packet drop probability the packet is either dropped or queued if the average queue size is between the minimum and maximum queue threshold.
* The packet is automatically dropped if the average queue size is greater than the maximum threshold.
Calculation of average queue size
The average queue size depends on the previous average as well as the current size of the queue. The calculation formula is given below:
where
is the user-configurable exponential weight factor,
is the old average and
is the current queue size. The previous average is more important for high values of
. Peaks and lows in queue size are smoothed by a high value. For low values of
, the average queue size is close to the current queue size.
References
{{Reflist
Network performance
da:Undgåelse af datanet-trafikforstoppelse#RED og WRED