|Table of contents|
4 IP ECN
5 Cisco AQM: Dynamic buffer limiting (DBL)
6 Good things about Active Queue Management (RED, WRED, ECN, Cisco DBL)
7 See also
8 External Links
TCP/IP congestion avoidance
Problems occur when many concurrent TCP-flows are experiencing port queue buffer tail-drops. Then TCP's automatic congestion avoidance is not enough. All flows that experience port queue buffer tail-drop, will begin a TCP retrain at the same moment - this is called TCP global synchronization.
One solution is to use RED (Random Early Detection) on network equipments port queue buffer. On network equipment ports with more than one queue buffer, WRED (Weighted Random Early Detection) could be used if available. RED indirectly signals to sender and receiver by deleting some packets , eg. when the average queue buffer lengths are more than 50% filled and deletes exponentially more and more packets, when the average queue buffer lengths are approaching 100%. The average queue buffer lengths are computed over 1 second at a time.
Some network equipment are equipped with ports that can follow and measure each flow (flowbased-RED/WRED) and are hereby able to signal to a too big bandwidth flow according to some QoS policy. A policy could divide the bandwidth among all flows by some criteria.
Another approach is to use IP ECN (Explicit Congestion Notification). ECN is only used when the two hosts signal that they want to use it. With this method, an ECN bit is used to signal that there is explicit congestion. This is better in some ways than the indirect packet delete congestion notification performed by the RED/WRED algorithms, but it requires explicit support by both hosts to be effective. Some outdated or buggy network equipment drops packets with the ECN bit set, rather than ignoring the bit.
Cisco AQM: Dynamic buffer limiting (DBL)
Cisco has taken a step further in their Catalyst 4000 series with engine IV. Engine IV has the possibility to classify all connection-oriented flows in "aggressive" (bad) and "adaptive" (good). It ensures that no flows fill the port queues for a long time. DBL can utilize IP ECN instead of packet-delete-signalling. See the sections in: Active Queue Management and Enabling Dynamic Buffer Limiting.