TCP Delay Jitter and Starvation
- Constant high buffer usage (long queue) causes delay. Variable buffer usage causes jitter. More aggressive flows can cause other flows to starve. No differentiated dropping occurs.
Another TCP-related phenomenon that reduces optimal throughput of network applications is TCP starvation. When multiple flows are being transmitted through a router, some of these flows may be much more aggressive than other flows. For instance, when the TCP transmit window increases for file-transfer applications, the TCP session can send a number of large packets to its destination. These packets immediately fill the queue on the router, and other, less aggressive flows can be starved because there is no differentiated treatment indicating which packets should be dropped. As a result, less aggressive flows are dropped at the output interface.
Tail drop is not the optimal mechanism for congestion avoidance and therefore should not be used. Instead, more intelligent congestion avoidance mechanisms should be used that are capable of slowing traffic before congestion occurs.
© 2006 Cisco Systems, Inc. Implement the DiffServ QoS Model 4-113
Continue reading here: Cisco IOS Traffic Policing and Shaping Mechanisms
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