Which three problems result from application mixing of UDP and TCP streams within a network with no QoS?
E. lower throughput
Correct Answer: ACE
It is a general best practice not to mix TCP-based traffic with UDP-based traffic (especially streaming video)
within a single service provider class due to the behaviors of these protocols during periods of congestion.
Specifically, TCP transmitters will throttle-back flows when drops have been detected. Although some UDP
applications have application-level windowing, flow control, and retransmission capabilities, most UDP
transmitters are completely oblivious to drops and thus never lower transmission rates due to dropping. When TCP flows are combined with UDP flows in a single service provider class and the class experiences
congestion, then TCP flows will continually lower their rates, potentially giving up their bandwidth to dropoblivious UDP flows. This effect is called TCP-starvation/UDP-dominance. This can increase latency and lower the overall throughput.
TCP-starvation/UDP-dominance likely occurs if (TCP-based) mission-critical data is assigned to the same
service provider class as (UDP-based) streaming video and the class experiences sustained congestion. Even if WRED is enabled on the service provider class, the same behavior would be observed, as WRED (for the most part) only affects TCP-based flows.
Granted, it is not always possible to separate TCP-based flows from UDP-based flows, but it is beneficial to be aware of this behavior when making such application-mixing decisions.