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Chapter 2
Introduction
E-NIT-CTC-20041213-0013 v0.5
13
Bandwidth versus QoS Quality of Service is really best noticed when the Best Effort service encounters
congestion. So a common question is "why not provide more bandwidth, use Best
Effort, and get rid of complicated QoS architectures?"
There are four answers:
First of all, it is less economic to use more bandwidth than to use QoS. Many
congestion problems can be resolved by using QoS.
The second reason is, Denial of Service (DoS) attacks can always fill links. Even
a 10Gbps link can be flooded by ten compromised gigabit ethernet hosts. QoS
allows Voice traffic to work perfectly even at the peak of a DoS incident.
The third reason is, a scavenger service (also known as a "worst effort" or "less
than best effort" service) gives Best Effort traffic such as web browsing priority
over traffic such as large downloads.
Last but not least, we can use quality of service to ameliorate the effect of TCP
unfriendly traffic, such as unauthenticated video (UDP). This amelioration can
prevent congestion collapse of Best Effort traffic due to excessive video load.
Using QoS for this function is in no way as satisfactory as modifying video
stream and video multicast protocols to become TCP friendly. But using QoS
does ameliorate the worst effect of these TCP unfriendly protocols.
Bandwidth does improve the latency for data, but may still require QoS for
congestion management and “guaranteed QoS”.