Computer Networks · free preview
Queuing Disciplines
A router's queue is where congestion becomes visible, and the discipline that empties it decides who suffers, first-in-first-out with tail drop is the simple default that lets one greedy flow capture any share it can grab, priority queuing lets marked traffic cut in line at the risk of starving the unmarked, and fair queuing isolates each flow in its own queue and approximates an ideal bit-by-bit round-robin by stamping every packet with the time it would finish under that ideal and serving the smallest stamp first, a formula that extends with weights into weighted fair queuing and that protects well-behaved flows from the rest.
1. The Default and Its Blindness
The Default and Its Blindness
2. Bit-by-Bit, Approximately
Bit-by-Bit, Approximately
3. Weights and What Fairness Buys
Weights and What Fairness Buys
4. When Queues Meet Windows
When Queues Meet Windows
5. The separable pair
A router runs FIFO with tail drop. Which statement about its two components is correct?
6. The starving queues
Why is priority queuing dangerous without admission control or pricing?
7. Packet-size unfairness
Two flows share a link under simple packet-by-packet round-robin. One sends 1000-byte packets, the other 500-byte packets. What happens?
8. The finish-time rule
In fair queuing's approximation of bit-by-bit round-robin, a packet arrives at tick 10 for a flow whose previous packet finished at tick 14, and the packet is 3 bits long. What is its finish time?
9. Isolation
A source floods the link far beyond its fair share. Under fair queuing, who suffers?
10. Weights
Under weighted fair queuing, flow A has weight 2 and flow B has weight 1, and both are backlogged. What share does A receive?
11. The tail's fate
What drop policy discards any packet that arrives to find the buffer full?
12. The idealized service
Fair queuing approximates which impossible ideal, in which the router would interleave one bit from each active flow in turn?
13. The smallest stamp
Under fair queuing, the router next transmits the queued packet with the smallest what?
14. The policy knob
What per-flow parameter does weighted fair queuing add so shares can be proportional rather than equal?
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