Computer Networks · free preview
Congestion Avoidance
Rather than waiting for buffers to overflow, avoidance schemes predict congestion and signal early, DECbit has routers set a congestion bit when the average queue length exceeds one and sources halve or grow their window depending on whether most packets arrived marked, random early detection computes a weighted average queue length and drops arriving packets with a probability that ramps between two thresholds so a few early losses prevent a flood of later ones, explicit congestion notification carries the signal in the header instead of a drop, and source-based schemes measure delay itself, Vegas comparing expected and actual rates against thresholds, BBR modeling bottleneck bandwidth and minimum round-trip time and pacing to that model, and DCTCP harvesting explicit marks in the datacenter to trim windows in proportion to measured congestion.
1. Setting the Bit
Setting the Bit
2. Dropping Early, On Purpose
Dropping Early, On Purpose
3. Marks, Not Corpses
Marks, Not Corpses
4. Listening to Delay Instead
Listening to Delay Instead
5. The one-bit warning
Under DECbit, when does a router set the congestion bit in a packet's header?
6. Why drop on purpose
What is the purpose of RED's dropping packets before the buffer is full?
7. The averaging formula
RED computes its average queue length as with small . Why average at all instead of using the instantaneous queue length?
8. Marking beats dropping
Why is explicit congestion notification preferred over RED-style dropping where both endpoints support it?
9. Vegas's arithmetic
A Vegas flow has a window of 8 packets, a BaseRTT of 100 ms, and measures an actual rate of 60 packets per second. What does the flow conclude?
10. DCTCP's gentleness
What makes DCTCP's window adjustment gentler than classic halving on loss?
11. The predicted event
Congestion avoidance schemes try to predict and signal what event before it happens?
12. The probability curve
In RED, between the minimum and maximum thresholds, the drop probability does what as average queue length grows?
13. The yardstick
In TCP Vegas, what is BaseRTT, the round-trip time used as the denominator of the expected rate?
14. The model pair
BBR models a path using which two measured quantities?
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