Moral Lessons from Week 4 (or 10)
Venkatesh Srinivas (S2 '18) & Sid Shanker (S1'18)
TCP: Review
TCP tries to estimate the bandwidth of a link.
Sending window over time
How big a window is "too big"?
Two Signals
- Packet Loss
- Round trip times
What happens when the queue fills up?
No active queue management
It turns out, managing the queue differently can help!
Introducing:
RED
Drop packets before the queue is full
Introducing:
RED
Drop packets before the queue is full
Maybe.
Packet
Packet
Packet :(
Packet
Packet
Packet
Packet
Packet
Packet
Packet
A queue using RED
With traditional droptail
With RED
But, sometimes, this would happen:
Problem: pretty much no packets going through
We only record the average when an item is added to the queue.
"What's an average?"
Turns out, the "spike" was having a HUGE impact on our average queue depth.
Solution: use time!
By decaying the average over time, we allow our algorithms to recover.
