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.

Solution: read the paper!