Computer System Engineering

Lecture 12 Outline

1. Introduction 
   • Last time: TCP CC. Massive success. Doesn’t require us to change the network, is something machines can opt-in to (don’t have to have reliable transport if you don’t need it), lets us prevent congestion in a distributed manner.
   • But:
     • Can result in long delays when routers have too much buffering.
     • Doesn’t work well in some scenarios (DCTCP).
     • Most important for today: Doesn’t react to congestion until queues are full.
   • Full queues = long delay.
   • Queues = necessary to absorb bursts.
   • Goal: Transient queues, not persistent queues.
   • Idea: Drop packets *before* the queues are full. TCP senders will back off before congestion is too bad.
2. DropTail 
   • The original queue management scheme. When a packet arrives, if the queue is full, drop it; else, enqueue it.
   • Simple (+).
   • Only drops packets when it needs to (+/-).
     • Remember: Dropped packet => retransmission, which wastes resources.
   • Synchronizes sources (-).
   • Not very fair (-).
   • Tends to result in mostly-full queues (-).
   • Bad for bursty traffic (-).
3. RED 
   • Active queue management scheme.
   • Idea: Drop packets before the queue is full to give senders an early signal.
   • Requires a measure of the average queue size, q_avg.