6.033 | Spring 2018 | Undergraduate
Computer System Engineering
Week 7: Networking Part III

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.
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Spring 2018
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