Congestion Control, High-Speed Networking

• Random Early Discard
• Speeding up:
  • Faster technologies
  • New challenges

Random Early Discard (RED)

• uses packet drop to notify sender of congestion (designed to work with TCP/IP networks)
• intent is that by dropping a packet "early", the source will slow down, avoiding dropping a lot of packets later.

RED: drop algorithm

• len: current length, L: average queue length, W: weight, typically 0.002, count: packets since last drop
• when packet arrives:
  • L = (1 - W) * L + W * len
  • if L <= Min, queue packet
  • if Min < L < Max
    • with probability P(L, count) drop packet, count = 0
    • otherwise queue the packet, count += 1
  • P increases until a packet is dropped: P' = MaxP * (L - Min) / (Max - Min),
    P = P' / (1 - count * P')
  • if Max <= L, drop the packet, count = 0

In-class exercise

• len = 700
• L = 599
• W = 0.002,
• count = 100
• Min = 500
• Max = 1000
• MaxP = 0.02
• compute:
  • the new average length L
  • the probability P of dropping the packet
• you may use appropriate approximations

Faster Networks

• TCP/IP developed for 56Kb/s land lines
• experience: Ethernet (10 Mb/s), modems (10-60Kb/s)
• new technologies:
  • 100 Mb/s Ethernet
  • ATM
  • Gigabit Ethernet
• what happens when things go faster?

When things go faster

• throughput increases, but
• speed of light remains (approximately) the same, so
• latency does not decrease significantly, so
• do things to avoid round-trips

Latency and Throughput

• throughput = size / latency
• what we really care about is latency = propagation + transmission + queue
• transmission = size / bandwidth

• throughput =~ size / propagation
• latency =~ propagation

What Breaks

• connection setup/teardown is slow
• resource reservation is slow
• feedback from the network is slow:
  • congestion control is unresponsive
  • must buffer lots of data in case of packet loss

Trading Througput for Latency

• Prefetch (takes more bandwidth)
• Cache (takes more memory)
• Reuse (saves latency)
• Forward Error Correction (takes more bandwidth, saves latency)