L22_best_effort_networks_layering

L22_best_effort_networks_layering - Best-Effort Networks...

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1 Best-Effort Networks and Layering Lecture 22 April 28, 2008 6.02 Spring 2008 Understanding network delays Little’s law Best-effort network: losses, corruption, reordering Coping using layered protocols Queues in Packet-Switched Networks • Queues absorb bursts of arriving packets Switch Queue • What happens if queue overflows? • Packets are dropped (lost) • Can we design a packet-switched network to avoid such losses? • Turns out we can’t (or shouldn’t); it would be a bad idea to try!
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2 Packets May Be Dropped in a Packet-Switched Network • Can’t usually provision link for peak rate • Peak-to-average very high for data traffic (provisioning for peak wastes bandwidth) • Moreover, may not be able to predict traffic patterns well • What if we use memory to simply buffer all packets? • Bad idea: long queues don’t help throughput • They only increase delay (intuitive; from Little’s law) Use queues only to absorb transients (hard problem because it’s hard to tell what a “transient” is in practice) • So, drop packets when queues overflow • [Or, drop packets pre-emptively to signal congestion early] • At a higher layer, detect and recover from loss • Not a new symptom: noise, interference/collisions, cause uncorrectable bit errors (so packet must be dropped) Understanding Network Delay (Latency) • Little’s law is an important queueing theory result • Relates mean number of packets (jobs) to processing rate and mean delay • Four independent causes of delay
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3 Understanding Little’s Law • E[N] = rate (or throughput) * delay (or latency) Suppose T is large and that S packets are forwarded in that time Let W = area under N(t) curve from 0 to T Then, rate = S/T; and mean number of pkts in queue, E[N] = W/T
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L22_best_effort_networks_layering - Best-Effort Networks...

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