414Lecture9

414Lecture9 - Lecture 9 Transport Layer Congestion Control...

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Lecture 9 Transport Layer Congestion Control ECSE 414 – Fall 2010
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ECSE 414, Lecture 10 2 Announcements Project description due today Midterm Exam, Tuesday, Oct 12 In class, closed book Example posted on WebCT Covers through Transport Layer Lecture 10 (next Tuesday) / Chapter 3 in K&R Homework Assignment #2 Due Thursday, October 7 Late assignments will not be accepted 2010 Michael Rabbat
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ECSE 414, Lecture 10 3 TCP Recap • Sequence & acknowledgement numbers Refer to bytes, not packets (e.g., ACK next byte expected) Cumulative acknowledgement • Adaptive timeout time Estimate average RTT and RTT variation TimeoutInterval = EstimatedRTT + 4*DevRTT Double TimeoutInterval after timer expiration • Fast retransmit After three duplicate ACKs, don’t wait for timeout • Flow control Communicate available ReceiveWindow size, avoid overflow • Connection Management Three-way handshaking and connection teardown 2010 Michael Rabbat
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ECSE 414, Lecture 10 4 Principles of Congestion Control Congestion: informally: “too many sources sending too much data too fast for network to handle” different from flow control! manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) 2010 Michael Rabbat
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ECSE 414, Lecture 10 5 Causes/costs of congestion: scenario 1 two senders, two receivers one router, infinite buffers no retransmission large delays when congested maximum achievable throughput unlimited shared output link buffers Host A λ in : original data Host B λ out 2010 Michael Rabbat R/2 R/2 R/2
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ECSE 414, Lecture 10 6 Causes/costs of congestion: scenario 2 one router, finite buffers sender retransmission of lost packet finite shared output link buffers Host A λ in : original data Host B λ out λ ' in : original data, plus retransmitted data 2010 Michael Rabbat
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ECSE 414, Lecture 10 7 Causes/costs of congestion: scenario 2 always: (goodput=throughput perceived by app) “perfect” retransmission only when loss: retransmission of delayed (not lost) packet makes larger (than perfect case) for same λ in λ out = λ in λ out > λ in
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This note was uploaded on 05/09/2011 for the course ECSE 414 taught by Professor Rabbat during the Fall '10 term at McGill.

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414Lecture9 - Lecture 9 Transport Layer Congestion Control...

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