12 - TCP and congestion control

12 - TCP and congestion control - Last Lecture TCP 1 2...

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Last Lecture: TCP 1. Multiplexing and Demultiplexing 2. Byte-stream service Stream of bytes sent and received, not stream of packets 3. Reliable data transfer A combination of go-back-N and selective repeat, and performance tuning heuristics 4. Connection management Connection establishment and tear down 5. Flow control Prevent sender from overflowing receiver 6. Congestion control (later) SUNY at Buffalo; CSE 489/589 – Modern Networking Concepts; Fall 2010; Instructor: Hung Q. Ngo 1
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This Lecture: TCP 1. Multiplexing and Demultiplexing 2. Byte-stream service Stream of bytes sent and received, not stream of packets 3. Reliable data transfer A combination of go-back-N and selective repeat, and performance tuning heuristics 4. Connection management Connection establishment and tear down 5. Flow control Prevent sender from overflowing receiver 6. Congestion control General principles & How TCP does it SUNY at Buffalo; CSE 489/589 – Modern Networking Concepts; Fall 2010; Instructor: Hung Q. Ngo 2
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What is Congestion Control? Flow control : Keep one fast sender from overflowing a slow receiver Congestion control: Keep a set of senders from overloading the network (or, more precisely, some routers in the network) SUNY at Buffalo; CSE 489/589 – Modern Networking Concepts; Fall 2010; Instructor: Hung Q. Ngo 3
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Congestion is Unavoidable SUNY at Buffalo; CSE 489/589 – Modern Networking Concepts; Fall 2010; Instructor: Hung Q. Ngo 4
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Especially After A Soccer Match Win SUNY at Buffalo; CSE 489/589 – Modern Networking Concepts; Fall 2010; Instructor: Hung Q. Ngo 5
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Network Congestion Is Unavoidable SUNY at Buffalo; CSE 489/589 – Modern Networking Concepts; Fall 2010; Instructor: Hung Q. Ngo 6 • Two packets arrive at the same time The router can only transmit one … and either buffer or drop the other • If many packets arrive in short period of time The router cannot keep up with arriving traffic … and the buffer may eventually overflow
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What Happens When Congestion Occurs? If nothing is done to alleviate the problem, then The router(s) has to drop packets More packets are lost Retransmissions inject more packets into the network … more packets are lost Packets not dropped wait in long queue End-to-end delay gets large Senders time out, retransmit more packets … queues become full, more packets dropped/lost This is a top-10 problem in Computer Networking Reliable data transfer is another SUNY at Buffalo; CSE 489/589 – Modern Networking Concepts; Fall 2010; Instructor: Hung Q. Ngo 7
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Ways to Deal With Congestion Reservations , like in circuit switching Pre-allocate bandwidths along paths Requires negotiation before sending packets Pricing Don’t drop packets for the high-bidders Requires a payment model Dynamic adjustment (TCP) Every sender infers the level of congestion And adapts its sending rate, for the greater good SUNY at Buffalo; CSE 489/589 – Modern Networking Concepts; Fall 2010; Instructor: Hung Q. Ngo 8
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