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Chapter 3 Transport Layer

Chapter 3 Transport Layer. Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. Our goals: understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control.

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Chapter 3 Transport Layer

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  1. Chapter 3Transport Layer Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009. Transport Layer

  2. Our goals: understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control learn about transport layer protocols in the Internet: UDP: connectionless transport TCP: connection-oriented transport TCP congestion control Chapter 3: Transport Layer Transport Layer

  3. 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6Principles of congestion control 3.7 TCP congestion control Chapter 3 outline Transport Layer

  4. 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) a top-10 problem! Principles of Congestion Control Transport Layer

  5. two senders, two receivers one router, infinite buffers no retransmission large delays when congested maximum achievable throughput lout : per-connection throughput lin : original data rate unlimited shared output link buffers Host A Host B Causes/costs of congestion: scenario 1 C is outgoing link rate. Transport Layer

  6. one router, finite buffers sender retransmission of lost packet Causes/costs of congestion: scenario 2 Host A lout lin : original data l'in : original data, plus retransmitted data Host B finite shared output link buffers l'in is the offered load. l'in > lin Transport Layer

  7. always: (goodput) “perfect” retransmission only when loss: retransmission of delayed (not lost) packet makes larger (than perfect case) for same l l l > = l l l R/2 in in in R/2 R/2 out out out R/3 lout lout lout R/4 R/2 R/2 R/2 lin lin lin Causes/costs of congestion: scenario 2 Retrans. Due to Loss No Loss Retrans. Due to Loss And Premature Timeout “costs” of congestion: • more work (retrans) for given “goodput” • unneeded retransmissions: link carries multiple copies of pkt Transport Layer

  8. four senders multihop paths timeout/retransmit l l in in Host A Host B Causes/costs of congestion: scenario 3 Q:what happens as and increase ? lout lin : original data l'in : original data, plus retransmitted data finite shared output link buffers Transport Layer

  9. Host A Host B Causes/costs of congestion: scenario 3 lout Another “cost” of congestion: • when packet dropped, any “upstream transmission capacity used for that packet was wasted! Transport Layer

  10. End-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP Network-assisted congestion control: routers provide feedback to end systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate sender should send at Approaches towards congestion control Two broad approaches towards congestion control: Transport Layer

  11. ABR: available bit rate: “elastic service” if sender’s path “underloaded”: sender should use available bandwidth if sender’s path congested: sender throttled to minimum guaranteed rate RM (resource management) cells: sent by sender, interspersed with data cells bits in RM cell set by switches (“network-assisted”) NI bit: no increase in rate (mild congestion) CI bit: congestion indication RM cells returned to sender by receiver, with bits intact Case study: ATM ABR congestion control Transport Layer

  12. two-byte ER (explicit rate) field in RM cell congested switch may lower ER value in cell sender’ send rate thus maximum supportable rate on path EFCI bit in data cells: set to 1 in congested switch if data cell immediately preceding RM cell has EFCI set, destination sets CI bit in returned RM cell Case study: ATM ABR congestion control Transport Layer

  13. 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control Chapter 3 outline Transport Layer

  14. TCP congestion control: additive increase, multiplicative decrease • Approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs • additive increase: increase cwnd by 1 MSS every RTT until loss detected • multiplicative decrease: cut cwnd in half after loss Saw tooth behavior: probing for bandwidth congestion window size time Transport Layer

  15. sender limits transmission: LastByteSent-LastByteAcked  cwnd Roughly, cwnd is dynamic, function of perceived network congestion How does sender perceive congestion? loss event = timeout or 3 duplicate acks TCP sender reduces rate (cwnd) after loss event three mechanisms: AIMD slow start conservative after timeout events cwnd rate = Bytes/sec RTT TCP Congestion Control: details Transport Layer

  16. When connection begins, cwnd = 1 MSS Example: MSS = 500 bytes & RTT = 200 msec initial rate = 20 kbps available bandwidth may be >> MSS/RTT desirable to quickly ramp up to respectable rate TCP Slow Start • When connection begins, increase rate exponentially fast until first loss event Transport Layer

  17. When connection begins, increase rate exponentially until first loss event: double cwnd every RTT done by incrementing cwnd for every ACK received Summary: initial rate is slow but ramps up exponentially fast time TCP Slow Start (more) Host A Host B one segment RTT two segments four segments Transport Layer

  18. After 3 dup ACKs: cwnd is cut in half window then grows linearly But after timeout event: cwnd instead set to 1 MSS; window then grows exponentially to a threshold (ssthresh), then grows linearly Refinement: inferring loss Philosophy: • 3 dup ACKs indicates network capable of delivering some segments • timeout indicates a “more alarming” congestion scenario Transport Layer

  19. Q: When should the exponential increase switch to linear? A: When cwnd gets to 1/2 of its value before timeout. Implementation: Variable Threshold (ssthresh) At loss event, ssthresh is set to 1/2 of cwnd just before loss event Refinement Transport Layer

  20. Summary: TCP Congestion Control • When cwnd is below ssthresh, sender in slow-start phase, window grows exponentially. • When cwnd is above ssthresh, sender is in congestion-avoidance phase, window grows linearly. • When a triple duplicate ACK occurs, ssthresh set to cwnd/2 and cwnd set to ssthresh. • When timeout occurs, ssthresh set to cwnd/2 and cwnd is set to 1 MSS. Transport Layer

  21. TCP sender congestion control Transport Layer

  22. TCP throughput • What’s the average throughput of TCP as a function of window size and RTT? • Ignore slow start • Let W be the window size when loss occurs. • When window is W, throughput is W/RTT • Just after loss, window drops to W/2, throughput to W/2RTT. • Average throughout: .75 W/RTT Transport Layer

  23. TCP Futures: TCP over “long, fat pipes” • Example: 1500 byte segments, 100ms RTT, want 10 Gbps throughput • Requires window size W = 83,333 in-flight segments • Throughput in terms of loss rate: • ➜ L = 2·10-10 Wow • New versions of TCP for high-speed Transport Layer

  24. Fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K TCP connection 1 bottleneck router capacity R TCP connection 2 TCP Fairness Transport Layer

  25. Two competing sessions (AIMD): Additive increase gives slope of 1, as throughout increases multiplicative decrease decreases throughput proportionally Why is TCP fair? equal bandwidth share R loss: decrease window by factor of 2 congestion avoidance: additive increase Connection 2 throughput loss: decrease window by factor of 2 congestion avoidance: additive increase Connection 1 throughput R Transport Layer

  26. Fairness and UDP Multimedia apps often do not use TCP do not want rate throttled by congestion control Instead use UDP: pump audio/video at constant rate, tolerate packet loss Research area: TCP friendly Fairness and parallel TCP connections nothing prevents app from opening parallel connections between 2 hosts. Web browsers do this Example: link of rate R supporting 9 connections; new app asks for 1 TCP, gets rate R/10 new app asks for 11 TCPs, gets R/2 ! Fairness (more) Transport Layer

  27. principles behind transport layer services: multiplexing, demultiplexing reliable data transfer flow control congestion control instantiation and implementation in the Internet UDP TCP Next: leaving the network “edge” (application, transport layers) into the network “core” Chapter 3: Summary Transport Layer

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