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Transport Layer 4

Transport Layer 4. 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

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Transport Layer 4

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  1. Transport Layer 4 2: Transport Layer 4

  2. 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: 2: Transport Layer 4

  3. end-end control (no network assistance) transmission rate limited by congestion window size, Congwin, over segments: w * MSS throughput = Bytes/sec RTT TCP Congestion Control Congwin • w segments, each with MSS bytes sent in one RTT: 2: Transport Layer 4

  4. two “phases” slow start congestion avoidance important variables: Congwin threshold: defines threshold between two slow start phase, congestion control phase “probing” for usable bandwidth: ideally: transmit as fast as possible (Congwin as large as possible) without loss increaseCongwin until loss (congestion) loss: decreaseCongwin, then begin probing (increasing) again TCP congestion control: 2: Transport Layer 4

  5. exponential increase (per RTT) in window size (not so slow!) loss event: timeout (Tahoe TCP) and/or or three duplicate ACKs (Reno TCP) Slowstart algorithm time TCP Slowstart Host A Host B one segment RTT initialize: Congwin = 1 for (each segment ACKed) Congwin++ until (loss event OR CongWin > threshold) two segments four segments 2: Transport Layer 4

  6. TCP Congestion Avoidance Congestion avoidance /* slowstart is over */ /* Congwin > threshold */ Until (loss event) { every w segments ACKed: Congwin++ } threshold = Congwin/2 Congwin = 1 perform slowstart 1 1: TCP Reno skips slowstart (fast recovery) after three duplicate ACKs 2: Transport Layer 4

  7. Fairness goal: if N TCP sessions share same bottleneck link, each should get 1/N of link capacity TCP congestion avoidance: AIMD:additive increase, multiplicative decrease increase window by 1 per RTT decrease window by factor of 2 on loss event TCP Fairness AIMD TCP connection 1 bottleneck router capacity R TCP connection 2 2: Transport Layer 4

  8. Two competing sessions: 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 2: Transport Layer 4

  9. 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 layer) into the network “core” Summary 2: Transport Layer 4

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