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CS551: Basic TCP Mechanisms

CS551: Basic TCP Mechanisms. Christos Papadopoulos ( http://netweb.usc.edu/cs551/). Introduction to TCP. Communication abstraction: Reliable Ordered Point-to-point Byte-stream Protocol implemented entirely at the ends Assumes unreliable, non-sequenced delivery Fate sharing Operations

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CS551: Basic TCP Mechanisms

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  1. CS551:Basic TCP Mechanisms Christos Papadopoulos (http://netweb.usc.edu/cs551/)

  2. Introduction to TCP • Communication abstraction: • Reliable • Ordered • Point-to-point • Byte-stream • Protocol implemented entirely at the ends • Assumes unreliable, non-sequenced delivery • Fate sharing • Operations • OPEN/LISTEN, CONNECT, SEND, RECEIVE, ABORT

  3. TCP Reliability Mechanism Sender Receiver 1 data 2 ack lost ack 3 2

  4. TCP Header Source port Destination port Sequence number Flags: SYN FIN RESET PUSH URG ACK Acknowledgement Advertised window Flags Hdr len 0 Checksum Urgent pointer Options (variable) Data

  5. TCP Mechanisms • Connection establishment • Sequence number selection • Connection tear-down • Round-trip estimation • Window flow control

  6. Connection Establishment A and B must agree on initial sequence number selection: Use 3-way handshake A B A B Aseqn SYN SYN+ACK-A Bseqn ACK-B ACK

  7. Connection Setup

  8. Sequence Number Selection • Initial sequence number (ISN) selection • Why not simply chose 0? • Must avoid overlap with earlier incarnation • Possible solutions • Assume non-volatile memory • Clock-based solutions • Requirements for ISN selection • Must operate correctly • Without synchronized clocks • Despite node failures

  9. ISN and Quiet Time • Assume upper bound on segment lifetime (MSL) • In TCP, this is 2 minutes • Upon startup, cannot assign sequence numbers for MSL seconds • Can still have sequence number overlap • If sequence number space not large enough for high-bandwidth connections

  10. Connection Tear-down • Normal termination • Allow unilateral close • Avoid sequence number overlap • TCP must continue to receive data even after closing • Cannot close connection immediately: what if a new connection restarts and uses same sequence number?

  11. Tear-down Packet Exchange Sender Receiver FIN FIN-ACK Data write Data ack FIN FIN-ACK

  12. Connection Tear-down

  13. Round-trip Time Estimation • Wait at least one RTT before retransmitting • Importance of accurate RTT estimators: • Low RTT -> unneeded retransmissions • High RTT -> poor throughput • RTT estimator must adapt to change in RTT • But not too fast, or too slow!

  14. Initial Round-trip Estimator Round trip times exponentially averaged: • New RTT = a (old RTT) + (1 - a) (new sample) • Recommended value for a: 0.8 - 0.9 • Retransmit timer set to b RTT, where b = 2 • Every time timer expires, RTO exponentially backed-off

  15. Retransmission Ambiguity A B A B Original transmission Original transmission RTO RTO ACK Sample RTT Sample RTT retransmission retransmission ACK

  16. Karn’s Retransmission Timeout Estimator • Accounts for retransmission ambiguity • If a segment has been retransmitted: • Don’t count RTT sample on ACKs for this segment • Keep backed off time-out for next packet • Reuse RTT estimate only after one successful transmission

  17. Jacobson’s Retransmission Timeout Estimator • Key observation: • Using b RTT for timeout doesn’t work • At high loads round trip variance is high • Solution: • If D denotes mean variation • Timeout = RTT + 4D

  18. Flow Control • Problem: Fast sender can overrun receiver • Packet loss, unnecessary retransmissions • Possible solutions: • Sender transmits at pre-negotiated rate • Sender limited to a window’s worth of unacknowledged data • Flow control different from congestion control

  19. Window Flow Control: Send window Sent but not acked Not yet sent Sequence numbers Next to be sent

  20. Window Flow Control: Receive Receive buffer Acked but not delivered to user Not yet acked Sequence numbers Gap window

  21. Window Advancement Issues • Advancing a full window • When the receive window fills up, how do things get started again? • Silly window syndrome • Fast sender, slow receiver • The small packet problem and Nagle’s algorithm • Delaying small packets to improve throughput

  22. TCP Extensions • Needed for high-bandwidth delay connections • Accurate round-trip time estimation • Window size limitations • Impact of loss • Implemented using TCP options • Timestamp • Protection from sequence number wraparound • Large windows

  23. Timestamp Extension • Used to improve timeout mechanism by more accurate measurement of RTT • When sending a packet, insert current timestamp into option • Receiver echoes timestamp in ACK

  24. Protection From Wraparound • Wraparound time vs. Link speed: • 1.5Mbps: 6.4 hours • 10Mbps: 57 minutes • 45Mbps: 13 minutes • 100Mbps: 6 minutes • 622Mbps: 55 seconds • 1.2Gbps: 28 seconds • Use timestamp to distinguish sequence number wraparound

  25. Large Windows • Apply scaling factor to advertised window • Specifies how many bits window must be shifted to the left • Scaling factor exchanged during connection setup

  26. TCP Congestion Control

  27. Congestion 10 Mbps • If both sources send full windows, we may get congestion collapse • Other forms of congestion collapse: • Retransmissions of large packets after loss of a single fragment • Non-feedback controlled sources 1.5 Mbps 100 Mbps

  28. Congestion Control and Avoidance • A mechanism which: • Uses network resources efficiently • Preserves fair network resource allocation • Prevents or avoids collapse • Congestion collapse is not just a theory • Has been frequently observed in many networks

  29. Congestion Response delay throughput load load

  30. Criteria • Efficiency: • System is most efficient at knee of throughput curve • Most throughput with low delay • Efficiency metric: power (throughputa/delay) • Fairness: • In the absence of knowing requirements, assume a fair allocation means equal allocation • Fairness index: (Sxi)2/n(Sxi2)

  31. Congestion Control Design • Avoidance or control? • Avoidance keeps system at knee of curve • But, to do that, need routers to send accurate signals (some feedback) • Sending host must adjust amount of data it puts in the network based on detected congestion • TCP uses its window to do this • But what’s the right strategy to increase/decrease window

  32. Feedback Control Model • We study this question using a feedback control model: • Reduce window when congestion is perceived • Increase window otherwise • Constraints: • Efficiency • Fairness • Stability or convergence (the system must not oscillate significantly)

  33. Linear Control Xi(t + 1) = ai + bi Xi(t) • Formulation allows for the feedback signal: • to be increased/decreased additively (by changing ai) • to be increased/decreased multiplicatively (by changing bi) • Which of the four combinations is optimal?

  34. TCP Congestion Control • A collection of interrelated mechanisms: • Slow start • Congestion avoidance • Accurate retransmission timeout estimation • Fast retransmit • Fast recovery

  35. Congestion Control • Underlying design principle: packet conservation • At equilibrium, inject packet into network only when one is removed • Basis for stability of physical systems

  36. TCP Congestion Control Basics • Keep a congestion window, cwnd • Denotes how much network is able to absorb • Sender’s maximum window: • Min (advertised window, cwnd) • Sender’s actual window: • Max window - unacknowledged segments

  37. Self-clocking • If we have large actual window, should we send data in one shot? • No, use acks to clock sending new data

  38. ..Self-clocking Pr Pb receiver sender Ab As Ar

  39. Slow Start • How do we get this clocking behavior to start? • Initialize cwnd = 1 • Upon receipt of every ack, cwnd = cwnd + 1 • Implications • Window actually increases in RTT * logW • Can overshoot window and cause packet loss

  40. Slow Start Example one RTT 0R 1 one pkt time 1R 1 2 3 2R 2 3 4 6 5 7 3R 4 5 6 7 8 10 12 14 9 11 13 15

  41. Slow Start Sequence Plot Data (KB) Slow start time

  42. Congestion Avoidance • Coarse grained timeout as loss indicator • If loss occurs when cwnd = W • Network can absorb 0.5W ~ W segments • Set cwnd to 0.5W (multiplicative decrease) • Needed to avoid exponential queue buildup • Upon receiving ACK • Increase cwnd by 1/cwnd (additive increase) • Multiplicative increase -> non-convergence

  43. Slow Start and Congestion Avoidance • If packet is lost we lose our self clocking as well • Need to implement slow-start and congestion avoidance together • When timeout occurs set ssthresh to 0.5w • If cwnd < ssthresh, use slow start • Else use congestion avoidance

  44. Congestion Avoidance Sequence Plot

  45. Congestion Window Congestion window time

  46. Nam Animations..

  47. Impact of Timeouts • Timeouts can cause sender to • Slow start • Retransmit a possibly large portion of the window • Bad for lossy high bandwidth-delay paths • Can leverage duplicate acks to: • Retransmit fewer segments (fast retransmit) • Advance cwnd more aggressively (fast recovery)

  48. Fast Retransmit • When can duplicate acks occur? • Loss • Packet re-ordering • Assume packet re-ordering is infrequent • Use receipt of 3 or more duplicate acks as indication of loss • Retransmit that segment before timeout

  49. Fast Retransmit Example [Fall96a] figure 3 fast retransmit after 3 dup ACKs fast retx helps a lot, but not always (if no dup ACKs)

  50. Fast Retransmit - 1 Drop window seqnum 14 1 ack 0 0 ack 28 2 1-2 29-30 ack 1-2 ack 29-30 4 3-6 ack 3-6 Actions after dupacks for pkt 13: 1. On 3rd dupack 13 enter fast rtx 2. Set ssthresh = 15/2 = 7 3. Set cwnd = 1, retransmit 14 4. Receiver cached 15-28, acks 28 5. cwnd++ continue with slow start 6. At pkt 35 enter congestion avoidance 8 7-14 ack 7-13 15 15-28 14 dupack-13 1

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