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TCP Variants

TCP Variants. TCP Algorithms:. Four intertwined algorithms used commonly in TCP implementations Slow Start - Every ack increases the sender’s window ( cwnd ) size by 1

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TCP Variants

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  1. TCP Variants

  2. TCP Algorithms: Four intertwined algorithms used commonly in TCP implementations • Slow Start - Every ack increases the sender’s window (cwnd) size by 1 • Congestion Avoidance - Reducing sender’s window size by half at experience of loss, and increase the sender’s window at the rate of about one packet per RTT (NOTE: not per ack) • Fast Retransmit - Don’t wait for retransmit timer to go off, retransmit packet if 3 duplicate acksreceived • Fast Recovery - Since duplicate ack came through, one packet has left the wire. Perform congestion avoidance, don’t jump down to slow start

  3. TCP Variants : • TCP-Tahoe: • implements the slow start, congestion avoidance, and fast retransmit algorithms • TCP-Reno: • implements the slow start, congestion avoidance, fast retransmit, and fast recovery algorithms

  4. segment 1 cwnd = 1 TCP Tahoe’s Fast Retransmit S R • Sender receives 3 dupACKS. • Sender infers that the segment is lost. • Sender re-sends the segment immediately! • Sender returns to slow-start. ACK 1 cwnd = 2 segment 2 segment 3 ACK 2 ACK 3 cwnd = 4 segment 4 segment 5 segment 6 segment 7 ACK 3 3 duplicate ACKs ACK 3 ACK 3 segment 4 fast-retransmit of segment 4

  5. Fast Recovery Concept: • After fast retransmit, reduce cwnd by half, and continue sending segments at this reduced level. Problems: • Sender has too many outstanding segments. • How does sender transmit packets on a dupACK? Inflate cwnd. cwnd (initial) ssthresh fast-retransmit fast-retransmit timeout new ACK new ACK Time Slow Start Congestion Avoidance “inflating” cwnd with dupACKs “deflating” cwnd with a new ACK

  6. Fast Retransmit & Fast Recovery (TCP Reno) • After receiving 3 dupACKS: • Retransmit the lost segment. • Set ssthresh = flight size/2. • Set cwnd = ssthresh, and ndupacks = 3. N.B. In Reno: send_win = min ( rwnd, cwnd + ndupacks ). • If dupACK arrives: • ++ ndupacks • Transmit new segment, if allowed. • If new ACK arrives: • ndupacks = 0 • Exit fast recovery. • If RTO expires: • ndupacks = 0 • Perform slow-start - ( ssthresh = flight size/2, cwnd = 1 )

  7. TCP Vegas • Developed by Brakmo, O'Malley & Peterson [SIGCOMM,1994] • Sender-side protocol • Inter-operates with other TCP variants • Relies on accurate timing to estimate congestion • In contrast to Reno – use only packet losses

  8. Vegas Congestion Avoidance • Adjust window size even before packet loss • Use fine-grained timers for accurate RTTs • Use timing information to compute • Expected throughput (best case throughput with minimum RTT, or baseRTT) • Actual throughput (using current RTT) • Adjust window based on difference between actual and expected throughput

  9. TCP Vegas Summary • Anywhere between 40%-70% better throughput than Reno • Only if router buffers are not heavily utilized • Under heavy congestion, Vegas behaves like Reno • Vegas is less aggressive than Reno in using up router buffers • Vegas throughput suffers when competing with other Reno-based data flows • Path rerouting changes the baseRTT, which can result in throughput loss • Fairness issues of Vegas still unresolved

  10. TCP SACK • Basic problem in TCP is that cumulative ACKS provide little information • Selective acknowledgement (SACK) essentially adds a bitmask of packets received • Implemented as a TCP option • Use is negotiated during handshake • Encoded as a set of received byte ranges in TCP header • (max of 4 ranges/often max of 3) • Note that ACK sequence numbers are still cumulative!

  11. Selective ACKnowledgements (SACK) • TCP SACK options specify the blocks of data received in terms of blocks • Block = contiguous range of data received • Left edge specified first sequence number • Right edge specifies last sequence number + 1 • Header has 40 bytes for TCP option • 2 bytes for TCP options Kind and Length • 38 bytes remaining – specify 4 block ranges at most

  12. TCP Westwood (TCPW) • Sender side only modification to TCP Reno • End to end bandwidth estimation to set congestion windows (cwnd) and slow start threshold (ssthresh) • Additive Increase Adaptive Decrease paradigm • Instead of blindly halving the congestion window after 3 DUP ACKs, adaptively set a slow start threshold and congestion window • Takes into account that duplicate ACKs arriving means a packet passed through network successfully • Increase throughput significantly for wireless networks and fairness for both wired and wireless networks.

  13. TCP Reno vs. TCP Westwood

  14. TCP Reno vs. TCP Westwood

  15. TCP Cubic • cwnd growth function : cubic function in terms of the elapsed time since the last loss event • Window growth rate independent of RTT: • keeps the protocol TCP friendly • guarantees RTT fairness • Behaves like TCP under high loss rate regions • Tackles the under utililization problem in low loss rate region • Ideal for high bandwidth-delay product networks

  16. Cwnd growth pattern • Steady state: • window grows very fast upon a window reduction • As it gets closer to Wmax, it slows down its growth • Wmaxis the window size just before the last window reduction • Probing state: • slow growth around Wmaxenhances the stability • fast growth away from Wmaxensures the scalability

  17. CUBIC cwnd curves comparison with TCP

  18. Questions?

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