ack jumps ayer ack 20 loss is repaired

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Unformatted text preview: h large values Fixed Slow ­start AI Time 68 26 11/12/13 Slow ­Start Solu7on (2) •  Eventually packet loss will occur when the network is congested –  Loss 7meout tells us cwnd is too large –  Next 7me, switch to AI beforehand –  Slowly adapt cwnd near right value •  In terms of cwnd: –  Expect loss for cwndC ≈ 2BD+queue –  Use ssthresh = cwndC/2 to switch to AI 69 Slow ­Start Solu7on (3) •  Combined behavior, ayer first 7me –  Most 7me spend near right value Window cwndC cwndIDEAL Fixed AI phase ssthresh Slow ­start AI Time 70 27 11/12/13 Slow ­Start (Doubling) Timeline Increment cwnd by 1 packet for each ACK 71 Addi7ve Increase Timeline Increment cwnd by 1 packet every cwnd ACKs (or 1 RTT) 72 28 11/12/13 TCP Tahoe (Implementa7on) •  Ini7al slow ­start (doubling) phase –  Start with cwnd = 1 (or small value) –  cwnd += 1 packet per ACK •  Later Addi7ve Increase phase –  cwnd += 1/cwnd packets per ACK –  Roughly adds 1 packet per RTT •  Switching threshold (ini7ally infinity) –  Switch to AI when cwnd > ssthresh –  Set ssthresh = cwnd/2 ayer loss –  Begin with slow ­start ayer 7meout 73 Timeout Misfortunes •  Why do a slow ­start ayer 7meout? –  Instead of MD cwnd (for AIMD) 74 29 11/12/13 Timeout Misfortunes •  Why do a slow ­start ayer 7meout? –  Instead of MD cwnd (for AIMD) •  Timeouts are sufficiently long that the ACK clock will have run down –  Slow ­start ramps up the ACK clock •  We need to detect loss before a 7meout to get to full AIMD –  Done in TCP Reno 75 Introduc7on to Computer Networks TCP Fast Retransmit / Fast Recovery (§6.5.10) Computer Science & Engineering 30 11/12/13 Inferring Loss from ACKs •  TCP uses a cumula7ve ACK –  Carries highest in ­order seq. number –  Normally a steady advance •  Duplicate ACKs give us hints about what data hasn’t arrived –  Tell us some new data did arrive, but it was not next segment –  Thus the next segment may be lost 79 Fast Retransmit •  Treat three duplicate ACKs as a loss –  Retransmit next expected segment –  Some repe77on allows for reordering, but s7ll detects loss quickly Ack 1 2 3 4 5 5 5 5 5 5 80 31 11/12/13 Fast Retransmit (2) . . . Third duplicate ACK, so send 14 Ack 10 Ack 11 Ack 12 Ack 13 Ack 13 Ack 13 Ack 13 Ack 13 . . . ACK jumps ayer Ack 20 loss is repaired . . . . . . Data 14 was lost earlier, but got 15 to 20 Data 20 . . . Data 14 Retransmission fills in the hole at 14 . . . 81 Fast Retransmit (3) •  It can repair single segment loss quickly, typically before a 7meout •  However, we have quiet 7me at the sender/receiver while wai7ng for the ACK to jump •  And we s7ll need to MD cwnd … 82 32 11/12/13 Inferring Non ­Loss from ACKs •  Duplicate ACKs also give us hints about what data has arrived –  Each new duplicate ACK means that some...
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This document was uploaded on 04/04/2014.

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