Strains of tcp including lossbased conges7on control

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Unformatted text preview: new segment has arrived –  It will be the segments ayer the loss –  Thus advancing the sliding window will not increase the number of segments stored in the network 83 Fast Recovery •  First fast retransmit, and MD cwnd •  Then pretend further duplicate ACKs are the expected ACKs –  Lets new segments be sent for ACKs –  Reconcile views when the ACK jumps Ack 1 2 3 4 5 5 5 5 5 5 84 33 11/12/13 Fast Recovery (2) Third duplicate ACK, so send 14 Set ssthresh, cwnd = cwnd/2 More ACKs advance window; may send segments before jump Ack 12 Ack 13 Ack 13 Ack 13 Ack 13 Ack 13 Ack 13 Ack 20 . . . . . . Exit Fast Recovery Data 14 was lost earlier, but got 15 to 20 Data 20 Data 14 Retransmission fills in the hole at 14 Data 21 Data 22 85 Fast Recovery (3) •  With fast retransmit, it repairs a single segment loss quickly and keeps the ACK clock running •  This allows us to realize AIMD –  No 7meouts or slow ­start ayer loss, just con7nue with a smaller cwnd •  TCP Reno combines slow ­start, fast retransmit and fast recovery –  Mul7plica7ve Decrease is ½ 86 34 11/12/13 TCP Reno TCP sawtooth ACK clock running MD of ½ , no slow ­start 87 TCP Reno, NewReno, and SACK •  Reno can repair one loss per RTT –  Mul7ple losses cause a 7meout •  NewReno further refines ACK heuris7cs –  Repairs mul7ple losses without 7meout •  SACK is a be`er idea –  Receiver sends ACK ranges so sender can retransmit without guesswork 88 35 11/12/13 Introduc7on to Computer Networks Explicit Conges7on No7fica7on (§5.3.4, §6.5.10) Computer Science & Engineering Conges7on Avoidance vs. Control •  Classic TCP drives the network into conges7on and then recovers –  Needs to see loss to slow down •  Would be be`er to use the network but avoid conges7on altogether! –  Reduces loss and delay •  But how can we do this? 91 36 11/12/13 Feedback Signals •  Delay and router signals can let us avoid conges7on Signal Example Protocol Pros / Cons Packet loss Classic TCP Cubic TCP (Linux) Hard to get wrong Hear about congestion late Packet delay Compound TCP (Windows) Hear about congestion early Need to infer congestion Router indication TCPs with Explicit Congestion Notification Hear about congestion early Require router support 92 ECN (Explicit Conges7on No7fica7on) •  Router detects the onset of conges7on via its queue –  When congested, it marks affected packets (IP header) 93 37 11/12/13 ECN (2) •  Marked packets arrive at receiver; treated as loss –  TCP receiver reliably informs...
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