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l14_feb28 - Lecture 14 High-speed TCP connections...

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1 Lecture 14 High-speed TCP connections Wraparound Keeping the pipeline full Estimating RTT Fairness of TCP congestion control Internet resource allocation and QoS
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2 Protection against wraparound What is wraparound: A byte with a sequence number x may be sent at one time and then on the same connection a byte with the same sequence number x may be sent again. Wrap Around: controlled by the 32-bit SequenceNum The maximum lifetime of an IP datagram is 120 sec thus we need to have a wraparound time at least 120 sec. For slow links OK but no longer sufficient for optical networks. Bandwidth & Time Until Wrap Around Bandwidth T1 (1.5Mbps) Ethernet (10Mbps) T3 (45Mbps) FDDI (100Mbps) STS-3 (155Mbps) STS-12 (622Mbps) STS-24 (1.2Gbps) Time Until Wrap Around 6.4 hours 57 minutes 13 minutes 6 minutes 4 minutes 55 seconds 28 seconds
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3 Keeping the pipe full The SequenceNum, the sequence number space (32 bits) should be twice as large as the window size (16 bits). It is. The window size (the number of bytes in transit) is given by the the AdvertisedWindow field (16 bits) . The higher the bandwidth the larger the window size to keep the pipe full . Essentially we regard the network as a storage system and the amount of data is equal to: ( bandwidth x delay )
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4 Required window size for a 100 msec RTT. Bandwidth T1 (1.5Mbps) Ethernet (10Mbps) T3 (45Mbps) FDDI (100Mbps) STS-3 (155Mbps) STS-12 (622Mbps) STS-24 (1.2Gbps) Delay x Bandwidth Product 18KB 122KB 549KB 1.2MB 1.8MB 7.4MB 14.8MB
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5 Original Algorithm for Adaptive Retransmission Measure SampleRTT for each segment/ACK pair Compute weighted average of RTT EstimatedRTT = α x EstimatedRTT + (1- α 29 x SampleRTT where 0.8 < α < 0.9 Set timeout based on EstimatedRTT TimeOut = 2 x EstimatedRTT
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6 Karn/Partridge Algorithm Do not sample RTT when re-transmitting Double timeout after each retransmission
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7 Karn/Partridge Algorithm S e n d e r S e n d e r R e c e i v e r R e c e i v e r S a m p l e R T T O r i g i n a l t r a n s m i s s i o n R e - t r a n s m i s s i o n A c k n o w l e d g m e n t O r i g i n a l t r a n s m i s s i o n A c k n o w l e d g m e n t R e - t r a n s m i s s i o n S a m p l e R T T
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8 Jacobson/Karels Algorithm New calculation for average RTT Diff = SampleRTT - EstimatedRTT EstimatedRTT = EstimatedRTT + ( δ x
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  • Fall '11
  • Staff
  • Transmission Control Protocol, congestion notification, window size, Explicit Congestion Notification, TCP congestion

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