lec12b - Fast Retransmit time-out period often relatively...

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Transport Layer 3-71 Fast Retransmit ! time-out period often relatively long: " long delay before resending lost packet ! detect lost segments via duplicate ACKs. " sender often sends many segments back-to- back " if segment is lost, there will likely be many duplicate ACKs. ! if sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost: " fast retransmit: resend segment before timer expires
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Transport Layer 3-72 Host A timeout Host B time X resend 2 nd segment Figure 3.37 Resending a segment after triple duplicate ACK
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Transport Layer 3-73 event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } else { increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) { resend segment with sequence number y } Fast retransmit algorithm: a duplicate ACK for already ACKed segment fast retransmit
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Transport Layer 3-75 TCP Flow Control ! receive side of TCP connection has a receive buffer: ! speed-matching service: matching the send rate to the receiving app s drain rate ! app process may be slow at reading from buffer sender won t overflow receiver s buffer by transmitting too much, too fast flow control
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Transport Layer 3-76 TCP Flow control: how it works (suppose TCP receiver discards out-of-order segments) ! spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] ! rcvr advertises spare room by including value of RcvWindow in segments !
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