module15-tcp3V2

module15-tcp3V2 - 1 TCP III - Error Control TCP Error...

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Unformatted text preview: 1 TCP III - Error Control TCP Error Control 2 ARQ Error Control • Two types of errors: – Lost packets – Damaged packets • Most Error Control techniques are based on: 1. Error Detection Scheme (Parity checks, CRC). 2. Retransmission Scheme. • Error control schemes that involve error detection and retransmission of lost or corrupted packets are referred to as Automatic Repeat Request (ARQ) error control . 3 Background: ARQ Error Control • The most common ARQ retransmission schemes: – Stop-and-Wait ARQ – Go-Back-N ARQ – Selective Repeat ARQ • The protocol for sending ACKs in all ARQ protocols are based on the sliding window flow control scheme • TCP uses a version of the Go-Back-N Protocol 4 Background: Go-Back-N ARQ P a c k e t • Go-Back-N sends cumulative acknowledgments A B P a c k e t 1 A C K 1 P a c k e t 3 P a c k e t 4 P a c k e t 2 P a c k e t 3 Packets 2,3,4,5 are retransmitted A C K 2 P a c k e t 2 loss P a c k e t 5 P a c k e t 4 A C K 1 A C K 1 A C K 1 Timeout for Packet 2 A C K 3 A C K 4 A C K 5 P a c k e t 5 A C K 6 5 Retransmissions in TCP • A TCP sender retransmits a segment when it assumes that the segment has been lost: 1. No ACK has been received and a timeout occurs 2. Multiple ACKs have been received for the same segment 6 Receiving duplicate ACKs • If three or more duplicate ACKs are received in a row, the TCP sender believes that a segment has been lost. • Then TCP performs a retransmission of what seems to be the missing segment, without waiting for a timeout to happen. • This can fix losses of single segments 1K SeqNo=0 AckNo=1024 AckNo=1024 1K SeqNo=1024 SeqNo=2048 1K AckNo=1024 SeqNo=3072 1K SeqNo=4096 1K 1. duplicate 2. duplicate AckNo=1024 SeqNo=1024 1K SeqNo=5120 1K 3. duplicate 7...
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This note was uploaded on 03/05/2012 for the course CS 458 taught by Professor Staff during the Fall '08 term at UVA.

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module15-tcp3V2 - 1 TCP III - Error Control TCP Error...

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