TCPIIIErrorControl.pptVIP

EL 537 - Fall 1997 TCP III - Error Control 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. 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 Background: Go-Back-N ARQ Go-Back-N sends cumulative acknowledgments Retransmissions in TCP A TCP sender retransmits a segment when it assumes that the segment has been lost: No ACK has been received and a timeout occurs Multiple ACKs have been received for the same segment 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 Retransmission Timer TCP sender maintains one retransmission timer for each connection When the timer reaches the retransmission timeout (RTO) value, the sender retransmits the first segment that has not been acknowledged The timer is started when When a packet with payload is transmitted and timer is not running When an ACK arrives that acknowledges new data, When a segment is retransmitted The timer is stopped when All segments are acknowledged How to set the timer Retransmission Timer: The setting of the retransmission timer is crucial for good performance of TCP Timeout value too small ? results in unnecessary retransmissions Timeout value too large ? long waiting time before a retransmission can be issued A problem is t