·TCP/IP Protocols (21) -- TCP Timeout and Retransmission

TCP provides a reliable transport layer. One of the ways it provides reliability is for each end to acknowledge the data it receives from the other end. But data segments and acknowledgments can get lost. TCP handles this by setting a timeout when it sends data, and if the data isn't acknowledged when the timeout expires, it retransmits the data. A critical element of any implementation is the timeout and retransmission strategy.

TCP manages four different timers for each connection.

1. A retransmission timer is used when expecting an acknowledgment from the other end. This chapter looks at this timer in detail, along with related issues such as congestion avoidance.

2. A persist timer keeps window size information flowing even if the other end closes its receive window. 

3. A keepalive timer detects when the other end on an otherwise idle connection crashes or reboots.

4. A 2MSL timer measures the time a connection has been in the TIME_WAIT state.

Round-Trip Time Measurement

Fundamental to TCP's timeout and retransmission is the measurement of the round-trip time (RTT) experienced on a given connection. We expect this can change over time, as routes might change and as network traffic changes, and TCP should track these changes and modify its timeout accordingly.

First TCP must measure the RTT between sending a byte with a particular sequence number and receiving an acknowledgment that covers that sequence number. Normally there is not a one-to-one correspondence between data segments and ACKs.

The timing is done by incrementing a counter every time the 500-ms TCP timer routine is invoked. This means that a segment whose acknowledgment arrives 550 rns after the segment was sent could end up with either a 1 tick RTT (implying 500 ms) or a 2 tick RTT (implying 1000 ms).

In addition to this tick counter for each connection, the starting sequence number of the data in the segment is also remembered. When an acknowledgment that includes this sequence number is received, the timer is turned off. If the data was not retransmitted when the ACK arrives, the smoothed RTT and smoothed mean deviation are updated based on this new measurement.