21.1 Introduction

21.1 Introduction

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. How is the timeout interval determined, and how frequently does a retransmission occur?

We've already seen two examples of timeout and retransmission: (1) In the ICMP port unreachable example in Section 6.5 we saw the TFTP client using UDP employing a simple (and poor) timeout and retransmission strategy: it assumed 5 seconds was an adequate timeout period and retransmitted every 5 seconds. (2) In the ARP example to a nonexistent host (Section 4.5), we saw that when TCP tried to establish the connection it retransmitted its SYN using a longer delay between each retransmission.

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. Chapter 22 describes this timer.

3. A keepalive timer detects when the other end on an otherwise idle connection crashes or reboots. Chapter 23 describes this timer.

4. A 2MSL timer measures the time a connection has been in the TIME_WAIT state. We described this state in Section 18.6.

In this chapter we start with a simple example of TCP's timeout and retransmission and then move to a larger example that lets us look at all the details involved in TCP's timer management. We look at how typical implementations measure the round-trip time of TCP segments and how TCP uses these measurements to estimate the retransmission timeout of the next segment it transmits. We then look at TCP's congestion avoidance ” what TCP does when packets are lost ” and follow through an actual example where packets are lost. We also look at the newer fast retransmit and fast recovery algorithms, and see how they let TCP detect lost packets faster than waiting for a timer to expire.