25.4 tcp_fasttimo Function

Team-Fly     TCP/IP Illustrated, Volume 2: The Implementation By Gary R. Wright, W. Richard Stevens Table of Contents Chapter 25.  TCP Timers 25.4 tcp_fasttimo Function The function tcp_fasttimo, shown in Figure 25.7, is called by pr_fasttimo every 200 ms. It handles only the delayed ACK timer. Figure 25.7. tcp_fasttimo function, which is called every 200 ms. Each Internet PCB on the TCP list that has a corresponding TCP control block is checked. If the TF_DELACK flag is set, it is cleared and the TF_ACKNOW flag is set instead. tcp_output is called, and since the TF_ACKNOW flag is set, an ACK is sent. How can TCP have an Internet PCB on its PCB list that doesn't have a TCP control block (the test at line 50)? When a socket is created (the PRU_ATTACH request, in response to the socket system call) we'll see in Figure 30.11 that the creation of the Internet PCB is done first, followed by the creation of the TCP control block. Between these two operations a high-priority clock interrupt can occur (Figure 1.13), which calls tcp_fasttimo. Team-Fly     Top