24.6 TCP State Transition Diagram

Team-Fly     TCP/IP Illustrated, Volume 2: The Implementation By Gary R. Wright, W. Richard Stevens Table of Contents Chapter 24.  TCP: Transmission Control Protocol 24.6 TCP State Transition Diagram Many of TCP's actions, in response to different types of segments arriving on a connection, can be summarized in a state transition diagram, shown in Figure 24.15. We also duplicate this diagram on one of the front end papers, for easy reference while reading the TCP chapters. Figure 24.15. TCP state transition diagram. These state transitions define the TCP finite state machine. Although the transition from LISTEN to SYN_SENT is allowed by TCP, there is no way to do this using the sockets API (i.e., a connect is not allowed after a listen). The t_state member of the control block holds the current state of a connection, with the values shown in Figure 24.16. Figure 24.16. t_state values. This figure also shows the tcp_outflags array, which contains the outgoing flags for tcp_output to use when the connection is in that state. Figure 24.16 also shows the numerical values of these constants since the code uses their numerical relationships. For example, the following two macros are defined: #define TCPS_HAVERCVDSYN(s) ((s) >= TCPS_SYN_RECEIVED) #define TCPS_HAVERCVDFIN(s) ((s) >= TCPS_TIME_WAIT) Similarly, we'll see that tcp_notify handles ICMP errors differently when the connection is not yet established, that is, when t_state is less than TCPS_ESTABLISHED. The name TCPS_HAVERCVDSYN is correct, but the name TCPS_HAVERCVDFIN is misleading. A FIN has also been received in the CLOSE_WAIT, CLOSING, and LAST_ACK states. We encounter this macro in Chapter 29. Half-Close When a process calls shutdown with a second argument of 1, it is called a half-close. TCP sends a FIN but allows the process to continue receiving on the socket. (Section 18.5 of Volume 1 contains examples of TCP's half-close.) For example, even though we label the ESTABLISHED state "data transfer," if the process does a half-close, moving the connection to the FIN_WAIT_1 and then the FIN_WAIT_2 states, data can continue to be received by the process in these two states. Team-Fly     Top