TCP IP Illustrated

Each end must transmit a syn and the syns must pass

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Unformatted text preview: n ARP request and reply are required (lines 7 and 8). Then the reset is sent in line 9. The client receives the reset and outputs that the connection was terminated by the foreign host. (The final message output by the Telnet client is not as informative as it could be.) 18.8 Simultaneous Open file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_conn.htm (22 of 37) [12/09/2001 14.47.16] Chapter 18. TCP Connection Establishment and Termination It is possible, although improbable, for two applications to both perform an active open to each other at the same time. Each end must transmit a SYN, and the SYNs must pass each other on the network. It also requires each end to have a local port number that is well known to the other end. This is called a simultaneous open. For example, one application on host A could have a local port of 7777 and perform an active open to port 8888 on host B. The application on host B would have a local port of 8888 and perform an active open to port 7777 on host A. This is not the same as connecting a Telnet client on host A to the Telnet server on host B, at the same time that a Telnet client on host B is connecting to the Telnet server on host A. In this Telnet scenario, both Telnet servers perform passive opens, not active opens, and the Telnet clients assign themselves an ephemeral port number, not a port number that is well known to the other Telnet server. TCP was purposely designed to handle simultaneous opens and the rule is that only one connection results from this, not two connections. (Other protocol suites, notably the OSI transport layer, create two connections in this scenario, not one.) When a simultaneous open occurs the state transitions differ from those shown in Figure 18.13. Both ends send a SYN at about the same time, entering the SYN_SENT state. When each end receives the SYN, the state changes to SYN_RCVD (Figure 18.12), and each end resends the SYN and acknowledges the received SYN. When each end receives the SYN plus t...
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