TCP IP Illustrated

Also notice that the port number for the established

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Unformatted text preview: .* The first line for port 23 is the ESTABLISHED connection. All four elements of the local and foreign address are filled in for this connection: the local IP address and port number, and the foreign IP address and port number. The local IP address corresponds to the interface on which the connection request arrived (the Ethernet interface, 140.252.13.33). The end point in the LISTEN state is left alone. This is the end point that the concurrent server uses to accept future connection requests. It is the TCP module in the kernel that creates the new end point in the ESTABLISHED state, when the incoming connection request arrives and is accepted. Also notice that the port number for the ESTABLISHED connection doesn't change: it's 23, the same as the LISTEN end point. We now initiate another Telnet client from the same client (slip) to this server. Here is the relevant netstat output: Proto tcp tcp RecvQ 0 0 SendForeign Address Local Address (state) Q 140.252.13.33.23 140.252.13.65.1030 ESTABLISHED 0 140.252.13.33.23 140.252.13.65.1029 ESTABLISHED 0 file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_conn.htm (28 of 37) [12/09/2001 14.47.16] Chapter 18. TCP Connection Establishment and Termination tcp 0 0 *.23 *.* LISTEN We now have two ESTABLISHED connections from the same host to the same server. Both have a local port number of 23. This is not a problem for TCP since the foreign port numbers are different. They must be different because each of the Telnet clients uses an ephemeral port, and the definition of an ephemeral port is one that is not currently in use on that host (slip). This example reiterates that TCP demultiplexes incoming segments using all four values that comprise the local and foreign addresses: destination IP address, destination port number, source IP address, and source port number. TCP cannot determine which process gets an incoming segment by looking at the destination port number only. Also, the only one of the three end points at port 23 that will receive incoming connection requests is the one in the LISTEN state. The end points in t...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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