TCP IP Illustrated

6 tcpdump output for connection establishment that

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Unformatted text preview: ni/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_conn.htm (6 of 37) [12/09/2001 14.47.16] Chapter 18. TCP Connection Establishment and Termination 1 0.0 2 5.814797 ( 5.8148) 3 29.815436 (24.0006) bsdi-1024 > svr4.discard: S 291008001:291008001(0) win 4096 <mss 1024> [tos 0x10] bsdi-1024 > svr4.discard: S 291008001:291008001(0) win 4096 <mss 1024> [tos 0x10] bsdi.l024 > svr4.discard: S 291008001:291008001(0) win 4096 <mss 1024> [tos 0x10] Figure 18.6 tcpdump output for connection establishment that times out. The interesting point in this output is how frequently the client's TCP sends a SYN to try to establish the connection. The second segment is sent 5.8 seconds after the first, and the third is sent 24 seconds after the second. As a side note, this example was run about 38 minutes after the client was rebooted. This corresponds with the initial sequence number of 291,008,001 (approximately 38 x 60 x 64000 x 2). Recall earlier in this chapter we said that typical Berkeley-derived systems initialize the initial sequence number to 1 and then increment it by 64,000 every half-second. Also, this is the first TCP connection since the system was bootstrapped, which is why the client's port number is 1024. What isn't shown in Figure 18.6 is how long the client's TCP keeps retransmitting before giving up. To see this we have to time the telnet command: bsdi % date ; telnet svr4 discard ; date Thu Sep 24 16:24:11 MST 1992 Trying 192.82.148.2... telnet: Unable to connect to remote host: Connection timed out Thu Sep 24 16:25:27 MST 1992 The time difference is 76 seconds. Most Berkeley-derived systems set a time limit of 75 seconds on the establishment of a new connection. We'll see in Section 21.4 that the third packet sent by the client would have timed out around 16:25:29, 48 seconds after it was sent, had the client not given up after 75 seconds. First Timeout Period One puzzling item in Figure 18.6 is that the first timeout period, 5.8 seconds, is clo...
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