TCP IP Illustrated

notice that despite the server saying the transfer

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Unformatted text preview: 01 14.47.49] Chapter 27. FTP: File Transfer Protocol byte beyond the last byte of urgent data. The FTP client purposely writes the first 3 bytes as urgent data, knowing the urgent pointer will (incorrectly) point to the next byte that is written (the data mark, DM, at sequence number 54). This first write with 3 bytes of urgent data is sent immediately, along with the urgent pointer, followed by the next 7 bytes. (The BSD FTP server does not have a problem with which interpretation of the urgent pointer is used by the client. When the server receives urgent data on the control connection it reads the next FTP command, looking for ABOR or STAT, ignoring any embedded Telnet commands.) Notice that despite the server saying the transfer was aborted (segment 18, on the control connection), the client receives 14 more segments of data (sequence numbers 1537 through 5120) on the data connection. These segments were probably queued in the network device driver on the server when the abort was received, but the client prints "1536 bytes received" meaning it ignores all the segments of data that it receives (segments 17 and later) after sending the abort (segments 14 and 15). In the case of a Telnet user typing the interrupt key, we saw in Figure 26.17 that by default the Unix client does not send the interrupt process command as urgent data. We said this was OK because there is little chance that the flow of data from the client to the server is stopped by flow control. With FTP the client is also sending an interrupt process command across the control connection, and since two connections are being used there is little chance that the control connection is stopped by flow control. Why does FTP send the interrupt process command as urgent data when Telnet does not? The answer is that FTP uses two connections, whereas Telnet uses one, and on some operating systems it may be hard for a process to monitor two connections for input at the same time. FTP assumes that these marginal...
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