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Unformatted text preview: is in the 2MSL wait, but from what we
showed in Section 18.6, the connection won't succeed. The reason is that the socket
pair for the connection request equals the socket pair from step 4 that is still in a
2MSL wait. The rules of TCP forbid the server from sending the SYN. There is no
way for the server to override this 2MSL wait of the socket pair before reusing the
same socket pair.
At this point the BSD server retries the connection request every 5 seconds, up to
18 times, for a total of 90 seconds. We see that segment 9 succeeds about 1 minute
later. (We mentioned in Chapter 18 that SVR4 uses an MSL of 30 seconds, for a
2MSL wait of 1 minute.) We don't see any SYNs from these failures in this time
line because the active opens fail and the server's TCP doesn't even send a SYN.
The reason the Host Requirements RFC recommends using the PORT command is to
avoid this 2MSL wait between successive uses of a data connection. By continually
changing the port number on one end, the problem we just showed disappears.
Text File Transfer: NVT ASCII Representation or Image?
Let's verify that the transmission of a text file uses NVT ASCII by default. This time we file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/ftp_file.htm (14 of 24) [12/09/2001 14.47.49] Chapter 27. FTP: File Transfer Protocol don't specify the -d flag, so we don't see the client commands, but notice that the client
still prints the server's responses:
sun % ftp bsdi
Connected to bsdi.
220 bsdi FTP server (Version 5.60)
we type RETURN
331 Password required for
we type our password
230 User rstevens logged in.
fetch a file
ftp> get hello.c
200 PORT command successful.
150 Opening ASCII mode data connection for hello.c (38
server says file contains 38 bytes
226 Transfer complete.
output by client
local: hello.c remote: hello.c
42 bytes received in 0.0037
42 bytes across data connection
seconds (11 Kbytes/s)
sun % ls -l hello....
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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.
- Spring '12