TCP IP Illustrated

252129 for a total of 75 seconds similar to figure 186

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Unformatted text preview: .29. This is rejected in line 2. The SMTP client on vangogh then tries the next IP address for sun: (line 3), and it also causes an RST to be returned (line 4). file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/smtp_sim.htm (12 of 23) [12/09/2001 14.47.52] Chapter 28. SMTP: Simple Mail Transfer Protocol The SMTP client doesn't try to differentiate between the different error returns from its active open on line 1, which is why it tries the other IP address on line 3. If the error had been something like "host unreachable" for the first attempt, it's possible that the second attempt could work. If the reason the SMTP client's active open fails is because the server host is down, we would see the client retransmit the SYN to IP address for a total of 75 seconds (similar to Figure 18.6), followed by the client sending another three SYNs to IP address for another 75 seconds. After 150 seconds the client would move on to the next MX record with the higher preference. VRFY and EXPN Commands The VRFY command verifies that a recipient address is OK, without actually sending mail. EXPN is intended to expand a mailing list, without sending mail to the list. Many SMTP implementations (such as Sendmail) consider the two the same, but we mentioned that newer versions of Sendmail do differentiate between the two. As a simple test we can connect to a newer version of Sendmail and see the difference. (We have removed the extraneous Telnet client output.) sun % telnet 25 220-vangogh.CS. Berkeley. EDU Sendmail 8.1C/6.32 ready at Tue, 3 Aug 1993 14: 59:12 -0700 220 ESMTP spoken here helo 250 vangogh.CS.Berkeley.EDU Hello [], pleased to meet you vrfy nosuchname 550 nosuchname... User unknown vrfy rstevens 250 Richard Stevens <rstevens@vangogh.CS.Berkeley.EDU> expn rstevens 250 Richard Stevens <> First notice that we purposely ty...
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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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