TCP IP Illustrated

Homenet2runtcpiptcp ip illustratedtcpinthtm 10 of 14

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Unformatted text preview: t; vangogh.login: 2 slip.1023 > vangogh.login: 2 vangogh.login > slip.1023: 4 P 1:2(1) ack P 2:3(1) ack P 3:4(1) ack P 5:6(1) ack slip.1023 > vangogh.login: . ack 2 vangogh.login > slip.1023: P 2:6(4) ack 4 slip.1023 > vangogh.login: . ack 6 type F2 key slip.1023 > vangogh.login: 6 slip.1023 > vangogh.login: 6 slip.1023 > vangogh.login: 6 vangogh.login > slip.1023: 5 vangogh.login > slip.1023: 1 P 4:5(1) ack P 5:6(1) ack P 6:7(1) ack P 6:8(2) ack P 8:10(2) ack slip.1023 > vangogh.login: . ack 10 Figure 19.7 Disabling the Nagle algorithm during an Rlogin session. It is instructive and more enlightening to take this output and construct the time line, file:///D|/Documents%20and%20Settings/bigini/Docu.../homenet2run/tcpip/tcp-ip-illustrated/tcp_int.htm (10 of 14) [12/09/2001 14.47.18] Chapter 19. TCP Interactive Data Flow knowing that some of the segments are crossing in the network. Also, this example requires careful examination of the sequence numbers, to follow the data flow. We show this in Figure 19.8. We have numbered the segments to correspond with the numbering in the tcpdump output in Figure 19.7. The first change we notice is that all 3 bytes are sent when they're ready (segments 1,2, and 3). "There is no delay-the Nagle algorithm has been disabled. The next packet we see in the tcpdump output (segment 4) contains byte 5 from the server with an ACK 4. This is wrong. The client immediately responds with an ACK 2 (it is not delayed), not an ACK 6, since it wasn't expecting byte 5 to arrive. It appears a data segment was lost. We show this with a dashed line in Figure 19.8. How do we know this lost segment contained bytes 2, 3, and 4, along with an ACK 3? The next byte we're expecting is byte number 2, as announced by segment 5. (Whenever TCP receives out-of-order data beyond the next expected sequence number, it normally responds with an acknowledgment specifying the sequence number of the next byte it expects to receive.) Also, since the missing segment contained bytes 2, 3, and 4, it means the server must have received segment 2, so the missing segment must have specified an ACK 3 (the sequence number of the...
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