TCP IP Illustrated

earlier bsd implementations incorrectly aborted a

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Unformatted text preview: sion q A received host unreachable or network unreachable is effectively ignored, since these two errors are considered transient. It could be that an intermediate router has gone down and it can take the routing protocols a few minutes to stabilize on an alternative route. During this period either of these two ICMP errors can occur, but they must not abort the connection. Instead, TCP keeps trying to send the data that caused the error, although it may eventually time out. (Recall in Figure 21.1 that TCP did not give up for 9 minutes.) Current Berkeley-based implementations record that the ICMP error occurred, and if the connection times out, the ICMP error is translated into a more relevant error code than "connection timed out." Earlier BSD implementations incorrectly aborted a connection whenever an ICMP host unreachable or network unreachable was received. An Example We can see how an ICMP host unreachable is handled by taking our dialup SLIP link down during the middle of a connection. We establish a connection from the host slip to the host aix. (From the figure on the inside front cover we see that this connection goes through our dialup SLIP link.) After establishing the connection and transferring some data, the dialup SLIP link between the routers sun and netb is taken down. This causes the default routing table entry on sun (which we showed in Section 9.2) to be removed. We expect sun to then respond to IP datagrams destined for the 140.252.1 Ethernet with an ICMP host unreachable. We want to see how TCP handles these ICMP errors. Here is the interactive session on the host slip: slip % sock aix echo test line test line another line another line line number 3 line number 3 the last line run our sock program type this line and it's echoed SLIP link is brought down here then type this line and watch retransmissions SLIP link is reestablished here and the line and its echo are exchanged SLIP link is brought down here, and not reestablished read e...
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