TCP IP Illustrated

5 seconds after the first request then again 24

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Unformatted text preview: ime shown in the tcpdump output is 29.5 seconds. But the output from the date commands before and after the telnet command shows that the connection request from the Telnet client appears to have given up after about 75 seconds. Indeed, we'll see later that most BSD implementations set a limit of 75 seconds for a TCP connection request to complete. In Chapter 18 when we see the sequence of TCP segments that is sent to establish the connection, we'll see that these ARP requests correspond one-to-one with the initial TCP SYN (synchronize) segment that TCP is trying to send. Note that on the wire we never see the TCP segments. All we can see are the ARP requests. Until an ARP reply comes back, the TCP segments can't be sent, since the destination hardware address isn't known. If we ran tcpdump in a filtering mode, looking only for TCP data, there would have been no output at all. ARP Cache Timeout A timeout is normally provided for entries in the ARP cache. (In Section 4.8 we'll see that the arp command allows an entry to be placed into the cache by the administrator that will never time out.) Berkeley-derived implementations normally have a timeout of 20 minutes for a completed entry and 3 minutes for an incomplete entry (We saw an incomplete entry in our previous example where we forced an ARP to a nonexistent host on the Ethernet.) These implementations normally restart the 20-minute timeout for an entry each time the entry is used. The Host Requirements RFC says that this timeout should occur even if the entry is in use, but most Berkeleyderived implementations do not do this-they restart the timeout each time the entry is referenced. 4.6 Proxy ARP Proxy ARP lets a router answer ARP requests on one of its networks for a host on another of its networks. This fools the sender of the ARP request into thinking that the router is the destination host, when in fact the destination host is "on the other side" of the router. The router is acting as a proxy agent for the destination host, relaying packets to it from other hosts. An example is the best way to describe proxy AR...
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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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