TCP IP Illustrated

2521334 at 00c0c29b26 bsdi 1402521335 at 00c06f2d40 ip

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Unformatted text preview: time=52. ms 64 bytes from svr4 (140.252.13.34): icmp_seq=l. time=90. ms ^? type interrupt key to stop --140.252.13.63 PING Statistics-2 packets transmitted, 6 packets received, -200% packet loss round-trip (ms) min/avg/max = 1/85/192 sun % arp -a check ARP cache again svr4 (140.252.13.34) at 0:0:c0:c2:9b:26 bsdi (140.252.13.35) at 0:0:c0:6f:2d:40 IP looks at the destination address (140.252.13.63), determines that it is the subnetdirected broadcast address, and sends the datagram to the link-layer broadcast address. We mentioned in Section 6.3 that this type of broadcast means all the hosts on the local network, including the sender. We see here that we do get a reply from the sending host (sun) in addition to the other hosts on the cable. In this example we've also shown the ARP cache before and after the ping of the broadcast address. This is to show the interaction between broadcasting and ARP. "The ARP cache is empty before we execute ping, but full afterward. (That is, there is one entry for every other host on the cable that responded to the echo request.) How did this happen when we said that the Ethernet frame is sent to the link-layer broadcast address (0xffffffff)? The sending of these frames by sun does not require ARP. If we watch ping using tcpdump, we see that it is the recipients of the broadcast frames that generate an ARP request to sun, before they can send their reply. This is because the reply is unicast. We said in Section 4.5 that the receiver of an ARP request (sun in this example) normally adds the requestor's IP address and hardware address to its ARP cache, in addition to sending an ARP reply. This is on the assumption that if the requestor is about to send us a packet, we'll probably want to send something back. Our use of ping is somewhat special because the type of programming interface that it uses (called "raw sockets" on most Unix implementations) always allows a datagram to be sent to the broadcast address. What if we use an application that w...
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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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