This preview shows page 1. Sign up to view the full content.
Unformatted text preview: time=52. ms
64 bytes from svr4 (184.108.40.206): icmp_seq=l. time=90. ms
^? type interrupt key to stop
--220.127.116.11 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 (18.104.22.168) at 0:0:c0:c2:9b:26
bsdi (22.214.171.124) at 0:0:c0:6f:2d:40
IP looks at the destination address (126.96.36.199), 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...
View Full Document
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.
- Spring '12