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Unformatted text preview: py of the broadcast packet
through some internal loopback mechanism. Since by definition the term "broadcast" means
all the hosts on the local network, it should include the sending host. (Referring to Figure
2.4 what is happening is that when the Ethernet driver recognizes that the destination
address is the broadcast address, the packet is sent onto the network and a copy is made and
passed to the loopback interface.)
Next, bsdi broadcasts the reply, while svr4 sends the reply only to the requestor.
Normally the reply should be unicast unless the source IP address of the request is 0.0.0.0,
which it isn't in this example. Therefore, sending the reply to the broadcast address is a
BSD/386 bug. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/icmp_int.htm (5 of 16) [12/09/2001 14.46.42] Chapter 6. ICMP: Internet Control Message Protocol The Host Requirements RFC says that a system must not send an address mask reply unless
it is an authoritative agent for address masks. (To be an authoritative agent it must be
specifically configured to send these replies. See Appendix E.) As we can see from this
example, however, most host implementations send a reply if they get a request. Some hosts
even send the wrong reply!
The final point is shown by the following example. We send an address mask request to our
own IP address and to the loopback address:
sun % icmpaddrmask sun
received mask = ff000000, from 18.104.22.168
sun % icmpaddrmask localhost
received mask = ff000000, from 127.0.0.1
In both cases the returned address mask corresponds to the loopback address, the class A
address 127.0.0.1. Again, referring to Figure 2.4 we see that IP datagrams sent to the host's
own IP address (22.214.171.124 in this example) are actually sent to the loop-back interface.
The ICMP address mask reply must correspond to the subnet mask of the interface on
which the request was received (since a multihomed host can have different subnet masks...
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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.
- Spring '12