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Unformatted text preview: uest is broadcast (line 1) and the RARP reply on line 2 is unicast. The
output on line 2, at sun, means the RARP reply contains the IP address for the host sun
On line 3 we see that once sun receives its IP address, it issues a TFTP read-request
(RRQ) for the file 8CFCOD21.SUN4C. (TFTP is the Trivial File Transfer Protocol. We
describe it in more detail in Chapter 15.) The eight hexadecimal digits in the filename are
the hex representation of the IP address 184.108.40.206 for the host sun. This is the IP
address that was returned in the RARP reply. The remainder of the filename, SUN4C,
indicates the type of system being bootstrapped.
tcpdump says that line 3 is an IP datagram of length 65, and not a UDP datagram
(which it really is), because we are running tcpdump with the -e flag, to see the
hardware-level addresses. Another point to notice in Figure 5.1 is that the length of the
Ethernet frame on line 2 appears to be shorter than the minimum (which we said was 60
bytes in Section 4.5.) The reason is that we are running tcpdump on the system that is
sending this Ethernet frame (bsdi). The application, rarpd, writes 42 bytes to the BSD
Packet Filter device (14 bytes for the Ethernet header and 28 bytes for the RARP reply)
and this is what tcpdump receives a copy of. But the Ethernet device driver pads this
short frame to the minimum size for transmission (60). Had we been running tcpdump on
another system, the length would have been 60.
We can see in this example that when this diskless system receives its IP address in an
RARP reply, it issues a TFTP request to read a bootstrap image. At this point we won't
go into additional detail about how diskless systems bootstrap themselves. (Chapter 16
describes the bootstrap sequence of a diskless X terminal using RARP, BOOTP, and
Figure 5.2 shows the resulting packets if there is no RARP server on the network. The
destination address of each packet is the Ethernet broadcast address. The Ethernet
address following who-is is the target hardware address,...
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- Spring '12