TCP IP Illustrated

14 001 80203f642 00c06f2d40 ip 65 sun26999

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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 (140.252.13.33). 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 140.252.13.33 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 TFTP.) 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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