TCP IP Illustrated

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Unformatted text preview: verse Address Resolution Protocol 5.1 Introduction When a system with a local disk is bootstrapped it normally obtains its IP address from a configuration file that's read from a disk file. But a system without a disk, such as an X terminal or a diskless workstation, needs some other way to obtain its IP address. Each system on a network has a unique hardware address, assigned by the manufacturer of the network interface. The principle of RARP is for the diskless system to read its unique hardware address from the interface card and send an RARP request (a broadcast frame on the network) asking for someone to reply with the diskless system's IP address (in an RARP reply). While the concept is simple, the implementation is often harder than ARP for reasons described later in this chapter. The official specification of RARP is RFC 903 [Finlayson et al. 1984]. 5.2 RARP Packet Format The format of an RARP packet is almost identical to an ARP packet (Figure 4.3). The only differences are that the frame type is 0x8035 for an RARP request or reply, and the op field has a value of 3 for an RARP request and 4 for an RARP reply. As with ARP, the RARP request is broadcast and the RARP reply is normally unicast. 5.3 RARP Examples In our internet we can force the host sun to bootstrap from the network, instead of its local disk. If we run an RARP server and tcpdump on the host bsdi we get the output shown in Figure 5.1. We use the -e flag to have tcpdump print the hardware addresses: 1 0.0 2 0.13 (0.13) 8:0:20:3:f6:42 ff:ff:ff:ff:ff:ff rarp 60: rarp who-is 8:0:20:3:f6:42 tell 8:0:20:3:f6:42 0:0:c0:6f:2d:40 8:0:20:3:f6:42 rarp 42: rarp reply 8:0:20:3:f6:42 at sun file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/rarp_rev.htm (1 of 5) [12/09/2001 14.46.40] Chapter 5. RARP: Reverse Address Resolution Protocol 3 0.14 (0.01) 8:0:20:3:f6:42 0:0:c0:6f:2d:40 ip 65: >sun.26999 > bsdi.tftp: 23 RRQ "8CFCOD21.SUN4C" Figure 5.1 RARP request and reply. The RARP req...
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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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