TCP IP Illustrated


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Unformatted text preview: l Multiple RARP Servers per Network Another complication is that RARP requests are sent as hardware-level broadcasts, as shown in Figure 5.2. This means they are not forwarded by routers. To allow diskless systems to bootstrap even when the RARP server host is down, multiple RARP servers are normally provided on a single network (e.g., a single cable). As the number of servers increases (to provide redundancy), the network traffic increases, since every server sends an RARP reply for every RARP request. The diskless system that sent the RARP request normally uses the first RARP reply that it receives. (We never had this problem with ARP, because only a single host sends an ARP reply.) Furthermore, there is a chance that each RARP server can try to respond at about the same time, increasing the probability of collisions on an Ethernet. 5.5 Summary RARP is used by many diskless systems to obtain their IP address when bootstrapped. The RARP packet format is nearly identical to the ARP packet. An RARP request is broadcast, identifying the sender's hardware address, asking for anyone to respond with the sender's IP address. The reply is normally unicast. Problems with RARP include its use of a link-layer broadcast, preventing most routers from forwarding an RARP request, and the minimal information returned: just the system's IP address. In Chapter 16 we'll see that BOOTP returns more information for the diskless system that is bootstrapping: its IP address, the name of a host to bootstrap from, and so on. While the RARP concept is simple, the implementation of an RARP server is system dependent. Hence not all TCP/IP implementations provide an RARP server. Exercises 5.1 Is a separate frame type field required for RARP? Could the same value be used for ARP and RARP 0x0806? 5.2 With multiple RARP servers on a network, how can they prevent their responses from colliding with each on the network? file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/rarp_rev.htm (5 of 5) [12/09/2001 14.46.40] Chapter 6. ICMP: Internet Control Message Protocol ICMP...
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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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