TCP IP Illustrated

When a request is received the relay agent places its

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Unformatted text preview: ystem with a disk is used as a router, it can probably run a BOOTP server itself. Alternatively, the common Unix BOOTP server (Appendix F) supports this relay mode, but again, if you can run a BOOTP server on the physical network, there's normally no need to forward the requests to yet another server on another network. What happens is that the router (also called the "BOOTP relay agent") listens tor BOOTP requests on the server's well-known port (67). When a request is received, the relay agent places its IP address into the gateway IP address field in the BOOTP request, and sends the request to the real BOOTP server. (The address placed by the relay agent into the gateway field is the IP address of the interface on which the request was received.) The relay agent also increments the hops field by one. (This is to prevent infinite loops in case the request is reforwarded. RFC 951 mentions that the request should probably be thrown away if the hop count reaches 3.) Since the outgoing request is a unicast datagram (as opposed to the original client request that was broadcast), it can follow any route to the real BOOTP server, passing through other routers. The real server gets the request, forms the BOOTP reply, and sends it back to the relay agent, not the client. The real server knows that the request has been forwarded, since the gateway field in the request is nonzero. The relay agent receives the reply and sends it to the client. 16.6 Vendor-Specific Information In Figure 16.2 we showed a 64-byte vendor-specific area, RFC 1533 [Alexander and Droms 1993] defines the format of this area. This area contains optional information for the server to return to the client. If information is provided, the first 4 bytes of this area are set to the IP address file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/bootp.htm (7 of 9) [12/09/2001 14.47.08] Chapter 16. BOOTP: Bootstrap Protocol This is called the magic cookie and means there is information in the area. The re...
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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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