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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Bootstrap Protocol (BOOTP)

BOOTP Vendor-Specific Area and Vendor Information Extensions 1 2 3 4 TCP/IP Dynamic Host Configuration Protocol (DHCP)

Relaying BOOTP Requests Using Broadcasts

The simplest case of relaying is when each network has a relay agent that knows the IP address of the BOOTP server. The relay agent “captures” the request in step 3 above, and sends it directly to the BOOTP server, wherever it may be on the network. The request is relayed as a regular unicast UDP message and routed to the BOOTP server. The BOOTP server's reply is routed back to the BOOTP relay agent just like any UDP message in an IP datagram, and the relay agent forwards the reply.

It is also possible to set up BOOTP relay agents to relay requests even if they don't know the BOOTP server's address. These agents take requests received on one network and relay them to the next, where they expect another agent to continue the relaying process until a BOOTP server is reached. For example, suppose we have a set of three networks. Network N1 is connected to Network N2 using Router RA, and N2 connects to N3 using Router RB. Both of these routers function as relay agents but don't know the IP address of the BOOTP server. Here's what would happen if a client on N1 sent a request and the server was on N3:

1. The client would send its request.
   
   
2. RA would capture the request and put its address into GIAddr. It would increment the Hops field to a value of 1 and then broadcast the request out on Network N2.
   
   
3. RB would capture this request. It would see there is already an address in GIAddr so it would leave that alone. It would increment the Hops field to 2 and broadcast the request on Network N3.
   
   
4. The BOOTP server would receive the request, process it and return the reply directly back to RA.
   
   
5. RA would relay the reply back to the client.

As you can see, the purpose of the Hops field is to ensure that errant requests don't circle around the network endlessly. Each relay agent increments it and if the value of 16 is ever exceeded, the request is dropped. You can also see that any relay agents other than the first are involved only for handling the request; the reply is sent unicast back to the agent closest to the client.

Incidentally, if this multiple-step relaying process sounds like IP routing (only using broadcasts), and the Hops field sounds like the Time To Live field in an IP datagram, then you've been paying attention. It is essentially the same idea.

BOOTP Vendor-Specific Area and Vendor Information Extensions 1 2 3 4 TCP/IP Dynamic Host Configuration Protocol (DHCP)