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BOOTP Relay Agents (Forwarding Agents)
(Page 2 of 4)
The Function of BOOTP Relay Agents
To make this all work, then, we need something to act as an intermediary between the client and the server: a BOOTP relay agent. The job of a BOOTP relay agent is to sit on a physical network where BOOTP clients may be located and act as a proxy for the BOOTP server. The agent gets its name because it relays messages between the client and server, and thus enables them to be on different networks.
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Note: BOOTP relay agents were originally called forwarding agents. That name was considered too easy to cause confusion between BOOTP operation and the general “forwarding” behavior of regular routers. RFC 1542 changed the name to make explicit the fact that BOOTP relaying was not the same as conventional IP datagram forwarding.In practice, a BOOTP relay agent is not usually a separate piece of hardware. It's a software module that runs on an existing piece of hardware that performs other functions. It is common for BOOTP relay agent functionality to be implemented on an IP router. In that case, the router is acting both as a “regular” router and also playing the role of a BOOTP agent. The forwarding functions required of a BOOTP relay agent are distinct from the normal IP datagram forwarding tasks of a router (though there are certain similarities as we will see.)
Naturally, the placement of the client and server on different networks and the presence of a relay agent changes the normal request/reply process of BOOTP significantly. A couple of specific fields in the BOOTP message format are used to control the process. RFC 951 was rather vague in describing how this process works, so RFC 1542 described it in much more detail.
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Key Concept: Since BOOTP uses broadcasts, the BOOTP client and BOOTP server must be on the same physical network to be able to hear each others broadcasted transmissions. For a client and server on different networks to communicate, a third party is required to facilitate the transaction: a BOOTP relay agent. This device, which is often a router, listens for transmissions from BOOTP clients and relays them to the BOOTP server. The server responds back to the agent, which then sends the server’s response back to the client.
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Version 3.0 - Version Date: September 20, 2005
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