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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.
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
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.
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 servers response back to the client.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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