Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Searchable, convenient, complete TCP/IP information.
The TCP/IP Guide

Custom Search







Table Of Contents  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)

Previous Topic/Section
BOOTP Vendor-Specific Area and Vendor Information Extensions
Previous Page
Pages in Current Topic/Section
123
4
Next Page
TCP/IP Dynamic Host Configuration Protocol (DHCP)
Next Topic/Section

BOOTP Relay Agents (Forwarding Agents)
(Page 4 of 4)

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.

 


Previous Topic/Section
BOOTP Vendor-Specific Area and Vendor Information Extensions
Previous Page
Pages in Current Topic/Section
123
4
Next Page
TCP/IP Dynamic Host Configuration Protocol (DHCP)
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.