Subject: [MOBILE-IP] FW: Mobile IP Agents as DHCP Proxies

From: "Kay, Rodney" <[email-masked]>
Date: Tue, 13 Jun 2000 12:21:42 -0500

> -----Original Message-----
> From: Kay, Rodney
> Sent: Monday, June 12, 2000 11:13 AM
> To:   [email-masked]'
> Subject:      Mobile IP Agents as DHCP Proxies
>
> I am reviewing the subject document, and have encountered a situation
> which doesn't make much sense to me.  Please forgive me if my question
> seems simplistic, but I am new to the IETF and chose your document due to
> the new mobile computing requirements of my place of employment.
>
> In 3.1(3) you indicate that the HA should compare the IP address in the
> DHCPOFFER with it's own when the DHCPOFFER includes the Mobile IP Home
> Agent option (type 68); and if these are disparate, then an error 136
> "Unknown Home Agent Address" should be issued.  My question - why would an
> HA receive a DHCPOFFER for which it had not issued a DHCPDISCOVER request
> as defined in [2]?
>
> Thank you for your time.
>
> Rodney H. Kay
> Vista Systems Manager
> Puget Sound Health Care System
>

---

From: Steven Glass - Solaris Software <[email-masked]>
Date: Tue, 13 Jun 2000 13:42:33 -0400

Rodney,

    Sorry for the delay in my response.  I got a fairly lengthy feedback from
someone yesterday, which combined with real work occupied more than the usual
block of time...

> DHCPOFFER with it's own when the DHCPOFFER includes the Mobile IP Home Agent
> option (type 68); and if these are disparate, then an error 136 "Unknown
> Home Agent Address" should be issued.  My question - why would an HA receive
> a DHCPOFFER for which it had not issued a DHCPDISCOVER request as defined in
> [2]?

    3.1(2) specifies the DHCP discover, so I assume you're asking why the
DHCPOFFER would have a different value for the Mobile IP Home Agent Address
option...

    The HA sends a DHCPDISCOVER with the Mobile IP Home Agent option set to
either the address in the home agent field of the registration request, or the
IP address of the interface on the mobile node's home link that will be
servicing the mobile node.

    The DHCP Server then builds a DHCPOFFER based on several things, but
specifically to the Mobile IP Home Agent option (see page 29-30 of 2131):

   Once the network address and lease have been determined, the server
   constructs a DHCPOFFER message with the offered configuration
   parameters.

   ...                         The configuration parameters MUST be
   selected by applying the following rules in the order given below.
   The network administrator is responsible for configuring multiple
   DHCP servers to ensure uniform responses from those servers.  The
   server MUST return to the client:

   ...

   o Parameters requested by the client, according to the following
     rules:

        -- IF the server has been explicitly configured with a default
           value for the parameter, the server MUST include that value
           in an appropriate option in the 'option' field, ELSE

        -- IF the server recognizes the parameter as a parameter
           defined in the Host Requirements Document, the server MUST
           include the default value for that parameter as given in the
           Host Requirements Document in an appropriate option in the
           'option' field, ELSE

        -- The server MUST NOT return a value for that parameter,

     The server MUST supply as many of the requested parameters as
     possible and MUST omit any parameters it cannot provide.  The
     server MUST include each requested parameter only once unless
     explicitly allowed in the DHCP Options and BOOTP Vendor
     Extensions document.

     ...

   o Any parameters specific to this client (as identified by
     the contents of 'chaddr' or 'client identifier' in the DHCPDISCOVER
     or DHCPREQUEST message), e.g., as configured by the network
     administrator,

     ...

   o Parameters with non-default values on the client's subnet.

    So if the DHCP Server has a different value configured for the home agent
address assigned to this client-id, it'll include the Mobile IP Home Agent
Address option with the value it's configured to send.

                              Cheers,
                                  Steve