Subject: [mobile-ip] Clarification required on DHAAD section

From: [email-masked]
Date: Fri, 25 Oct 2002 12:42:26 +0530

In section 11.4.1. Dynamic Home Agent Address Discovery, it is
mentioned that

" For example, if necessary, the mobile node MAY attempt its home
  registration with each of these home agents, in turn, by sending
  each a Binding Update and waiting for the matching Binding
  Acknowledgement, until its registration is accepted by one of
  these home agents. "

1. If the mobile node sends a BU with D bit set continuously to all
these Home Agents (in the listed order) without any delay, then
there is a high possibility of DAD failure at all Home Agents.
Assume that DupAddrDetectTransmits at Home network is 3 times, then
before the first HA completes the DAD, second HA will start DAD
and this will continue with all HA's. At last, the mobile node will
not be able to register with any of the HA.

Should we need to give some delay between BU's? At least, 1.5 times
longer than (RetransTimer * DupAddrDetectTransmits).

2. Otherwise, if the mobile node tries with each of these
Home Agents following the BU Retransmission rules, i.e., trying
the second HA only after sending MAX_BINDACK_TIMEOUT BU's with
exponential back-off to the first HA, then the mobile node has to
unnecessarily wait before it registers with the second HA, if the
first HA is down.

In this case, Can we not send BU first to the HA which has replied
for the DHAAD request always. Should we need to strictly follow the
listed order.

Thanks in Advance.
Srinivasan.D

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From: Jari Arkko <[email-masked]>
Date: Sat, 26 Oct 2002 13:49:31 +0300

> not be able to register with any of the HA.

Yes.

> longer than (RetransTimer * DupAddrDetectTransmits).

As you indicated further down in your e-mail it isn't clear that retransmission
rules can be applied in this case. So we need to add the following:

     "... The mobile node MUST, however, wait at least 1.5 times
      longer than (RetransTimer * DupAddrDetectTransmits) before
      sending a Binding Update to the next home agent."

(But note that there are further complications: DupAddrDetectTransmits
is a node configuration variable, and there are no means to ensure
we have the same value as is used on the home network. And RetransTimer
isn't communicated to mobile nodes unless they are, or have been on
the home network. So we are basically assuming the values that the
home network uses. But my personal recommendation is to not take
these further complications in account; there's little we can do
about them and in most cases the given expression is sufficient.)

> unnecessarily wait before it registers with the second HA, if the

Yes.

> for the DHAAD request always. Should we need to strictly follow the

I'd rather use the listed order. If it is out-of-date, perhaps
the router advertisement frequency on the home network should be
increased, or vendors should build better tracking of other home
agents using NUD probes.

Jari