Subject: [mobile-ip] Duplicate Address Detection

From: "Peter Grimstrup (TED)" <[email-masked]>
Date: Wed, 10 Oct 2001 10:53:44 +0200

Draft 15 and probably the earlier drafts says:

D bit MUST not be set without A and H bit (sec. 5.1)

Section 8.2 says nothing about what to do if D is set but H is not.
One could 1) discard packet or 2) ignore D-bit or ?

Secondly, section 9.1 doesn't mention what to do if AHD == 011.
One could 1) discard packet or 2) continue as if the A bit was 1?
Later in section 9.1, A-bit is in a way forced to 1.
To be consistent, one could release section 5.1 to D-bit must not
be set without H-bit - or maybe better: D-bit is ignored if the
H-bit is not set (solves section 8.2 question as well)

Thirdly, section 9.1 requires DAD unconditionally when D-bit is
set. Does this mean, that even if the HA is already defending
the MN (i.e. the BU is updating/refreshing an already existing
entry), the DAD must be executed again?

/Peter

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From: Brian Haley USG <[email-masked]>
Date: Wed, 10 Oct 2001 11:18:47 -0400

> One could 1) discard packet or 2) ignore D-bit or ?

In my opinion, since section 5.1 says "This bit is only valid when the
Home Registration (H) and Acknowledge (A) bits are also set...", we
should ignore the bit.  This follows the general notion of being
liberal in what we accept.

> Later in section 9.1, A-bit is in a way forced to 1.

I think you answered your own question here?  A home registration
MUST be acknowledged regardless of the A bit as section 9.1 says.

> H-bit is not set (solves section 8.2 question as well)

Covered above, section 5.1 already says this.

> entry), the DAD must be executed again?

DAD only needs to be done before the address is assigned to an interface,
I don't think you ever do it again (there is a reference to RFC 2462
in this text).

-Brian

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From: "Krishna Kumar" <[email-masked]>
Date: Thu, 11 Oct 2001 08:56:07 -0700

Hi Peter,

> entry), the DAD must be executed again?

Section 10.7 talks about this :

While the mobile node is away from home, it relies on the home agent
to participate in Duplicate Address Detection (DAD) to defend its
home address against stateless autoconfiguration performed by another
node.  Therefore, the mobile node SHOULD set the Duplicate Address
Detection (D) bit based on any requirements for DAD that would apply
to the mobile node if it were at home [17, 27].  If the mobile
node's recent Binding Update was accepted by the home agent, and the
lifetime for that Binding Update has not yet expired, the mobile node
SHOULD NOT set the `D' bit in the new Binding Update; the home agent
will already be defending the home address(es) of the mobile node and
does not need to perform DAD again.

Regards,

- KK

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From: "Peter Grimstrup (TED)" <[email-masked]>
Date: Thu, 11 Oct 2001 18:30:28 +0200

Hi Krishna,

Thanks for your answer, I have another comment

> does not need to perform DAD again.

Fine, but this is a description of the MN behaviour.
Generally, implementing a HA, I cannot trust the MN allways to behave
correctly. Whenever possible, the HA should act reasonably on "minor"
errors from the MN - if some reaction is not explicitly in the standard.

Maybe it's better to remove the D bit and let the HA decide when to do DAD
(according to RFC2462, I guess).

/Peter