Subject: Q: Negative acknowledgement in neighbor advertisements

From: Aki M Laukkanen <[email-masked]>
Date: Fri, 1 Dec 2000 18:51:13 +0200 (EET)

Just a small question, why is there not an option/flag for negative
acknowledgements in the neighbor advertisements? It could be used to
significantly shorten the delay for DAD in some link layers. Or is it
expected that link-layers guarantee the uniqueness of the interface
identifiers on the link so there is no need to perform DAD? Other uses
would be faster feedback for applications etc.

Aki

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From: Francis Dupont <[email-masked]>
Date: Fri, 01 Dec 2000 18:47:29 +0100

In your previous mail you wrote:

   Just a small question, why is there not an option/flag for negative
   acknowledgements in the neighbor advertisements?

=> who is supposed to send this? If the link is point-to-point then
DAD is not issue because the link-layer (PPP) is supposed to guarantee
no interface ID collision. If the link is not point-to-point then
no neighbor should answer on the behalf of others.

   Other uses would be faster feedback for applications etc.

=> on a point-to-point link most neighbor discovery functions are not used...

[email-masked]

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From: Aki M Laukkanen <[email-masked]>
Date: Sat, 2 Dec 2000 00:13:09 +0200 (EET)

> no neighbor should answer on the behalf of others.

Sorry, I should have been more specific and now I understand why
this has not been addressed. There are links, I am not sure how to
characterise them with one term, one-way shared links perhaps? These are
multicast capable but have a central point of control i.e. the base
station. Most wireless links could be modelled this way but few are in
practice. Hiperlan (/2) is one such link. Most wireless links could be made
to look like one.

In this case, negative acknowledgement, no neighbor is going to answer on
behalf of others but since the base station (Access Point) has knowledge of
all the attached hosts, it could provide a negative acknowledgement.

Aki

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From: Francis Dupont <[email-masked]>
Date: Sat, 02 Dec 2000 00:12:04 +0100

=> these link-layers have no knowledge of the network addresses.

=> no because it knows only the MAC addresses of attached nodes.
In order to provide what you describe you have to change the base station
in a NHS (NHRP server if you don't know the acronym)...

Regards

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From: Erik Nordmark <[email-masked]>
Date: Fri, 1 Dec 2000 16:13:02 -0800 (PST)

> all the attached hosts, it could provide a negative acknowledgement.

I understand that in such cases the base station would know the 
link-layer addresses assigned to all the attached hosts.

But how does it manage to track all the IPv6 addresses assigned to
the hosts? The IPv6 addresses could be assigned using
 - stateless addrconf (based on the link layer address)
 - dhcpv6 (not necessarily based on the link layer address)
 - manually configured (not necessarily based on the lla)

Since DAD doesn't know how the address was assigned and your optimization
can't possible work for anything but statelessly configured addresses,
I don't see how this can work.

*IF* there will be a link-layer (called "foo") where the base station can
ensure that the link layer addresses (actually, the 64 bit tokens) are
unique on the link, it would be possible to specify in the IPv6-over-foo RFC
that DAD isn't required for the statelessly configured IPv6 addresses (but
required for all other addresses).

   Erik

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From: Aki M Laukkanen <[email-masked]>
Date: Sat, 2 Dec 2000 11:19:55 +0200 (EET)

On Fri, 1 Dec 2000, Erik Nordmark wrote:

Erik,

> I don't see how this can work.

You are of course correct.

> required for all other addresses).

Not requiring DAD for link-local addresses is a win already.

         Upon reception of a new IP address, the client MUST perform
	 Duplicate Address Detection (DAD) as specified in RFC
	 2462 [19].  If the IP address has already been allocated to the

How about if all hosts are required to set the 'I' and 'Q' flags in the IP
address extension? In effect suggest addresses configured using SAA or other
mechanisms. DAD has already been done for these addresses if needed. Then
the server can either grant or deny the request. It is not explicitly said
in there but why would DAD be needed in that case?

Aki