Subject: ND - Neighbor Adv - 'O' Flag - clarification

From: "Murugan KAT" <[email-masked]>
Date: Sat, 2 Feb 2002 15:20:35 +0530

Hi,

As per ND specification the Neighbor Advertisement message contains  "O"
Flag (OverRide flag).
What is the purpose of this flag. Or when the nodes is supposed  set this
Flag. Or when the nodes are not supposed to set this flag.

I suppose whenever the receiver finds a difference between the sent link
address and the address what it has in its ND cache it can modify the value.

Why is this extra bit...?

Thanks,
KAT

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From: Francis Dupont <[email-masked]>
Date: Sun, 03 Feb 2002 15:55:31 +0100

In your previous mail you wrote:

=> the "O" flag is mainly about the link-layer address.

   What is the purpose of this flag.

=> this flag says if the cached link-layer address should be
overrided by a new one or not.

   Or when the nodes is supposed  set this Flag.

=> always when not in the following cases.

   Or when the nodes are not supposed to set this flag.
   
=> when the responder acts as a proxy or for an anycast address,
i.e. when there can be more than one valid responder.
The RFC 2461 has an explicit SHOULD/SHOULD NOT in the definition
of the flag.

=> no, look at the state machine in appendix. You should not do
what you'd like (so there are some MUSTs) because ND should be determinitisc.

=> to provide advertisements for proxies/anycasts in a deterministic way.
Please reread RFC 2461, this part is pretty clear!

Regards

[email-masked]

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From: Erik Nordmark <[email-masked]>
Date: Tue, 5 Feb 2002 10:04:35 +0100 (CET)

> Flag. Or when the nodes are not supposed to set this flag.

The mechanical aspects of when it is set are covered in RFC 2460.

> address and the address what it has in its ND cache it can modify the value.

Correct. That is what RFC 2460 says. In addition there is some effect
on the reachability state as specified in the rfc.

> Why is this extra bit...?

That question I don't think is clearly answered in the RFC.

The motivation is that there are two possible approaches for dealing
with information such as the link-layer address received in a ND packet:
1. The last received information takes precedence
2. The first received information takes precedence

Normally #1 makes sense - the link layer address might change e.g. due to
a replacement of the network adaptor or some form of failover between multiple
adapters.
But in the case of a neighbor solicitation #2 makes more sense - it is
desirable by default to pick the first entity to respond since there is
some correlation between the speed to respond and the load and/or speed of
the node.

Also, in the case of proxy neighbor advertisements it might make sense
to make the precedence be determined by the sender of the NA.
In the case of Mobile IP there is a home agent that proxies for
a mobile node that is away from home. But in the case when the mobile moves
home and start sending NA messages you'd like those message to take
precedence over any proxy advertisements sent by the HA.
(Of course the HA will be notified by the MN to stop sending the proxy
advertisement but before it does the above rule makes sense.)

The single override bit provides the flexibility to do all of the above.

  Erik

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From: Erik Nordmark <[email-masked]>
Date: Tue, 5 Feb 2002 10:04:35 +0100 (CET)

Erik

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From: "Murugan KAT" <[email-masked]>
Date: Thu, 7 Feb 2002 14:42:05 +0530

Thanks for your detailed answer.

-----Original Message-----
From: Erik Nordmark [mailto:[email-masked]]
Sent: Tuesday, 5 February 2002 2:35 PM
To: [email-masked]
Cc: [email-masked]
Subject: Re: ND - Neighbor Adv - 'O' Flag - clarification

a replacement of the network adaptor or some form of failover between
multiple

  Erik