Subject: RE: [DNA] [Issue X] LinkID v.s. Landmark Prefix

From: "Tero Kauppinen (JO/LMF)" <[email-masked]>
Date: Tue, 26 Jul 2005 12:57:37 +0200

I'm starting to get so confused in the middle of this LinkID v.s.
Landmark Prefix discussion that I'm afraid that I'm loosing my grasp
even with the basics.

I started wondering what will actually happen in the following scenario:

Let there be two routers, 'A' and 'B' and some access points. 'A'
advertises prefixes 1 and 2 whereas 'B' advertises prefix 3. After
connecting to an AP host, 'N', sends an RS and gets an RA (from 'A'),
and finally configures an address (basing on prefix 2). In the case of
LinkID 'N' gets the current link-id prefix from the RA, and in the case
of Landmark Prefix 'N' chooses a landmark prefix. Let's assume that
prefix 1 is both the smallest prefix and the prefix 'N' chooses to be
the landmark prefix. 

If we then say that 'A' stops advertising prefix 2 and zero-lifetime
messages disappear, what will be the outcome? I mean, 'N' could attach
to another AP, send an RA and learn that it's still in the same network.
However, even though 'N' still in the same network, the address 'N' is
using (prefix 2 based) is not valid anymore.

As I said, I'm so confused that I can't even be sure if this has already
been discussed on the list.

--
Tero

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From: Syam Madanapalli <[email-masked]>
Date: Tue, 26 Jul 2005 18:06:35 +0530

Hi Tero,

> to another AP, send an RA and learn that it's still in the same network.

Either Landmark or Linkid approaches detect that there is no link
change. I think, as far DNA is concerned the job is done. 

> using (prefix 2 based) is not valid anymore.

This is the host problem and is same problem if the router suddenly
stops advertising prefix 2. I looked into 2462bis and found the following 
text: 

5.5.3  Router Advertisement Processing

---

d) If the prefix advertised is not equal to the prefix of an address
      configured by stateless autoconfiguration already in the list of
      addresses associated with the interface (where "equal" means the
      two prefix lengths are the same and the first prefix-length bits
      of the prefixes are identical), and the Valid Lifetime is not 0,
      form an address (and add it to the list) by combining the
      advertised prefix with the link's interface identifier

If the node did not configure the address based on prefix 1 then
it should either wait till the prefix 2 life time expiry or if the router 
does not support forwarding for the prefix 2, an intelligent host can
configure new address using prefix 1 based on packet loss. I am
not sure about choosing new address based on packet loss.

---

From: Erik Nordmark <[email-masked]>
Date: Tue, 26 Jul 2005 06:16:22 -0700

> been discussed on the list.

My tongue in cheek answer is that this is a feature of RFC 2461 :-)
Even if there was no DNA, or there was DNA and the host didn't move to a 
different AP, the host would continue to use prefix2 until it expired.
This could be viewed as a feature in RFC 2461 in that even if a router 
fails and stops advertising a prefix for some time, it will not upset 
the configuration of the hosts unnecessarily. So when the administrator 
wants to remove a prefix, she has to explicitly configure the routers to 
drop the lifetime to zero (either abruptly or gradually).

The narrow path I've tried to follow is to realize the above issue, but 
yet make sure that DNA doesn't get any more confused by this then a host 
would get that doesn't move. For instance, by looking at how a 
non-moving host would behave when it attaches to the network.

But I might be making the wrong call here, and that in the light of 
intermittently connected hosts and lossy (radio) links, we should make 
the reverification of being connected to the same "place" a lot less 
forgiving.

    Erik