Subject: Re: IPv6 Link-Local Use Issue for Applications

From: Dan Lanciani <[email-masked]>
Date: Mon, 25 Aug 2003 18:16:24 -0400 (EDT)

Hans Kruse <[email-masked]> wrote:

|I think the following "rules" should go with this approach:
|
|- Assume DNS returns both PA and PUPI, then
|  if my node only has PUPI, I select the advertised PUPI,
|  if my node has a PA, I select the PA being advertised.

I assume that the "only" above implies that if you have both PUPI and PA
addresses you wish to select the PA address?  If so, this defeats the goal
of quasi-automatic connection stability in the face of changing PA addresses.
It would require setting up separate DNS entries with only PUPI addresses in
order to force a stable connection.

|- Obviously, if DNS only returns PUPI and I have both, I use PUPI,
|  and use PA if DNS only returns PA and I have both.
|
|- If there is no match (I have only PUPI and DNS returns PA, or vice 
|versa), I would suggest to fail the connection by default (even though it 
|might work), but I am not sure.

As long as this default can be changed without the cooperation on the
applications (i.e., a global option for the stack) it would be ok...

				Dan Lanciani
				ddl@danlan.*com
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From: Hans Kruse <[email-masked]>
Date: Mon, 25 Aug 2003 18:44:06 -0400

--On Monday, August 25, 2003 18:16 -0400 Dan Lanciani 
<[email-masked]> wrote:

>| I think the following "rules" should go with this approach:
>|
>| - Assume DNS returns both PA and PUPI, then
> of quasi-automatic connection stability in the face of changing PA
> addresses. It would require setting up separate DNS entries with only
> PUPI addresses in order to force a stable connection.

That is what I think needs to happen.  If you preferentially pick local 
addresses, you risk unnecessary failure during address referral. 
Advertising only PUPI for nodes that need stable connections forces the use 
of the local addresses without need to modify application behavior.  Of 
course, nothing would prevent an application from explicitly preferring 
PUPIs (or letting the user configure such override).

I am concerned about a default, out-of-the-box behavior that succeeds in 
the most common cases.

>
>| - Obviously, if DNS only returns PUPI and I have both, I use PUPI,
>| - If there is no match (I have only PUPI and DNS returns PA, or vice
>| versa), I would suggest to fail the connection by default (even though
>| it  might work), but I am not sure.
>

Again, a stack-wide override would make sense (registry, sysctl, etc).  As 
other overrides, their use may cause breakage, but I think that is an 
acceptable risk as long as the default behavior of the stack prevents such 
breakage.

Hans Kruse, Associate Professor
J. Warren McClure School of Communication Systems Management
Adjunct Associate Professor of Electrical Engineering and Computer Science
Ohio University, Athens, OH, 45701
740-593-4891 voice, 740-593-4889 fax
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---

From: "Chirayu Patel" <[email-masked]>
Date: Tue, 26 Aug 2003 09:50:03 +0530

> applications (i.e., a global option for the stack) it would be ok...

Modifications to the address selection policy table would help in achieving
this default without any change to the applications. Of course, if the
applications can always chose their own set of src/dest addresses to override
the stack defaults.

CP

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From: Keith Moore <[email-masked]>
Date: Tue, 26 Aug 2003 10:20:40 -0400

> this default without any change to the applications.

unless you get conflicting policy from different interfaces, which is entirely
possible.

per-link policy is the wrong way to think about this.  what you want is for
each network to be able to say "here's the best way to get here from there
given a variety of conditions"... and then minimize the number of conditions.

---

From: "Chirayu Patel" <[email-masked]>
Date: Tue, 26 Aug 2003 22:04:29 +0530

> unless you get conflicting policy from different interfaces, which is

I am saying that a modification to the policy table (which is per host, and
not per interface) is a solution. You can do it manually or through an
automated mechanism. 

If you are using an automated mechanism, as I had suggested, getting
conflicting policy from different interfaces implies that something wrong with
the network engineering. There should be a way to detect it.

> given a variety of conditions"... and then minimize the number of
conditions.

I am not able to understand the text. Can you provide an example? 

CP