Subject: No RS after valid lifetime times out

From: Christian Schild <[email-masked]>
Date: Thu, 30 Sep 2004 09:43:51 +0200

Hiho,

after we experimented a lot with (especially short) lifetimes on various
platforms and tested renumbering procedures, I have an urgent question. 

Shouldn't a client react when its valid lifetime times out and the
interface loses the address? 

There may be a number of reasons why RAs stop on a given link or why an
interface doesn't receive them anymore (improper configuration of timers
is just one of them). If so, first the preferred and then the valid
lifetime times out. Usually a system removes the associated address from
its interface.

This may leave the system without a global address while it may still
need one (and could get one). In this case, the system has to wait for
the next RA. This may take a (too) long time, if not infinite. 

I would have expected, that immediately when an interface removes its
(last) global address, it will try to obtain a new one, sending out an
immediate RS. This is not the case on various platforms.

RFC2462 does not give this advise or at least I was unable to find it.
Nor does 2462bis do. 

So long,
     Christian

-- 
JOIN - IPv6 reference center    Christian Schild
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From: Francis Dupont <[email-masked]>
Date: Thu, 30 Sep 2004 15:02:46 +0200

In your previous mail you wrote:

=> short prefix lifetimes are not the common case: prefix lifetimes
are at least in months and are announced in days. And don't forget
the 2 hour rule.

   
=> no but it can react to the lost of all default routers (default
lifetime is 600 seconds, maximum is 1800 seconds).

Regards

[email-masked]

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From: "Christian Schild (JOIN Project Team)" <[email-masked]>
Date: Thu, 30 Sep 2004 16:11:57 +0200

Hi Francis,

> are at least in months and are announced in days. And don't forget

I agree that in the common case there are no 'short' lifetimes, but I
wouldn't call it "months" (default is 7 days for preferred and 30 days
for valid). Especially I doubt that RAs are "announced in days" by
default. 

Still there are circumstances when short lifetimes are necessary, e.g.
during a renumbering event or when abusing RAs for router redundancy.

This all is not my point. My point is, that an interface doesn't try to
reconfigure itself when it loses its address. 

> lifetime is 600 seconds, maximum is 1800 seconds).

I agree, that might also be possible. But a quick check showed that
neither Linux, BSD nor Windows hosts send RSs' upon Router Lifetime
timeout either. 

But this advise is also lacking in RFC2462. If I searched it correctly,
the RFC does not talk about Router Lifetime at all.

And still, when the prefix' valid lifetime is shorter than the Router
Lifetime (Maximum is 18.2 hours? 16bit?), there are times when the
interface is lacking a global address.

Christian

---

From: Stig Venaas <[email-masked]>
Date: Thu, 30 Sep 2004 16:29:31 +0200

> lifetime is 600 seconds, maximum is 1800 seconds).

Or in other words. The lifetime should be much longer than the RA
period. Then you should only lose all addresses if no RA's are sent
by router, or link is broken. There should be no reason to send an RS
then.

There are some events that perhaps should trigger an RS, e.g when
moving to new network (dna).

Stig

---

From: Francis Dupont <[email-masked]>
Date: Thu, 30 Sep 2004 16:36:08 +0200

I agree that in the common case there are no 'short' lifetimes, but I
   wouldn't call it "months"

=> in fact this is more in years: I changed our prefixes twice in 5 years.

   (default is 7 days for preferred and 30 days
   for valid). Especially I doubt that RAs are "announced in days" by
   default. 
   
=> 7 days and 30 days are in days, aren't they?

=> I can't parse the second case.

=> a global address is not useful without a default router.

   
=> my point is that they could.

=> yes, everything about Router Lifetime is in RFC 2461.

=> 1800 seconds = 30 minutes.

=> IMHO it is a bad idea to add text in RFCs about cases that should
never happen in the real world, in particular in standard track RFCs
which are already far too large/complex.

Regards

---

From: Erik Nordmark <[email-masked]>
Date: Thu, 30 Sep 2004 09:22:41 -0700

> RFC2462 does not give this advise or at least I was unable to find it.

I think RFC 2461 is quite clear when a host can/should send an RS.
And the above case isn't listed.

 > interface loses the address?

It is currently unspecified whether or not the host should do things 
like immediately kill any TCP connections using the invalidated address, 
so different implementations might do things differently with respect to 
this issue.

But that wasn't the root of your question.
Doesn't your case fall in a misconfiguration of the routers?
For instance, I've tested things in the past with a periodic RA 
annoucement of 10 minutes with a valid lifetime of 5 minutes and 
observed that the loose the address after 5 minutes and regain it when 
hearing an RA 5 minutes later. That's a fine test case, but it wouldn't 
be a useful configuration for an operational network.

So why do you think we need to change something?

 > during a renumbering event or when abusing RAs for router redundancy.

But even if this is done, the lifetimes of the prefixes shouldn't be 
affected; it should only affect the advertised default router lifetime, 
right?

 > the RFC does not talk about Router Lifetime at all.

Correct, because the router lifetime is discussed in RFC 2461.

You seem to want to solve the problem when the routers have been 
configured with a bad set of parameters to use in the advertisements.
Why can't this be fixed by configuring the routers properly?

If you think the hosts need to do more work to handle misconfigured 
routers I fear it is a very slippery slope; for instance, should the 
hosts guard against the routers advertising incorrect prefixes as well?

    Erik