Subject: [mobile-ip] default router lifetime value in RAs

From: Keiichi SHIMA / 島慶一 <[email-masked]>
Date: Wed, 04 Jun 2003 15:36:09 +0900 (JST)

Hi all,

Do you have any idea about how to calculate the default router
lifetime specified in router advertisements, when we set advertisement
interval shorter than the value defined in RFC2461?

RFC2461 says, 

      AdvDefaultLifetime
                     The value to be placed in the Router Lifetime field
                     of Router Advertisements sent from the interface,
                     in seconds.  MUST be either zero or between
                     MaxRtrAdvInterval and 9000 seconds.  A value of
                     zero indicates that the router is not to be used as
                     a default router.
 
                     Default: 3 * MaxRtrAdvInterval
 
If we set MaxRtrAdvInterval to 0.07 as the mip6 spec says,
AdvDefaultLifetime becomes 0.21.  I don't think it is a proper value.

Should the mip6 spec define the lowest value for AdvDefaultLifetime?
For example, 12 seconds (the lowest value based on RFC2461, minimum
value of MaxRtrAdvInterval(4 sec) * 3) is a good candidate, I think.

How do you think?

---
Keiichi SHIMA
IIJ Research Laboratory <[email-masked]>
KAME Project <[email-masked]>

---

From: Jari Arkko <[email-masked]>
Date: Wed, 04 Jun 2003 11:09:46 +0300

> value of MaxRtrAdvInterval(4 sec) * 3) is a good candidate, I think.

The issue seems to be setting the default -- the text you quoted
from RFC 2461 allows you to set it to, say, 12 seconds if that's
what you like.

But regarding the default, why would you think 0.21 is not a proper
value? If MaxRtrAdvInterval is so low, you should have heard a few
RAs during that time, and if you haven't, well, maybe you *are* off
the link. Or are you concerned that due to temporary wireless
disturbances, there should be a longer default lifetime, to get
some hysteresis?

--Jari

---

From: Keiichi SHIMA / 島慶一 <[email-masked]>
Date: Wed, 04 Jun 2003 18:14:26 +0900 (JST)

Hi Jari,

From: Jari Arkko <[email-masked]>

> disturbances, there should be a longer default lifetime, to get

To tell the truth, I didn't think about it much.  I just think we have
no way to set the default router lifetime between 0.21 to 1 second
because the router lifetime field of RA packet is in unit of second.
We cannot send an RA with 0.21 router lifetime unless we extend the
format of RA.

Best Regards,

---
Keiichi SHIMA

---

From: Jari Arkko <[email-masked]>
Date: Wed, 04 Jun 2003 12:42:53 +0300

> We cannot send an RA with 0.21 router lifetime unless we extend the

Ah, of course... thanks for observing this. It certainly deserves
to be mentioned in the MIPv6 spec. That is, we should at least
have a note which says that default router lifetimes have to
be above 1 s due to RFC 2461, even if the MaxRtrAdvInterval
would indicate a smaller default.

--Jari

---

From: Keiichi SHIMA / 島慶一 <[email-masked]>
Date: Wed, 04 Jun 2003 19:19:46 +0900 (JST)

Hi,

From: Jari Arkko <[email-masked]>
Subject: Re: [mobile-ip] default router lifetime value in RAs

> would indicate a smaller default.

Thanks, Jari.  I'm fine with this.

---
Keiichi

---

From: Jari Arkko <[email-masked]>
Date: Mon, 16 Jun 2003 17:54:11 +0300

This issue has been registered as #308. We agreed
earlier that there is a potential problem when
MaxRtrAdvInterval is set very low, since one of
the fields in a RA can become zero due to lack
of precision (units are seconds). We also agreed
that the document should say default values should
be above 1 s. There was also some discussion
about the small MaxRtrAdvInterval values, but
I believe the WG took a decision on those earlier,
and we can't open that decision again at this stage...

Here's the proposed text change. Add the following
to the end of Section 7.5:

"Note that according to RFC 2461~\cite{rfc2461}, AdvDefaultLifetime is
by default based on the value of MaxRtrAdvInterval. AdvDefaultLifetime
is used in the Router Lifetime field of Router Advertisements. Given
that this field is expressed in seconds, a small MaxRtrAdvInterval
value can result in a zero value for this field. To prevent this,
routers SHOULD keep AdvDefaultLifetime in at least one second, even if
the use of MaxRtrAdvInterval would result in a smaller value."

--Jari