Keiichi Shima writes: 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.

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Jari Arkko responds to Keiich Shima: 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?

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Keiichi Shima responds to Jari Arkko: 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.

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Jari Arkko responds to Keiichi Shima: 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.

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Keiichi Shima writes: Thanks, Jari.  I'm fine with this.

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(Discussion by Francis Dupont and others that the small
MaxRtrAdvInterval values are not wise. But I think these have earlier
been decided by the WG after a lengthy discussion.)

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Jari Arkko writes: 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."

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