Suresh Krishnan wrote:
Hi Hesham/Dave/Erik,
I am not taking a stand on whether an explicit off-link flag is
necessary/useful or not, but I have encountered a scenario where the existing
algorithm specified in RFC4861 does not work very well. Let's say a router
wants to signal to the clients that 2001:dead:beef::/48 is on-link except for
2001:dead:beef:abcd::/64 that is off-link. How would it go about describing
this? I see two ways
a) Advertise the /48 with L=0 and send redirects for all addresses not on the
/64
Alternatively you could advertize a set of /47 through /64 prefixes as
on-link. I think in this case you'd end up with 15 or so prefixes being
advertized.
b) Advertise the /48 with L=1 and the /64 with Q(the new off-link flag)=0
I see b) as being more efficient than a)
I think b) is a lot more complex to understand, because one would have
to understand who wins when one RA says L=1 and onther says Q=1 and
their is partial of complete overlap between the prefixes in question.
Also, depending on that preference issues, a sane host implementation
might need to either have a positive (on-link) or negative (off-link)
list of prefixes, which means the host would have to do the expansion.
One can illustrate this complexity by modifying your example to have 3
sub-prefixes where
2001:dead:beef::/48 is on-link, except for
2001:dead:beef:ab00::/56 which is off-link, except for
2001:dead:beef:abcd::/64 which is on-link
P.S: I do not think that this scenario is very likely, just possible.
I agree. And my example might be event more unlikely, but possible.
Erik
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