Carsten Bormann a écrit :
On Apr 3, 2009, at 13:50, Peter Siklosi wrote:
In my mind, the simplest model would be to represent the cost as
ND in 6lowpan is (mainly) about the host-router interface.
I agree with the intention. I have hower purposefully pictured Router1
and 2 to have two different interfaces each - I can reason with them so.
I'm afraid 6lowpan ND may not uses such devices, though.
Does the host even have to know about the way the router metric is
computed?
No.
The host (if it wants to be just a host) should not need to know
about the routing protocol and its metrics.
I agree.
I have therefore proposed the ND metric to simply be a 16-bit
integer, with a mid-range default (i.e., 0x8000 if the number is
unsigned). Routing protocols may want to define a way how to compute
that integer at a router so that multiple routers present numbers
that make sense when used together by a host. (And that may not
exclude use of the value found via ND in bootstrapping a router.
Still outside the scope of ND proper.)
I agree with ND metric 16-bit, in this sense. Maybe 8bit would be
sufficient too. ?
BTW, standard IPv6 ND does not have such a metric for the following
stated reason:
Unlike in IPv4 Router Discovery, the Router Advertisement messages do
not contain a preference field. The preference field is not needed
to handle routers of different "stability"; the Neighbor
Unreachability Detection will detect dead routers and switch to a
working one.
Well yes, but there's something about default routes - which we seem to
consider here more than the specific routes.
For default routes (address of the default router) there are fields
"Router Lifetime". If each router decreased the lifetime it advertised
further down the network then Host would make a decision to prefer the
default router whose Router Lifetime is higher, and thus reach quicker
the Internet via the least decreased Router Lifetime - a potential reuse
without defining new Cost field.
Yet again, all this provided I understand ok the IP subnet structure in
6lowpan ND.
Alex
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