[rbridge] A thought about TTLs

[Eastlake III Donald-LDE008]

Rbridges are in a fundamentally different position in deciding shim TTLs
than IP hosts are in deciding IP TTLs. In particular, a host knows
almost nothing about the routing situation while an Rbridge has explicit
link state information.

Thus it would be reasonable to recommend a TTL based on the expected
number of hops for unicast or maximum expected number of hops for
multicast/broadcast/flooded or on the diameter of the network (maximum
number of Rbridge-Rbridge hops for the two most distant Rbridges) or the
like ... It would even be possible to have Rbridges trim back the TTL
for the copy forwarding to a particularly short branch of a tree or even
when forwarding along a unicast path if that path has gotten shorter.
Such adjustment is probably too much work to mandate but it seems to me
that it should be allowable behavior.

Thanks,
Donald