Date: Mon, 21 Oct 2002 17:03:58 -0400
From: Keith Moore <[EMAIL PROTECTED]>
Message-ID: <[EMAIL PROTECTED]>
| because if you want apps to work reliably under these
| conditions then you are essentially asking hosts to do routing in
| the absence of routing information
Sorry Keith, but what has routing got to do with anything? Address
scoping is really just yet another level of naming, it is an extension
to the address.
Given any random address, with or without scoping information, there's
always a possibility that the address won't work, for all kinds of
reasons, so apps better be prepared to deal with that one way or another.
That is, if you consider it important for an app to know which of several
possible addresses (perhaps all "global" address) will work before using
one, then you're right, there's a whole bunch of extra info that hosts would
need to somehow discover first, but in practice, that isn't what anything
does, "suck it and see" is the traditional approach - given multiple addresses,
pick one (at random, though first come tends to be better) and see if it
works, if so, then fine, if not, go on to the next.
There's nothing about scoping that changes that in the slightest. Of
course, the appropriate scope info has to be available with each address.
The DNS has no way to return that info (nor do we have any global naming
scheme for scopes that would allow it to), which is one reason why limited
scope addresses should never appear in the DNS, except where the limited
scope is (for other reasons) defined to be the universe (which is how we
get by putting our current global addresses in the DNS - the globe of
concern is "our globe" and all global addresses by definition refer to it).
Other means of locating suitable limited scope addresses need to provide
the scope information as a by product (which in many cases can be done by
simply using the appropriate scope identifier from the interface over
which the address was discovered).
I know you're concerned about apps (protocols) which pass around addresses
in the data (and consequently which aren't limited by the normal scope
control measures). Where that happens the apps just need to take care not
to pass an address to a node where it isn't known to be meaningful.
Since that's hard to determine, a simpler rule that works, is simply never
to pass an address of a lesser scope (more restricted scope) than the
address being used for the peer in the communications over which the
address is to be sent.
Certainly that's something that apps aren't doing now - but they should be.
It certainly isn't anything like as burdensome as the picture you have
been painting of the difficulties involved.
kre
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