Re: A simple question
2003-04-21 04:40:23
--On Sunday, 20 April, 2003 22:13 -0400 Keith Moore
<moore(_at_)cs(_dot_)utk(_dot_)edu> wrote:
At which point they might as well just select their global
address unless the destination address for a service is site
local. This decision could (should?) be in the hands of the
IP stack, unless the application specifically asks for such
control.
I disagree that the IP stack can supply reasonable defaults in
the face of multiple scopes. The criteria for choosing a
source address varies widely from one application to another.
Some applications need stable addresses, others need addresses
usable from all of their potential peers, others need to
choose the source address that results in the best performance
(where there is more than one meaning of 'best performance').
The default address selection rules are at best a guess.
We should let the network do routing, so that hosts and apps
aren't expected to make routing decisions.
Keith,
I don't understand your point here, and would like to. It seems
to me that your first paragraph ("I disagree...") suggests that
the applications need to specify the addresses/interfaces
because they know their needs and the stack can't figure it out.
And the second argues for pushing the decisions onto the
network, which has no information at all (and, if it had it,
could probably apply it only by changing addresses mid-flight --
NAT-like or worse). That suggests to me a conclusion of "not
application, not stack, not network either" which leaves very
few options.
If I ignore that apparent contradiction, I'm left with the
conclusion that we need some new addressing and routing
abstractions so that the applications can tell the stack (and
the stack might tell, or negotiate with, the network) what
considerations they need optimized and do so without specifying
specific addresses or inspecting components of addresses. It
seems to me that such abstractions would also eliminate one of
the arguments against site local: having an application able to
tell the stack:
* I need a local address
* I need an appropriate address for routing to <DNS name>
* I need a completely-global address
Are all far more reasonable than having the application itself
recognize particular addresses or prefixes and trying to figure
out which ones make sense in its context. It also _might_
suggest that we need to reexamine ICMP in the IPv6 context to be
sure that, if the stack needs information from the network to
make satisfactory choices along these lines, it can ask them
without, e.g., trying to set up one or more TCP connections (or
sending UDP packet using an unspecified protocol) and waiting
for the responses or timeouts.
Is that really where we need to be headed and, if so, is anyone
doing the work rather than being stuck on "local-good /
local-bad", "scope-necessary / scope-bad", types of arguments?
john
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