6to4 is a crapshoot, it can be reasonable or it can completely fail,
with everything in between. But it's never going to be better than
native IPv4, obviously.
No, not obviously - because if the application has a need to do
address referral then there are conditions in which the 6to4 address
will work better.
I was talking about performance. I guess even there there are a few
cases where 6to4 can do better than the IPv4 it's carried over, but
that should be rare.
More common than you might think, especially given that native IPv4 is
often subject to traffic filters that might not apply to 6to4.
I agree that this is an important issue, but I fail to see how this
relates to applications knowing about addresses, unless applications
are going to do their own performance testing, which I don't recommend.
p2p applications are doing this already.
Do you mean BitTorrent? Those applications don't really test
performance, but simply use different addresses concurrently. Which is
an excellent way to make the question of which address performs better
moot, by the way.
The applications that do that still have to deal with addresses
explicitly. p2p transfer of large file is sort of a special case since
you can generally afford to spend some time learning which paths are
more efficient.
As I said, a good start would be that API because once applications
use it, people working on better address selection etc have a place
to insert their stuff and improve performance of real applications
without having to rewrite the application.
I spent a fair amount of time trying to come up with an API that
would let applications push this decision-making to a lower layer
while allowing that lower layer to make those decisions effectively
based on the needs of that particular application. That API ended up
looking fairly complex. There are lots of factors that influence
address selection.
Yes, communicating application needs back and forth would be tricky.
For instance, suppose I have two links: one is 2 Mbps ADSL. This is
not so bad that an application can't run, but if my other link is 1000
Mbps, then the application would probably want to use that link. But
the application doesn't know if that's the case, it could also be a
GPRS link with enormous latency and virtually no bandwidth. So what
does the application say?
The application says something like "I need a minimum of X Mb/s
bandwidth" (which implies, e.g. don't use any interface that can't
provide that) or "I don't need much bandwidth but I do need a
connection that's going to stay up for awhile" (e.g. use the GPRS
interface or maybe the mobile IP in-care-of address) or "I need as much
bandwidth as possible but I don't expect the connection to need to stay
up very long". (use the fastest path available or maybe even more than
one path concurrently) or "I need the best latency I can get but the
amount of data transferred will be small and the duration will be short".
Keith
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