(It is actually harder to decide between multiple available global
prefixes, than to decide between global and site local - the former is
a difficult problem, the latter is almost trivial).
disagree. the app can choose any global prefix and reasonably expect
it to work, modulo link failures.
Nit: the vast majority of apps today bind to INADDR_ANY (or its IPv6
equivalent), so it's really the OS which is choosing the source address.
one of the problems introduced by scoped addresses is that this is no longer
sufficient for some kinds of apps.
If the app binds a specific source address, it is invariably
user-configurable and thus not an application problem.
or to put it more clearly, the only way an app can choose reasonably
between multiple addresses of varying scopes is to be explicitly configured as
to how to do so. otherwise, the app has no idea whether one scope or another
is more suitable for its purposes. any assumptions about whether, for
instance, site-locals are more secure or more stable than globals, or
whether the app will need to communicate with off-site nodes, need to be
explicitly configured.
of course, if the app weren't being forced to choose between addresses of
differing scopes then it wouldn't need to bind to specific source addresses
(no more than v4 apps do), and such configuration would not be necessary.
but when choosing between a global and a site local the app needs to
know whether the site local address will be valid for the hosts that need
to use it, and it has no way to know this.
Well, since one can easily determine whether the destination address is SL
or not, picking a source address of the correct class is trivial.
wrong again. it's not reasonable to assume that the source address will not
be used by off site nodes. (of course, specific apps can make that
assumption, but the architecture should not)
the app may also have to choose which interface to use with the site-
local prefix, and it has no good way to know this either.
Ah, but it'll have a different SL address on each interface, so that's no
worse than having multiple global addresses.
depends on what is driving the selection - I don't think it's reasonable
to assume that all interfaces on a host are within a single site.
Keith