>There's the question of how much burden do scoped addresses create for
>applications. For most applications, even on multi-sited hosts, there's
>no burden at all if they are coded using sockaddrs. Distributed
>applications that send around addresses do have to cognizant of scoped
>addresses. However this is not hard to deal with. For example I checked
>on how one such distributed application that is being built at MS
>handles this issue. The application looks at the scope of the
>correspondent and sends its addresses of matching scope. In other words,
>when talking to a global address you send your global addresses and when
>talking to a site-local address you send your site-local addresses.
application on multi-sited host is difficult, as
- not every protocols are friendly with scoped addresses (scope
identifier get stripped off during transit)
- even if you successfully pass around scoped addresses with scope
identifiers, scope identifiers (view of scope) differ by node.
>There's the question of implementation effort for the network stack to
>support scoped addresses. This is hard to quantify but I would say it
>was small. (And necessary regardless of site-locals, unless link-locals
>are next up for revisionist architecture.) The additional code in source
>& destination address selection was about 10 lines each. The additional
>code in route lookup and destination cache management was worse, maybe
>100 lines. A scope-id needs to be passed around as an argument to
>various functions so there was a very small impact in many other places.
i would comment that it is very hard to get multi-sited router
implementation right (specifically routing protocols).
see descriptions by NEC guys a couple of months ago - it is a big mess.
so my take is as follows:
- we should at least forbid nodes from joining multiple sites.
it will simplify things a lot.
- we should document behavior of site-border routers,
every gory details.
this is the bottom line.
i'm still a bit concerned about the use of scoped address in general,
as it will cause confusion when scoped address leaks out of site
in application payload (like email foo@bar@baz notation or whatever).
but i feel it too late to kill it.
itojun
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