RE: What's our target audience? (Was: Re: Shim6 proxies)

["Christian Huitema" <huitema@windows.microsoft.com>]

The discussion of target audience appears somewhat linked to the
possibility of supporting multiple IPv6 addresses (or locators) in a
site. We know that the multi-address solution does not scale well to
very large sites. The management burden increases with the number of
hosts and routers, and the potential number of providers and prefixes
also increases with the size of the site. It is fairly clear that, above
a certain size, site managers will insist for their very own "provider
independent" prefix.

Today, we have proof of support for multiple addresses in hosts.
Networking stacks are designed to handle multiple interfaces, and to
manage a different address (or set of addresses) for each interface.
Speaking for the products I know well, the IPv6 stack in XP, Windows
2003 and Vista implements the "strong host" model: by default, the
packets are sent through the interface corresponding to the chosen
source address. This is effectively a form of source address based
routing.

So, we have source address based route selection in the scope of a
single host. This is however a fairly small scope, a bit remote from the
"site multi-homing" goal of shim6. 

It is however relatively easy to imagine how the same model could be
extended to single-subnet sites. If multiple exit routers are attached
to the link, hosts will receive IPv6 router advertisements from each of
these routers, advertising different prefixes. The hosts can configure
as many addresses, and make sure that a different "default gateway" is
associated to each of the different source addresses. This is not
implemented now, but would not be very hard.

That extension would provide support for multi-homing in "single subnet
sites". This is still far from the general goal of supporting generic
site multi-homing, but that would be a first step. The solution would
apply to the home networks, and many small businesses.

I don't know whether we can realistically extend the scope of the
"multiple addresses" support beyond these small sites. In theory,
routers could manage some form of source address based route selection,
e.g. pick different default routes based on the provider prefix in the
source address of packets. But that requires adding the concept of
source based route selection in the intra-domain routing protocols. That
may take some time...

-- Christian Huitema