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[RRG] Hosts using routing
In the discussion of who selects the Ingress link (to the egress site,
just to confuse the language) the question of whether the hosts have
enough information comes up.
This then leads to the question of whether the hosts participate in routing.
There is a very strong tradition that we keep hosts out of routing.
While there are multiple factors, including control and policy issues,
there are two important and closely related issues that tend to cause us
to want to keep hosts out of the routing game.
Firstly, it is clearly desirable to be able to evolve the routing
protocols and routing system behavior without having to change every
host. The IS-IS vs OSPF issues are messy enough. They would be MUCH
harder if the selection also had to allow for what the hosts support.
Heck, the migration from RIP to either link state protocol in sites
would have been effectively impossible if the hosts needed to be able to
see and understand the routing of the day. (They would have understood
RIP.)
The second, related aspects is that good implementations of routing
protocols are complex. And given the information segregation, to really
have enough information using existing routing protocols hosts would
have to speak multiple routing protocols. Getting it right, robustly,
across all platforms, just seems extremely unlikely.
Unfortunately, this tends to lead to a situation where if we want the
hosts to have enough information to sensibly influence Ingress link
choices, we also seem to need to define a routing->host information
protocol to go with that.
Yours,
Joel
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