multiple routing tables review patch ready for simple testing.

[Bakul Shah]

On Tue, 29 Apr 2008 13:42:03 PDT Julian Elischer <julian at elischer.org>  wrote:
> 
> Interfaces are not however assigned to  FIB instance. each FIB may
> contain entries for each interface, and by default they do, but you
> can delete teh entries associated with a particular interface from
> a particular FIB so that fib will never use that interface.
> 
> An interface may however be present in entries from multiple FIBs
> in which case the INCOMING packets on that interface need to
> be disambiguated with respect to which FIB they belong to.

This confuses me....

The whole point of a FIB is to decide the *next* hop for a
given input packet. So questions.
1) A packet arrives on an interface.  If this interface is
   associated with more than one FIB, which FIB does it get
   given to?

2) If that decision is taken by a a packet 'classifier',
   isn't it in effect doing the job of a FIB (deciding the
   next hop, which happens to be a local FIB)?  Recall that
   basically a packet passes from a FIB to another FIB until
   it gets to its eventual destination.

3) When a local packets needs to be sent, which FIB gets it?
   Does setfib decides that?  If there a default FIB?

> This is a job for an outside entity (from the fibs).
> In this case a packet classifier such as pf or ipfw is ideal
> for the job. providing an outside mechanism for implementing
> whatever policy the admin wants to set up.

I believe having to use pf/ipfw will slow things down a bit
so the question is what does associating an interface with
multiple FIBs buy you?

> if you have several alias addresses on an interface it is possible
> that some FIBS know about some of them and others know about other
> addresses. New addresses when added are added to each FIB and
> whatever is adding them shoudl remove them from the ones that don't
> need it.  This may change but it fits in with how the current code
> works and keeps the diff to a manageable size.
> (and it suits what I need for work where a route manager daemon
> knows to do this.)

Wouldn't it make sense to treat each alias as on a separate
logical interface?  Then each logical interface belongs to
exactly one FIB.  On input you decide which logical inteface
a packet arrived on by looking at its destination MAC
address.  That reduces confusion quite a bit, at least in my
mind!  What does doing more than this buy you?