fib{,6}: questions and proposals

[Robin Jarry]

Hi Vladimir,

Medvedkin, Vladimir, Mar 19, 2024 at 18:16:
> > 2) Is it OK/safe to modify a fib from a control thread (read/write) 
> >    while it is used by data path threads (read only)?
>
> This part is a bit more complicated. In practice, I would say yes, 
> however, there is a possibility that if the lookup thread is preempted 
> in the middle of the lookup process, and at the same time the control 
> thread deletes the corresponding route, then the lookup result may 
> return outdated data. This problem is solved in LPM with RCU enabled. 
> I have plans to implement it in the near future in the FIB.

OK that's good to know, thanks.

> > 3) There is no public API to list/walk all configured routes in 
> >    a fib. Would that be possible/easy to implement?
>
> Yes, it already there. FIB under the hood uses rte_rib to hold 
> existing routes. So walking through can be implemented like:

I had tried it and got confusing results out of this. This must have 
been before I had realized that all addresses needed to be in host 
order...

I tried again and it works as advertised with a small missing detail: 
after configuring a default route, e.g.:

    rte_fib_add(fib, RTE_IPV4(2, 2, 0, 0), 16, RTE_IPV4(1, 2, 3, 4));
    rte_fib_add(fib, RTE_IPV4(3, 3, 3, 0), 24, RTE_IPV4(4, 3, 2, 1));
    rte_fib_add(fib, RTE_IPV4(0, 0, 0, 0), 0, RTE_IPV4(9, 9, 9, 9));

It is not returned by rte_rib_get_nxt() successive calls. I only see the 
other two routes:

    2.2.0.0/16 via 1.2.3.4
    3.3.3.0/24 via 4.3.2.1

Is this expected?

> > 4) In rte_fib, every IPv4 address (route *and* next hop) needs to be 
> >    in host order. This is not consistent with fib6 where addresses 
> >    are stored in network order. It took me quite a while to figure 
> >    out what was wrong with my code. 
>
> This API behavior was created in such a way that it is the same as 
> LPM.
>
> As for LPM, I think it was done this way for performance reasons 
> because in some scenarios you only working with the host order ipv4 
> addresses.

This should really be advertised in strong capital letters in the API 
docs. Or (preferably) hidden to the user. I don't see any valid scenario 
where you would work with host order IPv4 addresses.

Do you think we could change that API or at least add a flag at FIB/RIB 
creation to make it transparent to the user and consistent between IPv4 
and IPv6?

Thanks!