Re: fib{,6}: questions and proposals

["Medvedkin, Vladimir" <vladimir.medvedkin@intel.com>]

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Hi Robin,

Apologies for the delayed response

On 19/03/2024 20:38, Robin Jarry wrote:
> 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 result may return outdated data.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?

Yes, it is expected. It is also reflected in API: "Retrieve next more 
specific prefix ...". So, in your case you should explicitly lookup 0/0 
route.

IfindthismoreconvenientfordataplanestructureslikeDIR24-8,whereIneedto 
findgaps forsomegivensuperprefix.

>
>> > 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.
I just implemented lookup the same way as LPM. As for valid scenario, 
years ago I used an LPM/FIB lookup on a huge text log file(it was nginx 
logs if I remember correctly) with hundreds of million lines with IP 
addresses to resolve corresponding AS numbers for some statistics. The 
macro I used converted substrings with IPv4 into unsigned integers in 
host byte order. So, it is not always true that IPv4 are in network byte 
order.
>
> 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?

Yes, I will add FIB configuration option to allow BE IPv4 as an input 
for lookup function.

>
> Thanks!
>
-- 
Regards,
Vladimir

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