Issue with unsigned integer LOCAL_PREF

[Daniel Suchy]

Hello,
commercial BGP implementations (Juniper, Nokia, Cisco) usually 
implements silent saturation based on your point (2) in such cases, 
Quagga does this too (see route_value_adjust function [1]; which handles 
these overflows). It's not limited to locpref, also other attibutes can 
be modified by addition/subtraction.

I suggest to follow this practice also in Bird code, as this behavior is 
commonly expected here.

Length of relevant attributes are defined in RFC and I don't expect any 
move from 32b to 64b integers here - as this will break compability 
between speakers.

- Daniel


[1] 
https://github.com/Quagga/quagga/blob/88d6516676cbcefb6ecdc1828cf59ba3a6e5fe7b/bgpd/bgp_routemap.c#L148

On 12/1/20 5:51 PM, Ondrej Zajicek wrote:
> There are at least three reasonable behaviors for fixed-range
> arithmetics:
> 
> 1) silent overflow (current behavior)
> 
> 2) silent saturation (as you suggested)
> 
> 3) explicit (logged) error
> 
> Not really sure which is better. (1) is likely least astonishment (for
> fixed-range), (2) is most useful when overflows are intentional, (3) is
> most useful when overflows are unintentional.
> 
> I have minor preference for (3), but no strong preference either way to
> change anything. Also note that in cases when fixed-range attributes have
> lower range than 32bits (i.e. 16bit preference and 24bit ospf_metrics),
> we generate explicit error on overflow on assignment.
> 
> Or perhaps we should just move to arbitrary-range integers .. many new
> RFCs already use 64bit attributes anyways.