[Babel-users] Removing source-specific for older kernels [was: Higher CPU usage since 1.9.x leads to instability on slow devices]

[Christof Schulze]

On Tue, Jun 02, 2020 at 12:14:26PM +0200, Juliusz Chroboczek wrote:
>> To my eye, conflict_solution() [2] really stands out.
>
>Thanks a lot to both of you.
>
>That's the disambiguation code, due to Matthieu Boutier.  It's used for
>supporting source-specific routing in the case where the kernel doesn't
>support it:
>
>  - for IPv4, it is always used for source-specific routing;
>  - for IPv6, it is used with kernels earlier than 3.11.
>
>Now it turns out that:
>
>  - people don't do source-specific with IPv4 (they use NAT instead);
>  - people have upgraded to 3.11 or later now.
>
>In other words, this code shouldn't trigger in practice.  I have no idea
>why it triggers for you, perhaps you're running a very old kernel, perhaps
>there's a bug in the detection of the kernel version, or perhaps this part
>of the code triggers even when not needed.
>
>Since this code is not used in practice, I'm thinking of removing it
>altogether, which will simplify the codebase and avoid this kind of issues
>in the future.  How do people feel about that?  To restate, the effect
>will be:
>
>  - source-specific IPv4 routes will be ignored;
>  - source-specific IPv6 routes will be ignored on kernels before 3.11;
>  - no regression on BSD, which doesn't do source-specific in any case.
>
>You will still be able to do IPv6 source-specific if you run a Linux 3.11
>or later.  If you have routers running old kernels, you'll need to ensure
>you're announcing a non-specific default route.
3.11 is very old and it has bugs with IPv6 causing dropped ICMPv6 
packets. I doubt it is still in use in IPV6 based networks because the 
icmp breakage will overshadow UX of ipv6.

Source-specific routing for ipv4 could be interesting but I do not know 
of anyone actually using it ;) As you say. People will use nat.

Christof

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