maximum ipv4 bgp prefix length of /24 ?

Sun Oct 1 18:19:19 UTC 2023

On Sun, Oct 1, 2023 at 1:03 AM Saku Ytti <saku at ytti.fi> wrote:

> On Sun, 1 Oct 2023 at 06:07, Owen DeLong via NANOG <nanog at nanog.org>
> wrote:
>
> > Not sure why you think FIB compression is a risk or will be a mess. It’s
> a pretty straightforward task.
>
> Also people falsely assume that the parts they don't know about, are
> risk free and simple.
>
> While in reality there are tons of proprietary engineering choices to
> make devices perform in expected environments, not arbitrary
> environments. So already today you could in many cases construct
> specific FIB, which exposes these compromises and makes devices not
> perform.

I would go a step further; for any system of compression hoping to gain a
net positive space savings,
Godel's incompleteness theorem guarantees that there is at least one input
to the system that will result in no space savings whatsoever.

If your device is counting on FIB compression to deliver sufficient space
savings to allow a FIB of size > SRAM to fit into SRAM,
it really should have a reasonable, sane fallback mode for when the next
routing update happens to result in a FIB that is incompressible.

Unfortunately, many coders today have not read Godel, Escher, Bach: An
Eternal Golden Braid,
and like the unfortunate Crab, consider their FIB compression algorithms to
be unbreakable[0].

As I discovered many years ago, at web scale, even seemingly
highly-improbable sequences of bits end up happening frequently enough to
become problematic.

In short: if you count on FIB compression working at a compression ratio
greater than 1 in order for your network to function, you had better have a
good plan for what to do when your phone rings at 3am because your FIB has
just become incompressible.   ^_^;

Matt

[0]. https://genius.com/Douglas-hofstadter-contracrostipunctus-annotated
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