Re: Question about RFC6724

[JINMEI Tatuya / 神明達哉 <[e-mail]>]

On Mon, May 2, 2022 at 7:02 PM Brian E Carpenter
<[e-mail]> wrote:

> > I've just checked an implementation I'm relatively familiar with, and
> > found that it seems to completely ignore the "up to the length of S's
> > prefix" part:
> > https://github.com/freebsd/freebsd-src/blob/271f6d52a606e86c11b366082f11fe69350f24da/lib/libc/net/getaddrinfo.c#L924
> > that is, it compares the entire 128-bit IPv6 addresses and 32-bit IPv4
> > addresses.
>
> I think that makes a lot of sense. There's no real advantage in stopping
> the match at /64, and it's only by going beyond /96 that it can be
> useful for IPv4-mapped, if a host happens to have more than one IPv4
> address.

I suspect this implementation simply hasn't been updated since RFC3484,
rather than deliberately ignored the new text in RFC6724. But I
also suspect it wouldn't matter much in practice, since
- especially for IPv6, the interface ID length would be a constant
  (at least today, and will be so unless and until RFC4291 is updated
  to relax it), so the end result would be the same in practice.
- in the case of IPv4, the source address is often essentially unique,
  and the difference of CPL(S, D1) and CPL(S, D2) would not be that
  critical except for some unusally artifial cases.
- otherwise, rules using CommonPrefixLen are generally considered a
  "better than nothing" last resort, and an earlier rule would usually
  provide the selection winner.

--
jinmei