Re: Question about RFC6724

[JINMEI Tatuya / 神明達哉 <[e-mail]>]

On Sun, May 1, 2022 at 9:33 PM Brian E Carpenter
<[e-mail]> wrote:

> > 2.2.  Common Prefix Length
> >
> >    We define the common prefix length CommonPrefixLen(S, D) of a source
> >    address S and a destination address D as the length of the longest
> >    prefix (looking at the most significant, or leftmost, bits) that the
> >    two addresses have in common, up to the length of S's prefix (i.e.,
> >    the portion of the address not including the interface ID).  For
> >    example, CommonPrefixLen(fe80::1, fe80::2) is 64.
>
> The "interface ID" is simply a non-concept for IPv4-mapped IPv6 addresses. So what do implementations do?

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.

But I don't think the specification simply "doesn't work for
::ffff:0:0/96" (or IPv4 addresses).
Since 'S' is a local address, the implementation should also be able
to get the corresponding netmask
(which should be contiguous in practice, and can be converted to a
"prefix length").
An implementation could apply that prefix and compare the
corresponding IPv4 addresses
up to that length of bits.
Extending it to the form of IPv4-mapped IPv6 addresses would be straightforward,
although the RFC text could have been clearer for that case.

--
jinmei