Re: [Technical Errata Reported] RFC6724 (6971)

[David Farmer <[e-mail]>]

I've been thinking about this a bit more and accounting for the IPv6 Subnet
Model [RFC 5942];

On Wed, May 11, 2022 at 4:12 PM David Farmer <[e-mail]> wrote:

>
> Nevertheless, the CommonPrefixLen() should be no longer than the SA subnet
> prefix length, which is typically 64 for most IPv6 subnets,
>

Typically in IPv6, the prefix length will be 64. However, this MUST NOT be
assumed, as the prefix length is a parameter that MUST come from either a
valid Prefix Information Option (PIO) contained in a Router Advertisement
(RA) or from a manually configured prefix length. Based on the combination
of RFC 4291 and RFC 4682, PIOs with the A-Bit set are currently only valid
if they have a prefix length of 64, this is probably the most common
configuration, and why 64 is the typical prefix length. However, PIOs
without the A-Flag set are valid at any length, along with manually
configured prefix lengths of any length. It should be noted that the L-Bit
will commonly be set in most PIOs, denoting an on-link prefix, and the use
of Neighbor Discovery (ND) for communication with the on-link address.
However, if L-Bit is not set, this should still be considered a valid
off-link prefix, requiring communication with the off-link address to be
sent via the router sending the RA containing the PIO in question. See
RFC8028 for more details.

Further, DHCPv6 doesn't include a prefix length, normally a PIO that covers
the address will specify the prefix length for an address learned via
DHCPv6. However, if there is no PIO that covers the address learned via
DHCPv6, I believe the prefix length must be assumed to be 128.


> and in the case of IPv4-mapped addresses in ::ffff:0:0/96 is typically
> between 112 and 125, with 120 possibly being the most common, representing
> IPv4 subnet masks of 255.255.0.0 (/16), 255.255.255.248 (/29) and
> 255.255.255.0 (/24) respectively.
>



> Finally, it is technically possible for the CommonPrefixLen() to be 128,
> but I think this should only occur IFF the SA and the DA are in fact the
> same address.
>

and the SA prefix length is 128.

Thanks

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