At Tue, 11 Mar 2008 07:36:48 -0700,
Erik Nordmark <[EMAIL PROTECTED]> wrote:
> I agree that "subnet" by itself is underspecified; in some cases it
> should just be a prefix. I don't think this document uses subnet" to
> mean "link".
>
> However, I do think we should refer to "subnet prefix" in some places.
> If nothing else to make it easier for folks with an IPv4 background to
> understand what we are talking about. The "subnet prefix" term is
> specified in RFC 4291 (the addressing architecture) thus I don't think
> need to completely purge it.
Actually, I've just realized the word "subnet" might be the source of
confusion. I've looked at the FreeBSD's ifconfig source code to
recall why the default prefix length was chosen to be 64 and found the
following code fragment:
if (explicit_prefix == 0) {
/* Aggregatable address architecture defines all prefixes
are 64. So, it is convenient to set prefixlen to 64 if
it is not specified. */
setifprefixlen("64", 0, s, afp);
/* in6_getprefix("64", MASK) if MASK is available here... */
}
Although the notion of "aggregatable address" was deprecated, I think
the sense of this comment could still apply if the implementor refers
to RFC4291. That is, Section 2.5 gives the following structure
| n bits | 128-n bits |
+-------------------------------+---------------------------------+
| subnet prefix | interface ID |
+-------------------------------+---------------------------------+
and states as follows:
All Global Unicast addresses other than those that start with binary
000 have a 64-bit interface ID field (i.e., n + m = 64),
(this means the subnet prefix is 64).
And, as others pointed out, since the notion of "subnet" is not really
clear, it's not surprising even if the implementor interprets a
"subnet prefix" as on-link prefix.
Along with the context of this draft, I believe an IPv6 address should
be considered at the "minimum" level as described in RFC4291:
At a minimum, a node may
consider that unicast addresses (including its own) have no internal
structure:
| 128 bits |
+-----------------------------------------------------------------+
| node address |
+-----------------------------------------------------------------+
If this makes sense, I'd propose revising bullet #2 of Section 2 as
follows:
2. The configuration of an IPv6 address, whether through IPv6
stateless address autoconfiguration [RFC4862], DHCPv6
[RFC3315], or manual configuration does not imply that any
prefix is on- link. This means the address should initially be
considered the one having no internal structure as shown in
[RFC4291]. A host is explicitly told that prefixes or
addresses are on-link through the means specified in [RFC4861].
---
JINMEI, Tatuya
Internet Systems Consortium, Inc.
--------------------------------------------------------------------
IETF IPv6 working group mailing list
ipv6@ietf.org
Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6
--------------------------------------------------------------------
