Dave,
RFC 1812 Section 2.2.5.1 defines the case where multiple subnets are
associated with a single interface of a router thusly:
"The inventors of the subnet mechanism presumed that each piece of an
organization's network would have only a single subnet number. In
practice, it has often proven necessary or useful to have several
subnets share a single physical cable. For this reason, routers
should be capable of configuring multiple subnets on the same
physical interfaces, and treat them (from a routing or forwarding
perspective) as though they were distinct physical interfaces."
Is this what you mean by multiple subnets on a single link?
jak
----- Original Message -----
From: "Dave Thaler" <[EMAIL PROTECTED]>
To: "Julien Laganier" <[EMAIL PROTECTED]>; "INT Area"
<[EMAIL PROTECTED]>
Cc: "James Kempf" <[EMAIL PROTECTED]>; "NetLMM WG" <[EMAIL PROTECTED]>
Sent: Tuesday, August 08, 2006 1:13 PM
Subject: RE: IPv6 addressing model, per-MN subnet prefix, and broadcast
domain
If I understand the picture at bottom, this is fine.
That is, there can be multiple subnet prefixes per link,
and different hosts may be in different subsets of the
set of subnet prefixes. All of this is fine in the IP
addressing model.
Section 2.1 of the [draft-thaler-intarea-multilink-subnet-issues] draft
acknowledges this:
In December 1995, the original IP Version 6 Addressing Architecture
[RFC1884] was published, stating: "IPv6 continues the IPv4 model
that a subnet is associated with one link. Multiple subnets may be
assigned to the same link."
Thus it explicitly acknowledges that the current IPv4 model has been
that a subnet is associated with one link, and that IPv6 does not
change this model. Furthermore, a subnet is sometimes considered to
be only a subset of a link, when multiple subnets are assigned to
the same link.
-Dave
-----Original Message-----
From: Julien Laganier [mailto:[EMAIL PROTECTED]
Sent: Monday, August 07, 2006 6:21 AM
To: Dave Thaler; INT Area
Cc: James Kempf; NetLMM WG
Subject: IPv6 addressing model, per-MN subnet prefix, and broadcast
domain
Hi Dave, and other folks knowledgeable about IPv6 addressing model,
[EMAIL PROTECTED] CCed, please reply only to [EMAIL PROTECTED]
While working on the issues of which addressing model to use in
NetLMM, I think I got confused with issues involved the IPv6
addressing model (or its assumptions.)
I would therefore like to ask you if a potential NetLMM addressing
model (per-MN subnet prefix [RFC3314]) would, in some situations,
conflict with the IP addressing model.
Background
----------
Dave's draft on issues involved with multilink subnets
[draft-thaler-intarea-multilink-subnet-issues] list some assumptions
of the IP addressing model, but there might be other that are not
specific to multilink subnets. I'd therefore like to ask you about
possible conflicts between IPv6 and RFC3314 addressing model.
We are considering the situation of mobile nodes (MNs) attached to a
NetLMM domain. The NetLMM domain span multiple access links, each
served by a given access router (AR). A MN attaches to one link, and
hence to one AR.
( NetLMM domain )
/ | | | \
AR AR AR AR AR
/ \ \ / \ \
MN MN MN MN MN MN
If all of the MNs in the domain uses a common subnet prefix we
obviously end-up with a multilink subnet, which is problematic as
described in Dave's draft. Now a simple way to avoid multilink subnet
issues is to use a per-MN subnet prefix, as in the IETF
recommendation to 3GPP [RFC3314]. That way, each of the MN moves has
a different prefix and hence none of the prefix spans more than one
link, thus avoiding multilink subnet issues.
Issue
-----
Such model has however raised a question, which is orthogonal to
multi-link subnets issues. RFC3314 was proposed for use in a scenario
where the link between the MN and its AR is point-to-point. Now if we
consider a broadcast/multicast capable link-layer technology such as
Ethernet, then we would have a situation in which, on a given link,
the broadcast domain and hence the link-local scope are larger than
the any of the per-MN subnet prefixes scope (as illustrated below
when 3 MNs A, B and C are connected to one such link served by one AR
R).
A subnet prefix scope: -R-------A------------------
B subnet prefix scope: -R---------------B----------
C subnet prefix scope: -R------------------------C-
link-local scope: -R-------A-------B--------C-
L2 broadcast scope: -R-------A-------B--------C-
Do you think that this situation (i.e. link-local scope larger than
subnet prefix scope) would conflict with the IPv6 addressing model,
or any of its assumptions?
Many thanks in advance. Best regards,
--julien
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