draft-shirasaki-dualstack-service-02.txt CPE assigned Host Address Lifetime issue

["Green" <quartets@21cn.com>]

Hi, all:

I have a question regarding the following paragraph in
draft-shirasaki-dualstack-service-02.txt:

==>
2.4. Address Assignment

   CPE assigns global-scope /64 prefixes subnetted from the delegated
   prefix to its downstream interfaces. In the case where the delegated
   prefix has infinite lifetimes, the preferred and valid lifetimes of
   assigned /64 prefixes should be their default values in [RFC2461].
==>

According to this draft, the CPE gets a (non-link-local) prefix (possibly
/48) using DHCP from the BRAS after successful PPoE initiation. The CPE then
delegate the prefix's subnet to its host-facing links. The subnet prefixes
is advertised on the links using RA, with the prefered and valid lifetime
using default values in rfc2461, which is 7days and 30 days.

Certainly I understand these default value could be overrided by manual
configuration on CPE router, but since CPE router is supposed to be an
unmanaged device installed in customer home, such assumption can be dropped.

Problem may arise when the PPPoE session is terminated between the CPE and
BRAs, in a case other than CPE powering off or customer disconnect the hosts
(PCs), and then the PPPoE session between the two gets re-established.

In this case, the CPE will get a new prefix using DHCP from the BRAS
(Although Radius extension can be configured to make sure the same customer
line, or CPE MAC address, or user id will be assigned the same prefix in
some cases, generally I think the CPE will just get a new Prefix just like
nowadays's dial-up getting a new IPv4 address everytime you dial), and the
CPE will delegate new subnet prefixes to its links.

So now the hosts (PCs) will have an old  non-link-local IPv6 address, with
its lifetime still valid for the next week or month, and receives a new
non-link-local IPv6 address. When sending out packets, it will have to make
a default address selection based on the two source addresses. It will have
good chance of selecting the old address, which the BRAS doesn't have a clue
when forwarding packet back to this (destination) address.

DHCPv6 can reclaim the assigned addresses from its clients, but stateless
auto-configuration can't do the same, unless the Router Advertisement from
which this Prefix Information option was obtained has been authenticated,
(quoting from RFC 2462 - IPv6 Stateless Address Autoconfiguration 5.5.3
Router Advertisement Processing) which is obviously not the case between CPE
and hosts (Home PC). So, the CPE can't force the hosts to make the old
address expire explicitly, the above mentioned problem will occur.

Please comment, I am eager to know the answer.

And, I am very curious of the CPE used in NTT's deployment, is there any
more information on that available?

Green Guo Limin
limin.guo@ericsson.com
Ericsson China