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

[SHIRASAKI Yasuhiro <yasuhiro@nttv6.jp>]

On Fri, 13 Feb 2004 21:41:24 +0800,
"Green" <quartets@21cn.com> wrote:
> 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.

We've been using a Framed-IPv6-Prefix radius attribute for a persistent
prefix assignment in our current service, so there has been no change
of delegating prefixes. This is what infinity lifetimes of prefixes
in the DHCPv6-PD means in our service. But we might need to think
about the problem you pointed out when we'll start prefix-pool based
service.

> 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.

I've just checked two implementations and noticed a different behavior
against prefix changes. When the CPE changed advertising prefixes
in a flash, one implementation did the things as you mentioned,
but the another one has revoked an old prefix immidiately and started
communications with a new prefix only. If all hosts behave as
the latter implementation, we don't need any special treatment
with prefix changes...

Anyway, it's reasonable that CPEs have stable storage retain delegated
prefixes and advertise old prefixes with pltime = 0, vltime = 2hour,
when the delegated prefix was changed.

--
SHIRASAKI Yasuhiro @ NTT Communications
t: +81-3-6800-3262, f: +81-3-5365-2990