Subject: Re: [Issue 10] Warnings in Applicability Statement

From: Bernard Aboba <[email-masked]>
Date: Sun, 9 Feb 2003 22:43:43 -0800 (PST)

"Existing applications fundamentally assume addresses of
communicating peers are routable, relatively unchanging and unique. These
assumptions no longer hold with IPv4 link-local addresses."

They also may not hold for DHCPv4 assigned addresses. Without failover
or clustering support, DHCPv4 outages are more likely than many of us
would like to believe -- and so are address conflicts, if experience
at IETF meetings is any guide. For example, nowadays DHCPv4 servers
are often provided on devices without the "stable storage" required
by RFC 2131. When those devices reboot (which can be frequent) all
address state is lost.

DHCPv4 server configurations which provide multiple offers from
distinct address pools will result in address changes whenever the
renewing server is down or even heavily loaded. Often organizations
try to get around these problems by lengthening the lease time to days or
even weeks. Of course, private addresses are commonly assigned by
DHCPv4 mechanisms.

So let's not be under the illusion that outside of LLv4, there is a world
where IPv4 "routable, relatively unchanging and unique" addresses are
always available. If they were, we wouldn't need IPv6 :)

This means, among other things, that switching between LLv4 and DHCPv4
assigned addresses should be carefully thought through. If not handled
carefully, it can make the problem worse, not better.

I would recommend that the text be changed to:

"Existing applications fundamentally assume addresses of
communicating peers are routable, relatively unchanging and unique. These
assumptions no longer hold with IPv4 link-local addresses, particularly
on networks with an unstable DHCPv4 configuration or a substantial number
of hosts."

---

From: Bernard Aboba <[email-masked]>
Date: Mon, 10 Feb 2003 17:55:32 -0800 (PST)

> I suggest instead:
> communicating peers are routable, relatively unchanging and unique.
> These assumptions no longer hold with IPv4 link-local addresses,
> especially those with a substantial number of hosts.  On networks that
> do not provide stable DHCP configurations the introduction of IPv4
> link-local addresses may further decrease address stability due to
> hosts switching between LL addresses and DHCP-assigned addresses.

Sounds good to me.