Subject: Section 2.2: Issues with IPv4 LL and Roaming Behavior

From: Bernard Aboba <[email-masked]>
Date: Wed, 13 Aug 2003 14:23:33 -0700 (PDT)

I've noticed that portions of Section 2.2 are not very clear about the
intended behavior in the case of a host changing its point of attachment.
I'm concerned that the current language could do some damage -- resulting
in hosts that retain LLv4 addresses across subnet moves, or even do not
obtain a routable address when one was available, remaining perpetually
disconnected from the Internet.

2.2.  Claiming a Link-Local Address

"When a network interface transitions from an inactive to an active
state, the host does not have knowledge of what Link-Local IPv4
addresses may currently be in use on that link, since the network
interface may have been inactive when a conflicting address was claimed.

[BA] This sentence assumes that a host knows what network it
is connected to when the interfaces transitions from inactive to active.

When an interface transitions from inactive to active, the host not
only doesn't have knowledge of LLv4 addresses in use -- it typically
doesn't even know for sure whether it has connected to an alternative
point of attachment.  It is therefore appropriate to determine what
network the host is connected to (such as via probing for a
default gateway or attempting to obtain a routable address)
rather than merely probing for an IPv4LL conflict, potentially
on a different network than the one on which it was originally allocated.

I suggest that this be changed to:

"When a network interface transitions from an inactive to an active
state, the host does not have knowledge of what Link-Local IPv4
addresses may currently be in use on that link, since the point of
attachment may have changed or the network interface may have been
inactive when a conflicting address was claimed."

"Were it immediately to begin using a Link-Local IPv4 address which is
already in use by another host, this would be disruptive to that other
host."

Worse than that -- if a host were to assume autoconf without verification
whenever an interface came up, it would not obtain a routable address,
even when one was available.  The implication of the above two paragraphs
seems to be that doing a probe first would make it ok to use an LLv4
address when an interface comes up.  But this is still assuming that both
LLv4 and routable address are in use on the same interface.  Otherwise,
this is not correct.

Suggest this be changed to:

"Were the host to immediately begin using a Link-Local IPv4
address which is already in use by another host, this would be
disruptive to that other host.  Since it is possible that the
host has changed its point of attachment, a routable address may be
obtainable on the new network, and therefore it cannot be assumed that a
Link-Local IPv4 address is to be preferred."

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From: Robert Elz <[email-masked]>
Date: Thu, 14 Aug 2003 18:49:51 +0700

I agree those changes are worth making.

kre