Re: New Version Notification for draft-hinden-ipv4flag-00.txt

[Erik Kline <[e-mail]>]

I like this, and it seems similar to something that came out of some
sidebar conversations I was having in Singapore.

Here are a few changes I might suggest, followed by how I could see
implementing such a thing in an OS like Android.

    [1] I would propose making it an EFO bit and leave the reserved
bits for potentially more IPv6-critical information signaling.

    [2] I would not say that 0 means "IPv4 is Available on this
Router" but rather "this router makes no attestation about the
availability or lack thereof of IPv4".

The actual implementation that I could see being reasonable is as follows.

Because every OS starts IPv4 and IPv6 configuration paths in parallel
on link-up/link-attach, I think the most reasonable thing to do is to
interpret this flag as the "save your electrons flag".

In Android I think I would propose that we consider the bit only after
we have satisfactory global IPv6 provisioning, which I would define
as:

    [a] at least one valid (i.e. non-deprecated), non-ULA, GUA

    [b] at least one valid (i.e. non-deprecated) default route

    [c] at least one valid (i.e. non-deprecated) DNS server

Additionally, we could include a successful internet connectivity
check as a requirement, just to make sure we keep trying IPv4 until
any captive portal has been passed.

Finally: once IPv6 can be categorized as "working", and we saw no
other RAs with 0 in the extended flags options nor any RAs without
extended flags options, and we hadn't yet found any DHCPv4 server,
then we could use the "save your electrons" flag as a signal to stop
our DHCPv4 client.  Operating Systems that implement IPv4 link-local
communications (Android does not) might use this signal to disable
that as well.