Re: Stateful SLAAC (draft-ietf-v6ops-unique-ipv6-prefix-per-host)

Lorenzo Colitti <lorenzo@google.com> Fri, 10 November 2017 00:35 UTC

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From: Lorenzo Colitti <lorenzo@google.com>
Date: Fri, 10 Nov 2017 09:35:10 +0900
Message-ID: <CAKD1Yr0jOtHcTp5RKaj0vOuivmRFYwwEf_4NKfX1QD-vF3NxrA@mail.gmail.com>
Subject: Re: Stateful SLAAC (draft-ietf-v6ops-unique-ipv6-prefix-per-host)
To: Fernando Gont <fgont@si6networks.com>
Cc: IPv6 Operations <v6ops@ietf.org>, "6man-ads@tools.ietf.org" <6man-ads@ietf.org>, "v6ops-ads@ietf.org" <v6ops-ads@ietf.org>, "6man@ietf.org" <6man@ietf.org>, draft-ietf-v6ops-unique-ipv6-prefix-per-host@tools.ietf.org
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On Fri, Nov 10, 2017 at 2:48 AM, Fernando Gont <fgont@si6networks.com>;
wrote:

> > I don't see how this is a protocol change. Sending RAs unicast is
> > already allowed by RFC 4861, so this is just an operational practice.
>
> That's incorrect. We are talking about sending multicasted internetlayer
> packets to a unicast link-layer address. There's nothing in RFC4861 that
> suggests you should do that. RFC6085 says "you shouldn't drop packets if
> they are internet-layer multicast but link-layer unicast". But
> certainly, unless a protocol spec says otherwise, the normal mapping
> applies.
>

The intent of 6085 is clearly to allow this. The introduction says that the
purpose of the document is to allows mapping a multicast group to a
link-layer address "when it is clear that only one address is relevant".

> The additional text did not actually change this practice at all.> It
> > simply clarified what has always been a fundamental premise of this
> > operational practice: if you want to give each host its own prefix, you
> > need to ensure that the RA with that prefix is only received by that
> host.
>
> If you want to give each host it's own prefix, you do prefix delegation.
> So far, SLAAC doesn't do prefix delegation. If you want to do prefix
> delegation in slaac, write a std track document in 6man, not a BCP in
> v6ops.
>

That statement does not match reality. All IPv6-capable phones (well over
100 million at this point) have a /64 prefix that is dedicated to them by
the network, and the mechanisms used for that are RAs and SLAAC. See RFC
6459.


> > This is a bizarre claim. The first-hop router must always have fully
> > up-to-date state on all the prefixes it is sending RAs for, otherwise it
> > cannot fulfill its fundamental purpose of forwarding traffic to those
> > prefixes. The word "stateless" in SLAAC applies to addresses configured
> > to the host, not how routers route traffic.
>
> A SLAAC router need not maintain any sort of dynamic mapping. It's
> static configuration information of the sort "I'm announcing this prefix
> on this interface".. If you think otherwise, please point me the section
> in RFC4862 where this conceptual data structure is mentioned. (Note:
> Neighbor Cache, Destination Cache, etc. are all mentioned in RFC4861).
>
:
In many cases that's not correct. Consider DHCPv6 PD, which you cite as an
example above. IIRC there is nothing in any RFC that says that the
first-hop router of a requesting router needs to maintain a mapping between
delegating prefix and requesting router, but that mapping is 100% required
for DHCPv6 PD to work.

>     3) What happens if the SLAAC router crashes and reboots, loosing state
> >     of the "leased" prefixes?
> >
> > You seem to be assuming that the router does not store the prefixes in
> > stable storage.
>
> Routers store configuration info, not dynamic mappings.
>

Again, see the DHCPv6 PD example above.

>     4) How are prefixes selected? And, what's the minimum size of the pool
> >     of prefixes for the selection algorithm not to break due two "prefix
> >     collisions"? Does the selection algorithm have any specific
> properties?
> >
> > I see no reason why this should be in scope for this document.
>
> Actually, what's not in the scope of this document, and not within v6ops
> charter, is the protocol you are specifying to handle prefixes with SLAAC.


Protocols are out of the v6ops charter, and that's fine because there is no
protocol in this document. :-)