using ULA for 'hidden' v6 devices?

Owen DeLong owen at
Thu Jan 26 16:53:28 UTC 2012

On Jan 26, 2012, at 8:14 AM, Ray Soucy wrote:

> Inline
> On Thu, Jan 26, 2012 at 9:05 AM, Tim Chown <tjc at> wrote:
>> Thanks for the comments Ray, a couple of comments in-line.
>> On 26 Jan 2012, at 12:43, Ray Soucy wrote:
>>> Local traffic shouldn't need to touch the CPE regardless of ULA or
>>> GUA.  Also note that we already have the link local scope for traffic
>>> between hosts on the same link (which is all hosts in a typical home
>>> network); ULA only becomes useful if routing is involved which is not
>>> the typical deployment for the home.
>> The assumption in homenet is that it will become so.
> Does this mean we're also looking at residential allocations larger
> than a /64 as the norm?

We certainly should be. I still think that /48s for residential is the right answer.

My /48 is working quite nicely in my house.

>>> ULA is useful, on the other hand, if NPT is used.  NPT is not NAT, and
>>> doesn't have any of the nastiness of NAT.
>> Well, you still have address rewriting, but prefix-based.
> I think that the port rewriting, and as a consequence not being able
> to map to specific hosts easily, was the bigger problem with NAT.

No, the need for ALGs is the biggest problem with NAT. NPT does not resolve that issue.

Yes, port rewriting and other issues are also problematic, but, they are less problematic than the need for ALGs.

> As for the comments made by others regarding "helpers" for NAT, there
> really aren't many that are needed aside from older pre-NAT protocols
> like H.323 which decided it would be a good idea to use the IP in the
> packet payload for authentication.  Thankfully, over a decade of NAT
> has helped end this practice.

Yes, it has blocked innovation in protocols that can't easily engineer around NAT. Hopefully we can stop doing that soon.

>>> I think a lot of the question has to do with what the role of CPE will
>>> be going forward.  As long as we're talking dual-stack, having
>>> operational consistency between IPv4 and IPv6 makes sense.  If it's an
>>> IPv6-only environment, then things become a lot more flexible (do we
>>> even need CPE to include a firewall, or do we say host-based firewalls
>>> are sufficient, for example).
>> The initial assumption in homenet is a stateful firewall with hosts inside the homenet using PCP or something similar.
>> Tim
> So a CPE device with a stateful firewall that accepts a prefix via
> DHCPv6-PD and makes use of SLAAC for internal network(s) is the
> foundation, correct?

I would expect it to be a combination of SLAAC, DHCPv6, and/or DHCPv6-PD. Which combination may be vendor dependent, but, hopefully the norm will include support for downstream routers and possibly chosen address style configuration (allowing the user to pick an address for their host and configure it at the CPE) which would require DHCP support.

> Then use random a ULA allocation that exists to route internally
> (sounds a lot like a site-local scope; which I never understood the
> reason we abandoned).

I can actually see this as a reasonable use of ULA, but, I agree site-local scope would have been a better choice. The maybe you can maybe you cant route it nature of ULA is, IMHO it's only advantage over site-local and at the same time the greatest likelihood that it will be misused in a variety of harmful ways, not the least of which is to bring the brain-damage of NAT forward into the IPv6 enterprise.

> I'm just not seeing the value in adding ULA as a requirement unless
> bundled with NPT for a multi-homed environment, especially if a
> stateful firewall is already included.  If anything, it might slow
> down adoption due to increased complexity.

I don't believe it adds visible complexity. I think it should be relatively transparent to the end-user.

Basically, you have one prefix for communications within the house (ULA) and another prefix for communications outside. The prefix for external sessions may not be stable (may change periodically for operational or German reasons), but, the internal prefix remains stable and you can depend on it for configuring access to (e.g. printers, etc.).

Sure, service discovery (mDNS, et. al) should obviate the need for most such configuration, but, there will likely always be something that doesn't quite get SD right somehow.

Also, the ULA addresses don't mysteriously stop working when your connection to your ISP goes down, so, at least your LAN stuff doesn't die from ISP death.


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