Failover IPv6 with multiple PA prefixes (Was: IPv6 fc00::/7 - Unique local addresses)

Owen DeLong owen at
Mon Nov 1 20:04:28 CDT 2010

>> He may or may not be. I don't think it's such a bad idea.
> How about algorithmically generating these addresses, so that
> they're near unique, instead of having the overhead of a central
> registry, and a global routability expectation?
Why not just keep a low-overhead central registry and start accepting
that PI != global routability. Routability is a discussion between the
resource holder and their ISP(s).

ULA is the algorithmically generated problem and I think it's a generally
bad idea. It's basically an expectation of uniqueness where it may or
may not exist and the potential to fudge the level of routability into whatever
strange definition long-term creativity may evolve.

I think it's better to make GUA easy to get and remove the expectation
that GUA == Routable. (Ideally, we'd restore that eventually with a
scalable routing paradigm).

>>> Recently we've seen somebody (on either nanog or ipv6-ops) proposing to
>>> set valid lifetimes of 24 hours on ISP GUA prefixes. While a 24 hour
>>> outage is unusual for a always connected broadband service, it isn't
>>> for intermittently connected nodes and networks.
>> The upstream valid lifetime doesn't have a lot to do with what happens on
>> the internal network if you're smart.
> Residential end-users aren't "smart" and aren't network engineers - they
> pay people like us so that they don't have to be.
That still doesn't have a lot to do with enterprise failover which is what we
were talking about.

As to residential, residential end users mostly don't care if their network
goes down when their provider goes away. However, for those that do,
it still shouldn't be hard for the provider to uncouple circuit viability from
address presence.

>>> In effect people who suggest using PA GUAs or PI for all IPv6 addressing
>>> are saying you can't run IPv6 unless you have an available IPv6 ISP
>>> connection or you must be able to afford to be able to thrust upon the
>>> world occupation of a global route table slot. They're not realistic
>>> requirements for all potential users of IPv6. 
>> No...PI does not require an available IPv6 ISP connection at all. This
>> is a misstatement that does not become any less false no matter how
>> many times you repeat it.
> What if you don't have an IPv6 ISP connection? Where do you get your PA
> from? Link local isn't good enough, because it can't span more than a
> single link. Homes in the future are likely to have multiple networks -
> visitor segments, multicast segments for video, children
> segments, 6LowPAN for home sensor networks etc.
It gets configured in your router. Why is that such a difficult concept?

Your home gateway that talks to your internet connection can either
get it via DHCP-PD or static configuration. Either way, it could (should?)
be set up to hold the prefix until it gets told something different, possibly
even past the advertised valid time. It can delegate subnets using
DHCP-PD, but, the point is that the top level router at the site can
easily be coded/configured to keep the prefix regardless of the state of the
external link.

> You've stated you use link locals for this sort of thing, yet you'd be
> specifying the interface to use as well. That isn't much different to
> using a subnet number, embedded in the address, to specify either
> directly attached or remotely reachable subnets. The nice thing about
> doing it that way is that IPv6 applications are addressing scope
> agnostic - they just use the address supplied, and ask the
> underlying OS, which uses the local route table, to
> direct where the packets go and therefore select the outgoing interface.
> Link locals + interfaces is more complicated, because now socket
> options have to be invoked, and only in the case of when a link local
> address is specified, which also means performing an address type check
> for the interface option. This code has to be present in ever
> application, instead of letting the underlying OS taking care of how
> application packets are directed towards their destinations, and the
> applications not having to care.
No, I've stated that you could. I have stated that I use link locals for
a variety of things.

Usually for this type of thing, I'd use a legitimate GUA prefix whether
PI or PA.

>>> For the most common and scalable case of PA, external addressing
>>> dependencies reduce reliability, because you can't control them.
>>> Completely relying on external connectivity and addressing for your
>>> internal networks reduces their reliability and availability.
>> This is also false if you use any form of sanity in applying the assigned
>> PA prefix to your network.
> I suppose since they don't have the expertise, you could consider
> residential end-users insane. You can't make the insane sane just by
> telling them to be so. Preventing their "insanity" from breaking their
> Internet service, causing them to call your helpdesk, is the sane
> thing to do. That is achieved by making their Internet service work
> with the absolute least operational intervention on their part. It's
> hard enough to get them to enter their username/password via an
> embedded web server - to the point where some vendors supply setup CDs
> to automate the discovery of the device, avoiding the end user having
> to type an IP address URL into their browser.
Sure, but, why can't you set it up so that you either ship them pre-configured
hardware, have their hardware download it's configuration once each time
the CONFIGURATION MASTER RESET button is pushed, or, having the
hardware learn the configuration via DHCP-PD, but, keeping the configuration
until a newer configuration is received?

All of these provide zero-user-intervention ways to configure their
equipment such that they will have a valid PA address locally that
survives a link failure.

>>> In this common case of PA, how are you going to justify that "no IPv6
>>> without an IPv6 ISP" view to people who are very remote, such that even
>>> intermittent Internet access is very occasional; to people who run IPv6
>>> sensor networks and don't ever want them connected to the Internet; or
>>> 3rd world countries where just local connectivity provides a very
>>> significant benefit, when global connectivity just isn't affordable?
>>> These and similar are cases where only ISP PA or PI aren't acceptable.
>> Nobody is trying to. This is a fallacy of logic that you keep pushing,
>> but, it's still false. If I wire a PA prefix into my router, it doesn't go
>> away just because the ISP does. All that happens is that I can't
>> reach the internet from it, which is kind of true regardless of the
>> address space used at the point where your ISP goes away.
> You haven't ever tried to get a majority of residential end-users
> to update their firmware have you? You'll have the same luck getting a
> majority of them to "wire a PA prefix into" their routers. 
Why do they have to wire it in? Why can't I wire it in for them?

I know lots of companies that maintain control of the top-level CPE router
for just this reason.

>>> Permanent connectivity to the global IPv6 Internet, while common,
>>> should not be essential to being able to run IPv6, and neither should
>>> PI. All you should need to run IPv6 reliably is stable internal
>>> addressing. Global connectivity should be optional, and possibly only
>>> occasional.
>> Why shouldn't PI if it was easy to get? I still don't understand the
>> perceived advantage of ULA vs. PI other than the perception that
>> it is easier to get. If PI is just as easy to get, why is it a problem?
> It seems to me your main criticism of ULAs is that people would expect
> it to be globally routed if they paid enough money. Now you're saying
> that if PI is really easy to get, people *won't* have a global routing
> expectation of PI routability? I certainly would if I was given PI.
> What would be worse is that this "non-routable" PI won't come out of a
> specific prefix so that it can easily filtered, unlike ULAs.
If you find a provider that will route your PI, no harm done. You're paying
enough to get someone to listen to your routes and the internet is
accepting them for the time being.

At least your PI is subject to policy and the will of the community.

ULA, on the other hand, has no community oversight, no policy body,
and no restrictions whatsoever on who else uses "your ULA". Yet,
through creativity and luck, ULA will eventually get routed across
more and more of the internet until it starts to look like cheap easy
policy free GUA. At that point, the harm is not about your expectations,
the harm is about the failed expectations of the rest of the internet
with respect  to ULA.

>>>> 2) ULA brings with it (as do any options that include multiple
>>>> addresses) host-stack complexity and address-selection issues... 'do I
>>>> use ULA here or GUA when talking to the remote host?'
>>> There's an app for that (or rather a library routine called
>>> getaddrinfo() and an optional table it consults), and there's soon going
>>> to be a way to distribute it via DHCPv6 if the defaults don't suit -
>> Sure, now, how many applications have been coded to actually
>> pay attention to what getaddrinfo is telling them about address
>> selection order?
> All the ones I use - they all seem to use the first getaddrinfo()
> response. They should be attempting to successively connect() to all
> responses in the order that getaddrinfo() returns as connect()
> failures occur. I don't know if they are (as destination reachability
> is usually good), however if they aren't, then the application
> developers haven't used getaddrinfo() correctly. That behaviour
> wouldn't be exclusive to IPv6 though - IPv4 applications should also be
> attempting to connect() to successive addresses when multiple are
> returned. IOW, applications coping with multiple responses to
> getaddrinfo() is not an exclusive issue to IPv6.
There are well behaved and not so well behaved applications out
there with respect to getaddrinfo. I agree with you about the ideal,
but, counting on that is sort of like counting on the user to configure
something correctly... Not likely to reduce your help desk calls.

> I actually override the current default IPv6 address rules. Here's
> my /etc/gai.conf, which makes ULAs override GUAs as that currently
> isn't in the default address selection rules, and makes tunnelled IPv6
> preferred over native IPv4, as I don't currently have native IPv6. The
> MRS entries are the non-defaults, the rest are from the gai.conf manual
> page.
You do this for your residential customers? It's fun to watch how this
discussion gets steered back to enterprise for places where it works
better for you as an enterprise, but, residential customers are suddenly
the topic when I give an answer that solves the enterprise problem but
may not work for residential.


> --
> # Used for selecting source addresses
> #
> # label <prefix> <label>
> #
> label  ::1/128       0
> label  ::/0          1
> label  2002::/16     2
> label  2000::/3      2 # MRS
> label ::/96          3
> label ::ffff:0:0/96  4
> label fc00::/7       5 # ULA - MRS
> # Used for sorting destination addresses
> #
> # precedence <prefix> <precedence>
> #
> precendence  ::1/128       50
> precendence  ::/0          40
> precendence  fc00::/7      35 # ULA - MRS
> precendence  2000::/3      30 # MRS
> precendence  2002::/16     30
> precendence ::/96          20
> precendence ::ffff:0:0/96  10
> --

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