Re: [homenet] ULA scope [draft-ietf-6man-rfc3484-revise-05.txt]

Brian E Carpenter <brian.e.carpenter@gmail.com> Tue, 20 March 2012 21:26 UTC

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Date: Wed, 21 Mar 2012 10:25:57 +1300
From: Brian E Carpenter <brian.e.carpenter@gmail.com>
Organization: University of Auckland
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To: Anders Brandt <Anders_Brandt@sigmadesigns.com>
Subject: Re: [homenet] ULA scope [draft-ietf-6man-rfc3484-revise-05.txt]
References: <4EB3F3D6.4090302@innovationslab.net> <9B57C850BB53634CACEC56EF4853FF653B3C3777@TK5EX14MBXW601.wingroup.windeploy.ntdev.microsoft.com> <9B57C850BB53634CACEC56EF4853FF653B3EDB9E@TK5EX14MBXW601.wingroup.windeploy.ntdev.microsoft.com> <E6E7EE34-8244-40B6-84C1-C79E8BDE7921@nttv6.net> <4F3ABFBA.8060605@gmail.com> <29EBA88D-BDB1-464C-915F-B9063578DC51@nttv6.net> <9B57C850BB53634CACEC56EF4853FF653B45BB08@TK5EX14MBXW604.wingroup.windeploy.ntdev.microsoft.com> <C8827D58-5C69-4A44-B9CE-86791466814E@nttv6.net> <4F63896E.10607@gmail.com> <CAFtBC=8=__8GdtExB8oYgA7pOfjxNfXCLzuOXz7_UKCPhwjenw@mail.gmail.com> <5B6B2B64C9FE2A489045EEEADDAFF2C3043A22C2@XMB-RCD-109.cisco.com> <4F64026B.8080308@gmail.com> <9B57C850BB53634CACEC56EF4853FF653B4A639F@TK5EX14MBXW603.wingroup.windeploy.ntdev.microsoft.com> <CABOxzu0kXRg=xdeq143+FWBTFc=+dbJD4LdpOGPi1KmyJ9YmEA@mail.gmail.com> <CABOxzu0x97UmA+Fq9d3e-Wp_ruT0gUni0UxnzgvtzDddjceg-A@mail.gmail.com> <03F31C213F2C6941BFDDBB4336E9E6CD0ABC058C@cph-ex 1>
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On 2012-03-20 21:51, Anders Brandt wrote:
> Kerry Lynn writes:
> 
>> On Sat, Mar 17, 2012 at 2:22 AM, Dave Thaler <dthaler@microsoft.com>
>> wrote:
>>> Brian Carpenter writes:
>>> [...]
>>>> Let me be clear. If a local service has (for some reason) both a ULA
>>>> and a non- ULA global address, and the host has both, I think the
>>>> correct default behaviour is for the ULA address pair to be used.
>>> As I put into the doc, I don't think that's quite right.
>>>
>>> If both the source and dest ULAs are in the same /48 then I think the
>>> correct default is as you say (use ULA).
>>>
>>> If the source and dest ULAs are in different /48's then I think the
>>> correct default is instead to use the non-ULA global, since there's no
>>> guarantee of routability between different /48s.  So unless configured
>>> otherwise, one has to assume it's far more problematic than a non-ULA
>> global.
>> Do you mean "no guarantee of symmetric routability"?  The fact that the
>> packet arrived in the first place seems to indicate earlier policy choices (e.g.
>> the sender may not have a non-ULA global address, and the two /48s already
>> seem to share a common definition of "site").
>>
>> I am still relatively new to homenet and I am surely missing a lot of
>> background.  Has anyone discussed dealing with multiple /48 ULA prefixes
>> in a single site?
> 
> I would like to second Kerry.
> 
> It is a surprise to me that ULA addresses are not by default routable within the site.
> I can easily imagine a number of LLN border routers which autonomously allocate
> different ULA prefixes for use within their individual LLN subnets.

IMHO that should be a NOT RECOMMENDED behaviour. ULAs make sense if they
cover an entire enterprise or home network, but not if they cover a subset.

> Meeting a ULA address outside the local prefix will cause the LLN node to forward
> its IP packets to the default gateway (border router) of the LLN subnet. This way
> packets can travel between LLN subnets using normal routing with long-term stable
> ULA addresses. We need the stable addresses for control-style applications in LLNs.
> 
> Obviously it requires a routing protocol in the (homenet) LAN but are there other issues?

It doesn't just require a routing protocol; it also requires a routing policy
that knows which routers have to block the ULAs (plural). That seems a lot
more complex that a rule that says only a border router originates and delegates
a ULA prefix, because that border router would also know to block the
prefix across the border.

Anyway - maybe you should look at draft-liu-v6ops-ula-usage-analysis
and discuss it over on v6ops.

    Brian

> 
> Thanks,
>   Anders
>>> You'll find the above logic in the current 3484bis draft.
>>>
>>> -Dave
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