Re: [homenet] Let's make in-home ULA presence a MUST !?

James Woodyatt <> Wed, 15 October 2014 17:29 UTC

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Date: Wed, 15 Oct 2014 10:29:50 -0700
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From: James Woodyatt <>
To: HOMENET Working Group <>
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Subject: Re: [homenet] Let's make in-home ULA presence a MUST !?
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On Wed, Oct 15, 2014 at 4:14 AM, Sander Steffann <> wrote:

> [I wrote:]

> Consider a hypothetical router that has the regular automatic default
> behavior of commissioning a new standalone network while discovering any
> existing networks that it already possesses the credentials to join. Now
> consider what happens when devices of this category are continually losing
> and regaining their connectivity with the rest of the wireless network in
> the home. Let's imagine this happens many times per hour. How many days
> does it take before all your constrained-resource hosts have no space left
> in their route tables for all the deprecated but still valid ULA prefixes?
> Does it have to be a *new* standalone network (ULA prefix)? The router
> could just generate a ULA prefix once and reuse it whenever it needs to,
> right? Generating a new prefix on every connect/disconnect would indeed
> cause a mess...

No, the router can't do that. Consider that ULA prefixes may be advertised
through tunnels to one or more exterior private routing domains that
communicate with multiple home networks simultaneously. Because the locally
generated ULA prefix is copied from the commissioning router to all the
other routers in the home network, a router must therefore generate a new
ULA prefix at every network commissioning to preserve the property that ULA
prefixes are statistically unlikely to collide. Otherwise, every time a
single device is used to commission a new network with the same ULA prefix,
that prefix will collide with existing previously commissioned networks at
the exterior domain gateway.

james woodyatt <>
Nest Labs, Communications Engineering