Re: Updated IID length text

Lorenzo Colitti <lorenzo@google.com> Thu, 19 January 2017 06:21 UTC

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From: Lorenzo Colitti <lorenzo@google.com>
Date: Thu, 19 Jan 2017 15:21:10 +0900
Message-ID: <CAKD1Yr06uPj95XNzvFnwrmxnykVw5CKf3uO40eaiWzZ0uXmuRg@mail.gmail.com>
Subject: Re: Updated IID length text
To: Fernando Gont <fgont@si6networks.com>
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On Thu, Jan 19, 2017 at 2:38 PM, Fernando Gont <fgont@si6networks.com>
wrote:

> > You forgot the "where layer 2 ensures that there are no duplicate MAC
> > addresses on the network" part of this scenario.
>
> 1) AP != Network -- you might be assuming the network is simpler than it
> really is
>

In small networks, the probability of collision is low because there are
few devices. Large networks are usually built with a centralized control
plane, because otherwise roaming doesn't work, and in that sort of network,
MAC addresses have to be unique or devices don't get on the network.


> 2) How many nodes do you need in a 48-bit space for the probability of
> collisions to become a concern?
>

I never said the probability is unacceptable with 48 bits. That depends a
lot on the network circumstances. What I said that the increase in
probability when going down from 64 bits to 48 bits, which it is. If you
want 99.999% chance of no collisions, with 48 random bits I that puts you
between 10k and 100k devices. With 64 bits that's more like 10^35 devices.
That's an incredible difference. At 32 bits it's a joke - 1% chance of
collision at 10k devices.


> 3) If you are concerned about collisions in 48 bits as a result of
> random numbers, I'm curious why layer-3 concerns you more --
> particularly when, in layer-3 you do have a mechanism for detecting
> them, and one for recovering from them (whereas in layer-2, you don't).


Sigh. Consider 802.11 wifi. Dynamic MAC addresses are desirable for privacy
reasons. If there's a random MAC address collision, you don't get on the
network (most of the time; as discussed above). At that point you either
fail or you try again with a different random MAC address. That's your
retry mechanism. Once you've cleared that retry mechanism, L2 guarantees
that your EUI-64-based IPv6 address is unique.