Re: Reducing the battery impact of ND

Erik Nordmark <> Wed, 22 January 2014 21:44 UTC

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From: Erik Nordmark <>
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To: Lorenzo Colitti <>, Erik Nordmark <>
Subject: Re: Reducing the battery impact of ND
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On 1/21/14 12:34 PM, Lorenzo Colitti wrote:

> To my mind, sending an RS when the RA lifetime is about to expire is a
> robustness fix, not an efficiency fix. In a world where IPv6 is optional
> and IPv4 is the main protocol, just having your IPv6 address expire is
> not necessarily a problem. But in a world where IPv6 is the main
> protocol, then hosts will probably want to try harder to keep their IPv6
> address around.


Whether you call it "efficiency" or "robustness" doesn't make much 
difference from a technical or a process perspective.

The approach you advocate is insufficient to ensure better robustness 
for at least two reasons:
  - the lifetime(s) of the prefixes are independent of the lifetimes of 
the default routers. Hence in this approach a host can have default 
routers but no prefixes.
  - the reachability state in the NCEs is independent of the default 
router list - per RFC 4861 the router stays in the default router list 
even if it is unreachable. Thus your last router might be unreachable 
but with remaining lifetime - it was the next-to-last router in the list 
that was reachable.

Solving those without risking hosts sending lots of RSs during failure 
events or resulting in a steady state of RSs from hosts seems tricky.

>     FWIW I don't have a problem with an opt-in update to 4861. I even
>     think efficient-nd is the way to go because it reduces overall ND
>     multicast traffic and not just your narrow focus on mobile phone
>     battery life.
> The tradeoff is that efficient ND adds more state to the network about
> host addresses and changes a pretty core part of the IPv6 specs. I think
> that if we can reduce the battery impact of multicast traffic, we will
> have fixed most of the problem without doing either of those, and to my
> mind, that's a much better tradeoff.

Changing from periodic RAs to (effectively) periodic RSs as you propose 
would change RFC 4861 is a very fundamental way.

It may or may not be the right thing to do for the future, but I think 
that requires a bit more work then forcing some edits into a document 
which has almost completed the WG process. I think resilient-rs has 
significant benefits in its current form and scope.