Re: [Roll] WGLC for draft-thubert-6man-flow-label-for-rpl-03

Philip Levis <pal@cs.stanford.edu> Mon, 04 August 2014 18:24 UTC

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From: Philip Levis <pal@cs.stanford.edu>
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To: Michael Richardson <mcr+ietf@sandelman.ca>
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Cc: Routing Over Low power and Lossy networks <roll@ietf.org>, "ipv6@ietf.org" <ipv6@ietf.org>
Subject: Re: [Roll] WGLC for draft-thubert-6man-flow-label-for-rpl-03
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On Aug 4, 2014, at 7:02 AM, Michael Richardson <mcr+ietf@sandelman.ca> wrote:

> 
> Philip Levis <pal@cs.stanford.edu> wrote:
>> 5% or 10% isn't huge, but it is significant and valuable. I've just
>> been a bit confused as to why this proposal has been trotted around for
>> 4 years and yet no-one had yet even done this simple calculation, so WG
>> members could make an informed engineering decision. IMO, a 5-10%
> 
> my opinion:
> 
> 1) the exact calculation was hard to really do until 6tisch made it clearer
>   what the duty cycle was going to be.
> 
> 2) it took awhile to come back to figuring out how to deal with the flow
>   label issue.
> 
> 3) it is now clear exactly how close many of our packets are to the 127 byte
>   limit, and how much the HbH header hurts if it is the reason we go into
>   two fraglets.
> 
> 4) we had other things to worry about!

Michael,

1) doesn't depend on 6tisch, it's based on 15.4e timing, and could have been calculated for any other low power link layer. Years ago.

2) The proposal isn't substantively different -- in terms of bytes sent/saved -- from the first version of the proposal suggested in 2010.

3) The analysis so far hasn't even touched this issue.

4) The entire introduction/motivation for the proposal is saving energy! But no-one thought it important enough to actually estimate how much energy it would save (in a best case, typical case) before a last call?

Here's my worry -- the tradeoffs of low-power networks are very different than the kinds of networks the IETF typically deals with. It'll lead to protocol decisions a bit different than always-on networks. But that doesn't mean it should be used as a blanket justification or motivation. Otherwise it's just smoke and mirrors (energy is critical! battery is critical! we therefore need less CPU-intensive compression schemes! here's a new one. we need less CPU-intensive cryptographic schemes! here's a new one.). If energy is so important, and this would be such a big savings, then why the recalcitrance to estimate it?  It doesn't help my confidence in the engineering when some of the back-of-the-envelope analyses (Xavier's) have basic arithmetic errors that seem incognizant of how to measure energy. 

Phil