Re: [Roll] Benjamin Kaduk's Discuss on draft-ietf-roll-turnon-rfc8138-14: (with DISCUSS and COMMENT)

"Pascal Thubert (pthubert)" <pthubert@cisco.com> Fri, 11 September 2020 19:16 UTC

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From: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
To: Benjamin Kaduk <kaduk@mit.edu>
CC: Michael Richardson <mcr+ietf@sandelman.ca>, "Routing Over Low power and Lossy networks" <roll@ietf.org>, "roll-chairs@ietf.org" <roll-chairs@ietf.org>, Alvaro Retana <aretana.ietf@yahoo.com>, Ines Robles <mariainesrobles@googlemail.com>, "draft-ietf-roll-turnon-rfc8138@ietf.org" <draft-ietf-roll-turnon-rfc8138@ietf.org>, The IESG <iesg@ietf.org>
Thread-Topic: [Roll] Benjamin Kaduk's Discuss on draft-ietf-roll-turnon-rfc8138-14: (with DISCUSS and COMMENT)
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Subject: Re: [Roll] Benjamin Kaduk's Discuss on draft-ietf-roll-turnon-rfc8138-14: (with DISCUSS and COMMENT)
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Now I’m unsure Michael and I agree anymore. 

What will today’s developer code?

We ask him to test if mop is < 7

What value will the developer place in his code if the test returns false?

If there is no code the Boolean will be uninitialized and the result will depend on the type of variable and the compiler. 

Whatever the developer does the code will end up having a behavior, compression or not.

 Leaving it to the implementation will have some people choose true and others false. This is not what we want.

We want to control what the code does so we can expect it in the future and build our backward compatibility based on that sure knowledge.

Before the draft the default was no compression. Quite naturally since initially it did not exist.

Also we discussed on the ML that for RPLv2 all implementations MUST support the compression. 

In which case it is a better default for a coder today to decide to use the compression for mop 7, isn’t it?

I hope I make the case right. Just think you’re coding it!

Take care,

Pascal

> Le 11 sept. 2020 à 18:26, Benjamin Kaduk <kaduk@mit.edu> a écrit :
> 
> On Fri, Sep 11, 2020 at 12:23:50PM -0400, Michael Richardson wrote:
>> 
>> Benjamin Kaduk <kaduk@mit.edu> wrote:
>>> to MOPs 0..6; but the situation for MOP 7 seems slightly different.  If we
>>> were *just* leaving the bit undefined/free for reuse in that situation,
>>> that is probably also something that we can do in a normal "allocate a bit
>>> from an IANA registry" document without need for Updates.
>> 
>> Up to here, we agree.
>> 
>>> But that's not
>>> all we're doing; we're also saying that if you see MOP==7, then you have to
>>> use the 8138 header/compression/whatever-we-end-up-calling-it.  Yet we are
>>> *not* allocating MOP==7.
>> 
>> Tthat's exactly what we don't want to do.
>> 
>> We are saying NOTHING about rfc8138 when MOP==7.
>> Nor are we saying that the T-bit exists (or doesn't exist).
> 
> That's not how I read:
> 
>       For a MOP value of 7, the compression MUST be used by default
>   regardless of the setting of the "T" flag.
> 
> 
>> What behaviour is default and what behaviour is negotiated, and how it it
>> negotiated, and how the results are turned on, would be up to a document that
>> specifies MOP=7 (or larger mopex)
> 
> What you describe here is more along the lines of what I expected.
> 
> -Ben
> 
>> As an analogy, when we did the ToS->DSCP + bits-that-became-ECN
>> change, we did this for IP_version==4 and IP_version==6.
>> We specifically did not change it for IP_version==7 or 8.
>> 
>> --
>> Michael Richardson <mcr+IETF@sandelman.ca>   . o O ( IPv6 IøT consulting )
>>           Sandelman Software Works Inc, Ottawa and Worldwide
>> 
> 
>