Re: [Roll] using the flow label instead of hop by hop

Phil;

There is indeed lot of pressure for this in terms of header sizes and energy consumption in the *real world*.
And there is no hack in the proposed solution.
Simply we believe more in practical engineering and ML discussions than we trust in crystal balls. 

Cheers,

Pascal

-----Original Message-----
From: Philip Levis [mailto:pal@cs.stanford.edu] 
Sent: vendredi 19 octobre 2012 21:46
To: Owen Kirby
Cc: Pascal Thubert (pthubert); roll@ietf.org
Subject: Re: [Roll] using the flow label instead of hop by hop

I realize there's a lot of pressure for this in terms of header sizes, but this seems like exactly the kind of engineering hack/optimization that has painful long-term implications. Such as the Internet of Things prevents the flow label from ever being used effectively.

Phil

On Oct 19, 2012, at 11:59 AM, Owen Kirby wrote:

> Pascal,
> 
> I'm not entirely convinced that using the flow label like this is a good idea. Using the flow labels to carry the RPL HbH information works on the premise that packets being routed through the RPL domain wouldn't otherwise use the flow label. This assumption might be correct in a network of homogenous nodes with no external prefixes (eg: appendix A.5), but there may still be cases where packets being routed through the RPL domain might want to set the flow label. If this draft were to go forward, I would have some questions:
> 
> When a  forwarding node receives a packet with the flow label set, how does it determine whether the flow label contains an identifier of the 5-tuple, or it contains the RPL HbH information? To get it wrong would interfere with the forwarding behavior and lead to interoperability issues.
> 
> When packets are received from an external prefix, how does the ingress router handle the flow label? Would it destroy the original label, leave the original label in tact, or use IPv6-in-IPv6 encapsulation to preserve the label (ie: the inner header contains the original flow label, and the outer header contains the HbH information)?
> 
> How would the DODAG root rebuild the flow label from the 5-tuple if it encounters packets that have been fragmented at the IPv6 layer? The primary use of the flow label is so that routers don't have to reassemble IPv6 fragments when forwarding to determine the 5-tuple (which is a challenging thing for a router to do).
> 
> Cheers,
> Owen
> 
> On 18/10/2012 9:43 AM, Pascal Thubert (pthubert) wrote:
>> Hi
>>  
>> When I started this draft, we had a series of chats with Brian Carpenter and apparently reached a gentleman agreement that it was doable within the RPL domain to use the flow label and avoid the Hop by Hop option.
>>  
>> I published http://tools.ietf.org/html/draft-thubert-roll-flow-label-01 based on that discussion but did not get news from the group since then.
>>  
>> This technique has a number of advantages, in particular -it saves 
>> extra bytes for the RPL option and the HbH header.
>> -it also avoids the prescribed tunneling within the RPL domain for packets from the outside.
>> - it has an optimized compression with 6LoWPAN.
>>  
>> Is there interest in the group to continue? If so I'd be glad to have some discussion time at the next meeting.
>>  
>> Cheers,
>>  
>>  
>> 
>> Pascal Thubert
>> IPv6 Engineering
>> 
>> pthubert@cisco.com
>> Phone :+33 497 23 26 34
>> Mobile :+33 619 98 29 85
>> 
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