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

Owen Kirby <> Fri, 19 October 2012 18:59 UTC

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Date: Fri, 19 Oct 2012 11:59:51 -0700
From: Owen Kirby <>
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To: "Pascal Thubert (pthubert)" <>
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Subject: Re: [Roll] using the flow label instead of hop by hop
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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).


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 
> <> 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
> <>
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> Mobile :+33 619 98 29 85
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