[Roll] On 6LoRH issue #11 (was Proposed improvement in RH3-6LoRH...)

"Pascal Thubert (pthubert)" <pthubert@cisco.com> Sun, 31 January 2016 09:35 UTC

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From: "Pascal Thubert (pthubert)" <pthubert@cisco.com>
To: "S.V.R.Anand" <anand@ece.iisc.ernet.in>, "6lo@ietf.org" <6lo@ietf.org>
Thread-Topic: On 6LoRH issue #11 (was Proposed improvement in RH3-6LoRH...)
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Date: Sun, 31 Jan 2016 09:35:26 +0000
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Cc: "6tisch@ietf.org" <6tisch@ietf.org>, Routing Over Low power and Lossy networks <roll@ietf.org>
Subject: [Roll] On 6LoRH issue #11 (was Proposed improvement in RH3-6LoRH...)
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Hello Anand:

Thanks for this, please see below:

Having a symmetric path, in principle, certainly offers advantages when we want
the communicating nodes to use a well-defined path that has been set up to meet
the QoS requirements of an application. Such a pinned path can be appropriately
provisioned with required bandwidth resources along the entire path.


Ø  True but in radios, the links may be very asymmetrical, and the optimal return path may be very different.

Ø  So the pinned-down return path may be different actually.


I found the P2P RPL very appropriate for the current discussion. What is
interesting is that the Bidirectional Route presented here actually enables
creating a symmetric path between end nodes by passing on the complete path
information in its signaling messages. In this process, routing state is
installed along the path.


Ø  I hope that we start new on-demand work for RPL, based on this and AODV. It may be that the route is optimized for metrics that are mostly directional, one way or the other, if the traffic is so. Ideally we'd build and associated 2 DODAGs, one for each direction. This is discussed in https://tools.ietf.org/html/draft-thubert-roll-asymlink-00


As you noted, keeping signaling separate from data is certainly an elegant way.

There can however be contexts when in-band signaling that uses RH3 as per
RFC6554 data can be more efficient than using a signaling protocol, assuming
the security concerns are addressed as per RFC2460. This in-band approach is
attractive when we want to set up a rapid on-the-fly symmetric path along with
6TiSCH OTF making bandwidth reservation along the way for transactional message
exchanges. I am hinting at the possible PCE/NME based solution that (i) works
with the RPL DODAG, (ii) considers the hop distance between the communicating
node to assess energy costs, (iii) optimizes network resource availability, and
finally provides right inputs for the nodes. It may well be that dropping the
address is the right choice.


Ø  I can agree with that, and I used that sort of method in https://tools.ietf.org/html/draft-thubert-tree-discovery


I think I am doing too much of hand waving going on by leaving out all the
essential details. Hope I managed to vaguely convey the point.

I tend to feel that distributed, centralized and their combination might
co-exist to optimize network resources, and therefore retaining or dropping the
address in RH3-6LoRH can be made optional.



Ø  Could be, but should we do it now or when we have the actual need?

Ø  Do you have a design in mind that limits the implementation overhead of supporting both?


Am I making sense ?



Ø  To me, certainly :)


Anand




On Saturday 23 January 2016 02:34 PM, Pascal Thubert (pthubert) wrote:
>
> Hello Anand:
>
>
>
> 2 good points , that we need to continue in different threads if we do.
>
>
> The source routing optimization done by dropping the addresses on the way
> certainly has benefits. However, there could be loss of a natural "symmetric"
> property that one might want to enforce between communicating end nodes in the
> routing path. By symmetry I mean using the same path in both the directions of
> communication. Policy based routing, centralized routing, for instance, could
> be potential users of this property. May be this does not represent a common
> use case. But nevertheless, we have to be aware of this side effect which RFC6554
> swapping process does not have.
>
>
>
> Ø  Pascal: Yes, Simon made that point yesterday.
>
> Ø  It is generally not a good idea to reverse a routing header. The RH may have been used to stay away from the shortest path for some reason that is only valid on the way in (segment routing).
>
> Ø  P2P RPL reverses a path that is learnt reactively, so we have a real protocol for doing that sort of thing as opposed to an echo.
>
> Ø  Reversing a header is discouraged by RFC 2460 (for RH0) unless it is authenticated (which means AH). We do not authenticate the RH3, there are a number of reasons for that, general deprecation of AH, no use of AH in LLNs etc& Note that AH does not protect the RH on the way, it is just a validation at the receiver for the sole value of reversing it.
>
> Ø  A RPL domain is usually protected by L2 security and that secures both RPL itself and the RH in packets, at every hop
>
> Ø  The benefit of saving energy and lowering the chances of loss are seen as overwhelming compared to the value of possibly reversing the header
>
> Ø  Yet we might define a variation where we do not pop out the first entry as we go. I do not see a consensus on the value of doing it now.
>
>
>
>
>
> Going further, the DAO projection proposal
> (draft-thubert-roll-dao-projection-02.txt) will have several virtual roots
> inside the RPL domain. The automatic assumption of a well known root may not apply
> when nodes within RPL domain communicate with each other. I suppose it will
> have a bearing on the RH3-6LoRH performance.
>
>
>
> Ø  It is not really an assumption, but something we leverage as we go. The RPI is very much like a context indicator. If an address shares a lon prefix with a root, adding the RPI of that root is actually a compression technique.
>
>
>
> The above observations are not serious, but feels good to ponder over. Will be
> happy to receive your comments.
>
> Thanks Anand!
>
>
>
> Pascal
>
>
>
>
>
> On Monday 18 January 2016 11:24 PM, Pascal Thubert (pthubert) wrote:
> >
>
> > Dear all
>
> >
>
> >
>
> >
>
> > The picture below illustrates how the RH3 6LoRH works with draft 03 in a case like Root -> A -> B -> C -> leaf
>
> >
>
> >
>
> >
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> >
>
> >
>
> > The first 6LoRH is expected to be a full address (128 bits) to set up a reference and the next 6LoRH are expected to be smaller and just override the rightmost bits which form the delta from the reference.
>
> >
>
> >
>
> >
>
> > Proposal: we could consider that the 128bits source of the IP header before the RH3 is the reference to start with.
>
> >
>
> >
>
> >
>
> > With that even the first hop could be compressed the same way as the other hops. With RPL, the root is the encapsulator if IP in IP in used. Good thing, in that case the root is totally elided with the IP-in-IP 6LoRH.
>
> >
>
> >
>
> >
>
> > So this simple proposal saves up to 16 octets (thats in the case with a single subnet and all addresses differ only by the last 2 bytes). Im willing to add it in the next revision.
>
> >
>
> >
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> >
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> > Any opposition?
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> >
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> >
>
> >
>
> > Pascal
>
> >
>
> >
>
> >
>
> > --
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