Re: [6lowpan] Fwd: Re: "Advertize on Behalf" flag in ARO

Erik Nordmark <> Thu, 14 April 2011 23:29 UTC

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Date: Thu, 14 Apr 2011 16:30:14 -0700
From: Erik Nordmark <>
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Subject: Re: [6lowpan] Fwd: Re: "Advertize on Behalf" flag in ARO
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On 4/13/11 12:53 AM, Pascal Thubert (pthubert) wrote:
> Hi Erik:
> The RPL (DAO) sequence number allows the node to increment rapidly in
> case of rapid changes and then lazily when the situation is stable and
> DAO are scarce. The increase is strictly monotonous which is critical to
> us.
> A time stamp imposes a synchronization between the routers. We do not
> have such mechanism in RPL. A time unit (a granularity) must be agreed
> upon. Within that unit, movements go undetected so the time unit must be
> thin grained to cover rapid changes. Yet, depending on the medium, the
> time unit, and the size of the network, it is not necessarily
> easy/possible to guarantee a strictly monotonous value with a thin
> grained time unit. And we have limited space (2 octets) and have to deal
> with wrap around, which divides the space by at least 3.
> So RPL went for a sequence number.

But the unstated assumption that RPL made is that all host-to-router
protocols have to now be RPL aware. That doesn't sound like good design.
A host isn't aware of whether the routers speak IS-IS or OSPF, so why do
the hosts need to be aware of RPL by passing some TID around?

> I think ND has the same need as MIP for a TID == Sequence # . We know of
> MIP; We know of RPL. We know of the backbone router operation. We know
> we'll need the TID and we know exactly why. I think we should have it in
> the 6LowPAN ND spec right away to avoid interop issues when we add RPL
> and BR operations.

I don't see a need in 6lowpan-nd for a TID; the protocol works fine
without it.
I think RPL needs to be improved to deal with reality. Isn't there a
desire for RPL to handle 4861 hosts? Those would never know about a TID.