Re: [6tsch] Fwd: 6TiSCH: about join priority in IEEE802.15.4e

Xavier Vilajosana Guillen <> Wed, 18 September 2013 14:41 UTC

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Date: Wed, 18 Sep 2013 07:41:03 -0700
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From: Xavier Vilajosana Guillen <>
To: Thomas Watteyne <>
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Subject: Re: [6tsch] Fwd: 6TiSCH: about join priority in IEEE802.15.4e
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Hi Thomas,

when you say the "the "integer" portion of the rank in the join priority,"
you mean the result of the DAGRank(rank) function?


On Tue, Sep 17, 2013 at 6:59 PM, Thomas Watteyne <
> wrote:

> All,
> Please find below an e-mail exchange I had with Jonathan Simon, co-editor
> of the IEEE802.15.4e standard. He is confirming that driving the join
> priority field in the enhanced beacons from an upper layer does not break
> standards-compliance, which is great news.
> His second point is also very interesting: we could use just the "integer"
> portion of the rank in the join priority, so that the value announced is
> roughly equivalent to a hop count.
> Would that work?
> Thomas
> ---------- Forwarded message ----------
> From: Jonathan Simon <>
> Date: Tue, Sep 17, 2013 at 10:14 AM
> Subject: Re: 6TiSCH: about join priority in IEEE802.15.4e
> To: Thomas Watteyne <>
> Thomas -  The join priority was intended to be used in the way you
> describe.  Note that join priority is described somewhat vaguely in the
> text - the spec does not say what to use it for other beyond a MAC
> management layer function of selecting a prospective parent, and no service
> primitives are described to set or change it.   The spec does not use the
> proscriptive "shall" or "must" language, and I certainly intended it as a
> default in absence of other upper layer methods to manage it.
> So as to 1), updating the join priority to reflect a node's actual
> position in the network rather than its "birth" position is a good idea.
>  WirelessHART has this capability.
> For 2),  the important thing is that all devices in the network should be
> using join priority in the same way to avoid confusion. Making the
> objective function scale in integer hops seems intuitive and simple and
> matches the default description, but if each device was n "hops" from its
> parent, it would serve the same purpose.
> Hope this answers your questions. Feel free to forward this to the 6TiSCH
> group.
> Jonathan
>  --
> Jonathan Simon, Ph. D
> Director of Systems Engineering
> Linear Technology, Dust Networks product group
> 30695 Huntwood Ave
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> On Sep 12, 2013, at 12:53 PM, Thomas Watteyne wrote:
> Jonathan,
> We are discussing the interaction between IEEE802.15.4e and the RPL
> routing protocol in the 6TiSCH group (see minutes last call at [1]).
> We have some questions about the "Join Priority" field in the
> IEEE802.15.4e EB, in particular about how much an upper layer (6top [2])
> can influence its value. We want to make sure that we do not break
> compliance with the IEEE802.15.4e standard.
> For reference, this is the text in IEEE802.15.4e (p.90):
> The 1-octet Join Priority field can be used by a joining device to select
> among beaconing devices when multiple beacons are heard. The PAN
> coordinator’s join priority is zero. A lower value of join priority
> indicates that the device is the preferred one to connect to. The beaconing
> device’s join priority is the lowest join priority heard when it joined
> the network plus one.
> We have the following discussions:
>    1. After a node has joined, it might be moving to a different
>    topological location. In that case, it should update the Join Priority it
>    is announcing. Luckily, RPL maintains the node's rank, a concept very
>    similar to join priority.
>    *Question*: Can we have the 6top change the priority announced by
>    IEEE802.15.4e to the RPL rank (or a number calculated from the RPL rank)
>    after the node has joined?
>    2. RPL is very flexible about how rank is calculated. Depending on the
>    Objective Function used, the rank can be calculated differently. In
>    particular, the difference of the rank between a node and routing parent
>    can be different from 1.
>    *Question*: Can the join priority announced by node and its parent be
>    different by more than 1?
> Your input is very welcome,
> Thomas
> [1]
> [2]
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