# What data for which node?

**URL:** <https://forum.opencyphal.org/t/what-data-for-which-node/625>\
**Category:** Libcanard\
**Created:** [September 25, 2019, 10:58am UTC](https://forum.opencyphal.org/t/what-data-for-which-node/625 "2019-09-25T10:58:06Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![256shadesofgrey](https://forum.opencyphal.org/letter_avatar_proxy/v4/letter/2/b782af/32.png) [@256shadesofgrey](https://forum.opencyphal.org/u/256shadesofgrey)\
**Post date:** [September 25, 2019, 10:58am UTC](https://forum.opencyphal.org/t/what-data-for-which-node/625/1 "2019-09-25T10:58:06Z")

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Hello,

I want to implement the UAVCAN protocol via libcanard on a motor controller. What I couldn’t deduce from looking at the documentation is, how do nodes decide what information to take from the messages.

For example I want to use uavcan.equipment.esc.RPMCommand. As a message rather than service it does not have a destination node ID, so I’m assuming that the nodes just pick out the right entry in the data array? If that is the case, what is the standard way for a node to decide, which entry to pick? Does it have something to do with the node ID, is it arbitrary? Or am I misunderstanding something entirely?

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**Author:** ![pavel.kirienko](https://forum.opencyphal.org/user_avatar/forum.opencyphal.org/pavel.kirienko/32/9_2.png) [@pavel.kirienko](https://forum.opencyphal.org/u/pavel.kirienko)\
**Post date:** [October 2, 2019, 2:34am UTC](https://forum.opencyphal.org/t/what-data-for-which-node/625/2 "2019-10-02T02:34:57Z")

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> [@256shadesofgrey](#):
>
> As a message rather than service it does not have a destination node ID, so I’m assuming that the nodes just pick out the right entry in the data array?

Yes.

> [@256shadesofgrey](#):
>
> If that is the case, what is the standard way for a node to decide, which entry to pick?

The node is supposed to have a configuration parameter dictating which entry to pick. In the case of ESC, it’s usually named `uavcan.esc_index`, but it’s not a hard requirement.

> [@256shadesofgrey](#):
>
> Does it have something to do with the node ID

No. The node-ID is a transport-layer entity.