rpl: remove unnecessary check for multiple instances

[cgundogan]

If another instance id is received, the current implementation returns, because it does not support multiple instances. The check however is unnecessary in this case and will be removed anyway when I introduce multiple instances support.
Putting this in smaller PRs will reduce complexity.

[OlegHahm]

Couldn't this debug line useful anyway?

[cgundogan]

I don't see the importance of this debug message. If the rpl implementation did not join any instance yet, it will do it now with the information specified in this incoming DIO

[OlegHahm]

I thought it could be good to keep an overview if this node will join the dodag because it hasn't joined any dodag or because it prefers the dodag its DIO it is currently receiving.

[cgundogan]

this information cannot be retrieved here, because this part of the code handles the instances and not the dodags. The implementation will create any instance it encounters, if it did exist before, it will skip the creation process. This is all to this section of the code, therefore it was kind of too complex before..

[OlegHahm]

I don't get why this check is unnecessary in the first place.

[cgundogan]

The check is too complicated and does not allow more then one instance. With the proposed changes it will check, if the implementation joined the current DIO's instance. If not, it will create this instance and join the dodag (later in the code). Otherwise, it already joined the isntance and continues.

[OlegHahm]

But isn't this dependent on another patch then?

[cgundogan]

not really dependent.. there is in the momentan no code which utilizes multiple instance ids. So it will not hurt to remove the restrictions that block the creation of other than the first instance. And even if there were two different instance ids floating around, the current implemenation does not take this into account, because it uses rpl_get_my_dodag() throughout the code.

[BytesGalore]

I think if the node uses/takes advantage of more than one RPL instance the current check can be adopted to determine if the node is already using it.
@OlegHahm the check you mentioned for determine if the node joined due to a more preferable DODAG could be eventually done here [1].

[1] https://github.com/RIOT-OS/RIOT/blob/master/sys/net/routing/rpl/rpl_control_messages.c#L669

[OlegHahm]

So, what will happen if there's another instance ID and the node calls the join function?

[BytesGalore]

@OlegHahm this won't happen because the node can only be in one RPL instance

[OlegHahm]

Ah, because it is checked later in this function, right?

[BytesGalore]

Ok, strike my comment I just starred on wrong codelines.
When there is a new instance ID, the node will join the DODAG, e.g here [1], which will create a new DODAG here [2]. (if I'm not starred on wrong lines again 👓 )

[1] https://github.com/cgundogan/RIOT/blob/rpl_remove_multiple_instance_check/sys/net/routing/rpl/rpl_control_messages.c#L672
[2] https://github.com/RIOT-OS/RIOT/blob/master/sys/net/routing/rpl/rpl_dodag.c#L90

[OlegHahm]

Ok, I think I finally understood this. ACK and go.