Unexpected behavior of Z1 and Sky motes in Cooja simulation in DODAG formation

Question

I was exploring the Contiki Cooja simulator (with Contiki NG) and experimenting with a UDP server and several UDP clients (from examples/rpl-udp) with different mote types when I noticed something I didn't expect.

When I set the motes as type z1/sky, almost all client motes send DAOs only to the server. In fact the DODAG ends with a star topology (with several clients remaining isolated due to small neighbour table size in sky mote).

However, in the case of cooja motes, this does not happen; clients send DAOs to other clients as well.

Pattern of DAOs in a scenario with z1 motes

Is this intended and default? If it is the default, then why? How can this behaviour be altered?

Answer 1

The maximum routing table and neighbor table sizes are limited by compilation constants in Contiki-NG. On Sky and Z1 they are particularly small as these platforms do not have a lot RAM, and can cause some issues with RPL.

For instance, on Z1 the following defines are set. The neighbor table size of 8 in particular is much too small for many networks.

#define NBR_TABLE_CONF_MAX_NEIGHBORS    8
#define NETSTACK_MAX_ROUTE_ENTRIES      8

For platforms that do not set them explicitly, these constants are 16 and 16 respectively.

On Cooja they are much larger:

#define NBR_TABLE_CONF_MAX_NEIGHBORS    300
#define NETSTACK_MAX_ROUTE_ENTRIES      300