MAC trace & message packet

[Mickael Vera <Mickael.Vera@crm.mot.com>]

Hi,

I use the snapshot
I use MAC trace in a simulation with wireless nodes
and base stations.

What are packets with "message" in the info field :

s 0.002118081 _3_ MAC  --- 0 message 84 [0 ffffffff 0 800] -------
[4196352:255 -1:255 32 0] 
r 0.002454128 _4_ MAC  --- 0 message 32 [0 ffffffff 0 800] -------
[4196352:255 -1:255 32 0] 
r 0.002454175 _5_ MAC  --- 0 message 32 [0 ffffffff 0 800] -------
[4196352:255 -1:255 32 0] 
s 0.233696811 _5_ MAC  --- 1 message 84 [0 ffffffff 2 800] -------
[4196353:255 -1:255 32 0] 
r 0.234032858 _4_ MAC  --- 1 message 32 [0 ffffffff 2 800] -------
[4196353:255 -1:255 32 0] 
r 0.234032905 _3_ MAC  --- 1 message 32 [0 ffffffff 2 800] -------
[4196353:255 -1:255 32 0] 
s 1.268525105 _4_ MAC  --- 2 message 84 [0 ffffffff 1 800] -------
[8390656:255 -1:255 32 0] 
r 1.268861152 _5_ MAC  --- 2 message 32 [0 ffffffff 1 800] -------
[8390656:255 -1:255 32 0] 
r 1.268861152 _3_ MAC  --- 2 message 32 [0 ffffffff 1 800] -------
[8390656:255 -1:255 32 0] 

I don't understand their roles in my simulation.
I have seen in the sources that they are used by 
ivs agent:

/* ivs data packet; ctrl packets are sent back as "messages" */

but what is this agent ?
Why does the trace of simulation begins with this lines ?

-- 
Vera Mickael Stagiaire