what is the whack message in ipsec vpn?

Hi All, 

It is question for whack message in IPSEC VPN. 

I've follow message in a log for ipsec vpn. 

==== ipsec vpn log ====

2016:10:24-15:24:00 ahn105_master pluto[31978]: | *received whack message
2016:10:24-15:24:00 ahn105_master pluto[31978]: | get esp.68d824a4@192.168.100.101
2016:10:24-15:24:00 ahn105_master pluto[31978]: | current: 0 bytes
2016:10:24-15:24:00 ahn105_master pluto[31978]: | get outbound policy with reqid 16401
2016:10:24-15:24:00 ahn105_master pluto[31978]: | use_time: Jan 01 09:00:00 1970
2016:10:24-15:24:00 ahn105_master pluto[31978]: | get esp.76d27a8e@192.168.100.100
2016:10:24-15:24:00 ahn105_master pluto[31978]: | current: 0 bytes
2016:10:24-15:24:00 ahn105_master pluto[31978]: | get inbound policy with reqid 16401
2016:10:24-15:24:00 ahn105_master pluto[31978]: | use_time: Jan 01 09:00:00 1970
2016:10:24-15:24:00 ahn105_master pluto[31978]: | next event EVENT_REINIT_SECRET in 841 seconds

Questions. 

1. what's mean this message? is it generated in regularly for information proposed only?

2. The log shows that current is 0 bytes (line 3 and 7). is it normal ?

3. And what's mean the 0 bytes of current in line 3 and 7?

4. what is the mean of outbound policy and inbound policy?

Your quick advice would be much appreciated. 

Thanks