networking - OpenFlow actions=CONTROLLER:51 meaning -

i have network topology using mininet.the topology connected ryu simple_switch.py program.i have 6 switches , 6 hosts on network.

the contents of flow table on 6 swtiches given below.

table s1 nxst_flow reply (xid=0x4):  cookie=0x0, duration=10.97s, table=0, n_packets=11, n_bytes=561, priority=65535,dl_dst=01:80:c2:00:00:0e,dl_type=0x88cc actions=controller:51 table s2 nxst_flow reply (xid=0x4):  cookie=0x0, duration=10.947s, table=0, n_packets=11, n_bytes=561, priority=65535,dl_dst=01:80:c2:00:00:0e,dl_type=0x88cc actions=controller:51 table s3 nxst_flow reply (xid=0x4):  cookie=0x0, duration=10.921s, table=0, n_packets=11, n_bytes=561, priority=65535,dl_dst=01:80:c2:00:00:0e,dl_type=0x88cc actions=controller:51 table s4 nxst_flow reply (xid=0x4):  cookie=0x0, duration=10.893s, table=0, n_packets=11, n_bytes=561, priority=65535,dl_dst=01:80:c2:00:00:0e,dl_type=0x88cc actions=controller:51 table s5 nxst_flow reply (xid=0x4):  cookie=0x0, duration=10.86s, table=0, n_packets=11, n_bytes=561, priority=65535,dl_dst=01:80:c2:00:00:0e,dl_type=0x88cc actions=controller:51 table s6 nxst_flow reply (xid=0x4):  cookie=0x0, duration=10.834s, table=0, n_packets=11, n_bytes=561, priority=65535,dl_dst=01:80:c2:00:00:0e,dl_type=0x88cc actions=controller:51 

what action=controller:51 mean? mean send packets packet_in controller?

it means switch should send first 51 bytes of packet controller. @ max_len field in ofp_output_action details.