thanks your answer.
Maybe tcp keepalive???
comment please. DP recommad tcp keepalive vaule/
ex) # echo 600 > /proc/sys/net/ipv4/tcp_keepalive_time
# echo 60 > /proc/sys/net/ipv4/tcp_keepalive_intvl
# echo 20 > /proc/sys/net/ipv4/tcp_keepalive_probes