Why can't connect by udp?

Summary:

i open udp port by firewalld, and test by iperf and udp server, but i can't get response, ACK.

but netcat tool says connection success.

Content (required):

ubuntu@instance-****:~$ sudo firewall-cmd --list-all

public

 target: default

 icmp-block-inversion: no

 interfaces: 

 sources: 

 services: dhcpv6 dhcpv6-client http https imap ldap mysql ssh

 ports: 3000/tcp 80/tcp 22/tcp 433/tcp 3001/tcp 3002/tcp 3006/tcp 3007/tcp 25565/tcp 3306/tcp 5900-5910/tcp 8000-8900/tcp 30037/tcp 24454/tcp 51820/udp 24450-24453/udp 24455-24460/udp 24454/udp 25565/udp

 protocols: 

 forward: yes

 masquerade: no

 forward-ports: 

 source-ports: 

 icmp-blocks: 

 rich rules: 

ubuntu@instance-****:~$ netstat -nul

Active Internet connections (only servers)

Proto Recv-Q Send-Q Local Address      Foreign Address     State