OpenVPN UDP

Hi guys,
I'm trying to set-up a configuration like this one:

openvpn client -> nginx 1.11.3 -> openvpn server (udp port)

I configured nginx to accept connections on a udp port and reverse to upstream servers.
With this configuration,openvpn client does not complete the TLS ack...
I get a tcp dump session on nginx server and openvpn server but the result is the same : it stops with a client hello message (SSL) but openvpn does not continue with a server hello.

I never used Nginx before,so i just tried to reconfigure Openvpn and Nginx to TCP and all is fine,and vpn tunnel is established.
I think that in some way Nginx donesn't like UDP/SSL-TLS connections.

Is it right by design or do i miss something ?
Thanks in advance.
Cheers