ppp0 Link encap:Point-to-Point Protocol
inet addr:172.16.36.1 P-t-P:172.16.36.2 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1396 Metric:1
RX packets:27 errors:0 dropped:0 overruns:0 frame:0
TX packets:34 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:2260 (2.2 KiB) TX bytes:2337 (2.2 KiB)


ppp1 Link encap:Point-to-Point Protocol
inet addr:172.16.36.1 P-t-P:172.16.36.3 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1396 Metric:1
RX packets:61 errors:0 dropped:0 overruns:0 frame:0
TX packets:57 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:5382 (5.2 KiB) TX bytes:4588 (4.4 KiB)

I can ping 172.16.36.1 from 172.16.36.2, but i can not ping 172.16.36.2 from 172.16.36.1,and i can not ping 172.16.36.3 from 172.16.36.2

This is the system does not support, or I am not configured correctly

What are the firewall rules on the VPN server (172.16.36.1) which filter ICMP traffic ?
What are the rules in the FORWARD chain ? Seems that it filters at least ICMP traffic between VPN clients, which goes through the server.