[strongSwan] unable to allocate SPI's From kernel

[anand rao]

Hi,

   I am trying a host to host IKE v2 setup. I am facing following error whenever 
i tried to do "ipsec up host-host"

  initiating IKE_SA host-host[1] to 192.168.0.2
generating IKE_SA_INIT request 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) ]
sending packet: from 192.168.0.1[500] to 192.168.0.2[500]
received packet: from 192.168.0.2[500] to 192.168.0.1[500]
parsed IKE_SA_INIT response 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) ]
authentication of '192.168.0.1' (myself) with pre-shared key
establishing CHILD_SA host-host
unable to allocate SPIs from kernel

strongswan version I am using is 4.3.6
My kernel version is 2.6.21

Attached is the .config file of my kernel configuration. Please help me if I am 
missing anything.

also attached is my my ipsec.conf and strongswan.conf files.

-Anand