Netfilter nf_tables tunnel module

modulename: nft_tunnel.ko

configname: CONFIG_NFT_TUNNEL

Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>Network packet filtering framework (Netfilter)
└─>Core Netfilter Configuration
└─>Netfilter nf_tables tunnel module
In linux kernel since version 4.2 (release Date: 2015-08-30)  
This option adds the "tunnel" expression that you can use to set
tunneling policies.

source code:
net/netfilter/nft_tunnel.c