ip6tnl supports modes ip6ip6, ipip6, any.
Mode ipip6 is IPv4 over IPv6, and mode ip6ip6 is IPv6 over IPv6, and mode any supports both IPv4/IPv6 over IPv6.
When the ip6tnl module is loaded, the Linux kernel will create a default device, named ip6tnl0.
Example
ip link add name tun6 type ip6tnl local LOCAL_IPv6_ADDR remote REMOTE_IPv6_ADDR mode any ip link set dev tun6 up
Suggested syntax:
set interfaces tunnel tun30 encapsulation ip6tnl
https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/tree/ip/link_ip6tnl.c