You are likely to get licensing problems doing this with kernel code
The BSD and Linux kernel licenses wouldn't allow you to redistribute the
resulting BSD kernel item. You may also want to look hard at the existing BSD
IPv6 projects as a base for BSD IPv6 work.
> First,
> It will be done at the user level.
>
> Linux --> FreeBSD or FreeBSD --> Linux
> Is its portability easy?
The user level is similar so far. There is a sort of draft API in an
rfc draft and a set of conventions where everyone considers the draft
to be wrong at the moment
> then it will carried out at the kernel level.
>
> Linux --> FreeBSD or FreeBSD --> Linux
> is its portability easy?
No. Very very different. The whole buffering and conceptual flow is
very very different.
> I want to make an IPv6 island with Linux on my system.
> Does Linux IPv6 support for the interoperability with other
> IPv6 hosts?
Yes