Investigate: Evaluate semantics of IPv6 forwarding

[jovatn]

The way Infix handles the IPv6 forwarding primitive differs significantly from what's stated in ietf-ip.yang.
Is this how we want it?

Quote from Infix documentation:

For IPv6 the ability to route between interfaces is always enabled, instead this flag controls ...

Quote from ietf-ip.yang.

     leaf forwarding {
        type boolean;
        default false;
        description
          "Controls IPv6 packet forwarding of datagrams received by,
           but not addressed to, this interface.  IPv6 routers
           forward datagrams.  IPv6 hosts do not (except those
           source-routed via the host).";
        reference
          "RFC 4861: Neighbor Discovery for IP version 6 (IPv6)
                     Section 6.2.1, IsRouter";
      }

[troglobit]

CCB decision: use nftables.