However, our draft does no intend to make judge of 6rd, or other tunnel
techonogies. We actually would like to list serveral suitable tunnel
techonogies with brief introductions in order to complete our puzzle. As we
ready state in the draft, "ISATAP or VET are also be considered." Actually,
one thing we can do for next version is to expand this section a little bit,
to include brief introductions for Auto GRE, ISATAP and VET. However, we
also think 6rd is suitable and do not want to rule it out.