extbuf
    Points to a buffer that contains an extension header. This is
    either a valid pointer or a NULL pointer.
 extlen
    Specifies the length of the extension header to finish
    initializing. A valid value is any number greater than or equal
    to 0.
 offset
    Specifies the length of the existing extension header. Obtain
    this value from a prior call to inet6_opt_init() or inet6_opt_
    append().