This function determines the amount of space, in bytes, required
    for a routing header. Although the function returns the amount of
    space required, it does not allocate buffer space. This enables
    the application to allocate a larger buffer.
    If the application uses ancillary data, it must pass the returned
    length to CMSG_LEN() to determine the amount of memory required
    for the ancillary data object, including the cmsghdr structure.
                                   NOTE
       If an application wants to send other ancillary data
       objects, it must specify them to sendmsg() as a single msg_
       control buffer.