use more RFC4880, add a footer to read udc-unit backward or forward
use more RFC4880, add a footer to read udc-unit backward or forward

Open-UDC/open-udc,Open-UDC/open-udc,jbar/open-udc,jbar/open-udc,jbar/open-udc,Open-UDC/open-udc,jbar/open-udc,Open-UDC/open-udc,Open-UDC/open-udc,Open-UDC/open-udc,Open-UDC/open-udc