writing-task-part2.txt

2. ARP is implemented to find the destination MAC address
    when sending to the IP addresses that are in the same
    subnet with the source.
   If destination is not in the same subnet, check the
    routing table and route the packet. (destination MAC
    address is the corresponding next hop MAC address in
    the case.)

3. The routing algorithem "NRP" is based on the distance
    vector algorithem. The NRP packet is defined as below,
    each line stands for 8 bytes
     ________________________________________
    |num |flag|        MAC address           |
    |   IP prefix      |  slash   |   dist   |
    |   IP prefix      |  slash   |   dist   |
    |........................................|
    |___IP prefix______|__slash___|___dist___|

    The first line is the header and each line below is a
    record.

    -num: number of records, max is NRP_MAX_REC(185),
            considering the max length of ethernet frame.
            (it's a naive design for now.)
    -flag: NRP_NEW_PKT(1) or NRP_OLD_PKT(0), see below.

   When a host is newly added to the network, it adds its
    local devices to its routing table. Then it encapsulate
    its whole routing table into a NRP packet, and set the
    flag as NRP_NEW_PKT.
   When a host receives a NRP packet, it first uses records
    in the packet to update its own routing table. Then it
    do the following two things:
    (1) Check the flag of the packet. If it is NRP_NEW_PKT,
        it encapsulate its whole routing table into a NRP
        packet (flag = NRP_OLD_PKT), and send the packet
        back to the source.
    (2) It encapsulate all the updated routing items into a
        NRP packet (flag = NRP_OLD_PKT), and send it to all
        the neighbors that are not in the same subnet with
        it (by using all the devices except the device that
        received the packet to send).

4. An special ethernet frame type is defined for NRP packet
    (#define ETHERTYPE_NRP 0x1106). When hosts not running
    this protocol stack receives the frame, they check the
    ether_type field in the frame and they can't find the
    corresponding type. So they think the frame is damaged
    and they just discard it.