Data link / Application layers

Could anyone make a simple example of an implementation in C of 2 separate layers, application and data link? In the program i need to make, they communicate with pre-defined functions, and the data link layer uses stop-n-wait or sliding window, which ill implement. The data link will use UDP sockets, but they will be included in pre-defined functions, so all i need to know is how to make the structure of the 2 layers program for this situation. Thanks in advance.

Generally, the application layer is a packet within a bigger packet that is the data link. The purpose of the data link is to ensure the packet arrives correctly to the right application and complete with the correct content.

I don't have any direct example, but the data structure may be something like this:

Code:

---

struct header
{
    int magic;      // A magic number that indicates the start of a packet.
    int pktsize;    // Size of the payload (application) data.
    int hdrsize;  // Size of this header
    int pktcsum;  // checksum of the payload data.
    int recipient;  // Who it's for.
    int sender;    // Who sent it
    int hdrcsum;  // checksum of the header (it's last, so we can calculate it from the other components)
};

---

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Mats