It reads the file piece by piece (BUFFER_SIZE).

For each piece, it prepends a sequence number (0, 1, 2...).

It puts this data into a UDP packet and broadcasts it to everyone on the network.

After the last piece, it sends a final packet with a special sequence number (-1) to signal the end.

It immediately creates and opens the output file.

It listens for packets.

For each packet it receives, it reads the sequence number. If it's not -1, it writes the data chunk directly to the file.

When it sees the -1 packet, it closes the file and stops.

First, it reads the entire file into memory, splitting it into a list of chunks (slices). This makes it easy to re-send any specific chunk later.

It creates a random session ID for this transfer.

It broadcasts all the chunks one by one (the first pass). Each packet has a detailed header.

After the first pass, it sends a special End-Of-Broadcast (FLAG_EOB) packet.

It then waits and listens for NACK messages from receivers on a separate nackSocket.

If it receives NACKs (e.g., "I'm missing packets 42, 123, and 501 for session 98765"), it re-broadcasts only those specific chunks.

This "listen and repair" cycle repeats for a few rounds to ensure everyone gets the complete file.

It listens for packets and stores them in an in-memory map, using the sequence number as the key. This automatically handles out-of-order packets.

When it receives the FLAG_EOB packet, it checks its map to see if it has all the chunks from 0 to totalPackets.

If chunks are missing, it constructs and sends a NACK message to the sender, listing the sequence numbers of the missing chunks. It then waits for the re-transmissions.

If all chunks are present, it loops from 0 to totalPackets, retrieves the chunks from its map in the correct order, writes them to the output file, and closes it.