File Transfer over Raw TCP

The problem

TCP hides most of what makes reliable delivery work. To really understand it, the goal was to rebuild the mechanics from scratch: splitting data into numbered chunks, surviving lost and out-of-order packets, and proving the file arrived intact. Built for BigEndian Challenge 1.

Approach

• Built the server and client on Python's raw socket module with no helper libraries, including a receive loop that reads exact byte counts, since TCP is a stream with no message boundaries.
• Split each file into 1024-byte chunks with sequence numbers, and shuffled them before sending to simulate out-of-order delivery.
• Added packet-drop simulation on the server, either random (0 to 90% per chunk) or manual (choose exact chunks), with a control panel for it.
• The client finds the missing sequence numbers and sends a NACK. The server resends only those chunks, and the loop repeats until nothing is missing.
• The server computes a SHA-256 checksum of the original file and the client compares it with the reassembled file before accepting it.
• Wrote two live web dashboards, one for the server (drop controls, chunk grid, log) and one for the client (upload and progress).

Results

• End-to-end transfer that survives heavy simulated packet loss and still finishes with a verified checksum.
• Application-layer sequencing and reassembly that works even when chunks arrive out of order.
• Measures transfer speed end to end, including every retransmit round.
• Integers are packed in big-endian byte order on the wire, using struct.pack.

At a glance