Reliable Files Transfer

The source code and some specifications will be omitted per UCSC’s policy.

During the Winter 2025, I had a chance to take a network programming class of CSE156. One of my favorite’s class assignments was to create a program transferring files over UDP socket. The goal was to learn the importance and the reason why each TCP field exists, since the assignment forced me to basically reimplement a reliable protocol over unreliable transport link from scratch. The result was a packet structure that is similar to TCP packets, and control flows that assembled TCP.

Challenges

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Make a reliable transferring over unreliable transfer protocol (UDP).
Each UDP packet size must be lower than a specific number.
Can handle dropped packets, duplicate packets, unordered packets.
Can handle multiple transfers at once, so multiplexing.
Must be relatively fast for large files, so keeping an entire 1GB file buffer inside memory is out of the equation.

Stack

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C++ with standard library only. Writing on UDP must be done on socket-level only.

Implementation

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There are two abstraction layers.

           CLIENT                               SERVER
             |                                     ^
      filename,content                      filename,content
             v                                     |
    +-----------------+                   +-----------------+
    |   ReliableFTP   |                   |   ReliableFTP   |
    +-----------------+                   +-----------------+
             |                                     ^
      std::vector<char>                     std::vector<char>
             v                                     |
    +-----------------+                   +-----------------+
    |   ReliableUDP   |                   |   ReliableUDP   |
    +-----------------+                   +-----------------+
             |                                     ^
      std::vector<char>                     std::vector<char>
             v                                     |
+---------------------------------------------------------------+
|                              UDP                              |
+---------------------------------------------------------------+

This one breaks up files to bytes, and vice versa.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SIZE:int(4B)  |          FILENAME:string(SIZE B)              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|                     CONTENT:vector<char>                      |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This layer forms bytes to packets. Metadata (SNI/TYPE) is used for self-made reliable transfer protocol.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     SNI:int(4B)     |     SIZE:int(4B)       |  TYPE:char(1B) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|                     CONTENT:vector<char>                      |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

To handle the incoming packets, there must be a concrete algorithm resembling data with the assumption of abnormal sequence. The methods will be left out as this is the heart of the assignment.

Additional notes

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Not only did I learn more on socket programming and TCP, I also learned how to handle buffers. With many layers of abstraction and fragmentation from limited packet size, constructing partially complete buffer with interactiveness is a must.