File Transfer Protocol

Echo uses a sophisticated protocol called FileChunk for reliable UDP-based file transfers. The protocol uses Protocol Buffers (proto3) for efficient serialization.

Protocol Structure

LSB                                                                MSB
Byte:   1       2       3       4       5       6       7       8
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |                        version (uint32)                       |
     +-------+-------+-------+-------+-------+-------+-------+-------+
Byte:   9      10      11      12      13      14      15      16
     |                      chunkIndex (uint32)                      |
     +-------+-------+-------+-------+-------+-------+-------+-------+
Byte:  17      18      19      20      21
     |           totalChunks (uint32)        | isLastChunk (1 byte)  |
     +-------+-------+-------+-------+-------+-------+-------+-------+
Byte:  22+ (variable)
     | filename length (varint) |
     +--------------------------+
     | filename (N bytes)       |
     +--------------------------+
     | data length (varint)     |
     +--------------------------+
     | data (M bytes)           |
     +--------------------------+
     | checksum length (varint) |   ← only if `isLastChunk == true`
     +--------------------------+
     | checksum (SHA256 string) |
     +--------------------------+

Protocol Operation

Sender Process

1. Reads the file and splits it into 1024-byte chunks
2. Wraps each chunk in a FileChunk message
3. Sends chunks over UDP and waits for acknowledgment
4. Includes SHA-256 checksum with the final chunk

Receiver Process

1. Receives each FileChunk message
2. Writes chunk data to disk
3. Sends acknowledgment back to sender
4. Verifies complete file using checksum after receiving final chunk

Protocol Features

• Chunked UDP transfer for efficient data transmission
• Reliable delivery through acknowledgment system
• File integrity verification via SHA-256 checksums
• Flexible filename and size handling
• Efficient serialization using Protocol Buffers