US Patent 9398053 Efficient data transmission between computing devices

The subject disclosure is directed towards technology by which data transmission sizes are reduced when uploading files over a network. By processing hash values corresponding to a plurality of data blocks of a file to potentially be uploaded to a server, the server identifies any already known data block or blocks of the file. The server performs a server-local copy operation that writes the known data block into a server-local copy of the file. If applicable, the server returns hash values corresponding to unknown data blocks to a client, by which the client responds by uploading copies of the unknown data blocks. Accordingly, the client and the server maintain the server-local copy of the file by transferring only unknown data blocks.