Issues about Chunkupload

I'm writing a very simple console uploading program using most of the sample code from Dropbox.net Api. But I have two issues now.

When the network drops and reconnects, I get an error saying "lookup_failed/incorrect_offset/". Where I go wrong?

Another related question is,  say I have a 5120kb file to upload and the chunk is 1024kb. So when the third chunk is uploading,  suppose the network suddently drops after uploading 500kb, then where should I set the offset to continue uploading, from offset=3072 or from offset=3072+500= 3572?

Here is my ChunkUpload code:

       private async Task ChunkUpload(DropboxClient client, string srcfile, string dstfile)
        {
            if (File.Exists(srcfile))
            {
                Console.WriteLine("\nUploading file {0} to Dropbox path: {1}\n", srcfile, dstfile);
                // Chunk size is 1024KB.
                const int chunkSize = 1024 * 1024;

                FileStream fs = new FileStream(srcfile, FileMode.Open);
                byte[] data = new byte[fs.Length];
                fs.Read(data, 0, data.Length);
                fs.Close();

                using (var stream = new MemoryStream(data))
                {
                    int numChunks = (int)Math.Ceiling((double)stream.Length / chunkSize);

                    byte[] buffer = new byte[chunkSize];
                    string sessionId = null;

                    var idx = 0;
                   do
                    {
                        var byteRead = stream.Read(buffer, 0, chunkSize);
                        var percentage = 0;
                        int row = Console.CursorTop - 1;  //Set cursor position
                        Console.SetCursorPosition(0, row);
                        if ( (idx+1) == numChunks)  //Last chunk, 99% done!
                        {
                            percentage =  99;
                            Console.WriteLine("Uploaded {0}: {1}% of total {2}kb", idx, percentage, (int)Math.Ceiling((double)stream.Length));
                        }
                        else
                        {
                            percentage = 100 * (idx + 1) * chunkSize / (int)Math.Ceiling((double)stream.Length);
                            Console.WriteLine("Uploaded {0}: {1}% of total {2}kb", idx, percentage, (int)Math.Ceiling((double)stream.Length));
                        }
                        try
                        {
                            using (MemoryStream memStream = new MemoryStream(buffer, 0, byteRead))
                            {
                                if (idx == 0)
                                {
                                    var result = await client.Files.UploadSessionStartAsync(body: memStream);
                                    sessionId = result.SessionId;
                                }

                                else
                                {
                                    UploadSessionCursor cursor = new UploadSessionCursor(sessionId, (ulong)(chunkSize * idx));

                                    if (idx == numChunks - 1)
                                    {
                                        await client.Files.UploadSessionFinishAsync(cursor, new CommitInfo(dstfile), memStream);
                                        row = Console.CursorTop - 1;
                                        Console.SetCursorPosition(0, row);
                                        Console.WriteLine("Uploaded {0}: {1}% of total {2}kb", idx, 100, (int)Math.Ceiling((double)stream.Length));
                                    }
                                    else
                                    {
                                        await client.Files.UploadSessionAppendV2Async(cursor, body: memStream);
                                    }
                                }
                            }
                            idx++;
                        }
                        catch (Exception e)
                        {
                            Console.WriteLine("Error uploading:{0}", e.Message);
                            Console.WriteLine("SessionID: {0}", sessionId);
                            Console.WriteLine("\nMake sure everything is OK then press enter. \n");
                            Console.ReadLine();
                        }
                    } while (idx < numChunks);
                }
                Console.WriteLine("Upload OK! Press enter to exit...");
                Console.ReadLine();
            }
            else
            {
                Console.WriteLine("Wrong path:{0}", srcfile);
            }
        }