val b1 = source.readByte() and 0xff

    val isMasked = b1 and B1_FLAG_MASK != 0
    if (isMasked == isClient) {
      // Masked payloads must be read on the server. Unmasked payloads must be read on the client.
      throw ProtocolException(
        if (isClient) {
          "Server-sent frames must not be masked."
        } else {
          "Client-sent frames must be masked."