The datatable will post one or more sSortDir_x parameters to your controller, depending on how many columns are sorted on simultaneously in the table.
The specific columns that the table is sorted by are sent in the iSortCol_ parameters (again, one or more).
public class TableParameter
{
public string sEcho { get; set; }
public int iDisplayStart { get; set; }
public int iDisplayLength { get; set; }
public int iSortingCols { get; set; }
public int iSortCol_0 { get; set; } // the first (and usually only) column to be sorted by
public string sSortDir_0 { get; set; } // the direction of the first column sort (asc/desc)
public int iSortCol_1 { get; set; } // the second column to be sorted by
public string sSortDir_1 { get; set; } // the direction of the second column sort
// etc
}