Algorithm 1 The group table selection algorithm.

1:procedure TABLESELECT 2:last_arrival_time ← None; 3:last_output_channel ← None; 4:timeout ←δ; 5:while receive one packet do 6: key ← hash(packet); 7: current_time ← getTimeNow(); 8: if key not in keys of last_arrival_time then 9:  add ($key,current\_time$) to $last\_arrival\_time$; 10:  $output\_channel$ ← $\mathit{random}(group\_alive\_buckets(\left)\right)$; 11:  add ($key,output\_channel$) to $last\_output\_channel$; 12: else 13:  $last\_time$ ← $get(key,last\_arrival\_time)$; 14:  if $current\_time-last\_time\ge timeout$ then 15:    update $(key,current\_time)$ to $last\_arrival\_time$; 16:    $output\_channel\leftarrow \mathbf{random}(group\_alive\_buckets(\left)\right)$; 17:    update $(key,output\_channel)$ to $last\_output\_channel$; 18:  else 19:    update$(key,current\_time)$ to $last\_arrival\_time$; 20:    $last\_channel\leftarrow get(key,last\_output\_channel)$; 21:    if $last\_channel$ corresponding bucket is not alive then 22:      $output\_channel\leftarrow \mathbf{random}(group\_alive\_buckets(\left)\right)$; 23:      update $(key,output\_channel)$ to $last\_output\_channel$; 24:      send one port down message to controller; 25:    else 26:      $output\_channel\leftarrow last\_channel$; 27:    end if 28:  end if 29: end if 30: send packet to output_channel; 31:end while 32:end procedure