code: public void writeAckPrefix(long stamp) throws SIResourceException
      {
          if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled())
              SibTr.entry(tc, "writeAckPrefix", Long.valueOf(stamp));
  
          synchronized (this)
          {
              // SIB0105
              // Update controllable health state if poss
              if (stamp >= lastAckExpTick)
              {
                  getControlAdapter().getHealthState().updateHealth(HealthStateListener.ACK_EXPECTED_STATE,
                                                                    HealthState.GREEN);
                  // Disable the checking until another ackExpected is sent
                  lastAckExpTick = Long.MAX_VALUE;
              }
              if (stamp >= lastNackReceivedTick)
              {
                  getControlAdapter()
                                  .getHealthState().updateHealth(HealthStateListener.NACK_RECEIVED_STATE,
                                                                 HealthState.GREEN);
                  // Disable the checking until another ackExpected is sent
                  lastNackReceivedTick = Long.MAX_VALUE;
              }
  
              // Keep track of the highest ack we have received
              if (stamp > ack)
                  ack = stamp;
  
              // This will update the prefix if stamp > completedPrefix
              // It also combines any adjacent Completed ranges 
              oststream.setCompletedPrefix(stamp);
              long newCompletedPrefix = oststream.getCompletedPrefix();
  
              if (newCompletedPrefix > oack)
              {
                  oack = newCompletedPrefix;
                  upControl.sendAckMessage(null, null, null,
                                           newCompletedPrefix, priority, reliability, streamID, false);
              }
  
              if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled())
                  SibTr.exit(tc, "writeAckPrefix");
          }
      }
docs: This method is called when an Ack message is recieved from a downstream
  ME. It updates the ackPrefix of the stream and then passes the message
  up to the PubSubInputHandler which will aggregate the Acks from all
  InternalOutputStreams.
  
  @param stamp The ackPrefix