code: private void sendAck()
      {
          if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled())
              SibTr.entry(this, tc, "sendAck");
  
          long completedPrefix = oststream.getCompletedPrefix();
  
          // Now that all messages are delivered we need to send Acks
          // sendAck back to sending ME (RemoteMessageTransmitter)
          try
          {
              if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled())
                  SibTr.debug(tc, "sendAck up to: " + completedPrefix);
  
              upControl.sendAckMessage(
                                       streamSet.getRemoteMEUuid(),
                                       streamSet.getDestUuid(),
                                       streamSet.getBusUuid(),
                                       completedPrefix,
                                       priority,
                                       reliability,
                                       streamSet.getStreamID(),
                                       true);
              this._lastAckedTick = completedPrefix;
          } catch (SIResourceException e)
          {
              //FFDC
              FFDCFilter.processException(
                                          e,
                                          "com.ibm.ws.sib.processor.gd.GuaranteedTargetStream.sendAck",
                                          "1:2019:1.110",
                                          this);
  
              SibTr.exception(tc, e);
  
              //TODO don't know what to do here - rethrow?!
          }
          if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled())
              SibTr.exit(tc, "sendAck");
      }
docs: /*
  Utility method to send Ack up to completedPrefix