the scenario time at which the message is valid. The RTI manages delivery of messages to time-constrained federates, with messages from time-regulating federates delivered at the appropriate time using a Time Stamp Order queue. Time-constrained federates request permission from the RTI to advance their time. The RTI grants a time advance to a time-constrained federates when it is sure that the federate cannot receive a message with a time stamp in its past. Example of Lookahead, granted and advancing: A federate uses a fixed time step of 10 and has a Lookahead of 10. The federate is granted to logical time 50 by the RTI. The RTI thus guarantees that all messages with time step less or equal to 50 have been delivered to the federate. The federate now has all the necessary data to correctly calculate and send messages for the granted time plus Lookahead, i.e. 60. When the federate has sent all messages with timestamp 60, it requests to be advanced to time 60. It thereby promises not to send any messages with a timestamp less than 70. The RTI delivers all messages with timestamp less or equal to 60 to the federate. It then grants the federate to time 60. Etc. If at least one federate in the federation performs pacing, i.e. correlates their time advance requests with a real time clock, the federation may run in real time or scaled real time. Without pacing, the federation will run as fast as possible (e.g., federations that do not require human interaction at runtime nor interfaces with systems that depend upon a real-time clock can run as fast as computing resources will allow). Key services include: EnableTimeConstrained and EnableTimeRegulating that enables theses modes for a federate TimeAdvanceRequest whereby a federate requests to be advanced to a specified logical time TimeAdvancedGrant whereby the RTI informs a federate that it is granted to a specified logical time. EnableAsynchronousDelivery that enables delivery of Receive Order messages both when a federate is in the granted and advancing state. For event driven simulation it is also possible for a federate to request to be advanced to the next event using the following service: NextMessageRequest whereby a federate requests to be advanced to the timestamp of the next message due for delivery to the federate, or a specified logical time, whichever has a lower timestamp. Another important concept is Greatest Available Logical Time (GALT).