February 2002: The mDNSResponder code has a slight architectural change to improve efficiency. The mDNSResponder code previously called ScheduleNextTask() after every operation, to calculate the time at which it needed to be called back to perform its next timed operation. When the workload is light, and protocol operations are rare and far apart, this makes sense. However, on networks where there is a lot of mDNS traffic (or the CPU is slow), this leads to the following anomolous behaviour: mDNSResponder spends a lot of CPU time working out what to do next, when what it needs to do next should be obvious: Finish processing the big backlog of packets that have been received. To remedy this, mDNSResponder now only executes ScheduleNextTask() when there is no other obvious work waiting to be done. However, the mDNSResponder code does not have direct access to this knowledge. Only the platform layer below knows whether there are packets waiting to be processed. Only the client layer above knows whether it is in the process of performing a long sequence of back-to-back mDNS API calls. This means that the new architecture places an additional responsibility on the client layer and/or platform support layer. As long as they have immediate work to do, they should call the appropriate mDNSCore routines to accomplish that work. With each call, mDNSCore will do only what it immediately has to do to satisfy the call. Any optional work will be deferred. As soon as there is no more immediate work to do, the calling layer MUST call mDNS_Execute(). Failure to call mDNS_Execute() will lead to unreliable or incorrect operation. The value returned from mDNS_Execute() is the next time (in absolute platform time units) at which mDNS_Execute() MUST be called again to perform its next necessary operation (e.g. transmitting its next scheduled query packet, etc.) Note that the time returned is an absolute time, not the time *interval* between now and the next required call. For OS APIs that work in terms of intervals instead of absolute times, mDNSPlatformTimeNow() must be subtracted from the absolute time to get the interval between now and the next event. In a single-threaded application using a blocking select() call as its main synchronization point, this means that you should call mDNS_Execute() before calling select(), and the timeout value you pass to select() MUST NOT be larger than that indicated by the result returned from mDNS_Execute(). After the blocking select() call returns, you should do whatever work you have to do, and then, if mDNS packets were received, or mDNS API calls were made, be sure to call mDNS_Execute() again, and if necessary adjust your timeout value accordingly, before going back into the select() call. In an asynchronous or interrupt-driven application, there are three places that should call mDNS_Execute(): 1. After delivering received packets, the platform support layer should call mDNS_Execute(), and use the value returned to set the platform callback timer to fire at the indicated time. 2. After making any mDNS API call or series of calls, the client layer should call mDNS_Execute(), and use the value returned to set the platform callback timer to fire at the indicated time. 3. When the platform callback timer fires, it should call mDNS_Execute() (to allow mDNSCore to perform its necessary work) and then the timer routine use the result returned to reset itself to fire at the right time for the next scheduled event.