Cleaned up and added a couple of lines of comment about what 'Send' and 'Ack' really mean. All that really matters is distinguishing between outgoing packets and incoming packets. 'Send' and 'Ack' are unfortunate names reflecting my previous muddled thinking about this. So for 'Send' read outgoing, and for 'Ack', read incoming. A low-priority task is to change the naming to reflect this semantic change.