As I read RFC4342 I can't implement a packets per second implementation directly - there are several options but packets per second is not one of them. The mathematical equation allows it as you seem to agree by your first statement. The RFC doesn't. If we track the average packet size it works but then that is a whole lot more complex code to achieve exactly the same thing.
I disagree with this reading of the RFC. The RFC explicitly allows the sender to calculate X as if every packet had size 's = MSS'. As usual, the implementation can then implement any simplifications such an assumption would allow. An implementation that assumes 's = MSS' can therefore calculate a sending rate in packets per second.
Eddie
