if you have the DF flag set, and your packet is larger than the MTU along the route to the destination... your packet will get dropped.

That is the point! You can easily use binary search to find a proper MTU to use for your entire path this way. You only need to stop on some reasonable multiple, say 128 or 64, so a few packets is all you need to get there.

Start with 2048. If it works, double it. If not, halve it. Then binary search until the difference between "works" and "doesn't work" is your search granularity size (such as 128.)

What if the path changes?

What if the path changes?

Then you're either using a non-optimal (too small) MTU or it will drop, depending on whether the new route has a higher or lower MTU.

I would still go with 1280 for simplicity, or if you really want to do a MTU discovery, do your binary search between 1280 and 1500.

The reason being that 1280 is guaranteed by IPv6 (and every router that isn't in a museum is IPv6 capable, so it must have at least that MTU) but on the other hand you are not likely to get more than 1500 as this is what standard ethernet delivers, and there is "always some ethernet" in between you and the other computer. Unless everyone configures their cards and routers for jumbo frames, which probably won't happen.

Of course a granularity of 128 is way too coarse for such a small interval, especially since that would only leave the choice between 1280 and 1408. Note that 1280 is what the "start at 2048" binary search would eventually converge to, too (2048fail --->1024work ---> 1536fail ---> 1280work).