[jsnell]

You can measure RTT continuously, accurately, and even in the presence of packet loss just with selective acks and a little bit of extra bookkeeping in the sender.

It's hard to overstate how expensive TCP timestamps are. The thing is that they bloat every single packet including control packets. 2% of the world's bandwidth is being wasted on this.

The only reason for anyone to implement TCP timestamps today is that iOS clients have horrible receive window scaling if timestamps are disabled. (Well, that was the only reason a few years ago when I was still in the game of keeping up with the quirks of different TCP stacks.)

I wrote more on the subject at the time: https://www.snellman.net/blog/archive/2017-07-20-s3-mystery/

[toast0]

I would guess that Apple could be convinced to fix this, if someone has the right contact. The iOS TCP stack shows a lot of care, generally: they do path MTU probing well, and they've deployed MP-TCP (requires apps to enable it though), among other things I can't remember. Fixing performance if timestamps are disabled seems like something they'd do.