Why is iOS 17 setting TCP reserved bit?

Question

I started using iOS 17 beta and I am noticing that websites take long to load. I looked at my firewall logs and it looks like safari on new iOS is setting reserved bit in which firewall's IPS engine is blocking right away: Invalid TCP reserved bit. This is not happening on iOS 16.Is there some hidden TCP change with iOS 17 that is now secretly using those TCP reserved bits?

mikequinlan · Accepted Answer

Are you using iCloud Private Relay? Is it possible that their third-party content provider is setting that?

Ocha · Answer

Would be nice to see a TDP dump of before and after upgrade. Is there a way to get it on iOS?

Ocha · Answer

TIL there is TCP reserved bits

dtaht · Answer

Which TCP reserved bit? ECN? The reserved fields in the ack?

wmf · Answer

I wonder if this is L4S. https://developer.apple.com/documentation/network/testing_an...