Typo and move full stop to a more natural place.

s/identially/identically/

Can you explain what you mean by that? I'm arguably not the indended audience, but if I don't understand it, there's a chance some of the intended audience and or some interested readers won't either.

When a host receives a TCP packet to a port which is not listening, or no connection exists, or the header information is in some other way not acceptable, the host may respond with a RST (reset) packet. Some of these RST packets are sent from "regular" TCP processing (e.g. outside the sequence window) and others from non-open ports. Making these distinctable by carrying different markings - depending which code path was sending them - would give clues as to what ports/services may be reachable, and give rise to more targetted attacks.
As part if this change, when introducing this new capability (modified TCP header markings for sessions), the irregular codepath is addressed too, such that a 3rd party can not deduct which of the two different paths was sending out the RST, reducing an information side-channel.

Gotcha, thanks. Would "This value also uses ECN for RST replies to probes of non-open ports." mean the same? It looks clearer to me.

Was removing this intended as part of this review?

I'm confused, there is no change here on reviews.freebsd.org; it may be that this diff was uploaded just prior to a flurry of tcp related (including man page) changes, which I have not yet rebased the patch to...