I have reviewed this document as part of the security directorate's ongoing effort to review all IETF documents being processed by the IESG. These comments were written primarily for the benefit of the security area directors. Document editors and WG chairs should treat these comments just like any other last call comments.

The summary of the review is Ready.

I did an early review of this document in April of 2021. At that time, I said I'm not a TCP or ECN expert, so please take my comments with a proverbial grain of salt. Nothing has changed, I am still not a TCP or ECN expert.

The security considerations in the original draft contained a TODO about a potential covert channel; this document still contains the TODO. I think this should be resolved before publication, but I leave this to the AD. I have nothing new to add.