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.

This document builds on existing data flows defined in RFC 8724
(Static Context Header Compression and Fragmentation) used in 
Low-Power Wide Area Networks (LPWANs). It decribes a new method for
a sender to return bitmaps of lost SCHC fragments to the sender.
In this method the receiver can return more than one bitmap in an
"SCHC Compound ACK".

The Security Considerations section states that Security Considerations
of RFC 8724 apply, and I agree. No new issues appear to be introduced.