This document defines two new flags in IS-IS and OSPF to signal loss of reachability to an individual prefix in case of summarization. I think that it has a well defined scope and is almost ready for publication. 
In this regard, I noticed the normative reference to draft-ietf-lsr-ospf-prefix-extended-flags, which, I guess, will be published before this document.

I have only few minor comments for your consideration:

- In the Abstract, I suggest to replace 'In the presence of summarization,' with 'Summarization is often used in IGP to improve network efficiency, but'.

- In the Introduction, I suggest to swap the last two paragraphs, otherwise it is not clear how they are sequential.

- Section 4 on "Generation of the UPA" could be moved before section 2 on "Supporting UPA in IS-IS" and section 3 on "Supporting UPA in OSPF". I think it would be more logical.

- Section 6 on "Deployment Considerations for UPA" seems to discuss only the case of area/domain partition. I would also highlight what are the operational benefits of UPA, as briefly mentioned in the Introduction.

- In section 9 on "Security Considerations", you can also add the reference to RFC7794 and draft-ietf-lsr-ospf-prefix-extended-flags.