This document has been reviewed as part of the transport area review team's
ongoing effort to review key IETF documents. These comments were written
primarily for the transport area directors, but are copied to the document's
authors and WG to allow them to address any issues raised and also to the IETF
discussion list for information.

When done at the time of IETF Last Call, the authors should consider this
review as part of the last-call comments they receive. Please always CC
tsv-art@ietf.org if you reply to or forward this review.

This is a well-written and technically solid document that specifies a reasonable OAM mechanism for BIER environments.

No major issues observed.

Below are some minor / non-blocking comments that might help improve the document (note that I have a limited understanding of BIER and associated environments, so some of these issues could be based on misunderstandings from my end, or be completely obvious for the intended audience).

Section 3.1 & 4.5

The draft defines two reply modes: BIER and IP/UDP.
It would be helpful to include a short explanation of their intended use and trade-offs.
For instance, replying over IP/UDP may be intended for cases where the Echo Reply could exceed the BIER path MTU.
Are there any known drawbacks to using IP/UDP replies (e.g., loss of return-path symmetry or reduced accuracy)?
A short paragraph explaining when each mode is appropriate would likely be useful to both implementers and operators.

The document allows the Querier Timestamp Format (QTF) and Responder Timestamp Format (RTF) to differ and requires that senders of Echo Requests MUST be able to interpret both NTP and PTP formats.
This ensures interoperability, but the use of mixed formats may cause ambiguity or precision loss.
Some discussion around this might be useful, and perhaps even recommendations to senders of Echo Replies.
Perhaps something like: "Responders SHOULD use the same timestamp format as indicated in the QTF, when possible."