Original text:
The spin bit provides an additional method to measure per-flow RTT from observation points on the network path throughout the duration <del> of a connection.  Endpoint participation in spin bit signaling is </del>  optional in QUIC.  That is, while its location is fixed in this version of QUIC, an endpoint can unilaterally choose to not support "spinning" the bit.  Use of the spin bit for RTT measurement by acknowledgment signaling is carried inside QUIC's encrypted payload, and ACK manipulation is impossible.  Specifically, heuristics attempting to distinguish ACK-only packets from payload-carrying <del> packets based on packet size are likely to fail, and are not </del>  recommended to use as a way to construe internals of QUIC's operation as those mechanisms can change, e.g., due to the use of extensions. QUIC is deliberately designed to fail rather than persist when the network cannot support its operation.  For HTTP/3, this extends to recommending a fallback to TCP-based versions of HTTP rather than <del> persisting with a QUIC connection that might be unstable.  And I- D.ietf-quic-applicability recommends TCP fallback for other protocols on the basis that this is preferable to sudden connection errors and time outs.  Furthermore, wide deployment of NATs with this behavior </del>  hinders the use of QUIC's migration function, which relies on the ability to change the connection ID any time during the lifetime of a QUIC connection.

Reviewer comments:
I set the ready to merge label but there are merge conflict, so please don't merge before accepting my suggestions...
[Ding2015] and [IPIM] are in the references of the manageability draft, but aren't actually... referenced anywhere. That suggests either some text is missing or these entries are spurious.
Yes this has been already flagged by and is fixed in (which we will all merge on Monday when Brain is back). Thanks anyway!
Unused Reference: 'Ding2015' is defined on line 1272, but no explicit reference was found in the text Unused Reference: 'IPIM' is defined on line 1293, but no explicit reference was found in the text
Duplicate reference: draft-ietf-quic-applicability, mentioned in 'QUIC-APPLICABILITY', was also mentioned in 'I-D.ietf-quic-applicability'.
There are two paragraphs that include 2119-language: RFC 2119 keyword, line 578: '... RECOMMENDED. First, it only provid...' RFC 2119 keyword, line 969: '... NOT RECOMMENDED....' Should these be made lowercase or is a 2119 paragraph needed?
I see what was done. Looks ready to go.

Revised text:
The spin bit provides an additional method to measure per-flow RTT from observation points on the network path throughout the duration  <ins> of a connection, following the principles of protocol measurability laid out in IPIM.  Endpoint participation in spin bit signaling is </ins> optional in QUIC.  That is, while its location is fixed in this version of QUIC, an endpoint can unilaterally choose to not support "spinning" the bit.  Use of the spin bit for RTT measurement by acknowledgment signaling is carried inside QUIC's encrypted payload, and ACK manipulation is impossible.  Specifically, heuristics attempting to distinguish ACK-only packets from payload-carrying  <ins> packets based on packet size are very likely to fail, and are not </ins> recommended to use as a way to construe internals of QUIC's operation as those mechanisms can change, e.g., due to the use of extensions. QUIC is deliberately designed to fail rather than persist when the network cannot support its operation.  For HTTP/3, this extends to recommending a fallback to TCP-based versions of HTTP rather than  <ins> persisting with a QUIC connection that might be unstable.  And QUIC- APPLICABILITY recommends TCP fallback for other protocols on the basis that this is preferable to sudden connection errors and time outs.  Furthermore, wide deployment of NATs with this behavior </ins> hinders the use of QUIC's migration function, which relies on the ability to change the connection ID any time during the lifetime of a QUIC connection.