Original text:
3.8. <del> The round-trip time of QUIC flows can be inferred by observation once per flow, during the handshake, as in passive TCP measurement; this requires parsing of the QUIC packet header and recognition of the handshake, as illustrated in handshake.  It can also be inferred during the flow's lifetime, if the endpoints use the spin bit facility described below and in Section 17.3.1 of QUIC-TRANSPORT. </del>  3.8.1. on each packet sent. When in use, the latency spin bit in each direction changes value <del> once per round-trip time (RTT) any time that both endpoints are sending packets continuously.  An on-path observer can observe the time difference between edges (changes from 1 to 0 or 0 to 1) in the spin bit signal in a single direction to measure one sample of end- to-end RTT.  This mechanism follows the principles of protocol measurability laid out in IPIM. </del>  Note that this measurement, as with passive RTT measurement for TCP, includes any transport protocol delay (e.g., delayed sending of Limited RTT measurement is possible by passive observation of QUIC traffic; see sec-rtt.  No passive measurement of loss is possible <del> with the present wire image.  Extremely limited observation of upstream congestion may be possible via the observation of CE markings on ECN-enabled QUIC traffic. </del>  4.2.

Reviewer comments:
Refers to quicwg. This also addresses /extremely limited/ ECN visibility (limited for sure, but we don't need to say "extremely"). Fixed multiple definitions of RTT Refered to metrics, etc for encrypted traffic wrt network-layer measurement via a REF.
Thanks for the suggestions from Ian and Mirjay, they were of course incorporated.

Revised text:
3.8.  <ins> The round-trip time (RTT) of QUIC flows can be inferred by observation once per flow, during the handshake, as in passive TCP measurement; this requires parsing of the QUIC packet header and recognition of the handshake, as illustrated in handshake.  It can also be inferred during the flow's lifetime, if the endpoints use the spin bit facility described below and in Section 17.3.1 of QUIC- TRANSPORT. </ins> 3.8.1. on each packet sent. When in use, the latency spin bit in each direction changes value  <ins> once per RTT any time that both endpoints are sending packets continuously.  An on-path observer can observe the time difference between edges (changes from 1 to 0 or 0 to 1) in the spin bit signal in a single direction to measure one sample of end-to-end RTT.  This mechanism follows the principles of protocol measurability laid out in IPIM. </ins> Note that this measurement, as with passive RTT measurement for TCP, includes any transport protocol delay (e.g., delayed sending of Limited RTT measurement is possible by passive observation of QUIC traffic; see sec-rtt.  No passive measurement of loss is possible  <ins> with the present wire image.  Limited observation of upstream congestion may be possible via the observation of CE markings on ECN- enabled QUIC traffic. On-path devices can also make measurements of RTT, loss and other performance metrics when information is carried in an additional network-layer packet header (Section 6 of I-D.ietf-tsvwg-transport- encrypt describes use of operations, administration and management (OAM) information).  Using network-layer approaches also has the advantage that common observation and analysis tools can be consistently used by multiple transport protocols, however, these techniques are often limited to measurements within one or multiple cooperating domains. </ins> 4.2.