Change:
9.1.3. <del> The ACK Delay field of the ACK frame is relative to the largest acknowledged packet number (see Section 13.2.5 of QUIC-TRANSPORT). When using paths with different transmission delays, the reported host delay will most of the time relate to the path with the shortest latency.  To collect ACK delays on all the paths, hosts should rely on time stamps as described in QUIC-Timestamp. </del> <ins> When SPNS is in use, accurate RTT estimation requires more careful considerations.  According to QUIC-RECOVERY, an endpoint generates an RTT sample on receiving an ACK frame that meets the following two conditions: (1) the largest acknowledged packet number is newly acknowledged, and (2) at least one of the newly acknowledged packets was ack-eliciting.  The RTT sample, latest_rtt is calculated as the time elapsed since the largest acknowledged packet was sent. However, when applying the above algorithm with SPNS, one may encounter the following issues: (1) RTT of some paths are not updated timely if ACKs are mostly returned from other paths, and (2) ACK frames depend on the largest received packet of the connection, not the path, and the resulted RTT sample may be the sum of the one-way delays of two different paths.  One solution for accurate RTT measurements is to employ time-stamps as described in QUIC-Timestamp. If one chooses not to use time-stamps but wants to get reasonable estimation of RTTs on multiple paths with single packet number space, the following practices can be used: For packet receiver (ACK sender): Maintain an ACK threshold and an ACK timer for each path.  A path should send an ACK when it receives ack-eliciting-threshold number of ack-eliciting packets (e.g., two) on this path, and an ack- eliciting packet must be acknowledged within MAX_ACK_DELAY. Write ACK frame based on the largest received packet of the path. Start the ACK ranges with the largest received packet number of that path, which means that the "Largest Acknowledged" field is the path's largest packet number and the "ACK Delay" field is the delay time of the path's largest received packet. For packet sender (ACK receiver): Maintain an unacked list for each path to retrieve the packets that has been sent when an ACK is received.  It can coexist with the unacked list of the connection layer or packet number space layer. Generate RTT sample for a path when the following conditions are met: (1) the largest acknowledged packet number is newly acknowledged by the ACK received from this path, and (2) at least one of the newly acknowledged packets was ack-eliciting. </ins> 9.1.4.

Discussion:
For issue , add some considerations for RTT computation in our single pns implementation. Not sure if it is easy enough to understand. Please feel free to change.
Yes, this needs more discussion. Also maybe we want to move this text rather in the "Implementation Considerations" section?
NAME NAME updated this PR. Can we merge it? Note that this PR is only for SPNS and part of that respective section. Based on his report, the strategy seems reasonable. For ACK_MP we have the following text in the packet scheduling section (7.):
Comment by NAME (on PR ): I think we expand this section a bit as we found getting the delay right was non-trivial. QUIC time-stamp would work for sure. But if one decides not to use QUIC timestamp, we probably want to explain what is needed to be done here.
I have submitted PR for this one.
Closing as got merged.
I'm ok with the current version, just a few editorial hints.Seems like a reasonable tradeoff, easy to implement.