Original text:
The RTT for a network path may change over time.  If a path's actual RTT decreases, the min_rtt will adapt immediately on the first low <del> sample.  If the path's actual RTT increases, the min_rtt will not adapt to it, allowing future RTT samples that are smaller than the new RTT to be included in smoothed_rtt. </del>  5.3.

Reviewer comments:
I believe this covers the discussion on the issue. It also introduces some MUSTs which should've always been there. I don't have a strong opinion on the use of RFC 2119 language here, and I'm happy to remove all of that language if folks think that's better. I would like not to bikeshed this, but I would also like to be on a beach right now, enjoying a lovely drink and watching the sunset.
NAME wrote the following in his WGLC review (): Splitting this off from as a potential design issue.
This seems like a reasonable thing to do.
I'd support a can or a MAY, as an example when you might reset min_rtt, but I don't think we need a SHOULD here.
minrtt is currently only used for filtering unexpectedly large ackdelay's reported by the peer. If the path RTT truly inflates then the peer will be able to report much larger ackdelay values and get a much lower RTT sample accepted. But I don't think this is a security problem in practice. The only other consideration is that in future QUIC implementations may reuse this computed value for other purposes like delay based congestion control. It's hard to make a judgement call on how important it is to reset minrtt. I guess I'd be happy with a SHOULD but I can live with a MAY.
Jana made a good argument that persistent congestion could be declared due to an overly small RTT, so resetting minrtt could avoid an endpoint repeatedly declaring persistent congestion. That being said, the RTT samples also need to be too low to spuriously declare persistent congestion, so this is increasingly sounding like an attack scenario, rather than having a poor estimate of minrtt. I think a SHOULD is Ok now, but I'd like to state the best rationale we have for why it's a SHOULD and neither a MUST or a MAY.
I think there's one tricky point in resetting the minrtt after handshake confirmation. That is that all the packets can be acked with delays. IIRC, one of the reasons we were happy with using minrtt for capping latestrtt is that Initial and Handshake packets do not have ackdelays. If we are to suggest that an endpoint MAY reset minrtt when persistent congestion is declared, I think we should consider the risk of not having good estimate of minrtt, as we would have during the handshake.
I don't see that as a risk here. min_rtt doesn't consider any discretionary delays.
NAME That's exactly the problem. will be inflated by , unless and until the peer sends an immediate acknowledgement. And until that happens, will as well.
I think that's OK. In practice, immediate ACKs do happen, and even at low rates, this should resolve to a "true" minimum. And if it doesn't, then you have worse performance than you might had otherwise. You have to have a pretty poor network (or a terrible ACK generation regime) for minrtt not to eventually resolve to a useful minimum. Explaining this risk is fine, but the endpoint that does this made the choice to throw out minrtt and that's just a consequence of that.
On one hand I agree that we can expect to see immediate ACKs, especially when HTTP/3 is used. At the same time, I am not sure if resetting minrtt when observing persistent congestion is better than other cases where we might reset minrtt, but that we do not currently mention. Change of IPTTL could be one of the examples in this regard (IIRC NAME pointed out that we do not use that value for detecting path changes). To summarize, I think talking about resetting minrtt might be fine, though I am not sure if that discussion should be tied to persistent congestion.
To give a specific example, one problem that I know with the current definition of minrtt is that it cannot detect and correct RTT estimate of a congested path. The sole purpose of minrtt is to discard excessive ack-delays being reported by the receiver. This works by capping the lower bound of adjustedrtt to minrtt. The problem here is that minrtt is a value that is typically collected when the path is almost idle. Once the path becomes congested, the true RTT becomes much greater than minrtt. For instance, if RTT-on-idle is 10ms and if RTT-on-congestion is 50ms, minrtt would be 10ms, and therefore cannot be used to correct RTT estimates when the peer reports an excessive ackdelay of, say 25ms. The RTT estimate would become 25ms in such case. I tend to think that this might be a bigger problem of the original issue, and therefore that we should state that an endpoint can periodically re-collect minrtt estimates, if we are to make a change to the current definition of minrtt.
Based on agreement on the resolution in , which introduces normative language, let's mark this design.
Seems fine. I'm not 100% on the strength of the recommendation to reset after persistent congestion, but it's sensible advice.The resetting of min RTT is also specified with LEDBAT or BBR, which differ from each other and from this proposed spec. This is a complex issue, because resetting min RTT in LEDBAT "because of congestion" is a variation of the latecomer advantage. But as long as the text applies only to RENO as specified here, then I am fine with it.I like the cautionary text. Just one suggestion but LGTM regardless. Thank you for working on this.

Revised text:
The RTT for a network path may change over time.  If a path's actual RTT decreases, the min_rtt will adapt immediately on the first low  <ins> sample.  If the path's actual RTT increases however, the min_rtt will not adapt to it, allowing future RTT samples that are smaller than the new RTT to be included in smoothed_rtt. Endpoints SHOULD set the min_rtt to the newest RTT sample after persistent congestion is established.  This is to allow a connection to reset its estimate of min_rtt and smoothed_rtt (smoothed-rtt) after a disruptive network event, and because it is possible that an increase in path delay resulted in persistent congestion being incorrectly declared. Endpoints MAY re-establish the min_rtt at other times in the connection, such as when traffic volume is low and an acknowledgement is received with a low acknowledgement delay.  Implementations SHOULD NOT refresh the min_rtt value too often, since the actual minimum RTT of the path is not frequently observable. </ins> 5.3.