Original text:
whether a threshold has been met and an acknowledgement is to be sent in response. <del> 7. </del>  On sending an update to the peer's "max_ack_delay", an endpoint can use this new value in later computations of its Probe Timeout (PTO)

Reviewer comments:
by adding a one byte IMMEDIATE_ACK frame.
editorial nit: my preference would be for frame diagrams to use the notation in QUIC transport. I see that ACK_FREQUENCY is defined using ASCII art, so perhaps you want to consider a separate follow up PR to switch styles after this lands.
Thanks Lucas, I filed to update them.
They currently use ascii art. Will write this up once is merged.
sgtm, thanks!
When sending a PTO packet, a sender wants to induce an immediate ACK. The only trick I know to induce such an ACK is intentionally skip the packet number. However, the problem of that approach is that it relies on the receiver not ignoring reorders. When an ACKFREQUENCY frame previously sent had set the bit, then it become impossible for a sender to induce an immediate ACK when sending a PTO packet. At the moment, the only solution is to bundle an ACK_FREQUENCY packet in each PTO packet, but that seems like a bit of unnecessary overhead to me. Should we have a dedicated frame for inducing immediate ACKs instead?
That's an option. Another option is to revisit the max_reordering idea discussed in . If the max reordering was 5, then you could skip 5 PNs. I do feel like it'd be nice to allow specifying an int for max reordering, since that matches up with adaptive packet threshold loss detection, but as I expressed in , I think getting the details right are deceptively difficult, so I'm torn.
This does seem like a real problem. I like the idea of a single "ELICIT_ACK" type frame. With one byte a sender could effectively mark its PTOs ensuring they get an ACK without relying on the trick. An explicit frame is also easier to implement for the sender than the packet number skipping.
There's quite a bit of discussion on this on the tcpm mailing list at the moment, ie: URL I'm increasingly thinking it's worth using an unused protected bit in the header for this. It'd end up competing with the 2-bit packet loss measurements, but given those are experimental and may never be widely deployed, I think it might be worth it. For example, I believe this could allow an implementation to not include maxackdelay in PTO if it set the "ACK-pull"(or whatever it's called) bit on the final packet. That already seems possible if more than the threshold number of ack-eliciting packets are outstanding.
I've thought more about this, and I think most of the time a 1-byte frame would be acceptable, but it definitely adds some implementation complexity, so I'd be curious which design others prefer? If we can decide, then I'll be happy to write a PR(or NAME can). Besides PTO, a use case would be making the last packet before becoming app-limited elicit an immediate ACK. This could allow skipping maxackdelay for all PTOs(see ), though always doing it could be a bit much in some cases. For example, browsers commonly send a series of requests, but to the transport it seems app-limited between them, because it doesn't know the browser has another request to send. It might be useful for 'chirping' as well, but I've never tried that, so it's mostly speculation.
We don't currently have any intention of deploying them either, and I think that's unlikely to change. I haven't seen a lot of good evidence that they are terribly helpful, and content providers don't seem to have a good incentive for implementing it. For us at least, the difference in complexity for a frame versus a bit in the header is negligible. A frame is actually be slightly easier to implement and losing a byte here and there isn't so bad.
I strongly prefer using the header bit. Although it will be a bit more work to implement, the fact that I won't end up with a 1-byte STREAM frame that I have to split off at the end of the packet will be worth the effort.
Is testimony of network operators not taken as evidence? I remember a line of people, from different companies -- BT, ATT, and so on, at IETF 101 practically begging for some information in QUIC packets to be exposed for monitoring and troubleshooting.
I do not see anyone propose a 1-byte STREAM frame. I believe what is meant is a new frame type. If anything, using a new frame type to elicit an ACK is easier than using a header bit, as it involves fewer parts of the code to change.
Nobody is proposing a 1-byte STREAM frame (you can't just make up stream data anyway). The problem is that a new frame would consume bytes (or probably one byte) in the packet payload. Given that you set this signal on a PTO, and PTOs are often retransmissions of packets, this would require you to repackage (and, most likely, cut off a single byte from a STREAM frame). Using a header bit would solve that problem.
NAME and I discussed this and there are number of ways to fix this, but given this issue of not being able to elicit an immediate ACK by skipping packet numbers is introduced by the draft, we feel it's important it be fixed in some way. One more option is new STREAM frame codepoints that elicit an immediate ACK.
I'm ready to write a PR, but I'd appreciate thoughts on whether the existing PING frame should be repurposed or we should add a new frame(ACK, PONG, etc). I lean towards a new frame, since there are enough code-points and I believe there are use cases for the existing PING frame that does not elicit an immediate ACK.
+1. While I would not be opposed to repurposing the existing PING frame, I do not see the necessity of doing that. Let's simply consume one code point of the remaining 33.
Thanks, now for bikeshedding time. Some possible names: ACKME ACKPULL (from TCP) FAST_ACK PONG
Putting on my bikeshedding hat: I don't think we should prefix the frame name with ACK since that almost makes it looks like yet another variant of the ACK frame. The "pull" TCP terminology is fine (e.g. PULLACK) but we could also be more explicit with something like SOLICIT_ACK.
SCTP has something similar and calls it SACK-IMMEDIATELY (RFC 7053).
Thanks, I'd be happy with ACKIMMEDIATELY or IMMEDIATEACK
For those two options I'd prefer IMMEDIATE_ACK for the same reasoning as above -- so it appears less like an ACK variant.
IMMEDIATEACK sounds good to me. Alternatively, I'd be also fine with something like ACKNOW.
IMMEDIATE_ACK it is, PR coming.

Revised text:
whether a threshold has been met and an acknowledgement is to be sent in response.  <ins> 8. </ins> On sending an update to the peer's "max_ack_delay", an endpoint can use this new value in later computations of its Probe Timeout (PTO)