Code change:
ack-eliciting packets, to avoid an expensive consecutive PTO expiration due to a single lost datagram. <del> It is possible that the sender has no new or previously-sent data to send.  As an example, consider the following sequence of events: new </del> <ins> When the PTO timer expires, and there is new or previously sent unacknowledged data, it MUST be sent.  Data that was previously sent with Initial encryption MUST be sent before Handshake data and data previously sent at Handshake encryption MUST be sent before any ApplicationData data. It is possible the sender has no new or previously-sent data to send. As an example, consider the following sequence of events: new </ins> application data is sent in a STREAM frame, deemed lost, then retransmitted in a new packet, and then the original transmission is <del> acknowledged.  In the absence of any new application data, a PTO timer expiration now would find the sender with no new or previously- sent data to send. When there is no data to send, the sender SHOULD send a PING or other ack-eliciting frame in a single packet, re-arming the PTO timer. </del> <ins> acknowledged.  When there is no data to send, the sender SHOULD send a PING or other ack-eliciting frame in a single packet, re-arming the PTO timer. </ins> Alternatively, instead of sending an ack-eliciting packet, the sender MAY mark any packets still in flight as lost.  Doing so avoids

Related discussion:
text: It makes things slightly simpler at the client, but it adds about as much code to the server. Additional code has to be added at the server to the PTO response to only send a PING and not a retransmission when the path is not yet validated. Second, I think it does open up an interesting attack vector that is currently absent. NAME notes that there is amplification, but I don't think byte amplification is the interesting one here, since the total load is going to be < 1 MTU. The interesting amplification here is packet amplification, which is a useful attack since UDP and packet processing costs at endpoints / clients is a relatively high cost. And you can get a fair number of packets back from the server for spoofing a couple of packets, in spite of the exponential backoff. I'll grant that the attack is a bit of a stretch, and does require that the attacker guess the server's CID and packet number. Though, as NAME pointed out to me offline, we allow the server to use a zero-length CID. I don't think this change is necessary. And on balance, I don't think this change is a good one.
I do like the idea of sending a PING frame from the server as a signal to the client that it is blocked. I think we can suggest that as an optimization that a clever server might implement under the 3x limit, or when it reaches the 3x limit.
NAME While endpoints can do that, I am not sure if it is a good idea to endorse such a behavior in the specification. The reason we have so far been fine with having the 3x limit being applied until the Handshake packets are exchanged is because we assume that the size of ServerHello would not be that large compared to the size of the ClientHello, and therefore that the chance of hitting 3x limit before path validation is minimal. It's a hack. If we are to address the corner cases that the server hits after sending 3x amount of data, then I think we should fix the hack than putting another hack (i.e. the PING that signals 3x limit) above the existing hack.