Re: [quicwg/base-drafts] Change PTO to be per packet number space (#3066)

[Martin Thomson <[e-mail]>]

martinthomson commented on this pull request.

I think that if we're going to use this in resolving the key discard thing, then we'll need stronger support for the advice about sending in multiple spaces.

> @@ -438,11 +438,13 @@ and larger thresholds increase loss detection delay.
 A Probe Timeout (PTO) triggers sending one or two probe datagrams when
 ack-eliciting packets are not acknowledged within the expected period of
 time or the handshake has not been completed.  A PTO enables a connection to
-recover from loss of tail packets or acknowledgements. The PTO algorithm used
-in QUIC implements the reliability functions of Tail Loss Probe
-{{?TLP=I-D.dukkipati-tcpm-tcp-loss-probe}} {{?RACK}}, RTO {{?RFC5681}} and
-F-RTO algorithms for TCP {{?RFC5682}}, and the timeout computation is based on
-TCP's retransmission timeout period {{?RFC6298}}.
+recover from loss of tail packets or acknowledgements.  As with loss
+detection, the probe timeout is per packet number space.
+
+The PTO algorithm used in QUIC implements the reliability functions of
+Tail Loss Probe {{?TLP=I-D.dukkipati-tcpm-tcp-loss-probe}} {{?RACK}},

Is `{{?RACK}}` another citation for Tail Loss Probe?

> @@ -438,11 +438,13 @@ and larger thresholds increase loss detection delay.
 A Probe Timeout (PTO) triggers sending one or two probe datagrams when
 ack-eliciting packets are not acknowledged within the expected period of
 time or the handshake has not been completed.  A PTO enables a connection to
-recover from loss of tail packets or acknowledgements. The PTO algorithm used
-in QUIC implements the reliability functions of Tail Loss Probe
-{{?TLP=I-D.dukkipati-tcpm-tcp-loss-probe}} {{?RACK}}, RTO {{?RFC5681}} and
-F-RTO algorithms for TCP {{?RFC5682}}, and the timeout computation is based on
-TCP's retransmission timeout period {{?RFC6298}}.
+recover from loss of tail packets or acknowledgements.  As with loss
+detection, the probe timeout is per packet number space.
+
+The PTO algorithm used in QUIC implements the reliability functions of
+Tail Loss Probe {{?TLP=I-D.dukkipati-tcpm-tcp-loss-probe}} {{?RACK}},
+RTO {{?RFC5681}} and F-RTO algorithms for TCP {{?RFC5682}}, and the timeout

The commas here are odd.  I think that you could start a new sentence here:

```suggestion
RTO {{?RFC5681}}, and F-RTO  {{?RFC5682}} algorithms for TCP.  The timeout
```

Yeah, this is old text, but it's really confusing still.

> @@ -496,9 +500,7 @@ be considered an RTT sample.
 
 Until the server has validated the client's address on the path, the amount of
 data it can send is limited, as specified in Section 8.1 of {{QUIC-TRANSPORT}}.
-Data at Initial encryption MUST be retransmitted before Handshake data and
-data at Handshake encryption MUST be retransmitted before any ApplicationData
-data.  If no data can be sent, then the PTO alarm MUST NOT be armed until
+If no data can be sent, then the PTO alarm MUST NOT be armed until

```suggestion
If no data can be sent, then the PTO alarm MUST NOT be armed for the corresponding packet number space until
```

> @@ -522,9 +524,14 @@ removed from bytes in flight when the Initial and Handshake keys are discarded.
 ### Sending Probe Packets
 
 When a PTO timer expires, a sender MUST send at least one ack-eliciting packet
-as a probe, unless there is no data available to send.  An endpoint MAY send up
-to two full-sized datagrams containing ack-eliciting packets, to avoid an
-expensive consecutive PTO expiration due to a single lost datagram.
+in the packet number space as a probe, unless there is no data available to
+send.  An endpoint MAY send up to two full-sized datagrams containing
+ack-eliciting packets, to avoid an expensive consecutive PTO expiration due

I think that we can drop the "containing ack-eliciting packets" here, it's redundant with the first sentence.

> @@ -522,9 +524,14 @@ removed from bytes in flight when the Initial and Handshake keys are discarded.
 ### Sending Probe Packets
 
 When a PTO timer expires, a sender MUST send at least one ack-eliciting packet
-as a probe, unless there is no data available to send.  An endpoint MAY send up
-to two full-sized datagrams containing ack-eliciting packets, to avoid an
-expensive consecutive PTO expiration due to a single lost datagram.
+in the packet number space as a probe, unless there is no data available to
+send.  An endpoint MAY send up to two full-sized datagrams containing
+ack-eliciting packets, to avoid an expensive consecutive PTO expiration due
+to a single lost datagram.
+
+In addition to sending data in the packet number space for which the timer
+expired, the sender SHOULD coalesce ack-eliciting packets from all other packet
+number spaces with inflight data if sending coalesced packets is supported.

This is important, so I think that it needs more support.  Questions that arise are: What happens if I don't do this?  Under what conditions would it be fine not to send in both spaces?

> @@ -933,7 +940,7 @@ loss_detection_timer:
 pto_count:
 : The number of times a PTO has been sent without receiving an ack.
 
-time_of_last_sent_ack_eliciting_packet:
+time_of_last_sent_ack_eliciting_packet[kPacketNumberSpace]:

```suggestion
time_of_last_sent_ack_eliciting_packet\[kPacketNumberSpace]:
```

> @@ -1096,12 +1103,20 @@ Pseudocode for SetLossDetectionTimer follows:
 // Returns the earliest loss_time and the packet number
 // space it's from.  Returns 0 if all times are 0.
 GetEarliestLossTime():
-  time = loss_time[Initial]
+  return GetEarliestTimeAndSpace(loss_time)
+
+// Returns the earliest time_of_last_sent_ack_eliciting_packet
+// and the packet number space it's from.
+GetEarliestAckElicitingTime():
+  return GetEarliestTimeAndSpace(loss_time)

You have two identical functions here that don't seem to do anything other than indirect over the global.  I don't get this change.

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