Re: [quicwg/base-drafts] Simplify TLP and RTO into Probe Timeout (#2114)

janaiyengar <notifications@github.com> Thu, 13 December 2018 21:05 UTC

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Date: Thu, 13 Dec 2018 21:05:55 +0000
From: janaiyengar <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Simplify TLP and RTO into Probe Timeout (#2114)
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janaiyengar commented on this pull request.



>  
-To reduce latency, it is RECOMMENDED that the sender set and allow the TLP timer
-to fire twice before setting an RTO timer. In other words, when the TLP timer
-expires the first time, a TLP packet is sent, and it is RECOMMENDED that the TLP
-timer be scheduled for a second time. When the TLP timer expires the second
-time, a second TLP packet is sent, and an RTO timer SHOULD be scheduled {{rto}}.
+There is no requirement on the clock granularity. The PTO value MUST be set to

I've changed this, but also added some text at the definition of granularity.

>  
-* When the final TLP packet is sent, the RTO period is set to max(SRTT +
-  4*RTTVAR + MaxAckDelay, kMinRTOTimeout)
+Consecutive PTO periods increase exponentially, and as a result, connection
+recovery latency increases exponentially as packets continue to be dropped in
+the network.  Sending two packets on PTO expiration increases resilience to
+packet drop in both directions, thus reducing the probability of consecutive PTO

I'll remove "in both directions" -- more trouble than it's worth.

>  
-QUIC also diverges from TCP by including MaxAckDelay in the RTO period. Since
-QUIC corrects for this delay in its SRTT and RTTVAR computations, it is
-necessary to add this delay explicitly in the TLP and RTO computation.
+Delivery or loss of packets in flight is established when an acknowledgement is
+received that newly acknowledges one or more packets.  An endpoint might receive
+an acknowledgement after multiple consecutive PTO periods.

I think this is not required. There's mention of consecutive PTOs later on where it's more relevant.

> @@ -722,14 +695,13 @@ Pseudocode for OnAckReceived and UpdateRtt follow:
       // Find the smallest newly acknowledged packet
       smallest_newly_acked =
         FindSmallestNewlyAcked(newly_acked_packets)
-      // If any packets sent prior to RTO were acked, then the
-      // RTO was spurious. Otherwise, inform congestion control.
-      if (rto_count > 0 &&
-            smallest_newly_acked > largest_sent_before_rto):
-        OnRetransmissionTimeoutVerified(smallest_newly_acked)
+      // If any packets sent prior to PTO were acked, then the
+      // PTO was spurious. Otherwise, inform congestion control.
+      if (pto_count > 0 &&
+            smallest_newly_acked > largest_sent_before_pto):

@martinthomson : @ianswett is trying to capture the difference in congestion response that the current loss recovery spec has for TLP vs. RTO. The idea is to say that for a couple of PTOs, there will be a smaller reduction in cwnd, and after that, the cwnd will be collapsed. I can write that up.

>  
-## Retransmission Timeout
+A PTO expiration is classified as spurious or valid when an ACK frame is
+received that newly acknowledges packets in flight, see {{pto-loss}}.  On a
+valid PTO, the congestion window MUST be reduced to the minimum congestion
+window and slow start is re-entered.

I'm going to simplify this further. I think that having a verification is unnecessary; the behavior ends up being the same whether a PTO is verified or not. I can retain this notion of reducing the cwnd differently for the first 2 PTOs and then collapsing it to min_cwnd after that.

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