Merge pull request #3654 from quicwg/ianswett-pto-loss-reorg

Move a PTO section up

draft-ietf-quic-recovery.md

@@ -478,8 +478,13 @@ 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.
+time or the server may not have validated the client's address.  A PTO enables
+a connection to recover from loss of tail packets or acknowledgements.
+
+A PTO timer expiration event does not indicate packet loss and MUST NOT cause
+prior unacknowledged packets to be marked as lost. When an acknowledgement
+is received that newly acknowledges packets, loss detection proceeds as
+dictated by packet and time threshold mechanisms; see {{ack-loss-detection}}.
 
 As with loss detection, the probe timeout is per packet number space.
 The PTO algorithm used in QUIC implements the reliability functions of
@@ -661,16 +666,6 @@ and ensures the highest priority frames arrive first.  Sending different
 payloads each time reduces the chances of spurious retransmission.
 
 
-### Loss Detection {#pto-loss}
-
-Delivery or loss of packets in flight is established when an ACK frame is
-received that newly acknowledges one or more packets.
-
-A PTO timer expiration event does not indicate packet loss and MUST NOT cause
-prior unacknowledged packets to be marked as lost. When an acknowledgement
-is received that newly acknowledges packets, loss detection proceeds as
-dictated by packet and time threshold mechanisms; see {{ack-loss-detection}}.
-
 ## Handling Retry Packets
 
 A Retry packet causes a client to send another Initial packet, effectively