target = "https://www.rfc-editor.org/rfc/rfc9002#section-6.2.1"

# 6.2.1.  Computing PTO
#
# When an ack-eliciting packet is transmitted, the sender schedules a
# timer for the PTO period as follows:
# 
# PTO = smoothed_rtt + max(4*rttvar, kGranularity) + max_ack_delay
# 
# The PTO period is the amount of time that a sender ought to wait for
# an acknowledgment of a sent packet.  This time period includes the
# estimated network RTT (smoothed_rtt), the variation in the estimate
# (4*rttvar), and max_ack_delay, to account for the maximum time by
# which a receiver might delay sending an acknowledgment.
# 
# When the PTO is armed for Initial or Handshake packet number spaces,
# the max_ack_delay in the PTO period computation is set to 0, since
# the peer is expected to not delay these packets intentionally; see
# Section 13.2.1 of [QUIC-TRANSPORT].
# 
# The PTO period MUST be at least kGranularity to avoid the timer
# expiring immediately.
# 
# When ack-eliciting packets in multiple packet number spaces are in
# flight, the timer MUST be set to the earlier value of the Initial and
# Handshake packet number spaces.
# 
# An endpoint MUST NOT set its PTO timer for the Application Data
# packet number space until the handshake is confirmed.  Doing so
# prevents the endpoint from retransmitting information in packets when
# either the peer does not yet have the keys to process them or the
# endpoint does not yet have the keys to process their acknowledgments.
# For example, this can happen when a client sends 0-RTT packets to the
# server; it does so without knowing whether the server will be able to
# decrypt them.  Similarly, this can happen when a server sends 1-RTT
# packets before confirming that the client has verified the server's
# certificate and can therefore read these 1-RTT packets.
# 
# A sender SHOULD restart its PTO timer every time an ack-eliciting
# packet is sent or acknowledged, or when Initial or Handshake keys are
# discarded (Section 4.9 of [QUIC-TLS]).  This ensures the PTO is
# always set based on the latest estimate of the RTT and for the
# correct packet across packet number spaces.
# 
# When a PTO timer expires, the PTO backoff MUST be increased,
# resulting in the PTO period being set to twice its current value.
# The PTO backoff factor is reset when an acknowledgment is received,
# except in the following case.  A server might take longer to respond
# to packets during the handshake than otherwise.  To protect such a
# server from repeated client probes, the PTO backoff is not reset at a
# client that is not yet certain that the server has finished
# validating the client's address.  That is, a client does not reset
# the PTO backoff factor on receiving acknowledgments in Initial
# packets.
# 
# This exponential reduction in the sender's rate is important because
# consecutive PTOs might be caused by loss of packets or
# acknowledgments due to severe congestion.  Even when there are ack-
# eliciting packets in flight in multiple packet number spaces, the
# exponential increase in PTO occurs across all spaces to prevent
# excess load on the network.  For example, a timeout in the Initial
# packet number space doubles the length of the timeout in the
# Handshake packet number space.
# 
# The total length of time over which consecutive PTOs expire is
# limited by the idle timeout.
# 
# The PTO timer MUST NOT be set if a timer is set for time threshold
# loss detection; see Section 6.1.2.  A timer that is set for time
# threshold loss detection will expire earlier than the PTO timer in
# most cases and is less likely to spuriously retransmit data.

[[spec]]
level = "MUST"
quote = '''
The PTO period MUST be at least kGranularity to avoid the timer
expiring immediately.
'''

[[spec]]
level = "MUST"
quote = '''
When ack-eliciting packets in multiple packet number spaces are in
flight, the timer MUST be set to the earlier value of the Initial and
Handshake packet number spaces.
'''

[[spec]]
level = "MUST"
quote = '''
An endpoint MUST NOT set its PTO timer for the Application Data
packet number space until the handshake is confirmed.
'''

[[spec]]
level = "SHOULD"
quote = '''
A sender SHOULD restart its PTO timer every time an ack-eliciting
packet is sent or acknowledged, or when Initial or Handshake keys are
discarded (Section 4.9 of [QUIC-TLS]).
'''

[[spec]]
level = "MUST"
quote = '''
When a PTO timer expires, the PTO backoff MUST be increased,
resulting in the PTO period being set to twice its current value.
'''

[[spec]]
level = "MUST"
quote = '''
The PTO timer MUST NOT be set if a timer is set for time threshold
loss detection; see Section 6.1.2.
'''