target = "https://www.rfc-editor.org/rfc/rfc9002#section-6.2"

# 6.2.  Probe Timeout
#
# A Probe Timeout (PTO) triggers the sending of one or two probe
# datagrams when ack-eliciting packets are not acknowledged within the
# expected period of 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 acknowledgments.
# 
# As with loss detection, the PTO is per packet number space.  That is,
# a PTO value is computed per packet number space.
# 
# A PTO timer expiration event does not indicate packet loss and MUST
# NOT cause prior unacknowledged packets to be marked as lost.  When an
# acknowledgment is received that newly acknowledges packets, loss
# detection proceeds as dictated by the packet and time threshold
# mechanisms; see Section 6.1.
# 
# The PTO algorithm used in QUIC implements the reliability functions
# of Tail Loss Probe [RFC8985], RTO [RFC5681], and F-RTO algorithms for
# TCP [RFC5682].  The timeout computation is based on TCP's RTO period
# [RFC6298].

[[spec]]
level = "MUST"
quote = '''
A PTO timer expiration event does not indicate packet loss and MUST
NOT cause prior unacknowledged packets to be marked as lost.
'''