target = "https://www.rfc-editor.org/rfc/rfc9002#section-5.1"

# 5.1.  Generating RTT Samples
#
# An endpoint generates an RTT sample on receiving an ACK frame that
# meets the following two conditions:
# 
# *  the largest acknowledged packet number is newly acknowledged, and
# 
# *  at least one of the newly acknowledged packets was ack-eliciting.
# 
# The RTT sample, latest_rtt, is generated as the time elapsed since
# the largest acknowledged packet was sent:
# 
# latest_rtt = ack_time - send_time_of_largest_acked
# 
# An RTT sample is generated using only the largest acknowledged packet
# in the received ACK frame.  This is because a peer reports
# acknowledgment delays for only the largest acknowledged packet in an
# ACK frame.  While the reported acknowledgment delay is not used by
# the RTT sample measurement, it is used to adjust the RTT sample in
# subsequent computations of smoothed_rtt and rttvar (Section 5.3).
# 
# To avoid generating multiple RTT samples for a single packet, an ACK
# frame SHOULD NOT be used to update RTT estimates if it does not newly
# acknowledge the largest acknowledged packet.
# 
# An RTT sample MUST NOT be generated on receiving an ACK frame that
# does not newly acknowledge at least one ack-eliciting packet.  A peer
# usually does not send an ACK frame when only non-ack-eliciting
# packets are received.  Therefore, an ACK frame that contains
# acknowledgments for only non-ack-eliciting packets could include an
# arbitrarily large ACK Delay value.  Ignoring such ACK frames avoids
# complications in subsequent smoothed_rtt and rttvar computations.
# 
# A sender might generate multiple RTT samples per RTT when multiple
# ACK frames are received within an RTT.  As suggested in [RFC6298],
# doing so might result in inadequate history in smoothed_rtt and
# rttvar.  Ensuring that RTT estimates retain sufficient history is an
# open research question.

[[spec]]
level = "SHOULD"
quote = '''
To avoid generating multiple RTT samples for a single packet, an ACK
frame SHOULD NOT be used to update RTT estimates if it does not newly
acknowledge the largest acknowledged packet.
'''

[[spec]]
level = "MUST"
quote = '''
An RTT sample MUST NOT be generated on receiving an ACK frame that
does not newly acknowledge at least one ack-eliciting packet.
'''