target = "https://www.rfc-editor.org/rfc/rfc9000#section-19.3"

# 19.3.  ACK Frames
#
# Receivers send ACK frames (types 0x02 and 0x03) to inform senders of
# packets they have received and processed.  The ACK frame contains one
# or more ACK Ranges.  ACK Ranges identify acknowledged packets.  If
# the frame type is 0x03, ACK frames also contain the cumulative count
# of QUIC packets with associated ECN marks received on the connection
# up until this point.  QUIC implementations MUST properly handle both
# types, and, if they have enabled ECN for packets they send, they
# SHOULD use the information in the ECN section to manage their
# congestion state.
# 
# QUIC acknowledgments are irrevocable.  Once acknowledged, a packet
# remains acknowledged, even if it does not appear in a future ACK
# frame.  This is unlike reneging for TCP Selective Acknowledgments
# (SACKs) [RFC2018].
# 
# Packets from different packet number spaces can be identified using
# the same numeric value.  An acknowledgment for a packet needs to
# indicate both a packet number and a packet number space.  This is
# accomplished by having each ACK frame only acknowledge packet numbers
# in the same space as the packet in which the ACK frame is contained.
# 
# Version Negotiation and Retry packets cannot be acknowledged because
# they do not contain a packet number.  Rather than relying on ACK
# frames, these packets are implicitly acknowledged by the next Initial
# packet sent by the client.
# 
# ACK frames are formatted as shown in Figure 25.
# 
# ACK Frame {
#   Type (i) = 0x02..0x03,
#   Largest Acknowledged (i),
#   ACK Delay (i),
#   ACK Range Count (i),
#   First ACK Range (i),
#   ACK Range (..) ...,
#   [ECN Counts (..)],
# }
# 
#                      Figure 25: ACK Frame Format
# 
# ACK frames contain the following fields:
# 
# Largest Acknowledged:  A variable-length integer representing the
#    largest packet number the peer is acknowledging; this is usually
#    the largest packet number that the peer has received prior to
#    generating the ACK frame.  Unlike the packet number in the QUIC
#    long or short header, the value in an ACK frame is not truncated.
# 
# ACK Delay:  A variable-length integer encoding the acknowledgment
#    delay in microseconds; see Section 13.2.5.  It is decoded by
#    multiplying the value in the field by 2 to the power of the
#    ack_delay_exponent transport parameter sent by the sender of the
#    ACK frame; see Section 18.2.  Compared to simply expressing the
#    delay as an integer, this encoding allows for a larger range of
#    values within the same number of bytes, at the cost of lower
#    resolution.
# 
# ACK Range Count:  A variable-length integer specifying the number of
#    ACK Range fields in the frame.
# 
# First ACK Range:  A variable-length integer indicating the number of
#    contiguous packets preceding the Largest Acknowledged that are
#    being acknowledged.  That is, the smallest packet acknowledged in
#    the range is determined by subtracting the First ACK Range value
#    from the Largest Acknowledged field.
# 
# ACK Ranges:  Contains additional ranges of packets that are
#    alternately not acknowledged (Gap) and acknowledged (ACK Range);
#    see Section 19.3.1.
# 
# ECN Counts:  The three ECN counts; see Section 19.3.2.

[[spec]]
level = "MUST"
quote = '''
QUIC implementations MUST properly handle both
types, and, if they have enabled ECN for packets they send, they
SHOULD use the information in the ECN section to manage their
congestion state.
'''