target = "https://www.rfc-editor.org/rfc/rfc9002#section-6.2.4"

# 6.2.4.  Sending Probe Packets
#
# When a PTO timer expires, a sender MUST send at least one ack-
# eliciting packet in the packet number space as a probe.  An endpoint
# MAY send up to two full-sized datagrams containing ack-eliciting
# packets to avoid an expensive consecutive PTO expiration due to a
# single lost datagram or to transmit data from multiple packet number
# spaces.  All probe packets sent on a PTO MUST be ack-eliciting.
# 
# In addition to sending data in the packet number space for which the
# timer expired, the sender SHOULD send ack-eliciting packets from
# other packet number spaces with in-flight data, coalescing packets if
# possible.  This is particularly valuable when the server has both
# Initial and Handshake data in flight or when the client has both
# Handshake and Application Data in flight because the peer might only
# have receive keys for one of the two packet number spaces.
# 
# If the sender wants to elicit a faster acknowledgment on PTO, it can
# skip a packet number to eliminate the acknowledgment delay.
# 
# An endpoint SHOULD include new data in packets that are sent on PTO
# expiration.  Previously sent data MAY be sent if no new data can be
# sent.  Implementations MAY use alternative strategies for determining
# the content of probe packets, including sending new or retransmitted
# data based on the application's priorities.
# 
# It is possible the sender has no new or previously sent data to send.
# As an example, consider the following sequence of events: new
# application data is sent in a STREAM frame, deemed lost, then
# retransmitted in a new packet, and then the original transmission is
# acknowledged.  When there is no data to send, the sender SHOULD send
# a PING or other ack-eliciting frame in a single packet, rearming the
# PTO timer.
# 
# Alternatively, instead of sending an ack-eliciting packet, the sender
# MAY mark any packets still in flight as lost.  Doing so avoids
# sending an additional packet but increases the risk that loss is
# declared too aggressively, resulting in an unnecessary rate reduction
# by the congestion controller.
# 
# Consecutive PTO periods increase exponentially, and as a result,
# connection recovery latency increases exponentially as packets
# continue to be dropped in the network.  Sending two packets on PTO
# expiration increases resilience to packet drops, thus reducing the
# probability of consecutive PTO events.
# 
# When the PTO timer expires multiple times and new data cannot be
# sent, implementations must choose between sending the same payload
# every time or sending different payloads.  Sending the same payload
# may be simpler and ensures the highest priority frames arrive first.
# Sending different payloads each time reduces the chances of spurious
# retransmission.

[[spec]]
level = "MUST"
quote = '''
When a PTO timer expires, a sender MUST send at least one ack-
eliciting packet in the packet number space as a probe.
'''

[[spec]]
level = "MAY"
quote = '''
An endpoint
MAY send up to two full-sized datagrams containing ack-eliciting
packets to avoid an expensive consecutive PTO expiration due to a
single lost datagram or to transmit data from multiple packet number
spaces.
'''

[[spec]]
level = "MUST"
quote = '''
All probe packets sent on a PTO MUST be ack-eliciting.
'''

[[spec]]
level = "SHOULD"
quote = '''
In addition to sending data in the packet number space for which the
timer expired, the sender SHOULD send ack-eliciting packets from
other packet number spaces with in-flight data, coalescing packets if
possible.
'''

[[spec]]
level = "SHOULD"
quote = '''
An endpoint SHOULD include new data in packets that are sent on PTO
expiration.
'''

[[spec]]
level = "MAY"
quote = '''
Previously sent data MAY be sent if no new data can be
sent.
'''

[[spec]]
level = "MAY"
quote = '''
Implementations MAY use alternative strategies for determining
the content of probe packets, including sending new or retransmitted
data based on the application's priorities.
'''

[[spec]]
level = "SHOULD"
quote = '''
When there is no data to send, the sender SHOULD send
a PING or other ack-eliciting frame in a single packet, rearming the
PTO timer.
'''

[[spec]]
level = "MAY"
quote = '''
Alternatively, instead of sending an ack-eliciting packet, the sender
MAY mark any packets still in flight as lost.
'''