rfc9959v2.txt		rfc9959.txt
	skipping to change at line 13 ¶		skipping to change at line 13 ¶
	Request for Comments: 9959 Thales Alenia Space		Request for Comments: 9959 Thales Alenia Space
	Category: Standards Track E. Stephan		Category: Standards Track E. Stephan
	ISSN: 2070-1721 Orange		ISSN: 2070-1721 Orange
	G. Fairhurst		G. Fairhurst
	R. Secchi		R. Secchi
	University of Aberdeen		University of Aberdeen
	C. Huitema		C. Huitema
	Private Octopus Inc.		Private Octopus Inc.
	April 2026		April 2026
	Convergence of Congestion Control from Retained State		Careful Resume: Convergence of Congestion Control from Retained State
	Abstract		Abstract
	This document specifies a cautious method for Internet transports		This document specifies a cautious method for Internet transports
	that enables fast startup of congestion control for a wide range of		that enables fast startup of Congestion Control (CC) for a wide range
	connections, known as "Careful Resume". It reuses a set of computed		of connections, known as "Careful Resume". It reuses a set of
	congestion control parameters that are based on previously observed		computed CC parameters that are based on previously observed path
	path characteristics between the same pair of transport endpoints.		characteristics between the same pair of transport endpoints. These
	These parameters are saved, allowing them to be later used to modify		parameters are saved, allowing them to be later used to modify the CC
	the congestion control behaviour of a subsequent connection.		behaviour of a subsequent connection.
	This document describes the assumptions and defines the requirements		This document describes the assumptions and defines the requirements
	for how a sender utilises these parameters to provide opportunities		for how a sender utilises these parameters to provide opportunities
	for a connection to more rapidly get up to speed and utilise		for a connection to more rapidly get up to speed and utilise
	available capacity. It discusses how the use of this method impacts		available capacity. It discusses how the use of this method impacts
	the capacity at a shared network bottleneck and the safe response		the capacity at a shared network bottleneck and the safe response
	that is needed after any indication that the new rate is		that is needed after any indication that the new rate is
	inappropriate.		inappropriate.
	Status of This Memo		Status of This Memo
	skipping to change at line 126 ¶		skipping to change at line 126 ¶
	All Internet transports are required to either use a Congestion		All Internet transports are required to either use a Congestion
	Control (CC) algorithm or to constrain their rate of transmission		Control (CC) algorithm or to constrain their rate of transmission
	[RFC2914] [RFC8085]. In 2010, a survey of alternative CC algorithms		[RFC2914] [RFC8085]. In 2010, a survey of alternative CC algorithms
	[RFC5783] noted that there are challenges when a CC algorithm		[RFC5783] noted that there are challenges when a CC algorithm
	operates across an Internet path with a high and/or varying		operates across an Internet path with a high and/or varying
	Bandwidth-Delay Product (BDP). The specified method targets a		Bandwidth-Delay Product (BDP). The specified method targets a
	solution for these challenges.		solution for these challenges.
	A CC algorithm typically takes time to ramp up the sending rate.		A CC algorithm typically takes time to ramp up the sending rate.
	This is called the "Slow-Start Phase" and is informally known as the		This is called the "Slow Start Phase" and is informally known as the
	time to "get up to speed". This defines a time during which a sender		time to "get up to speed". This defines a time during which a sender
	intentionally uses less capacity than might be available, with the		intentionally uses less capacity than might be available, with the
	intention to avoid or limit overshoot of the available capacity for		intention to avoid or limit overshoot of the available capacity for
	the path. In the context of CC, a path is associated with the end-		the path. In the context of CC, a path is associated with the end-
	to-end communication between a pair of transport endpoints, each		to-end communication between a pair of transport endpoints, each
	identified by a source IP address and a unicast or anycast		identified by a source IP address and a unicast or anycast
	destination IP address. (This document does not define support for		destination IP address. (This document does not define support for
	broadcast or multicast destination addresses.) A path can also be		broadcast or multicast destination addresses.) A path can also be
	associated with a specific Differentiated Services Code Point (DSCP).		associated with a specific Differentiated Services Code Point (DSCP).
	Below the transport layer, a specific path could be realised in		Below the transport layer, a specific path could be realised in
	skipping to change at line 160 ¶		skipping to change at line 160 ¶
	the capacity at a common bottleneck [RFC2914]).		the capacity at a common bottleneck [RFC2914]).
	A separate instance of a CC algorithm typically executes over a		A separate instance of a CC algorithm typically executes over a
	transport path. This seeks to avoid an increase in the queuing		transport path. This seeks to avoid an increase in the queuing
	(latency or jitter) and/or congestion packet loss for the flow. In		(latency or jitter) and/or congestion packet loss for the flow. In
	the case of a multipath transport, there can be more than one path		the case of a multipath transport, there can be more than one path
	with a separate CC context for each path.		with a separate CC context for each path.
	This document specifies Careful Resume, a method that seeks to reduce		This document specifies Careful Resume, a method that seeks to reduce
	the time to complete a transfer when the sending rate is limited by		the time to complete a transfer when the sending rate is limited by
	the congestion controller using the congestion window (CWND). That		the congestion controller using the congestion window (CWND).
	is, when a transfer seeks to send significantly more data than		Specifically, this is when a transfer seeks to send significantly
	allowed by the initial congestion window (IW) and where the BDP of		more data than allowed by the initial congestion window (IW) and
	the path is also significantly more than the product of the IW and		where the BDP of the path is also significantly more than the product
	path Round Trip Time (RTT).		of the IW and path Round Trip Time (RTT).
	Careful Resume introduces an alternative method to select initial CC		Careful Resume introduces an alternative method to select initial CC
	parameters that seeks to more rapidly and safely grow the sending		parameters that seeks to more rapidly and safely grow the sending
	rate controlled by the CWND.		rate controlled by the CWND.
	Careful Resume is based on temporal sharing (sometimes known as		Careful Resume is based on temporal sharing (sometimes known as
	"caching") of a saved set of CC parameters that relate to previous		"caching") of a saved set of CC parameters that relate to previous
	observations of the same path. The parameters are saved and used to		observations of the same path. The parameters are saved and used to
	modify the CC behaviour of a subsequent connection between the same		modify the CC behaviour of a subsequent connection between the same
	endpoints.		endpoints.
	skipping to change at line 213 ¶		skipping to change at line 213 ¶
	1.2. Receiver Preference		1.2. Receiver Preference
	Whilst the sender could take optimisation decisions without		Whilst the sender could take optimisation decisions without
	considering the receiver's preference, there are cases where a		considering the receiver's preference, there are cases where a
	receiver could have information that is not available at the sender		receiver could have information that is not available at the sender
	or might benefit from understanding that Careful Resume might be		or might benefit from understanding that Careful Resume might be
	used. In these cases, a receiver could use a transport mechanism to		used. In these cases, a receiver could use a transport mechanism to
	explicitly ask to either enable or inhibit Careful Resume when an		explicitly ask to either enable or inhibit Careful Resume when an
	application initiates a new connection.		application initiates a new connection.
	Examples where a receiver might request to inhibit using Careful		Receivers might request the ability to inhibit the use of Careful
	Resume include:		Resume in some situations, for example:
	1. a receiver that can predict the pattern of traffic (e.g., insight		1. when a receiver can predict the pattern of traffic (e.g., insight
	into the volume of data to be sent, the expected length of a		into the volume of data to be sent, the expected length of a
	connection, or the requested maximum transfer rate);		connection, or the requested maximum transfer rate);
	2. a receiver with a local indication that a path/local interface		2. when a receiver with a local indication that a path/local
	has changed since the CC parameters were saved;		interface has changed since the CC parameters were saved;
	3. knowledge of the current hardware limitations at a receiver;		3. when a receiver has knowledge of the current hardware
			limitations;
	4. a receiver that can predict additional capacity will be needed		4. when a receiver can predict additional capacity will be needed
	for other concurrent or later flows (i.e., it prefers to activate		for other concurrent or later flows (i.e., it prefers to activate
	Careful Resume for a different connection).		Careful Resume for a different connection).
	1.3. Transport Protocol Interaction		1.3. Transport Protocol Interaction
	The CWND is one factor that limits the sending rate of a transport		The CWND is one factor that limits the sending rate of a transport
	protocol. Other mechanisms also constrain the maximum sending rate.		protocol. Other mechanisms also constrain the maximum sending rate.
	These include the sender pacing rate and the receiver-advertised		These include the sender pacing rate and the receiver-advertised
	window [RFC9293] or flow credit [RFC9000]; see Section 4.7.		window [RFC9293] or flow control credit [RFC9000]; see Section 4.7.
	1.4. Examples of Scenarios of Interest		1.4. Examples of Scenarios of Interest
	This section provides a set of examples where Careful Resume is		This section provides a set of examples where Careful Resume is
	expected to improve performance. Either endpoint can assume the role		expected to improve performance. Either endpoint can assume the role
	of a sender or a receiver. Careful Resume can also be independently		of a sender or a receiver. Careful Resume can also be independently
	used for each direction of a bidirectional connection.		used for each direction of a bidirectional connection.
	For example, consider an application that uses a series of		For example, consider an application that uses a series of
	connections over a path: Without a new method, each connection would		connections over a path: Without a new method, each connection would
	skipping to change at line 290 ¶		skipping to change at line 291 ¶
	changed so much that the saved values are no longer valid. We		changed so much that the saved values are no longer valid. We
	describe that as the "Reconnaissance Phase". During that phase,		describe that as the "Reconnaissance Phase". During that phase,
	the sender will not send more data than allowed for any new		the sender will not send more data than allowed for any new
	connection, e.g., using the recommended maximum IW for the first		connection, e.g., using the recommended maximum IW for the first
	RTT of transmitting data [RFC6928] [RFC9002]. The sender will		RTT of transmitting data [RFC6928] [RFC9002]. The sender will
	only proceed with the resume process if the reconnaissance		only proceed with the resume process if the reconnaissance
	succeeds. If it fails (for example, if previous packets in a		succeeds. If it fails (for example, if previous packets in a
	connection experience congestion or the RTT is significantly		connection experience congestion or the RTT is significantly
	different), the sender will follow the standard process for a new		different), the sender will follow the standard process for a new
	connection. This provides some protection against aggravating		connection. This provides some protection against aggravating
	severe congestion and to establish the minimum RTT.		severe congestion and allows establishing the minimum RTT.
	2. The second precaution is to cautiously use the saved parameters		2. The second precaution is to cautiously use the saved parameters
	when resuming. This is called the "Unvalidated Phase". For		when resuming. This is called the "Unvalidated Phase". For
	example, the jump in the size of CWND/rate is restricted to a		example, the jump in the size of CWND/rate is restricted to a
	fraction (1/2) of the saved_cwnd, to avoid starving other flows		fraction (1/2) of the saved_cwnd, to avoid starving other flows
	that may have started or increased their capacity after the last		that may have started or increased their capacity after the last
	capacity measurement. The same principle applies for CC		capacity measurement. The same principle applies for CC
	algorithms that use different parameters to classic TCP CC: i.e.,		algorithms that use different parameters to classic TCP CC: i.e.,
	a sender must not send faster than allowed by a fraction of the		a sender must not send faster than allowed by a fraction of the
	saved CC parameters. For example, a connection using a rate-		saved CC parameters. For example, a connection using a rate-
	skipping to change at line 314 ¶		skipping to change at line 315 ¶
	does not exceed the previously used rate. This is intended to		does not exceed the previously used rate. This is intended to
	avoid a sudden influx of packets that could result in building a		avoid a sudden influx of packets that could result in building a
	bottleneck queue and disrupting existing flows. Successful		bottleneck queue and disrupting existing flows. Successful
	validation can allow further increases of the CWND, after first		validation can allow further increases of the CWND, after first
	validating that the used rate did not result in congestion.		validating that the used rate did not result in congestion.
	3. The third precaution is to enter a "Safe Retreat Phase" if the		3. The third precaution is to enter a "Safe Retreat Phase" if the
	validation fails, for example, if congestion is detected during		validation fails, for example, if congestion is detected during
	validation. The risk here is that the trial use of the saved CC		validation. The risk here is that the trial use of the saved CC
	parameters could have disrupted existing connections. For		parameters could have disrupted existing connections. For
	example, consider a connection using Reno CC: When exiting "slow		example, consider a connection using Reno CC: When exiting "Slow
	start" mode due to loss, Reno would normally update the CWND to a		Start" mode due to loss, Reno would normally update the CWND to a
	"slow start threshold" set to half the volume of data in flight.		"Slow Start threshold" set to half the volume of data in flight.
	However, during this validation, the CWND is restored from the		However, during this validation, the CWND is restored from the
	saved CC parameters. The resultant sending rate could be much		saved CC parameters. The resultant sending rate could be much
	larger than the value that would have been reached by a		larger than the value that would have been reached by a
	"standard" slow start process, resulting in an overload of the		"standard" Slow Start process, resulting in an overload of the
	path that potentially could cause significant congestion to other		path that potentially could cause significant congestion to other
	flows. Instead of continuing with that "too large" value, the		flows. Instead of continuing with that "too large" value, the
	retreat process resets the congestion window (rate) to a value no		retreat process resets the congestion window (rate) to a value no
	greater than what a standard process would have discovered. For		greater than what a standard process would have discovered. For
	other CC algorithms, such as Cubic [RFC9438] or BBR, the		other CC algorithms, such as CUBIC [RFC9438] or BBR, the
	implementation details may differ, but the principle remains:		implementation details may differ, but the principle remains:
	Trying and failing should not unduly disadvantage existing		Trying and failing should not unduly disadvantage existing
	connections that share a common bottleneck (e.g., resulting in		connections that share a common bottleneck (e.g., resulting in
	starving these connections).		starving these connections).
	2. Language, Notation, and Terms		2. Language, Notation, and Terms
	This section provides a brief summary of key terms and the		This section provides a brief summary of key terms and the
	requirements language.		requirements language.
	skipping to change at line 370 ¶		skipping to change at line 371 ¶
	information could improve the path differentiation, it could reduce		information could improve the path differentiation, it could reduce
	the reusability of saved parameters.		the reusability of saved parameters.
	The saved CC parameters can only be used to modify the startup when		The saved CC parameters can only be used to modify the startup when
	the Remote Endpoint is not known to have changed (see Section 3.2).		the Remote Endpoint is not known to have changed (see Section 3.2).
	2.3. Logging Support		2.3. Logging Support
	This document defines triggers to support logging key events. For		This document defines triggers to support logging key events. For
	example, [LOG] provides definitions that enable a Careful Resume		example, [LOG] provides definitions that enable a Careful Resume
	implementation to generate qlog events when using QUIC.		implementation to generate QLOG events when using QUIC.
	2.4. Notation and Terms		2.4. Notation and Terms
	The document uses language drawn from a range of RFCs. The following		The document uses language drawn from a range of RFCs. The following
	terms are defined:		terms are defined:
	ACK: The indication at the transport layer that the Remote Endpoint		ACK: The indication at the transport layer that the Remote Endpoint
	has correctly received the acknowledged data. In a CC algorithm,		has correctly received the acknowledged data. In a CC algorithm,
	an ACK also confirms that the acknowledged data is no longer in		an ACK also confirms that the acknowledged data is no longer in
	flight.		flight.
	Beta: A scaling factor between 0.5 and 1; the default value is 0.5.		Beta: A scaling factor between 0.5 and 1; the default value is 0.5.
	Careful Resume (CR): The method specified in this document to select		Careful Resume: The method specified in this document to select
	initial CC parameters and to more rapidly and safely increase the		initial CC parameters and to more rapidly and safely increase the
	initial sending rate.		initial sending rate.
	Congestion Control (CC) parameters: A set of saved CC parameters		Congestion Control (CC) parameters: A set of saved CC parameters
	from observing the capacity of an established connection (see		from observing the capacity of an established connection (see
	Section 1.1).		Section 1.1).
	congestion window (CWND): The congestion window or equivalent CC		congestion window (CWND): The congestion window or equivalent CC
	variable limiting the maximum sending rate (see [RFC5681]).		variable limiting the maximum sending rate (see [RFC5681]).
	skipping to change at line 470 ¶		skipping to change at line 471 ¶
	* Observing (saved_cwnd): The currently utilised capacity for the		* Observing (saved_cwnd): The currently utilised capacity for the
	connection is measured as the volume of bytes sent during an RTT		connection is measured as the volume of bytes sent during an RTT
	and is recorded in the saved_cwnd. This could be computed by		and is recorded in the saved_cwnd. This could be computed by
	measuring the volume of data acknowledged in one RTT. If the		measuring the volume of data acknowledged in one RTT. If the
	measured CWND is less than four times the IW, the sender can		measured CWND is less than four times the IW, the sender can
	choose to not save the CC parameters, because the additional		choose to not save the CC parameters, because the additional
	actions associated with performing Careful Resume for a small CWND		actions associated with performing Careful Resume for a small CWND
	would not justify its use.		would not justify its use.
	* Observing (saved_rtt): The minimum RTT at the time of observation		* Observing (saved_rtt): The minimum RTT at the time of observation
	is saved as the saved_rrt.		is saved as the saved_rtt.
	* Observing (updating saved CC parameters): A sender MUST NOT retain		* Observing (updating saved CC parameters): A sender MUST NOT retain
	more than one set of CC parameters for a Remote Endpoint, but the		more than one set of CC parameters for a Remote Endpoint, but the
	set of CC parameters SHOULD be updated (or replaced) after a later		set of CC parameters SHOULD be updated (or replaced) after a later
	observation of a path to the same Remote Endpoint.		observation of a path to the same Remote Endpoint.
	Implementation notes are provided in Section 4.1.		Implementation notes are provided in Section 4.1.
	3.2. Reconnaissance Phase		3.2. Reconnaissance Phase
	skipping to change at line 495 ¶		skipping to change at line 496 ¶
	The sender enters the Reconnaissance Phase after connection setup		The sender enters the Reconnaissance Phase after connection setup
	(using normal CC). In this phase, the CWND is initialised to the IW,		(using normal CC). In this phase, the CWND is initialised to the IW,
	and the sender transmits any initial data.		and the sender transmits any initial data.
	In the Reconnaissance Phase, the sender performs the following		In the Reconnaissance Phase, the sender performs the following
	action:		action:
	* Reconnaissance Phase (received acknowledgment): The CWND is		* Reconnaissance Phase (received acknowledgment): The CWND is
	increased using normal CC as each ACK confirms delivery of		increased using normal CC as each ACK confirms delivery of
	previously unacknowledged data (i.e., the base congestion control		previously unacknowledged data (i.e., the base CC algorithm
	algorithm continues to operate normally).		continues to operate normally).
	The sender exits the Reconnaissance Phase and stops using Careful		The sender exits the Reconnaissance Phase and stops using Careful
	Resume when one of the following events occurs:		Resume when one of the following events occurs:
	* Reconnaissance Phase (detected congestion): If the sender detects		* Reconnaissance Phase (detected congestion): If the sender detects
	congestion (e.g., packet loss or ECN-CE marking), this indicates		congestion (e.g., packet loss or ECN-CE marking), this indicates
	that use of the saved CC parameters is inappropriate. The sender		that use of the saved CC parameters is inappropriate. The sender
	stops using Careful Resume and MUST continue using normal CC to		stops using Careful Resume and MUST continue using normal CC to
	react to the detected congestion.		react to the detected congestion.
	skipping to change at line 520 ¶		skipping to change at line 521 ¶
	Careful Resume and return to use normal CC.		Careful Resume and return to use normal CC.
	* Reconnaissance Phase (path change): If the Remote Endpoint is not		* Reconnaissance Phase (path change): If the Remote Endpoint is not
	the same as any saved_remote_endpoint, or the sender receives a		the same as any saved_remote_endpoint, or the sender receives a
	signal from the local stack indicating that the path is now		signal from the local stack indicating that the path is now
	different to the observed path, the sender MUST stop using Careful		different to the observed path, the sender MUST stop using Careful
	Resume and returns to use normal CC.		Resume and returns to use normal CC.
	* Reconnaissance Phase (lifetime of saved CC parameters): The CC		* Reconnaissance Phase (lifetime of saved CC parameters): The CC
	parameters are temporal. If the Lifetime of the observed CC		parameters are temporal. If the Lifetime of the observed CC
	parameters is exceeded, this set of CC parameters is not used; the		parameters is exceeded, this set of CC parameters is not used.
	saved CC parameters are deleted and MUST stop using Careful Resume		The saved CC parameters are deleted, and the sender MUST stop
	and returns to use normal CC.		using Careful Resume and return to using normal CC.
	* Reconnaissance Phase (minimum RTT too small): If the minimum RTT		* Reconnaissance Phase (minimum RTT too small): If the minimum RTT
	recorded in the Reconnaissance Phase is less than or equal to		recorded in the Reconnaissance Phase is less than or equal to
	(saved_rtt / 2) (see Section 4.2.1), the sender MUST stop using		(saved_rtt / 2) (see Section 4.2.1), the sender MUST stop using
	Careful Resume (e.g., logged as rtt_not_validated in [LOG]) and		Careful Resume (e.g., logged as rtt_not_validated in [LOG]) and
	returns to use normal CC. This is because the calculation of a		returns to use normal CC. This is because the calculation of a
	sending rate from a saved_cwnd is directly impacted by the RTT;		sending rate from a saved_cwnd is directly impacted by the RTT;
	therefore, a significant change in the RTT is a strong indication		therefore, a significant change in the RTT is a strong indication
	that the previously observed CC parameters are not valid for the		that the previously observed CC parameters are not valid for the
	current path.		current path.
	skipping to change at line 620 ¶		skipping to change at line 621 ¶
	packet): The sender enters the Validating Phase when an ACK is		packet): The sender enters the Validating Phase when an ACK is
	received that acknowledges the first packet number (or higher)		received that acknowledges the first packet number (or higher)
	that was sent in the Unvalidated Phase (e.g., logged as		that was sent in the Unvalidated Phase (e.g., logged as
	first_unvalidated_packet_acknowledged in [LOG]).		first_unvalidated_packet_acknowledged in [LOG]).
	* Unvalidated Phase (limiting time in the Unvalidated Phase): The		* Unvalidated Phase (limiting time in the Unvalidated Phase): The
	sender enters the Validating Phase if more than one RTT has		sender enters the Validating Phase if more than one RTT has
	elapsed while in the Unvalidated Phase (e.g., logged as		elapsed while in the Unvalidated Phase (e.g., logged as
	rtt_exceeded in [LOG]).		rtt_exceeded in [LOG]).
	Unvalidated Phase (check flight_size): Upon any of these events (and		Unvalidated Phase (check flight_size): Upon any of these events, and
	after processing any Acknowledgments that update the PipeSize and		after processing any Acknowledgments that update the PipeSize and
	flight_size), the sender checks if (flight_size is less than the IW)		flight_size, the sender checks if the flight_size is less than the IW
	or (flight_size is less than or equal to the PipeSize) then the CWND		or if the flight_size is less than or equal to the PipeSize, and if
	is reset to the PipeSize (e.g., logged as rate_limited in [LOG]) and		true, resets the CWND to the PipeSize (e.g., logged as rate_limited
	the sender stops using Careful Resume and returns to use normal CC.		in [LOG]) and stops using Careful Resume and returns to use normal
	In the absence of detected congestion, the CWND is not reduced below		CC. In the absence of detected congestion, the CWND is not reduced
	the IW. (The PipeSize does not include the part of the jump_cwnd		below the IW. (The PipeSize does not include the part of the
	that was not utilised.) Otherwise, the CWND MUST be set to the		jump_cwnd that was not utilised.) Otherwise, the CWND MUST be set to
	flight_size and the sender progresses to the Validating Phase.		the flight_size and the sender progresses to the Validating Phase.
	Implementation notes are provided in Section 4.3.		Implementation notes are provided in Section 4.3.
	Notes for BBR are provided in Appendix C.1.		Notes for BBR are provided in Appendix C.1.
	3.4. Validating Phase		3.4. Validating Phase
	The Validating Phase checks whether all packets sent in the		The Validating Phase checks whether all packets sent in the
	Unvalidated Phase were received without inducing congestion. The		Unvalidated Phase were received without inducing congestion. The
	CWND remains unvalidated and the sender typically remains in this		CWND remains unvalidated and the sender typically remains in this
	phase for one RTT.		phase for one RTT.
	In the Validating Phase, the sender performs the following actions:		In the Validating Phase, the sender performs the following actions:
	* Validating Phase (tracking PipeSize): The PipeSize is increased by		* Validating Phase (tracking PipeSize): The PipeSize is increased by
	the volume of acknowledged data for each received ACK that		the volume of acknowledged data for each received ACK that
	indicates a packet was successfully sent over the path.		indicates a packet was successfully sent over the path.
	* Validating Phase (updating the CWND): The CWND is updated using		* Validating Phase (updating the CWND): The CWND is updated using
	the normal rules for the current congestion controller. This		the normal rules for the current congestion controller. This
	typically will use "slow start", which allows the CWND to be		typically will use "Slow Start", which allows the CWND to be
	increased for each received ACK that indicates a packet has been		increased for each received ACK that indicates a packet has been
	successfully sent across the path.		successfully sent across the path.
	The sender exits the Validating Phase when one of the following		The sender exits the Validating Phase when one of the following
	events occurs:		events occurs:
	* Validating Phase (detected congestion): If the sender determines		* Validating Phase (detected congestion): If the sender determines
	that congestion was detected (e.g., packet loss or ECN-CE		that congestion was detected (e.g., packet loss or ECN-CE
	marking), Careful Resume enters the Safe Retreat Phase (e.g.,		marking), Careful Resume enters the Safe Retreat Phase (e.g.,
	logged as either packet_loss or ECN_CE in [LOG]).		logged as either packet_loss or ECN_CE in [LOG]).
	skipping to change at line 720 ¶		skipping to change at line 721 ¶
	* Safe Retreat Phase (acknowledgment of unvalidated packets): When		* Safe Retreat Phase (acknowledgment of unvalidated packets): When
	the last packet (or a later packet) sent during the Unvalidated		the last packet (or a later packet) sent during the Unvalidated
	Phase has been acknowledged, the sender stops using Careful Resume		Phase has been acknowledged, the sender stops using Careful Resume
	and returns to use normal CC.		and returns to use normal CC.
	On leaving the Safe Retreat Phase, the ssthresh MUST be set to no		On leaving the Safe Retreat Phase, the ssthresh MUST be set to no
	larger than the most recently measured PipeSize * Beta, where Beta is		larger than the most recently measured PipeSize * Beta, where Beta is
	a scaling factor between 0.5 and 1. The default value is 0.5, chosen		a scaling factor between 0.5 and 1. The default value is 0.5, chosen
	to reduce the probability of inducing a second round of congestion.		to reduce the probability of inducing a second round of congestion.
	Cubic defines a Beta__cubic of 0.7 [RFC9438] (e.g., logged as		CUBIC defines a Beta__cubic of 0.7 [RFC9438] (e.g., logged as
	exit_recovery in [LOG]).		exit_recovery in [LOG]).
	Implementation notes are provided in Section 4.5.		Implementation notes are provided in Section 4.5.
	Notes for BBR are described in Appendix C.3.		Notes for BBR are described in Appendix C.3.
	3.6. Detecting Persistent Congestion While Using Careful Resume		3.6. Detecting Persistent Congestion While Using Careful Resume
	A sender that experiences persistent congestion (e.g., a		A sender that experiences persistent congestion (e.g., a
	Retransmission Time Out (RTO) or expiry in TCP) ceases to use Careful		Retransmission Time Out (RTO) or expiry in TCP) ceases to use Careful
	skipping to change at line 743 ¶		skipping to change at line 744 ¶
	cause it to enter the Drain state while the "carefully-resuming" flag		cause it to enter the Drain state while the "carefully-resuming" flag
	is set to True; see Appendix C.3.		is set to True; see Appendix C.3.
	As in loss recovery, data sent in the Unvalidated Phase could be		As in loss recovery, data sent in the Unvalidated Phase could be
	later acknowledged after an RTO event.		later acknowledged after an RTO event.
	3.7. Returning to Use Normal CC		3.7. Returning to Use Normal CC
	After exiting Careful Resume, the sender returns to using the normal		After exiting Careful Resume, the sender returns to using the normal
	CC algorithm (e.g., in congestion avoidance when the CWND is more		CC algorithm (e.g., in congestion avoidance when the CWND is more
	than ssthresh, or slow start when less than or equal to ssthresh).		than ssthresh, or Slow Start when less than or equal to ssthresh).
	Implementation notes are provided in Section 4.6.		Implementation notes are provided in Section 4.6.
	4. Implementation Notes and Guidelines		4. Implementation Notes and Guidelines
	This section provides guidance for implementation and use.		This section provides guidance for implementation and use.
	4.1. Observing the Path Capacity		4.1. Observing the Path Capacity
	There are various approaches to measuring the capacity used by a		There are various approaches to measuring the capacity used by a
	connection. Congestion controllers, such as such as Reno [RFC5681]		connection. Congestion controllers, such as Reno [RFC5681] or CUBIC
	or Cubic [RFC9438], could estimate the capacity based on the CWND,		[RFC9438], could estimate the capacity based on the CWND,
	flight_size, acknowledged rate, etc. A different approach could		flight_size, acknowledged rate, etc. A different approach could
	estimate the same parameters for a rate-based congestion controller,		estimate the same parameters for a rate-based congestion controller,
	such as BBR [BBR-CC], or by observing the rate at which data is		such as BBR [BBR-CC], or by observing the rate at which data is
	acknowledged by the Remote Endpoint.		acknowledged by the Remote Endpoint.
	Implementations are required to calculate a saved_rtt, measuring the		Implementations are required to calculate a saved_rtt, measuring the
	minimum RTT while observing the capacity. For example, this could be		minimum RTT while observing the capacity. For example, this could be
	the minimum of a set RTT of measurements measured over the previous 5		the minimum of a set RTT of measurements measured over the previous 5
	minutes.		minutes.
	* There are cases where the current CWND does not reflect the path		* There are cases where the current CWND does not reflect the path
	capacity. At the end of slow start, the CWND can be significantly		capacity. At the end of Slow Start, the CWND can be significantly
	larger than needed to fully utilise the path (i.e., a CWND		larger than needed to fully utilise the path (i.e., a CWND
	overshoot). It is inappropriate to use an overshoot in the CWND		overshoot). It is inappropriate to use an overshoot in the CWND
	as a basis for estimating the capacity. In most cases, the CWND		as a basis for estimating the capacity. In most cases, the CWND
	will converge to a stable value after several more RTTs. One		will converge to a stable value after several more RTTs. One
	mitigation when a connection is in Slow Start could be to set the		mitigation when a connection is in Slow Start could be to set the
	saved_cwnd based on the validated pipe size (i.e., CWND / 2).		saved_cwnd based on the validated pipe size (i.e., CWND / 2).
	* When the sender is rate limited or in the RTT following a burst of		* When the sender is rate limited or in the RTT following a burst of
	transmission, a sender typically transmits less data than allowed		transmission, a sender typically transmits less data than allowed
	by the CWND. Such observations could be discounted when		by the CWND. Such observations could be discounted when
	skipping to change at line 819 ¶		skipping to change at line 820 ¶
	A sender that is rate limited [RFC7661] sends insufficient data to be		A sender that is rate limited [RFC7661] sends insufficient data to be
	able to validate transmission at a higher rate. Such a sender is		able to validate transmission at a higher rate. Such a sender is
	allowed to remain in the Reconnaissance Phase and to not transition		allowed to remain in the Reconnaissance Phase and to not transition
	to the Unvalidated Phase until there is more data in the transmission		to the Unvalidated Phase until there is more data in the transmission
	buffer than would normally be permitted by the CC algorithm.		buffer than would normally be permitted by the CC algorithm.
	4.2.1. Confirming the Path		4.2.1. Confirming the Path
	Path characteristics can change over time for many reasons. This can		Path characteristics can change over time for many reasons. This can
	result in the previously observed CC parameters becoming irrelevant.		result in the previously observed CC parameters becoming irrelevant.
	To help confirm the path, the sender compares the saved_rrt with each		To help confirm the path, the sender compares the saved_rtt with each
	current RTT sample.		current RTT sample.
	If the current RTT sample is less than a half of the saved_rrt, this		If the current RTT sample is less than a half of the saved_rtt, this
	is regarded as too small. This is an indicator of a path change.		is regarded as too small. This is an indicator of a path change.
	This factor of two arises because the jump_cwnd is calculated as half		This factor of two arises because the jump_cwnd is calculated as half
	the measured saved_cwnd and the sending rate ought not to exceed the		the measured saved_cwnd and the sending rate ought not to exceed the
	observed rate when the saved_cwnd was measured.		observed rate when the saved_cwnd was measured.
	If the current RTT is larger than the saved_rtt, this would result in		If the current RTT is larger than the saved_rtt, this would result in
	a proportionally lower rate for the unvalidated packets, because the		a proportionally lower rate for the unvalidated packets, because the
	transmission is paced based on the current RTT. Hence, this rate is		transmission is paced based on the current RTT. Hence, this rate is
	still safe. If the current RTT has been incorrectly measured as		still safe. If the current RTT has been incorrectly measured as
	larger than the actual path RTT, the sender will receive an ACK for		larger than the actual path RTT, the sender will receive an ACK for
	an unvalidated packet before it has completed the Unvalidated Phase.		an unvalidated packet before it has completed the Unvalidated Phase.
	This ACK resets the CWND to reflect the flight_size, and the sender		This ACK resets the CWND to reflect the flight_size, and the sender
	then enters the Validating Phase. The flight_size reflects the		then enters the Validating Phase. The flight_size reflects the
	amount of outstanding data in the network rather than the maximum		amount of outstanding data in the network rather than the maximum
	that is permitted by the CWND.		that is permitted by the CWND.
	A current RTT that is more than ten times the saved_rrt is indicative		A current RTT that is more than ten times the saved_rtt is indicative
	of a path change. The value of ten accommodates both increases in		of a path change. The value of ten accommodates both increases in
	latency from buffering on a path and any variation between RTT		latency from buffering on a path and any variation between RTT
	samples.		samples.
	Note 1: In the Reconnaissance Phase, the sender calculates a minimum		Note 1: In the Reconnaissance Phase, the sender calculates a minimum
	RTT over the phase and checks this on entry to the Unvalidated Phase.		RTT over the phase and checks this on entry to the Unvalidated Phase.
	This avoids a need to check after each current RTT sample.		This avoids a need to check after each current RTT sample.
	Note 2: During the Unvalidated Phase, the minimum RTT cannot		Note 2: During the Unvalidated Phase, the minimum RTT cannot
	increase, and hence the minimum RTT can never be larger than		increase, and hence the minimum RTT can never be larger than
	(saved_rtt x 10) during the Unvalidated Phase.		(saved_rtt x 10) during the Unvalidated Phase.
	The sender also verifies that the initial data was acknowledged. Any		The sender also verifies that the initial data was acknowledged. Any
	loss could be indicative of persistent congestion. If a sender in		loss could be indicative of persistent congestion. If a sender in
	the Reconnaissance Phase detects congestion, it stops using Careful		the Reconnaissance Phase detects congestion, it stops using Careful
	Resume and returns to using normal CC. Some transport protocols		Resume and returns to using normal CC. Some transport protocols
	implement CC mechanisms that infer potential congestion from an		implement CC mechanisms that infer potential congestion from an
	increase in the current RTT. Designs need to consider if such an		increase in the current RTT. Designs need to consider whether such
	indication is a suitable trigger to revert to stop using Careful		an indication is a suitable trigger to stop using Careful Resume and
	Resume.		revert to using normal CC.
	4.3. Safety in the Unvalidated Phase		4.3. Safety in the Unvalidated Phase
	This section considers the safety for using saved CC parameters to		This section considers the safety for using saved CC parameters to
	tentatively update the CWND. This seeks to avoid starving other		tentatively update the CWND. This seeks to avoid starving other
	flows that could have either started or increased their use of		flows that could have either started or increased their use of
	capacity since observing the capacity of a path.		capacity since observing the capacity of a path.
	To avoid inducing significant congestion to any connections that have		To avoid inducing significant congestion to any connections that have
	started to use a shared bottleneck, a sender must not directly use		started to use a shared bottleneck, a sender must not directly use
	skipping to change at line 903 ¶		skipping to change at line 904 ¶
	period. However, [RFC7661] targets rate-limited connections using		period. However, [RFC7661] targets rate-limited connections using
	normal CC. Careful Resume includes additional mechanisms to avoid		normal CC. Careful Resume includes additional mechanisms to avoid
	and mitigate the effects of overshoot, and therefore a longer period		and mitigate the effects of overshoot, and therefore a longer period
	can be justified when using a saved_cwnd with Careful Resume.		can be justified when using a saved_cwnd with Careful Resume.
	When the path characteristics are known to be dynamic, or the path		When the path characteristics are known to be dynamic, or the path
	varies, a small Lifetime is desirable (e.g., measured in minutes).		varies, a small Lifetime is desirable (e.g., measured in minutes).
	For stable paths, and where the sender does not expect the path to be		For stable paths, and where the sender does not expect the path to be
	shared by many senders, a longer Lifetime (e.g., measured in hours)		shared by many senders, a longer Lifetime (e.g., measured in hours)
	could be used. A bottleneck that is shared by a large number of		could be used. A bottleneck that is shared by a large number of
	senders brings greater risk that CR connections could contribute		senders brings greater risk that Careful Resume connections could
	congestion that leads to prolonged overload with starvation. This		contribute congestion that leads to prolonged overload with
	can be mitigated by setting a small Lifetime.		starvation. This can be mitigated by setting a small Lifetime.
	4.3.2. Pacing in the Unvalidated Phase		4.3.2. Pacing in the Unvalidated Phase
	A sender needs to avoid any step increase in the CWND resulting in a		A sender needs to avoid any step increase in the CWND resulting in a
	burst of packets that is greater than the size of the CC algorithm's		burst of packets that is greater than the size of the CC algorithm's
	IW. This is consistent with [RFC8085] and [RFC9000].		IW. This is consistent with [RFC8085] and [RFC9000].
	Pacing packets as a function of the current RTT, rather than the		Pacing packets as a function of the current RTT, rather than the
	saved_rrt, provides additional safety during the Unvalidated Phase,		saved_rtt, provides additional safety during the Unvalidated Phase,
	because it avoids a smaller saved_rrt inflating the sending rate.		because it avoids a smaller saved_rtt inflating the sending rate.
	The lower bound to the minimum acceptable current RTT avoids sending		The lower bound to the minimum acceptable current RTT avoids sending
	unvalidated packets at a rate that would be higher than was		unvalidated packets at a rate that would be higher than was
	previously observed.		previously observed.
	The following example provides a relevant pacing rhythm: An Inter-		The following example provides a relevant pacing rhythm: An Inter-
	packet Transmission Time (ITT) is determined by using the current		packet Transmission Time (ITT) is determined by using the current
	Maximum Packet Size (MPS), including headers, the saved_cwnd, and the		Maximum Packet Size (MPS), including headers, the saved_cwnd, and the
	current RTT. A safety margin can be configured to avoid sending more		current RTT. A safety margin can be configured to avoid sending more
	than a maximum (max_jump):		than a maximum (max_jump):
	skipping to change at line 986 ¶		skipping to change at line 987 ¶
	for unvalidated packets.		for unvalidated packets.
	The Safe Retreat Phase sets a safe CWND value to drain any		The Safe Retreat Phase sets a safe CWND value to drain any
	unvalidated packets from the path after a packet loss has been		unvalidated packets from the path after a packet loss has been
	detected or when ACKs that indicate the sent packets were marked as		detected or when ACKs that indicate the sent packets were marked as
	ECN-CE. The CC parameters that were used are invalid and are		ECN-CE. The CC parameters that were used are invalid and are
	removed.		removed.
	The Safe Retreat reaction differs from a traditional reaction to		The Safe Retreat reaction differs from a traditional reaction to
	detected congestion, because a jump_cwnd can result in a		detected congestion, because a jump_cwnd can result in a
	significantly higher rate than would be allowed by Slow-Start. Such		significantly higher rate than would be allowed by Slow Start. Such
	a jump could aggressively feed a congested bottleneck, resulting in		a jump could aggressively feed a congested bottleneck, resulting in
	overshoot where a disproportionate number of packets from existing		overshoot where a disproportionate number of packets from existing
	flows are displaced from the buffer at the congested bottleneck. For		flows are displaced from the buffer at the congested bottleneck. For
	this reason, a sender in the Safe Retreat Phase needs to react to		this reason, a sender in the Safe Retreat Phase needs to react to
	detected congestion by reducing the CWND significantly below the		detected congestion by reducing the CWND significantly below the
	saved_cwnd.		saved_cwnd.
	During loss recovery, a receiver can cumulatively acknowledge data		During loss recovery, a receiver can cumulatively acknowledge data
	that was previously sent in the Unvalidated Phase in addition to		that was previously sent in the Unvalidated Phase in addition to
	acknowledging the successful retransmission of data. [RFC3465]		acknowledging the successful retransmission of data. [RFC3465]
	skipping to change at line 1115 ¶		skipping to change at line 1116 ¶
	if a server is configured to send using Careful Resume, there is an		if a server is configured to send using Careful Resume, there is an
	onus to appropriately manage the use of saved CC parameters (see		onus to appropriately manage the use of saved CC parameters (see
	Section 4.2).		Section 4.2).
	The way in which this is realised will depend upon the design choices		The way in which this is realised will depend upon the design choices
	in configuring the network and the servers. On the one hand, if all		in configuring the network and the servers. On the one hand, if all
	the servers responding to a given IP address share the same location		the servers responding to a given IP address share the same location
	(e.g., are in the same data center), then a method could be provided		(e.g., are in the same data center), then a method could be provided
	to coordinate their sharing of the CC parameters that are used to		to coordinate their sharing of the CC parameters that are used to
	send data using Careful Resume. On the other hand, if the service		send data using Careful Resume. On the other hand, if the service
	configuration is such that subsquent use of the IP anycast address		configuration is such that subsequent use of the IP anycast address
	might result in a very different path to a server (e.g., at a		might result in a very different path to a server (e.g., at a
	different location where the path would be unable to support the same		different location where the path would be unable to support the same
	capacity), a sender should not use Careful Resume based on saved CC		capacity), a sender should not use Careful Resume based on saved CC
	parameters.		parameters.
	6. IANA Considerations		6. IANA Considerations
	This document has no IANA actions.		This document has no IANA actions.
	7. Security Considerations		7. Security Considerations
	The security considerations are the same as for other sender-based		The security considerations are the same as for other sender-based CC
	congestion control methods. Such methods rely on the receiver		methods. Such methods rely on the receiver appropriately
	appropriately acknowledging receipt of data. The ability of an on-		acknowledging receipt of data. The ability of an on-path or off-path
	path or off-path attacker to influence congestion control depends		attacker to influence CC depends upon the security properties of the
	upon the security properties of the transport protocol being used.		transport protocol being used.
	8. References		8. References
	8.1. Normative References		8.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	skipping to change at line 1283 ¶		skipping to change at line 1284 ¶
	restart-00>.		restart-00>.
	Appendix A. Notes on the Careful Resume Phases		Appendix A. Notes on the Careful Resume Phases
	The table below is provided to illustrate the operation of Careful		The table below is provided to illustrate the operation of Careful
	Resume. This table is informative; please refer to the body of the		Resume. This table is informative; please refer to the body of the
	document for the normative specification. The description is based		document for the normative specification. The description is based
	on a normal CC that uses Reno. The PipeSize tracks the validated		on a normal CC that uses Reno. The PipeSize tracks the validated
	CWND.		CWND.
	The table uses the following abbreviations:		The phases in Table 1 correspond to Sections 3.2 through 3.5. This
			table also uses the following abbreviations:
	SS = Slow-Start		SS = Slow Start
	FS = flight_size		FS = flight_size
	PS = PipeSize		PS = PipeSize
	ACK = highest acknowledged packet		ACK = highest acknowledged packet
			CC =Congestion Control
			CR = Careful Resume
			IW = Initial congestion Window
			CWND = Congestion Window
	+======+=========+============+=============+===========+===========+		+=======+=========+============+============+===========+===========+
	|Phase |Normal |Recon. |Unvalidated |Validating |Safe Ret. |		|Phase |Normal |Recon. |Unvalidated |Validating |Safe Ret. |
	+======+=========+============+=============+===========+===========+		+=======+=========+============+============+===========+===========+
	| |Observing|Confirm path|Send faster |Validate |Drain path;|		| |Observing|Confirm path|Send faster |Validate |Drain path;|
	| |CC params| |using |new CWND; |Update PS |		| |CC params| |using |new CWND; |Update PS |
	| | | |saved_cwnd |Update PS | |		| | | |saved_cwnd |Update PS | |
	+------+---------+------------+-------------+-----------+-----------+		+-------+---------+------------+------------+-----------+-----------+
	|On | - |CWND=IW |PS=FS |If (FS>PS) |CWND=(PS/2)|		|On | - |CWND=IW |PS=FS |If (FS>PS) |CWND=(PS/2)|
	|entry:| | |jump_cwnd |{CWND=FS} | |		|entry: | | |jump_cwnd |{CWND=FS} | |
	| | | |=saved_cwnd |else | |		| | | |=saved_cwnd |else | |
	| | | |/2; CWND |{CWND=PS; | |		| | | |/2; CWND |{CWND=PS; | |
	| | | |=jump_cwnd |use Normal | |		| | | |=jump_cwnd |use Normal | |
	| | | | |CC} | |		| | | | |CC} | |
	+------+---------+------------+-------------+-----------+-----------+		+-------+---------+------------+------------+-----------+-----------+
	|CWND: |When in |CWND |CWND is not |CWND can |CWND is not|		|CWND: |When in |CWND |CWND is not |CWND can |CWND is not|
	| |observe, |increases |increased |increase |increased |		| |observe, |increases |increased |increase |increased |
	| |measure |using SS | |using | |		| |measure |using SS | |using | |
	| |saved | | |normal CC | |		| |saved | | |normal CC | |
	| |_cwnd | | | | |		| |_cwnd | | | | |
	+------+---------+------------+-------------+-----------+-----------+		+-------+---------+------------+------------+-----------+-----------+
	|PS: | - | - |PS+=ACked |		|PS: | - | - |PS+=ACKed |
	+------+---------+------------+-------------+-----------+-----------+		+-------+---------+------------+------------+-----------+-----------+
	|RTT: |Measure |Measure | - | - | - |		|RTT: |Measure |Measure | - | - | - |
	| |saved_rtt|current RTT | | | |		| |saved_rtt|current RTT | | | |
	+------+---------+------------+-------------+-----------+-----------+		+-------+---------+------------+------------+-----------+-----------+
	|If |Normal CC|Normal CC; |Enter Safe Retreat | - |		|If loss|Normal CC|Normal CC; |Enter Safe Retreat | - |
	|loss | |CR is not | | |		|or ECN-| |CR is not | | |
	|or | |allowed | | |		|CE: | |allowed | | |
	|ECNCE:| | | | |		+-------+---------+------------+------------+-----------+-----------+
	+------+---------+------------+-------------+-----------+-----------+		|Next |Observing|If (CWND, |If (FS>CWND |If (ACK >= |If (ACK >= |
	|Next |Observing|If (CWND, |If (FS>CWND |If (ACK >= |If (ACK >= |		|Phase: |(as |Lifetime, |or >1 RTT |last |last |
	|Phase:|(as |Lifetime, |or >1 RTT |last |last |		| |needed) |and RTT |has passed |unvalidated|unvalidated|
	| |needed) |and RTT |has passed |unvalidated|unvalidated|		| | |confirmed), |or ACK >= |packet), |packet), |
	| | |confirmed), |or ACK >= |packet), |packet), |		| | |enter |first |use Normal |ssthresh = |
	| | |enter |first |use Normal |ssthresh = |		| | |Unvalidated;|unvalidated |CC |PS x Beta; |
	| | |Unvalidated;|unvalidated |CC |PS x Beta; |		| | |else |packet), If | |and use |
	| | |else |packet), If | |and use |		| | |CWND=Max |((FS>CWND) | |Normal CC |
	| | |CWND=Max |((FS>CWND) | |Normal CC |		| | |(PS,IW); and|or | | |
	| | |(PS,IW); and|or (FS<=PS)) | | |		| | |use Normal |(FS<=PS)) | | |
	| | |use Normal |cwnd=PS; and | | |		| | |CC |cwnd=PS; | | |
	| | |CC |use Normal | | |		| | | |and use | | |
	| | | |CC else | | |		| | | |Normal CC | | |
	| | | |cwnd=FS; | | |		| | | |else | | |
	| | | |enter | | |		| | | |cwnd=FS; | | |
	| | | |Validating | | |		| | | |enter | | |
	+------+---------+------------+-------------+-----------+-----------+		| | | |Validating | | |
			+-------+---------+------------+------------+-----------+-----------+
	Table 1: Illustration of the Operation of Careful Resume		Table 1: Illustration of the Operation of Careful Resume
	Appendix B. Examples of the Careful Resume Phases		Appendix B. Examples of the Careful Resume Phases
	The following examples consider an implementation that keeps track of		The following examples consider an implementation that keeps track of
	transmitted data in terms of packets and provide informative examples		transmitted data in terms of packets and provide informative examples
	of use.		of use.
	In the Unvalidated Phase, the first unvalidated packet corresponds to		In the Unvalidated Phase, the first unvalidated packet corresponds to
	skipping to change at line 1390 ¶		skipping to change at line 1397 ¶
	When the first unvalidated packet is acknowledged (packet number 30),		When the first unvalidated packet is acknowledged (packet number 30),
	the sender enters the Validating Phase. (This transition would also		the sender enters the Validating Phase. (This transition would also
	occur if the flight_size increased to equal the CWND.) During this		occur if the flight_size increased to equal the CWND.) During this
	phase, the CWND can be increased for each ACK that acknowledges an		phase, the CWND can be increased for each ACK that acknowledges an
	unvalidated packet, because this indicates that the packet was		unvalidated packet, because this indicates that the packet was
	validated.		validated.
	When an ACK is received that acknowledges the last packet that was		When an ACK is received that acknowledges the last packet that was
	sent in the Unvalidated Phase, the sender stops using Careful Resume.		sent in the Unvalidated Phase, the sender stops using Careful Resume.
	For example, if the CWND is less than ssthresh, a Reno or Cubic		For example, if the CWND is less than ssthresh, a Reno or CUBIC
	sender using normal CC is permitted to use Slow-Start to grow the		sender using normal CC is permitted to use Slow Start to grow the
	CWND towards the ssthresh and will then enter congestion avoidance.		CWND towards the ssthresh and will then enter congestion avoidance.
	B.2. Example with No Loss, Rate Limited		B.2. Example with No Loss, Rate Limited
	A rate-limited sender will not fully utilise the available CWND when		A rate-limited sender will not fully utilise the available CWND when
	using Careful Resume, and the CWND is therefore reset on entry to the		using Careful Resume, and the CWND is therefore reset on entry to the
	Validating Phase, as described below.		Validating Phase, as described below.
	The sender starts by sending up to IW packets (10) in the		The sender starts by sending up to IW packets (10) in the
	Reconnaissance Phase. It commences as described in the first		Reconnaissance Phase. It commences as described in the first
	skipping to change at line 1464 ¶		skipping to change at line 1471 ¶
	acknowledged. This conservative CWND calculation ensures the sender		acknowledged. This conservative CWND calculation ensures the sender
	drains the path after this potentially severe congestion event.		drains the path after this potentially severe congestion event.
	There is no further increase in the CWND in this phase.		There is no further increase in the CWND in this phase.
	The sender continues to receive ACKs that acknowledge the remaining		The sender continues to receive ACKs that acknowledge the remaining
	86 (121-35) unvalidated packets. Recall that the 35th unvalidated		86 (121-35) unvalidated packets. Recall that the 35th unvalidated
	packet was lost and had packet number 64 (29+35). The PipeSize		packet was lost and had packet number 64 (29+35). The PipeSize
	tracks the capacity discovered by ACKs that acknowledge the		tracks the capacity discovered by ACKs that acknowledge the
	unvalidated packets (i.e., the PipeSize is increased for each		unvalidated packets (i.e., the PipeSize is increased for each
	received ACK that acknowledges new data). Although this PipeSize		received ACK that acknowledges new data). Although this PipeSize
	cannot be used to safety initialise the CWND (because it was measured		cannot be used to safely initialise the CWND (because it was measured
	when the sender had aggressively created overload), the estimated		when the sender had aggressively created overload), the estimated
	PipeSize (which, in this case, is 121-1 = 120 packets) can be used to		PipeSize (which, in this case, is 121-1 = 120 packets) can be used to
	set the ssthresh on exit from Safe Retreat, since it does indicate a		set the ssthresh on exit from Safe Retreat, since it does indicate a
	measured upper limit to the current capacity.		measured upper limit to the current capacity.
	At the point where all the unvalidated packets that were sent in the		At the point where all the unvalidated packets that were sent in the
	Unvalidated Phase have been either acknowledged or have been declared		Unvalidated Phase have been either acknowledged or have been declared
	lost, the sender updates the ssthresh to be no larger than the		lost, the sender updates the ssthresh to be no larger than the
	recently measured PipeSize multiplied by Beta (the final action of		recently measured PipeSize multiplied by Beta (the final action of
	the Safe Retreat Phase), and the sender stops using Careful Resume.		the Safe Retreat Phase), and the sender stops using Careful Resume.
	Because the CWND will now be less than ssthresh, a sender using		Because the CWND will now be less than ssthresh, a sender using
	normal CC is permitted to use Slow-Start to grow the CWND towards the		normal CC is permitted to use Slow Start to grow the CWND towards the
	ssthresh, after which it will enter congestion avoidance.		ssthresh, after which it will enter congestion avoidance.
	Appendix C. Implementation Notes for Using BBR		Appendix C. Implementation Notes for Using BBR
	Bottleneck Bandwidth and Round-trip propagation time (BBR) uses		Bottleneck Bandwidth and Round-trip propagation time (BBR) uses
	recent measurements of a transport connection's delivery rate, Round		recent measurements of a transport connection's delivery rate, Round
	Trip Time (RTT), and packet loss rate to build an explicit model of		Trip Time (RTT), and packet loss rate to build an explicit model of
	the network path. BBR then uses this model to control both how fast		the network path. BBR then uses this model to control both how fast
	it sends data and the maximum volume of data it allows in flight in		it sends data and the maximum volume of data it allows in flight in
	the network at any time [BBR-CC].		the network at any time [BBR-CC].
End of changes. 42 change blocks.
	120 lines changed or deleted		127 lines changed or added
This html diff was produced by rfcdiff 1.48.