BBR Congestion Control
draft-cardwell-iccrg-bbr-congestion-control-01

Document Type Active Internet-Draft (individual)
Authors Neal Cardwell  , Yuchung Cheng  , Soheil Yeganeh  , Ian Swett  , Van Jacobson 
Last updated 2021-11-07
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Internet Congestion Control Research Group                   N. Cardwell
Internet-Draft                                                  Y. Cheng
Intended status: Experimental                          S. Hassas Yeganeh
Expires: 12 May 2022                                            I. Swett
                                                             V. Jacobson
                                                                  Google
                                                         8 November 2021

                         BBR Congestion Control
             draft-cardwell-iccrg-bbr-congestion-control-01

Abstract

   This document specifies the BBR congestion control algorithm.  BBR
   ("Bottleneck Bandwidth and Round-trip propagation time") uses recent
   measurements of a transport connection's delivery rate, round-trip
   time, and packet loss rate to build an explicit model of 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 the
   network at any time.  Relative to loss-based congestion control
   algorithms such as Reno [RFC5681] or CUBIC [RFC8312], BBR offers
   substantially higher throughput for bottlenecks with shallow buffers
   or random losses, and substantially lower queueing delays for
   bottlenecks with deep buffers (avoiding "bufferbloat").  BBR can be
   implemented in any transport protocol that supports packet-delivery
   acknowledgment.  Thus far, open source implementations are available
   for TCP [RFC793] and QUIC [RFC9000].  This document specifies version
   2 of the BBR algorithm, also sometimes referred to as BBRv2 or bbr2.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 12 May 2022.

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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Transport Connection State  . . . . . . . . . . . . . . .   5
     2.2.  Per-Packet State  . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Per-ACK Rate Sample State . . . . . . . . . . . . . . . .   5
     2.4.  Output Control Parameters . . . . . . . . . . . . . . . .   6
     2.5.  Pacing State and Parameters . . . . . . . . . . . . . . .   6
     2.6.  cwnd State and Parameters . . . . . . . . . . . . . . . .   7
     2.7.  General Algorithm State . . . . . . . . . . . . . . . . .   7
     2.8.  Core Algorithm Design Parameters  . . . . . . . . . . . .   7
     2.9.  Network Path Model Parameters . . . . . . . . . . . . . .   8
       2.9.1.  Data Rate Network Path Model Parameters . . . . . . .   8
       2.9.2.  Data Volume Network Path Model Parameters . . . . . .   8
     2.10. State for Responding to Congestion  . . . . . . . . . . .   9
     2.11. Estimating BBR.max_bw . . . . . . . . . . . . . . . . . .  10
     2.12. Estimating BBR.extra_acked  . . . . . . . . . . . . . . .  10
     2.13. Startup Parameters and State  . . . . . . . . . . . . . .  10
     2.14. ProbeRTT and min_rtt Parameters and State . . . . . . . .  10
       2.14.1.  Parameters for Estimating BBR.min_rtt  . . . . . . .  10
       2.14.2.  Parameters for Scheduling ProbeRTT . . . . . . . . .  11
   3.  Design Overview . . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  High-Level Design Goals . . . . . . . . . . . . . . . . .  11
     3.2.  Algorithm Overview  . . . . . . . . . . . . . . . . . . .  12
     3.3.  State Machine Overview  . . . . . . . . . . . . . . . . .  13
     3.4.  Network Path Model Overview . . . . . . . . . . . . . . .  13
       3.4.1.  High-Level Design Goals for the Network Path Model  .  13
       3.4.2.  Time Scales for the Network Model . . . . . . . . . .  14
     3.5.  Control Parameter Overview  . . . . . . . . . . . . . . .  15
     3.6.  Environment and Usage . . . . . . . . . . . . . . . . . .  15
   4.  Detailed Algorithm  . . . . . . . . . . . . . . . . . . . . .  15
     4.1.  State Machine . . . . . . . . . . . . . . . . . . . . . .  15
       4.1.1.  State Transition Diagram  . . . . . . . . . . . . . .  15

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       4.1.2.  State Machine Operation Overview  . . . . . . . . . .  16
       4.1.3.  State Machine Tactics . . . . . . . . . . . . . . . .  17
     4.2.  Algorithm Organization  . . . . . . . . . . . . . . . . .  18
       4.2.1.  Initialization  . . . . . . . . . . . . . . . . . . .  18
       4.2.2.  Per-Transmit Steps  . . . . . . . . . . . . . . . . .  18
       4.2.3.  Per-ACK Steps . . . . . . . . . . . . . . . . . . . .  19
       4.2.4.  Per-Loss Steps  . . . . . . . . . . . . . . . . . . .  19
     4.3.  State Machine Operation . . . . . . . . . . . . . . . . .  19
       4.3.1.  Startup . . . . . . . . . . . . . . . . . . . . . . .  19
       4.3.2.  Drain . . . . . . . . . . . . . . . . . . . . . . . .  22
       4.3.3.  ProbeBW . . . . . . . . . . . . . . . . . . . . . . .  23
       4.3.4.  ProbeRTT  . . . . . . . . . . . . . . . . . . . . . .  33
     4.4.  Restarting From Idle  . . . . . . . . . . . . . . . . . .  38
       4.4.1.  Setting Pacing Rate in ProbeBW  . . . . . . . . . . .  38
       4.4.2.  Checking for ProberRTT Completion . . . . . . . . . .  39
       4.4.3.  Logic . . . . . . . . . . . . . . . . . . . . . . . .  39
     4.5.  Updating Network Path Model Parameters  . . . . . . . . .  39
       4.5.1.  Tracking Packet-Timed Round Trips and
               BBR.round_count . . . . . . . . . . . . . . . . . . .  40
       4.5.2.  BBR.max_bw  . . . . . . . . . . . . . . . . . . . . .  41
       4.5.3.  BBR.min_rtt . . . . . . . . . . . . . . . . . . . . .  43
       4.5.4.  BBR.offload_budget  . . . . . . . . . . . . . . . . .  45
       4.5.5.  BBR.extra_acked . . . . . . . . . . . . . . . . . . .  46
       4.5.6.  Updating the Model Upon Packet Loss . . . . . . . . .  47
     4.6.  Updating Control Parameters . . . . . . . . . . . . . . .  51
       4.6.1.  Summary of Control Behavior in the State Machine  . .  51
       4.6.2.  Pacing Rate: BBR.pacing_rate  . . . . . . . . . . . .  53
       4.6.3.  Send Quantum: BBR.send_quantum  . . . . . . . . . . .  54
       4.6.4.  Congestion Window . . . . . . . . . . . . . . . . . .  55
   5.  Implementation Status . . . . . . . . . . . . . . . . . . . .  60
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  62
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  62
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  62
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  62
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  62
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  63
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  65

1.  Introduction

   The Internet has traditionally used loss-based congestion control
   algorithms like Reno ([Jac88], [Jac90], [WS95] [RFC5681]) and CUBIC
   ([HRX08], [RFC8312]).  These algorithms worked well for many years
   because they were sufficiently well-matched to the prevalent range of
   bandwidth-delay products and degrees of buffering in Internet paths.
   As the Internet has evolved, loss-based congestion control is
   increasingly problematic in several important scenarios:

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   1.  Shallow buffers: In shallow buffers, packet loss can happen even
       when a link has low utilization.  With high-speed, long-haul
       links employing commodity switches with shallow buffers, loss-
       based congestion control can cause abysmal throughput because it
       overreacts, multiplicatively decreasing the sending rate upon
       packet loss, and only slowly growing its sending rate thereafter.
       This can happen even if the packet loss arises from transient
       traffic bursts when the link is mostly idle.

   2.  Deep buffers: At the edge of today's Internet, loss-based
       congestion control can cause the problem of "bufferbloat", by
       repeatedly filling deep buffers in last-mile links and causing
       high queuing delays.

   3.  Dynamic traffic workloads: With buffers of any depth, dynamic
       mixes of newly-entering flows or flights of data from recently
       idle flows can cause frequent packet loss.  In such scenarios
       loss-based congestion control can fail to maintain its fair share
       of bandwidth, leading to poor application performance.

   In both the shallow-buffer (1.) or dynamic-traffic (3.) scenarios
   mentioned above it is difficult to achieve full throughput with loss-
   based congestion control in practice: for CUBIC, sustaining 10Gbps
   over 100ms RTT needs a packet loss rate below 0.000003% (i.e., more
   than 40 seconds between packet losses), and over a 100ms RTT path a
   more feasible loss rate like 1% can only sustain at most 3 Mbps
   [RFC8312].  These limitations apply no matter what the bottleneck
   link is capable of or what the connection's fair share is.
   Furthermore, failure to reach the fair share can cause poor
   throughpout and poor tail latency for latency-sensitive applications.

   The BBR ("Bottleneck Bandwidth and Round-trip propagation time")
   congestion control algorithm is a model-based algorithm that takes an
   approach different from loss-based congestion control: BBR uses
   recent measurements of a transport connection's delivery rate, round-
   trip time, and packet loss rate to build an explicit model of the
   network path, including its estimated available bandwidth, bandwidth-
   delay product, and the maximum volume of data that the connection can
   place in-flight in the network without causing excessive queue
   pressure.  It then uses this model in order to guide its control
   behavior in seeking high throughput and low queue pressure.

   This document describes the current version of the BBR algorithm,
   BBRv2.  The previous version of the algorithm, BBRv1, was described
   previously at a high level [CCGHJ16][CCGHJ17].  The implications of
   BBR in allowing high utilization of high-speed networks with shallow
   buffers have been discussed in other work [MM19].  Active work on the
   BBR algorithm is continuing.

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   This document is organized as follows.  Section 2 provides various
   definitions that will be used throughout this document.  Section 3
   provides an overview of the design of the BBR algorithm, and section
   4 describes the BBR algorithm in detail, including BBR's network path
   model, control parameters, and state machine.  Section 5 describes
   the implementation status, section 6 describes security
   considerations, section 7 notes that there are no IANA
   considerations, and section 8 closes with Acknowledgments.

2.  Terminology

   This document defines state variables and constants for the BBR
   algorithm.

   The variables starting with C, P, or rs not defined below are defined
   in [draft-cheng-iccrg-delivery-rate-estimation].

2.1.  Transport Connection State

   C.delivered: The total amount of data (tracked in octets or in
   packets) delivered so far over the lifetime of the transport
   connection C.

   SMSS: The Sender Maximum Segment Size.

   is_cwnd_limited: True if the connection has fully utilized its cwnd
   at any point in the last packet-timed round trip.

   InitialCwnd: The initial congestion window set by the transport
   protocol implementation for the connection at initialization time.

2.2.  Per-Packet State

   packet.delivered: C.delivered when the given packet was sent by
   transport connection C.

   packet.departure_time: The earliest pacing departure time for the
   given packet.

   packet.tx_in_flight: The volume of data that was estimated to be in
   flight at the time of the transmission of the packet.

2.3.  Per-ACK Rate Sample State

   rs.delivered: The volume of data delivered between the transmission
   of the packet that has just been ACKed and the current time.

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   rs.delivery_rate: The delivery rate (aka bandwidth) sample obtained
   from the packet that has just been ACKed.

   rs.rtt: The RTT sample calculated based on the most recently-sent
   segment of the segments that have just been ACKed.

   rs.newly_acked: The volume of data cumulatively or selectively
   acknowledged upon the ACK that was just received.  (This quantity is
   referred to as "DeliveredData" in [RFC6937].)

   rs.newly_lost: The volume of data newly marked lost upon the ACK that
   was just received.

   rs.tx_in_flight: The volume of data that was estimated to be in
   flight at the time of the transmission of the packet that has just
   been ACKed (the most recently sent segment among segments ACKed by
   the ACK that was just received).

   rs.lost: The volume of data that was declared lost between the
   transmission and acknowledgement of the packet that has just been
   ACKed (the most recently sent segment among segments ACKed by the ACK
   that was just received).

2.4.  Output Control Parameters

   cwnd: The transport sender's congestion window, which limits the
   amount of data in flight.

   BBR.pacing_rate: The current pacing rate for a BBR flow, which
   controls inter-packet spacing.

   BBR.send_quantum: The maximum size of a data aggregate scheduled and
   transmitted together.

2.5.  Pacing State and Parameters

   BBR.pacing_gain: The dynamic gain factor used to scale BBR.bw to
   produce BBR.pacing_rate.

   BBRPacingMarginPercent: The static discount factor of 1% used to
   scale BBR.bw to produce BBR.pacing_rate.

   BBR.next_departure_time: The earliest pacing departure time for the
   next packet BBR schedules for transmission.

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2.6.  cwnd State and Parameters

   BBR.cwnd_gain: The dynamic gain factor used to scale the estimated
   BDP to produce a congestion window (cwnd).

   BBRStartupPacingGain: A constant specifying the minimum gain value
   for calculating the pacing rate that will allow the sending rate to
   double each round (4*ln(2) ~= 2.77) [BBRStartupPacingGain]; used in
   Startup mode for BBR.pacing_gain.

   BBRStartupCwndGain: A constant specifying the minimum gain value for
   calculating the cwnd that will allow the sending rate to double each
   round (2.0); used in Startup mode for BBR.cwnd_gain.

   BBR.packet_conservation: A boolean indicating whether BBR is
   currently using packet conservation dynamics to bound cwnd.

2.7.  General Algorithm State

   BBR.state: The current state of a BBR flow in the BBR state machine.

   BBR.round_count: Count of packet-timed round trips elapsed so far.

   BBR.round_start: A boolean that BBR sets to true once per packet-
   timed round trip, on ACKs that advance BBR.round_count.

   BBR.next_round_delivered: packet.delivered value denoting the end of
   a packet-timed round trip.

   BBR.idle_restart: A boolean that is true if and only if a connection
   is restarting after being idle.

2.8.  Core Algorithm Design Parameters

   BBRLossThresh: The maximum tolerated per-round-trip packet loss rate
   when probing for bandwidth (the default is 2%).

   BBRBeta: The default multiplicative decrease to make upon each round
   trip during which the connection detects packet loss (the value is
   0.7).

   BBRHeadroom: The multiplicative factor to apply to BBR.inflight_hi
   when attempting to leave free headroom in the path (e.g. free space
   in the bottleneck buffer or free time slots in the bottleneck link)
   that can be used by cross traffic (the value is 0.85).

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   BBRMinPipeCwnd: The minimal cwnd value BBR targets, to allow
   pipelining with TCP endpoints that follow an "ACK every other packet"
   delayed-ACK policy: 4 * SMSS.

2.9.  Network Path Model Parameters

2.9.1.  Data Rate Network Path Model Parameters

   The data rate model parameters together estimate both the sending
   rate required to reach the full bandwidth available to the flow
   (BBR.max_bw), and the maximum pacing rate control parameter that is
   consistent with the queue pressure objective (BBR.bw).

   BBR.max_bw: The windowed maximum recent bandwidth sample - obtained
   using the BBR delivery rate sampling algorithm [draft-cheng-iccrg-
   delivery-rate-estimation] - measured during the current or previous
   bandwidth probing cycle (or during Startup, if the flow is still in
   that state).  (Part of the long-term model.)

   BBR.bw_hi: The long-term maximum sending bandwidth that the algorithm
   estimates will produce acceptable queue pressure, based on signals in
   the current or previous bandwidth probing cycle, as measured by loss.
   (Part of the long-term model.)

   BBR.bw_lo: The short-term maximum sending bandwidth that the
   algorithm estimates is safe for matching the current network path
   delivery rate, based on any loss signals in the current bandwidth
   probing cycle.  This is generally lower than max_bw or bw_hi (thus
   the name).  (Part of the short-term model.)

   BBR.bw: The maximum sending bandwidth that the algorithm estimates is
   appropriate for matching the current network path delivery rate,
   given all available signals in the model, at any time scale.  It is
   the min() of max_bw, bw_hi, and bw_lo.

2.9.2.  Data Volume Network Path Model Parameters

   The data volume model parameters together estimate both the volume of
   in-flight data required to reach the full bandwidth available to the
   flow (BBR.max_inflight), and the maximum volume of data that is
   consistent with the queue pressure objective (cwnd).

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   BBR.min_rtt: The windowed minimum round-trip time sample measured
   over the last MinRTTFilterLen = 10 seconds.  This attempts to
   estimate the two-way propagation delay of the network path when all
   connections sharing a bottleneck are using BBR, but also allows BBR
   to estimate the value required for a bdp estimate that allows full
   throughput if there are legacy loss-based Reno or CUBIC flows sharing
   the bottleneck.

   BBR.bdp: The estimate of the network path's BDP (Bandwidth-Delay
   Product), computed as: BBR.bdp = BBR.bw * BBR.min_rtt.

   BBR.extra_acked: A volume of data that is the estimate of the recent
   degree of aggregation in the network path.

   BBR.offload_budget: The estimate of the minimum volume of data
   necessary to achieve full throughput when using sender (TSO/GSO) and
   receiver (LRO, GRO) host offload mechanisms.

   BBR.max_inflight: The estimate of the volume of in-flight data
   required to fully utilize the bottleneck bandwidth available to the
   flow, based on the BDP estimate (BBR.bdp), the aggregation estimate
   (BBR.extra_acked), the offload budget (BBR.offload_budget), and
   BBRMinPipeCwnd.

   BBR.inflight_hi: Analogous to BBR.bw_hi, the long-term maximum volume
   of in-flight data that the algorithm estimates will produce
   acceptable queue pressure, based on signals in the current or
   previous bandwidth probing cycle, as measured by loss.  That is, if a
   flow is probing for bandwidth, and observes that sending a particular
   volume of in-flight data causes a loss rate higher than the loss rate
   objective, it sets inflight_hi to that volume of data.  (Part of the
   long-term model.)

   BBR.inflight_lo: Analogous to BBR.bw_lo, the short-term maximum
   volume of in-flight data that the algorithm estimates is safe for
   matching the current network path delivery process, based on any loss
   signals in the current bandwidth probing cycle.  This is generally
   lower than max_inflight or inflight_hi (thus the name).  (Part of the
   short-term model.)

2.10.  State for Responding to Congestion

   BBR.bw_latest: a 1-round-trip max of delivered bandwidth
   (rs.delivery_rate).

   BBR.inflight_latest: a 1-round-trip max of delivered volume of data
   (rs.delivered).

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2.11.  Estimating BBR.max_bw

   BBR.MaxBwFilter: The filter for tracking the maximum recent
   rs.delivery_rate sample, for estimating BBR.max_bw.

   MaxBwFilterLen: The filter window length for BBR.MaxBwFilter = 2
   (representing up to 2 ProbeBW cycles, the current cycle and the
   previous full cycle).

   BBR.cycle_count: The virtual time used by the BBR.max_bw filter
   window.  Note that BBR.cycle_count only needs to be tracked with a
   single bit, since the BBR.MaxBwFilter only needs to track samples
   from two time slots: the previous ProbeBW cycle and the current
   ProbeBW cycle.

2.12.  Estimating BBR.extra_acked

   BBR.extra_acked_interval_start: the start of the time interval for
   estimating the excess amount of data acknowledged due to aggregation
   effects.

   BBR.extra_acked_delivered: the volume of data marked as delivered
   since BBR.extra_acked_interval_start.

   BBR.ExtraACKedFilter: the max filter tracking the recent maximum
   degree of aggregation in the path.

   BBRExtraAckedFilterLen = The window length of the
   BBR.ExtraACKedFilter max filter window: 10 (in units of packet-timed
   round trips).

2.13.  Startup Parameters and State

   BBR.filled_pipe: A boolean that records whether BBR estimates that it
   has ever fully utilized its available bandwidth ("filled the pipe").

   BBR.full_bw: A recent baseline BBR.max_bw to estimate if BBR has
   "filled the pipe" in Startup.

   BBR.full_bw_count: The number of non-app-limited round trips without
   large increases in BBR.full_bw.

2.14.  ProbeRTT and min_rtt Parameters and State

2.14.1.  Parameters for Estimating BBR.min_rtt

   BBR.min_rtt_stamp: The wall clock time at which the current
   BBR.min_rtt sample was obtained.

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   MinRTTFilterLen: A constant specifying the length of the BBR.min_rtt
   min filter window, MinRTTFilterLen is 10 secs.

2.14.2.  Parameters for Scheduling ProbeRTT

   BBRProbeRTTCwndGain = A constant specifying the gain value for
   calculating the cwnd during ProbeRTT: 0.5 (meaning that ProbeRTT
   attempts to reduce in-flight data to 50% of the estimated BDP).

   ProbeRTTDuration: A constant specifying the minimum duration for
   which ProbeRTT state holds inflight to BBRMinPipeCwnd or fewer
   packets: 200 ms.

   ProbeRTTInterval: A constant specifying the minimum time interval
   between ProbeRTT states: 5 secs.

   BBR.probe_rtt_min_delay: The minimum RTT sample recorded in the last
   ProbeRTTInterval.

   BBR.probe_rtt_min_stamp: The wall clock time at which the current
   BBR.probe_rtt_min_delay sample was obtained.

   BBR.probe_rtt_expired: A boolean recording whether the
   BBR.probe_rtt_min_delay has expired and is due for a refresh with an
   application idle period or a transition into ProbeRTT state.

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Design Overview

3.1.  High-Level Design Goals

   The high-level goal of BBR is to achieve both:

   1.  The full throughput (or approximate fair share thereof) available
       to a flow

       *  Achieved in a fast and scalable manner (using bandwidth in
          O(log(BDP)) time).

       *  Achieved with average packet loss rates of up to 1%.

   2.  Low queue pressure (low queuing delay and low packet loss).

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   These goals are in tension: sending faster improves the odds of
   achieving (1) but reduces the odds of achieving (2), while sending
   slower improves the odds of achieving (2) but reduces the odds of
   achieving (1).  Thus the algorithm cannot maximize throughput or
   minimize queue pressure independently, and must jointly optimize
   both.

   To try to achieve these goals, and seek an operating point with high
   throughput and low delay [K79] [GK81], BBR aims to adapt its sending
   process to match the network delivery process, in two dimensions:

   1.  data rate: the rate at which the flow sends data should ideally
       match the rate at which the network delivers the flow's data (the
       available bottleneck bandwidth)

   2.  data volume: the amount of unacknowledged data in flight in the
       network should ideally match the bandwidth-delay product (BDP) of
       the path

   Both the control of the data rate (via the pacing rate) and data
   volume (directly via the congestion window or cwnd; and indirectly
   via the pacing rate) are important.  A mismatch in either dimension
   can cause the sender to fail to meet its high-level design goals:

   1.  volume mismatch: If a sender perfectly matches its sending rate
       to the available bandwidth, but its volume of in-flight data
       exceeds the BDP, then the sender can maintain a large standing
       queue, increasing network latency and risking packet loss.

   2.  rate mismatch: If a sender's volume of in-flight data matches the
       BDP perfectly but its sending rate exceeds the available
       bottleneck bandwidth (e.g. the sender transmits a BDP of data in
       an unpaced fashion, at the sender's link rate), then up to a full
       BDP of data can burst into the bottleneck queue, causing high
       delay and/or high loss.

3.2.  Algorithm Overview

   Based on the rationale above, BBR tries to spend most of its time
   matching its sending process (data rate and data volume) to the
   network path's delivery process.  To do this, it explores the
   2-dimensional control parameter space of (1) data rate ("bandwidth"
   or "throughput") and (2) data volume ("in-flight data"), with a goal
   of finding the maximum values of each control parameter that are
   consistent with its objective for queue pressure.

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   Depending on what signals a given network path manifests at a given
   time, the objective for queue pressure is measured in terms of the
   most strict among:

   *  the amount of data that is estimated to be queued in the
      bottleneck buffer (data_in_flight - estimated_BDP): the objective
      is to maintain this amount at or below 1.5 * estimated_BDP

   *  the packet loss rate: the objective is a maximum per-round-trip
      packet loss rate of BBRLossThresh=2% (and an average packet loss
      rate considerably lower)

3.3.  State Machine Overview

   BBR varies its control parameters with a simple state machine that
   aims for high throughput, low latency, and an approximately fair
   sharing of bandwidth, while maintaining an up-to-date model of the
   network path.

   A BBR flow starts in the Startup state, and ramps up its sending rate
   quickly, to rapidly estimate the maximum available bandwidth
   (BBR.max_bw).  When it estimates the bottleneck bandwidth has been
   fully utilized, it enters the Drain state to drain the estimated
   queue.  In steady state a BBR flow mostly uses the ProbeBW states, to
   periodically briefly send faster to probe for higher capacity and
   then briefly send slower to try to drain any resulting queue.  If
   needed, it briefly enters the ProbeRTT state, to lower the sending
   rate to probe for lower BBR.min_rtt samples.  The detailed behavior
   for each state is described below.

3.4.  Network Path Model Overview

3.4.1.  High-Level Design Goals for the Network Path Model

   At a high level, the BBR model is trying to reflect two aspects of
   the network path:

   *  Model what's required for achieving full throughput: Estimate the
      minimum data rate and data volume required to fully utilize the
      fair share of the bottleneck bandwidth available to the flow.
      This must incorporate estimates of the maximum available bandwidth
      (BBR.max_bw), the BDP of the path (BBR.bdp), and the requirements
      of any offload features on the end hosts or mechanisms on the
      network path that produce aggregation effects (summing up to
      BBR.max_inflight).

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   *  Model what's permitted for achieving low queue pressure: Estimate
      the maximum data rate (BBR.bw) and data volume (cwnd) consistent
      with the queue pressure objective, as measured by the estimated
      degree of queuing and packet loss.

   Note that those two aspects are in tension: the highest throughput is
   available to the flow when it sends as fast as possible and occupies
   as many bottleneck buffer slots as possible; the lowest que pressure
   is achieved by the flow when it sends as slow as possible and
   occupies as few bottleneck buffer slots as possible.  To resolve the
   tension, the algorithm aims to achieve the maximum throughput
   achievable while still meeting the queue pressure objective.

3.4.2.  Time Scales for the Network Model

   At a high level, the BBR model is trying to reflect the properties of
   the network path on two different time scales:

3.4.2.1.  Long-term model

   One goal is for BBR to maintain high average utilization of the fair
   share of the available bandwidth, over long time intervals.  This
   requires estimates of the path's data rate and volume capacities that
   are robust over long time intervals.  This means being robust to
   congestion signals that may be noisy or may reflect short-term
   congestion that has already abated by the time an ACK arrives.  This
   also means providing a robust history of the best recently-achievable
   performance on the path so that the flow can quickly and robustly aim
   to re-probe that level of performance whenever it decides to probe
   the capacity of the path.

3.4.2.2.  Short-term model

   A second goal of BBR is to react to every congestion signal,
   including loss, as if it may indicate a persistent/long-term increase
   in congestion and/or decrease in the bandwidth available to the flow,
   because that may indeed be the case.

3.4.2.3.  Time Scale Strategy

   BBR sequentially alternates between spending most of its time using
   short-term models to conservatively respect all congestion signals in
   case they represent persistent congestion, but periodically using its
   long-term model to robustly probe the limits of the available path
   capacity in case the congestion has abated and more capacity is
   available.

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3.5.  Control Parameter Overview

   BBR uses its model to control the connection's sending behavior.
   Rather than using a single control parameter, like the cwnd parameter
   that limits the volume of in-flight data in the Reno and CUBIC
   congestion control algorithms, BBR uses three distinct control
   parameters:

   1.  pacing rate: the maximum rate at which BBR sends data.

   2.  send quantum: the maximum size of any aggregate that the
       transport sender implementation may need to transmit as a unit to
       amortize per-packet transmission overheads.

   3.  cwnd: the maximum volume of data BBR allows in-flight in the
       network at any time.

3.6.  Environment and Usage

   BBR is a congestion control algorithm that is agnostic to transport-
   layer and link-layer technologies, requires only sender-side changes,
   and does not require changes in the network.  Open source
   implementations of BBR are available for the TCP [RFC793] and QUIC
   [RFC9000] transport protocols, and these implementations have been
   used in production for a large volume of Internet traffic.

4.  Detailed Algorithm

4.1.  State Machine

   BBR implements a state machine that uses the network path model to
   guide its decisions, and the control parameters to enact its
   decisions.

4.1.1.  State Transition Diagram

   The following state transition diagram summarizes the flow of control
   and the relationship between the different states:

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                |
                V
       +---> Startup  ------------+
       |        |                 |
       |        V                 |
       |     Drain  --------------+
       |        |                 |
       |        V                 |
       +---> ProbeBW_DOWN  -------+
       | ^      |                 |
       | |      V                 |
       | |   ProbeBW_CRUISE ------+
       | |      |                 |
       | |      V                 |
       | |   ProbeBW_REFILL  -----+
       | |      |                 |
       | |      V                 |
       | |   ProbeBW_UP  ---------+
       | |      |                 |
       | +------+                 |
       |                          |
       +---- ProbeRTT <-----------+

4.1.2.  State Machine Operation Overview

   When starting up, BBR probes to try to quickly build a model of the
   network path; to adapt to later changes to the path or its traffic,
   BBR must continue to probe to update its model.  If the available
   bottleneck bandwidth increases, BBR must send faster to discover
   this.  Likewise, if the round-trip propagation delay changes, this
   changes the BDP, and thus BBR must send slower to get inflight below
   the new BDP in order to measure the new BBR.min_rtt.  Thus, BBR's
   state machine runs periodic, sequential experiments, sending faster
   to check for BBR.bw increases or sending slower to yield bandwidth,
   drain the queue, and check for BBR.min_rtt decreases.  The frequency,
   magnitude, duration, and structure of these experiments differ
   depending on what's already known (startup or steady-state) and
   application sending behavior (intermittent or continuous).

   This state machine has several goals:

   *  Achieve high throughput by efficiently utilizing available
      bandwidth.

   *  Achieve low latency and packet loss rates by keeping queues
      bounded and small.

   *  Share bandwidth with other flows in an approximately fair manner.

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   *  Feed samples to the model estimators to refresh and update the
      model.

4.1.3.  State Machine Tactics

   In the BBR framework, at any given time the sender can choose one of
   the following tactics:

   *  Acceleration: Send faster then the network is delivering data: to
      probe the bw (https://docs.google.com/document/
      d/1_EqkO9UjJ2yD0SEiICeHLMVT285L2Tkd9kkybIMDWfY/
      edit#heading=h.etm5bmltrkme), the maximum bandwidth available to
      the flow

   *  Cruising: Send at the same rate the network is delivering data:
      try to match the sending rate to the flow's current available
      bandwidth, to try to achieve high utilization of the available
      bandwidth without increasing queue pressure

   *  Deceleration: Send slower than the network is delivering data: to
      reduce the amount of data in flight, with a number of overlapping
      motivations:

      -  Reducing queuing delay: to reduce queuing delay, to reduce
         latency for request/response cross-traffic (e.g.  RPC, web
         traffic).

      -  Reducing packet loss: to reduce packet loss, to reduce tail
         latency for request/response cross-traffic (e.g.  RPC, web
         traffic) and improve coexistence with Reno/CUBIC.

      -  Probing BBR.min_rtt: to probe the path's BBR.min_rtt

      -  Bandwidth convergence: to aid bandwidth fairness convergence,
         by leaving unused capacity in the bottleneck link or bottleneck
         buffer, to allow other flows that may have lower sending rates
         to discover and utilize the unused capacity

      -  Burst tolerance: to allow bursty arrivals of cross-traffic
         (e.g. short web or RPC requests) to be able to share the
         bottleneck link without causing excessive queuing delay or
         packet loss

   Throughout the lifetime of a BBR flow, it sequentially cycles through
   all three tactics, to measure the network path and try to optimize
   its operating point.

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   BBR's state machine uses two control mechanisms.  Primarily, it uses
   the pacing_gain (see the "Pacing Rate" section), which controls how
   fast packets are sent relative to BBR.bw.  A pacing_gain > 1
   decreases inter-packet time and increases inflight.  A pacing_gain <
   1 has the opposite effect, increasing inter-packet time and while
   aiming to decrease inflight.  Second, if the state machine needs to
   quickly reduce inflight to a particular absolute value, it uses the
   cwnd.

4.2.  Algorithm Organization

   The BBR algorithm is an event-driven algorithm that executes steps
   upon the following events: connection initialization, upon each ACK,
   upon the transmission of each quantum, and upon loss detection
   events.  All of the sub-steps invoked referenced below are described
   below.

4.2.1.  Initialization

   Upon transport connection initialization, BBR executes its
   initialization steps:

     BBROnInit():
       init_windowed_max_filter(filter=BBR.MaxBwFilter, value=0, time=0)
       BBR.min_rtt = SRTT ? SRTT : Inf
       BBR.min_rtt_stamp = Now()
       BBR.probe_rtt_done_stamp = 0
       BBR.probe_rtt_round_done = false
       BBR.prior_cwnd = 0
       BBR.idle_restart = false
       BBR.extra_acked_interval_start = Now()
       BBR.extra_acked_delivered = 0
       BBRResetCongestionSignals()
       BBRResetLowerBounds()
       BBRInitRoundCounting()
       BBRInitFullPipe()
       BBRInitPacingRate()
       BBREnterStartup()

4.2.2.  Per-Transmit Steps

   When transmitting, BBR merely needs to check for the case where the
   flow is restarting from idle:

     BBROnTransmit():
       BBRHandleRestartFromIdle()

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4.2.3.  Per-ACK Steps

   On every ACK, the BBR algorithm executes the following
   BBRUpdateOnACK() steps in order to update its network path model,
   update its state machine, and adjust its control parameters to adapt
   to the updated model:

     BBRUpdateOnACK():
       BBRUpdateModelAndState()
       BBRUpdateControlParameters()

     BBRUpdateModelAndState():
       BBRUpdateLatestDeliverySignals()
       BBRUpdateCongestionSignals()
       BBRUpdateACKAggregation()
       BBRCheckStartupDone()
       BBRCheckDrain()
       BBRUpdateProbeBWCyclePhase()
       BBRUpdateMinRTT()
       BBRCheckProbeRTT()
       BBRAdvanceLatestDeliverySignals()
       BBRBoundBWForModel()

     BBRUpdateControlParameters():
       BBRSetPacingRate()
       BBRSetSendQuantum()
       BBRSetCwnd()

4.2.4.  Per-Loss Steps

   On every packet loss event, where some sequence range "packet" is
   marked lost, the BBR algorithm executes the following
   BBRUpdateOnLoss() steps in order to update its network path model

     BBRUpdateOnLoss(packet):
       BBRHandleLostPacket(packet)

4.3.  State Machine Operation

4.3.1.  Startup

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4.3.1.1.  Startup Dynamics

   When a BBR flow starts up, it performs its first (and most rapid)
   sequential probe/drain process in the Startup and Drain states.
   Network link bandwidths currently span a range of at least 11 orders
   of magnitude, from a few bps to 200 Gbps.  To quickly learn
   BBR.max_bw, given this huge range to explore, BBR's Startup state
   does an exponential search of the rate space, doubling the sending
   rate each round.  This finds BBR.max_bw in O(log_2(BDP)) round trips.

   To achieve this rapid probing in the smoothest possible fashion, in
   Startup BBR uses the minimum gain values that will allow the sending
   rate to double each round: in Startup BBR sets BBR.pacing_gain to
   BBRStartupPacingGain (2.77) [BBRStartupPacingGain] and BBR.cwnd_gain
   to BBRStartupCwndGain (2).

   When initializing a connection, or upon any later entry into Startup
   mode, BBR executes the following BBREnterStartup() steps:

     BBREnterStartup():
       BBR.state = Startup
       BBR.pacing_gain = BBRStartupPacingGain
       BBR.cwnd_gain = BBRStartupCwndGain

   As BBR grows its sending rate rapidly, it obtains higher delivery
   rate samples, BBR.max_bw increases, and the pacing rate and cwnd both
   adapt by smoothly growing in proportion.  Once the pipe is full, a
   queue typically forms, but the cwnd_gain bounds any queue to
   (cwnd_gain - 1) * estimated_BDP, which is approximately (2.77 - 1) *
   estimated_BDP = 1.77 * estimated_BDP.  The immediately following
   Drain state is designed to quickly drain that queue.

   During Startup, BBR estimates whether the pipe is full using two
   estimators.  The first looks for a plateau in the BBR.max_bw
   estimate.  The second looks for packet loss.  The following
   subsections discuss these estimators.

     BBRCheckStartupDone():
       BBRCheckStartupFullBandwidth()
       BBRCheckStartupHighLoss()
       if (BBR.state == Startup and BBR.filled_pipe)
         BBREnterDrain()

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4.3.1.2.  Estimating When Startup has Filled the Pipe Based on Bandwidth
          Plateau

   During Startup, BBR estimates whether the pipe is full by looking for
   a plateau in the BBR.max_bw estimate.  The output of this "full pipe"
   estimator is tracked in BBR.filled_pipe, a boolean that records
   whether BBR estimates that it has ever fully utilized its available
   bandwidth ("filled the pipe").  If BBR notices that there are several
   (three) rounds where attempts to double the delivery rate actually
   result in little increase (less than 25 percent), then it estimates
   that it has reached BBR.max_bw, sets BBR.filled_pipe to true, exits
   Startup and enters Drain.

   Upon connection initialization the full pipe estimator runs:

     BBRInitFullPipe():
       BBR.filled_pipe = false
       BBR.full_bw = 0
       BBR.full_bw_count = 0

   Once per round trip, upon an ACK that acknowledges new data, and when
   the delivery rate sample is not application-limited (see [draft-
   cheng-iccrg-delivery-rate-estimation]), BBR runs the "full pipe"
   estimator, if needed:

     BBRCheckStartupFullBandwidth():
       if BBR.filled_pipe or
          !BBR.round_start or rs.is_app_limited
         return  /* no need to check for a full pipe now */
       if (BBR.max_bw >= BBR.full_bw * 1.25)  /* still growing? */
         BBR.full_bw = BBR.max_bw    /* record new baseline level */
         BBR.full_bw_count = 0
         return
       BBR.full_bw_count++ /* another round w/o much growth */
       if (BBR.full_bw_count >= 3)
         BBR.filled_pipe = true

   BBR waits three rounds to have solid evidence that the sender is not
   detecting a delivery-rate plateau that was temporarily imposed by the
   receive window.  Allowing three rounds provides time for the
   receiver's receive-window auto-tuning to open up the receive window
   and for the BBR sender to realize that BBR.max_bw should be higher:
   in the first round the receive-window auto-tuning algorithm grows the
   receive window; in the second round the sender fills the higher
   receive window; in the third round the sender gets higher delivery-
   rate samples.  This three-round threshold was validated by YouTube
   experimental data.

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4.3.1.3.  Estimating When Startup has Filled the Pipe Based on Packet
          Loss

   A second method BBR uses for estimating the bottleneck is full is by
   looking at sustained packet losses Specifically for a case where the
   following criteria are all met:

   *  The connection has been in fast recovery for at least one full
      round trip.

   *  The loss rate over the time scale of a single full round trip
      exceeds loss_thresh (2%).

   *  There are at least BBRStartupFullLossCnt=3 discontiguous sequence
      ranges lost in that round trip.

   If these criteria are all met, then BBRCheckStartupHighLoss() sets
   BBR.filled_pipe = true and exits Startup and enters Drain.

   The algorithm waits until all three criteria are met to filter out
   noise from burst losses, and to try to ensure the bottleneck is fully
   utilized on a sustained basis, and the full bottleneck bandwidth has
   been measured, before attempting to drain the level of in-flight data
   to the estimated BDP.

4.3.2.  Drain

   Upon exiting Startup, BBR enters its Drain state.  In Drain, BBR aims
   to quickly drain any queue created in Startup by switching to a
   pacing_gain well below 1.0, until any estimated queue has been
   drained.  It uses a pacing_gain that is the inverse of the value used
   during Startup, chosen to try to drain the queue in one round
   [BBRDrainPacingGain]:

     BBREnterDrain():
       BBR.state = Drain
       BBR.pacing_gain = 1/BBRStartupCwndGain  /* pace slowly */
       BBR.cwnd_gain = BBRStartupCwndGain      /* maintain cwnd */

   In Drain, when the amount of data in flight is less than or equal to
   the estimated BDP, meaning BBR estimates that the queue has been
   fully drained, then BBR exits Drain and enters ProbeBW.  To implement
   this, upon every ACK BBR executes:

     BBRCheckDrain():
       if (BBR.state == Drain and packets_in_flight <= BBRInflight(1.0))
         BBREnterProbeBW()  /* BBR estimates the queue was drained */

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4.3.3.  ProbeBW

   Long-lived BBR flows tend to spend the vast majority of their time in
   the ProbeBW states.  In the ProbeBW states, a BBR flow sequentially
   accelerates, decelerates, and cruises, to measure the network path,
   improve its operating point (increase throughput and reduce queue
   pressure), and converge toward a more fair allocation of bottleneck
   bandwidth.  To do this, the flow sequentially cycles through all
   three tactics: trying to send faster than, slower than, and at the
   same rate as the network delivery process.  To achieve this, a BBR
   flow in ProbeBW mode cycles through the four Probe bw states - DOWN,
   CRUISE, REFILL, and UP - described below in turn.

4.3.3.1.  ProbeBW_DOWN

   In the ProbeBW_DOWN phase of the cycle, a BBR flow pursues the
   deceleration tactic, to try to send slower than the network is
   delivering data, to reduce the amount of data in flight, with all of
   the standard motivations for the deceleration tactic (discussed in
   "State Machine Tactics", above).  It does this by switching to a
   BBR.pacing_gain of 0.75, sending at 75% of BBR.bw.

   Exit conditions: The flow exits this phase and enters CRUISE when the
   flow estimates that both of the following conditions have been met:

   *  There is free headroom: If inflight_hi is set, then BBR remains in
      DOWN at least until the volume of in-flight data is less than or
      equal to BBRHeadroom*BBR.inflight_hi.  The goal of this constraint
      is to ensure that in cases where loss signals suggest an upper
      limit on the volume of in-flight data, then the flow attempts to
      leave some free headroom in the path (e.g. free space in the
      bottleneck buffer or free time slots in the bottleneck link) that
      can be used by cross traffic (both for volume-based convergence of
      bandwidth shares and for burst tolerance).

   *  At least BBR.min_rtt has elapsed in ProbeBW_DOWN.

4.3.3.2.  ProbeBW_CRUISE

   In the ProbeBW_CRUISE phase of the cycle, a BBR flow pursues the
   "cruising" tactic (discussed in "State Machine Tactics", above),
   attempting to send at the same rate the network is delivering data.
   It tries to match the sending rate to the flow's current available
   bandwidth, to try to achieve high utilization of the available
   bandwidth without increasing queue pressure.  It does this by
   switching to a pacing_gain of 1.0, sending at 100% of BBR.bw.
   Notably, while in this state it responds to concrete congestion
   signals (loss) by reducing BBR.bw_lo and BBR.inflight_lo, because

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   these signals suggest that the available bandwidth and deliverable
   volume of in-flight data have likely reduced, and the flow needs to
   change to adapt, slowing down to match the latest delivery process.

   Exit conditions: The connection adaptively holds this state until it
   decides that it is time to probe for bandwidth, at which time it
   enters ProbeBW_REFILL (see "Time Scale for Bandwidth Probing",
   below).

4.3.3.3.  ProbeBW_REFILL

   The goal of the ProbeBW_REFILL state is to "refill the pipe", to try
   to fully utilize the network bottleneck without creating any
   significant queue pressure.

   To do this, BBR first resets the short-term model parameters bw_lo
   and inflight_lo, setting both to "Infinity".  This is the key moment
   in the BBR time scale strategy (see "Time Scale Strategy", above)
   where the flow pivots, discarding its conservative short-term bw_lo
   and inflight_lo parameters and beginning to robustly probe the
   bottleneck's long-term available bandwidth.  During this time bw_hi
   and inflight_hi, if set, constrain the connection.

   During ProbeBW_REFILL BBR uses a BBR.pacing_gain of 1.0, to send at a
   rate that matches the current estimated available bandwidth, for one
   packet-timed round trip.  The goal is to fully utilize the bottleneck
   link before transitioning into ProbeBW_UP and significantly
   increasing the chances of causing a loss signal.  The motivating
   insight is that, as soon as a flow starts acceleration, sending
   faster than the available bandwidth, it will start building a queue
   at the bottleneck.  And if the buffer is shallow enough, then the
   flow can cause loss signals very shortly after the first accelerating
   packets arrive at the bottleneck.  If the flow were to neglect to
   fill the pipe before it causes this loss signal, then these very
   quick signals of excess queue could cause the flow's estimate of the
   path's capacity (i.e. inflight_hi) to significantly underestimate.
   In particular, if the flow were to transition directly from
   ProbeBW_CRUISE to ProbeBW_UP, the volume of in-flight data (at the
   time the first accelerating packets were sent) may often be still
   very close to the volume of in-flight data maintained in CRUISE,
   which may be only BBRHeadroom*inflight_hi.

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   Exit conditions: The flow exits ProbeBW_REFILL after one packet-timed
   round trip, and enters UP.  This is because after one full round trip
   of sending in ProbeBW_REFILL the flow can safely estimate that the
   pipe is as full as its BBR.bdp estimate permits.  And
   correspondingly, at this point the flow starts to see bandwidth
   samples reflecting its ProbeBW_REFILL behavior, which may be putting
   too much data in flight.

4.3.3.4.  ProbeBW_UP

   After ProbeBW_REFILL refills the pipe, ProbeBW_UP probes for possible
   increases in available bandwidth by using a BBR.pacing_gain of 1.25,
   sending faster than the current estimated available bandwidth.

   If the flow has not set BBR.inflight_hi or BBR.bw_hi, it tries to
   raise the volume of in-flight data to at least BBR.pacing_gain *
   BBR.bdp = 1.25 * BBR.bdp; note that this may take more than
   BBR.min_rtt if BBR.min_rtt is small (e.g. on a LAN).

   If the flow has set BBR.inflight_hi or BBR.bw_hi, it moves to an
   operating point based on those limits and then gradually increases
   the upper volume bound (BBR.inflight_hi) and rate bound (BBR.bw_hi)
   using the following approach:

   *  bw_hi: The flow raises bw_hi to the latest measured bandwidth
      sample if the latest measured bandwidth sample is above bw_hi and
      the loss rate for the sample is not above the BBRLossThresh.

   *  inflight_hi: The flow raises inflight_hi in ProbeBW_UP in a manner
      that is slow and cautious at first, but increasingly rapid and
      bold over time.  The initial caution is motivated by the fact that
      a given BBR flow may be sharing a shallow buffer with thousands of
      other flows, so that the buffer space available to the flow may be
      quite tight - even just a single packet.  The increasingly rapid
      growth over time is motivated by the fact that in a high-speed WAN
      the increase in available bandwidth (and thus the estimated BDP)
      may require the flow to grow the volume of its inflight data by up
      to O(1,000,000); even a quite typical BDP like 10Gbps * 100ms is
      82,563 packets.  BBR takes an approach where the additive increase
      to BBR.inflight_hi exponentially doubles each round trip; in each
      successive round trip, inflight_hi grows by 1, 2, 4, 8, 16, etc,
      with the increases spread uniformly across the entire round trip.
      This helps allow BBR to utilize a larger BDP in O(log(BDP)) round
      trips, meeting the design goal for scalable utilization of newly-
      available bandwidth.

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   Exit conditions: The BBR flow ends ProbeBW_UP bandwidth probing and
   transitions to ProbeBW_DOWN to try to drain the bottleneck queue when
   any of the following conditions are met:

   1.  Estimated queue: The flow has been in ProbeBW_UP for at least
       1*min_rtt, and the estimated queue is high enough that the flow
       judges it has robustly probed for available bandwidth
       (packets_in_flight > 1.25 * BBR.bdp).

   2.  Loss: The current loss rate exceeds loss_thresh (2%).

4.3.3.5.  Time Scale for Bandwidth Probing

   Choosing the time scale for probing bandwidth is tied to the question
   of how to coexist with legacy Reno/CUBIC flows, since probing for
   bandwidth runs a significant risk of causing packet loss, and causing
   packet loss can significantly limit the throughput of such legacy
   Reno/CUBIC flows.

4.3.3.5.1.  Bandwidth Probing and Coexistence with Reno/CUBIC

   BBR has an explicit strategy for coexistence with Reno/CUBIC: to try
   to behave in a manner so that Reno/CUBIC flows coexisting with BBR
   can continue to work well in the primary contexts where they do
   today:

   *  Intra-datacenter/LAN traffic: we want Reno/CUBIC to be able to
      perform well in 100M through 40G enterprise and datacenter
      Ethernet

      -  BDP = 40 Gbps * 20 us / (1514 bytes) ~= 66 packets

   *  Public Internet last mile traffic: we want Reno/CUBIC to be able
      to support up to 25Mbps (for 4K Video) at an RTT of 30ms, typical
      parameters for common CDNs for large video services:

      -  BDP = 25Mbps * 30 ms / (1514 bytes) ~= 62 packets

   The challenge in meeting these goals is that Reno/CUBIC need long
   periods of no loss to utilize large BDPs.  The good news is that in
   the environments where Reno/CUBIC work well today (mentioned above),
   the BDPs are small, roughly ~100 packets or less.

4.3.3.5.2.  A Dual-Time-Scale Approach for Coexistence

   The BBR strategy has several aspects:

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   1.  The highest priority is to estimate the bandwidth available to
       the BBR flow in question.

   2.  Secondarily, a given BBR flow adapts (within bounds) the
       frequency at which it probes bandwidth and knowingly risks packet
       loss, to allow Reno/CUBIC to reach a bandwidth at least as high
       as that given BBR flow.

   To adapt the frequency of bandwidth probing, BBR considers two time
   scales: a BBR-native time scale, and a bounded Reno-conscious time
   scale:

   *  T_bbr: BBR-native time-scale

      -  T_bbr = uniformly randomly distributed between 2 and 3 secs

   *  T_reno: Reno-coexistence time scale

      -  T_reno_bound = pick_randomly_either({62, 63})

      -  T_reno = min(BDP in packets, 62) round trips

   *  T_probe: The time between bandwidth probe UP phases:

      -  T_probe = min(T_bbr, T_reno)

   This dual-time-scale approach is similar to that used by CUBIC, which
   has a CUBIC-native time scale given by a cubic curve, and a "Reno
   emulation" module that estimates what cwnd would give the flow Reno-
   equivalent throughput.  At any given moment, CUBIC choose the cwnd
   implied by the more aggressive strategy.

   We randomize both the T_bbr and T_reno parameters, for better mixing
   and fairness convergence.

4.3.3.5.3.  Design Considerations for Choosing Constant Parameters

   We design the maximum wall-clock bounds of BBR-native inter-
   bandwidth-probe wall clock time, T_bbr, to be:

   *  Higher than 2 sec to try to avoid causing loss for a long enough
      time to allow Reno flow with RTT=30ms to get 25Mbps (4K video)
      throughput.  For this workload, given the Reno sawtooth that
      raises cwnd from roughly BDP to 2*BDP, one MSS per round trip, the
      inter-bandwidth-probe time must be at least: BDP * RTT = 25Mbps *
      .030 sec / (1514 bytes) * 0.030 sec = 1.9secs

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   *  Lower than 3 sec to ensure flows can start probing in a reasonable
      amount of time to discover unutilized bw on human-scale
      interactive time-scales (e.g. perhaps traffic from a competing web
      page download is now complete).

   The maximum round-trip bounds of the Reno-coexistence time scale,
   T_reno, are chosen to be 62-63 with the following considerations in
   mind:

   *  Choosing a value smaller than roughly 60 would imply that when BBR
      flows coexisted with Reno/CUBIC flows (e.g.  Netflix Reno flows)
      on public Internet broadband links, the Reno/CUBIC flows would not
      be able to achieve enough bandwidth to show 4K video.

   *  Choosing a value larger than roughly 65 would prevent BBR from
      reaching its goal of tolerating 1% loss per round trip.  Given
      that the steady-state (non-bandwidth-probing) BBR response to a
      round trip with X% packet loss is to reduce the sending rate by X%
      (see the "Updating the Model Upon Packet Loss" section), this
      means that the BBR sending rate after N rounds of packet loss at a
      rate loss_rate is (1 - loss_rate)^N.  This means that for a flow
      that encounters 1% loss in 65 round trips of ProbeBW_CRUISE, and
      then doubles its cwnd (back to BBR.inflight_hi) in ProbeBW_REFILL
      and ProbeBW_UP, it will be able to restore and reprobe its
      original sending rate, since: BBW.max_bw * (1 - loss_rate)^N * 2 =
      BBR.max_bw * (1 - .01)^65 ~= 1.04 * BBR.max_bw.  That is, the flow
      will be able to fully respond to packet loss signals in
      ProbeBW_CRUISE while also fully re-measuring its maximum
      achievable throughput in ProbeBW_UP.

   The resulting behavior is that for BBR flows with small BDPs, the
   bandwidth probing will be on roughly the same time scale as Reno/
   CUBIC; flows with large BDPs will intentionally probe more rapidly/
   frequently than Reno/CUBIC would (roughly every 62 round trips for
   low-RTT flows, or 2-3 secs for high-RTT flows).

   The considerations above for timing bandwidth probing can be
   implemented as follows:

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     /* Is it time to transition from DOWN or CRUISE to REFILL? */
     BBRCheckTimeToProbeBW():
       if (BBRHasElapsedInPhase(BBR.bw_probe_wait) ||
           BBRIsRenoCoexistenceProbeTime())
         BBRStartProbeBW_REFILL()
         return true
       return false

     /* Randomized decision about how long to wait until
      * probing for bandwidth, using round count and wall clock.
      */
     BBRPickProbeWait():
       /* Decide random round-trip bound for wait: */
       BBR.rounds_since_bw_probe =
         random_int_between(0, 1); /* 0 or 1 */
       /* Decide the random wall clock bound for wait: */
       BBR.bw_probe_wait =
         2sec + random_float_between(0.0, 1.0) /* 0..1 sec */

     BBRIsRenoCoexistenceProbeTime():
       reno_rounds = BBRTargetInflight()
       rounds = min(reno_rounds, 63)
       return BBR.rounds_since_bw_probe >= rounds

     /* How much data do we want in flight?
      * Our estimated BDP, unless congestion cut cwnd. */
     BBRTargetInflight()
       return min(BBR.bdp, cwnd)

4.3.3.6.  ProbeBW Algorithm Details

   BBR's ProbeBW algorithm operates as follows.

   Upon entering ProbeBW, BBR executes:

     BBREnterProbeBW():
       BBRStartProbeBW_DOWN()

   The core logic for entering each state:

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     BBRStartProbeBW_DOWN():
       BBRResetCongestionSignals()
       BBR.probe_up_cnt = Infinity /* not growing inflight_hi */
       BBRPickProbeWait()
       BBR.cycle_stamp = Now()  /* start wall clock */
       BBR.ack_phase  = ACKS_PROBE_STOPPING
       BBRStartRound()
       BBR.state = ProbeBW_DOWN

     BBRStartProbeBW_CRUISE():
       BBR.state = ProbeBW_CRUISE

     BBRStartProbeBW_REFILL():
       BBRResetLowerBounds()
       BBR.bw_probe_up_rounds = 0
       BBR.bw_probe_up_acks = 0
       BBR.ack_phase = ACKS_REFILLING
       BBRStartRound()
       BBR.state = ProbeBW_REFILL

     BBRStartProbeBW_UP():
       BBR.ack_phase = ACKS_PROBE_STARTING
       BBRStartRound()
       BBR.cycle_stamp = Now() /* start wall clock */
       BBR.state = ProbeBW_UP
       BBRRaiseInflightHiSlope()

   BBR executes the following BBRUpdateProbeBWCyclePhase() logic on each
   ACK that ACKs or SACKs new data, to advance the ProbeBW state
   machine:

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     /* The core state machine logic for ProbeBW: */
     BBRUpdateProbeBWCyclePhase():
       if (!BBR.filled_pipe)
         return  /* only handling steady-state behavior here */
       BBRAdaptUpperBounds()
       if (!IsInAProbeBWState())
         return /* only handling ProbeBW states here: */

       switch (state)

       ProbeBW_DOWN:
         if (BBRCheckTimeToProbeBW())
           return /* already decided state transition */
         if (BBRCheckTimeToCruise())
           BBRStartProbeBW_CRUISE()

       ProbeBW_CRUISE:
         if (BBRCheckTimeToProbeBW())
           return /* already decided state transition */

       ProbeBW_REFILL:
         /* After one round of REFILL, start UP */
         if (BBR.round_start)
           BBR.bw_probe_samples = 1
           BBRStartProbeBW_UP()

       ProbeBW_UP:
         if (BBRHasElapsedInPhase(BBR.min_rtt) and
             inflight > BBRInflight(BBR.max_bw, 1.25))
           BBRStartProbeBW_DOWN()

   The ancillary logic to implement the ProbeBW state machine:

     IsInAProbeBWState()
       state = BBR.state
       return (state == ProbeBW_DOWN or
               state == ProbeBW_CRUISE or
               state == ProbeBW_REFILL or
               state == ProbeBW_UP)

     /* Time to transition from DOWN to CRUISE? */
     BBRCheckTimeToCruise():
       if (inflight > BBRInflightWithHeadroom())
         return false /* not enough headroom */
       if (inflight <= BBRInflight(BBR.max_bw, 1.0))
         return true  /* inflight <= estimated BDP */
       if (BBRHasElapsedInPhase(BBR.min_rtt))
         Return true  /* at least a round in DOWN */

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     BBRHasElapsedInPhase(interval):
       return Now() > BBR.cycle_stamp + interval

     /* Return a volume of data that tries to leave free
      * headroom in the bottleneck buffer or link for
      * other flows, for fairness convergence and lower
      * RTTs and loss */
     BBRInflightWithHeadroom():
       if (BBR.inflight_hi == Infinity)
         return Infinity
       headroom = max(1, BBRHeadroom * BBR.inflight_hi)
         return max(BBR.inflight_hi - headroom,
                    BBRMinPipeCwnd)

     /* Raise inflight_hi slope if appropriate. */
     BBRRaiseInflightHiSlope():
       growth_this_round = 1MSS << BBR.bw_probe_up_rounds
       BBR.bw_probe_up_rounds = min(BBR.bw_probe_up_rounds + 1, 30)
       BBR.probe_up_cnt = max(cwnd / growth_this_round, 1)

     /* Increase inflight_hi if appropriate. */
     BBRProbeInflightHiUpward():
       if (!is_cwnd_limited or cwnd < BBR.inflight_hi)
         return  /* not fully using inflight_hi, so don't grow it */
      BBR.bw_probe_up_acks += rs.newly_acked
      if (BBR.bw_probe_up_acks >= BBR.probe_up_cnt)
        delta = BBR.bw_probe_up_acks / BBR.probe_up_cnt
        BBR.bw_probe_up_acks -= delta * BBR.bw_probe_up_cnt
        BBR.inflight_hi += delta
      if (BBR.round_start)
        BBRRaiseInflightHiSlope()

     /* Track ACK state and update BBR.max_bw window and
      * BBR.inflight_hi and BBR.bw_hi. */
     BBRAdaptUpperBounds():
       if (BBR.ack_phase == ACKS_PROBE_STARTING and BBR.round_start)
         /* starting to get bw probing samples */
         BBR.ack_phase = ACKS_PROBE_FEEDBACK
       if (BBR.ack_phase == ACKS_PROBE_STOPPING and BBR.round_start)
         /* end of samples from bw probing phase */
         if (IsInAProbeBWState() and !rs.is_app_limited)
           BBRAdvanceMaxBwFilter()

       if (!CheckInflightTooHigh())
         /* Loss rate is safe. Adjust upper bounds upward. */
         if (BBR.inflight_hi == Infinity or BBR.bw_hi == Infinity)
           return /* no upper bounds to raise */
         if (rs.tx_in_flight > BBR.inflight_hi)

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           BBR.inflight_hi = rs.tx_in_flight
         if (rs.delivery_rate > BBR.bw_hi)
           BBR.bw_hi = rs.bw
         if (BBR.state == ProbeBW_UP)
           BBRProbeInflightHiUpward()

4.3.4.  ProbeRTT

4.3.4.1.  ProbeRTT Overview

   To help probe for BBR.min_rtt, on an as-needed basis BBR flows enter
   the ProbeRTT state to try to cooperate to periodically drain the
   bottleneck queue - and thus improve their BBR.min_rtt estimate of the
   unloaded two-way propagation delay.

   A critical point is that before BBR raises its BBR.min_rtt estimate
   (which would in turn raise its maximum permissible cwnd), it first
   enters ProbeRTT to try to make a concerted and coordinated effort to
   drain the bottleneck queue and make a robust BBR.min_rtt measurement.
   This allows the BBR.min_rtt estimates of ensembles of BBR flows to
   converge avoiding feedback loops of ever-increasing queues and RTT
   samples.

   The ProbeRTT state works in concert with BBR.min_rtt estimation.  Up
   to once every ProbeRTTInterval = 5 seconds, the flow enters ProbeRTT,
   decelerating by setting its cwnd_gain to BBRProbeRTTCwndGain = 0.5 to
   reduce its volume of inflight data to half of its estimated BDP, to
   try to allow the flow to measure the unloaded two-way propagation
   delay.

   There are two main motivations for making the MinRTTFilterLen roughly
   twice the ProbeRTTInterval.  First, this ensures that during a
   ProbeRTT episode the flow will "remember" the BBR.min_rtt value it
   measured during the previous ProbeRTT episode, providing a robust bdp
   estimate for the cwnd = 0.5*bdp calculation, increasing the
   likelihood of fully draining the bottleneck queue.  Second, this
   allows the flow's BBR.min_rtt filter window to generally include RTT
   samples from two ProbeTT episodes, providing a more robust estimate.

   The algorithm for ProbeRTT is as follows:

   Entry conditions: In any state other than ProbeRTT itself, if the
   BBR.probe_rtt_min_delay estimate has not been updated (i.e., by
   getting a lower RTT measurement) for more than ProbeRTTInterval = 5
   seconds, then BBR enters ProbeRTT and reduces the BBR.cwnd_gain to
   BBRProbeRTTCwndGain = 0.5.

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   Exit conditions: After maintaining the volume of in-flight data at
   BBRProbeRTTCwndGain*BBR.bdp for at least ProbeRTTDuration (200 ms)
   and at least one round trip, BBR leaves ProbeRTT and transitions to
   ProbeBW if it estimates the pipe was filled already, or Startup
   otherwise.

4.3.4.2.  ProbeRTT Design Rationale

   BBR is designed to have ProbeRTT sacrifice no more than roughly 2% of
   a flow's available bandwidth.  It is also designed to spend the vast
   majority of its time (at least roughly 96 percent) in ProbeBW and the
   rest in ProbeRTT, based on a set of tradeoffs.  ProbeRTT lasts long
   enough (at least ProbeRTTDuration = 200 ms) to allow flows with
   different RTTs to have overlapping ProbeRTT states, while still being
   short enough to bound the throughput penalty of ProbeRTT's cwnd
   capping to roughly 2%, with the average throughput targeted at:

     throughput = (200ms*0.5*BBR.bw + (5s - 200ms)*BBR.bw) / 5s
                = (.1s + 4.8s)/5s * BBR.bw = 0.98 * BBR.bw

   As discussed above, BBR's BBR.min_rtt filter window, MinRTTFilterLen,
   and time interval between ProbeRTT states, ProbeRTTInterval, work in
   concert.  BBR uses a MinRTTFilterLen equal to or longer than
   ProbeRTTInterval to allow the filter window to include at least one
   ProbeRTT.

   To allow coordination with other BBR flows, each flow MUST use the
   standard ProbeRTTInterval of 5 secs.

   An ProbeRTTInterval of 5 secs is short enough to allow quick
   convergence if traffic levels or routes change, but long enough so
   that interactive applications (e.g., Web, remote procedure calls,
   video chunks) often have natural silences or low-rate periods within
   the window where the flow's rate is low enough for long enough to
   drain its queue in the bottleneck.  Then the BBR.probe_rtt_min_delay
   filter opportunistically picks up these measurements, and the
   BBR.probe_rtt_min_delay estimate refreshes without requiring
   ProbeRTT.  This way, flows typically need only pay the 2 percent
   throughput penalty if there are multiple bulk flows busy sending over
   the entire ProbeRTTInterval window.

   As an optimization, when restarting from idle and finding that the
   BBR.probe_rtt_min_delay has expired, BBR does not enter ProbeRTT; the
   idleness is deemed a sufficient attempt to coordinate to drain the
   queue.

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4.3.4.3.  Calculating the rs.rtt RTT Sample

   Upon transmitting each packet, BBR (or the associated transport
   protocol) stores in per-packet data the wall-clock scheduled
   transmission time of the packet in packet.departure_time (see the
   "Pacing Rate: BBR.pacing_rate" section for how this is calculated).

   For every ACK that newly acknowledges some data (whether cumulatively
   or selectively), the sender's BBR implementation (or the associated
   transport protocol implementation) attempts to calculate an RTT
   sample.  The sender MUST consider any potential retransmission
   ambiguities that can arise in some transport protocols.  If some of
   the acknowledged data was not retransmitted, or some of the data was
   retransmitted but the sender can still unambiguously determine the
   RTT of the data (e.g. if the transport supports [RFC7323] TCP
   timestamps or an equivalent mechanism), then the sender calculates an
   RTT sample, rs.rtt, as follows:

     rs.rtt = Now() - packet.departure_time

4.3.4.4.  ProbeRTT Logic

   On every ACK BBR executes BBRUpdateMinRTT() to update its ProbeRTT
   scheduling state (BBR.probe_rtt_min_delay and
   BBR.probe_rtt_min_stamp) and its BBR.min_rtt estimate:

     BBRUpdateMinRTT()
       BBR.probe_rtt_expired =
         Now() > BBR.probe_rtt_min_stamp + ProbeRTTInterval
       if (rs.rtt >= 0 and
           (rs.rtt < BBR.probe_rtt_min_delay or
            BBR.probe_rtt_expired))
          BBR.probe_rtt_min_delay = rs.rtt
          BBR.probe_rtt_min_stamp = Now()

       min_rtt_expired =
         Now() > BBR.min_rtt_stamp + MinRTTFilterLen
       if (BBR.probe_rtt_min_delay < BBR.min_rtt or
           min_rtt_expired)
         BBR.min_rtt       = BBR.probe_rtt_min_delay
         BBR.min_rtt_stamp = BBR.probe_rtt_min_stamp

   Here BBR.probe_rtt_expired is a boolean recording whether the
   BBR.probe_rtt_min_delay has expired and is due for a refresh, via
   either an application idle period or a transition into ProbeRTT
   state.

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   On every ACK BBR executes BBRCheckProbeRTT() to handle the steps
   related to the ProbeRTT state as follows:

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     BBRCheckProbeRTT():
       if (BBR.state != ProbeRTT and
           BBR.probe_rtt_expired and
           not BBR.idle_restart)
         BBREnterProbeRTT()
         BBRSaveCwnd()
         BBR.probe_rtt_done_stamp = 0
         BBR.ack_phase = ACKS_PROBE_STOPPING
         BBRStartRound()
       if (BBR.state == ProbeRTT)
         BBRHandleProbeRTT()
       if (rs.delivered > 0)
         BBR.idle_restart = false

     BBREnterProbeRTT():
       BBR.state = ProbeRTT
       BBR.pacing_gain = 1
       BBR.cwnd_gain = BBRProbeRTTCwndGain  /* 0.5 */

     BBRHandleProbeRTT():
       /* Ignore low rate samples during ProbeRTT: */
       MarkConnectionAppLimited()
       if (BBR.probe_rtt_done_stamp == 0 and
           packets_in_flight <= BBRProbeRTTCwnd())
         /* Wait for at least ProbeRTTDuration to elapse: */
         BBR.probe_rtt_done_stamp =
           Now() + ProbeRTTDuration
         /* Wait for at least one round to elapse: */
         BBR.probe_rtt_round_done = false
         BBRStartRound()
       else if (BBR.probe_rtt_done_stamp != 0)
         if (BBR.round_start)
           BBR.probe_rtt_round_done = true
         if (BBR.probe_rtt_round_done)
           BBRCheckProbeRTTDone()

     BBRCheckProbeRTTDone():
       if (BBR.probe_rtt_done_stamp != 0 and
           Now() > BBR.probe_rtt_done_stamp)
         /* schedule next ProbeRTT: */
         BBR.probe_rtt_min_stamp = Now()
         BBRRestoreCwnd()
         BBRExitProbeRTT()

     MarkConnectionAppLimited():
       C.app_limited =
         (C.delivered + packets_in_flight) ? : 1

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4.3.4.5.  Exiting ProbeRTT

   When exiting ProbeRTT, BBR transitions to ProbeBW if it estimates the
   pipe was filled already, or Startup otherwise.

   When transitioning out of ProbeRTT, BBR calls BBRResetLowerBounds()
   to reset the lower bounds, since any congestion encountered in
   ProbeRTT may have pulled the short-term model far below the capacity
   of the path.

   But the algorithm is cautious in timing the next bandwidth probe:
   raising inflight after ProbeRTT may cause loss, so the algorithm
   resets the bandwidth-probing clock by starting the cycle at
   ProbeBW_DOWN().  But then as an optimization, since the connection is
   exiting ProbeRTT, we know that infligh is already below the estimated
   BDP, so the connection can proceed immediately to ProbeBW_CRUISE.

   To summarize, the logic for exiting ProbeRTT is as follows:

     BBRExitProbeRTT():
       BBRResetLowerBounds()
       if (BBR.filled_pipe)
         BBRStartProbeBW_DOWN()
         BBRStartProbeBW_CRUISE()
       else
         BBREnterStartup()

4.4.  Restarting From Idle

4.4.1.  Setting Pacing Rate in ProbeBW

   When restarting from idle in ProbeBW states, BBR leaves its cwnd as-
   is and paces packets at exactly BBR.bw, aiming to return as quickly
   as possible to its target operating point of rate balance and a full
   pipe.  Specifically, if the flow's BBR.state is ProbeBW, and the flow
   is application-limited, and there are no packets in flight currently,
   then at the moment the flow sends one or more packets BBR sets
   BBR.pacing_rate to exactly BBR.bw.  More precisely, the BBR algorithm
   takes the following steps in BBRHandleRestartFromIdle() before
   sending a packet for a flow.

   The "Restarting Idle Connections" section of [RFC5681] suggests
   restarting from idle by slow-starting from the initial window.
   However, this approach was assuming a congestion control algorithm
   that had no estimate of the bottleneck bandwidth and no pacing, and
   thus resorted to relying on slow-starting driven by an ACK clock.
   The long (log_2(BDP)*RTT) delays required to reach full utilization
   with that "slow start after idle" approach caused many large

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   deployments to disable this mechanism, resulting in a "BDP-scale
   line-rate burst" approach instead.  Instead of these two approaches,
   BBR restarts by pacing at BBR.bw, typically achieving approximate
   rate balance and a full pipe after only one BBR.min_rtt has elapsed.

4.4.2.  Checking for ProberRTT Completion

   As an optimization, when restarting from idle BBR checks to see if
   the connection is in ProbeRTT and has met the exit conditions for
   ProbeRTT.  If a connection goes idle during ProbeRTT then often it
   will have met those exit conditions by the time it restarts, so that
   the connection can restore the cwnd to its full value before it
   starts transmitting a new flight of data.

4.4.3.  Logic

   The BBR algorithm takes the following steps in
   BBRHandleRestartFromIdle() before sending a packet for a flow:

     BBRHandleRestartFromIdle():
       if (packets_in_flight == 0 and C.app_limited)
         BBR.idle_restart = true
            BBR.extra_acked_interval_start = Now()
         if (IsInAProbeBWState())
           BBRSetPacingRateWithGain(1)
         else if (BBR.state == ProbeRTT)
           BBRCheckProbeRTTDone()

4.5.  Updating Network Path Model Parameters

   BBR is a model-based congestion control algorithm: it is based on an
   explicit model of the network path over which a transport flow
   travels.  The following is a summary of each parameter, including its
   meaning and how the algorithm calculates and uses its value.  We can
   group the parameter into three groups:

   *  core state machine parameters

   *  parameters to model the data rate

   *  parameters to model the volume of in-flight data

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4.5.1.  Tracking Packet-Timed Round Trips and BBR.round_count

   Several aspects of the BBR algorithm depend on counting the progress
   of "packet-timed" round trips, which start at the transmission of
   some segment, and then end at the acknowledgement of that segment.
   BBR.round_count is a count of the number of these "packet-timed"
   round trips elapsed so far.  BBR uses this virtual BBR.round_count
   because it is more robust than using wall clock time.  In particular,
   arbitrary intervals of wall clock time can elapse due to application
   idleness, variations in RTTs, or timer delays for retransmission
   timeouts, causing wall-clock-timed model parameter estimates to "time
   out" or to be "forgotten" too quickly to provide robustness.

   BBR counts packet-timed round trips by recording state about a
   sentinel packet, and waiting for an ACK of any data packet that was
   sent after that sentinel packet, using the following pseudocode:

   Upon connection initialization:

     BBRInitRoundCounting():
       BBR.next_round_delivered = 0
       BBR.round_start = false
       BBR.round_count = 0

   Upon sending each packet, the rate estimation algorithm [draft-cheng-
   iccrg-delivery-rate-estimation] records the amount of data thus far
   acknowledged as delivered:

     packet.delivered = C.delivered

   Upon receiving an ACK for a given data packet, the rate estimation
   algorithm [draft-cheng-iccrg-delivery-rate-estimation] updates the
   amount of data thus far acknowledged as delivered:

       C.delivered += packet.size

   Upon receiving an ACK for a given data packet, the BBR algorithm
   first executes the following logic to see if a round trip has
   elapsed, and if so, increment the count of such round trips elapsed:

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     BBRUpdateRound():
       if (packet.delivered >= BBR.next_round_delivered)
         BBRStartRound()
         BBR.round_count++
         BBR.rounds_since_probe++
         BBR.round_start = true
       else
         BBR.round_start = false

     BBRStartRound():
       BBR.next_round_delivered = C.delivered

4.5.2.  BBR.max_bw

   BBR.max_bw is BBR's estimate of the maximum bottleneck bandwidth
   available to data transmissions for the transport flow.  At any time,
   a transport connection's data transmissions experience some slowest
   link or bottleneck.  The bottleneck's delivery rate determines the
   connection's maximum data-delivery rate.  BBR tries to closely match
   its sending rate to this bottleneck delivery rate to help seek "rate
   balance", where the flow's packet arrival rate at the bottleneck
   equals the departure rate.  The bottleneck rate varies over the life
   of a connection, so BBR continually estimates BBR.max_bw using recent
   signals.

4.5.2.1.  Delivery Rate Samples for Estimating BBR.max_bw

   Since calculating delivery rate samples is subtle, and the samples
   are useful independent of congestion control, the approach BBR uses
   for measuring each single delivery rate sample is specified in a
   separate Internet Draft [draft-cheng-iccrg-delivery-rate-estimation].

4.5.2.2.  BBR.max_bw Max Filter

   Delivery rate samples are often below the typical bottleneck
   bandwidth available to the flow, due to "noise" introduced by random
   variation in physical transmission processes (e.g. radio link layer
   noise) or queues or along the network path.  To filter these effects
   BBR uses a max filter: BBR estimates BBR.max_bw using the windowed
   maximum recent delivery rate sample seen by the connection over
   recent history.

   The BBR.max_bw max filter window covers a time period extending over
   the past two ProbeBW cycles.  The BBR.max_bw max filter window length
   is driven by trade-offs among several considerations:

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   *  It is long enough to cover at least one entire ProbeBW cycle (see
      the "ProbeBW" section).  This ensures that the window contains at
      least some delivery rate samples that are the result of data
      transmitted with a super-unity pacing_gain (a pacing_gain larger
      than 1.0).  Such super-unity delivery rate samples are
      instrumental in revealing the path's underlying available
      bandwidth even when there is noise from delivery rate shortfalls
      due to aggregation delays, queuing delays from variable cross-
      traffic, lossy link layers with uncorrected losses, or short-term
      buffer exhaustion (e.g., brief coincident bursts in a shallow
      buffer).

   *  It aims to be long enough to cover short-term fluctuations in the
      network's delivery rate due to the aforementioned sources of
      noise.  In particular, the delivery rate for radio link layers
      (e.g., wifi and cellular technologies) can be highly variable, and
      the filter window needs to be long enough to remember "good"
      delivery rate samples in order to be robust to such variations.

   *  It aims to be short enough to respond in a timely manner to
      sustained reductions in the bandwidth available to a flow, whether
      this is because other flows are using a larger share of the
      bottleneck, or the bottleneck link service rate has reduced due to
      layer 1 or layer 2 changes, policy changes, or routing changes.
      In any of these cases, existing BBR flows traversing the
      bottleneck should, in a timely manner, reduce their BBR.max_bw
      estimates and thus pacing rate and in-flight data, in order to
      match the sending behavior to the new available bandwidth.

4.5.2.3.  BBR.max_bw and Application-limited Delivery Rate Samples

   Transmissions can be application-limited, meaning the transmission
   rate is limited by the application rather than the congestion control
   algorithm.  This is quite common because of request/response traffic.
   When there is a transmission opportunity but no data to send, the
   delivery rate sampler marks the corresponding bandwidth sample(s) as
   application-limited [draft-cheng-iccrg-delivery-rate-estimation].
   The BBR.max_bw estimator carefully decides which samples to include
   in the bandwidth model to ensure that BBR.max_bw reflects network
   limits, not application limits.  By default, the estimator discards
   application-limited samples, since by definition they reflect
   application limits.  However, the estimator does use application-
   limited samples if the measured delivery rate happens to be larger
   than the current BBR.max_bw estimate, since this indicates the
   current BBR.Max_bw estimate is too low.

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4.5.2.4.  Updating the BBR.max_bw Max Filter

   For every ACK that acknowledges some data packets as delivered, BBR
   invokes BBRUpdateMaxBw() to update the BBR.max_bw estimator as
   follows (here rs.delivery_rate is the delivery rate sample obtained
   from the ACK that is being processed, as specified in [draft-cheng-
   iccrg-delivery-rate-estimation]):

     BBRUpdateMaxBw()
       BBRUpdateRound()
       if (rs.delivery_rate >= BBR.max_bw || !rs.is_app_limited)
           BBR.max_bw = update_windowed_max_filter(
                         filter=BBR.MaxBwFilter,
                         value=rs.delivery_rate,
                         time=BBR.cycle_count,
                         window_length=MaxBwFilterLen)

4.5.2.5.  Tracking Time for the BBR.max_bw Max Filter

   BBR tracks time for the BBR.max_bw filter window using a virtual
   (non-wall-clock) time tracked by counting the cyclical progression
   through ProbeBW cycles.  Each time through the Probe bw cycle, one
   round trip after exiting ProbeBW_UP (the point at which the flow has
   its best chance to measure the highest throughput of the cycle), BBR
   increments BBR.cycle_count, the virtual time used by the BBR.max_bw
   filter window.  Note that BBR.cycle_count only needs to be tracked
   with a single bit, since the BBR.max_bw filter only needs to track
   samples from two time slots: the previous ProbeBW cycle and the
   current ProbeBW cycle:

     BBRAdvanceMaxBwFilter():
       BBR.cycle_count++

4.5.3.  BBR.min_rtt

   BBR.min_rtt is BBR's estimate of the round-trip propagation delay of
   the path over which a transport connection is sending.  The path's
   round-trip propagation delay determines the minimum amount of time
   over which the connection must be willing to sustain transmissions at
   the BBR.bw rate, and thus the minimum amount of data needed in-
   flight, for the connection to reach full utilization (a "Full Pipe").
   The round-trip propagation delay can vary over the life of a
   connection, so BBR continually estimates BBR.min_rtt using recent
   round-trip delay samples.

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4.5.3.1.  Round-Trip Time Samples for Estimating BBR.min_rtt

   For every data packet a connection sends, BBR calculates an RTT
   sample that measures the time interval from sending a data packet
   until that packet is acknowledged.

   For the most part, the same considerations and mechanisms that apply
   to RTT estimation for the purposes of retransmission timeout
   calculations [RFC6298] apply to BBR RTT samples.  Namely, BBR does
   not use RTT samples based on the transmission time of retransmitted
   packets, since these are ambiguous, and thus unreliable.  Also, BBR
   calculates RTT samples using both cumulative and selective
   acknowledgments (if the transport supports [RFC2018] SACK options or
   an equivalent mechanism), or transport-layer timestamps (if the
   transport supports [RFC7323] TCP timestamps or an equivalent
   mechanism).

   The only divergence from RTT estimation for retransmission timeouts
   is in the case where a given acknowledgment ACKs more than one data
   packet.  In order to be conservative and schedule long timeouts to
   avoid spurious retransmissions, the maximum among such potential RTT
   samples is typically used for computing retransmission timeouts;
   i.e., SRTT is typically calculated using the data packet with the
   earliest transmission time.  By contrast, in order for BBR to try to
   reach the minimum amount of data in flight to fill the pipe, BBR uses
   the minimum among such potential RTT samples; i.e., BBR calculates
   the RTT using the data packet with the latest transmission time.

4.5.3.2.  BBR.min_rtt Min Filter

   RTT samples tend to be above the round-trip propagation delay of the
   path, due to "noise" introduced by random variation in physical
   transmission processes (e.g. radio link layer noise), queues along
   the network path, the receiver's delayed ack strategy, ack
   aggregation, etc.  Thus to filter out these effects BBR uses a min
   filter: BBR estimates BBR.min_rtt using the minimum recent RTT sample
   seen by the connection over that past MinRTTFilterLen seconds.  (Many
   of the same network effects that can decrease delivery rate
   measurements can increase RTT samples, which is why BBR's min-
   filtering approach for RTTs is the complement of its max-filtering
   approach for delivery rates.)

   The length of the BBR.min_rtt min filter window is MinRTTFilterLen =
   10 secs.  This is driven by trade-offs among several considerations:

   *  The MinRTTFilterLen is longer than ProbeRTTInterval, so that it
      covers an entire ProbeRTT cycle (see the "ProbeRTT" section
      below).  This helps ensure that the window can contain RTT samples

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      that are the result of data transmitted with inflight below the
      estimated BDP of the flow.  Such RTT samples are important for
      helping to reveal the path's underlying two-way propagation delay
      even when the aforementioned "noise" effects can often obscure it.

   *  The MinRTTFilterLen aims to be long enough to avoid needing to cut
      in-flight and throughput often.  Measuring two-way propagation
      delay requires in-flight to be at or below BDP, which risks some
      amount of underutilization, so BBR uses a filter window long
      enough that such underutilization events can be rare.

   *  The MinRTTFilterLen aims to be long enough that many applications
      have a "natural" moment of silence or low utilization that can cut
      in-flight below BDP and naturally serve to refresh the
      BBR.min_rtt, without requiring BBR to force an artificial cut in
      in-flight.  This applies to many popular applications, including
      Web, RPC, chunked audio or video traffic.

   *  The MinRTTFilterLen aims to be short enough to respond in a timely
      manner to real increases in the two-way propagation delay of the
      path, e.g. due to route changes, which are expected to typically
      happen on longer time scales.

   A BBR implementation MAY use a generic windowed min filter to track
   BBR.min_rtt.  However, a significant savings in space and improvement
   in freshness can be achieved by integrating the BBR.min_rtt
   estimation into the ProbeRTT state machine, so this document
   discusses that approach in the ProbeRTT section.

4.5.4.  BBR.offload_budget

   BBR.offload_budget is the estimate of the minimum volume of data
   necessary to achieve full throughput using sender (TSO/GSO) and
   receiver (LRO, GRO) host offload mechanisms, computed as follows:

       BBRUpdateOffloadBudget():
         BBR.offload_budget = 3 * BBR.send_quantum

   The factor of 3 is chosen to allow maintaining at least:

   *  1 quantum in the sending host's queuing discipline layer

   *  1 quantum being segmented in the sending host TSO/GSO engine

   *  1 quantum being reassembled or otherwise remaining unacknowledged
      due to the receiver host's LRO/GRO/delayed-ACK engine

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4.5.5.  BBR.extra_acked

   BBR.extra_acked is a volume of data that is the estimate of the
   recent degree of aggregation in the network path.  For each ACK, the
   algorithm computes a sample of the estimated extra ACKed data beyond
   the amount of data that the sender expected to be ACKed over the
   timescale of a round-trip, given the BBR.bw.  Then it computes
   BBR.extra_acked as the windowed maximum sample over the last
   BBRExtraAckedFilterLen=10 packet-timed round-trips.  If the ACK rate
   falls below the expected bandwidth, then the algorithm estimates an
   aggregation episode has terminated, and resets the sampling interval
   to start from the current time.

   The BBR.extra_acked thus reflects the recently-measured magnitude of
   data and ACK aggregation effects such as batching and slotting at
   shared-medium L2 hops (wifi, cellular, DOCSIS), as well as end-host
   offload mechanisms (TSO, GSO, LRO, GRO), and end host or middlebox
   ACK decimation/thinning.

   BBR augments its cwnd by BBR.extra_acked to allow the connection to
   keep sending during inter-ACK silences, to an extent that matches the
   recently measured degree of aggregation.

   More precisely, this is computed as:

     BBRUpdateACKAggregation():
       /* Find excess ACKed beyond expected amount over this interval */
       interval = (Now() - BBR.extra_acked_interval_start)
       expected_delivered = BBR.bw * interval
       /* Reset interval if ACK rate is below expected rate: */
       if (BBR.extra_acked_delivered <= expected_delivered)
           BBR.extra_acked_delivered = 0
           BBR.extra_acked_interval_start = Now()
           expected_delivered = 0
       BBR.extra_acked_delivered += rs.newly_acked
       extra = BBR.extra_acked_delivered - expected_delivered
       extra = min(extra, cwnd)
       BBR.extra_acked =
         update_windowed_max_filter(
           filter=BBR.ExtraACKedFilter,
           value=extra,
           time=BBR.round_count,
           window_length=BBRExtraAckedFilterLen)

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4.5.6.  Updating the Model Upon Packet Loss

   In every state, BBR responds to (filtered) congestion signals,
   including loss.  The response to those congestion signals depends on
   the flow's current state, since the information that the flow can
   infer depends on what the flow was doing when the flow experienced
   the signal.

4.5.6.1.  Probing for Bandwidth In Startup

   In Startup, if the congestion signals meet the Startup exit criteria,
   the flow exits Startup and enters Drain.

4.5.6.2.  Probing for Bandwidth In ProbeBW

   BBR searches for the maximum volume of data that can be sensibly
   placed in-flight in the network.  A key precondition is that the flow
   is actually trying robustly to find that operating point.  To
   implement this, when a flow is in ProbeBW, and an ACK covers data
   sent in one of the accelerating phases (REFILL or UP), and the ACK
   indicates that the loss rate over the past round trip exceeds the
   queue pressure objective, and the flow is not application limited,
   and has not yet responded to congestion signals from the most recent
   REFILL or UP phase, then the flow estimates that the volume of data
   it allowed in flight exceeded what matches the current delivery
   process on the path, and reduces BBR.inflight_hi:

     /* Do loss signals suggest inflight is too high
      * If so, react. */
     CheckInflightTooHigh():
       if (IsInflightTooHigh(rs))
         if (BBR.bw_probe_samples)
           BBRHandleInflightTooHigh()
         return true  /* inflight too high */
       else
         return false /* inflight not too high */

     IsInflightTooHigh():
       return (rs.lost > rs.tx_in_flight * BBRLossThresh)

     BBRHandleInflightTooHigh():
       BBR.bw_probe_samples = 0;  /* only react once per bw probe */
       if (!rs.is_app_limited)
         BBR.inflight_hi = max(rs.tx_in_flight,
                               BBRTargetInflight() * BBRBeta))
       If (BBR.state == ProbeBW_UP)
         BBRStartProbeBW_DOWN()

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   Here rs.tx_in_flight is the amount of data that was estimated to be
   in flight when the most recently ACKed packet was sent.  And the
   BBRBeta (0.7x) bound is to try to ensure that BBR does not react more
   dramatically than CUBIC's 0.7x multiplicative decrease factor.

   Some loss detection algorithms, including algorithms like RACK
   [RFC8985] that delay loss marking while waiting for potential
   reordering to resolve, may mark packets as lost long after the loss
   itself happened.  In such cases, the tx_in_flight for the delivered
   sequence range that allowed the loss to be detected may be
   considerably smaller than the tx_in_flight of the lost packet itself.
   In such cases using the former tx_in_flight rather than the latter
   can cause BBR.inflight_hi to be significantly underestimated.  To
   avoid such issues, BBR processes each loss detection event to more
   precisely estimate the volume of in-flight data at which loss rates
   cross BBRLossThresh, noting that this may have happened mid-way
   through some packet.  To estimate this value, we can solve for
   "lost_prefix" in the following equation, where inflight_prev
   represents the volume of in-flight data preceding this packet,
   lost_prev represents the data lost among that previous in-flight
   data:

    lost                     /  inflight                     >= BBRLossThresh
   (lost_prev + lost_prefix) / (inflight_prev + lost_prefix) >= BBRLossThresh
   /* solving for lost_prefix we arrive at: */
   lost_prefix = (BBRLossThresh * inflight_prev - lost_prev) / (1 - BBRLossThresh)

   In pseudocode:

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     BBRHandleLostPacket(packet):
       if (!BBR.bw_probe_samples)
         return /* not a packet sent while probing bandwidth */
       rs.tx_in_flight = packet.tx_in_flight /* inflight at transmit */
       rs.lost = C.lost - packet.lost /* data lost since transmit */
       rs.is_app_limited = packet.is_app_limited;
       if (IsInflightTooHigh(rs))
         rs.tx_in_flight = BBRInflightHiFromLostPacket(rs, packet)
         BBRHandleInflightTooHigh(rs)

     /* At what prefix of packet did losses exceed BBRLossThresh? */
     BBRInflightHiFromLostPacket(rs, packet):
       size = packet.size
       /* What was in flight before this packet? */
       inflight_prev = rs.tx_in_flight - size
       /* What was lost before this packet? */
       lost_prev = rs.lost - size
       lost_prefix = (BBRLossThresh * inflight_prev - lost_prev) /
                     (1 - BBRLossThresh)
       /* At what inflight value did losses cross BBRLossThresh? */
       inflight = inflight_prev + lost_prefix
       return inflight

4.5.6.3.  When not Probing for Bandwidth

   When not explicitly accelerating to probe for bandwidth (Drain,
   ProbeRTT, ProbeBW_DOWN, ProbeBW_CRUISE), BBR responds to loss by
   slowing down to some extent.  This is because loss suggests that the
   available bandwidth and safe volume of in-flight data may have
   decreased recently, and the flow needs to adapt, slowing down toward
   the latest delivery process.  BBR flows implement this response by
   reducing the short-term model parameters, BBR.bw_lo and
   BBR.inflight_lo.

   When encountering packet loss when the flow is not probing for
   bandwidth, the strategy is to gradually adapt to the current measured
   delivery process (the rate and volume of data that is delivered
   through the network path over the last round trip).  This applies
   generally: whether in fast recovery, RTO recovery, TLP recovery;
   whether application-limited or not.

   There are two key parameters the algorithm tracks, to measure the
   current delivery process:

   BBR.bw_latest: a 1-round-trip max of delivered bandwidth
   (rs.delivery_rate).

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   BBR.inflight_latest: a 1-round-trip max of delivered volume of data
   (rs.delivered).

   Upon the ACK at the end of each round that encountered a newly-marked
   loss, the flow updates its model (bw_lo and inflight_lo) as follows:

         bw_lo     = max(       bw_latest, BBRBeta *       bw_lo )
   inflight_lo     = max( inflight_latest, BBRBeta * inflight_lo )

   This logic can be represented as follows:

     /* Near start of ACK processing: */
     BBRUpdateLatestDeliverySignals():
       BBR.loss_round_start = 0
       BBR.bw_latest       = max(BBR.bw_latest,       rs.delivery_rate)
       BBR.inflight_latest = max(BBR.inflight_latest, rs.delivered)
       if (rs.prior_delivered >= BBR.loss_round_delivered)
         BBR.loss_round_delivered = C.delivered
         BBR.loss_round_start = 1

     /* Near end of ACK processing: */
     BBRAdvanceLatestDeliverySignals():
       if (BBR.loss_round_start)
         BBR.bw_latest       = rs.delivery_rate
         BBR.inflight_latest = rs.delivered

     BBRResetCongestionSignals():
       BBR.loss_in_round = 0
       BBR.bw_latest = 0
       BBR.inflight_latest = 0

     /* Update congestion state on every ACK */
     BBRUpdateCongestionSignals():
       BBRUpdateMaxBw()
       if (rs.losses > 0)
         BBR.loss_in_round = 1
       if (!BBR.loss_round_start)
         return  /* wait until end of round trip */
       BBRAdaptLowerBoundsFromCongestion()
       BBR.loss_in_round = 0

     /* Once per round-trip respond to congestion */
     BBRAdaptLowerBoundsFromCongestion():
       if (BBRIsProbingBW())
         return
       if (BBR.loss_in_round())
         BBRInitLowerBounds()
         BBRLossLowerBounds()

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     /* Handle the first congestion episode in this cycle */
     BBRInitLowerBounds():
       if (BBR.bw_lo == Infinity)
         BBR.bw_lo = BBR.max_bw
       if (BBR.inflight_lo == Infinity)
         BBR.inflight_lo = cwnd

     /* Adjust model once per round based on loss */
     BBRLossLowerBounds()
       BBR.bw_lo       = max(BBR.bw_latest,
                             BBRBeta * BBR.bw_lo)
       BBR.inflight_lo = max(BBR.inflight_latest,
                             BBRBeta * BBR.infligh_lo)

     BBRResetLowerBounds():
       BBR.bw_lo       = Infinity
       BBR.inflight_lo = Infinity

     BBRBoundBWForModel():
       BBR.bw = min(BBR.max_bw, BBR.bw_lo, BBR.bw_hi)

4.6.  Updating Control Parameters

   BBR uses three distinct but interrelated control parameters: pacing
   rate, send quantum, and congestion window (cwnd).

4.6.1.  Summary of Control Behavior in the State Machine

   The following table summarizes how BBR modulates the control
   parameters in each state.  In the table below, the semantics of the
   columns are as follows:

   *  State: the state in the BBR state machine, as depicted in the
      "State Transition Diagram" section above.

   *  Tactic: The tactic chosen from the "State Machine Tactics"
      subsection above: "accel" refers to acceleration, "decel" to
      deceleration, and "cruise" to cruising.

   *  Pacing Gain: the value used for BBR.pacing_gain in the given
      state.

   *  Cwnd Gain: the value used for BBR.cwnd_gain in the given state.

   *  Rate Cap: the rate values applied as bounds on the BBR.max_bw
      value applied to compute BBR.bw.

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   *  Volume Cap: the volume values applied as bounds on the
      BBR.max_inflight value to compute cwnd.

   The control behavior can be summarized as follows:

+-----------------+--------+--------+------+--------+------------------+
| State           | Tactic | Pacing | Cwnd | Rate   | Volume           |
|                 |        | Gain   | Gain | Cap    | Cap              |
+-----------------+--------+--------+------+--------+------------------+
| Startup         | accel  | 2.77   | 2    |        |                  |
|                 |        |        |      |        |                  |
+-----------------+--------+--------+------+--------+------------------+
| Drain           | decel  | 0.5    | 2    | bw_hi, | inflight_hi,     |
|                 |        |        |      | bw_lo  | inflight_lo      |
+-----------------+--------+--------+------+--------+------------------+
| ProbeBW_DOWN    | decel  | 0.75   | 2    | bw_hi, | inflight_hi,     |
|                 |        |        |      | bw_lo  | inflight_lo      |
+-----------------+--------+--------+------+--------+------------------+
| ProbeBW_CRUISE  | cruise | 1.0    | 2    | bw_hi, | 0.85*inflight_hi |
|                 |        |        |      | bw_lo  | inflight_lo      |
+-----------------+--------+--------+------+--------+------------------+
| ProbeBW_REFILL  | accel  | 1.0    | 2    | bw_hi  | inflight_hi      |
|                 |        |        |      |        |                  |
+-----------------+--------+--------+------+--------+------------------+
| ProbeBW_UP      | accel  | 1.25   | 2    | bw_hi  | inflight_hi      |
|                 |        |        |      |        |                  |
+-----------------+--------+--------+------+--------+------------------+
| ProbeRTT        | decel  | 1.0    | 0.5  | bw_hi, | 0.85*inflight_hi |
|                 |        |        |      | bw_lo  | inflight_lo      |
+-----------------+--------+--------+------+--------+------------------+

   Upon processing each ACK, BBR uses the values in the table above to
   compute BBR.bw in BBRBoundBWForModel(), and the cwnd in
   BBRBoundCwndForModel().

   The pacing gain values are chosen so that the average of the
   ProbeBW_UP pacing gain of 1.25 and ProbeBW_DOWN pacing gain of 0.75
   is a value of 1.0.  Since the ProbeBW_CRUISE and ProbeBW_REFILL
   states also have a pacing gain of 1.0, on average during the entire
   cycle the pacing gain is 1.0.  This is done to allow for the average
   pacing rate to equal BBR.bw, the estimated available bandwidth, and
   thus allow for the flow -- to the extent that its BBR.bw estimate is
   accurate -- to maintain high utilization, while maintaining a small,
   well-bounded queue.

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4.6.2.  Pacing Rate: BBR.pacing_rate

   To help match the packet-arrival rate to the bottleneck bandwidth
   available to the flow, BBR paces data packets.  Pacing enforces a
   maximum rate at which BBR schedules quanta of packets for
   transmission.

   The sending host implements pacing by maintaining inter-quantum
   spacing at the time each packet is scheduled for departure,
   calculating the next departure time for a packet for a given flow
   (BBR.next_departure_time) as a function of the most recent packet
   size and the current pacing rate, as follows:

     BBR.next_departure_time = max(Now(), BBR.next_departure_time)
     packet.departure_time = BBR.next_departure_time
     pacing_delay = packet.size / BBR.pacing_rate
     BBR.next_departure_time = BBR.next_departure_time + pacing_delay

   To adapt to the bottleneck, in general BBR sets the pacing rate to be
   proportional to bw, with a dynamic gain, or scaling factor of
   proportionality, called pacing_gain.

   When a BBR flow starts it has no bw estimate (bw is 0).  So in this
   case it sets an initial pacing rate based on the transport sender
   implementation's initial congestion window ("InitialCwnd", e.g. from
   [RFC6928]), the initial SRTT (smoothed round-trip time) after the
   first non-zero RTT sample, and the initial pacing_gain:

     BBRInitPacingRate():
       nominal_bandwidth = InitialCwnd / (SRTT ? SRTT : 1ms)
       BBR.pacing_rate =  BBRStartupPacingGain * nominal_bandwidth

   After initialization, on each data ACK BBR updates its pacing rate to
   be proportional to bw, as long as it estimates that it has filled the
   pipe (BBR.filled_pipe is true; see the "Startup" section for
   details), or doing so increases the pacing rate.  Limiting the pacing
   rate updates in this way helps the connection probe robustly for
   bandwidth until it estimates it has reached its full available
   bandwidth ("filled the pipe").  In particular, this prevents the
   pacing rate from being reduced when the connection has only seen
   application-limited bandwidth samples.  BBR updates the pacing rate
   on each ACK by executing the BBRSetPacingRate() step as follows:

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     BBRSetPacingRateWithGain(pacing_gain):
       rate = pacing_gain * bw * (100 - BBRPacingMarginPercent) / 100
       if (BBR.filled_pipe || rate > BBR.pacing_rate)
         BBR.pacing_rate = rate

     BBRSetPacingRate():
       BBRSetPacingRateWithGain(BBR.pacing_gain)

   To help drive the network toward lower queues and low latency while
   maintaining high utilization, the BBRPacingMarginPercent constant of
   1 aims to cause BBR to pace at 1% below the bw, on average.

4.6.3.  Send Quantum: BBR.send_quantum

   In order to amortize per-packet overheads involved in the sending
   process (host CPU, NIC processing, and interrupt processing delays),
   high-performance transport sender implementations (e.g., Linux TCP)
   often schedule an aggregate containing multiple packets (multiple
   SMSS) worth of data as a single quantum (using TSO, GSO, or other
   offload mechanisms).  The BBR congestion control algorithm makes this
   control decision explicitly, dynamically calculating a quantum
   control parameter that specifies the maximum size of these
   transmission aggregates.  This decision is based on a trade-off:

   *  A smaller quantum is preferred at lower data rates because it
      results in shorter packet bursts, shorter queues, lower queueing
      delays, and lower rates of packet loss.

   *  A bigger quantum can be required at higher data rates because it
      results in lower CPU overheads at the sending and receiving hosts,
      who can ship larger amounts of data with a single trip through the
      networking stack.

   On each ACK, BBR runs BBRSetSendQuantum() to update BBR.send_quantum
   as follows:

     BBRSetSendQuantum():
       if (BBR.pacing_rate < 1.2 Mbps)
         floor = 1 * SMSS
       else
         floor = 2 * SMSS
       BBR.send_quantum = min(BBR.pacing_rate * 1ms, 64KBytes)
       BBR.send_quantum = max(BBR.send_quantum, floor)

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   A BBR implementation MAY use alternate approaches to select a
   BBR.send_quantum, as appropriate for the CPU overheads anticipated
   for senders and receivers, and buffering considerations anticipated
   in the network path.  However, for the sake of the network and other
   users, a BBR implementation SHOULD attempt to use the smallest
   feasible quanta.

4.6.4.  Congestion Window

   The congestion window, or cwnd, controls the maximum volume of data
   BBR allows in flight in the network at any time.  It is the maximum
   volume of in-flight data that the algorithm estimates is appropriate
   for matching the current network path delivery process, given all
   available signals in the model, at any time scale.  BBR adapts the
   cwnd based on its model of the network path and the state machine's
   decisions about how to probe that path.

   By default, BBR grows its cwnd to meet its BBR.max_inflight, which
   models what's required for achieving full throughput, and as such is
   scaled to adapt to the estimated BDP computed from its path model.
   But BBR's selection of cwnd is designed to explicitly trade off among
   competing considerations that dynamically adapt to various
   conditions.  So in loss recovery BBR more conservatively adjusts its
   sending behavior based on more recent delivery samples, and if BBR
   needs to re-probe the current BBR.min_rtt of the path then it cuts
   its cwnd accordingly.  The following sections describe the various
   considerations that impact cwnd.

4.6.4.1.  Initial cwnd

   BBR generally uses measurements to build a model of the network path
   and then adapts control decisions to the path based on that model.
   As such, the selection of the initial cwnd is considered to be
   outside the scope of the BBR algorithm, since at initialization there
   are no measurements yet upon which BBR can operate.  Thus, at
   initialization, BBR uses the transport sender implementation's
   initial congestion window (e.g. from [RFC6298] for TCP).

4.6.4.2.  Computing BBR.max_inflight

   The BBR BBR.max_inflight is the upper bound on the volume of data BBR
   allows in flight.  This bound is always in place, and dominates when
   all other considerations have been satisfied: the flow is not in loss
   recovery, does not need to probe BBR.min_rtt, and has accumulated
   confidence in its model parameters by receiving enough ACKs to
   gradually grow the current cwnd to meet the BBR.max_inflight.

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   On each ACK, BBR calculates the BBR.max_inflight in
   BBRUpdateMaxInflight() as follows:

     BBRBDPMultiple(gain):
       if (BBR.min_rtt == Inf)
         return InitialCwnd /* no valid RTT samples yet */
       BBR.bdp = BBR.bw * BBR.min_rtt
       return gain * BBR.bdp

     BBRQuantizationBudget(inflight)
       BBRUpdateOffloadBudget()
       inflight = max(inflight, BBR.offload_budget)
       inflight = max(inflight, BBRMinPipeCwnd)
         if (BBR.state == ProbeBW && BBR.cycle_idx == ProbeBW_UP)
         inflight += 2
       return inflight

     BBRInflight(gain):
       inflight = BBRBDPMultiple(gain)
       return BBRQuantizationBudget(inflight)

     BBRUpdateMaxInflight():
       BBRUpdateAggregationBudget()
       inflight = BBRBDPMultiple(BBR.cwnd_gain)
       inflight += BBR.extra_acked
       BBR.max_inflight = BBRQuantizationBudget(inflight)

   The "estimated_bdp" term tries to allow enough packets in flight to
   fully utilize the estimated BDP of the path, by allowing the flow to
   send at BBR.bw for a duration of BBR.min_rtt.  Scaling up the BDP by
   BBR.cwnd_gain bounds in-flight data to a small multiple of the BDP,
   to handle common network and receiver behavior, such as delayed,
   stretched, or aggregated ACKs [A15].  The "quanta" term allows enough
   quanta in flight on the sending and receiving hosts to reach high
   throughput even in environments using offload mechanisms.

4.6.4.3.  Minimum cwnd for Pipelining

   For BBR.max_inflight, BBR imposes a floor of BBRMinPipeCwnd (4
   packets, i.e. 4 * SMSS).  This floor helps ensure that even at very
   low BDPs, and with a transport like TCP where a receiver may ACK only
   every alternate SMSS of data, there are enough packets in flight to
   maintain full pipelining.  In particular BBR tries to allow at least
   2 data packets in flight and ACKs for at least 2 data packets on the
   path from receiver to sender.

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4.6.4.4.  Modulating cwnd in Loss Recovery

   BBR interprets loss as a hint that there may be recent changes in
   path behavior that are not yet fully reflected in its model of the
   path, and thus it needs to be more conservative.

   Upon a retransmission timeout (RTO), BBR conservatively reduces cwnd
   to a value that will allow 1 SMSS to be transmitted.  Then BBR
   gradually increases cwnd using the normal approach outlined below in
   "Core cwnd Adjustment Mechanism".

   When a BBR sender detects packet loss but there are still packets in
   flight, on the first round of the loss-repair process BBR temporarily
   reduces the cwnd to match the current delivery rate as ACKs arrive.
   On second and later rounds of loss repair, it ensures the sending
   rate never exceeds twice the current delivery rate as ACKs arrive.

   When BBR exits loss recovery it restores the cwnd to the "last known
   good" value that cwnd held before entering recovery.  This applies
   equally whether the flow exits loss recovery because it finishes
   repairing all losses or because it executes an "undo" event after
   inferring that a loss recovery event was spurious.

   There are several ways to implement this high-level design for
   updating cwnd in loss recovery.  One is as follows:

   Upon retransmission timeout (RTO):

     BBROnEnterRTO():
       BBR.prior_cwnd = BBRSaveCwnd()
       cwnd = packets_in_flight + 1

   Upon entering Fast Recovery, set cwnd to the number of packets still
   in flight (allowing at least one for a fast retransmit):

     BBROnEnterFastRecovery():
       BBR.prior_cwnd = BBRSaveCwnd()
       cwnd = packets_in_flight + max(rs.newly_acked, 1)
       BBR.packet_conservation = true

   Upon every ACK in Fast Recovery, run the following
   BBRModulateCwndForRecovery() steps, which help ensure packet
   conservation on the first round of recovery, and sending at no more
   than twice the current delivery rate on later rounds of recovery
   (given that "rs.newly_acked" packets were newly marked ACKed or
   SACKed and "rs.newly_lost" were newly marked lost):

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     BBRModulateCwndForRecovery():
       if (rs.newly_lost > 0)
         cwnd = max(cwnd - rs.newly_lost, 1)
       if (BBR.packet_conservation)
         cwnd = max(cwnd, packets_in_flight + rs.newly_acked)

   After one round-trip in Fast Recovery:

     BBR.packet_conservation = false

   Upon exiting loss recovery (RTO recovery or Fast Recovery), either by
   repairing all losses or undoing recovery, BBR restores the best-known
   cwnd value we had upon entering loss recovery:

     BBR.packet_conservation = false
     BBRRestoreCwnd()

   Note that exiting loss recovery happens during ACK processing, and at
   the end of ACK processing BBRBoundCwndForModel() will bound the cwnd
   based on the current model parameters.  Thus the cwnd and pacing rate
   after loss recovery will generally be smaller than the values
   entering loss recovery.

   The BBRSaveCwnd() and BBRRestoreCwnd() helpers help remember and
   restore the last-known good cwnd (the latest cwnd unmodulated by loss
   recovery or ProbeRTT), and is defined as follows:

     BBRSaveCwnd():
       if (!InLossRecovery() and BBR.state != ProbeRTT)
         return cwnd
       else
         return max(BBR.prior_cwnd, cwnd)

     BBRRestoreCwnd():
       cwnd = max(cwnd, BBR.prior_cwnd)

4.6.4.5.  Modulating cwnd in ProbeRTT

   If BBR decides it needs to enter the ProbeRTT state (see the
   "ProbeRTT" section below), its goal is to quickly reduce the volume
   of in-flight data and drain the bottleneck queue, thereby allowing
   measurement of BBR.min_rtt.  To implement this mode, BBR bounds the
   cwnd to BBRMinPipeCwnd, the minimal value that allows pipelining (see
   the "Minimum cwnd for Pipelining" section, above):

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     BBRProbeRTTCwnd():
       probe_rtt_cwnd = BBRBDPMultiple(BBR.bw, BBRProbeRTTCwndGain)
       probe_rtt_cwnd = max(probe_rtt_cwnd, BBRMinPipeCwnd)
       return probe_rtt_cwnd

     BBRBoundCwndForProbeRTT():
       if (BBR.state == ProbeRTT)
         cwnd = min(cwnd, BBRProbeRTTCwnd())

4.6.4.6.  Core cwnd Adjustment Mechanism

   The network path and traffic traveling over it can make sudden
   dramatic changes.  To adapt to these changes smoothly and robustly,
   and reduce packet losses in such cases, BBR uses a conservative
   strategy.  When cwnd is above the BBR.max_inflight derived from BBR's
   path model, BBR cuts the cwnd immediately to the BBR.max_inflight.
   When cwnd is below BBR.max_inflight, BBR raises the cwnd gradually
   and cautiously, increasing cwnd by no more than the amount of data
   acknowledged (cumulatively or selectively) upon each ACK.

   Specifically, on each ACK that acknowledges "rs.newly_acked" packets
   as newly ACKed or SACKed, BBR runs the following BBRSetCwnd() steps
   to update cwnd:

     BBRSetCwnd():
       BBRUpdateMaxInflight()
       BBRModulateCwndForRecovery()
       if (!BBR.packet_conservation) {
         if (BBR.filled_pipe)
           cwnd = min(cwnd + rs.newly_acked, BBR.max_inflight)
         else if (cwnd < BBR.max_inflight || C.delivered < InitialCwnd)
           cwnd = cwnd + rs.newly_acked
         cwnd = max(cwnd, BBRMinPipeCwnd)
       }
       BBRBoundCwndForProbeRTT()
       BBRBoundCwndForModel()

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   There are several considerations embodied in the logic above.  If BBR
   has measured enough samples to achieve confidence that it has filled
   the pipe (see the description of BBR.filled_pipe in the "Startup"
   section below), then it increases its cwnd based on the number of
   packets delivered, while bounding its cwnd to be no larger than the
   BBR.max_inflight adapted to the estimated BDP.  Otherwise, if the
   cwnd is below the BBR.max_inflight, or the sender has marked so
   little data delivered (less than InitialCwnd) that it does not yet
   judge its BBR.max_bw estimate and BBR.max_inflight as useful, then it
   increases cwnd without bounding it to be below BBR.max_inflight.
   Finally, BBR imposes a floor of BBRMinPipeCwnd in order to allow
   pipelining even with small BDPs (see the "Minimum cwnd for
   Pipelining" section, above).

4.6.4.7.  Bounding cwnd Based on Recent Congestion

   Finally, BBR bounds the cwnd based on recent congestion, as outlined
   in the "Volume Cap" column of the table in the "Summary of Control
   Behavior in the State Machine" section:

     BBRBoundCwndForModel():
       cap = Infinity
       if (IsInAProbeBWState() and
           BBR.state != ProbeBW_CRUISE)
         cap = BBR.inflight_hi
       else if (BBR.state == ProbeRTT or
                BBR.state == ProbeBW_CRUISE)
         cap = BBRInflightWithHeadroom()

       /* apply inflight_lo (possibly infinite): */
       cap = min(cap, BBR.inflight_lo)
       cap = max(cap, BBRMinPipeCwnd)
       cwnd = min(cwnd, cap)

5.  Implementation Status

   This section records the status of known implementations of the
   algorithm defined by this specification at the time of posting of
   this Internet-Draft, and is based on a proposal described in
   [RFC7942].  The description of implementations in this section is
   intended to assist the IETF in its decision processes in progressing
   drafts to RFCs.  Please note that the listing of any individual
   implementation here does not imply endorsement by the IETF.
   Furthermore, no effort has been spent to verify the information
   presented here that was supplied by IETF contributors.  This is not
   intended as, and must not be construed to be, a catalog of available
   implementations or their features.  Readers are advised to note that
   other implementations may exist.

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   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   As of the time of writing, the following implementations of BBR have
   been publicly released:

   *  Linux TCP

      -  Source code URL:

         o  https://github.com/google/bbr/blob/v2alpha/README.md

         o  https://github.com/google/bbr/blob/v2alpha/net/ipv4/
            tcp_bbr2.c

      -  Source: Google

      -  Maturity: production

      -  License: dual-licensed: GPLv2 / BSD

      -  Contact: https://groups.google.com/d/forum/bbr-dev

      -  Last updated: August 21, 2021

   *  QUIC

      -  Source code URLs:

         o  https://cs.chromium.org/chromium/src/net/third_party/quiche/
            src/quic/core/congestion_control/bbr2_sender.cc

         o  https://cs.chromium.org/chromium/src/net/third_party/quiche/
            src/quic/core/congestion_control/bbr2_sender.h

      -  Source: Google

      -  Maturity: production

      -  License: BSD-style

      -  Contact: https://groups.google.com/d/forum/bbr-dev

      -  Last updated: October 21, 2021

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6.  Security Considerations

   This proposal makes no changes to the underlying security of
   transport protocols or congestion control algorithms.  BBR shares the
   same security considerations as the existing standard congestion
   control algorithm [RFC5681].

7.  IANA Considerations

   This document has no IANA actions.  Here we are using that phrase,
   suggested by [RFC5226], because BBR does not modify or extend the
   wire format of any network protocol, nor does it add new dependencies
   on assigned numbers.  BBR involves only a change to the congestion
   control algorithm of a transport sender, and does not involve changes
   in the network, the receiver, or any network protocol.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

8.  Acknowledgments

   The authors are grateful to Len Kleinrock for his work on the theory
   underlying congestion control.  We are indebted to Larry Brakmo for
   pioneering work on the Vegas [BP95] and New Vegas [B15] congestion
   control algorithms, which presaged many elements of BBR, and for
   Larry's advice and guidance during BBR's early development.  The
   authors would also like to thank Kevin Yang, Priyaranjan Jha, Yousuk
   Seung, Luke Hsiao for their work on TCP BBR; Jana Iyengar, Victor
   Vasiliev, Bin Wu for their work on QUIC BBR; and Matt Mathis for his
   research work on the BBR algorithm and its implications [MM19].  We
   would also like to thank C.  Stephen Gunn, Eric Dumazet, Nandita
   Dukkipati, Pawel Jurczyk, Biren Roy, David Wetherall, Amin Vahdat,
   Leonidas Kontothanassis, and the YouTube, google.com, Bandwidth
   Enforcer, and Google SRE teams for their invaluable help and support.
   We would like to thank Randall R.  Stewart, Jim Warner, Loganaden
   Velvindron, and Hiren Panchasara for feedback and suggestions on
   earlier versions of this document.

9.  References

9.1.  Normative References

   [RFC793]   Postel, J., "Transmission Control Protocol", September
              1981.

   [RFC2018]  Mathis, M. and J. Mahdavi, "TCP Selective Acknowledgment
              Options", RFC 2018, October 1996,
              <http://www.rfc-editor.org/rfc/rfc2018.txt>.

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   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, "TCP Extensions for High Performance",
              September 2014.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997,
              <http://www.rfc-editor.org/rfc/rfc2119.txt>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", May 2008.

   [RFC6298]  Paxson, V., "Computing TCP's Retransmission Timer",
              RFC 6298, June 2011,
              <https://wiki.tools.ietf.org/html/rfc6298>.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009,
              <https://tools.ietf.org/html/rfc5681>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", July 2016.

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              February 2018, <https://tools.ietf.org/html/rfc8312>.

   [RFC8985]  Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The
              RACK-TLP Loss Detection Algorithm for TCP", RFC 8985,
              DOI 10.17487/RFC8985, February 2021,
              <https://www.rfc-editor.org/info/rfc8985>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

9.2.  Informative References

   [draft-cheng-iccrg-delivery-rate-estimation]
              Cheng, Y., Cardwell, N., Hassas Yeganeh, S., and V.
              Jacobson, "Delivery Rate Estimation", Work in Progress,
              Internet-Draft, draft-cheng-iccrg-delivery-rate-
              estimation, November 2021, <https://tools.ietf.org/html/
              draft-cheng-iccrg-delivery-rate-estimation>.

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   [CCGHJ16]  Cardwell, N., Cheng, Y., Gunn, C., Hassas Yeganeh, S., and
              V. Jacobson, "BBR: Congestion-Based Congestion Control",
              ACM Queue Oct 2016, October 2016,
              <http://queue.acm.org/detail.cfm?id=3022184>.

   [CCGHJ17]  Cardwell, N., Cheng, Y., Gunn, C., Hassas Yeganeh, S., and
              V. Jacobson, "BBR: Congestion-Based Congestion Control",
              Communications of the ACM Feb 2017, February 2017,
              <https://cacm.acm.org/magazines/2017/2/212428-bbr-
              congestion-based-congestion-control/pdf>.

   [MM19]     Mathis, M. and J. Mahdavi, "Deprecating The TCP
              Macroscopic Model", Computer Communication Review, vol.
              49, no. 5, pp. 63-68 , October 2019.

   [BBRStartupPacingGain]
              Cardwell, N., Cheng, Y., Hassas Yeganeh, S., and V.
              Jacobson, "BBR Startup Pacing Gain: a Derivation", June
              2018, <https://github.com/google/bbr/blob/master/Documenta
              tion/startup/gain/analysis/bbr_startup_gain.pdf>.

   [BBRDrainPacingGain]
              Cardwell, N., Cheng, Y., Hassas Yeganeh, S., and V.
              Jacobson, "BBR Drain Pacing Gain: a Derivation", September
              2021, <https://github.com/google/bbr/blob/master/Documenta
              tion/startup/gain/analysis/bbr_drain_gain.pdf>.

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Authors' Addresses

   Neal Cardwell
   Google

   Email: ncardwell@google.com

   Yuchung Cheng
   Google

   Email: ycheng@google.com

   Soheil Hassas Yeganeh
   Google

   Email: soheil@google.com

Cardwell, et al.           Expires 12 May 2022                 [Page 65]
Internet-Draft                     BBR                     November 2021

   Ian Swett
   Google

   Email: ianswett@google.com

   Van Jacobson
   Google

   Email: vanj@google.com

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