Internet Engineering Task Force (IETF)                       S. Shalunov
Request for Comments: 6817                                      G. Hazel
Category: Experimental                                  BitTorrent, Inc.
ISSN: 2070-1721                                               J. Iyengar
                                           Franklin and Marshall College
                                                           M. Kuehlewind
                                                 University of Stuttgart
                                                           December 2012

             Low Extra Delay Background Transport (LEDBAT)


   Low Extra Delay Background Transport (LEDBAT) is an experimental
   delay-based congestion control algorithm that seeks to utilize the
   available bandwidth on an end-to-end path while limiting the
   consequent increase in queueing delay on that path.  LEDBAT uses
   changes in one-way delay measurements to limit congestion that the
   flow itself induces in the network.  LEDBAT is designed for use by
   background bulk-transfer applications to be no more aggressive than
   standard TCP congestion control (as specified in RFC 5681) and to
   yield in the presence of competing flows, thus limiting interference
   with the network performance of competing flows.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................4
      1.1. Requirements Notation ......................................4
      1.2. Design Goals ...............................................4
      1.3. Applicability ..............................................5
   2. LEDBAT Congestion Control .......................................6
      2.1. Overview ...................................................6
      2.2. Preliminaries ..............................................6
      2.3. Receiver-Side Operation ....................................7
      2.4. Sender-Side Operation ......................................7
           2.4.1. An Overview .........................................7
           2.4.2. The Complete Sender Algorithm .......................8
      2.5. Parameter Values ..........................................11
   3. Understanding LEDBAT Mechanisms ................................13
      3.1. Delay Estimation ..........................................13
           3.1.1. Estimating Base Delay ..............................13
           3.1.2. Estimating Queueing Delay ..........................13
      3.2. Managing the Congestion Window ............................14
           3.2.1. Window Increase: Probing for More Bandwidth ........14
           3.2.2. Window Decrease: Responding to Congestion ..........14
      3.3. Choosing the Queuing Delay Target .........................15
   4. Discussion .....................................................15
      4.1. Framing and ACK Frequency Considerations ..................15
      4.2. Competing with TCP Flows ..................................15
      4.3. Competing with Non-TCP Flows ..............................16
      4.4. Fairness among LEDBAT Flows ...............................16
   5. Open Areas for Experimentation .................................17
      5.1. Network Effects and Monitoring ............................17
      5.2. Parameter Values ..........................................18
      5.3. Filters ...................................................19
      5.4. Framing ...................................................19
   6. Security Considerations ........................................19
   7. Acknowledgements ...............................................20
   8. References .....................................................20
      8.1. Normative References ......................................20
      8.2. Informative References ....................................20
   Appendix A. Measurement Errors ....................................22
     A.1. Clock Offset ...............................................22
     A.2. Clock Skew .................................................22

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1.  Introduction

   TCP congestion control [RFC5681] seeks to share bandwidth at a
   bottleneck link equitably among flows competing at the bottleneck,
   and it is the predominant congestion control mechanism used on the
   Internet.  However, not all applications seek an equitable share of
   network throughput.  "Background" applications, such as software
   updates or file-sharing applications, seek to operate without
   interfering with the performance of more interactive and delay-
   and/or bandwidth-sensitive "foreground" applications.  Standard TCP
   congestion control, as specified in [RFC5681], may be too aggressive
   for use with such background applications.

   Low Extra Delay Background Transport (LEDBAT) is an experimental
   delay-based congestion control mechanism that reacts early to
   congestion in the network, thus enabling "background" applications to
   use the network while avoiding interference with the network
   performance of competing flows.  A LEDBAT sender uses one-way delay
   measurements to estimate the amount of queueing on the data path,
   controls the LEDBAT flow's congestion window based on this estimate,
   and minimizes interference with competing flows by adding low extra
   queueing delay on the end-to-end path.

   Delay-based congestion control protocols, such as TCP-Vegas
   [Bra94][Low02], are generally designed to achieve more, not less
   throughput than standard TCP, and often outperform TCP under
   particular network settings.  For further discussion on Lower-than-
   Best-Effort transport protocols see [RFC6297].  In contrast, LEDBAT
   is designed to be no more aggressive than TCP [RFC5681]; LEDBAT is a
   "scavenger" congestion control mechanism that seeks to utilize all
   available bandwidth and yields quickly when competing with standard
   TCP at a bottleneck link.

   In the rest of this document, we refer to congestion control
   specified in [RFC5681] as "standard TCP".

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.2.  Design Goals

   LEDBAT congestion control seeks to achieve the following goals:

   1.  to utilize end-to-end available bandwidth and to maintain low
       queueing delay when no other traffic is present,

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   2.  to add limited queuing delay to that induced by concurrent flows,

   3.  to yield quickly to standard TCP flows that share the same
       bottleneck link.

1.3.  Applicability

   LEDBAT is a "scavenger" congestion control mechanism that is
   motivated primarily by background bulk-transfer applications, such as
   large file transfers (as with file-sharing applications) and software
   updates.  It can be used with any application that seeks to minimize
   its impact on the network and on other interactive delay- and/or
   bandwidth-sensitive network applications.  LEDBAT is expected to work
   well when the sender and/or receiver is connected via a residential
   access network.

   LEDBAT can be used as part of a transport protocol or as part of an
   application, as long as the data transmission mechanisms are capable
   of carrying timestamps and acknowledging data frequently.  LEDBAT can
   be used with TCP, Stream Control Transmission Protocol (SCTP), and
   Datagram Congestion Control Protocol (DCCP), with appropriate
   extensions where necessary; and it can be used with proprietary
   application protocols, such as those built on top of UDP for peer-to-
   peer (P2P) applications.

   When used with an ECN-capable framing protocol, LEDBAT should react
   to an Explicit Congestion Notification (ECN) mark as it would to a
   loss, as specified in [RFC3168].

   LEDBAT is designed to reduce buildup of a standing queue by long-
   lived LEDBAT flows at a link with a tail-drop FIFO queue, so as to
   avoid persistently delaying other flows sharing the queue.  If Active
   Queue Management (AQM) is configured to drop or ECN-mark packets
   before the LEDBAT flow starts reacting to persistent queue buildup,
   LEDBAT reverts to standard TCP behavior rather than yielding to other
   TCP flows.  However, such an AQM is still desirable since it keeps
   queuing delay low, achieving an outcome that is in line with LEDBAT's
   goals.  Additionally, a LEDBAT transport that supports ECN enjoys the
   advantages that an ECN-capable TCP enjoys over an ECN-agnostic TCP;
   avoiding losses and possible retransmissions.  Weighted Fair Queuing
   (WFQ), as employed by some home gateways, seeks to isolate and
   protect delay-sensitive flows from delays due to standing queues
   built up by concurrent long-lived flows.  Consequently, while it
   prevents LEDBAT from yielding to other TCP flows, it again achieves
   an outcome that is in line with LEDBAT's goals [Sch10].

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2.  LEDBAT Congestion Control

2.1.  Overview

   A standard TCP sender increases its congestion window until a loss
   occurs [RFC5681] or an ECN mark is received [RFC3168], which, in the
   absence of link errors in the network, occurs only when the queue at
   the bottleneck link on the end-to-end path overflows or an AQM is
   applied.  Since packet loss or marking at the bottleneck link is
   expected to be preceded by an increase in the queueing delay at the
   bottleneck link, LEDBAT congestion control uses this increase in
   queueing delay as an early signal of congestion, enabling it to
   respond to congestion earlier than standard TCP and enabling it to
   yield bandwidth to a competing TCP flow.

   LEDBAT employs one-way delay measurements to estimate queueing delay.
   When the estimated queueing delay is less than a predetermined
   target, LEDBAT infers that the network is not yet congested and
   increases its sending rate to utilize any spare capacity in the
   network.  When the estimated queueing delay becomes greater than the
   predetermined target, LEDBAT decreases its sending rate as a response
   to potential congestion in the network.

2.2.  Preliminaries

   A LEDBAT sender uses a congestion window (cwnd) to gate the amount of
   data that the sender can send into the network in one round-trip time
   (RTT).  A sender MAY maintain its cwnd in bytes or in packets; this
   document uses cwnd in bytes.  LEDBAT requires that each data segment
   carries a "timestamp" from the sender, based on which the receiver
   computes the one-way delay from the sender and sends this computed
   value back to the sender.

   In addition to the LEDBAT mechanism described below, we note that a
   slow start mechanism can be used as specified in [RFC5681].  Since
   slow start leads to faster increase in the window than that specified
   in LEDBAT, conservative congestion control implementations employing
   LEDBAT may skip slow start altogether and start with an initial
   window of INIT_CWND * MSS.  (INIT_CWND is described later in
   Section 2.5.)

   The term "MSS", or the sender's Maximum Segment Size, used in this
   document refers to the size of the largest segment that the sender
   can transmit.  The value of MSS can be based on the path MTU
   discovery [RFC4821] algorithm and/or on other factors.

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2.3.  Receiver-Side Operation

   A LEDBAT receiver calculates the one-way delay from the sender to the
   receiver based on its own system time and timestamps in the received
   data packets.  The receiver then feeds the computed one-way delay
   back to the sender in the next acknowledgement.  A LEDBAT receiver
   operates as follows:

   on data_packet:
       remote_timestamp = data_packet.timestamp
       acknowledgement.delay = local_timestamp() - remote_timestamp
       # fill in other fields of acknowledgement

   A receiver may choose to delay sending an ACK and may combine
   acknowledgements for more than one data packet into a single ACK
   packet, as with delayed ACKs in standard TCP, for example.  In such
   cases, the receiver MAY bundle all the delay samples into one ACK
   packet and MUST transmit the samples in the order generated.  When
   multiple delay samples are bundled within a single ACK, the sender
   applies these bundled delay samples at once during its cwnd
   adjustment (discussed in the next section).  Since the sender's
   adjustment may be sensitive to the order in which the delay samples
   are applied, the computed delay samples should be available to the
   sender in the order they were generated at the receiver.

2.4.  Sender-Side Operation

2.4.1.  An Overview

   As a first approximation, a LEDBAT sender operates as shown below;
   the complete algorithm is specified later in Section 2.4.2.  TARGET
   is the maximum queueing delay that LEDBAT itself may introduce in the
   network, and GAIN determines the rate at which the cwnd responds to
   changes in queueing delay; both constants are specified later.
   off_target is a normalized value representing the difference between
   the measured current queueing delay and the predetermined TARGET
   delay. off_target can be positive or negative; consequently, cwnd
   increases or decreases in proportion to off_target.

   on initialization:
       base_delay = +INFINITY

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   on acknowledgement:
       current_delay = acknowledgement.delay
       base_delay = min(base_delay, current_delay)
       queuing_delay = current_delay - base_delay
       off_target = (TARGET - queuing_delay) / TARGET
       cwnd += GAIN * off_target * bytes_newly_acked * MSS / cwnd

   The simplified mechanism above ignores multiple delay samples in an
   acknowledgement, noise filtering, base delay expiration, and sender
   idle times, which we now take into account in our complete sender
   algorithm below.

2.4.2.  The Complete Sender Algorithm

   update_current_delay() maintains a list of one-way delay
   measurements, of which a filtered value is used as an estimate of the
   current end-to-end delay. update_base_delay() maintains a list of
   one-way delay minima over a number of one-minute intervals, to
   measure and to track changes in the base delay of the end-to-end
   path.  Both of these lists are maintained per LEDBAT flow.

   We note this algorithm assumes that slight random fluctuations exist
   in inter-packet arrival times at the bottleneck queue, to allow a
   LEDBAT sender to correctly measure the base delay.  See Section 4.4
   for a more complete discussion.

   The full sender-side algorithm is given below:

   on initialization:
       # cwnd is the amount of data that is allowed to be
       # outstanding in an RTT and is defined in bytes.
       # CTO is the congestion timeout value.

       create current_delays list with CURRENT_FILTER elements
       create base_delays list with BASE_HISTORY number of elements
       initialize elements in base_delays to +INFINITY
       initialize elements in current_delays according to FILTER()
       last_rollover = -INFINITY # More than a minute in the past
       flightsize = 0
       cwnd = INIT_CWND * MSS
       CTO = 1 second

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   on acknowledgement:
       # flightsize is the amount of data outstanding before this ACK
       #    was received and is updated later;
       # bytes_newly_acked is the number of bytes that this ACK
       #    newly acknowledges, and it MAY be set to MSS.

       for each delay sample in the acknowledgement:
           delay = acknowledgement.delay

       queuing_delay = FILTER(current_delays) - MIN(base_delays)
       off_target = (TARGET - queuing_delay) / TARGET
       cwnd += GAIN * off_target * bytes_newly_acked * MSS / cwnd
       max_allowed_cwnd = flightsize + ALLOWED_INCREASE * MSS
       cwnd = min(cwnd, max_allowed_cwnd)
       cwnd = max(cwnd, MIN_CWND * MSS)
       flightsize = flightsize - bytes_newly_acked

   on data loss:
       # at most once per RTT
       cwnd = min (cwnd, max (cwnd/2, MIN_CWND * MSS))
       if data lost is not to be retransmitted:
           flightsize = flightsize - bytes_not_to_be_retransmitted

   if no ACKs are received within a CTO:
       # extreme congestion, or significant RTT change.
       # set cwnd to 1MSS and backoff the congestion timer.
       cwnd = 1 * MSS
       CTO = 2 * CTO

    # implements an RTT estimation mechanism using data
    # transmission times and ACK reception times,
    # which is used to implement a congestion timeout (CTO).
    # If implementing LEDBAT in TCP, sender SHOULD use
    # mechanisms described in RFC 6298 [RFC6298],
    # and the CTO would be the same as the retransmission timeout (RTO).

       # Maintain a list of CURRENT_FILTER last delays observed.
       delete first item in current_delays list
       append delay to current_delays list

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       # Maintain BASE_HISTORY delay-minima.
       # Each minimum is measured over a period of a minute.
       # 'now' is the current system time
       if round_to_minute(now) != round_to_minute(last_rollover)
           last_rollover = now
           delete first item in base_delays list
           append delay to base_delays list
           base_delays.tail = MIN(base_delays.tail, delay)

   The LEDBAT sender seeks to extract the actual delay estimate from the
   current_delay samples by implementing FILTER() to eliminate any
   outliers.  Different types of filters MAY be used for FILTER() -- a
   NULL filter, that does not filter at all, is a reasonable candidate
   as well, since LEDBAT's use of a linear controller for cwnd increase
   and decrease may allow it to recover quickly from errors induced by
   bad samples.  Another example of a filter is the exponentially
   weighted moving average (EWMA) function, with weights that enable
   agile tracking of changing network delay.  A simple MIN filter
   applied over a small window (much smaller than BASE_HISTORY) may also
   provide robustness to large delay peaks, as may occur with delayed
   ACKs in TCP.  Care should be taken that the filter used, while
   providing robustness to noise, remains sensitive to persistent
   congestion signals.

   We note that when multiple delay samples are bundled within a single
   ACK, the sender's resulting cwnd may be slightly different than when
   the samples are sent individually in separate ACKs.  The cwnd is
   adjusted based on the total number of bytes ACKed and the final
   filtered value of queueing_delay, irrespective of the number of delay
   samples in an ACK.

   To implement an approximate minimum over the past few minutes, a
   LEDBAT sender stores BASE_HISTORY separate minima -- one each for the
   last BASE_HISTORY-1 minutes, and one for the running current minute.
   At the end of the current minute, the window moves -- the earliest
   minimum is dropped and the latest minimum is added.  If the
   connection is idle for a given minute, no data is available for the
   one-way delay and, therefore, a value of +INFINITY has to be stored
   in the list.  If the connection has been idle for BASE_HISTORY
   minutes, all minima in the list are thus set to +INFINITY and
   measurement begins anew.  LEDBAT thus requires that during idle
   periods, an implementation must maintain the base delay list.

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   LEDBAT restricts cwnd growth after a period of inactivity.  When the
   sender is application-limited, the sender's cwnd is clamped down
   using max_allowed_cwnd to a little more than flightsize.  To be TCP-
   friendly, LEDBAT halves its cwnd on data loss.

   LEDBAT uses a congestion timeout (CTO) to avoid transmitting data
   during periods of heavy congestion and to avoid congestion collapse.
   A CTO is used to detect heavy congestion indicated by loss of all
   outstanding data or acknowledgements, resulting in reduction of the
   cwnd to 1 MSS and an exponential backoff of the CTO interval.  This
   backoff of the CTO value avoids sending more data into an overloaded
   queue, and it also allows the sender to cope with sudden changes in
   the RTT of the path.  The function of a CTO is similar to that of an
   retransmission timeout (RTO) in TCP [RFC6298], but since LEDBAT
   separates reliability from congestion control, a retransmission need
   not be triggered by a CTO.  LEDBAT, however, does not preclude a CTO
   from triggering retransmissions, as could be the case if LEDBAT
   congestion control were to be used with TCP framing and reliability.

   The CTO is a gating mechanism that ensures exponential backoff of
   sending rate under heavy congestion, and it may be implemented with
   or without a timer.  An implementation choosing to avoid timers may
   consider using a "next-time-to-send" variable, set based on the CTO,
   to control the earliest time a sender may transmit without receiving
   any ACKs.  A maximum value MAY be placed on the CTO, and if placed,
   it MUST be at least 60 seconds.

2.5.  Parameter Values

   TARGET MUST be 100 milliseconds or less, and this choice of value is
   explained further in Section 3.3.  Note that using the same TARGET
   value across LEDBAT flows enables equitable sharing of the bottleneck
   bandwidth.  A flow with a higher TARGET value than other competing
   LEDBAT flows may get a larger share of the bottleneck bandwidth.  It
   is possible to consider the use of different TARGET values for
   implementing a relative priority between two competing LEDBAT flows
   by setting a higher TARGET value for the higher-priority flow.

   ALLOWED_INCREASE SHOULD be 1, and it MUST be greater than 0.  An
   ALLOWED_INCREASE of 0 results in no cwnd growth at all, and an
   ALLOWED_INCREASE of 1 allows and limits the cwnd increase based on
   flightsize in the previous RTT.  An ALLOWED_INCREASE greater than 1
   MAY be used when interactions between LEDBAT and the framing protocol
   provide a clear reason for doing so.

   GAIN MUST be set to 1 or less.  A GAIN of 1 limits the maximum cwnd
   ramp-up to the same rate as TCP Reno in Congestion Avoidance.  While
   this document specifies the use of the same GAIN for both cwnd

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   increase (when off_target is greater than zero) and decrease (when
   off_target is less than zero), implementations MAY use a higher GAIN
   for cwnd decrease than for the increase; our justification follows.
   When a competing non-LEDBAT flow increases its sending rate, the
   LEDBAT sender may only measure a small amount of additional delay and
   decrease the sending rate slowly.  To ensure no impact on a competing
   non-LEDBAT flow, the LEDBAT flow should decrease its sending rate at
   least as quickly as the competing flow increases its sending rate.  A
   higher decrease-GAIN MAY be used to allow the LEDBAT flow to decrease
   its sending rate faster than the competing flow's increase rate.

   The size of the base_delays list, BASE_HISTORY, SHOULD be 10.  If the
   actual base delay decreases, due to a route change, for instance, a
   LEDBAT sender adapts immediately, irrespective of the value of
   BASE_HISTORY.  If the actual base delay increases, however, a LEDBAT
   sender will take BASE_HISTORY minutes to adapt and may wrongly infer
   a little more extra delay than intended (TARGET) in the meanwhile.  A
   value for BASE_HISTORY is thus a trade-off: a higher value may yield
   a more accurate measurement when the base delay is unchanging, and a
   lower value results in a quicker response to actual increase in base

   A LEDBAT sender uses the current_delays list to maintain only delay
   measurements made within an RTT amount of time in the past, seeking
   to eliminate noise spikes in its measurement of the current one-way
   delay through the network.  The size of this list, CURRENT_FILTER,
   may be variable, and it depends on the FILTER() function as well as
   the number of successful measurements made within an RTT amount of
   time in the past.  The sender should seek to gather enough delay
   samples in each RTT so as to have statistical confidence in the
   measurements.  While the number of delay samples required for such
   confidence will vary depending on network conditions, the sender
   SHOULD use at least 4 delay samples in each RTT, unless the number of
   samples is lower due to a small congestion window.  The value of
   CURRENT_FILTER will depend on the filter being employed, but
   CURRENT_FILTER MUST be limited such that samples in the list are not
   older than an RTT in the past.

   INIT_CWND and MIN_CWND SHOULD both be 2.  An INIT_CWND of 2 should
   help seed FILTER() at the sender when there are no samples at the
   beginning of a flow, and a MIN_CWND of 2 allows FILTER() to use more
   than a single instantaneous delay estimate while not being too
   aggressive.  Slight deviations may be warranted, for example, when
   these values of INIT_CWND and MIN_CWND interact poorly with the
   framing protocol.  However, INIT_CWND and MIN_CWND MUST be no larger
   than the corresponding values specified for TCP [RFC5681].

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3.  Understanding LEDBAT Mechanisms

   This section describes the delay estimation and window management
   mechanisms used in LEDBAT.

3.1.  Delay Estimation

   LEDBAT estimates congestion in the direction of the data flow, and to
   avoid measuring additional delay from, e.g., queue buildup on the
   reverse path (or ACK path) or reordering, LEDBAT uses one-way delay
   estimates.  LEDBAT assumes that measurements are done with data
   packets, thus avoiding the need for separate measurement packets and
   avoiding the pitfall of measurement packets being treated differently
   from the data packets in the network.

   End-to-end delay can be decomposed into transmission (or
   serialization) delay, propagation (or speed-of-light) delay, queueing
   delay, and processing delay.  On any given path, barring some noise,
   all delay components except for queueing delay are constant.  To
   observe an increase in the queueing delay in the network, a LEDBAT
   sender separates the queueing delay component from the rest of the
   end-to-end delay, as described below.

3.1.1.  Estimating Base Delay

   Since queuing delay is always additive to the end-to-end delay,
   LEDBAT estimates the sum of the constant delay components, which we
   call "base delay", to be the minimum delay observed on the end-to-end

   To respond to true changes in the base delay, as can be caused by a
   route change, LEDBAT uses only recent measurements in estimating the
   base delay.  The duration of the observation window itself is a
   trade-off between robustness of measurement and responsiveness to
   change -- a larger observation window increases the chances that the
   true base delay will be detected (as long as the true base delay is
   unchanged), whereas a smaller observation window results in faster
   response to true changes in the base delay.

3.1.2.  Estimating Queueing Delay

   Assuming that the base delay is constant (in the absence of any route
   changes), the queueing delay is represented by the variable component
   of the measured end-to-end delay.  LEDBAT measures queueing delay as
   simply the difference between an end-to-end delay measurement and the
   current estimate of base delay.  The queueing delay should be

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   filtered (depending on the usage scenario) to eliminate noise in the
   delay estimation, such as due to spikes in processing delay at a node
   on the path.

3.2.  Managing the Congestion Window

   LEDBAT uses a simple linear controller to determine the sending rate
   as a function of the delay estimate, where the response of the sender
   is proportional to the difference between the current queueing delay
   estimate and the target.

3.2.1.  Window Increase: Probing for More Bandwidth

   When the queuing delay is smaller than a delay target value, as
   specified by the TARGET parameter in this document, a LEDBAT sender
   will increase its congestion window proportionally to the relative
   difference between the current queueing delay and the delay target.
   As the current queuing delay gets closer to TARGET, LEDBAT's window
   growth gets slower.  To compete fairly with concurrent TCP flows, we
   set the highest rate of LEDBAT's window growth (when the current
   queueing delay estimate is zero) to be the same as TCP's (increase of
   one packet per RTT).  In other words, a LEDBAT flow never ramps up
   faster than a competing TCP flow over the same path.  The TARGET
   value specifies the maximum extra queuing delay that LEDBAT will
   induce.  If the current queuing delay equals the TARGET value, LEDBAT
   tries to maintain this extra delay.

3.2.2.  Window Decrease: Responding to Congestion

   When a sender's queueing delay estimate is higher than the target,
   the LEDBAT flow's rate should be reduced.  LEDBAT's linear controller
   allows the sender to decrease the window proportional to the
   difference between the target and the current queueing delay.

   Unlike TCP-like loss-based congestion control, LEDBAT seeks to avoid
   losses and so a LEDBAT sender is not expected to normally rely on
   losses to determine the sending rate.  However, when data loss does
   occur, LEDBAT must respond as standard TCP does; even if the queueing
   delay estimates indicate otherwise, a loss is assumed to be a strong
   indication of congestion.  Thus, to deal with severe congestion when
   packets are dropped in the network, and to provide a fallback against
   incorrect queuing delay estimates, a LEDBAT sender halves its
   congestion window when a loss event is detected.  As with TCP New-
   Reno, LEDBAT reduces its cwnd by half at most once per RTT.

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3.3.  Choosing the Queuing Delay Target

   The International Telecommunication Union's (ITU's) Recommendation
   G.114 defines a one-way delay of 150 ms to be acceptable for most
   user voice applications [g114].  Thus, the delay induced by LEDBAT
   must be well below 150 ms to limit its impact on concurrent delay-
   sensitive traffic sharing the same bottleneck queue.  A target that
   is too low, on the other hand, increases the sensitivity of the
   sender's algorithm to noise in the one-way delays and in the delay
   measurement process, and may lead to reduced throughput for the
   LEDBAT flow and to under-utilization of the bottleneck link.

   Our recommendation of 100 ms or less as the target is a trade-off
   between these considerations.  Anecdotal evidence indicates that this
   value works well -- LEDBAT has been implemented and successfully
   deployed with a target value of 100 ms in two BitTorrent
   implementations: as the exclusive congestion control mechanism in
   BitTorrent Delivery Network Accelerator (DNA), and as an experimental
   mechanism in uTorrent [uTorrent].

4.  Discussion

4.1.  Framing and ACK Frequency Considerations

   While the actual framing and wire format of the protocols using
   LEDBAT are outside the scope of this document, we briefly consider
   the data framing and ACK frequency needs of LEDBAT mechanisms.

   To compute the data path's one-way delay, our discussion of LEDBAT
   assumes a framing that allows the sender to timestamp packets and for
   the receiver to convey the measured one-way delay back to the sender
   in ACK packets.  LEDBAT does not require this particular method, but
   it does require unambiguous delay estimates using data and ACK

   A LEDBAT receiver may send an ACK as frequently as one for every data
   packet received or less frequently; LEDBAT does require that the
   receiver MUST transmit at least one ACK in every RTT.

4.2.  Competing with TCP Flows

   LEDBAT is designed to respond to congestion indications earlier than
   loss-based standard TCP [RFC5681].  A LEDBAT flow gets more
   aggressive as the queueing delay estimate gets lower; since the
   queueing delay estimate is non-negative, LEDBAT is most aggressive
   when the queueing delay estimate is zero.  In this case, LEDBAT ramps
   up its congestion window at the same rate as standard TCP [RFC5681].
   LEDBAT may reduce its rate earlier than standard TCP and always

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   halves its congestion window on loss.  Thus, in the worst case, where
   the delay estimates are completely and consistently off, a LEDBAT
   flow falls back to standard TCP behavior, and is no more aggressive
   than standard TCP [RFC5681].

4.3.  Competing with Non-TCP Flows

   While LEDBAT yields to all high-load flows, both TCP and non-TCP,
   LEDBAT may not yield to low-load and latency-sensitive traffic that
   do not induce a measurable delay at the bottleneck queue, such as
   Voice over IP (VoIP) traffic.  While such flows will experience
   additional delay due to any concurrent LEDBAT flows, the TARGET delay
   sets a limit to the total amount of additional delay that all the
   concurrent LEDBAT flows will jointly induce.  If the TARGET delay is
   higher than what the bottleneck queue can sustain, the LEDBAT flows
   should experience loss and will fall back to standard loss-based TCP
   behavior.  Thus, in the worst case, LEDBAT will add no more latency
   than standard TCP when competing with non-TCP flows.  In the common
   case however, we expect LEDBAT flows to add TARGET amount of delay,
   which ought to be within the delay tolerance for most latency-
   sensitive applications, including VoIP applications.

4.4.  Fairness among LEDBAT Flows

   The primary design goals of LEDBAT are focused on the aggregate
   behavior of LEDBAT flows when they compete with standard TCP.  Since
   LEDBAT is designed for background traffic, we consider link
   utilization to be more important than fairness amongst LEDBAT flows.
   Nevertheless, we now consider fairness issues that might arise
   amongst competing LEDBAT flows.

   LEDBAT as described so far lacks a mechanism specifically designed to
   equalize utilization amongst LEDBAT flows.  Anecdotally observed
   behavior of existing implementations indicates that a rough
   equalization does occur since in most environments some amount of
   randomness in the inter-packet transmission times exists, as
   explained further below.

   Delay-based congestion control systems suffer from the possibility of
   latecomers incorrectly measuring and using a higher base-delay than
   an active flow that started earlier.  Consider that a bottleneck is
   saturated by a single LEDBAT flow, and the flow therefore maintains
   the bottleneck queue at TARGET delay.  When a new LEDBAT flow arrives
   at the bottleneck, it might incorrectly include the steady queueing
   delay in its measurement of the base delay on the path.  The new flow
   has an inflated estimate of the base delay, and may now effectively
   build on top of the existing, already maximal, queueing delay.  As
   the latecomer flow builds up, the old flow sees the true queueing

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   delay and backs off, while the latecomer keeps building up, using up
   the entire link's capacity, and effectively shutting the old flow
   out.  This advantage is called the "latecomer's advantage".

   In the worst case, if the first flow yields at the same rate as the
   new flow increases its sending rate, the new flow will see constant
   end-to-end delay, which it assumes is the base delay, until the first
   flow backs off completely.  As a result, by the time the second flow
   stops increasing its cwnd, it would have added twice the target
   queueing delay to the network.

   This advantage can be reduced if the first flow yields and empties
   the bottleneck queue faster than the incoming flow increases its
   occupancy in the queue.  In such a case, the latecomer might measure
   correctly a delay that is closer to the base delay.  While such a
   reduction might be achieved through a multiplicative decrease of the
   congestion window, this may cause strong fluctuations in flow
   throughput during the flow's steady state.  Thus, we do not recommend
   a multiplicative decrease scheme.

   We note that in certain use-case scenarios, it is possible for a
   later LEDBAT flow to gain an unfair advantage over an existing one
   [Car10].  In practice, this concern ought to be alleviated by the
   burstiness of network traffic: all that's needed to measure the base
   delay is one small gap in transmission schedules between the LEDBAT
   flows.  These gaps can occur for a number of reasons such as latency
   introduced due to application sending patterns, OS scheduling at the
   sender, processing delay at the sender or any network node, and link
   contention.  When such a gap occurs in the first sender's
   transmission while the latecomer is starting, base delay is
   immediately correctly measured.  With a small number of LEDBAT flows,
   system noise may sufficiently regulate the latecomer's advantage.

5.  Open Areas for Experimentation

   We now outline some areas that need experimentation in the Internet
   and under different network scenarios.  These experiments should help
   the community understand LEDBAT's dynamics and should help towards
   further standardization of LEDBAT and LEDBAT-related documents.

5.1.  Network Effects and Monitoring

   Further study is required to fully understand the behavior and
   convergence properties of LEDBAT in networks with non-tail-drop, non-
   FIFO queues, in networks with frequent route changes, and in networks
   with network-level load balancing.  These studies should have two

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   broad goals: (i) to understand the effects of different network
   mechanisms on LEDBAT, and (ii) to understand the impact of LEDBAT on
   the network.

   Network mechanisms and dynamics can influence LEDBAT flows in
   unintended ways.  For instance, frequent route changes that result in
   increasing base delays may, in the worst case, throttle a LEDBAT
   flow's throughput significantly.  The influence of different network
   traffic management mechanisms on LEDBAT throughput should be studied.

   An increasing number of LEDBAT flows in the network will likely
   result in operator-visible network effects as well, and these should
   thus be studied.  For instance, as long as the bottleneck queue in a
   network is larger than TARGET (in terms of delay), we expect that
   both the average queueing delay and loss rate in the network should
   reduce as LEDBAT traffic increasingly dominates the traffic mix in
   the network.  Note that for bottleneck queues that are smaller than
   TARGET, LEDBAT will appear to behave very similar to standard TCP and
   its flow-level behavior may not be distinguishable from that of
   standard TCP.

   We note that a network operator may be able to verify the operation
   of a LEDBAT flow by monitoring per-flow behavior and queues in the
   network -- when the queueing delay at a bottleneck queue is above
   TARGET as specified in this document, LEDBAT flows should be expected
   to back off and reduce their sending rate.

5.2.  Parameter Values

   The throughput and response of LEDBAT to the proposed parameter
   values of TARGET, decrease-GAIN, BASE_HISTORY, INIT_CWND, and
   MIN_CWND should be evaluated with different types of competing
   traffic in different network settings, including with different AQM
   schemes at the bottleneck queue.  TARGET controls LEDBAT's added
   latency, while decrease-GAIN controls LEDBAT's response to competing
   traffic.  Since LEDBAT is intended to be minimally intrusive to
   competing traffic, the impact of TARGET and decrease-GAIN on delay-
   sensitive traffic should be studied.  TARGET also impacts the growth
   rate of the congestion window when off_target is smaller than 1.
   This impact of TARGET on the rate of cwnd growth should be studied.
   The amount of history maintained by the base delay estimator,
   BASE_HISTORY, influences the responsiveness of LEDBAT to changing
   network conditions.  LEDBAT's responsiveness and throughput should be
   evaluated in the wide area and under conditions where abrupt changes
   in base delay might occur, such as with route changes and with
   cellular handovers.  The impact and efficacy of these parameters
   should be carefully studied with tests over the Internet.

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5.3.  Filters

   LEDBAT's effectiveness depends on a sender's ability to accurately
   estimate end-to-end queueing delay from delay samples.  Consequently,
   the filtering algorithm used for this estimation, FILTER(), is an
   important candidate for experiments.  This document suggests the use
   of NULL, EWMA, and MIN filters for estimating the current delay; the
   efficacy of these and other possible filters for this estimate should
   be investigated.  FILTER() may also impact cwnd dynamics when delay
   samples are bundled in ACKs, since cwnd adaption is done once per ACK
   irrespective of the number of delay samples in the ACK.  This impact
   should be studied when the different filters are considered.

5.4.  Framing

   This document defines only a congestion control algorithm and assumes
   that framing mechanisms for exchanging delay information exist within
   the protocol in which LEDBAT is being implemented.  If implemented in
   a new protocol, both the sender and receiver may be LEDBAT-aware, but
   if implemented in an existing protocol that is capable of providing
   one-way delay information, LEDBAT may be implemented as a sender-
   side-only modification.  In either case, the parent protocol may
   interact with LEDBAT's algorithms; for instance, the rate of ACK
   feedback to the data sender may be dictated by other protocol
   parameters, but will interact with the LEDBAT flow's dynamics.
   Careful experimentation is necessary to understand and integrate
   LEDBAT into both new and existing protocols.

6.  Security Considerations

   LEDBAT's aggressiveness is contingent on the delay estimates and on
   the TARGET delay value.  If these parameter values at the sender are
   compromised such that delay estimates are artificially set to zero
   and the TARGET delay value is set to +INFINITY, the LEDBAT algorithm
   deteriorates to TCP-like behavior.  Thus, while LEDBAT is sensitive
   to these parameters, the algorithm is fundamentally limited in the
   worst case to be as aggressive as standard TCP.

   A man in the middle may be able to change queueing delay on a network
   path, and/or modify the timestamps transmitted by a LEDBAT sender
   and/or modify the delays reported by a LEDBAT receiver, thus causing
   a LEDBAT flow to back off even when there's no congestion.  A
   protocol using LEDBAT ought to minimize the risk of such man-in-the-
   middle attacks by at least authenticating the timestamp field in the
   data packets and the delay field in the ACK packets.

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   LEDBAT is not known to introduce any new concerns with privacy,
   integrity, or other security issues for flows that use it.  LEDBAT is
   compatible with use of IPsec and Transport Layer Security (TLS) /
   Datagram Transport Layer Security (DTLS).

7.  Acknowledgements

   We thank folks in the LEDBAT working group for their comments and
   feedback.  Special thanks to Murari Sridharan and Rolf Winter for
   their patient and untiring shepherding.

8.  References

8.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3168]   Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
               of Explicit Congestion Notification (ECN) to IP",
               RFC 3168, September 2001.

   [RFC4821]   Mathis, M. and J. Heffner, "Packetization Layer Path MTU
               Discovery", RFC 4821, March 2007.

   [RFC5681]   Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
               Control", RFC 5681, September 2009.

   [RFC6298]   Paxson, V., Allman, M., Chu, J., and M. Sargent,
               "Computing TCP's Retransmission Timer", RFC 6298,
               June 2011.

8.2.  Informative References

   [Bra94]     Brakmo, L., O'Malley, S., and L. Peterson, "TCP Vegas:
               New techniques for congestion detection and avoidance",
               Proceedings of SIGCOMM '94, pages 24-35, August 1994.

   [Car10]     Carofiglio, G., Muscariello, L., Rossi, D., Testa, C.,
               and S. Valenti, "Rethinking Low Extra Delay Background
               Transport Protocols", October 2010,

   [Low02]     Low, S., Peterson, L., and L. Wang, "Understanding TCP
               Vegas: A Duality Model", JACM 49 (2), March 2002.

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RFC 6817                         LEDBAT                    December 2012

   [RFC5905]   Mills, D., Martin, J., Burbank, J., and W. Kasch,
               "Network Time Protocol Version 4: Protocol and Algorithms
               Specification", RFC 5905, June 2010.

   [RFC6297]   Welzl, M. and D. Ros, "A Survey of Lower-than-Best-Effort
               Transport Protocols", RFC 6297, June 2011.

   [Sch10]     Schneider, J., Wagner, J., Winter, R., and H. Kolbe, "Out
               of my Way -- Evaluating Low Extra Delay Background
               Transport in an ADSL Access Network", Proceedings of 22nd
               International Teletraffic Congress (ITC22), September

               SYSTEMS AND NETWORKS; International telephone connections
               and circuits - General; Recommendations on the
               transmission quality for an entire international
               telephone connection; One-way transmission time", ITU-T
               Recommendation G.114, 05/2003.

   [uTorrent]  Hazel, G., "uTorrent Transport Protocol library",
               July 2012, <http://github.com/bittorrent/libutp>.

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Appendix A.  Measurement Errors

   LEDBAT measures and uses one-way delays, and we now consider
   measurement errors in timestamp generation and use.  In this section,
   we use the same locally linear clock model and the same terminology
   as Network Time Protocol (NTP) [RFC5905].  In particular, NTP uses
   the terms "offset" to refer to the difference between measured time
   and true time, and "skew" to refer to difference of clock rate from
   the true rate.  A clock thus has two time measurement errors: a fixed
   offset from the true time, and a skew.  We now consider these errors
   in the context of LEDBAT.

A.1.  Clock Offset

   The offset of the clocks, both the sender's and the receiver's, shows
   up as a fixed error in LEDBAT's one-way delay measurement.  The
   offset in the measured one-way delay is simply the difference in
   offsets between the receiver's and the sender's clocks.  LEDBAT,
   however, does not use this estimate directly, but uses the difference
   between the measured one-way delay and a measured base delay.  Since
   the offset error (difference of clock offsets) is the same for the
   measured one-way delay and the base delay, the offsets cancel each
   other out in the queuing delay estimate, which LEDBAT uses for its
   window computations.  Clock offset error thus has no impact on

A.2.  Clock Skew

   Clock skew generally shows up as a linearly changing error in a time
   estimate.  Similar to the offset, the skew of LEDBAT's one-way delay
   estimate is thus the difference between the two clocks' skews.
   Unlike the offset, however, skew does not cancel out when the queuing
   delay estimate is computed, since it causes the two clocks' offsets
   to change over time.

   While the offset could be large, with some clocks off by minutes or
   even hours or more, skew is typically small.  Typical skews of
   untrained clocks seem to be around 100-200 parts per million (ppm)
   [RFC5905], where a skew of 100 ppm translates to an error
   accumulation of 6 milliseconds per minute.  This accumulation is
   limited in LEDBAT, since any error accumulation is limited to the
   amount of history maintained by the base delay estimator, as dictated
   by the BASE_HISTORY parameter.  The effects of clock skew error on
   LEDBAT should generally be insignificant unless the skew is unusually
   high, or unless extreme values have been chosen for TARGET (extremely
   low) and BASE_HISTORY (extremely large).  Nevertheless, we now
   consider the possible impact of skew on LEDBAT behavior.

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   Clock skew can manifest in two ways: the sender's clock can be faster
   than the receiver's clock, or the receiver's clock can be faster than
   the sender's clock.  In the first case, the measured one-way delay
   will decrease as the sender's clock drifts forward.  While this drift
   can lead to an artificially low estimate of the queueing delay, the
   drift should also lead to a lower base delay measurement, which
   consequently absorbs the erroneous reduction in the one-way delay

   In the second case, the one-way delay estimate will artificially
   increase with time.  This increase can reduce a LEDBAT flow's
   throughput unnecessarily.  In this case, a skew correction mechanism
   can be beneficial.

   We now discuss an example clock skew correction mechanism.  In this
   example, the receiver sends back raw (sending and receiving)
   timestamps.  Using this information, the sender can estimate one-way
   delays in both directions, and the sender can also compute and
   maintain an estimate of the base delay as would be observed by the
   receiver.  If the sender detects the receiver reducing its estimate
   of the base delay, it may infer that this reduction is due to clock
   drift.  The sender then compensates by increasing its base delay
   estimate by the same amount.  To apply this mechanism, timestamps
   need to be transmitted in both directions.

   We now outline a few other ideas that can be used for skew

   o  Skew correction with faster virtual clock:

      Since having a faster clock on the sender will result in
      continuous updates of the base delay, a faster virtual clock can
      be used for sender timestamping.  This virtual clock can be
      computed from the default machine clock through a linear
      transformation.  For instance, with a 500 ppm speed-up the
      sender's clock is very likely to be faster than a receiver's
      clock.  Consequently, LEDBAT will benefit from the implicit
      correction when updating the base delay.

   o  Skew correction with estimating drift:

      A LEDBAT sender maintains a history of base delay minima.  This
      history can provide a base to compute the clock skew difference
      between the two hosts.  The slope of a linear function fitted to
      the set of minima base delays gives an estimate of the clock skew.
      This estimation can be used to correct the clocks.  If the other
      endpoint is doing the same, the clock should be corrected by half
      of the estimated skew amount.

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   o  Byzantine skew correction:

      When it is known that each host maintains long-lived connections
      to a number of different other hosts, a byzantine scheme can be
      used to estimate the skew with respect to the true time.  Namely,
      a host calculates the skew difference for each of the peer hosts
      as described with the previous approach, then take the median of
      the skew differences.  While this scheme is not universally
      applicable, it combines well with other schemes, since it is
      essentially a clock training mechanism.  The scheme also corrects
      fast, since state is preserved between connections.

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Authors' Addresses

   Stanislav Shalunov
   BitTorrent, Inc.
   303 Second St., Suite S200
   San Francisco, CA  94107

   EMail: shalunov@shlang.com
   URI:   http://shlang.com

   Greg Hazel
   BitTorrent, Inc.
   303 Second St., Suite S200
   San Francisco, CA  94107

   EMail: greg@bittorrent.com

   Janardhan Iyengar
   Franklin and Marshall College
   415 Harrisburg Ave.
   Lancaster, PA  17603

   EMail: jiyengar@fandm.edu

   Mirja Kuehlewind
   University of Stuttgart

   EMail: mirja.kuehlewind@ikr.uni-stuttgart.de

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