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Low Extra Delay Background Transport (LEDBAT)

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6817.
Authors Greg Hazel , Jana Iyengar , Mirja Kühlewind , Stanislav Shalunov
Last updated 2012-05-24 (Latest revision 2011-10-31)
Replaces draft-shalunov-ledbat-congestion
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state WG Document
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Shepherd write-up Show Last changed 2012-04-23
IESG IESG state Became RFC 6817 (Experimental)
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Needs a YES.
Responsible AD Wesley Eddy
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Send notices to,,
LEDBAT WG                                                    S. Shalunov
Internet-Draft                                                  G. Hazel
Intended status: Experimental                             BitTorrent Inc
Expires: May 3, 2012                                          J. Iyengar
                                           Franklin and Marshall College
                                                           M. Kuehlewind
                                                 University of Stuttgart
                                                        October 31, 2011

             Low Extra Delay Background Transport (LEDBAT)


   LEDBAT is an experimental delay-based congestion control algorithm
   that attempts to utilize the available bandwidth on an end-to-end
   path while limiting the consequent increase in queueing delay on the
   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; it is
   designed to be no more aggressive than TCP congestion control and to
   yield in the presence of any competing flows when latency builds,
   thus limiting interference with the network performance of the
   competing flows.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 3, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.

Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Design Goals . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  4
   3.  LEDBAT Congestion Control  . . . . . . . . . . . . . . . . . .  4
     3.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Preliminaries  . . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Receiver-Side Operation  . . . . . . . . . . . . . . . . .  5
     3.4.  Sender-Side Operation  . . . . . . . . . . . . . . . . . .  6
       3.4.1.  An Overview  . . . . . . . . . . . . . . . . . . . . .  6
       3.4.2.  The Complete Sender Algorithm  . . . . . . . . . . . .  6
     3.5.  Parameter Values . . . . . . . . . . . . . . . . . . . . .  9
   4.  Understanding LEDBAT Mechanisms  . . . . . . . . . . . . . . . 11
     4.1.  Delay Estimation . . . . . . . . . . . . . . . . . . . . . 11
       4.1.1.  Estimating Base Delay  . . . . . . . . . . . . . . . . 11
       4.1.2.  Estimating Queueing Delay  . . . . . . . . . . . . . . 12
     4.2.  Managing the Congestion Window . . . . . . . . . . . . . . 12
       4.2.1.  Window Increase: Probing For More Bandwidth  . . . . . 12
       4.2.2.  Window Decrease: Responding To Congestion  . . . . . . 12
     4.3.  Choosing The Queuing Delay Target  . . . . . . . . . . . . 13
   5.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Framing and Ack Frequency Considerations . . . . . . . . . 13
     5.2.  Competing With TCP Flows . . . . . . . . . . . . . . . . . 13
     5.3.  Fairness Among LEDBAT Flows  . . . . . . . . . . . . . . . 14
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Timestamp errors  . . . . . . . . . . . . . . . . . . 16
     A.1.  Clock offset . . . . . . . . . . . . . . . . . . . . . . . 17
     A.2.  Clock skew . . . . . . . . . . . . . . . . . . . . . . . . 17
     A.3.  Clock skew correction mechanism  . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

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1.  Requirements notation

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

2.  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.  Not all applications seek an equitable share of network
   throughput, however "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 and standard TCP
   may be too aggressive for use with such background applications.

   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 avoidoing
   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 when latency builds 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.  In contrast, LEDBAT is designed to be
   no more aggressive than TCP; 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.

2.1.  Design Goals

   LEDBAT congestion control seeks to:
   1.  utilize end-to-end available bandwidth, and maintain low queueing
       delay when no other traffic is present,
   2.  add little to the queuing delay induced by concurrent flows,
   3.  quickly yield to flows using standard TCP congestion control that
       share the same bottleneck link,

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2.2.  Applicability

   LEDBAT is a "scavenger" congestion control mechanism that is
   primarily motivated 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 appropriate extensions where necessary, with TCP, SCTP,
   and DCCP, and with proprietary application protocols such as those
   built on top of UDP for P2P applications.

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

   LEDBAT is designed to reduce build-up 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 starts reacting to persistent queue build-up,
   LEDBAT reverts to standard TCP behavior, rather than yield 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].

   Further study is required to fully understand the behaviour of LEDBAT
   with non-drop-tail, non-FIFO queues.

3.  LEDBAT Congestion Control

3.1.  Overview

   A standard TCP sender increases its congestion window until a loss
   occurs [RFC5681] or an ECN mark is received [RFC3168], which, in the

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   absence of any Active Queue Management (AQM) and link errors in the
   network, occurs only when the queue at the bottleneck link on the
   end-to-end path overflows.  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 pre-determined
   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 a
   pre-determined target, LEDBAT decreases its sending rate quickly as a
   response to potential congestion in the network.

3.2.  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 roundtrip 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 3.5.)

   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.

3.3.  Receiver-Side Operation

   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

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   A receiver MAY send more than one delay sample in an acknowledgment.
   For instance, a receiver that delays acknowledgments, i.e., sends an
   acknowledgment less frequently than once per data packet, MAY send
   all the one-way delay samples that it gathers in one acknowledgment.

3.4.  Sender-Side Operation

3.4.1.  An Overview

   As a first approximation, a LEDBAT sender operates as shown below;
   the complete algorithm is specified later in Section 3.4.2.  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 pre-determined TARGET
   queuing delay. off_target can be positive or negative, and
   consequently, cwnd increases or decreases in proportion to

   on initialization:
       base_delay = +INFINITY

   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
   acknowledgment, noise filtering, base delay expiration, and sender
   idle times, which we now take into account in our complete sender
   algorithm below.

3.4.2.  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

   This algorithm restricts cwnd growth after a period of inactivity,
   where the cwnd is clamped down to a little more than flightsize using

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   max_allowed_cwnd.  To be TCP-friendly on data loss, LEDBAT halves its
   cwnd.  The full sender-side algorithm is given below:

 on initialization:
     # cwnd is the amount of data that is allowed to send in one 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 appropriate to FILTER()
     last_rollover = -INFINITY # More than a minute in the past
     flightsize = 0
     cwnd = INIT_CWND * MSS
     CTO = 1 second

 on acknowledgment:
     # 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 acknowledgment:
         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

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       # 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 <xref target= 'RFC6298'/>,
       # and the CTO would be the same as the RTO.

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

       # 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 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 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.

   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

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   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.

   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 acknowledgments, 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 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
   60 seconds or more.

   We note that LEDBAT assumes random fluctuations in inter-packet
   transmission times.  That will help to measure the correct base delay
   because the bottleneck runs empty from time to time; see section
   Section 5.3 for a discussion.

3.5.  Parameter Values

   TARGET MUST be 100 milliseconds or less, and this choice of value is
   explained further in Section 4.3.  Note that using the same TARGET
   value across LEDBAT flows enables equitable sharing of the bottleneck
   bandwidth---flows with a higher TARGET 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

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   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
   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 tradeoff: 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 a 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 depends on the FILTER() function as well as the
   number of successful measurements made within a 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, we recommend that the sender
   SHOULD use at least 4 samples in each RTT, unless the number of
   samples is lower due to a small congestion window.  Thus, subject to
   congestion window constraints, the number of delay samples in each
   RTT SHOULD be at least 4.  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

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   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].

4.  Understanding LEDBAT Mechanisms

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

4.1.  Delay Estimation

   LEDBAT estimates congestion in the direction of the data flow, and to
   avoid measuring additional delay from e.g. queue build-up on the
   reverse path (or ack path) or reordering, LEDBAT uses one-way delay
   estimates.  LEDBAT assumes 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.

4.1.1.  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
   tradeoff 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.

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4.1.2.  Estimating Queueing Delay

   Given that the base delay is constant, 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 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.

4.2.  Managing the Congestion Window

4.2.1.  Window Increase: Probing For More Bandwidth

   A LEDBAT sender increases its congestion window if the queuing delay
   is smaller than a target value, proportionally to the relative
   difference between the current queueing delay and the delay target.
   To be friendly to competing TCP flows, we set this highest rate of
   window growth to be the same as TCP's.  In other words, a LEDBAT flow
   thus never ramps up faster than a competing TCP flow over the same
   path.  As closer the extra delay gets to the TARGET value, as slower
   LEDBAT will increase the window.

4.2.2.  Window Decrease: Responding To Congestion

   When the sender's queueing delay estimate is higher than the target,
   the LEDBAT flow's rate should be reduced.  LEDBAT uses a simple
   linear controller to determine the sending rate as a function of the
   delay estimate, where the response is proportional to the difference
   between the current queueing delay estimate and the target.  This
   allows to decrease the window only slightly while probing and leads
   to a quite stable state with high link utilization.  In limited
   experiments with Bittorrent nodes, this controller seems to work

   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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4.3.  Choosing The Queuing Delay Target

   The queueing delay target is a tradeoff.  A target that is too low
   might result in under-utilization of the bottleneck link, because of
   the noise in the delay measurement e.g in a mobile scenario, and may
   also be more sensitive to error in the measured delay.  The
   International Telecommunication Union's (ITU's) Recommendation G.114
   defines a delay of 150 ms to be acceptable for most user voice
   applications.  Thus the extra delay induced by LEDBAT must be below
   150 ms to reduce impact on delay-sentive applications.  If the TARGET
   value is larger than the maximum delay the queue can induce, LEDBAT
   will fallback to the same behavior than standard TCP (see section
   Section 5.2).

   Our recommendation of 100 ms or less as the target is based on 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---BitTorrent
   DNA as the exclusive congestion control mechanism and in uTorrent as
   an experimental mechanism.

5.  Discussion

5.1.  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.

5.2.  Competing With TCP Flows

   LEDBAT is designed to respond to congestion indications earlier than
   loss-based TCP.  A LEDBAT flow is more aggressive when the queueing
   delay estimate is lower; since the queueing delay estimate is non-
   negative, LEDBAT is most aggressive when its queuing delay estimate
   is zero.  In this case, LEDBAT ramps up its congestion window at the

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   same rate as TCP does.  LEDBAT reduces its rate earlier than TCP
   does, always halving the congestion window on loss.  Thus, in the
   worst case where the delay estimates are completely and consistently
   off, a LEDBAT flow falls back to TCP mechanisms as it will be at most
   as aggressive as a TCP flow and halves on loss.

5.3.  Fairness Among LEDBAT Flows

   The primary design goals of LEDBAT are focussed 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 enviroments some amount of
   randomness in the inter-packet transmission times exist, as explained
   further below.

   Delay-based congestion control systems suffer from the possibility of
   late-comers incorrectly measuring and using a higher base-delay than
   an active flow that started earlier.  Suppose a LEDBAT flow is the
   only flow on the bottleneck, which the flow saturates, steadily
   maintaining the queueing delay at a target delay.  When a new LEDBAT
   flow arrives, it might incorrectly measure the current end-to-end
   delay, including the queueing delay being maintained by the first
   LEDBAT flow, as its base delay, and the incoming flow might now
   effectively seek to build on top of the existing, already maximal
   queueing delay.  As the second flow builds up, the first flow sees
   the true queueing delay and backs off, while the late-comer keeps
   building up, using up the entire link's capacity; this advantage is
   called the "late-comer's advantage".

   In the worse 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 the first flow yields quickly
   enough to empty the bottleneck queue faster than the incoming flow
   increases its occupancy in the queue; as a result, the late-comer
   might measure a delay closer to the base delay.  While such a
   reduction might be achieved through a multiplicative decrease of the

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   congestion window, this might cause stronger fluctuations in flow
   throughput during 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 may 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 late-comer is starting, base delay is
   immediately correctly measured.  With a small number of LEDBAT flows,
   system noise may sufficiently regulate the late-comer's advantage.

6.  IANA Considerations

   There are no IANA considerations for this document.

7.  Security Considerations

   A network on the path might choose to cause higher delay measurements
   than the real queuing delay so that LEDBAT backs off even when
   there's no congestion present.  While shaping of traffic into an
   artificially narrow bottleneck by increasing the queueing delay
   cannot be trivially counteracted, a protocol using LEDBAT should seek
   to minimize the risk of such an attack by authenticating the
   timestamp and delay fields in the packets.

   LEDBAT is not known to introduce any new concerns with privacy,
   integrity, or other security issues for flows that use it.  It should
   be compatible with use of IPsec and TLS/DTLS.

8.  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.

9.  References

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9.1.  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.

9.2.  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", arXiv:1010.5623v1, September 2010.

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

   [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

Appendix A.  Timestamp errors

   One-way delay measurement needs to deal with timestamp errors.  We'll
   use the same locally linear clock model and the same terminology as
   Network Time Protocol (NTP).  This model is valid for any
   differentiable clocks.  NTP uses the term "offset" to refer to
   difference from true time and "skew" to refer to difference of clock
   rate from the true rate.  The clock will thus have a fixed offset
   from the true time and a skew.  We'll consider what we need to do

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   about the offset and the skew separately.

A.1.  Clock offset

   First, consider the case of zero skew.  The offset of each of the two
   clocks shows up as a fixed error in one-way delay measurement.  The
   difference of the offsets is the absolute error of the one-way delay
   estimate.  We won't use this estimate directly, however.  We'll use
   the difference between that and a base delay.  Because the error
   (difference of clock offsets) is the same for the current and base
   delay, it cancels from the queuing delay estimate, which is what
   we'll use.  Clock offset is thus irrelevant to the design.

A.2.  Clock skew

   The clock skew manifests in a linearly changing error in the time
   estimate.  For a given pair of clocks, the difference in skews is the
   skew of the one-way delay estimate.  Unlike the offset, this no
   longer cancels in the computation of the queuing delay estimate.  On
   the other hand, while the offset could be huge, with some clocks off
   by minutes or even hours or more, the skew is typically small.  For
   example, NTP is designed to work with most clocks, yet it gives up
   when the skew is more than 500 parts per million (PPM).  Typical
   skews of clocks that have never been trained seem to often be around
   100-200 PPM.  Previously trained clocks could have 10-20 PPM skew due
   to temperature changes.  A 100-PPM skew means accumulating 6
   milliseconds of error per minute.  The base delay updates mostly
   takes care of clock skew unless the skew is unusually high or extreme
   values have been chosen for TARGET and BASE_HISTORY so that the clock
   skew in BASE_DELAY minutes is larger than the TARGET.

   Clock skew can be in two directions: either the sender's clock is
   faster than the receiver's, or vice versa.

   If the sender's clock is faster the one-way delay measurement will
   get more and more reduced by the clock drift over time.  Whenever
   there is no additional delay the base delay will be updated by a
   smaller one-way delay value and the skew is compensated.  If a
   competing flow introduces additional queueing delay LEDBAT will
   anyway get out of the way quickly and an overestimated one-way delay
   will just speed-up the back-off.

   When the receiver clock runs faster, the raw delay estimate will
   drift up with time.  This can suppress the throughput unnecessarily.
   In this case a skew correction mechanim can be benefital.  Further
   condersiderations based on a deployed implementation and LEDBAT
   specific preconditions are given in the next section.

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A.3.  Clock skew correction mechanism

   The following paragraph describes the deployed clock skew correction
   mechanism in the BitTorrent implementation for documentation purpose.

   In the BitTorrent implementation, the receiver sends back raw
   (sending and receiving) timestamps.  Using this information, the
   sender can estimate the one-way delays in both directions, and 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 base delay, it infers that this reduction is due to
   clock drift.  The sender can be compensated by increasing its base
   delay by the same amount.  To apply this mechanism however,
   timestamps need to be transmitted in both directions.

   The following considerations can be used for an alternative
   implementation as a reference:
   o  Skew correction with faster virtual clock:
      Since having a faster clock on the sender will continuousely
      update the base delay, a faster virtual clock for sender
      timestamping can be applied.  This virual clock can be computed
      from the default machine clock through a linear transformation.
      E.g. with a 500 PPM speed-up the sender's clock is very likely to
      be faster than any receiver's clock and thus LEDBAT will benefit
      from the implicit correction when updating the base delay.

   o  Skew correction with estimating drift:
      With LEDBAT the history of base delay minima is already kept for
      each minute.  This 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.

   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,
      calculate 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 acts the
      fastest, since the state is preserved between connections.

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Authors' Addresses

   Stanislav Shalunov
   BitTorrent Inc
   612 Howard St, Suite 400
   San Francisco, CA  94105


   Greg Hazel
   BitTorrent Inc
   612 Howard St, Suite 400
   San Francisco, CA  94105


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


   Mirja Kuehlewind
   University of Stuttgart


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