LEDBAT WG                                                    S. Shalunov
Internet-Draft                                                  G. Hazel
Intended status: Experimental                             BitTorrent Inc
Expires: September 15, 2011                                   J. Iyengar
                                           Franklin and Marshall College
                                                           M. Kuehlewind
                                                 University of Stuttgart
                                                          March 14, 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 induced in the network by the LEDBAT flow.  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 competing TCP flows, thus limiting
   interference with the network performance of the competing TCP 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
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   This Internet-Draft will expire on September 15, 2011.

Copyright Notice

   Copyright (c) 2011 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

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   Provisions Relating to IETF Documents
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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  . . . . . . . . . . . . . . . . . .  5
       3.4.1.  An Overview  . . . . . . . . . . . . . . . . . . . . .  5
       3.4.2.  The Complete Sender Algorithm  . . . . . . . . . . . .  6
     3.5.  Parameter Values . . . . . . . . . . . . . . . . . . . . .  7
   4.  Understanding LEDBAT Mechanisms  . . . . . . . . . . . . . . .  8
     4.1.  Delay Estimation . . . . . . . . . . . . . . . . . . . . .  8
       4.1.1.  Estimating Base Delay  . . . . . . . . . . . . . . . .  8
       4.1.2.  Estimating Queueing Delay  . . . . . . . . . . . . . .  9
     4.2.  Managing the Congestion Window . . . . . . . . . . . . . .  9
       4.2.1.  Window Increase: Probing For More Bandwidth  . . . . .  9
       4.2.2.  Window Decrease: Responding To Congestion  . . . . . .  9
     4.3.  Choosing The Queuing Delay Target  . . . . . . . . . . . . 10
   5.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Framing and Ack Frequency Considerations . . . . . . . . . 10
     5.2.  Competing With TCP Flows . . . . . . . . . . . . . . . . . 11
     5.3.  Fairness Among LEDBAT Flows  . . . . . . . . . . . . . . . 11
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Appendix A.  Timestamp errors  . . . . . . . . . . . . . . . . . . 13
     A.1.  Clock offset . . . . . . . . . . . . . . . . . . . . . . . 13
     A.2.  Clock skew . . . . . . . . . . . . . . . . . . . . . . . . 13
     A.3.  Clock skew correction mechanism  . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

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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 network
   use by applications that seek to avoid interfering with the network
   performance of TCP flows.  LEDBAT uses one-way delay measurements to
   determine congestion on the data path, and keeps latency across the
   bottleneck link in the end-to-end path low while attempting to
   utilize the available bandwidth on the end-to-end path.

   Delay-based congestion control protocols, such as TCP-Vegas [Bra94],
   are generally designed to achieve more, not less throughput than
   standard TCP, and often outperform TCP under particular network
   settings.  LEDBAT, however, 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.  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 TCP flows by
   adding low extra queueing delay on the end-to-end path.

2.1.  Design Goals

   LEDBAT congestion control seeks to:
   1.  keep delay low when no other traffic is present,
   2.  add little to the queuing delays induced by TCP traffic,
   3.  quickly yield to flows using standard TCP congestion control that
       share the same bottleneck link,

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   4.  utilize end-to-end available bandwidth, and
   5.  operate well in networks with FIFO queues and tail-drop queue

2.2.  Applicability

   LEDBAT is a "scavenger" congestion control mechanism---a LEDBAT flow
   seeks to utilize all available bandwidth and to yield quickly to a
   competing standard TCP flow---and 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.

   This document specifies a congestion control mechanism that can be
   used as part of a transport protocol or as part of an application.
   LEDBAT can be used where 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 atop UDP for P2P applications.

3.  LEDBAT Congestion Control

3.1.  Overview

   A standard TCP sender increases its congestion window until a loss
   occurs [RFC5681], which, in the absence of any Active Queue
   Management (AQM) in the network, occurs only when the queue at the
   bottleneck link on the end-to-end path overflows.  Since packet loss
   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

   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.

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3.2.  Preliminaries

   A LEDBAT sender uses a congestion window (cwnd) that gates the amount
   of data that the sender can send into the network in one RTT.  A
   sender can choose to maintain a 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 MIN_CWND MSS (MIN_CWND is described later in Section 3.5.

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

3.4.  Sender-Side Operation

3.4.1.  An Overview

   As a first approximation, a LEDAT 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 can introduce in the
   network, and GAIN determines the rate at which the congestion window
   responds to changes in queueing delay; both constants are specified
   later.  Since off_target can be positive or negative, the congestion
   window (cwnd) increases or decreases in proportion to off_target.

   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 / cwnd

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3.4.2.  The Complete Sender Algorithm

   The simplified mechanism above ignores noise filtering and base delay
   expiration, which we now take into account in our complete sender
   algorithm below. update_base_delay() maintains a list of one-way
   delay minima over a number of one-minute intervals, to measure and
   track changes in the base delay of the end-to-end path.
   update_current_delay() maintains a list of one-way delay
   measurements, of which the minimum is used as an estimate of the
   current end-to-end delay.  Note that while this document uses the
   minimum to filter any noise in the one-way delay, a different and
   more sophisticated filter MAY be used.

   This complete algorithm restricts cwnd growth after a period of
   inactivity, where the cwnd is clamped down to a little more than
   flightsize using max_allowed_cwnd.  Finally, to be TCP-friendly on
   data loss, LEDBAT halves its congestion window.  The full sender-side
   algorithm is given below:

   on initialization:
       create current_delays list with CURRENT_FILTER elements
       create base_delays list with BASE_HISTORY number of elements
       inialize elements in current_delays and base_delays to +INFINITY
       last_rollover = -INFINITY # More than a minute in the past.
       cwnd = INIT_CWND * MSS

   on acknowledgement:
       # flightsize is the amount of data oustanding before this ack
       #    was received and is updated later by update_flightsize();
       # bytes_newly_acked is the number of bytes that this ack
       #    newly acknowledges, and it MAY be set to MSS; and
       # cwnd is in bytes.

       delay = acknowledgement.delay
       queuing_delay = MIN(current_delays) - MIN(base_delays)
       off_target = (TARGET - queuing_delay) / TARGET
       cwnd += GAIN * off_target * bytes_newly_acked / cwnd
       max_allowed_cwnd = flightsize + ALLOWED_INCREASE * MSS
       cwnd = min(cwnd, max_allowed_cwnd)
       cwnd = max(cwnd, MIN_CWND * MSS)
       update_flightsize()  #subtracts bytes_newly_acked from flightsize

   on data loss:
       # atmost once per RTT
       cwnd = cwnd/2
       cwnd = max(cwnd, MIN_CWND * MSS)

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     # 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 min delays. Each represents a minute.
     if round_to_minute(now) != round_to_minute(last_rollover)
         last_rollover = now
         delete first item in base_delays list
         append delay to tail of base_delays list
         tail of base_delays list = MIN(tail of base_delays list, delay)

   Note, that random fluctuations in inter-packet transmission time is
   assumed; see section Section 5.3 for a discussion.

   To implement an approximate minimum over the last N minutes, a LEDBAT
   sender stores BASE_HISTORY+1 separate minima---BASE_HISTORY for the
   last BASE_HISTORY 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.  When the
   connection is idle for a given minute, no data is available for the
   one-way delay and, therefore, no minimum is stored.  When the
   connection has been idle for N minutes, the measurement begins anew.
   Thus even if no data has sent and consequently no acknowledgements
   have been received the implementation have to mainatin the base delay

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 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 congestion window ramp-up to the same

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   rate as TCP Reno in Congestion Avoidance.  BASE_HISTORY SHOULD be 10;
   MIN_CWND SHOULD be 2, and it MUST be at least 1.  INIT_CWND SHOULD be
   2, and it MUST be at least 1.  The choice of MIN_CWND and INIT_CWND
   are strongly connected to the framing protocol; a larger MIN_CWND
   and/or INIT_CWND MAY be used if the framing protocol allows it.  For
   instance, TCP senders may use a larger INIT_CWND as specified in

   A LEDBAT sender uses the current_delays list to maintain 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 depends on the number of successful measurements
   made within an RTT amount of time in the past.  CURRENT_FILTER SHOULD
   be 1, and it MUST be at least 1 and limited such that no samples in
   list are older than an RTT in the past.  Note that after a long
   silent period, a LEDBAT sender will still have at least 1
   current_delay sample; any current_delay samples older than an RTT
   MUST NOT be used in computing queueing_delay.  A simple
   implementation with a fixed-size list replaces measurements in the
   list that are older than an RTT with +INFINITY.

4.  Understanding LEDBAT Mechanisms

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

4.1.  Delay Estimation

   To observe an increase in the queueing delay in the network, LEDBAT
   separates the queueing delay component from the rest of the end-to-
   end delay.  This section explains how LEDBAT decomposes the observed
   changes in end-to-end delay into these two components.

   LEDBAT estimates congestion in the direction of the data flow, and to
   avoid measuring queue build-up on the reverse path (or ack path),
   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.

4.1.1.  Estimating Base Delay

   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.  Since

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   queuing delay is always additive to the end-to-end delay, we estimate
   the sum of the constant delay components, which we call "base delay",
   to be the minimum delay observed on the end-to-end path.  Using the
   minimum observed delay also allows LEDBAT to eliminate noise in the
   delay estimation, such as due to spikes in processing delay at a node
   on the path.

   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.

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.

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

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 detemine 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.  In
   limited experiments with Bittorrent nodes, this controller seems to
   work well.

   Unlike TCP-like loss-based congestion control, LEDBAT does not induce
   losses and so a LEDBAT sender is not expected to normally rely on

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   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.

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, especially
   if the LEDBAT flow is the only flow on the link, 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.

   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 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.

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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
   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 and is as aggressive
   as a TCP flow.

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

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   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
   congestion window, this might cause stronger fluctuations in flow
   throughput during steady state.

   In practice, however, this concern seems 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 OS scheduling at the sender, processing delay at
   the sender or any network node, and link contention.  When such a gap
   occurs while the late-comer is starting, base delay is immediately
   correctly measured.  With a small number of LEDBAT flows, system
   noise seems to 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.

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.

   [RFC3390]  Allman, M., Floyd, S., and C. Partridge, "Increasing TCP's
              Initial Window", RFC 3390, October 2002.

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

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.

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
   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

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   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 senders'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.  This will
   happen continuously as LEBDAT is design to keep the queue empty.  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.

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 implemtation the receiver sends back the raw
   (sending and receiving) timestamps.  Based on this imfomation and the
   system time at feedback receiption the sender can estimated the one-
   way delay in both directions.  Thus the sender can run the same base
   delay calculation algorithm it runs for itself for the receiver as
   well.  If the sender can detect the receiver reducing its base delay,
   it can infer that this is due to clock drift.  The sender can be
   compensated for by increasing the it's base delay by the same amount.
   To apply this mechanism symmetrical framing is need (i.e., same
   information about delays transmitted in both way).

   The following considerations can be used for an alternative
   implementation as a reference:

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   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.

Authors' Addresses

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

   Email: shalunov@bittorrent.com
   URI:   http://shlang.com

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

   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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