rLEDBAT: receiver-driven Low Extra Delay Background Transport for TCP

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Network Working Group                                         M. Bagnulo
Internet-Draft                                        A. Garcia-Martinez
Intended status: Experimental                                       UC3M
Expires: February 26, 2021                                 G. Montenegro
                                                      P. Balasubramanian
                                                         August 25, 2020

 rLEDBAT: receiver-driven Low Extra Delay Background Transport for TCP


   This document specifies the rLEDBAT, a set of mechanisms that enable
   the execution of a less-than-best-effort congestion control algorithm
   for TCP at the receiver end.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Motivations for rLEDBAT . . . . . . . . . . . . . . . . . . .   3
   3.  rLEDBAT mechanisms  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Controlling the receive window  . . . . . . . . . . . . .   5
       3.1.1.  Avoiding window shrinking . . . . . . . . . . . . . .   6
       3.1.2.  Window Scale Option . . . . . . . . . . . . . . . . .   6
     3.2.  Measuring delays  . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Measuring the RTT to estimate the queueing delay  . .   7
       3.2.2.  Measuring one way delay to estimate the queueing
               delay . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Detecting packet losses and retransmissions . . . . . . .  12
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   LEDBAT (Low Extra Delay Background Transport) [RFC6817] is a
   congestion-control algorithm that implements a less-than-best-effort
   (LBE) traffic class.

   When LEDBAT traffic shares a bottleneck with one or more TCP
   connections using standard congestion control algorithms such as
   Cubic [RFC8312] (hereafter standard-TCP for short), it reduces its
   sending rate earlier and more aggressively than standard-TCP
   congestion control, allowing standard-TCP traffic to use more of the
   available capacity.  In the absence of competing standard-TCP
   traffic, LEDBAT aims to make an efficient use of the available
   capacity, while keeping the queuing delay within predefined bounds.

   LEDBAT reacts both to packet loss and to variations in delay.
   Regarding to packet loss, LEDBAT reacts with a multiplicative
   decrease, similar to most TCP congestion controllers.  Regarding
   delay, LEDBAT aims for a target queueing delay.  When the measured
   current queueing delay is below the target, LEDBAT increases the
   sending rate and when the delay is above the target, it reduces the
   sending rate.  LEDBAT estimates the queuing delay by subtracting the
   measured current one-way delay from the estimated base one-way delay
   (i.e. the one-way delay in the absence of queues).

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   The LEDBAT specification [RFC6817] defines the LEDBAT congestion-
   control algorithm, implemented in the sender to control its sending
   rate.  LEDBAT is specified in a protocol and layer agnostic manner.

   LEDBAT++ [I-D.balasubramanian-iccrg-ledbatplusplus] is also an LBE
   congestion control algorithm which is inspired in LEDBAT while
   addressing several problems identified with the original LEDBAT
   specification.  In particular the differences between LEDBAT and
   LEDBAT++ include: i) LEDBAT++ uses the round-trip-time (RTT) (as
   opposed to the one way delay used in LEDBAT) to estimate the queuing
   delay; ii) LEDBAT++ uses an Additive Increase/Multiplicative Decrease
   algorithm to achieve inter-LEDBAT++ fairness and avoid the late-comer
   advantage observed in LEDBAT; iii) LEDBAT++ performs periodic
   slowdowns to improve the measurement of the base delay; iv) LEDBAT++
   is defined for TCP.

   In this note, we describe rLEDBAT, a set of mechanisms that enable
   the execution of an LBE delay-based congestion control algorithm such
   as LEDBAT or LEDBAT++ in the receiver end of a TCP connection.

2.  Motivations for rLEDBAT

   rLEDBAT enables new use cases and new deployment models, fostering
   the use of LBE traffic and benefitting the global Internet by
   improving overall allocation of resources.  The following scenarios
   are enabled by rLEDBAT:

      Content Delivery Networks and more sophisticated file distribution
      scenarios: Consider the case where the source of a file to be
      distributed (e.g., a software developer that wishes to distribute
      a software update) would prefer to use LBE and it enables LEDBAT/
      LEDBAT++ in the servers containing the source file.  However,
      because the file is being distributed through a CDN which
      surrogates do not support LBE congestion control, the result is
      that the file transfers, originated from CDN surrogates will not
      be using LBE.  Interestingly enough, in the case of the software
      update, the developer may also control the software performing the
      download in the client, the receiver of the file, but because
      current LEDBAT/LEDBAT++ are sender-based algorithms, controlling
      the client is not enough to enable LBE congestion control in the
      communication. rLEDBAT would enable the use of LBE traffic class
      for file distribution in this setup.

      Interference from proxies and other middleboxes: Proxies and other
      middleboxes are a commonplace in the Internet.  For instance, in
      the case of mobile networks, proxies are frequently used.  In the
      case of enterprise networks, it is common to deploy corporate
      proxies for filtering and firewalling.  In the case of satellite

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      links, Performance Enhancement Proxies (PEPs) are deployed to
      mitigate the effect of the long delay in TCP connection.  These
      proxies terminate the TCP connection on both ends and prevent the
      use of LBE congestion control in the segment between the proxy and
      the sink of the content, the client.  By enabling rLEDBAT, clients
      would be able to enable LBE traffic between them and the proxy.

      Receiver-defined preferences.  It is frequent that the bottleneck
      of the communication is the access link.  This is particularly
      true in the case of mobile devices.  It is then especially
      relevant for mobile devices to properly manage the capacity of the
      access link.  With current technologies, it is possible for the
      mobile device to use different congestion control algorithms
      expressing different preferences for the traffic.  For instance, a
      device can choose to use standard-TCP for some traffic and to use
      LEDBAT/LEDBAT++ for other traffic.  However, this would only
      affect the outgoing traffic since both standard-TCP and LEDBAT/
      LEDBAT++ are sender-driven.  The mobile device has no means to
      manage the traffic in the down-link, which is in most cases, the
      communication bottleneck for a typical eye-ball end-user. rLEDBAT
      enables the mobile device to selectively use LBE traffic class for
      some of the incoming traffic.  For instance, by using rLEDBAT, a
      user can use regular standard-TCP/UDP for video stream (e.g.,
      Youtube) and use rLEDBAT for other background file download.

3.  rLEDBAT mechanisms

   rLEDBAT provides the mechanisms to implement an LBE congestion
   control algorithm at the receiver-end of a TCP connection.  The
   rLEDBAT receiver controls the sender's rate through the Receive
   Window announced to the receiver in the TCP header.

   rLEDBAT assumes that the sender is a standard TCP sender. rLEDBAT
   does not require any rLEDBAT-specific modifications to the TCP
   sender.  The envisioned deployment model for rLEDBAT is that the
   clients implement rLEDBAT and this enable rLEDBAT in communications
   with existent standard TCP senders.  In particular, the sender MUST
   implement [I-D.ietf-tcpm-rfc793bis] and it also MUST implement the
   Time Stamp Option as defined in [RFC7323].  Also, the sender SHOULD
   implement some of the standard congestion control mechanisms, such as
   Cubic [RFC8312] or New Reno [RFC5681].

   rLEDBAT does not defines a new congestion control algorithm.  The LBE
   congestion control algorithm executed in the rLEDBAT receiver is
   defined in other documents.  The rLEDBAT receiver MUST use an LBE
   congestion control algorithm.  Because rLEDBAT assumes a standard TCP
   sender, the sender will be using a "best effort" congestion control
   algorithm (such as Cubic or New Reno).  Since rLEDBAT uses the

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   Receive Window to control the sender's rate and the sender calculates
   the sender's window as the minimum of the Receive window and the
   congestion window, rLEDBAT will only be effective as long as the
   congestion control algorithm executed in the receiver yields a
   smaller window than the one calculated by the sender.  This is
   normally the case when the receiver is using an LBE congestion
   control algorithm.  The rLEDBAT receiver SHOULD use the LEDBAT
   congestion control algorithm [RFC6817] or the LEDBAT++ congestion
   control algorithm [I-D.balasubramanian-iccrg-ledbatplusplus].  The
   rLEDBAT MAY use other LBE congestion control algorithms defined
   elsewhere.  Irrespectively of which congestion control algorithm is
   executed in the receiver, an rLEDBAT connection will never be more
   aggressive than standard TCP since it is always bounded by the
   congestion control algorithm executed at the sender.

   rLEDBAT is essentially composed of three types of mechanisms, namely,
   those that provide the means to measure the packet delay (either the
   round trip time or the one way delay, depending on the selected
   algorithm), mechanisms to detect packet loss and the means to
   manipulate the Receive Window to control the sender's rate.  We
   describe them next.

3.1.  Controlling the receive window

   rLEDBAT uses the Receive Window (RCV.WND) of TCP to enable the
   receiver to control the sender's rate.  [I-D.ietf-tcpm-rfc793bis]
   defines that the RCV.WND is used to announce the available receive
   buffer to the sender for flow control purposes.  In order to avoid
   confusion, we will call fc.WND the value that a standard RFC793bis
   TCP receiver calculates to set in the receive window for flow control
   purposes.  We call rl.WND the window value calculated by rLEDBAT
   algorithm and we call RCV.WND the value actually included in the
   Receive Window field of the TCP header.  For a RFC793bis receiver,
   RCV.WND == fc.WND.

   In the case of rLEDBAT receiver, the rLEDBAT receiver MUST NOT set
   the RCV.WND to a value larger than fc.WND and it SHOULD set the
   RCV.WND to the minimum of rl.WND and fc.WND, honoring both.

   When using rLEDBAT, two congestion controllers are in action in the
   flow of data from the sender to the receiver, namely, the congestion
   control algorithm of TCP in the sender side and the LBE congestion
   control algorithm executed in the receiver and conveyed to the sender
   through the RCV.WND.  In the normal TCP operation, the sender uses
   the minimum of the congestion window cwnd and the receiver window
   RCV.WND to calculate the sender's window SND.WND.  This is also true
   for rLEDBAT, as the sender is a regular TCP sender.  This guarantees
   that the rLEDBAT flow will never transmit more aggressively than a

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   TCP flow, as the sender's congestion window limits the sending rate.
   Moreover, because a LBE congestion control algorithm such as LEDBAT/
   LEDBAT++ is designed to react earlier and more aggressively to
   congestion than regular TCP congestion control, the rl.WND contained
   in the RCV.WND field of TCP will be in general smaller than the
   congestion window calculated by the TCP sender, implying that the
   rLEDBAT congestion control algorithm will be effectively controlling
   the sender's window.

   In summary, the sender's window is: SND.WND = min(cwnd, rl.WND,

3.1.1.  Avoiding window shrinking

   The LEDBAT/LEDBAT++ algorithm executed in a rLEDBAT receiver
   increases or decreases the rl.WND according to congestion signals
   (variations on the estimations of the queueing delay and packet
   loss).  If the new congestion window is smaller than the current one
   then directly announcing it in the RCV.WND may result in shrinking
   the window, i.e., moving the right window edge to the left.
   Shrinking the window is discouraged as per [I-D.ietf-tcpm-rfc793bis],
   as it may cause unnecessary packet loss and performance penalty.  To
   be consistent with [I-D.ietf-tcpm-rfc793bis], the rLEDBAT receiver
   SHOULD NOT shrink the receive window.

   In order to avoid window shrinking, upon the reception of a data
   packet, the announced window can be reduced in the number of bytes
   contained in the packet at most.  This may fall short to honor the
   new calculated value of the rl.WND.  So, in order to reduce the
   window as dictated by the rLEDBAT algorithm, the receiver will
   progressively reduce the advertised RCV.WND, always honoring that the
   reduction is less or equal than the received bytes, until the target
   window determined by the rLEDBAT algorithm is reached.  This implies
   that it may take up to one RTT for the rLEDBAT receiver to drain
   enough in-flight bytes to completely close its receive window without
   shrinking it.  This is more than sufficient to honor the window
   output from the LEDBAT/LEDBAT++ algorithms since they only allows to
   perform at most one multiplicative decrease per RTT.

3.1.2.  Window Scale Option

   The Window Scale (WS) option [RFC7323] is a mean to increase the
   maximum window size permitted by the Receive Window.  The use of the
   WS option implies that the changes in the window are expressed in the
   units resulting of the WS option used in the TCP connection.  This
   means that the rLEDBAT client will have to accumulate the increases
   resulting from the different received packets, and only convey a
   change in the window when the accumulated sum of increases is equal

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   or higher than one unit used to express the receive window according
   to the WS option in place for the TCP connection.

   Changes in the receive window that are smaller than 1 MSS are
   unlikely to have any immediate impact on the sender's rate, as usual
   TCP segmentation practice results in sending full segments (i.e.,
   segments of size equal to the MSS).  So, accumulating changes in the
   receive window until completing a full MSS in the sender or in the
   receiver makes little difference.

   Current WS option specification [RFC7323] defines that allowed values
   for the WS option are between 0 and 14.  Assuming a MSS around 1500
   bytes, WS option values between 0 and 11 result in the receive window
   being expressed in units that are about 1 MSS or smaller.  So, WS
   option values between 0 and 11 have no impact in rLEDBAT.

   WS option values higher than 11 can affect the dynamics of rLEDBAT,
   since control may become too coarse (e.g., with WS of 14, a change in
   one unit of the receive window implies a change of 10 MSS in the
   effective window).

   For the above reasons, the rLEDBAT client SHOULD set WS option values
   lower than 12.  Additional experimentation is required to explore the
   impact of larger WS values in rLEDBAT dynamics.

   Note that the recommendation for rLEDBAT to set the WS option value
   to lower values does not precludes the communication with servers
   that set the WS option values to larger values, since the WS option
   value used is set independently for each direction of the TCP

3.2.  Measuring delays

   Both LEDBAT and LEDBAT++ measure base and current delays to estimate
   the queueing delay.  LEDBAT uses the one way delay while LEDBAT++
   uses the round trip time.  In the next sections we describe how
   rLEDBAt mechanisms enable the receiver to measure the one way delay
   or the round trip time, whatever needed depending on the congestion
   control algorithm used.

3.2.1.  Measuring the RTT to estimate the queueing delay

   LEDBAT++ uses the round trip time (RTT) to estimate the queueing
   delay.  In order to estimate the queueing delay using the RTT, the
   rLEDBAT receiver estimates the base RTT (i.e., the constant
   components of the RTT) and also measures the current RTT.  By
   subtracting these two values, we obtain the queuing delay to be used
   by the rLEDBAT controller.

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   LEDBAT++ discovers the base RTT (RTTb) by taking the minimum value of
   the measured RTTs over a period of time.  The current RTT (RTTc) is
   estimated using a number of recent samples and applying a filter,
   such as the minimum (or the mean) of the last k samples.  Using the
   RTT to estimate the queueing delay has a number of shortcomings and
   difficulties that we discuss next.

   The queuing delay measured using the RTT includes also the queueing
   delay experienced by the return packets in the direction from the
   rLEDBAT receiver to the sender.  This is a fundamental limitation of
   this approach.  The impact of this error is that the rLEDBAT
   controller will also react to congestion in the reverse path
   direction which results in an even more conservative mechanism.

   In order to measure the RTT, the rLEDBAT client MUST enable the Time
   Stamp (TS) option [RFC7323].  By matching the TSVal value carried in
   outgoing packets with the TSecr value observed in incoming packets,
   it is possible to measure the RTT.  This allows the rLEDBAT receiver
   to measure the RTT even if it is acting as a pure receiver.  In a
   pure receiver there is no data flowing from the rLEDBAT receiver to
   the sender, making impossible to match data packets with
   acknowledgements packets to measure the RTT, as it is usually done in
   TCP for other purposes.

   Depending on the frequency of the local clock used to generate the
   values included in the TS option, several packets may carry the same
   TSVal value.  If that happens, the rLEDBAT receiver will be unable to
   match the different outgoing packets carrying the same TSVal value
   with the different incoming packets carrying also the same TSecr
   value.  However, it is not necessary for rLEDBAT to use all packets
   to estimate the RTT and sampling a subset of in-flight packets per
   RTT is enough to properly assess the queueing delay.  The RTT MUST
   then be calculated as the time since the first packet with a given
   TSVal was sent and the first packet that was received with the same
   value contained in the TSecr.  Other packets with repeated TS values
   SHOULD NOT be used for the RTT calculation.

   Several issues must be addressed in order to avoid an artificial
   increase of the observed RTT.  Different issues emerge depending
   whether the rLEDBAT capable host is sending data packets or pure ACKs
   to measure the RTT.  We next consider the issues separately.  Measuring RTT sending pure ACKs

   In this scenario, the rLEDBAT node (node A) sends a pure ACK to the
   other endpoint of the TCP connection (node B), including the TS
   option.  Upon the reception of the TS Option, host B will copy the
   value of the TSVal into the TSecr field of the TS option and include

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   that option into the next data packet towards host A.  However, there
   are two reasons why B may not send a packet immediately back to A,
   artificially increasing the measured RTT.  The first reason is when A
   has no data to send.  The second is when A has no available window to
   put more packets in-flight.  We describe next how each of these cases
   is addressed.

   The case where the host B has no data to send when it receives the
   pure Acknowledgement is expected to be rare in the rLEDBAT use cases.
   rLEDBAT will be used mostly for background file transfers so the
   expected common case is that the sender will have data to send
   throughout the lifetime of the communication.  However, if, for
   example, the file is structured in blocks of data, it may be the case
   that seldom, the sender will have to wait until the next block is
   available to proceed with the data transfer and momentarily lack of
   data to send.  To address this situation, the filter used by the
   congestion control algorithm executed in the receiver SHOULD discard
   the larger samples (e.g. a min filter would achieve this) when
   measuring the RTT using pure ACK packets.

   The limitation of available sender's window to send more packets can
   come either from the TCP congestion window in host B or from the
   announced receive window from the rLEDBAT in host A.  Normally, the
   receive window will be the one to limit the sender's transmission
   rate, since the LBE congestion control algorithm used by the rLEDBAT
   node is designed to be more restrictive on the sender's rate than
   standard-TCP.  If the limiting factor is the congestion window in the
   sender, it is less relevant if rLEDBAT further reduces the receive
   window due to a bloated RTT measurement, since the rLEDBAT is not
   actively controlling the sender's rate.  Nevertheless, the proposed
   approach to discard larger samples would also address this issue.

   To address the case in which the limiting factor is the receive
   window announced by rLEDBAT, the congestion control algorithm at the
   receiver SHOULD discard the RTT measurements done using pure ACK
   packets while reducing the window and avoid including bloated samples
   in the queueing delay estimation.  The rLEDBAT receiver is aware
   whether a given TSVal value was sent in a pure ACK packet where the
   window was reduced, and if so, it can discard the corresponding RTT
   measurement.  Measuring the RTT sending data packets

   In the case that the rLEDBAT node is sending data packets and
   matching them with pure ACKs to measure the RTT, a factor that can
   artificially increase the RTT measured is the presence of delayed
   Acknowledgements.  According to the TS option generation rules
   [RFC7323], the value included in the TSecr for a delayed ACK is the

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   one in the TSVal field of the earliest unacknowledged segment.  This
   may artificially increase the measured RTT.

   If both endpoints of the connection are sending data packets,
   Acknowledgments are piggybacked into the data packets and they are
   not delayed.  Delayed ACKs only increase the RTT measurement in the
   case that the sender has no data to send.  Since the expected use
   case for rLEDBAT is that the sender will be sending background
   traffic to the rLEDBAT receiver, the cases where delayed ACKs
   increase the measured RTT are expected to be rare.

   Nevertheless, for those measurements done using data packets sent by
   the rLEDBAT node matching pure ACKs sent from the other endpoint of
   the connection, they will result in an increased RTT.  The additional
   increase in the measured RTT will range between the transmission
   delay of on packet and 500 ms.  The reason for this is that delayed
   ACKs are generated every second data packet received and not delayed
   more than 500 ms according to [I-D.ietf-tcpm-rfc793bis].  The rLEDBAT
   receiver MAY discard the RTT measurements done using data packets
   from the rLEBDAT receiver and matching pure ACKs, especially if it
   has recent measurements done using other packet combinations.Also,
   applying a filter that discard larger samples would also address this
   issue (e.g. a min filter).

3.2.2.  Measuring one way delay to estimate the queueing delay

   The LEDBAT algorithm uses the one-way delay of packets as input.  A
   TCP receiver can measure the delay of incoming packets directly (as
   opposed to the sender-based LEDBAT, where the receiver measures the
   one-way delay and needs to convey it to the sender).

   In the case of TCP, the receiver can use the Time Stamp option to
   measure the one way delay by subtracting the time stamp contained in
   the incoming packet from the local time at which the packet has
   arrived.  As noted in [RFC6817] the clock offset between the clock of
   the sender and the clock in the receiver does not affect the LEDBAT
   operation, since LEDBAT uses the difference between the base one way
   delay and the current one way delay to estimate the queuing delay,
   effectively canceling the clock offset error in the queueing delay
   estimation.  There are however two other issues that the rLEDBAT
   receiver needs to take into account in order to properly estimate the
   one way delay, namely, the units in which the received timestamps are
   expressed and the clock skew.  We address them next.

   In order to measure the one way delay using TCP timestamps, the
   rLEDBAT receiver needs to discover the units in which the values of
   the TS option are expressed and second, to account for the skew
   between the two clocks of the endpoints of the TCP connection.  Note

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   that a mismatch of 100 ppm (parts per million) in the estimation at
   the receiver of the clock rate of the sender accounts for 6 ms of
   variation per minute in the measured delay for a communication, just
   one order of magnitude below the target set for controlling the rate
   by rLEDBAT.  Typical skew for untrained clocks is reported to be
   around 100-200 ppm [RFC6817].

   In order to learn both the TS units and the clock skew, the rLEDBAT
   receiver compares how much local time has elapsed between the sender
   has issued two packets with different TS values.  By comparing the
   local time difference and the TS value difference, the receiver can
   assess the TS units and relative clock skews.  In order for this to
   be accurate, the packets carrying the different TS values should
   experience equal (or at least similar delay) when traveling from the
   sender to the receiver, as any difference in the experienced delays
   would introduce error in the unit/skew estimation.  One possible
   approach is to select packets that experienced the minimum delay
   (i.e. close to zero queueing delay) to make the estimations.

   An additional difficulty regarding the estimation of the TS units and
   clock skew in the context of (r)LEDBAT is that the LEDBAT congestion
   controller actions directly affect the (queueing) delay experienced
   by packets.  In particular, if there is an error in the estimation of
   the TS units/skew, the LEDBAT controller will attempt to compensate
   it by reducing/increasing the load.  The result is that the LEDBAT
   operation interferes with the TS units/clock skew measurements.
   Because of this, measurements are more accurate when there is no
   traffic in the connection (in addition to the packets used for the
   measurements).  The problem is that the receiver is unaware if the
   sender is injecting traffic at any point in time, and
   opportunistically seize quiet intervals to preform measurements.  The
   receiver can however, force periodic slowdowns, reducing the
   announced receive window to a few packets and perform the
   measurements then.

   It is possible for the rLEDBAT receiver to perform multiple
   measurements to assess both the TS units and the relative clock skew
   during the lifetime of the connection, in order to obtain more
   accurate results.  Clock skew measurements are more accurate if the
   time period used to discover the skew is larger, as the impact of the
   skew becomes more apparent.  Due to the same logic, accurately
   learning the clock skew is more pressing as the time separating the
   two delays to compare increases.  It is a reasonable approach for the
   rLEDBAT receiver to perform an early discovery of the TS units (and
   the clock skew) using the first few packets of the TCP connection and
   then improve the accuracy of the TS units/clock skew estimation using
   periodic measurements later in the lifetime of the connection.

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3.3.  Detecting packet losses and retransmissions

   The rLEDBAT receiver is capable of detecting retransmitted packets in
   the following way.  We call RCV.HGH the highest sequence number
   correspondent to a received byte of data (not assuming that all bytes
   with smaller sequence numbers have been received already, there may
   be holes) and we call TSV.HGH the TSVal value corresponding to the
   segment in which that byte was carried.  SEG.SEQ stands for the
   sequence number of a newly received segment and we call TSV.SEQ the
   TSVal value of the newly received segment.

   If SEG.SEQ < RCV.HGH and TSV.SEQ > TSV.HGH then the newly received
   segment is a retransmission.  This is so because the newly received
   segment was generated later than another already received segment
   which contained data with a larger sequence number.  This means that
   this segment was lost and was retransmitted.

   The proposed mechanism to detect retransmissions at the receiver
   fails when there are window tail drops.  If all packets in the tail
   of the window are lost, the receiver will not be able to detect a
   mismatch between the sequence numbers of the packets and the order of
   the timestamps.  In this case, rLEDBAT will not react to losses but
   the TCP congestion controller at the sender will, most likely
   reducing its window to 1MSS and take over the control of the sending
   rate, until slow start ramps up and catches the current value of the
   rLEDBAT window.

4.  Security Considerations

5.  IANA Considerations

6.  Acknowledgements

   This work was supported by the EU through the H2020 5G-RANGE project
   and by the Spanish Ministry of Economy and Competitiveness through
   the 5G-City project (TEC2016-76795-C6-3-R).

7.  Informative References

              Balasubramanian, P., Ertugay, O., and D. Havey, "LEDBAT++:
              Congestion Control for Background Traffic", draft-
              balasubramanian-iccrg-ledbatplusplus-01 (work in
              progress), November 2019.

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              Eddy, W., "Transmission Control Protocol Specification",
              draft-ietf-tcpm-rfc793bis-18 (work in progress), August

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,

   [RFC6817]  Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
              "Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
              DOI 10.17487/RFC6817, December 2012,

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,

   [RFC8312]  Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
              R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
              RFC 8312, DOI 10.17487/RFC8312, February 2018,

Authors' Addresses

   Marcelo Bagnulo

   Email: marcelo@it.uc3m.es

   Alberto Garcia-Martinez

   Email: alberto@it.uc3m.es

   Gabriel Montenegro

   Email: Gabriel.Montenegro@microsoft.com

   Praveen Balasubramanian

   Email: pravb@microsoft.com

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