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  <front>
    <title abbrev="rLEDBAT">rLEDBAT: receiver-driven Receiver-Driven Low Extra Delay Background Transport for TCP
	</title> TCP</title>
    <seriesInfo name="RFC" value="9840"/>
    <author fullname="Marcelo Bagnulo" initials="M." surname="Bagnulo">
      <organization>Universidad Carlos III de Madrid</organization>
      <address>
        <email>marcelo@it.uc3m.es</email>
      </address>
    </author>
    <author fullname="Alberto Garcia-Martinez" García-Martínez" initials="A." surname="Garcia-Martinez"> surname="García-Martínez">
      <organization>Universidad Carlos III de Madrid</organization>
      <address>
        <email>alberto@it.uc3m.es</email>
      </address>
    </author>

    <author fullname="Gabriel Montenegro" initials="G." surname="Montenegro">
      <address>
        <email>g.e.montenegro@hotmail.com</email>
      </address>
    </author>
    <author fullname="Praveen Balasubramanian " initials="P." surname="Balasubramanian">
      <organization>Confluent</organization>
      <address>
        <email>pravb.ietf@gmail.com</email>
      </address>
    </author>
    <date year="2025" /> month="September"/>

    <workgroup>Internet Congestion Control</workgroup>

<keyword>Congestion control</keyword>
<keyword>scavenger/less-than-best-effort traffic</keyword>

    <abstract>
      <t> This document specifies rLEDBAT, receiver-driven Low Extra Delay Background Transport (rLEDBAT) -- a set of mechanisms that enable the execution of a less-than-best-effort congestion control algorithm for TCP at the receiver end. This document is a product of the Internet Congestion Control Research Group (ICCRG) of the Internet Research Task Force (IRTF).
      </t>
    </abstract>
  </front>
  <middle>
    <section title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>LEDBAT (Low Extra Delay Background Transport) <xref target="RFC6817" /> format="default"/> is a congestion-control congestion control algorithm used for less-than-best-effort (LBE) traffic.</t>
      <t>When LEDBAT traffic shares a bottleneck with other traffic using standard congestion control algorithms (for example, TCP traffic using Cubic<xref CUBIC <xref target="RFC9438" />, format="default"/>, hereafter referred to as standard-TCP "standard-TCP" for short), it reduces its sending rate earlier and more aggressively than standard-TCP congestion control, allowing other non-background traffic to use more of the available capacity. In the absence of competing traffic, LEDBAT aims to make an efficient use of the available capacity, while keeping the queuing delay within predefined bounds.</t>
      <t>LEDBAT reacts both to both packet loss and to variations in delay.  With respect to packet loss, LEDBAT reacts with a multiplicative decrease, similar to most TCP congestion controllers. Regarding delay, LEDBAT aims for a target queueing queuing delay. When the measured current queueing queuing delay is below the target, LEDBAT increases the sending rate 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. (i.e., the one-way delay in the absence of queues). </t>
      <t>The LEDBAT specification <xref target="RFC6817" /> format="default"/> defines the LEDBAT congestion-control congestion control algorithm, implemented in the sender to control its sending rate. LEDBAT is specified in a protocol protocol-agnostic and layer agnostic layer-agnostic manner.</t>
      <t>LEDBAT++ <xref target="I-D.irtf-iccrg-ledbat-plus-plus" /> format="default"/> is also an LBE congestion control algorithm which that is inspired by LEDBAT while addressing several problems identified with the original LEDBAT specification. In particular particular, the differences between LEDBAT and LEDBAT++ include: i) LEDBAT++ include the following:</t>

 <ol spacing="normal" type="%i)">
 <li>LEDBAT++ uses the round-trip-time round-trip time (RTT) (as opposed to the one way one-way delay used in LEDBAT) to estimate the queuing delay; ii) LEDBAT++ delay.</li>
 <li>LEDBAT++ uses an Additive Increase/Multiplicative Decrease additive increase/multiplicative decrease algorithm to achieve inter-LEDBAT++ fairness and avoid the late-comer latecomer advantage observed in LEDBAT; iii) LEDBAT++ LEDBAT.</li>
 <li>LEDBAT++ performs periodic slowdowns to improve the measurement of the base delay; iv) LEDBAT++ delay.</li>
 <li>LEDBAT++ is defined for TCP.</t> TCP.</li>
 </ol>
      <t>In this specification, we describe rLEDBAT, receiver-driven Low Extra Delay Background Transport (rLEDBAT) -- a set of mechanisms that enable the execution of an LBE delay-based congestion control algorithm such as LEDBAT or LEDBAT++ at the receiver end of a TCP connection.</t>
      <t> The consensus of the Internet Congestion Control Research Group (ICCRG) is to publish this document to encourage further experimentation and review of rLEDBAT. This document is not an IETF product and is not a standard. an Internet Standards Track specification. The status of this document is experimental. Experimental. In section 4 titled Experiment Considerations, <xref target="sect-5" format="default"/> ("<xref target="sect-5" format="title"/>"), we describe the purpose of the experiment and its current status.  </t>
    </section>

    <section title="Motivations numbered="true" toc="default">
      <name>Conventions and Terminology</name>
       <t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
       "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",
       "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
       "<bcp14>SHOULD NOT</bcp14>",
       "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
       "<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document
       are to be interpreted as described in BCP&nbsp;14
       <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only
       when, they appear in all capitals, as shown here.</t>
      <t>We use the following abbreviations throughout the text and include them here for the reader's convenience:</t>
      <dl spacing="normal" newline="false">
        <dt>RCV.WND:</dt><dd>The value included in the Receive Window field of
        the TCP header (the computation of which is modified by its
        specification).</dd>
        <dt>SND.WND:</dt><dd>The TCP sender's window.</dd>
        <dt>cwnd:</dt><dd>The congestion window as computed by the congestion
        control algorithm running at the TCP sender.</dd>
        <dt>RLWND:</dt><dd>The window value calculated by the rLEDBAT algorithm.</dd>
        <dt>fcwnd:</dt><dd>The value that a standard-TCP receiver compliant with
        <xref target="RFC9293"/>
        calculates to set in the receive window for flow control
        purposes.</dd>
        <dt>RCV.HGH:</dt><dd>The highest sequence number corresponding to a
        received byte of data at one point in time.</dd>
        <dt>TSV.HGH:</dt><dd>The Timestamp Value (TSval) <xref target="RFC7323" format="default"/> corresponding to the
        segment in which RCV.HGH was carried at that point in time.</dd>
        <dt>SEG.SEQ:</dt><dd>The sequence number of the last received segment.</dd>
        <dt>TSV.SEQ:</dt><dd>The TSval of the last received segment.</dd>
      </dl>
    </section>
    <section numbered="true" toc="default">
      <name>Motivations for rLEDBAT"> rLEDBAT</name>
      <t>rLEDBAT enables new use cases and new deployment models, fostering the use of LBE traffic. The following scenarios are enabled by rLEDBAT:
		<list>
			<t>Content
      </t>
      <dl spacing="normal" newline="true">
        <dt>Content Delivery Networks (CDNs) and more sophisticated file distribution scenarios: Consider scenarios:</dt>
<dd>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 that does not implement 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 client (the receiver of the file, 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 &nbsp;rLEDBAT would enable the use of an LBE traffic class for file distribution in this setup. </t>
	    	<t>Interference setup.</dd>
        <dt>Interference from proxies and other middleboxes: Proxies middleboxes:</dt>
<dd>Proxies and other middleboxes are 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 links, Performance Enhancement Enhancing Proxies (PEPs) are deployed to mitigate the effect of the long delay delays in a 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 can then enable LBE traffic between them and the proxy.</t>
	    	<t>Receiver-defined preferences. It is frequent that proxy.</dd>
          <dt>Receiver-defined preferences:</dt>
<dd>Frequently, the bottleneck of the communication access link is the access link. communication bottleneck. 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 traffic, since both standard-TCP and LEDBAT/LEDBAT++ are sender-driven. driven by the sender. The mobile device has no means to manage the traffic in the down-link, downlink, which is is, in most cases, the communication bottleneck for a typical eye-ball end-user. rLEDBAT "eyeball" end user. &nbsp;rLEDBAT enables the mobile device to selectively use an LBE traffic class for some of the incoming traffic. For instance, by using rLEDBAT, a user can use regular standard-TCP/UDP for a video stream (e.g., Youtube) YouTube) and use rLEDBAT for other background file download.</t>
		</list></t> downloads.</dd>
      </dl>
    </section>
    <section title="rLEDBAT mechanisms"> numbered="true" toc="default">
      <name>rLEDBAT Mechanisms</name>
      <t>rLEDBAT provides the mechanisms to implement an LBE congestion control algorithm at the receiver-end receiver end of a TCP connection. The rLEDBAT receiver controls the sender's rate through the Receive Window receive window announced by the receiver in the TCP header.</t>
      <t>rLEDBAT assumes that the sender is a standard TCP standard-TCP sender. rLEDBAT &nbsp;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 enables rLEDBAT in communications with existent standard TCP existing standard-TCP senders.  In particular, the sender MUST <bcp14>MUST</bcp14> implement <xref target="RFC9293" /> format="default"/> and it also MUST <bcp14>MUST</bcp14> implement the Time Stamp Option TCP Timestamps (TS) option as defined in <xref target="RFC7323" />. format="default"/>. Also, the sender should implement some of the standard congestion control mechanisms, such as Cubic CUBIC <xref target="RFC9438" /> format="default"/> or New Reno NewReno <xref target="RFC5681" />. </t> format="default"/> <xref target="RFC6582"/>.</t>
      <t>rLEDBAT does not define a new congestion control algorithm. The LBE congestion control algorithm executed in the rLEDBAT receiver is defined in other documents. The rLEDBAT receiver MUST <bcp14>MUST</bcp14> use an LBE congestion control algorithm. Because rLEDBAT assumes a standard TCP standard-TCP sender, the sender will be using a "best effort" congestion control algorithm (such as Cubic CUBIC or New Reno). NewReno). Since rLEDBAT uses the Receive Window receive window to control the sender's rate and the sender calculates the sender's window as the minimum of the Receive 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 <bcp14>SHOULD</bcp14> use the LEDBAT congestion control algorithm <xref target="RFC6817" /> format="default"/> or the LEDBAT++ congestion control algorithm <xref target="I-D.irtf-iccrg-ledbat-plus-plus" />. The rLEDBAT MAY format="default"/>. &nbsp;rLEDBAT <bcp14>MAY</bcp14> use other LBE congestion control algorithms defined elsewhere. Irrespective of which congestion control algorithm is executed in the receiver, an a rLEDBAT connection will never be more aggressive than standard-TCP standard-TCP, since it is always bounded by the congestion control algorithm executed at the sender.</t>
      <t>rLEDBAT is essentially composed of three types of mechanisms, namely, namely
those that provide the means to measure the packet delay (either the round trip time RTT or the one way one-way delay, depending on the selected algorithm), mechanisms to detect packet loss loss, and the means to manipulate the Receive Window receive window to control the sender's rate. The former first two provide input to the LBE congestion control algorithm algorithm, while the latter third uses the congestion window computed by the LBE congestion control algorithm to manipulate the Receive receive window, as depicted in the figure.</t> <xref target="fig1"/>.</t>
      <figure title="The anchor="fig1">
        <name>The rLEDBAT architecture."> Architecture</name>
        <artwork align="center"><![CDATA[ align="center" name="" type="" alt=""><![CDATA[
+------------------------------------------+
|   TCP receiver Receiver                           |
|                      +-----------------+ |
|                      |  +------------+ | |
|   +---------------------|     RTT    | | |
|   |                  |  | Estimation | | |
|   |                  |  +------------+ | |
|   |                  |                 | |
|   |                  |  +------------+ | |
|   |      +--------------| Loss, RTX  | | |
|   |      |           |  | Detection  | | |
|   |      |           |  +------------+ | |
|   v      v           |                 | |
| +----------------+   |                 | |
| | LBE Congestion |   |    rLEDBAT      | |
| |    Control     |   |                 | |
| +----------------+   |                 | |
|       |              |  +------------+ | |
|       |              |  | RCV-WND RCV.WND    | | |
|       +---------------->| Control    | | |
|                      |  +------------+ | |
|                      +-----------------+ |
+------------------------------------------+
]]></artwork>
      </figure>
      <t>We next describe each of the rLEDBAT components next.</t> components.</t>
      <section title="Controlling numbered="true" toc="default">
        <name>Controlling the receive window"> Receive Window</name>
        <t>rLEDBAT uses the Receive Window (RCV.WND) of TCP receive window (RCV.WND) to enable the receiver to control the sender's rate.  <xref target="RFC9293" /> defines format="default"/> specifies 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 fcwnd the value that a standard RFC793bis TCP standard-TCP receiver compliant with <xref target="RFC9293"/> calculates to set in the receive window for flow control purposes. We call RLWND the window value calculated by the rLEDBAT algorithm algorithm, and we call RCV.WND the value actually included in the Receive Window field of the TCP header. For a RFC793bis receiver, receiver compliant with <xref target="RFC9293"/>, RCV.WND == fcwnd.</t>
        <t>In the case of rLEDBAT receiver, the rLEDBAT receiver, this receiver MUST NOT <bcp14>MUST NOT</bcp14> set the RCV.WND to a value larger than fcwnd and it SHOULD <bcp14>SHOULD</bcp14> set the RCV.WND to the minimum of RLWND and fcwnd, honoring both.</t>
        <t>When using rLEDBAT, two congestion controllers are in action in the flow of data from the sender to the receiver, namely, namely the TCP congestion control algorithm of TCP in on 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 standard-TCP flow, as the sender's congestion window limits the sending rate. Moreover, because a an LBE congestion control algorithm such as LEDBAT/LEDBAT++ is designed to react earlier and more aggressively to congestion than regular TCP congestion control, the RLWND contained in the TCP RCV.WND field of TCP will generally 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. One exception to this scenario is that at the beginning of the connection, when there is no information to set RLWND, then, RLWND is set to its maximum value, so that the sending rate of the sender is governed by the flow control algorithm of the receiver and the TCP slow start mechanism of the sender.</t>
        <t>In summary, the sender's window is: is SND.WND = min(cwnd, RLWND, fcwnd)</t>
        <section title="Avoiding window shrinking"> numbered="true" toc="default">
          <name>Avoiding Window Shrinking</name>
          <t>The LEDBAT/LEDBAT++ algorithm executed in a rLEDBAT receiver increases or decreases RLWND according to congestion signals (variations on in the estimated queueing queuing delay and packet loss).

			If RLWND is decreased and directly announced in RCV.WND, this could lead to an announced window that is smaller than what is currently in use. This so called 'shrinking so-called "shrinking the window' window" is discouraged as per <xref target="RFC9293" />, format="default"/>, as it may cause unnecessary packet loss and performance penalty. penalties. To be consistent with <xref target="RFC9293" />, format="default"/>, the rLEDBAT receiver SHOULD NOT <bcp14>SHOULD NOT</bcp14> shrink the receive window. </t>
          <t>In order to avoid window shrinking, the receiver MUST <bcp14>MUST</bcp14> only reduce RCV.WND by the number of bytes upon of contained in a received data packet. This may fall short to honor the new calculated value of the RLWND immediately. However, the receiver SHOULD <bcp14>SHOULD</bcp14> progressively reduce the advertised RCV.WND, always honoring that the reduction is less than or equal than to 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 sufficient to honor the window output from the LEDBAT/LEDBAT++ algorithms algorithms, since they are only allow allowed to perform at most one multiplicative decrease per RTT.</t>
        </section>
        <section title="Setting numbered="true" toc="default">
          <name>Setting the Window Scale Option"> Option</name>
          <t>The Window Scale (WS) option <xref target="RFC7323" /> format="default"/> is a means to increase the maximum window size permitted by the Receive Window. receive window. The WS option defines a scale factor which that restricts the granularity of the receive window that can be announced. This means that the rLEDBAT client will have to accumulate the increases resulting from multiple received packets, packets and only convey a change in the window when the accumulated sum of increases is equal to or higher than one increase step as imposed by the scaling factor according to the WS option in place for the TCP connection.</t>
          <t>Changes in the receive window that are smaller than 1 MSS (Maximum Segment Size) are unlikely to have any immediate impact on the sender's rate, as usual rate. As usual, TCP's segmentation practice results in sending full segments (i.e., segments of size equal to the MSS). Current WS option specification <xref target="RFC7323" /> format="default"/>, which defines the WS option, specifies that allowed values for the WS option are between 0 and 14. Assuming a an MSS of 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 (unless packets smaller than the MSS are being exchanged).</t>
          <t>WS option values higher than 11 can affect the dynamics of rLEDBAT, since control may become too coarse (e.g., with a WS option value of 14, a change in one unit of the receive window implies a change of 10 MSS in the effective window).</t> window).
</t>
          <t>For the above reasons, the rLEDBAT client SHOULD <bcp14>SHOULD</bcp14> set WS option values lower than 12. Additional experimentation is required to explore the impact of larger WS values on rLEDBAT dynamics.</t>
          <t>Note that the recommendation for rLEDBAT to set the WS option value values to lower values does not precludes the preclude communication with servers that set the WS option values to larger values, since the WS option value is values are set independently for each direction of the TCP connection.</t>
        </section>
      </section>
      <section title="Measuring delays"> numbered="true" toc="default">
        <name>Measuring Delays</name>
        <t>Both LEDBAT and LEDBAT++ measure base and current delays to estimate the queueing queuing delay. LEDBAT uses the one way delay one-way delay, while LEDBAT++ uses the round trip time. RTT. In the next sections sections, we describe how rLEDBAT mechanisms enable the receiver to measure the one way one-way delay or the round trip time, whatever RTT -- whichever is needed needed, depending on the congestion control algorithm used.</t>
        <section title="Measuring numbered="true" toc="default">
          <name>Measuring RTT to estimate Estimate the queueing delay"> Queuing Delay</name>
          <t>LEDBAT++ uses the round trip time (RTT) RTT to estimate the queueing queuing delay. In order to estimate the queueing queuing delay using RTT, the rLEDBAT receiver estimates the base RTT (i.e., the constant components of RTT) and also measures the current RTT. By subtracting these two values, we obtain the queuing delay to be used by the rLEDBAT controller.</t>
          <t>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 RTT to estimate the queueing queuing delay has a number of shortcomings and difficulties that we discuss next.</t> difficulties, as discussed below.</t>
          <t>The queuing delay measured using RTT includes also includes the queueing queuing 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 limitation is that the rLEDBAT controller will also react to congestion in the reverse path direction which results direction, resulting in an even more conservative mechanism.</t>
          <t>In order to measure RTT, the rLEDBAT client MUST <bcp14>MUST</bcp14> enable the Time Stamp (TS) TS option <xref target="RFC7323" />. format="default"/>. By matching the TSVal value TSval carried in outgoing packets with the TSecr Timestamp Echo Reply (TSecr) value <xref target="RFC7323" format="default"/> observed in incoming packets, it is possible to measure RTT. This allows the rLEDBAT receiver to measure RTT even if it is acting as a pure receiver. In a pure receiver receiver, there is no data flowing from the rLEDBAT receiver to the sender, making it impossible to match data packets with acknowledgements Acknowledgment packets to measure RTT, as it in contrast to what is usually done in TCP for other purposes.</t>
          <t>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. TSval. If that happens, the rLEDBAT receiver will be unable to match the different outgoing packets carrying the same TSVal value TSval with the different incoming packets carrying also carrying the same TSecr value. However, it is not necessary for rLEDBAT to use all packets to estimate RTT RTT, and sampling a subset of in-flight packets per RTT is enough to properly assess the queueing queuing delay. RTT MUST <bcp14>MUST</bcp14> then be calculated as the time since the first packet with a given TSVal 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 <bcp14>SHOULD NOT</bcp14> be used for RTT calculation. calculations. </t>
          <t>Several issues must be addressed in order to avoid an artificial increase of in the observed RTT. Different issues emerge emerge, depending on whether the rLEDBAT capable
rLEDBAT-capable host is sending data packets or pure ACKs to measure RTT. We next consider the these issues separately.</t>
          <section title="Measuring numbered="true" toc="default">
            <name>Measuring RTT sending pure ACKs"> When Sending Pure ACKs</name>
            <t>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, option, host B will copy the value of the TSVal TSval into the TSecr field of the TS option and include that option into in 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. in flight. We describe next describe how each of these cases is addressed.</t>
            <t>The case where the host B has no data to send when it receives the pure Acknowledgement Acknowledgment is expected to be rare in the rLEDBAT use cases. rLEDBAT &nbsp;rLEDBAT will be used mostly for background file transfers 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 the sender seldomly will seldom have to wait until the next block is available to proceed with the data transfer. To address this situation, the filter used by the congestion control algorithm executed in the receiver SHOULD <bcp14>SHOULD</bcp14> discard outliers (e.g. (e.g., a min MIN filter <xref target="RFC6817"/> would achieve this) when measuring RTT using pure ACK packets.</t>
            <t>This limitation of the sender's window can come either from either 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 node is not actively controlling the sender's rate. Nevertheless, the proposed approach to discard larger samples would also address this issue.</t>
            <t>To address the case in which the limiting factor is the receive window announced by rLEDBAT, the congestion control algorithm at the receiver SHOULD <bcp14>SHOULD</bcp14> discard RTT measurements during the window reduction phase that are triggered by pure ACK packets. The rLEDBAT receiver is aware of whether a given TSVal value TSval was sent in a pure ACK packet where the window was reduced, and if so, it can discard the corresponding RTT measurement. </t>
          </section>
          <section title="Measuring numbered="true" toc="default">
            <name>Measuring RTT when sending data packets"> When Sending Data Packets</name>
            <t>In the case that the rLEDBAT node is sending data packets and matching them with pure ACKs to measure RTT, a factor that can artificially increase the RTT measured is the presence of delayed Acknowledgements. Acknowledgments.
		According to the TS option generation rules <xref target="RFC7323" />, format="default"/>,
		the value included in the TSecr for a delayed ACK is the one in the TSVal TSval field of the earliest unacknowledged segment.
		This may artificially increase the measured RTT. </t>
            <t>If both endpoints of the connection are sending data packets, Acknowledgments are piggybacked into onto the data packets and they are not delayed. Delayed ACKs only increase RTT measurements 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.</t>
            <t>Nevertheless, measurements based on data packets from the rLEDBAT node matching pure ACKs from the other end will result in an increased RTT sample. The additional increase in the measured RTT will be up to 500 ms. The reason for this This is that because delayed ACKs are generated every second data packet received and not delayed more than 500 ms according to <xref target="RFC9293" />. format="default"/>. The rLEDBAT receiver MAY <bcp14>MAY</bcp14> discard RTT measurements done using data packets from the rLEBDAT rLEDBAT receiver and matching pure ACKs, especially if it has recent measurements done using other packet combinations. Also, applying Applying a filter (e.g., a MIN filter) that discards outliers would also address this issue (e.g. a min filter).</t> issue.</t>
          </section>
        </section>
        <section title="Measuring one way delay numbered="true" toc="default">
          <name>Measuring One-Way Delay to estimate Estimate the queueing delay"> Queuing Delay</name>
          <t>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).</t>
          <t>In the case of TCP, the receiver can use the TimeStamp TS option to measure the one way one-way delay by subtracting the timestamp contained in the incoming packet from the local time at which the packet has arrived. As noted in <xref target="RFC6817" /> format="default"/>, the clock offset between the sender's clock of the sender and the receiver's clock in the receiver does not affect the LEDBAT operation, since LEDBAT uses the difference between the base one way one-way delay and the current one way one-way delay to estimate the queuing delay, effectively canceling "canceling out" the clock offset error in the queueing queuing delay estimation. There are however are, however, two other issues that the rLEDBAT receiver needs to take into account in order to properly estimate the one way one-way delay, namely, namely the units in which the received timestamps are expressed and the clock skew. We address them next.</t> These issues are addressed below.</t>
          <t>In order to measure the one way one-way delay using TCP timestamps, the rLEDBAT receiver, first, receiver first needs to discover the units of values in the TS option and, second, and then needs to account for the skew between the two endpoint clocks. Note that a mismatch of 100 ppm (parts per million) in the estimation of the sender's clock rate accounts for 6 ms of variation per minute in the measured delay. This is just one order of magnitude below the target delay set by rLEDBAT (or potentially more if the target is set to lower values, which is possible). Typical skew for untrained clocks is reported to be around 100-200 ppm <xref target="RFC6817" />.</t> format="default"/>.</t>
          <t>In order to learn both the TS units and the clock skew, the rLEDBAT receiver measures how much local time has elapsed between two packets with different TS values issued by the sender. 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) similar) delay when traveling from the sender to the receiver, as any difference in the experienced delays would introduce an error in the unit/skew estimation. One possible approach is to select packets that experienced the minimum minimal delay (i.e., queuing delay (i.e. close to zero queueing delay) zero) to make the estimations.</t>
          <t>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) (queuing) 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 for 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 of whether the sender is injecting traffic at any point in time, and so, time; it is therefore unable to use these quiet intervals to perform measurements. The receiver can can, however, force periodic slowdowns, reducing the
announced receive window to a few packets and perform performing the measurements then.</t> at that time.</t>
          <t>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.  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. </t>
        </section>
      </section>
      <section title="Detecting packet losses numbered="true" toc="default">
        <name>Detecting Packet Losses and retransmissions"> Retransmissions</name>
        <t>The rLEDBAT receiver is capable of detecting retransmitted packets in the following way. as follows. We call RCV.HGH the highest sequence number corresponding to a received byte of data (not assuming that all bytes with smaller sequence numbers have been received already, there may be holes) holes), and we call TSV.HGH the TSVal value TSval corresponding to the segment in which that byte was carried. SEG.SEQ stands for the sequence number of a newly received segment segment, and we call TSV.SEQ the TSVal value TSval of the newly received segment.</t>
        <t>If SEG.SEQ &lt; RCV.HGH and TSV.SEQ > TSV.HGH &gt; 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 already-received segment which that contained data with a larger sequence number. This means that this segment was lost and was retransmitted.</t>
        <t>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 losses; however, the TCP congestion controller at the sender will, most likely reducing its window to 1MSS 1 MSS and take taking over the control of the sending rate, rate until slow start ramps up and catches the current value of the rLEDBAT window.</t>
      </section>
    </section>
    <section title="Experiment Considerations"> numbered="true" anchor="sect-5" toc="default">
      <name>Experiment Considerations</name>
      <t>The status of this document is Experimental. The general purpose of the proposed experiment is to gain more experience running rLEDBAT over different network paths to see if the proposed rLEDBAT parameters perform well in different situations. Specifically, we would like to learn about the following aspects of the rLEDBAT mechanism: </t>
	<t><list>
		<t>- Interaction
      <ul spacing="normal">
        <li>
          <t>Interaction between the sender sender's and the receiver Congestion receiver's congestion control algorithms. rLEDBAT &nbsp;rLEDBAT posits that because the rLEDBAT receiver is using a less-than-best-effort congestion control algorithm, the receiver receiver's congestion control algorithm will expose a smaller congestion window (conveyed though through the Receive Window) receive window) than the one resulting from the congestion control algorithm executed at the sender. One of the purposes of the experiment is to learn how these two algorithms
interact and if the assumption that the receiver side is always controlling the sender's rate (and making rLEDBAT effective) holds. The experiment should include the different congestion control algorithms that are currently widely used in the Internet, including Cubic, BBR CUBIC, Bottleneck Bandwidth and Round-trip propagation time (BBR), and LEDBAT(++).</t>
		<t>- Interaction
        </li>
        <li>
          <t>Interaction between rLEDBAT and Active Queue Management techniques such as Codel, PIE Controlled Delay (CoDel); Proportional Integral controller Enhanced (PIE); and L4S.</t>
		<t>- How the Low Latency, Low Loss, and Scalable Throughput (L4S).
</t>
        </li>
        <li>
          <t>How rLEDBAT should resume after a period during which there was no incoming traffic and the information about the rLEDBAT state information is potentially dated.</t>
	</list></t>
        </li>
      </ul>
      <section title="Status numbered="true" toc="default">
        <name>Status of the experiment Experiment at the time Time of this writing.">
		<t>Currently there are This Writing</name>
        <t>Currently, the following implementations of rLEDBAT that can be used for experimentation:
			<list>
				<t>- Windows experimentation:</t>
        <ul spacing="normal">
          <li>
            <t>Windows 11. rLEDBAT &nbsp;rLEDBAT is available in Microsoft's Windows 11 22H2 since October 2023 <xref target="Windows11" />.</t>
				<t>- Windows format="default"/>.</t>
          </li>
          <li>
            <t>Windows Server 2022. rLEDBAT &nbsp;rLEDBAT is available in Microsoft's Windows Server 2022 since September 2022 <xref target="WindowsServer" />.</t>
				<t>- Apple. rLEDBAT format="default"/>.</t>
          </li>
          <li>
            <t>Apple. &nbsp;rLEDBAT is available in MacOS macOS and iOS since 2021   <xref target="Apple" />.</t>
				<t>- Linux format="default"/>.</t>
          </li>
          <li>
            <t>Linux implementation, open source, available since 2022 at https://github.com/net-research/rledbat_module.</t>
				<t>- ns3 <xref target="rledbat_module"/>.</t>
          </li>
          <li>
            <t>ns3 implementation, open source, available since 2020 at https://github.com/manas11/implementation-of-rLEDBAT-in-ns-3.</t>
			</list></t> <xref target="rLEDBAT-in-ns-3"/>.</t>
          </li>
        </ul>
        <t>In addition, rLEDBAT has been deployed by Microsoft in at wide scale in the following services:
				<list>
					<t>- BITS
        </t>
        <ul spacing="normal">
          <li>
            <t>BITS (Background Intelligent Transfer Service)</t>
					<t>- DO
          </li>
          <li>
            <t>DO (Delivery Optimization) service</t>
					<t>- Windows update #
          </li>
          <li>
            <t>Windows update: using DO</t>
					<t>- Windows Store #
          </li>
          <li>
            <t>Windows Store: using DO</t>
					<t>- OneDrive</t>
					<t>- Windows
          </li>
          <li>
            <t>OneDrive</t>
          </li>
          <li>
            <t>Windows Error Reporting # Reporting: wermgr.exe; werfault.exe</t>
					<t>- System
          </li>
          <li>
            <t>System Center Configuration Manager (SCCM)</t>
					<t>- Windows
          </li>
          <li>
            <t>Windows Media Player</t>
					<t>- Microsoft
          </li>
          <li>
            <t>Microsoft Office</t>
					<t>- Xbox
          </li>
          <li>
            <t>Xbox (download games) # games): using DO</t>
				</list> </t>
          </li>
        </ul>

        <t> Some initial experiments involving rLEDBAT have been reported in <xref target="COMNET3" />. format="default"/>. Experiments involving the interaction of between LEDBAT++ and BBR are presented in <xref target="COMNET2" />. format="default"/>. An experimental evaluation of the LEDBAT++ algorithm is presented in <xref target="COMNET1" />. format="default"/>. As LEDBAT++ is one of the less-than-best-effort congestion control algorithms that rLEDBAT relies on, the results regarding how LEDBAT++ interaction interacts with other congestion control algorithms are relevant for the understanding of rLEDBAT as well.</t>
      </section>
    </section>
    <section title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>Overall, we believe that rLEDBAT does not introduce any new vulnerabilities to existing TCP endpoints, as it relies on existing TCP knobs, notably the Receive Window receive window and timestamps. </t>
      <t>Specifically, rLEDBAT uses RCV.WND to modulate the rate of the sender. An attacker wishing to starve a flow can simply reduce the RCV.WND, irrespective of whether rLEDBAT is being used or not.</t>
      <t> We can further ask ourselves whether the attacker can use the rLEDBAT mechanisms in place to force the rLEDBAT receiver to reduce the RCV WND. RCV.WND. There are two ways an attacker can do that. One this:</t>

   <ul spacing="normal">
      <li>One would be to introduce an artificial delay to the packets either by either actually delaying the packets or modifying the Timestamps. timestamps. This would cause the rLEDBAT receiver to believe that a queue is building up and reduce the RCV.WND. Note that an attacker to do that so, an attacker must be on path, so if that is the case, it is probably more direct to simply reduce the RCV.WND.</t>
	       <t> The RCV.WND.</li>
      <li>The other option would be for the attacker to make the rLEDBAT receiver believe that a loss has occurred. To do that, this, it basically needs to retransmit an old packet (to be precise, it needs to transmit a packet with the right correct sequence number and the right correct port and IP numbers). This means that the attacker can achieve a reduction of incoming traffic to the rLEDBAT receiver not only by modifying the RCV.WND field of the packets originated from the rLEDBAT host, host but also by injecting packets with the proper sequence number in the other direction. This may slightly expand the attack surface.</t> surface.</li>
   </ul>
    </section>
    <section title="IANA Considerations">
		<t>No actions are required from IANA.</t>
    </section>

    <section title="Acknowledgements"> numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>This work was supported by the EU through the StandICT projects RXQ, CCI and CEL6, the NGI Pointer RIM project and the H2020 5G-RANGE project and by the Spanish Ministry of Economy and Competitiveness through the 5G-City project (TEC2016-76795-C6-3-R).</t>

	<t>We would like to thank ICCRG chairs Reese Enghardt and Vidhi Goel for their support on this work. We would also like to thank Daniel Havey for his help. We would like to thank Colin Perkins, Mirja Kuehlewind, and Vidhi Goel for their reviews and comments on earlier versions of this document.</t> document has no IANA actions.</t>
    </section>
  </middle>
  <back>

  <displayreference target="I-D.irtf-iccrg-ledbat-plus-plus" to="LEDBAT++"/>

    <references>
      <name>References</name>
      <references title="Informative References">
       <?rfc include='reference.RFC.9293'?>

  	 <?rfc include='reference.I-D.irtf-iccrg-ledbat-plus-plus" ?>
       <?rfc include="reference.RFC.6817" ?>
       <?rfc include="reference.RFC.7323" ?>
       <?rfc include="reference.RFC.9438"?>
  	 <?rfc include="reference.RFC.5681"?> anchor="sec-normative-references">
        <name>Normative References</name>
        <xi:include
href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include
href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
    </references>
      <references anchor="sec-informative-references">
      <name>Informative References</name>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9293.xml"/>

<!-- draft-irtf-iccrg-ledbat-plus-plus (I-D Exists) -->
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.irtf-iccrg-ledbat-plus-plus.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6817.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7323.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9438.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5681.xml"/>
      <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.6582.xml"/>

      <reference anchor="Windows11" > target="https://learn.microsoft.com/en-us/windows/deployment/do/whats-new-do">
        <front>
          <title>What's new in Delivery Optimization</title>
  	 	            <author initials="C.F." surname="Forsmann" fullname="Carmen">
  	 	                <organization />
          <author>
            <organization>Microsoft</organization>
          </author>
          <date year="2023" /> month="October" year="2024"/>
        </front>
  	 	        <seriesInfo name="Microsoft Documentation" value="https://learn.microsoft.com/en-us/windows/deployment/do/whats-new-do" />
  	 	        <refcontent></refcontent>
        <refcontent>Microsoft Windows Documentation</refcontent>
      </reference>

      <reference anchor="WindowsServer" > target="https://techcommunity.microsoft.com/t5/networking-blog/ledbat-background-data-transfer-for-windows/ba-p/3639278">
        <front>
          <title>LEDBAT Background Data Transfer for Windows</title>
          <author initials="D.H." initials="D" surname="Havey" fullname="Daniel">
  	   	 	                <organization />
            <organization/>
          </author>
          <date year="2022" /> month="September" year="2022"/>
        </front>
  	   	 	        <seriesInfo name="Microsoft Blog" value="https://techcommunity.microsoft.com/t5/networking-blog/ledbat-background-data-transfer-for-windows/ba-p/3639278" />
  	   	 	        <refcontent></refcontent>
        <refcontent>Microsoft Networking Blog</refcontent>
      </reference>

      <reference anchor="Apple" > target="https://developer.apple.com/videos/play/wwdc2021/10239/">
        <front>
          <title>Reduce network delays for your app</title>
          <author initials="S.C." surname="Stuart" fullname="Cheshire">
	     	   	 	                <organization /> initials="S" surname="Cheshire" fullname="Stuart Cheshire">
            <organization/>
          </author>
          <author initials="V.G." surname="Vidhi" fullname=" initials="V" surname="Goel" fullname="Vidhi Goel ">
	     	   	 	                <organization />
            <organization/>
          </author>
          <date year="2021" /> year="2021"/>
        </front>
	     	   	 	        <seriesInfo name="WWDC21" value="https://developer.apple.com/videos/play/wwdc2021/10239/" />
	     	   	 	        <refcontent></refcontent>
        <refcontent>Apple Worldwide Developers Conference (WWDC2021), Video</refcontent>
      </reference>

      <reference anchor="COMNET3" > anchor="COMNET3">
        <front>
          <title> Design, implementation and validation of a receiver-driven less-than-best-effort transport </title>
          <author initials="M.B." initials="M" surname="Bagnulo" fullname="Marcelo Bagnulo">
  				                <organization />
            <organization/>
          </author>
          <author initials="A.G." surname="Garcia-Martinez" initials="A" surname="García-Martínez" fullname="Alberto Garcia-Martinez">
  				                <organization /> García-Martínez">
            <organization/>
          </author>
          <author initials="A.M." surname="Mandalari" fullname="Anna Maria Mandalari">
  				                <organization />
            <organization/>
          </author>
          <author initials="P.B," initials="P" surname="Balasubramanian" fullname="Praveen Balasubramanian">
  				                <organization />
            <organization/>
          </author>
          <author initials="D.H." initials="D" surname="Havey" fullname="Daniel Havey">
  				                <organization />
            <organization/>
          </author>
          <author initials="G.M." initials="G" surname="Montenegro" fullname="Gabriel Montenegro">
  				                <organization />
            <organization/>
          </author>
          <date year="2022" /> month="September" year="2023"/>
        </front>
        <refcontent>Computer Networks, vol. 233</refcontent>
        <seriesInfo name="Computer Networks" value="Volume 233" />
  				        <refcontent></refcontent> name="DOI" value="10.1016/j.comnet.2023.109841"/>
      </reference>

      <reference anchor="COMNET2" > anchor="COMNET2">
        <front>
          <title>When less is more: BBR versus LEDBAT++</title>
          <author initials="M.B." initials="M" surname="Bagnulo" fullname="Marcelo Bagnulo">
  						                <organization />
            <organization/>
          </author>
          <author initials="A.G." surname="Garcia-Martinez" initials="A" surname="García-Martínez" fullname="Alberto Garcia-Martinez">
  						                <organization /> García-Martínez">
            <organization/>
          </author>
          <date year="2022" /> month="December" year="2022"/>
        </front>
        <refcontent>Computer Networks, vol. 219</refcontent>
        <seriesInfo name="Computer Networks" value="Volume 219" />
  						        <refcontent></refcontent> name="DOI" value="10.1016/j.comnet.2022.109460"/>
      </reference>

      <reference anchor="COMNET1" > anchor="COMNET1">
        <front>
          <title>An experimental evaluation of LEDBAT++ </title>
          <author initials="M.B." initials="M" surname="Bagnulo" fullname="Marcelo Bagnulo">
  								                <organization />
            <organization/>
          </author>
          <author initials="A.G." surname="Garcia-Martinez" initials="A" surname="García-Martínez" fullname="Alberto Garcia-Martinez">
  								                <organization /> García-Martínez">
            <organization/>
          </author>
          <date year="2022" /> month="July" year="2022"/>
        </front>
        <refcontent>Computer Networks, vol. 212</refcontent>
        <seriesInfo name='Computer Networks' value="Volume 212"/>
  								        <refcontent></refcontent> name="DOI" value="10.1016/j.comnet.2022.109036"/>
      </reference>

   <reference anchor="rledbat_module"
              target="https://github.com/net-research/rledbat_module">
     <front>
       <title>rledbat_module</title>
       <author/>
       <date month="September" day="9" year="2022"/>
     </front>
     <refcontent>commit d82ff20</refcontent>
   </reference>

   <reference anchor="rLEDBAT-in-ns-3"
              target="https://github.com/manas11/implementation-of-rLEDBAT-in-ns-3">
     <front>
       <title>Implementation-of-rLEDBAT-in-ns-3</title>
       <author/>
       <date month="June" day="24" year="2020"/>
     </front>
     <refcontent>commit 2ab34ad</refcontent>
   </reference>

    </references>
  </references>
    <section title="Terminology">

	<t>We use the following abreviations thoughout the text. We include a short list for the reader's convenence:</t>
		<t><list>
			<t>RCV.WND: the value included in the Receive Window field of the TCP header (which computation is modified by numbered="true" toc="default">
      <name>rLEDBAT Pseudocode</name>
      <t>In this specification)</t>
			<t>SND.WND: The TCP sender's window</t>
			<t>cwnd: the consgestion window as computed by the congestion control algorithm running at the TCP sender.</t>
			<t>RLWND: the window value calculated by rLEDBAT algorithm</t>
			<t>fcwnd: the value that a standard RFC793bis TCP receiver calculates to set in the receive window for flow control purposes.</t>
			<t>RCV.HGH: the highest sequence number corresponding to a received byte of data at one point in time</t>
			<t>TSV.HGH: TSV.HGH the TSVal value corresponding to the segment in which RCV.HGH was carried at that point in time</t>
			<t>SEG.SEQ: the sequence number of the last received segment</t>
			<t>TSV.SEQ: the TSVal value of the last received segment</t>
		</list></t>
</section>

<section title="rLEDBAT pseudo-code">

	<t>We next section, we describe how to integrate the proposed rLEDBAT mechanisms and  an LBE delay-based congestion control algorithm such as LEDBAT or LEDBAT++. We &nbsp;We describe the integrated  algorithm as two procedures, procedures: one that is executed when a packet is received by a rLEDBAT-enabled endpoint (Figure 2) (<xref target="fig2"/>) and another that is executed when the rLEDBAT-enabled endpoint sends a packet (Figure 3). (<xref target="fig3"/>). At the beginning, RLWND is set to its maximum value, so that the sending rate of the sender is governed by the flow control algorithm of the receiver and the TCP slow start mechanism of the sender, and the ackedBytes variable is set to 0. </t>
      <t>We assume that the LBE congestion control algorithm defines a WindowIncrease() function and a WindowDecrease() function. For example, in the case of LEDBAT++, the WindowIncrease() function is an additive increase, while the WindowDecrease() function is a multiplicative decrease. In the case of the WindowIncrease(), WindowIncrease() function, we assume that it takes as input the current window size and the number of bytes that were acknowledged since the last window update (ackedBytes) and returns as output the updated window size. In the case of WindowDecrease(), the WindowDecrease() function, it takes as input the current window size and returns the updated window size. </t>
      <t>The data structures used in the algorithms are as follows. The sentList sendList is a list that contains  the TSval and the local send time of each packet sent by the rLEDBAT-enabled endpoint. The TSecr field of the packets received by the rLEDBAT-enabled endpoint are is matched with the sendList to compute the RTT.</t>
      <t>The RTT values computed for each received packet are stored in the RTTlist, which contains also contains the received TSecr (to avoid using multiple packets with the same TSecr for RTT calculations, only the first packet received for a given TSecr is used to compute the RTT). It also contains the local time at which the packet was received, to allow selecting the RTTs measured in a given period (e.g., in the last 10 minutes). RTTlist is initialized with all its values to its maximum.</t>
      <figure title="Procedure executed when anchor="fig2">
        <name>Procedure Executed When a packet is received">

<sourcecode> Packet Is Received</name>
        <sourcecode type="pseudocode"><![CDATA[
procedure receivePacket()
  //Looks for first sent packet with same TSval as TSecr, and, and
  //returns time difference
  receivedRTT = computeRTT(sentList, computeRTT(sendList, receivedTSecr, receivedTime)

  //Inserts minimum value for a given receivedTSecr
  //note
  //Note that many received packets may contain same receivedTSecr
  insertRTT (RTTlist, receivedRTT, receivedTSecr, receivedTime)

  filteredRTT = minLastKMeasures(RTTlist, K=4)
  baseRTT = minLastNSeconds(RTTlist, N=180)
  qd = filteredRTT - baseRTT

  //ackedBytes is the number of bytes that can be used to reduce
  //the Receive Window receive window - without shrinking it - if necessary
  ackedBytes = ackedBytes + receiveBytes

  if retransmittedPacketDetected then
        RLWND = DecreaseWindow(RLWND) // Only  //Only once per RTT
  end if
  if qd &lt; < T then
        RLWND = IncreaseWindow(RLWND, ackedBytes)
  else
        RLWND = DecreaseWindow(RLWND)
  end if
end procedure
</sourcecode>
]]></sourcecode>
      </figure>

      <figure title="Procedure executed when anchor="fig3">
        <name>Procedure Executed When a packet is sent">
<sourcecode> Packet Is Sent</name>
        <sourcecode type="pseudocode"><![CDATA[
procedure SENDPACKET
  if (RLWND > RLWNDPrevious) or (RLWND - RLWNDPrevious &lt; < ackedBytes)
  then
        RLWNDPrevious = RLWND
  else
        RLWNDPrevious = RLWND - ackedBytes
  end if
  ackedBytes = 0
  RLWNDPrevious = RLWND

  //Compute the RWND RLWND to include in the packet
  RLWND = min(RLWND, fcwnd)
end procedure
</sourcecode>
]]></sourcecode>
      </figure>
    </section>
    <section numbered="false" toc="default">
      <name>Acknowledgments</name>
      <t>This work was supported by the EU through the StandICT projects RXQ, CCI, and CEL6; the NGI Pointer RIM project; and the H2020 5G-RANGE project; and by the Spanish Ministry of Economy and Competitiveness through the 5G-City project (TEC2016-76795-C6-3-R).</t>
      <t>We would like to thank ICCRG chairs <contact fullname="Reese Enghardt"/> and <contact fullname="Vidhi Goel"/> for their support on this work. We would also like to thank <contact fullname="Daniel Havey"/> for his help. We would like to thank <contact fullname="Colin Perkins"/>, <contact fullname="Mirja Kühlewind"/>, and <contact fullname="Vidhi Goel"/> for their reviews and comments on earlier draft versions of this document.</t>
    </section>
  </back>
</rfc>