rfc9840xml2.original.xml   rfc9840.xml 
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<front> <front>
<title abbrev="rLEDBAT">rLEDBAT: receiver-driven Low Extra Delay Background <title abbrev="rLEDBAT">rLEDBAT: Receiver-Driven Low Extra Delay Background
Transport for TCP Transport for TCP</title>
</title> <seriesInfo name="RFC" value="9840"/>
<author fullname="Marcelo Bagnulo" initials="M." surname="Bagnulo"> <author fullname="Marcelo Bagnulo" initials="M." surname="Bagnulo">
<organization>Universidad Carlos III de Madrid</organization> <organization>Universidad Carlos III de Madrid</organization>
<address> <address>
<email>marcelo@it.uc3m.es</email> <email>marcelo@it.uc3m.es</email>
</address> </address>
</author> </author>
<author fullname="Alberto García-Martínez" initials="A." surname="García-Mar
<author fullname="Alberto Garcia-Martinez" initials="A." surname="Garcia-Mar tínez">
tinez">
<organization>Universidad Carlos III de Madrid</organization> <organization>Universidad Carlos III de Madrid</organization>
<address> <address>
<email>alberto@it.uc3m.es</email> <email>alberto@it.uc3m.es</email>
</address> </address>
</author> </author>
<author fullname="Gabriel Montenegro" initials="G." surname="Montenegro"> <author fullname="Gabriel Montenegro" initials="G." surname="Montenegro">
<address> <address>
<email>g.e.montenegro@hotmail.com</email> <email>g.e.montenegro@hotmail.com</email>
</address> </address>
skipping to change at line 44 skipping to change at line 33
<address> <address>
<email>alberto@it.uc3m.es</email> <email>alberto@it.uc3m.es</email>
</address> </address>
</author> </author>
<author fullname="Gabriel Montenegro" initials="G." surname="Montenegro"> <author fullname="Gabriel Montenegro" initials="G." surname="Montenegro">
<address> <address>
<email>g.e.montenegro@hotmail.com</email> <email>g.e.montenegro@hotmail.com</email>
</address> </address>
</author> </author>
<author fullname="Praveen Balasubramanian " initials="P." surname="Balasubra manian"> <author fullname="Praveen Balasubramanian " initials="P." surname="Balasubra manian">
<organization>Confluent</organization> <organization>Confluent</organization>
<address> <address>
<email>pravb.ietf@gmail.com</email> <email>pravb.ietf@gmail.com</email>
</address> </address>
</author> </author>
<date year="2025" month="September"/>
<date year="2025" /> <workgroup>Internet Congestion Control</workgroup>
<keyword>Congestion control</keyword>
<keyword>scavenger/less-than-best-effort traffic</keyword>
<abstract> <abstract>
<t> This document specifies rLEDBAT, a set of mechanisms that enable the e xecution 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 Rese arch Group (ICCRG) of the Internet Research Task Force (IRTF). <t> This document specifies receiver-driven Low Extra Delay Background Tra nsport (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 docu ment is a product of the Internet Congestion Control Research Group (ICCRG) of t he Internet Research Task Force (IRTF).
</t> </t>
</abstract> </abstract>
</front> </front>
<middle> <middle>
<section title="Introduction"> <section numbered="true" toc="default">
<name>Introduction</name>
<t>LEDBAT (Low Extra Delay Background Transport) <xref target="RFC6817" / <t>LEDBAT (Low Extra Delay Background Transport) <xref target="RFC6817" fo
> is a congestion-control algorithm used for less-than-best-effort (LBE) traffic rmat="default"/> is a congestion control algorithm used for less-than-best-effor
.</t> t (LBE) traffic.</t>
<t>When LEDBAT traffic shares a bottleneck with other traffic using standa
<t>When LEDBAT traffic shares a bottleneck with other traffic using stand rd congestion control algorithms (for example, TCP traffic using CUBIC <xref tar
ard congestion control algorithms (for example, TCP traffic using Cubic<xref tar get="RFC9438" format="default"/>, hereafter referred to as "standard-TCP" for sh
get="RFC9438" />, hereafter referred as standard-TCP for short), it reduces its ort), it reduces its sending rate earlier and more aggressively than standard-TC
sending rate earlier and more aggressively than standard-TCP congestion control, P congestion control, allowing other non-background traffic to use more of the a
allowing other non-background traffic to use more of the available capacity. In vailable capacity. In the absence of competing traffic, LEDBAT aims to make effi
the absence of competing traffic, LEDBAT aims to make an efficient use of the a cient use of the available capacity, while keeping the queuing delay within pred
vailable capacity, while keeping the queuing delay within predefined bounds.</t> efined bounds.</t>
<t>LEDBAT reacts to both packet loss and variations in delay. With respec
<t>LEDBAT reacts both to packet loss and to variations in delay. With re t to packet loss, LEDBAT reacts with a multiplicative decrease, similar to most
spect to packet loss, LEDBAT reacts with a multiplicative decrease, similar to m TCP congestion controllers. Regarding delay, LEDBAT aims for a target queuing de
ost TCP congestion controllers. Regarding delay, LEDBAT aims for a target queuei lay. When the measured current queuing delay is below the target, LEDBAT increas
ng delay. When the measured current queueing delay is below the target, LEDBAT i es the sending rate, and when the delay is above the target, it reduces the send
ncreases the sending rate and when the delay is above the target, it reduces the ing rate. LEDBAT estimates the queuing delay by subtracting the measured current
sending rate. LEDBAT estimates the queuing delay by subtracting the measured cu one-way delay from the estimated base one-way delay (i.e., the one-way delay in
rrent one-way delay from the estimated base one-way delay (i.e. the one-way dela the absence of queues). </t>
y in the absence of queues). </t> <t>The LEDBAT specification <xref target="RFC6817" format="default"/> defi
nes the LEDBAT congestion control algorithm, implemented in the sender to contro
<t>The LEDBAT specification <xref target="RFC6817" /> defines the LEDBAT l its sending rate. LEDBAT is specified in a protocol-agnostic and layer-agnosti
congestion-control algorithm, implemented in the sender to control its sending r c manner.</t>
ate. LEDBAT is specified in a protocol and layer agnostic manner.</t> <t>LEDBAT++ <xref target="I-D.irtf-iccrg-ledbat-plus-plus" format="default
"/> is also an LBE congestion control algorithm that is inspired by LEDBAT while
<t>LEDBAT++ <xref target="I-D.irtf-iccrg-ledbat-plus-plus" /> is also an addressing several problems identified with the original LEDBAT specification.
LBE congestion control algorithm which is inspired by LEDBAT while addressing se In particular, the differences between LEDBAT and LEDBAT++ include the following
veral problems identified with the original LEDBAT specification. In particular :</t>
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 a
lgorithm to achieve inter-LEDBAT++ fairness and avoid the late-comer advantage o
bserved in LEDBAT; iii) LEDBAT++ performs periodic slowdowns to improve the meas
urement of the base delay; iv) LEDBAT++ is defined for TCP.</t>
<t>In this specification, we describe rLEDBAT, a set of mechanisms that e
nable the execution of an LBE delay-based congestion control algorithm such as L
EDBAT or LEDBAT++ at the receiver end of a TCP connection.</t>
<t> The consensus of the Internet Congestion Control Research Group (ICCR
G) is to publish this document to encourage further experimentation and review o
f rLEDBAT. This document is not an IETF product and is not a standard. The statu
s of this document is experimental. In section 4 titled Experiment Consideration
s, we describe the purpose of the experiment and its current status. </t>
</section>
<section title="Motivations for rLEDBAT">
<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 Delivery Networks and more sophisticated file
distribution scenarios: Consider the case where the source of a file to be distr
ibuted (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 t
hat does not implement LBE congestion control, the result is that the file trans
fers 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 cu
rrent LEDBAT/LEDBAT++ are sender-based algorithms, controlling the client is not
enough to enable LBE congestion control in the communication. rLEDBAT would ena
ble the use of LBE traffic class for file distribution in this setup. </t>
<t>Interference from proxies and other middleboxes: Proxies and o
ther middleboxes are commonplace in the Internet. For instance, in the case of m
obile 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 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 ena
bling rLEDBAT, clients would be able to enable LBE traffic between them and the
proxy.</t>
<t>Receiver-defined preferences. It is frequent that the bottlene
ck of the communication is the access link. This is particularly true in the cas
e of mobile devices. It is then especially relevant for mobile devices to proper
ly manage the capacity of the access link. With current technologies, it is poss
ible for the mobile device to use different congestion control algorithms expres
sing 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 an
d LEDBAT/LEDBAT++ are sender-driven. The mobile device has no means to manage th
e 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 selective
ly use LBE traffic class for some of the incoming traffic. For instance, by usin
g rLEDBAT, a user can use regular standard-TCP/UDP for video stream (e.g., Youtu
be) and use rLEDBAT for other background file download.</t>
</list></t>
</section>
<section title="rLEDBAT mechanisms">
<t>rLEDBAT provides the mechanisms to implement an LBE congestion control
algorithm at the receiver-end of a TCP connection. The rLEDBAT receiver control
s the sender's rate through the Receive Window announced by the receiver in the
TCP header.</t>
<t>rLEDBAT assumes that the sender is a standard TCP sender. rLEDBAT does
not require any rLEDBAT-specific modifications to the TCP sender. The envisione
d deployment model for rLEDBAT is that the clients implement rLEDBAT and this en
ables rLEDBAT in communications with existent standard TCP senders. In particul
ar, the sender MUST implement <xref target="RFC9293" /> and it also MUST impleme
nt the Time Stamp Option as defined in <xref target="RFC7323" />. Also, the send
er should implement some of the standard congestion control mechanisms, such as
Cubic <xref target="RFC9438" /> or New Reno <xref target="RFC5681" />. </t>
<t>rLEDBAT does not define a new congestion control algorithm. The LBE co
ngestion control algorithm executed in the rLEDBAT receiver is defined in other
documents. The rLEDBAT receiver MUST use an LBE congestion control algorithm. Be
cause rLEDBAT assumes a standard TCP sender, the sender will be using a "best ef
fort" congestion control algorithm (such as Cubic or New Reno). Since rLEDBAT us
es the 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 exec
uted in the receiver yields a smaller window than the one calculated by the send
er. This is normally the case when the receiver is using an LBE congestion contr
ol algorithm. The rLEDBAT receiver SHOULD use the LEDBAT congestion control algo
rithm <xref target="RFC6817" /> or the LEDBAT++ congestion control algorithm <xr
ef target="I-D.irtf-iccrg-ledbat-plus-plus" />. The rLEDBAT MAY use other LBE co
ngestion control algorithms defined elsewhere. Irrespective 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 congestio
n control algorithm executed at the sender.</t>
<t>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 d
etect packet loss and the means to manipulate the Receive Window to control the
sender's rate. The former provide input to the LBE congestion control algorithm
while the latter uses the congestion window computed by the LBE congestion contr
ol algorithm to manipulate the Receive window, as depicted in the figure.</t>
<figure title="The rLEDBAT architecture."> <ol spacing="normal" type="%i)">
<artwork align="center"><![CDATA[ <li>LEDBAT++ uses the round-trip time (RTT) (as opposed to the one-way delay us
ed in LEDBAT) to estimate the queuing delay.</li>
<li>LEDBAT++ uses an additive increase/multiplicative decrease algorithm to ach
ieve inter-LEDBAT++ fairness and avoid the latecomer advantage observed in LEDBA
T.</li>
<li>LEDBAT++ performs periodic slowdowns to improve the measurement of the base
delay.</li>
<li>LEDBAT++ is defined for TCP.</li>
</ol>
<t>In this specification, we describe receiver-driven Low Extra Delay Back
ground Transport (rLEDBAT) -- a set of mechanisms that enable the execution of a
n 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 an Internet Standards
Track specification. The status of this document is Experimental. In <xref targe
t="sect-5" format="default"/> ("<xref target="sect-5" format="title"/>"), we des
cribe the purpose of the experiment and its current status. </t>
</section>
<section 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</name>
<t>rLEDBAT enables new use cases and new deployment models, fostering the
use of LBE traffic. The following scenarios are enabled by rLEDBAT:
</t>
<dl spacing="normal" newline="true">
<dt>Content Delivery Networks (CDNs) and more sophisticated file distrib
ution scenarios:</dt>
<dd>Consider the case where the source of a file to be distributed (e.g., a soft
ware developer that wishes to distribute a software update) would prefer to use
LBE and enables LEDBAT/LEDBAT++ in the servers containing the source file. Howev
er, 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 CD
N surrogates will not be using LBE. Interestingly enough, in the case of the sof
tware update, the developer may also control the software performing the downloa
d 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. &nbsp;rLEDBAT would enable the use of a
n LBE traffic class for file distribution in this setup.</dd>
<dt>Interference from proxies and other 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 ent
erprise networks, it is common to deploy corporate proxies for filtering and fir
ewalling. In the case of satellite links, Performance Enhancing Proxies (PEPs) a
re deployed to mitigate the effect of long delays in a TCP connection. These pro
xies terminate the TCP connection on both ends and prevent the use of LBE conges
tion control in the segment between the proxy and the sink of the content, the c
lient. By enabling rLEDBAT, clients can then enable LBE traffic between them and
the proxy.</dd>
<dt>Receiver-defined preferences:</dt>
<dd>Frequently, the access link is the communication bottleneck. This is particu
larly true in the case of mobile devices. It is then especially relevant for mob
ile devices to properly manage the capacity of the access link. With current tec
hnologies, it is possible for the mobile device to use different congestion cont
rol algorithms expressing different preferences for the traffic. For instance, a
device can choose to use standard-TCP for some traffic and use LEDBAT/LEDBAT++
for other traffic. However, this would only affect the outgoing traffic, since b
oth standard-TCP and LEDBAT/LEDBAT++ are driven by the sender. The mobile device
has no means to manage the traffic in the downlink, which is, in most cases, th
e communication bottleneck for a typical "eyeball" end user. &nbsp;rLEDBAT enabl
es the mobile device to selectively use an LBE traffic class for some of the inc
oming traffic. For instance, by using rLEDBAT, a user can use regular standard-T
CP/UDP for a video stream (e.g., YouTube) and use rLEDBAT for other background f
ile downloads.</dd>
</dl>
</section>
<section numbered="true" toc="default">
<name>rLEDBAT Mechanisms</name>
<t>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 by the receiver in the T
CP header.</t>
<t>rLEDBAT assumes that the sender is a standard-TCP sender. &nbsp;rLEDBAT
does not require any rLEDBAT-specific modifications to the TCP sender. The envi
sioned deployment model for rLEDBAT is that the clients implement rLEDBAT and th
is enables rLEDBAT in communications with existing standard-TCP senders. In par
ticular, the sender <bcp14>MUST</bcp14> implement <xref target="RFC9293" format=
"default"/> and also <bcp14>MUST</bcp14> implement the TCP Timestamps (TS) optio
n as defined in <xref target="RFC7323" format="default"/>. Also, the sender shou
ld implement some of the standard congestion control mechanisms, such as CUBIC <
xref target="RFC9438" format="default"/> or NewReno <xref target="RFC5681" forma
t="default"/> <xref target="RFC6582"/>.</t>
<t>rLEDBAT does not define a new congestion control algorithm. The definit
ion of the actual LBE congestion control algorithm executed in the rLEDBAT recei
ver is beyond the scope of this document. The rLEDBAT receiver <bcp14>MUST</bcp1
4> use an LBE congestion control algorithm. Because rLEDBAT assumes a standard-T
CP sender, the sender will be using a "best effort" congestion control algorithm
(such as CUBIC or NewReno). Since rLEDBAT uses the receive window to control th
e 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 small
er 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
<bcp14>SHOULD</bcp14> use the LEDBAT congestion control algorithm <xref target="
RFC6817" format="default"/> or the LEDBAT++ congestion control algorithm <xref t
arget="I-D.irtf-iccrg-ledbat-plus-plus" format="default"/>. &nbsp;rLEDBAT <bcp14
>MAY</bcp14> use other LBE congestion control algorithms defined elsewhere. Irre
spective of which congestion control algorithm is executed in the receiver, a rL
EDBAT connection will never be more aggressive than standard-TCP, since it is al
ways bounded by the congestion control algorithm executed at the sender.</t>
<t>rLEDBAT is essentially composed of three types of mechanisms, namely
those that provide the means to measure the packet delay (either the RTT 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 ra
te. The first two provide input to the LBE congestion control algorithm, while t
he third uses the congestion window computed by the LBE congestion control algor
ithm to manipulate the receive window, as depicted in <xref target="fig1"/>.</t>
<figure anchor="fig1">
<name>The rLEDBAT Architecture</name>
<artwork align="center" name="" type="" alt=""><![CDATA[
+------------------------------------------+ +------------------------------------------+
| TCP receiver | | TCP Receiver |
| +-----------------+ | | +-----------------+ |
| | +------------+ | | | | +------------+ | |
| +---------------------| RTT | | | | +---------------------| RTT | | |
| | | | Estimation | | | | | | | Estimation | | |
| | | +------------+ | | | | | +------------+ | |
| | | | | | | | | |
| | | +------------+ | | | | | +------------+ | |
| | +--------------| Loss, RTX | | | | | +--------------| Loss, RTX | | |
| | | | | Detection | | | | | | | | Detection | | |
| | | | +------------+ | | | | | | +------------+ | |
| v v | | | | v v | | |
| +----------------+ | | | | +----------------+ | | |
| | LBE Congestion | | rLEDBAT | | | | LBE Congestion | | rLEDBAT | |
| | Control | | | | | | Control | | | |
| +----------------+ | | | | +----------------+ | | |
| | | +------------+ | | | | | +------------+ | |
| | | | RCV-WND | | | | | | | RCV.WND | | |
| +---------------->| Control | | | | +---------------->| Control | | |
| | +------------+ | | | | +------------+ | |
| +-----------------+ | | +-----------------+ |
+------------------------------------------+ +------------------------------------------+
]]></artwork> ]]></artwork>
</figure>
<t>We next describe each of the rLEDBAT components.</t>
<section numbered="true" toc="default">
<name>Controlling the Receive Window</name>
<t>rLEDBAT uses the TCP receive window (RCV.WND) to enable the receiver
to control the sender's rate. <xref target="RFC9293" format="default"/> specifi
es that the RCV.WND is used to announce the available receive buffer to the send
er for flow control purposes. In order to avoid confusion, we will call fcwnd th
e value that a standard-TCP receiver compliant with <xref target="RFC9293"/> cal
culates to set in the receive window for flow control purposes. We call RLWND th
e window value calculated by the rLEDBAT algorithm, and we call RCV.WND the valu
e actually included in the Receive Window field of the TCP header. For a receive
r compliant with <xref target="RFC9293"/>, RCV.WND == fcwnd.</t>
<t>In the case of the rLEDBAT receiver, this receiver <bcp14>MUST NOT</b
cp14> set the RCV.WND to a value larger than fcwnd and <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 f
low of data from the sender to the receiver, namely the TCP congestion control a
lgorithm on the sender side and the LBE congestion control algorithm executed i
n the receiver and conveyed to the sender through the RCV.WND. In the normal TCP
operation, the sender uses the minimum of the cwnd and the RCV.WND to calculate
the SND.WND. This is also true for rLEDBAT, as the sender is a regular TCP send
er. 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 r
ate. Moreover, because 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 will gener
ally be smaller than the congestion window calculated by the TCP sender, implyin
g 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, RLWND is set to its
maximum value, so that the sending rate of the sender is governed by the flow co
ntrol algorithm of the receiver and the TCP slow start mechanism of the sender.<
/t>
<t>In summary, the sender's window is SND.WND = min(cwnd, RLWND, fcwnd)<
/t>
<section numbered="true" toc="default">
<name>Avoiding Window Shrinking</name>
<t>The LEDBAT/LEDBAT++ algorithm executed in a rLEDBAT receiver increa
ses or decreases RLWND according to congestion signals (variations in the estima
ted queuing delay and packet loss).
</figure> 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
<t>We describe each of the rLEDBAT components next.</t> use. This so-called "shrinking the window" is discouraged as per <xref target="
RFC9293" format="default"/>, as it may cause unnecessary packet loss and perform
<section title="Controlling the receive window"> ance penalties. To be consistent with <xref target="RFC9293" format="default"/>,
the rLEDBAT receiver <bcp14>SHOULD NOT</bcp14> shrink the receive window. </t>
<t>rLEDBAT uses the Receive Window (RCV.WND) of TCP to enable the receive <t>In order to avoid window shrinking, the receiver <bcp14>MUST</bcp14
r to control the sender's rate. <xref target="RFC9293" /> defines that the RCV. > only reduce RCV.WND by the number of bytes contained in a received data packet
WND is used to announce the available receive buffer to the sender for flow cont . This may fall short to honor the new calculated value of the RLWND immediately
rol purposes. In order to avoid confusion, we will call fcwnd the value that a s . However, the receiver <bcp14>SHOULD</bcp14> progressively reduce the advertise
tandard RFC793bis TCP receiver calculates to set in the receive window for flow d RCV.WND, always honoring that the reduction is less than or equal to the recei
control purposes. We call RLWND the window value calculated by rLEDBAT algorithm ved bytes, until the target window determined by the rLEDBAT algorithm is reache
and we call RCV.WND the value actually included in the Receive Window field of d.
the TCP header. For a RFC793bis receiver, RCV.WND == fcwnd.</t> This implies that it may take up to one RTT for the rLEDBAT receiver to drain en
<t>In the case of rLEDBAT receiver, the rLEDBAT receiver MUST NOT set the ough in-flight bytes to completely close its receive window without shrinking it
RCV.WND to a value larger than fcwnd and it SHOULD set the RCV.WND to the minim . This is sufficient to honor the window output from the LEDBAT/LEDBAT++ algorit
um of RLWND and fcwnd, honoring both.</t> hms, since they are only allowed to perform at most one multiplicative decrease
per RTT.</t>
<t>When using rLEDBAT, two congestion controllers are in action in the fl </section>
ow of data from the sender to the receiver, namely, the congestion control algor <section numbered="true" toc="default">
ithm of TCP in the sender side and the LBE congestion control algorithm execute <name>Setting the Window Scale Option</name>
d in the receiver and conveyed to the sender through the RCV.WND. In the normal <t>The Window Scale (WS) option <xref target="RFC7323" format="default
TCP operation, the sender uses the minimum of the congestion window cwnd and the "/> is a means to increase the maximum window size permitted by the receive wind
receiver window RCV.WND to calculate the sender's window SND.WND. This is also ow. The WS option defines a scale factor that restricts the granularity of the r
true for rLEDBAT, as the sender is a regular TCP sender. This guarantees that th eceive window that can be announced. This means that the rLEDBAT client will hav
e rLEDBAT flow will never transmit more aggressively than a standard-TCP flow, a e to accumulate the increases resulting from multiple received packets and only
s the sender's congestion window limits the sending rate. Moreover, because a LB convey a change in the window when the accumulated sum of increases is equal to
E congestion control algorithm such as LEDBAT/LEDBAT++ is designed to react earl or higher than one increase step as imposed by the scaling factor according to t
ier and more aggressively to congestion than regular TCP congestion control, the he WS option in place for the TCP connection.</t>
RLWND contained in the RCV.WND field of TCP will be in general smaller than the <t>Changes in the receive window that are smaller than 1 MSS (Maximum
congestion window calculated by the TCP sender, implying that the rLEDBAT conge Segment Size) are unlikely to have any immediate impact on the sender's rate. As
stion control algorithm will be effectively controlling the sender's window. On usual, TCP's segmentation practice results in sending full segments (i.e., segm
e exception to this is at the beginning of the connection, when there is no info ents of size equal to the MSS). <xref target="RFC7323" format="default"/>, which
rmation to set RLWND, then, RLWND is set to its maximum value, so that the sendi defines the WS option, specifies that allowed values for the WS option are betw
ng rate of the sender is governed by the flow control algorithm of the receiver een 0 and 14. Assuming an MSS of around 1500 bytes, WS option values between 0 a
and the TCP slow start mechanism of the sender.</t> nd 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 (un
<t>In summary, the sender's window is: SND.WND = min(cwnd, RLWND, fcwnd)< less packets smaller than the MSS are being exchanged).</t>
/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 chan
<section title="Avoiding window shrinking"> ge in one unit of the receive window implies a change of 10 MSS in the effective
window).
<t>The LEDBAT/LEDBAT++ algorithm executed in a rLEDBAT receiver i </t>
ncreases or decreases RLWND according to congestion signals (variations on the e <t>For the above reasons, the rLEDBAT client <bcp14>SHOULD</bcp14> set
stimated queueing delay and packet loss). WS option values lower than 12. Additional experimentation is required to explo
re the impact of larger WS values on rLEDBAT dynamics.</t>
If RLWND is decreased and directly announced in RCV.WND, <t>Note that the recommendation for rLEDBAT to set the WS option value
this could lead to an announced window that is smaller than what is currently in s to lower values does not preclude communication with servers that set the WS o
use. This so called 'shrinking the window' is discouraged as per <xref target=" ption values to larger values, since WS option values are set independently for
RFC9293" />, as it may cause unnecessary packet loss and performance penalty. To each direction of the TCP connection.</t>
be consistent with <xref target="RFC9293" />, the rLEDBAT receiver SHOULD NOT s </section>
hrink the receive window. </t> </section>
<section numbered="true" toc="default">
<t>In order to avoid window shrinking, the receiver MUST only red <name>Measuring Delays</name>
uce RCV.WND by the number of bytes upon of a received data packet. This may fall <t>Both LEDBAT and LEDBAT++ measure base and current delays to estimate
short to honor the new calculated value of the RLWND immediately. However, the the queuing delay. LEDBAT uses the one-way delay, while LEDBAT++ uses the RTT. I
receiver SHOULD progressively reduce the advertised RCV.WND, always honoring tha n the next sections, we describe how rLEDBAT mechanisms enable the receiver to m
t the reduction is less or equal than the received bytes, until the target windo easure the one-way delay or the RTT -- whichever is needed, depending on the con
w determined by the rLEDBAT algorithm is reached. gestion control algorithm used.</t>
This implies that it may take up to one RTT for the rLEDBAT receiver to drain en <section numbered="true" toc="default">
ough in-flight bytes to completely close its receive window without shrinking it <name>Measuring RTT to Estimate the Queuing Delay</name>
. This is sufficient to honor the window output from the LEDBAT/LEDBAT++ algorit <t>LEDBAT++ uses the RTT to estimate the queuing delay. In order to es
hms since they only allow to perform at most one multiplicative decrease per RTT timate the queuing delay using RTT, the rLEDBAT receiver estimates the base RTT
.</t> (i.e., the constant components of RTT) and also measures the current RTT. By sub
</section> tracting these two values, we obtain the queuing delay to be used by the rLEDBAT
controller.</t>
<section title="Setting the Window Scale Option"> <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
<t>The Window Scale (WS) option <xref target="RFC7323" /> is a me using a number of recent samples and applying a filter, such as the minimum (or
ans to increase the maximum window size permitted by the Receive Window. The WS the mean) of the last k samples. Using RTT to estimate the queuing delay has a n
option defines a scale factor which restricts the granularity of the receive win umber of shortcomings and difficulties, as discussed below.</t>
dow that can be announced. This means that the rLEDBAT client will have to accum <t>The queuing delay measured using RTT also includes the queuing dela
ulate the increases resulting from multiple received packets, and only convey a y experienced by the return packets in the direction from the rLEDBAT receiver t
change in the window when the accumulated sum of increases is equal or higher th o the sender. This is a fundamental limitation of this approach. The impact of t
an one increase step as imposed by the scaling factor according to the WS option his limitation is that the rLEDBAT controller will also react to congestion in t
in place for the TCP connection.</t> he reverse path direction, resulting in an even more conservative mechanism.</t>
<t>In order to measure RTT, the rLEDBAT client <bcp14>MUST</bcp14> ena
<t>Changes in the receive window that are smaller than 1 MSS are ble the TS option <xref target="RFC7323" format="default"/>. By matching the TSv
unlikely to have any immediate impact on the sender's rate, as usual TCP's segme al carried in outgoing packets with the Timestamp Echo Reply (TSecr) value <xref
ntation practice results in sending full segments (i.e., segments of size equal target="RFC7323" format="default"/> observed in incoming packets, it is possibl
to the MSS). Current WS option specification <xref target="RFC7323" /> defines t e to measure RTT. This allows the rLEDBAT receiver to measure RTT even if it is
hat allowed values for the WS option are between 0 and 14. Assuming a MSS around acting as a pure receiver. In a pure receiver, there is no data flowing from the
1500 bytes, WS option values between 0 and 11 result in the receive window bein rLEDBAT receiver to the sender, making it impossible to match data packets with
g expressed in units that are about 1 MSS or smaller. So, WS option values betwe Acknowledgment packets to measure RTT, in contrast to what is usually done in T
en 0 and 11 have no impact in rLEDBAT (unless packets smaller than the MSS are b CP for other purposes.</t>
eing exchanged).</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. If t
<t>WS option values higher than 11 can affect the dynamics of rLE hat happens, the rLEDBAT receiver will be unable to match the different outgoing
DBAT, since control may become too coarse (e.g., with WS of 14, a change in one packets carrying the same TSval with the different incoming packets also carryi
unit of the receive window implies a change of 10 MSS in the effective window).< ng the same TSecr value. However, it is not necessary for rLEDBAT to use all pac
/t> kets to estimate RTT, and sampling a subset of in-flight packets per RTT is enou
<t>For the above reasons, the rLEDBAT client SHOULD set WS option gh to properly assess the queuing delay. RTT <bcp14>MUST</bcp14> then be calcula
values lower than 12. Additional experimentation is required to explore the imp ted as the time since the first packet with a given TSval was sent and the first
act of larger WS values on rLEDBAT dynamics.</t> packet that was received with the same value contained in the TSecr. Other pack
<t>Note that the recommendation for rLEDBAT to set the WS option ets with repeated TS values <bcp14>SHOULD NOT</bcp14> be used for RTT calculatio
value to lower values does not precludes the communication with servers that set ns. </t>
the WS option values to larger values, since the WS option value is set indepen <t>Several issues must be addressed in order to avoid an artificial in
dently for each direction of the TCP connection.</t> crease in the observed RTT. Different issues emerge, depending on whether the
</section> rLEDBAT-capable host is sending data packets or pure ACKs to measure RTT. We nex
</section> t consider these issues separately.</t>
<section numbered="true" toc="default">
<section title="Measuring delays"> <name>Measuring RTT When Sending Pure ACKs</name>
<t>In this scenario, the rLEDBAT node (node A) sends a pure ACK to t
<t>Both LEDBAT and LEDBAT++ measure base and current delays to estimate t he other endpoint of the TCP connection (node B), including the TS option. Upon
he queueing delay. LEDBAT uses the one way delay while LEDBAT++ uses the round t the reception of the TS option, host B will copy the value of the TSval into the
rip time. In the next sections we describe how rLEDBAT mechanisms enable the rec TSecr field of the TS option and include that option in the next data packet to
eiver to measure the one way delay or the round trip time, whatever is needed de wards host A. However, there are two reasons why B may not send a packet immedia
pending on the congestion control algorithm used.</t> tely back to A, artificially increasing the measured RTT. The first reason is wh
en A has no data to send.
<section title="Measuring RTT to estimate the queueing delay"> The second is when A has no available window to put more packets in flight. We n
ext describe how each of these cases is addressed.</t>
<t>LEDBAT++ uses the round trip time (RTT) to estimate the queueing delay <t>The case where host B has no data to send when it receives the pu
. In order to estimate the queueing delay using RTT, the rLEDBAT receiver estima re Acknowledgment is expected to be rare in the rLEDBAT use cases. &nbsp;rLEDBAT
tes the base RTT (i.e., the constant components of RTT) and also measures the cu will be used mostly for background file transfers, so the expected common case
rrent RTT. By subtracting these two values, we obtain the queuing delay to be us is that the sender will have data to send throughout the lifetime of the communi
ed by the rLEDBAT controller.</t> cation. However, if, for example, the file is structured in blocks of data, it m
ay be the case that the sender will seldom have to wait until the next block is
<t>LEDBAT++ discovers the base RTT (RTTb) by taking the minimum value of available to proceed with the data transfer. To address this situation, the filt
the measured RTTs over a period of time. The current RTT (RTTc) is estimated usi er used by the congestion control algorithm executed in the receiver <bcp14>SHOU
ng a number of recent samples and applying a filter, such as the minimum (or the LD</bcp14> discard outliers (e.g., a MIN filter <xref target="RFC6817"/> would a
mean) of the last k samples. Using RTT to estimate the queueing delay has a num chieve this) when measuring RTT using pure ACK packets.</t>
ber of shortcomings and difficulties that we discuss next.</t> <t>This limitation of the sender's window can come from either the T
CP congestion window in host B or the announced receive window from rLEDBAT in h
<t>The queuing delay measured using RTT includes also the queueing delay ost A. Normally, the receive window will be the one to limit the sender's transm
experienced by the return packets in the direction from the rLEDBAT receiver to ission rate, since the LBE congestion control algorithm used by the rLEDBAT node
the sender. This is a fundamental limitation of this approach. The impact of thi is designed to be more restrictive on the sender's rate than standard-TCP. If t
s error is that the rLEDBAT controller will also react to congestion in the reve he limiting factor is the congestion window in the sender, it is less relevant i
rse path direction which results in an even more conservative mechanism.</t> f rLEDBAT further reduces the receive window due to a bloated RTT measurement, s
ince the rLEDBAT node is not actively controlling the sender's rate. Nevertheles
<t>In order to measure RTT, the rLEDBAT client MUST enable the Time Stamp s, the proposed approach to discard larger samples would also address this issue
(TS) option <xref target="RFC7323" />. By matching the TSVal value carried in o .</t>
utgoing packets with the TSecr value observed in incoming packets, it is possibl <t>To address the case in which the limiting factor is the receive w
e to measure RTT. This allows the rLEDBAT receiver to measure RTT even if it is indow announced by rLEDBAT, the congestion control algorithm at the receiver <bc
acting as a pure receiver. In a pure receiver there is no data flowing from the p14>SHOULD</bcp14> discard RTT measurements during the window reduction phase th
rLEDBAT receiver to the sender, making impossible to match data packets with ack at are triggered by pure ACK packets. The rLEDBAT receiver is aware of whether a
nowledgements packets to measure RTT, as it is usually done in TCP for other pur given TSval was sent in a pure ACK packet where the window was reduced, and if
poses.</t> so, it can discard the corresponding RTT measurement. </t>
</section>
<t>Depending on the frequency of the local clock used to generate the val <section numbered="true" toc="default">
ues included in the TS option, several packets may carry the same TSVal value. I <name>Measuring RTT When Sending Data Packets</name>
f that happens, the rLEDBAT receiver will be unable to match the different outgo <t>In the case that the rLEDBAT node is sending data packets and mat
ing packets carrying the same TSVal value with the different incoming packets ca ching them with pure ACKs to measure RTT, a factor that can artificially increas
rrying also the same TSecr value. However, it is not necessary for rLEDBAT to us e the RTT measured is the presence of delayed Acknowledgments.
e all packets to estimate RTT and sampling a subset of in-flight packets per RTT According to the TS option generation rules <xref target="RFC7323
is enough to properly assess the queueing delay. RTT MUST then be calculated as " format="default"/>,
the time since the first packet with a given TSVal was sent and the first packe the value included in the TSecr for a delayed ACK is the one in t
t that was received with the same value contained in the TSecr. Other packets wi he TSval field of the earliest unacknowledged segment.
th repeated TS values SHOULD NOT be used for RTT calculation. </t>
<t>Several issues must be addressed in order to avoid an artificial incre
ase of the observed RTT. Different issues emerge depending whether the rLEDBAT c
apable host is sending data packets or pure ACKs to measure RTT. We next conside
r the issues separately.</t>
<section title="Measuring RTT sending pure ACKs">
<t>In this scenario, the rLEDBAT node (node A) sends a pure ACK t
o the other endpoint of the TCP connection (node B), including the TS option. Up
on 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 that option into the next data pack
et towards host A. However, there are two reasons why B may not send a packet im
mediately 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 d
escribe next how each of these cases is addressed.</t>
<t>The case where the host B has no data to send when it receives the pur
e 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 t
he case that the sender seldomly will have to wait until the next block is avail
able to proceed with the data transfer. To address this situation, the filter us
ed by the congestion control algorithm executed in the receiver SHOULD discard o
utliers (e.g. a min filter would achieve this) when measuring RTT using pure ACK
packets.</t>
<t>This limitation of the sender's window can come either from the TCP co
ngestion 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 tr
ansmission 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 releva
nt if rLEDBAT further reduces the receive window due to a bloated RTT measuremen
t, since the rLEDBAT node is not actively controlling the sender's rate. Neverth
eless, the proposed approach to discard larger samples would also address this i
ssue.</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 d
iscard RTT measurements during the window reduction phase that are triggered by
pure ACK packets. 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 discar
d the corresponding RTT measurement. </t>
</section>
<section title="Measuring RTT when sending data packets">
<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.
According to the TS option generation rules <xref target="RFC7323
" />,
the value included in the TSecr for a delayed ACK is the one in t
he TSVal field of the earliest unacknowledged segment.
This may artificially increase the measured RTT. </t> This may artificially increase the measured RTT. </t>
<t>If both endpoints of the connection are sending data packets, Ack
<t>If both endpoints of the connection are sending data packets, nowledgments are piggybacked onto the data packets and they are not delayed. Del
Acknowledgments are piggybacked into the data packets and they are not delayed. ayed ACKs only increase RTT measurements in the case that the sender has no data
Delayed ACKs only increase RTT measurements in the case that the sender has no d to send. Since the expected use case for rLEDBAT is that the sender will be sen
ata to send. Since the expected use case for rLEDBAT is that the sender will be ding background traffic to the rLEDBAT receiver, the cases where delayed ACKs in
sending background traffic to the rLEDBAT receiver, the cases where delayed ACKs crease the measured RTT are expected to be rare.</t>
increase the measured RTT are expected to be rare.</t> <t>Nevertheless, measurements based on data packets from the rLEDBAT
<t>Nevertheless, measurements based on data packets from the rLED node matching pure ACKs from the other end will result in an increased RTT samp
BAT node matching pure ACKs from the other end will result in an increased RTT s le. The additional increase in the measured RTT will be up to 500 ms. This is be
ample. The additional increase in the measured RTT will be up to 500 ms. The rea cause delayed ACKs are generated every second data packet received and not delay
son for this is that delayed ACKs are generated every second data packet receive ed more than 500 ms according to <xref target="RFC9293" format="default"/>. The
d and not delayed more than 500 ms according to <xref target="RFC9293" />. The r rLEDBAT receiver <bcp14>MAY</bcp14> discard RTT measurements done using data pac
LEDBAT receiver MAY discard RTT measurements done using data packets from the rL kets from the rLEDBAT receiver and matching pure ACKs, especially if it has rece
EBDAT receiver and matching pure ACKs, especially if it has recent measurements nt measurements done using other packet combinations. Applying a filter (e.g., a
done using other packet combinations. Also, applying a filter that discards outl MIN filter) that discards outliers would also address this issue.</t>
iers would also address this issue (e.g. a min filter).</t> </section>
</section> </section>
</section> <section numbered="true" toc="default">
<name>Measuring One-Way Delay to Estimate the Queuing Delay</name>
<section title="Measuring one way delay to estimate the queueing <t>The LEDBAT algorithm uses the one-way delay of packets as input. A
delay"> TCP receiver can measure the delay of incoming packets directly (as opposed to t
<t>The LEDBAT algorithm uses the one-way delay of packets as inpu he sender-based LEDBAT, where the receiver measures the one-way delay and needs
t. A TCP receiver can measure the delay of incoming packets directly (as opposed to convey it to the sender).</t>
to the sender-based LEDBAT, where the receiver measures the one-way delay and n <t>In the case of TCP, the receiver can use the TS option to measure t
eeds to convey it to the sender).</t> he one-way delay by subtracting the timestamp contained in the incoming packet f
<t>In the case of TCP, the receiver can use the TimeStamp option rom the local time at which the packet has arrived. As noted in <xref target="RF
to measure the one way delay by subtracting the timestamp contained in the incom C6817" format="default"/>, the clock offset between the sender's clock and the r
ing packet from the local time at which the packet has arrived. As noted in <xre eceiver's clock does not affect the LEDBAT operation, since LEDBAT uses the diff
f target="RFC6817" /> the clock offset between the clock of the sender and the c erence between the base one-way delay and the current one-way delay to estimate
lock in the receiver does not affect the LEDBAT operation, since LEDBAT uses the the queuing delay, effectively "canceling out" the clock offset error in the que
difference between the base one way delay and the current one way delay to esti uing delay estimation. There are, however, two other issues that the rLEDBAT rec
mate the queuing delay, effectively canceling the clock offset error in the queu eiver needs to take into account in order to properly estimate the one-way delay
eing delay estimation. There are however two other issues that the rLEDBAT recei , namely the units in which the received timestamps are expressed and the clock
ver needs to take into account in order to properly estimate the one way delay, skew. These issues are addressed below.</t>
namely, the units in which the received timestamps are expressed and the clock s <t>In order to measure the one-way delay using TCP timestamps, the rLE
kew. We address them next.</t> DBAT receiver first needs to discover the units of values in the TS option and t
hen needs to account for the skew between the two endpoint clocks. Note that a m
<t>In order to measure the one way delay using TCP timestamps, th ismatch of 100 ppm (parts per million) in the estimation of the sender's clock r
e rLEDBAT receiver, first, needs to discover the units of values in the TS optio ate accounts for 6 ms of variation per minute in the measured delay. This is jus
n and, second, needs to account for the skew between the two endpoint clocks. No t one order of magnitude below the target delay set by rLEDBAT (or potentially m
te that a mismatch of 100 ppm (parts per million) in the estimation of the sende ore if the target is set to lower values, which is possible). Typical skew for u
r's clock rate accounts for 6 ms of variation per minute in the measured delay. ntrained clocks is reported to be around 100-200 ppm <xref target="RFC6817" form
This just one order of magnitude below the target delay set by rLEDBAT (or poten at="default"/>.</t>
tially more if the target is set to lower values, which is possible). Typical sk <t>In order to learn both the TS units and the clock skew, the rLEDBAT
ew for untrained clocks is reported to be around 100-200 ppm <xref target="RFC68 receiver measures how much local time has elapsed between two packets with diff
17" />.</t> erent TS values issued by the sender. By comparing the local time difference and
<t>In order to learn both the TS units and the clock skew, the rL the TS value difference, the receiver can assess the TS units and relative cloc
EDBAT receiver measures how much local time has elapsed between two packets with k skews. In order for this to be accurate, the packets carrying the different TS
different TS values issued by the sender. By comparing the local time differenc values should experience equal (or at least similar) delay when traveling from
e and the TS value difference, the receiver can assess the TS units and relative the sender to the receiver, as any difference in the experienced delays would in
clock skews. In order for this to be accurate, the packets carrying the differe troduce an error in the unit/skew estimation. One possible approach is to select
nt TS values should experience equal (or at least similar delay) when traveling packets that experienced minimal delay (i.e., queuing delay close to zero) to m
from the sender to the receiver, as any difference in the experienced delays wou ake the estimations.</t>
ld introduce error in the unit/skew estimation. One possible approach is to sele <t>An additional difficulty regarding the estimation of the TS units a
ct packets that experienced the minimum delay (i.e. close to zero queueing delay nd clock skew in the context of (r)LEDBAT is that the LEDBAT congestion controll
) to make the estimations.</t> er actions directly affect the (queuing) delay experienced by packets. In partic
<t>An additional difficulty regarding the estimation of the TS un ular, if there is an error in the estimation of the TS units/skew, the LEDBAT co
its and clock skew in the context of (r)LEDBAT is that the LEDBAT congestion con ntroller will attempt to compensate for it by reducing/increasing the load. The
troller actions directly affect the (queueing) delay experienced by packets. In result is that the LEDBAT operation interferes with the TS units/clock skew meas
particular, if there is an error in the estimation of the TS units/skew, the LED urements. Because of this, measurements are more accurate when there is no traff
BAT controller will attempt to compensate it by reducing/increasing the load. Th ic in the connection (in addition to the packets used for the measurements). The
e result is that the LEDBAT operation interferes with the TS units/clock skew me problem is that the receiver is unaware of whether the sender is injecting traf
asurements. Because of this, measurements are more accurate when there is no tra fic at any point in time; it is therefore unable to use these quiet intervals to
ffic in the connection (in addition to the packets used for the measurements). T perform measurements. The receiver can, however, force periodic slowdowns, redu
he problem is that the receiver is unaware if the sender is injecting traffic at cing the
any point in time, and so, it is unable to use these quiet intervals to perform announced receive window to a few packets and performing the measurements at tha
measurements. The receiver can however, force periodic slowdowns, reducing the t time.</t>
announced receive window to a few packets and perform the measurements then.</t> <t>It is possible for the rLEDBAT receiver to perform multiple measure
<t>It is possible for the rLEDBAT receiver to perform multiple me ments to assess both the TS units and the relative clock skew during the lifetim
asurements to assess both the TS units and the relative clock skew during the li e of the connection, in order to obtain more accurate results. Clock skew measur
fetime of the connection, in order to obtain more accurate results. Clock skew m ements are more accurate if the time period used to discover the skew is larger,
easurements are more accurate if the time period used to discover the skew is la as the impact of the skew becomes more apparent. It is a reasonable approach f
rger, as the impact of the skew becomes more apparent. It is a reasonable appro or the rLEDBAT receiver to perform an early discovery of the TS units (and the c
ach for the rLEDBAT receiver to perform an early discovery of the TS units (and lock skew) using the first few packets of the TCP connection and then improve th
the clock skew) using the first few packets of the TCP connection and then impro e accuracy of the TS units/clock skew estimation using periodic measurements lat
ve the accuracy of the TS units/clock skew estimation using periodic measurement er in the lifetime of the connection. </t>
s later in the lifetime of the connection. </t> </section>
</section>
</section> <section numbered="true" toc="default">
<name>Detecting Packet Losses and Retransmissions</name>
</section> <t>The rLEDBAT receiver is capable of detecting retransmitted packets as
follows. We call RCV.HGH the highest sequence number corresponding to a receive
<section title="Detecting packet losses and retransmissions"> d byte of data (not assuming that all bytes with smaller sequence numbers have b
een received already, there may be holes), and we call TSV.HGH the TSval corresp
<t>The rLEDBAT receiver is capable of detecting retransmitted packets in onding to the segment in which that byte was carried. SEG.SEQ stands for the seq
the following way. We call RCV.HGH the highest sequence number corresponding to uence number of a newly received segment, and we call TSV.SEQ the TSval of the n
a received byte of data (not assuming that all bytes with smaller sequence numbe ewly received segment.</t>
rs have been received already, there may be holes) and we call TSV.HGH the TSVal <t>If SEG.SEQ &lt; RCV.HGH and TSV.SEQ &gt; TSV.HGH, then the newly rece
value corresponding to the segment in which that byte was carried. SEG.SEQ stan ived segment is a retransmission. This is so because the newly received segment
ds for the sequence number of a newly received segment and we call TSV.SEQ the T was generated later than another already-received segment that contained data wi
SVal value of the newly received segment.</t> th a larger sequence number. This means that this segment was lost and was retra
<t>If SEG.SEQ &lt; RCV.HGH and TSV.SEQ > TSV.HGH then the newly received nsmitted.</t>
segment is a retransmission. This is so because the newly received segment was g <t>The proposed mechanism to detect retransmissions at the receiver fail
enerated later than another already received segment which contained data with a s when there are window tail drops. If all packets in the tail of the window are
larger sequence number. This means that this segment was lost and was retransmi lost, the receiver will not be able to detect a mismatch between the sequence n
tted.</t> umbers of the packets and the order of the timestamps. In this case, rLEDBAT wil
l not react to losses; however, the TCP congestion controller at the sender will
<t>The proposed mechanism to detect retransmissions at the receiver fails , most likely reducing its window to 1 MSS and taking over the control of the se
when there are window tail drops. If all packets in the tail of the window are nding rate until slow start ramps up and catches the current value of the rLEDBA
lost, the receiver will not be able to detect a mismatch between the sequence nu T window.</t>
mbers of the packets and the order of the timestamps. In this case, rLEDBAT will </section>
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.
</t>
</section>
</section>
<section title="Experiment Considerations">
<t>The status of this document is Experimental. The general purpose of th
e proposed experiment is to gain more experience running rLEDBAT over different
network paths to see if the proposed rLEDBAT parameters perform well in differen
t situations. Specifically, we would like to learn about the following aspects o
f the rLEDBAT mechanism: </t>
<t><list>
<t>- Interaction between the sender and the receiver Congestion c
ontrol algorithms. rLEDBAT posits that because the rLEDBAT receiver is using a l
ess-than-best-effort congestion control algorithm, the receiver congestion contr
ol algorithm will expose a smaller congestion window (conveyed though the Receiv
e Window) than the one resulting from the congestion control algorithm executed
at the sender. One of the purposes of the experiment is learn how these two inte
ract and if the assumption that the receiver side is always controlling the send
er's rate (and making rLEDBAT effective) holds. The experiment should include th
e different congestion control algorithms that are currently widely used in the
Internet, including Cubic, BBR and LEDBAT(++).</t>
<t>- Interaction between rLEDBAT and Active Queue Management tech
niques such as Codel, PIE and L4S.</t>
<t>- How the rLEDBAT should resume after a period during which th
ere was no incoming traffic and the information about the rLEDBAT state informat
ion is potentially dated.</t>
</list></t>
<section title="Status of the experiment at the time of this writing.">
<t>Currently there are the following implementations of rLEDBAT t
hat can be used for experimentation:
<list>
<t>- Windows 11. rLEDBAT is available in Microsof
t's Windows 11 22H2 since October 2023 <xref target="Windows11" />.</t>
<t>- Windows Server 2022. rLEDBAT is available in
Microsoft's Windows Server 2022 since September 2022 <xref target="WindowsServe
r" />.</t>
<t>- Apple. rLEDBAT is available in MacOS and iOS
since 2021 <xref target="Apple" />.</t>
<t>- Linux implementation, open source, available
since 2022 at https://github.com/net-research/rledbat_module.</t>
<t>- ns3 implementation, open source, available s
ince 2020 at https://github.com/manas11/implementation-of-rLEDBAT-in-ns-3.</t>
</list></t>
<t>In addition, rLEDBAT has been deployed by Microsoft in
wide scale in the following services:
<list>
<t>- BITS (Background Intelligent Transfe
r Service)</t>
<t>- DO (Delivery Optimization) service</
t>
<t>- Windows update # using DO</t>
<t>- Windows Store # using DO</t>
<t>- OneDrive</t>
<t>- Windows Error Reporting # wermgr.exe
; werfault.exe</t>
<t>- System Center Configuration Manager
(SCCM)</t>
<t>- Windows Media Player</t>
<t>- Microsoft Office</t>
<t>- Xbox (download games) # using DO</t>
</list> </t>
<t> Some initial experiments involving rLEDBAT have been
reported in <xref target="COMNET3" />. Experiments involving the interaction of
LEDBAT++ and BBR are presented in <xref target="COMNET2" />. An experimental eva
luation of the LEDBAT++ algorithm is presented in <xref target="COMNET1" />. As
LEDBAT++ is one of the less-than-best-effort congestion control algorithms that
rLEDBAT relies on, the results regarding LEDBAT++ interaction with other congest
ion control algorithms are relevant for the understanding of rLEDBAT as well.</t
>
</section>
</section>
<section title="Security Considerations">
<t>Overall, we believe that rLEDBAT does not introduce any new vu
lnerabilities to existing TCP endpoints, as it relies on existing TCP knobs, not
ably the 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, irrespecti
ve 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. There
are two ways an attacker can do that. One would be to introduce an artificial de
lay to the packets either by actually delaying the packets or modifying the Time
stamps. This would cause the rLEDBAT receiver to believe that a queue is buildin
g up and reduce the RCV.WND. Note that an attacker to do that must be on path, s
o if that is the case, it is probably more direct to simply reduce the RCV.WND.<
/t>
<t> The other option would be for the attacker to make the rLEDBAT
receiver believe that a loss has occurred. To do that, it basically needs to re
transmit an old packet (to be precise, it needs to transmit a packet with the ri
ght sequence number and the right port and IP numbers). This means that the atta
cker can achieve a reduction of incoming traffic to the rLEDBAT receiver not onl
y by modifying the RCV.WND field of the packets originated from the rLEDBAT host
, but also by injecting packets with the proper sequence number in the other dir
ection. This may slightly expand the attack surface.</t>
</section> </section>
<section 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 n
etwork 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>
<ul spacing="normal">
<li>
<t>Interaction between the sender's and receiver's congestion control
algorithms. &nbsp;rLEDBAT posits that because the rLEDBAT receiver is using a le
ss-than-best-effort congestion control algorithm, the receiver's congestion cont
rol algorithm will expose a smaller congestion window (conveyed through the rece
ive window) than the one resulting from the congestion control algorithm execute
d 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 includ
e the different congestion control algorithms that are currently widely used in
the Internet, including CUBIC, Bottleneck Bandwidth and Round-trip propagation t
ime (BBR), and LEDBAT(++).</t>
</li>
<li>
<t>Interaction between rLEDBAT and Active Queue Management techniques
such as Controlled Delay (CoDel); Proportional Integral controller Enhanced (PIE
); and 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 pote
ntially dated.</t>
</li>
</ul>
<section numbered="true" toc="default">
<name>Status of the Experiment at the Time of This Writing</name>
<t>Currently, the following implementations of rLEDBAT can be used for e
xperimentation:</t>
<ul spacing="normal">
<li>
<t>Windows 11. &nbsp;rLEDBAT is available in Microsoft's Windows 11
22H2 since October 2023 <xref target="Windows11" format="default"/>.</t>
</li>
<li>
<t>Windows Server 2022. &nbsp;rLEDBAT is available in Microsoft's Wi
ndows Server 2022 since September 2022 <xref target="WindowsServer" format="defa
ult"/>.</t>
</li>
<li>
<t>Apple. &nbsp;rLEDBAT is available in macOS and iOS since 2021 <
xref target="Apple" format="default"/>.</t>
</li>
<li>
<t>Linux implementation, open source, available since 2022 <xref tar
get="rledbat_module"/>.</t>
</li>
<li>
<t>ns3 implementation, open source, available since 2020 <xref targe
t="rLEDBAT-in-ns-3"/>.</t>
</li>
</ul>
<t>In addition, rLEDBAT has been deployed by Microsoft at wide scale in
the following services:
</t>
<ul spacing="normal">
<li>
<t>BITS (Background Intelligent Transfer Service)</t>
</li>
<li>
<t>DO (Delivery Optimization) service</t>
</li>
<li>
<t>Windows update: using DO</t>
</li>
<li>
<t>Windows Store: using DO</t>
</li>
<li>
<t>OneDrive</t>
</li>
<li>
<t>Windows Error Reporting: wermgr.exe; werfault.exe</t>
</li>
<li>
<t>System Center Configuration Manager (SCCM)</t>
</li>
<li>
<t>Windows Media Player</t>
</li>
<li>
<t>Microsoft Office</t>
</li>
<li>
<t>Xbox (download games): using DO</t>
</li>
</ul>
<section title="IANA Considerations"> <t> Some initial experiments involving rLEDBAT have been reported in <xr
<t>No actions are required from IANA.</t> ef target="COMNET3" format="default"/>. Experiments involving the interaction be
tween LEDBAT++ and BBR are presented in <xref target="COMNET2" format="default"/
>. An experimental evaluation of the LEDBAT++ algorithm is presented in <xref ta
rget="COMNET1" format="default"/>. As LEDBAT++ is one of the less-than-best-effo
rt congestion control algorithms that rLEDBAT relies on, the results regarding h
ow LEDBAT++ interacts with other congestion control algorithms are relevant for
the understanding of rLEDBAT as well.</t>
</section>
</section> </section>
<section numbered="true" toc="default">
<name>Security Considerations</name>
<t>Overall, we believe that rLEDBAT does not introduce any new vulnerabili
ties to existing TCP endpoints, as it relies on existing TCP knobs, notably the
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. There a
re two ways an attacker can do this:</t>
<section title="Acknowledgements"> <ul spacing="normal">
<li>One would be to introduce an artificial delay to the packets by either
<t>This work was supported by the EU through the StandICT projects RXQ, C actually delaying the packets or modifying the timestamps. This would cause the
CI and CEL6, the NGI Pointer RIM project and the H2020 5G-RANGE project and by t rLEDBAT receiver to believe that a queue is building up and reduce the RCV.WND.
he Spanish Ministry of Economy and Competitiveness through the 5G-City project ( Note that to do so, an attacker must be on path, so if that is the case, it is
TEC2016-76795-C6-3-R).</t> probably more direct to simply reduce the RCV.WND.</li>
<li>The other option would be for the attacker to make the rLEDBAT receive
<t>We would like to thank ICCRG chairs Reese Enghardt and Vidhi Goel for r believe that a loss has occurred. To do this, it basically needs to retransmit
their support on this work. We would also like to thank Daniel Havey for his hel an old packet (to be precise, it needs to transmit a packet with the correct se
p. We would like to thank Colin Perkins, Mirja Kuehlewind, and Vidhi Goel for th quence number and the correct port and IP numbers). This means that the attacker
eir reviews and comments on earlier versions of this document.</t> 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 but
also by injecting packets with the proper sequence number in the other directio
n. This may slightly expand the attack surface.</li>
</ul>
</section>
<section numbered="true" toc="default">
<name>IANA Considerations</name>
<t>This document has no IANA actions.</t>
</section> </section>
</middle> </middle>
<back> <back>
<references title="Informative References"> <displayreference target="I-D.irtf-iccrg-ledbat-plus-plus" to="LEDBAT++"/>
<?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"?>
<reference anchor="Windows11" >
<front>
<title>What's new in Delivery Optimization</title>
<author initials="C.F." surname="Forsmann" fullname="
Carmen">
<organization />
</author>
<date year="2023" />
</front>
<seriesInfo name="Microsoft Documentation" value="https:/
/learn.microsoft.com/en-us/windows/deployment/do/whats-new-do" />
<refcontent></refcontent>
</reference>
<reference anchor="WindowsServer" >
<front>
<title>LEDBAT Background Data Transfer for Wi
ndows</title>
<author initials="D.H." surname="Havey" fulln
ame="Daniel">
<organization />
</author>
<date year="2022" />
</front>
<seriesInfo name="Microsoft Blog" value="https://
techcommunity.microsoft.com/t5/networking-blog/ledbat-background-data-transfer-f
or-windows/ba-p/3639278" />
<refcontent></refcontent>
</reference>
<reference anchor="Apple" >
<front>
<title>Reduce network delays for your
app</title>
<author initials="S.C." surname="Stua
rt" fullname="Cheshire">
<organization />
</author>
<author initials="V.G." surname="Vidh
i" fullname=" Goel ">
<organization />
</author>
<date year="2021" />
</front>
<seriesInfo name="WWDC21" value="https://
developer.apple.com/videos/play/wwdc2021/10239/" />
<refcontent></refcontent>
</reference>
<reference anchor="COMNET3" >
<front>
<title> Design, implementation and va
lidation of a receiver-driven less-than-best-effort transport </title>
<author initials="M.B." surname="Bagn
ulo" fullname="Marcelo Bagnulo">
<organization />
</author>
<author initials="A.G." surname="Garc
ia-Martinez" fullname="Alberto Garcia-Martinez">
<organization />
</author>
<author initials="A.M." surname="Mand
alari" fullname="Anna Maria Mandalari">
<organization />
</author>
<author initials="P.B," surname="Bala
subramanian" fullname="Praveen Balasubramanian">
<organization />
</author>
<author initials="D.H." surname="Have
y" fullname="Daniel Havey">
<organization />
</author>
<author initials="G.M." surname="Mont
enegro" fullname="Gabriel Montenegro">
<organization />
</author>
<date year="2022" />
</front>
<seriesInfo name="Computer Networks" valu
e="Volume 233" />
<refcontent></refcontent>
</reference>
<reference anchor="COMNET2" >
<front>
<title>When less is m
ore: BBR versus LEDBAT++</title>
<author initials="M.B
." surname="Bagnulo" fullname="Marcelo Bagnulo">
<organization />
</author>
<author initials="A.G
." surname="Garcia-Martinez" fullname="Alberto Garcia-Martinez">
<organization />
</author>
<date year="2022" />
</front>
<seriesInfo name="Compute
r Networks" value="Volume 219" />
<refcontent></refcontent>
</reference>
<reference anchor="COMNET <references>
1" > <name>References</name>
<front> <references anchor="sec-normative-references">
<titl <name>Normative References</name>
e>An experimental evaluation of LEDBAT++ </title> <xi:include
<auth href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
or initials="M.B." surname="Bagnulo" fullname="Marcelo Bagnulo"> <xi:include
< href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
organization /> </references>
</aut <references anchor="sec-informative-references">
hor> <name>Informative References</name>
<auth <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.929
or initials="A.G." surname="Garcia-Martinez" fullname="Alberto Garcia-Martinez"> 3.xml"/>
<
organization />
</aut
hor>
<date
year="2022" />
</front>
<seri
esInfo name='Computer Networks' value="Volume 212"/>
<refconte
nt></refcontent>
</reference>
</references> <!-- draft-irtf-iccrg-ledbat-plus-plus (I-D Exists) -->
<xi:include href="https://bib.ietf.org/public/rfc/bibxml3/reference.I-D.ir
tf-iccrg-ledbat-plus-plus.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.681
7.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.732
3.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.943
8.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.568
1.xml"/>
<xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.658
2.xml"/>
<section title="Terminology"> <reference anchor="Windows11" target="https://learn.microsoft.com/en-us/wi
ndows/deployment/do/whats-new-do">
<front>
<title>What's new in Delivery Optimization</title>
<author>
<organization>Microsoft</organization>
</author>
<date month="October" year="2024"/>
</front>
<refcontent>Microsoft Windows Documentation</refcontent>
</reference>
<t>We use the following abreviations thoughout the text. We include a sho <reference anchor="WindowsServer" target="https://techcommunity.microsoft.
rt list for the reader's convenence:</t> com/t5/networking-blog/ledbat-background-data-transfer-for-windows/ba-p/3639278"
<t><list> >
<t>RCV.WND: the value included in the Receive Window fiel <front>
d of the TCP header (which computation is modified by this specification)</t> <title>LEDBAT Background Data Transfer for Windows</title>
<t>SND.WND: The TCP sender's window</t> <author initials="D" surname="Havey" fullname="Daniel">
<t>cwnd: the consgestion window as computed by the conges <organization/>
tion control algorithm running at the TCP sender.</t> </author>
<t>RLWND: the window value calculated by rLEDBAT algorith <date month="September" year="2022"/>
m</t> </front>
<t>fcwnd: the value that a standard RFC793bis TCP receive <refcontent>Microsoft Networking Blog</refcontent>
r calculates to set in the receive window for flow control purposes.</t> </reference>
<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 segm
ent</t>
<t>TSV.SEQ: the TSVal value of the last received segment<
/t>
</list></t>
</section>
<section title="rLEDBAT pseudo-code"> <reference anchor="Apple" target="https://developer.apple.com/videos/play/
wwdc2021/10239/">
<front>
<title>Reduce network delays for your app</title>
<author initials="S" surname="Cheshire" fullname="Stuart Cheshire">
<organization/>
</author>
<author initials="V" surname="Goel" fullname="Vidhi Goel ">
<organization/>
</author>
<date year="2021"/>
</front>
<refcontent>Apple Worldwide Developers Conference (WWDC2021), Video</ref
content>
</reference>
<t>We next describe how to integrate the proposed rLEDBAT mechanisms and <reference anchor="COMNET3">
an LBE delay-based congestion control algorithm such as LEDBAT or LEDBAT++. We <front>
describe the integrated algorithm as two procedures, one that is executed when <title> Design, implementation and validation of a receiver-driven les
a packet is received by a rLEDBAT-enabled endpoint (Figure 2) and another that i s-than-best-effort transport </title>
s executed when the rLEDBAT-enabled endpoint sends a packet (Figure 3). At the b <author initials="M" surname="Bagnulo" fullname="Marcelo Bagnulo">
eginning, RLWND is set to its maximum value, so that the sending rate of the sen <organization/>
der is governed by the flow control algorithm of the receiver and the TCP slow s </author>
tart mechanism of the sender, and the ackedBytes variable is set to 0. </t> <author initials="A" surname="García-Martínez" fullname="Alberto Garcí
a-Martínez">
<organization/>
</author>
<author initials="A.M." surname="Mandalari" fullname="Anna Maria Manda
lari">
<organization/>
</author>
<author initials="P" surname="Balasubramanian" fullname="Praveen Balas
ubramanian">
<organization/>
</author>
<author initials="D" surname="Havey" fullname="Daniel Havey">
<organization/>
</author>
<author initials="G" surname="Montenegro" fullname="Gabriel Montenegro
">
<organization/>
</author>
<date month="September" year="2023"/>
</front>
<refcontent>Computer Networks, vol. 233</refcontent>
<seriesInfo name="DOI" value="10.1016/j.comnet.2023.109841"/>
</reference>
<t>We assume that the LBE congestion control algorithm defines a WindowIn <reference anchor="COMNET2">
crease() function and a WindowDecrease() function. For example, in the case of L <front>
EDBAT++, the WindowIncrease() function is an additive increase, while the Window <title>When less is more: BBR versus LEDBAT++</title>
Decrease() function is a multiplicative decrease. In the case of the WindowIncre <author initials="M" surname="Bagnulo" fullname="Marcelo Bagnulo">
ase(), we assume that it takes as input the current window size and the number o <organization/>
f bytes that were acknowledged since the last window update (ackedBytes) and ret </author>
urns as output the updated window size. In the case of WindowDecrease(), it take <author initials="A" surname="García-Martínez" fullname="Alberto Garcí
s as input the current window size and returns the updated window size. </t> a-Martínez">
<organization/>
</author>
<date month="December" year="2022"/>
</front>
<refcontent>Computer Networks, vol. 219</refcontent>
<seriesInfo name="DOI" value="10.1016/j.comnet.2022.109460"/>
</reference>
<t>The data structures used in the algorithms are as follows. The sentLis <reference anchor="COMNET1">
t is a list that contains the TSval and the local send time of each packet sent <front>
by the rLEDBAT-enabled endpoint. The TSecr field of the packets received by the <title>An experimental evaluation of LEDBAT++ </title>
rLEDBAT-enabled endpoint are matched with the sendList to compute the RTT.</t> <author initials="M" surname="Bagnulo" fullname="Marcelo Bagnulo">
<organization/>
</author>
<author initials="A" surname="García-Martínez" fullname="Alberto Garcí
a-Martínez">
<organization/>
</author>
<date month="July" year="2022"/>
</front>
<refcontent>Computer Networks, vol. 212</refcontent>
<seriesInfo name="DOI" value="10.1016/j.comnet.2022.109036"/>
</reference>
<t>The RTT values computed for each received packet are stored in the RTT <reference anchor="rledbat_module"
list, which contains also the received TSecr (to avoid using multiple packets wi target="https://github.com/net-research/rledbat_module">
th the same TSecr for RTT calculations, only the first packet received for a giv <front>
en TSecr is used to compute the RTT). It also contains the local time at which t <title>rledbat_module</title>
he packet was received, to allow selecting the RTTs measured in a given period ( <author/>
e.g., in the last 10 minutes). RTTlist is initialized with all its values to its <date month="September" day="9" year="2022"/>
maximum.</t> </front>
<refcontent>commit d82ff20</refcontent>
</reference>
<figure title="Procedure executed when a packet is received"> <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>
<sourcecode> </references>
</references>
<section numbered="true" toc="default">
<name>rLEDBAT Pseudocode</name>
<t>In this section, we describe how to integrate the proposed rLEDBAT mech
anisms and an LBE delay-based congestion control algorithm such as LEDBAT or LE
DBAT++. &nbsp;We describe the integrated algorithm as two procedures: one that
is executed when a packet is received by a rLEDBAT-enabled endpoint (<xref targe
t="fig2"/>) and another that is executed when the rLEDBAT-enabled endpoint sends
a packet (<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 a
lgorithm 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 WindowInc
rease() function and a WindowDecrease() function. For example, in the case of LE
DBAT++, the WindowIncrease() function is an additive increase, while the WindowD
ecrease() function is a multiplicative decrease. In the case of the WindowIncrea
se() 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 the WindowDecreas
e() 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 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 is matched with the sendList to compute the RTT.</t>
<t>The RTT values computed for each received packet are stored in the RTTl
ist, which also contains the received TSecr (to avoid using multiple packets wit
h the same TSecr for RTT calculations, only the first packet received for a give
n TSecr is used to compute the RTT). It also contains the local time at which th
e 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 anchor="fig2">
<name>Procedure Executed When a Packet Is Received</name>
<sourcecode type="pseudocode"><![CDATA[
procedure receivePacket() procedure receivePacket()
//Looks for first sent packet with same TSval as TSecr, and, //Looks for first sent packet with same TSval as TSecr, and
//returns time difference //returns time difference
receivedRTT = computeRTT(sentList, receivedTSecr, receivedTime) receivedRTT = computeRTT(sendList, receivedTSecr, receivedTime)
//Inserts minimum value for a given receivedTSecr //Inserts minimum value for a given receivedTSecr
//note that many received packets may contain same receivedTSecr //Note that many received packets may contain same receivedTSecr
insertRTT (RTTlist, receivedRTT, receivedTSecr, receivedTime) insertRTT (RTTlist, receivedRTT, receivedTSecr, receivedTime)
filteredRTT = minLastKMeasures(RTTlist, K=4) filteredRTT = minLastKMeasures(RTTlist, K=4)
baseRTT = minLastNSeconds(RTTlist, N=180) baseRTT = minLastNSeconds(RTTlist, N=180)
qd = filteredRTT - baseRTT qd = filteredRTT - baseRTT
//ackedBytes is the number of bytes that can be used to reduce //ackedBytes is the number of bytes that can be used to reduce
//the Receive Window - without shrinking it - if necessary //the receive window - without shrinking it - if necessary
ackedBytes = ackedBytes + receiveBytes ackedBytes = ackedBytes + receiveBytes
if retransmittedPacketDetected then if retransmittedPacketDetected then
RLWND = DecreaseWindow(RLWND) // Only once per RTT RLWND = DecreaseWindow(RLWND) //Only once per RTT
end if end if
if qd &lt; T then if qd < T then
RLWND = IncreaseWindow(RLWND, ackedBytes) RLWND = IncreaseWindow(RLWND, ackedBytes)
else else
RLWND = DecreaseWindow(RLWND) RLWND = DecreaseWindow(RLWND)
end if end if
end procedure end procedure
</sourcecode> ]]></sourcecode>
</figure> </figure>
<figure title="Procedure executed when a packet is sent"> <figure anchor="fig3">
<sourcecode> <name>Procedure Executed When a Packet Is Sent</name>
<sourcecode type="pseudocode"><![CDATA[
procedure SENDPACKET procedure SENDPACKET
if (RLWND > RLWNDPrevious) or (RLWND - RLWNDPrevious &lt; ackedBytes) if (RLWND > RLWNDPrevious) or (RLWND - RLWNDPrevious < ackedBytes)
then then
RLWNDPrevious = RLWND RLWNDPrevious = RLWND
else else
RLWNDPrevious = RLWND - ackedBytes RLWNDPrevious = RLWND - ackedBytes
end if end if
ackedBytes = 0 ackedBytes = 0
RLWNDPrevious = RLWND RLWNDPrevious = RLWND
//Compute the RWND to include in the packet //Compute the RLWND to include in the packet
RLWND = min(RLWND, fcwnd) RLWND = min(RLWND, fcwnd)
end procedure end procedure
</sourcecode> ]]></sourcecode>
</figure> </figure>
</section> </section>
<section numbered="false" toc="default">
<name>Acknowledgments</name>
<t>This work was supported by the EU through the StandICT projects RXQ, CC
I, 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 lik
e to thank <contact fullname="Colin Perkins"/>, <contact fullname="Mirja Kühlewi
nd"/>, and <contact fullname="Vidhi Goel"/> for their reviews and comments on ea
rlier draft versions of this document.</t>
</section>
</back> </back>
</rfc> </rfc>
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