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IETF RFC 9840
Last modified on Thursday, September 18th, 2025
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Internet Research Task Force (IRTF) M. Bagnulo
Request for Comments: 9840 A. García-Martínez
Category: Experimental Universidad Carlos III de Madrid
ISSN: 2070-1721 G. Montenegro
P. Balasubramanian
Confluent
September 2025
rLEDBAT: Receiver-Driven Low Extra Delay Background Transport for TCP
 Abstract
This document specifies 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).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Research Task
Force (IRTF). The IRTF publishes the results of Internet-related
research and development activities. These results might not be
suitable for deployment. This RFC represents the consensus of the
Internet Congestion Control Research Group of the Internet Research
Task Force (IRTF). Documents approved for publication by the IRSG
are not candidates for any level of Internet Standard; see Section 2
of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9840.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction
2. Conventions and Terminology
3. Motivations for rLEDBAT
4. rLEDBAT Mechanisms
4.1. Controlling the Receive Window
4.1.1. Avoiding Window Shrinking
4.1.2. Setting the Window Scale Option
4.2. Measuring Delays
4.2.1. Measuring RTT to Estimate the Queuing Delay
4.2.2. Measuring One-Way Delay to Estimate the Queuing Delay
4.3. Detecting Packet Losses and Retransmissions
5. Experiment Considerations
5.1. Status of the Experiment at the Time of This Writing
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. rLEDBAT Pseudocode
Acknowledgments
Authors' Addresses
1. Introduction
LEDBAT (Low Extra Delay Background Transport) [RFC 6817] is a
congestion control algorithm used for less-than-best-effort (LBE)
traffic.
When LEDBAT traffic shares a bottleneck with other traffic using
standard congestion control algorithms (for example, TCP traffic
using CUBIC [RFC 9438], hereafter referred to as "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 efficient use of the available
capacity, while keeping the queuing delay within predefined bounds.
LEDBAT reacts to both packet loss and 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 queuing delay. When the measured current queuing
delay is below the target, LEDBAT increases the sending rate, and
when the delay is above the target, it reduces the sending rate.
LEDBAT estimates the queuing delay by subtracting the measured
current one-way delay from the estimated base one-way delay (i.e.,
the one-way delay in the absence of queues).
The LEDBAT specification [RFC 6817] defines the LEDBAT congestion
control algorithm, implemented in the sender to control its sending
rate. LEDBAT is specified in a protocol-agnostic and layer-agnostic
manner.
LEDBAT++ [LEDBAT++] is also an LBE congestion control algorithm that
is inspired by LEDBAT while addressing several problems identified
with the original LEDBAT specification. In particular, the
differences between LEDBAT and LEDBAT++ include the following:
i) LEDBAT++ uses the round-trip time (RTT) (as opposed to the one-
way delay used in LEDBAT) to estimate the queuing delay.
ii) LEDBAT++ uses an additive increase/multiplicative decrease
algorithm to achieve inter-LEDBAT++ fairness and avoid the
latecomer advantage observed in LEDBAT.
iii) LEDBAT++ performs periodic slowdowns to improve the measurement
of the base delay.
iv) LEDBAT++ is defined for TCP.
In this specification, we describe 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.
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 Section 5 ("Experiment
Considerations"), we describe the purpose of the experiment and its
current status.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
We use the following abbreviations throughout the text and include
them here for the reader's convenience:
RCV.WND: The value included in the Receive Window field of the TCP
header (the computation of which is modified by its
specification).
SND.WND: The TCP sender's window.
cwnd: The congestion window as computed by the congestion control
algorithm running at the TCP sender.
RLWND: The window value calculated by the rLEDBAT algorithm.
fcwnd: The value that a standard-TCP receiver compliant with
[RFC 9293] calculates to set in the receive window for flow control
purposes.
RCV.HGH: The highest sequence number corresponding to a received
byte of data at one point in time.
TSV.HGH: The Timestamp Value (TSval) [RFC 7323] corresponding to the
segment in which RCV.HGH was carried at that point in time.
SEG.SEQ: The sequence number of the last received segment.
TSV.SEQ: The TSval of the last received segment.
3. Motivations for rLEDBAT
rLEDBAT enables new use cases and new deployment models, fostering
the use of LBE traffic. The following scenarios are enabled by
rLEDBAT:
Content Delivery Networks (CDNs) and more sophisticated file
distribution scenarios:
Consider the case where the source of a file to be distributed
(e.g., a software developer that wishes to distribute a software
update) would prefer to use LBE and 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 receiver of the file), but because current LEDBAT/
LEDBAT++ are sender-based algorithms, controlling the client is
not enough to enable LBE congestion control in the
communication. rLEDBAT would enable the use of an LBE traffic
class for file distribution in this setup.
Interference from proxies and other middleboxes:
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 Enhancing Proxies (PEPs) are
deployed to mitigate the effect of long 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 can then enable LBE traffic between them
and the proxy.
Receiver-defined preferences:
Frequently, the access link is the 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 use LEDBAT/LEDBAT++ for other traffic. However, this would
only affect the outgoing traffic, since both 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, the communication bottleneck for a typical "eyeball" end
user. 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) and use rLEDBAT for other background file
downloads.
4. rLEDBAT Mechanisms
rLEDBAT provides the mechanisms to implement an LBE congestion
control algorithm at the receiver end of a TCP connection. The
rLEDBAT receiver controls the sender's rate through the receive
window announced by the receiver in the TCP header.
rLEDBAT assumes that the sender is a standard-TCP sender. rLEDBAT
does not require any rLEDBAT-specific modifications to the TCP
sender. The envisioned deployment model for rLEDBAT is that the
clients implement rLEDBAT and this enables rLEDBAT in communications
with existing standard-TCP senders. In particular, the sender MUST
implement [RFC 9293] and also MUST implement the TCP Timestamps (TS)
option as defined in [RFC 7323]. Also, the sender should implement
some of the standard congestion control mechanisms, such as CUBIC
[RFC 9438] or NewReno [RFC 5681] [RFC 6582].
rLEDBAT does not define a new congestion control algorithm. The
definition of the actual LBE congestion control algorithm executed in
the rLEDBAT receiver is beyond the scope of this document. The
rLEDBAT receiver MUST use an LBE congestion control algorithm.
Because rLEDBAT assumes a standard-TCP sender, the sender will be
using a "best effort" congestion control algorithm (such as CUBIC or
NewReno). Since rLEDBAT uses 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
executed in the receiver yields a smaller window than the one
calculated by the sender. This is normally the case when the
receiver is using an LBE congestion control algorithm. The rLEDBAT
receiver SHOULD use the LEDBAT congestion control algorithm [RFC 6817]
or the LEDBAT++ congestion control algorithm [LEDBAT++]. rLEDBAT MAY
use other LBE congestion control algorithms defined elsewhere.
Irrespective of which congestion control algorithm is executed in the
receiver, a rLEDBAT connection will never be more aggressive than
standard-TCP, since it is always bounded by the congestion control
algorithm executed at the sender.
rLEDBAT is essentially composed of three types of mechanisms, namely
those that provide the means to measure the packet delay (either the
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 rate. The first two provide
input to the LBE congestion control algorithm, while the third uses
the congestion window computed by the LBE congestion control
algorithm to manipulate the receive window, as depicted in Figure 1.
+------------------------------------------+
| TCP Receiver |
| +-----------------+ |
| | +------------+ | |
| +---------------------| RTT | | |
| | | | Estimation | | |
| | | +------------+ | |
| | | | |
| | | +------------+ | |
| | +--------------| Loss, RTX | | |
| | | | | Detection | | |
| | | | +------------+ | |
| v v | | |
| +----------------+ | | |
| | LBE Congestion | | rLEDBAT | |
| | Control | | | |
| +----------------+ | | |
| | | +------------+ | |
| | | | RCV.WND | | |
| +---------------->| Control | | |
| | +------------+ | |
| +-----------------+ |
+------------------------------------------+
Figure 1: The rLEDBAT Architecture
We next describe each of the rLEDBAT components.
4.1. Controlling the Receive Window
rLEDBAT uses the TCP receive window (RCV.WND) to enable the receiver
to control the sender's rate. [RFC 9293] 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-TCP receiver compliant with [RFC 9293]
calculates to set in the receive window for flow control purposes.
We call RLWND the window value calculated by the rLEDBAT algorithm,
and we call RCV.WND the value actually included in the Receive Window
field of the TCP header. For a receiver compliant with [RFC 9293],
RCV.WND == fcwnd.
In the case of the rLEDBAT receiver, this receiver MUST NOT set the
RCV.WND to a value larger than fcwnd and SHOULD set the RCV.WND to
the minimum of RLWND and fcwnd, honoring both.
When using rLEDBAT, two congestion controllers are in action in the
flow of data from the sender to the receiver, namely the TCP
congestion control algorithm 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 cwnd and the RCV.WND to calculate the
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 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
generally be 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, 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.
In summary, the sender's window is SND.WND = min(cwnd, RLWND, fcwnd)
4.1.1. Avoiding Window Shrinking
The LEDBAT/LEDBAT++ algorithm executed in a rLEDBAT receiver
increases or decreases RLWND according to congestion signals
(variations in the estimated 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 the window" is discouraged as per
[RFC 9293], as it may cause unnecessary packet loss and performance
penalties. To be consistent with [RFC 9293], the rLEDBAT receiver
SHOULD NOT shrink the receive window.
In order to avoid window shrinking, the receiver MUST only reduce
RCV.WND by the number of bytes contained in a received data packet.
This may fall short to honor the new calculated value of the RLWND
immediately. However, the receiver SHOULD progressively reduce the
advertised RCV.WND, always honoring that the reduction is less than
or equal 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, since they are only allowed to perform at most one
multiplicative decrease per RTT.
4.1.2. Setting the Window Scale Option
The Window Scale (WS) option [RFC 7323] is a means to increase the
maximum window size permitted by the receive window. The WS option
defines a scale factor 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 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.
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, TCP's segmentation practice results in
sending full segments (i.e., segments of size equal to the MSS).
[RFC 7323], which defines the WS option, specifies that allowed values
for the WS option are between 0 and 14. Assuming 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).
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).
For the above reasons, the rLEDBAT client SHOULD set WS option values
lower than 12. Additional experimentation is required to explore the
impact of larger WS values on rLEDBAT dynamics.
Note that the recommendation for rLEDBAT to set the WS option values
to lower values does not preclude communication with servers that set
the WS option values to larger values, since WS option values are set
independently for each direction of the TCP connection.
4.2. Measuring Delays
Both LEDBAT and LEDBAT++ measure base and current delays to estimate
the queuing delay. LEDBAT uses the one-way delay, while LEDBAT++
uses the RTT. In the next sections, we describe how rLEDBAT
mechanisms enable the receiver to measure the one-way delay or the
RTT -- whichever is needed, depending on the congestion control
algorithm used.
4.2.1. Measuring RTT to Estimate the Queuing Delay
LEDBAT++ uses the RTT to estimate the queuing delay. In order to
estimate the 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.
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 queuing delay has a number of shortcomings and
difficulties, as discussed below.
The queuing delay measured using RTT also includes the 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 limitation is that the rLEDBAT
controller will also react to congestion in the reverse path
direction, resulting in an even more conservative mechanism.
In order to measure RTT, the rLEDBAT client MUST enable the TS option
[RFC 7323]. By matching the TSval carried in outgoing packets with
the Timestamp Echo Reply (TSecr) value [RFC 7323] 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, there is no data flowing from the rLEDBAT receiver
to the sender, making it impossible to match data packets with
Acknowledgment packets to measure RTT, in contrast to what is usually
done in TCP for other purposes.
Depending on the frequency of the local clock used to generate the
values included in the TS option, several packets may carry the same
TSval. If that happens, the rLEDBAT receiver will be unable to match
the different outgoing packets carrying the same TSval with the
different incoming packets also carrying the same TSecr value.
However, it is not necessary for rLEDBAT to use all packets to
estimate RTT, and sampling a subset of in-flight packets per RTT is
enough to properly assess the queuing delay. RTT MUST then be
calculated as the time since the first packet with a given TSval was
sent and the first packet that was received with the same value
contained in the TSecr. Other packets with repeated TS values SHOULD
NOT be used for RTT calculations.
Several issues must be addressed in order to avoid an artificial
increase in the observed RTT. Different issues emerge, depending on
whether the rLEDBAT-capable host is sending data packets or pure ACKs
to measure RTT. We next consider these issues separately.
4.2.1.1. Measuring RTT When Sending Pure ACKs
In this scenario, the rLEDBAT node (node A) sends a pure ACK to the
other endpoint of the TCP connection (node B), including the TS
option. Upon the reception of the TS option, host B will copy the
value of the TSval into the TSecr field of the TS option and include
that option 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. We next describe how each of these cases
is addressed.
The case where host B has no data to send when it receives the pure
Acknowledgment is expected to be rare in the rLEDBAT use
cases. rLEDBAT will be used mostly for background file transfers, so
the expected common case is that the sender will have data to send
throughout the lifetime of the communication. However, if, for
example, the file is structured in blocks of data, it may be the case
that the sender 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 discard outliers (e.g., a MIN filter
[RFC 6817] would achieve this) when measuring RTT using pure ACK
packets.
This limitation of the sender's window can come from either the TCP
congestion window in host B or the announced receive window from
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.
To address the case in which the limiting factor is the receive
window announced by rLEDBAT, the congestion control algorithm at the
receiver SHOULD discard RTT measurements during the window reduction
phase that are triggered by pure ACK packets. The rLEDBAT receiver
is aware of whether a given TSval was sent in a pure ACK packet where
the window was reduced, and if so, it can discard the corresponding
RTT measurement.
4.2.1.2. Measuring RTT When Sending Data Packets
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
Acknowledgments. According to the TS option generation rules
[RFC 7323], the value included in the TSecr for a delayed ACK is the
one in the TSval field of the earliest unacknowledged segment. This
may artificially increase the measured RTT.
If both endpoints of the connection are sending data packets,
Acknowledgments are piggybacked 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.
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. This is because delayed ACKs are generated
every second data packet received and not delayed more than 500 ms
according to [RFC 9293]. The rLEDBAT receiver MAY discard RTT
measurements done using data packets from the rLEDBAT receiver and
matching pure ACKs, especially if it has recent measurements done
using other packet combinations. Applying a filter (e.g., a MIN
filter) that discards outliers would also address this issue.
4.2.2. Measuring One-Way Delay to Estimate the Queuing Delay
The LEDBAT algorithm uses the one-way delay of packets as input. A
TCP receiver can measure the delay of incoming packets directly (as
opposed to the sender-based LEDBAT, where the receiver measures the
one-way delay and needs to convey it to the sender).
In the case of TCP, the receiver can use the TS option to measure the
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 [RFC 6817], the clock offset between the sender's clock and the
receiver's clock does not affect the LEDBAT operation, since LEDBAT
uses the difference between the base one-way delay and the current
one-way delay to estimate the queuing delay, effectively "canceling
out" the clock offset error in the queuing delay estimation. There
are, however, two other issues that the rLEDBAT receiver needs to
take into account in order to properly estimate the one-way delay,
namely the units in which the received timestamps are expressed and
the clock skew. These issues are addressed below.
In order to measure the one-way delay using TCP timestamps, the
rLEDBAT receiver first needs to discover the units of values in the
TS option 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 [RFC 6817].
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 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 minimal delay (i.e.,
queuing delay close to zero) to make the estimations.
An additional difficulty regarding the estimation of the TS units and
clock skew in the context of (r)LEDBAT is that the LEDBAT congestion
controller actions directly affect the (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 of whether the sender is injecting traffic at any point in
time; it is therefore unable to use these quiet intervals to perform
measurements. The receiver can, however, force periodic slowdowns,
reducing the announced receive window to a few packets and performing
the measurements at that time.
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.
4.3. Detecting Packet Losses and Retransmissions
The rLEDBAT receiver is capable of detecting retransmitted packets 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), and
we call TSV.HGH the TSval corresponding to the segment in which that
byte was carried. SEG.SEQ stands for the sequence number of a newly
received segment, and we call TSV.SEQ the TSval of the newly received
segment.
If SEG.SEQ < RCV.HGH and TSV.SEQ > TSV.HGH, then the newly received
segment is a retransmission. This is so because the newly received
segment was generated later than another already-received segment
that contained data with a larger sequence number. This means that
this segment was lost and was retransmitted.
The proposed mechanism to detect retransmissions at the receiver
fails when there are window tail drops. If all packets in the tail
of the window are lost, the receiver will not be able to detect a
mismatch between the sequence numbers of the packets and the order of
the timestamps. In this case, rLEDBAT will not react to losses;
however, the TCP congestion controller at the sender will, most
likely reducing its window to 1 MSS and taking over the control of
the sending rate until slow start ramps up and catches the current
value of the rLEDBAT window.
5. Experiment Considerations
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:
* Interaction between the sender's and receiver's congestion control
algorithms. rLEDBAT posits that because the rLEDBAT receiver is
using a less-than-best-effort congestion control algorithm, the
receiver's congestion control algorithm will expose a smaller
congestion window (conveyed through the 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, Bottleneck Bandwidth and
Round-trip propagation time (BBR), and LEDBAT(++).
* 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).
* 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.
5.1. Status of the Experiment at the Time of This Writing
Currently, the following implementations of rLEDBAT can be used for
experimentation:
* Windows 11. rLEDBAT is available in Microsoft's Windows 11 22H2
since October 2023 [Windows11].
* Windows Server 2022. rLEDBAT is available in Microsoft's Windows
Server 2022 since September 2022 [WindowsServer].
* Apple. rLEDBAT is available in macOS and iOS since 2021 [Apple].
* Linux implementation, open source, available since 2022
[rledbat_module].
* ns3 implementation, open source, available since 2020
[rLEDBAT-in-ns-3].
In addition, rLEDBAT has been deployed by Microsoft at wide scale in
the following services:
* BITS (Background Intelligent Transfer Service)
* DO (Delivery Optimization) service
* Windows update: using DO
* Windows Store: using DO
* OneDrive
* Windows Error Reporting: wermgr.exe; werfault.exe
* System Center Configuration Manager (SCCM)
* Windows Media Player
* Microsoft Office
* Xbox (download games): using DO
Some initial experiments involving rLEDBAT have been reported in
[COMNET3]. Experiments involving the interaction between LEDBAT++
and BBR are presented in [COMNET2]. An experimental evaluation of
the LEDBAT++ algorithm is presented in [COMNET1]. As LEDBAT++ is one
of the less-than-best-effort congestion control algorithms that
rLEDBAT relies on, the results regarding how LEDBAT++ interacts with
other congestion control algorithms are relevant for the
understanding of rLEDBAT as well.
6. Security Considerations
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 and timestamps.
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.
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 this:
* One would be to introduce an artificial delay to the packets by
either actually delaying the packets or modifying the timestamps.
This would cause the rLEDBAT receiver to believe that a queue is
building up and reduce the RCV.WND. Note that to do so, an
attacker must be on path, so if that is the case, it is probably
more direct to simply reduce the RCV.WND.
* The other option would be for the attacker to make the rLEDBAT
receiver believe that a loss has occurred. To do this, it
basically needs to retransmit an old packet (to be precise, it
needs to transmit a packet with the correct sequence number and
the 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 but also by injecting packets
with the proper sequence number in the other direction. This may
slightly expand the attack surface.
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
8.2. Informative References
[Apple] Cheshire, S. and V. Goel, "Reduce network delays for your
app", Apple Worldwide Developers Conference (WWDC2021),
Video, 2021,
<https://developer.apple.com/videos/play/wwdc2021/10239/>.
[COMNET1] Bagnulo, M. and A. García-Martínez, "An experimental
evaluation of LEDBAT++", Computer Networks, vol. 212,
DOI 10.1016/j.comnet.2022.109036, July 2022,
<https://doi.org/10.1016/j.comnet.2022.109036>.
[COMNET2] Bagnulo, M. and A. García-Martínez, "When less is more:
BBR versus LEDBAT++", Computer Networks, vol. 219,
DOI 10.1016/j.comnet.2022.109460, December 2022,
<https://doi.org/10.1016/j.comnet.2022.109460>.
[COMNET3] Bagnulo, M., García-Martínez, A., Mandalari, A.M.,
Balasubramanian, P., Havey, D., and G. Montenegro,
"Design, implementation and validation of a receiver-
driven less-than-best-effort transport", Computer
Networks, vol. 233, DOI 10.1016/j.comnet.2023.109841,
September 2023,
<https://doi.org/10.1016/j.comnet.2023.109841>.
[LEDBAT++] Balasubramanian, P., Ertugay, O., Havey, D., and M.
Bagnulo, "LEDBAT++: Congestion Control for Background
Traffic", Work in Progress, Internet-Draft, draft-irtf-
iccrg-ledbat-plus-plus-03, 9 September 2025,
<https://datatracker.ietf.org/doc/html/draft-irtf-iccrg-
ledbat-plus-plus-03>.
[RFC 5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC 5681, September 2009,
<https://www.rfc-editor.org/info/RFC 5681>.
[RFC 6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
NewReno Modification to TCP's Fast Recovery Algorithm",
RFC 6582, DOI 10.17487/RFC 6582, April 2012,
<https://www.rfc-editor.org/info/RFC 6582>.
[RFC 6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC 6817, December 2012,
<https://www.rfc-editor.org/info/RFC 6817>.
[RFC 7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC 7323, September 2014,
<https://www.rfc-editor.org/info/RFC 7323>.
[RFC 9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC 9293, August 2022,
<https://www.rfc-editor.org/info/RFC 9293>.
[RFC 9438] Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed.,
"CUBIC for Fast and Long-Distance Networks", RFC 9438,
DOI 10.17487/RFC 9438, August 2023,
<https://www.rfc-editor.org/info/RFC 9438>.
[rLEDBAT-in-ns-3]
"Implementation-of-rLEDBAT-in-ns-3", commit 2ab34ad, 24
June 2020, <https://github.com/manas11/implementation-of-
rLEDBAT-in-ns-3>.
[rledbat_module]
"rledbat_module", commit d82ff20, 9 September 2022,
<https://github.com/net-research/rledbat_module>.
[Windows11]
Microsoft, "What's new in Delivery Optimization",
Microsoft Windows Documentation, October 2024,
<https://learn.microsoft.com/en-us/windows/deployment/do/
whats-new-do>.
[WindowsServer]
Havey, D., "LEDBAT Background Data Transfer for Windows",
Microsoft Networking Blog, September 2022,
<https://techcommunity.microsoft.com/t5/networking-blog/
ledbat-background-data-transfer-for-windows/ba-p/3639278>.
Appendix A. rLEDBAT Pseudocode
In this section, we describe how to integrate the proposed rLEDBAT
mechanisms and an LBE delay-based congestion control algorithm such
as LEDBAT or LEDBAT++. We describe the integrated algorithm as two
procedures: one that is executed when a packet is received by a
rLEDBAT-enabled endpoint (Figure 2) and another that is executed when
the rLEDBAT-enabled endpoint sends a packet (Figure 3). 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.
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()
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 WindowDecrease() function, it takes as input the
current window size and returns the updated window size.
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.
The RTT values computed for each received packet are stored in the
RTTlist, which 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.
procedure receivePacket()
//Looks for first sent packet with same TSval as TSecr, and
//returns time difference
receivedRTT = computeRTT(sendList, receivedTSecr, receivedTime)
//Inserts minimum value for a given receivedTSecr
//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 - without shrinking it - if necessary
ackedBytes = ackedBytes + receiveBytes
if retransmittedPacketDetected then
RLWND = DecreaseWindow(RLWND) //Only once per RTT
end if
if qd < T then
RLWND = IncreaseWindow(RLWND, ackedBytes)
else
RLWND = DecreaseWindow(RLWND)
end if
end procedure
Figure 2: Procedure Executed When a Packet Is Received
procedure SENDPACKET
if (RLWND > RLWNDPrevious) or (RLWND - RLWNDPrevious < ackedBytes)
then
RLWNDPrevious = RLWND
else
RLWNDPrevious = RLWND - ackedBytes
end if
ackedBytes = 0
RLWNDPrevious = RLWND
//Compute the RLWND to include in the packet
RLWND = min(RLWND, fcwnd)
end procedure
Figure 3: Procedure Executed When a Packet Is Sent
Acknowledgments
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).
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 Kühlewind,
and Vidhi Goel for their reviews and comments on earlier draft
versions of this document.
Authors' Addresses
Marcelo Bagnulo
Universidad Carlos III de Madrid
Email: marcelo@it.uc3m.es
Alberto García-Martínez
Universidad Carlos III de Madrid
Email: alberto@it.uc3m.es
Gabriel Montenegro
Email: g.e.montenegro@hotmail.com
Praveen Balasubramanian
Confluent
Email: pravb.ietf@gmail.com
RFC TOTAL SIZE: 48230 bytes
PUBLICATION DATE: Thursday, September 18th, 2025
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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