Network Working Group M. Bagnulo
Internet-Draft A. Garcia-Martinez
Intended status: Experimental Universidad Carlos III de Madrid
Expires: 6 August 2025 G. Montenegro
P. Balasubramanian
Confluent
2 February 2025
rLEDBAT: receiver-driven Low Extra Delay Background Transport for TCP
draft-irtf-iccrg-rledbat-10
Abstract
This document specifies 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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 6 August 2025.
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. Code Components
Bagnulo, et al. Expires 6 August 2025 [Page 1]
Internet-Draft rLEDBAT February 2025
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Motivations for rLEDBAT . . . . . . . . . . . . . . . . . . . 3
3. rLEDBAT mechanisms . . . . . . . . . . . . . . . . . . . . . 4
3.1. Controlling the receive window . . . . . . . . . . . . . 6
3.1.1. Avoiding window shrinking . . . . . . . . . . . . . . 7
3.1.2. Setting the Window Scale Option . . . . . . . . . . . 8
3.2. Measuring delays . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Measuring RTT to estimate the queueing delay . . . . 9
3.2.2. Measuring one way delay to estimate the queueing
delay . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Detecting packet losses and retransmissions . . . . . . . 13
4. Experiment Considerations . . . . . . . . . . . . . . . . . . 13
4.1. Status of the experiment at the time of this writing. . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
8. Informative References . . . . . . . . . . . . . . . . . . . 16
Appendix A. Terminology . . . . . . . . . . . . . . . . . . . . 17
Appendix B. rLEDBAT pseudo-code . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction
LEDBAT (Low Extra Delay Background Transport) [RFC6817] 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[RFC9438], hereafter referred 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 an efficient use of the
available capacity, while keeping the queuing delay within predefined
bounds.
LEDBAT reacts both to packet loss and to variations in delay. With
respect to packet loss, LEDBAT reacts with a multiplicative decrease,
similar to most TCP congestion controllers. Regarding delay, LEDBAT
aims for a target queueing delay. When the measured current queueing
delay is below the target, LEDBAT increases the sending rate and when
Bagnulo, et al. Expires 6 August 2025 [Page 2]
Internet-Draft rLEDBAT February 2025
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 [RFC6817] defines the LEDBAT congestion-
control algorithm, implemented in the sender to control its sending
rate. LEDBAT is specified in a protocol and layer agnostic manner.
LEDBAT++ [I-D.irtf-iccrg-ledbat-plus-plus] is also an LBE congestion
control algorithm which is inspired by LEDBAT while addressing
several problems identified with the original LEDBAT specification.
In particular the differences between LEDBAT and LEDBAT++ include: i)
LEDBAT++ uses the round-trip-time (RTT) (as opposed to the one way
delay used in LEDBAT) to estimate the queuing delay; ii) LEDBAT++
uses an Additive Increase/Multiplicative Decrease algorithm to
achieve inter-LEDBAT++ fairness and avoid the late-comer advantage
observed in LEDBAT; iii) LEDBAT++ performs periodic slowdowns to
improve the measurement of the base delay; iv) LEDBAT++ is defined
for TCP.
In this specification, we describe 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 a standard. The status of this document is
experimental. In section 4 titled Experiment Considerations, we
describe the purpose of the experiment and its current status.
2. 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 and more sophisticated file distribution
scenarios: Consider the case where the source of a file to be
distributed (e.g., a software developer that wishes to distribute
a software update) would prefer to use LBE and it enables LEDBAT/
LEDBAT++ in the servers containing the source file. However,
because the file is being distributed through a CDN 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
Bagnulo, et al. Expires 6 August 2025 [Page 3]
Internet-Draft rLEDBAT February 2025
developer may also control the software performing the download in
the client, the receiver of the file, but because current LEDBAT/
LEDBAT++ are sender-based algorithms, controlling the client is
not enough to enable LBE congestion control in the communication.
rLEDBAT would enable the use of LBE traffic class for file
distribution in this setup.
Interference from proxies and other middleboxes: Proxies and other
middleboxes are commonplace in the Internet. For instance, in the
case of mobile networks, proxies are frequently used. In the case
of enterprise networks, it is common to deploy corporate proxies
for filtering and firewalling. In the case of satellite links,
Performance Enhancement Proxies (PEPs) are deployed to mitigate
the effect of the long delay in TCP connection. These proxies
terminate the TCP connection on both ends and prevent the use of
LBE congestion control in the segment between the proxy and the
sink of the content, the client. By enabling rLEDBAT, clients
would be able to enable LBE traffic between them and the proxy.
Receiver-defined preferences. It is frequent that the bottleneck
of the communication is the access link. This is particularly
true in the case of mobile devices. It is then especially
relevant for mobile devices to properly manage the capacity of the
access link. With current technologies, it is possible for the
mobile device to use different congestion control algorithms
expressing different preferences for the traffic. For instance, a
device can choose to use standard-TCP for some traffic and to use
LEDBAT/LEDBAT++ for other traffic. However, this would only
affect the outgoing traffic since both standard-TCP and LEDBAT/
LEDBAT++ are sender-driven. The mobile device has no means to
manage the traffic in the down-link, which is in most cases, the
communication bottleneck for a typical eye-ball end-user. rLEDBAT
enables the mobile device to selectively use LBE traffic class for
some of the incoming traffic. For instance, by using rLEDBAT, a
user can use regular standard-TCP/UDP for video stream (e.g.,
Youtube) and use rLEDBAT for other background file download.
3. rLEDBAT mechanisms
rLEDBAT provides the mechanisms to implement an LBE congestion
control algorithm at the receiver-end of a TCP connection. The
rLEDBAT receiver controls the sender's rate through the Receive
Window announced 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
Bagnulo, et al. Expires 6 August 2025 [Page 4]
Internet-Draft rLEDBAT February 2025
with existent standard TCP senders. In particular, the sender MUST
implement [RFC9293] and it also MUST implement the Time Stamp Option
as defined in [RFC7323]. Also, the sender should implement some of
the standard congestion control mechanisms, such as Cubic [RFC9438]
or New Reno [RFC5681].
rLEDBAT does not define a new congestion control algorithm. The LBE
congestion control algorithm executed in the rLEDBAT receiver is
defined in other documents. The rLEDBAT receiver MUST use an LBE
congestion control algorithm. Because rLEDBAT assumes a standard TCP
sender, the sender will be using a "best effort" congestion control
algorithm (such as Cubic or New Reno). Since rLEDBAT uses the
Receive Window to control the sender's rate and the sender calculates
the sender's window as the minimum of the Receive window and the
congestion window, rLEDBAT will only be effective as long as the
congestion control algorithm executed in the receiver yields a
smaller window than the one calculated by the sender. This is
normally the case when the receiver is using an LBE congestion
control algorithm. The rLEDBAT receiver SHOULD use the LEDBAT
congestion control algorithm [RFC6817] or the LEDBAT++ congestion
control algorithm [I-D.irtf-iccrg-ledbat-plus-plus]. The rLEDBAT MAY
use other LBE congestion 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 congestion control
algorithm executed at the sender.
rLEDBAT is essentially composed of three types of mechanisms, namely,
those that provide the means to measure the packet delay (either the
round trip time or the one way delay, depending on the selected
algorithm), mechanisms to detect packet loss and the means to
manipulate the Receive Window to control the sender's rate. The
former provide input to the LBE congestion control algorithm while
the latter uses the congestion window computed by the LBE congestion
control algorithm to manipulate the Receive window, as depicted in
the figure.
Bagnulo, et al. Expires 6 August 2025 [Page 5]
Internet-Draft rLEDBAT February 2025
+------------------------------------------+
| TCP receiver |
| +-----------------+ |
| | +------------+ | |
| +---------------------| RTT | | |
| | | | Estimation | | |
| | | +------------+ | |
| | | | |
| | | +------------+ | |
| | +--------------| Loss, RTX | | |
| | | | | Detection | | |
| | | | +------------+ | |
| v v | | |
| +----------------+ | | |
| | LBE Congestion | | rLEDBAT | |
| | Control | | | |
| +----------------+ | | |
| | | +------------+ | |
| | | | RCV-WND | | |
| +---------------->| Control | | |
| | +------------+ | |
| +-----------------+ |
+------------------------------------------+
Figure 1: The rLEDBAT architecture.
We describe each of the rLEDBAT components next.
3.1. Controlling the receive window
rLEDBAT uses the Receive Window (RCV.WND) of TCP to enable the
receiver to control the sender's rate. [RFC9293] defines that the
RCV.WND is used to announce the available receive buffer to the
sender for flow control purposes. In order to avoid confusion, we
will call fcwnd the value that a standard RFC793bis TCP receiver
calculates to set in the receive window for flow control purposes.
We call RLWND the window value calculated by rLEDBAT algorithm and we
call RCV.WND the value actually included in the Receive Window field
of the TCP header. For a RFC793bis receiver, RCV.WND == fcwnd.
In the case of rLEDBAT receiver, the rLEDBAT receiver MUST NOT set
the RCV.WND to a value larger than fcwnd and it 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 congestion
control algorithm of TCP in the sender side and the LBE congestion
control algorithm executed in the receiver and conveyed to the sender
Bagnulo, et al. Expires 6 August 2025 [Page 6]
Internet-Draft rLEDBAT February 2025
through the RCV.WND. In the normal TCP operation, the sender uses
the minimum of the congestion window cwnd and the receiver window
RCV.WND to calculate the sender's window SND.WND. This is also true
for rLEDBAT, as the sender is a regular TCP sender. This guarantees
that the rLEDBAT flow will never transmit more aggressively than a
standard-TCP flow, as the sender's congestion window limits the
sending rate. Moreover, because a 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 RCV.WND field of TCP will be in general
smaller than the congestion window calculated by the TCP sender,
implying that the rLEDBAT congestion control algorithm will be
effectively controlling the sender's window. One exception to this
is at the beginning of the connection, when there is no information
to set RLWND, then, RLWND is set to its maximum value, so that the
sending rate of the sender is governed by the flow control algorithm
of the receiver and the TCP slow start mechanism of the sender.
In summary, the sender's window is: SND.WND = min(cwnd, RLWND, fcwnd)
3.1.1. Avoiding window shrinking
The LEDBAT/LEDBAT++ algorithm executed in a rLEDBAT receiver
increases or decreases RLWND according to congestion signals
(variations on the estimated queueing 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
[RFC9293], as it may cause unnecessary packet loss and performance
penalty. To be consistent with [RFC9293], 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 upon of 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 or
equal than the received bytes, until the target window determined by
the rLEDBAT algorithm is reached. This implies that it may take up
to one RTT for the rLEDBAT receiver to drain enough in-flight bytes
to completely close its receive window without shrinking it. This is
sufficient to honor the window output from the LEDBAT/LEDBAT++
algorithms since they only allow to perform at most one
multiplicative decrease per RTT.
Bagnulo, et al. Expires 6 August 2025 [Page 7]
Internet-Draft rLEDBAT February 2025
3.1.2. Setting the Window Scale Option
The Window Scale (WS) option [RFC7323] is a means to increase the
maximum window size permitted by the Receive Window. The WS option
defines a scale factor which 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 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 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). Current WS option specification
[RFC7323] defines that allowed values for the WS option are between 0
and 14. Assuming a MSS around 1500 bytes, WS option values between 0
and 11 result in the receive window being expressed in units that are
about 1 MSS or smaller. So, WS option values between 0 and 11 have
no impact in rLEDBAT (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 WS of 14, a change in
one unit of the receive window implies a change of 10 MSS in the
effective window).
For the above reasons, the rLEDBAT client SHOULD set WS option values
lower than 12. Additional experimentation is required to explore the
impact of larger WS values on rLEDBAT dynamics.
Note that the recommendation for rLEDBAT to set the WS option value
to lower values does not precludes the communication with servers
that set the WS option values to larger values, since the WS option
value is set independently for each direction of the TCP connection.
3.2. Measuring delays
Both LEDBAT and LEDBAT++ measure base and current delays to estimate
the queueing delay. LEDBAT uses the one way delay while LEDBAT++
uses the round trip time. In the next sections we describe how
rLEDBAT mechanisms enable the receiver to measure the one way delay
or the round trip time, whatever is needed depending on the
congestion control algorithm used.
Bagnulo, et al. Expires 6 August 2025 [Page 8]
Internet-Draft rLEDBAT February 2025
3.2.1. Measuring RTT to estimate the queueing delay
LEDBAT++ uses the round trip time (RTT) to estimate the queueing
delay. In order to estimate the queueing delay using 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 queueing delay has a number of shortcomings and
difficulties that we discuss next.
The queuing delay measured using RTT includes also the queueing delay
experienced by the return packets in the direction from the rLEDBAT
receiver to the sender. This is a fundamental limitation of this
approach. The impact of this error is that the rLEDBAT controller
will also react to congestion in the reverse path direction which
results in an even more conservative mechanism.
In order to measure RTT, the rLEDBAT client MUST enable the Time
Stamp (TS) option [RFC7323]. By matching the TSVal value carried in
outgoing packets with the TSecr value observed in incoming packets,
it is possible to measure 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 impossible to match data packets with acknowledgements
packets to measure RTT, as it is usually done in TCP for other
purposes.
Depending on the frequency of the local clock used to generate the
values included in the TS option, several packets may carry the same
TSVal value. If that happens, the rLEDBAT receiver will be unable to
match the different outgoing packets carrying the same TSVal value
with the different incoming packets carrying also the same TSecr
value. However, it is not necessary for rLEDBAT to use all packets
to estimate RTT and sampling a subset of in-flight packets per RTT is
enough to properly assess the queueing 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 calculation.
Bagnulo, et al. Expires 6 August 2025 [Page 9]
Internet-Draft rLEDBAT February 2025
Several issues must be addressed in order to avoid an artificial
increase of the observed RTT. Different issues emerge depending
whether the rLEDBAT capable host is sending data packets or pure ACKs
to measure RTT. We next consider the issues separately.
3.2.1.1. Measuring RTT sending pure ACKs
In this scenario, the rLEDBAT node (node A) sends a pure ACK to the
other endpoint of the TCP connection (node B), including the TS
option. Upon the reception of the TS Option, host B will copy the
value of the TSVal into the TSecr field of the TS option and include
that option into the next data packet towards host A. However, there
are two reasons why B may not send a packet immediately back to A,
artificially increasing the measured RTT. The first reason is when A
has no data to send. The second is when A has no available window to
put more packets in-flight. We describe next how each of these cases
is addressed.
The case where the host B has no data to send when it receives the
pure Acknowledgement is expected to be rare in the rLEDBAT use cases.
rLEDBAT will be used mostly for background file transfers so the
expected common case is that the sender will have data to send
throughout the lifetime of the communication. However, if, for
example, the file is structured in blocks of data, it may be the case
that the sender seldomly will 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
would achieve this) when measuring RTT using pure ACK packets.
This limitation of the sender's window can come either from the TCP
congestion window in host B or from the announced receive window from
the rLEDBAT in host A. Normally, the receive window will be the one
to limit the sender's transmission rate, since the LBE congestion
control algorithm used by the rLEDBAT node is designed to be more
restrictive on the sender's rate than standard-TCP. If the limiting
factor is the congestion window in the sender, it is less relevant if
rLEDBAT further reduces the receive window due to a bloated RTT
measurement, since the rLEDBAT node is not actively controlling the
sender's rate. Nevertheless, the proposed approach to discard larger
samples would also address this issue.
Bagnulo, et al. Expires 6 August 2025 [Page 10]
Internet-Draft rLEDBAT February 2025
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 whether a given TSVal value was sent in a pure ACK packet
where the window was reduced, and if so, it can discard the
corresponding RTT measurement.
3.2.1.2. 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
Acknowledgements. According to the TS option generation rules
[RFC7323], 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 into 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. The reason for this is that delayed ACKs are
generated every second data packet received and not delayed more than
500 ms according to [RFC9293]. The rLEDBAT receiver MAY discard RTT
measurements done using data packets from the rLEBDAT receiver and
matching pure ACKs, especially if it has recent measurements done
using other packet combinations. Also, applying a filter that
discards outliers would also address this issue (e.g. a min filter).
3.2.2. Measuring one way delay to estimate the queueing delay
The LEDBAT algorithm uses the one-way delay of packets as input. A
TCP receiver can measure the delay of incoming packets directly (as
opposed to the sender-based LEDBAT, where the receiver measures the
one-way delay and needs to convey it to the sender).
In the case of TCP, the receiver can use the TimeStamp 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 [RFC6817] the clock offset between the clock of
Bagnulo, et al. Expires 6 August 2025 [Page 11]
Internet-Draft rLEDBAT February 2025
the sender and the clock in the receiver does not affect the LEDBAT
operation, since LEDBAT uses the difference between the base one way
delay and the current one way delay to estimate the queuing delay,
effectively canceling the clock offset error in the queueing delay
estimation. There are however two other issues that the rLEDBAT
receiver needs to take into account in order to properly estimate the
one way delay, namely, the units in which the received timestamps are
expressed and the clock skew. We address them next.
In order to measure the one way delay using TCP timestamps, the
rLEDBAT receiver, first, needs to discover the units of values in the
TS option and, second, 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 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
[RFC6817].
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 error in the unit/skew estimation. One possible
approach is to select packets that experienced the minimum delay
(i.e. close to zero queueing delay) to make the estimations.
An additional difficulty regarding the estimation of the TS units and
clock skew in the context of (r)LEDBAT is that the LEDBAT congestion
controller actions directly affect the (queueing) delay experienced
by packets. In particular, if there is an error in the estimation of
the TS units/skew, the LEDBAT controller will attempt to compensate
it by reducing/increasing the load. The result is that the LEDBAT
operation interferes with the TS units/clock skew measurements.
Because of this, measurements are more accurate when there is no
traffic in the connection (in addition to the packets used for the
measurements). The problem is that the receiver is unaware if the
sender is injecting traffic at any point in time, and so, it is
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 perform the
measurements then.
Bagnulo, et al. Expires 6 August 2025 [Page 12]
Internet-Draft rLEDBAT February 2025
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.
3.3. Detecting packet losses and retransmissions
The rLEDBAT receiver is capable of detecting retransmitted packets in
the following way. We call RCV.HGH the highest sequence number
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 value corresponding to the
segment in which that byte was carried. SEG.SEQ stands for the
sequence number of a newly received segment and we call TSV.SEQ the
TSVal value of the newly received segment.
If SEG.SEQ < RCV.HGH and TSV.SEQ > TSV.HGH then the newly received
segment is a retransmission. This is so because the newly received
segment was generated later than another already received segment
which contained data with a larger sequence number. This means that
this segment was lost and was retransmitted.
The proposed mechanism to detect retransmissions at the receiver
fails when there are window tail drops. If all packets in the tail
of the window are lost, the receiver will not be able to detect a
mismatch between the sequence numbers of the packets and the order of
the timestamps. In this case, rLEDBAT will not react to losses but
the TCP congestion controller at the sender will, most likely
reducing its window to 1MSS and take over the control of the sending
rate, until slow start ramps up and catches the current value of the
rLEDBAT window.
4. 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:
Bagnulo, et al. Expires 6 August 2025 [Page 13]
Internet-Draft rLEDBAT February 2025
- Interaction between the sender and the receiver Congestion
control algorithms. rLEDBAT posits that because the rLEDBAT
receiver is using a less-than-best-effort congestion control
algorithm, the receiver congestion control algorithm will expose a
smaller congestion window (conveyed though the Receive 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 interact and if the assumption that the
receiver side is always controlling the sender's rate (and making
rLEDBAT effective) holds. The experiment should include the
different congestion control algorithms that are currently widely
used in the Internet, including Cubic, BBR and LEDBAT(++).
- Interaction between rLEDBAT and Active Queue Management
techniques such as Codel, PIE and L4S.
- How the rLEDBAT should resume after a period during which there
was no incoming traffic and the information about the rLEDBAT
state information is potentially dated.
4.1. Status of the experiment at the time of this writing.
Currently there are the following implementations of rLEDBAT that 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 at
https://github.com/net-research/rledbat_module.
- ns3 implementation, open source, available since 2020 at
https://github.com/manas11/implementation-of-rLEDBAT-in-ns-3.
In addition, rLEDBAT has been deployed by Microsoft in wide scale in
the following services:
- BITS (Background Intelligent Transfer Service)
- DO (Delivery Optimization) service
- Windows update # using DO
Bagnulo, et al. Expires 6 August 2025 [Page 14]
Internet-Draft rLEDBAT February 2025
- 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 of 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 LEDBAT++ interaction with other
congestion control algorithms are relevant for the understanding of
rLEDBAT as well.
5. 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 that. One would be to
introduce an artificial delay to the packets either by 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 an attacker to do that 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 that, it basically
needs to retransmit an old packet (to be precise, it needs to
transmit a packet with the right sequence number and the right port
and IP numbers). This means that the attacker can achieve a
Bagnulo, et al. Expires 6 August 2025 [Page 15]
Internet-Draft rLEDBAT February 2025
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.
6. IANA Considerations
No actions are required from IANA.
7. Acknowledgements
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
Kuehlewind, and Vidhi Goel for their reviews and comments on earlier
versions of this document.
8. Informative References
[Apple] Stuart, S.C. and V.G. Vidhi, "Reduce network delays for
your app", WWDC21 https://developer.apple.com/videos/play/
wwdc2021/10239/, 2021.
[COMNET1] Bagnulo, M.B. and A.G. Garcia-Martinez, "An experimental
evaluation of LEDBAT++", Computer Networks Volume 212,
2022.
[COMNET2] Bagnulo, M.B. and A.G. Garcia-Martinez, "When less is
more: BBR versus LEDBAT++", Computer Networks Volume 219,
2022.
[COMNET3] Bagnulo, M.B., Garcia-Martinez, A.G., Mandalari, A.M.,
Balasubramanian, P.B,., Havey, D.H., and G.M. Montenegro,
"Design, implementation and validation of a receiver-
driven less-than-best-effort transport", Computer
Networks Volume 233, 2022.
Bagnulo, et al. Expires 6 August 2025 [Page 16]
Internet-Draft rLEDBAT February 2025
[I-D.irtf-iccrg-ledbat-plus-plus]
Balasubramanian, P., Ertugay, O., and D. Havey, "LEDBAT++:
Congestion Control for Background Traffic", Work in
Progress, Internet-Draft, draft-irtf-iccrg-ledbat-plus-
plus-01, 25 August 2020,
<https://datatracker.ietf.org/doc/html/draft-irtf-iccrg-
ledbat-plus-plus-01>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012,
<https://www.rfc-editor.org/info/rfc6817>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/info/rfc9293>.
[RFC9438] Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed.,
"CUBIC for Fast and Long-Distance Networks", RFC 9438,
DOI 10.17487/RFC9438, August 2023,
<https://www.rfc-editor.org/info/rfc9438>.
[Windows11]
Forsmann, C.F., "What's new in Delivery Optimization",
Microsoft Documentation https://learn.microsoft.com/en-
us/windows/deployment/do/whats-new-do, 2023.
[WindowsServer]
Havey, D.H., "LEDBAT Background Data Transfer for
Windows", Microsoft Blog
https://techcommunity.microsoft.com/t5/networking-
blog/ledbat-background-data-transfer-for-windows/ba-
p/3639278, 2022.
Appendix A. Terminology
We use the following abreviations thoughout the text. We include a
short list for the reader's convenence:
Bagnulo, et al. Expires 6 August 2025 [Page 17]
Internet-Draft rLEDBAT February 2025
RCV.WND: the value included in the Receive Window field of the TCP
header (which computation is modified by this specification)
SND.WND: The TCP sender's window
cwnd: the consgestion window as computed by the congestion control
algorithm running at the TCP sender.
RLWND: the window value calculated by rLEDBAT algorithm
fcwnd: the value that a standard RFC793bis TCP receiver 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: TSV.HGH the TSVal value 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 value of the last received segment
Appendix B. rLEDBAT pseudo-code
We next 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(), we
assume that it takes as input the current window size and the number
of bytes that were acknowledged since the last window update
(ackedBytes) and returns as output the updated window size. In the
case of WindowDecrease(), it takes as input the current window size
and returns the updated window size.
Bagnulo, et al. Expires 6 August 2025 [Page 18]
Internet-Draft rLEDBAT February 2025
The data structures used in the algorithms are as follows. The
sentList is a list that contains the TSval and the local send time of
each packet sent by the rLEDBAT-enabled endpoint. The TSecr field of
the packets received by the rLEDBAT-enabled endpoint are matched with
the sendList to compute the RTT.
The RTT values computed for each received packet are stored in the
RTTlist, which contains also 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(sentList, 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
Bagnulo, et al. Expires 6 August 2025 [Page 19]
Internet-Draft rLEDBAT February 2025
procedure SENDPACKET
if (RLWND > RLWNDPrevious) or (RLWND - RLWNDPrevious < ackedBytes)
then
RLWNDPrevious = RLWND
else
RLWNDPrevious = RLWND - ackedBytes
end if
ackedBytes = 0
RLWNDPrevious = RLWND
//Compute the RWND to include in the packet
RLWND = min(RLWND, fcwnd)
end procedure
Figure 3: Procedure executed when a packet is sent
Authors' Addresses
Marcelo Bagnulo
Universidad Carlos III de Madrid
Email: marcelo@it.uc3m.es
Alberto Garcia-Martinez
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
Bagnulo, et al. Expires 6 August 2025 [Page 20]