Network Working Group M. Bagnulo Internet-Draft A. Garcia-Martinez Intended status: Experimental UC3M Expires: January 9, 2020 G. Montenegro P. Balasubramanian Microsoft July 8, 2019 rLEDBAT: receiver-driven Low Extra Delay Background Transport for TCP draft-bagnulo-iccrg-rledbat-00.txt Abstract This document specifies the rLEDBAT, a receiver-driven, less-than- best-effort congestion control algorithm for TCP. 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 January 9, 2020. Copyright Notice Copyright (c) 2019 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 extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Bagnulo, et al. Expires January 9, 2020 [Page 1] Internet-Draft rLEDBAT July 2019 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Motivations for rLEDBAT . . . . . . . . . . . . . . . . . . . 3 3. rLEDBAT overview . . . . . . . . . . . . . . . . . . . . . . 4 4. rLEDBAT design rationale . . . . . . . . . . . . . . . . . . 5 4.1. Controlling the receive window . . . . . . . . . . . . . 5 4.1.1. Avoiding window shrinking . . . . . . . . . . . . . . 6 4.1.2. Window Scale Option . . . . . . . . . . . . . . . . . 6 4.2. Using the RTT to estimate the queueing delay . . . . . . 7 4.3. Inter-rLEDBAT fairness . . . . . . . . . . . . . . . . . 9 4.4. Reacting to packet loss . . . . . . . . . . . . . . . . . 9 4.5. Bootstrapping . . . . . . . . . . . . . . . . . . . . . . 10 4.6. Reaction to path changes . . . . . . . . . . . . . . . . 11 5. rLEDBAT algorithm . . . . . . . . . . . . . . . . . . . . . . 11 5.1. Data structures . . . . . . . . . . . . . . . . . . . . . 11 5.2. Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 12 5.3. rLEDBAT parameters . . . . . . . . . . . . . . . . . . . 13 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 9. Informative References . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction LEDBAT (Low Extra Delay Background Transport) [RFC6817] is a congestion-control algorithm that implements a less-than-best-effort (LBE) traffic class. When LEDBAT traffic shares a bottleneck with one or more TCP connections using standard congestion control algorithms such as Cubic [RFC8312] (hereafter standard-TCP for short), it reduces its sending rate earlier and more aggressively than standard-TCP congestion control, allowing standard-TCP traffic to use more of the available capacity. In the absence of competing standard-TCP traffic, LEDBAT aims to make an efficient use of the available capacity, while keeping the queuing delay within predefined bounds. LEDBAT reacts both to packet loss and to variations in delay. Regarding to packet loss, LEDBAT reacts with a multiplicative decrease, similar to most TCP congestion controllers. Regarding delay, LEDBAT aims for a target queueing delay. When the measured current queueing delay is below the target, LEDBAT increases the sending rate and when the delay is above the target, it reduces the sending rate. LEDBAT estimates the queuing delay by subtracting the measured current one-way delay from the estimated base one-way delay (i.e. the one way delay in the absence of queues). Bagnulo, et al. Expires January 9, 2020 [Page 2] Internet-Draft rLEDBAT July 2019 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. In this document, we describe rLEDBAT, a receiver-based, less-than- best-effort congestion control algorithm. rLEDBAT is inspired in LEDBAT but with the following differences: rLEDBAT is implemented in the TCP receiver and controls the sending rate of the sender through the TCP Receiver Window. rLEDBAT uses the round-trip-time (RTT) to estimate the queuing delay. rLEDBAT uses an Additive Increase/Multiplicative Decrease algorithm to achieve inter-(r)LEDBAT fairness and avoid the late- comer advantage observed in LEDBAT. 2. Motivations for rLEDBAT rLEDBAT enables new use cases and new deployment models, fostering the use of LBE traffic and benefitting the global Internet by improving overall allocation of resources. The following scenarios are enabled by rLEDBAT: Content Delivery Networks and more sophisticated file distribution scenarios: Consider the case where the source of a file to be distributed (e.g., a software developer that wishes to distribute a software update) would prefer to use LEDBAT and it enables LEDBAT in the servers containing the source file. However, because the file is being distributed through a CDN which surrogates do not support LEDBAT, the result is that the file transfers, originated from CDN surrogates will not be using LEDBAT. Interestingly enough, in the case of the software update, the developer also controls the software performing the download in the client, the receiver of the file, but because current LEDBAT is a sender-based algorithm, controlling the client is not enough to enable LEDBAT in the communication. rLEDBAT would enable the use of LBE traffic class for file distribution in this setup. Interference from proxies and other middleboxes: Proxies and other middleboxes are a commonplace in the Internet. For instance, in the case of mobile networks, proxies are frequently used. In the case of enterprise networks, it is common to deploy corporate proxies for filtering and firewalling. In the case of satellite 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 Bagnulo, et al. Expires January 9, 2020 [Page 3] Internet-Draft rLEDBAT July 2019 use of LEDBAT 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 for other traffic. However, this would only affect the outgoing traffic since both standard-TCP and LEDBAT are sender- driven. The mobile device has no means to manage the traffic in the down-link, which is in most cases, the most critical hop 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 overview rLEDBAT is a congestion control mechanism implemented at the receiver-end of a TCP connection. The rLEDBAT receiver controls the sender's rate through the Receive Window announced to the receiver in the TCP header. rLEDBAT implements an Additive Increase/Multiplicative decrease that reacts to both delay and packet loss. Similarly to LEDBAT, rLEDBAT limits the queueing delay in the path to a target delay T. rLEDBAT uses the RTT to estimate the queueing delay. The rLEDBAT receiver uses the TCP TimeStamp option to measure the RTT. rLEDBAT estimates the Base RTT (i.e. the RTT when there is no queuing delay) as the minimum observed RTT in the last n minutes. rLEDBAT estimation of the queuing delay (qd) is obtained subtracting the Base RTT from latest sample(s) of the RTT. The rLEDBAT algorithm at the receiver calculates a window value (rl.WND) which is then conveyed to the sender though the RECEIVE WINDOW field of the TCP header. We describe next how rl.WND value is calculated. Suppose that the rl.WND was last updated at time t0 and its current value is then rl.WND(t0) and at time t1 a packet is received. The rLEDBAT receiver updates rl.WND as follows: Bagnulo, et al. Expires January 9, 2020 [Page 4] Internet-Draft rLEDBAT July 2019 if qd < T, then rl.WND(t1) = rl.WND(t0) + alpha*MSS/rl.WND(t0) if qd > T, then rl.WND(t1) = rl.WND(t0)*betad with MSS being the Maximum Segment Size of the TCP connection, and alpha and betad being the additive increase and multiplicative decrease parameters respectively. This base algorithm results that while the queueing delay is below the target T, the congestion window increases by alpha * MSS$ per RTT, (with alpha > 1) while if the queueing delay is above the target T, the congestion control window is multiplied by betad, with 0 < betad < 1. The multiplicative reduction is applied at most one per RTT. rLEDBAT also performs a multiplicative decrease (with parameter betal) in case there is packet loss. Packet loss are detected at the receiver through the observation of retransmitted packets. Retransmissions of the sender are detected at the receiver by observing the sequence number of the segment and the timestamp value, as we describe later on. 4. rLEDBAT design rationale 4.1. Controlling the receive window rLEDBAT uses the Receive Window (RCV.WND) of TCP to enable the receiver to control the sender's rate. [I-D.ietf-tcpm-rfc793bis] defines that the RCV.WND is used to announce the available receive buffer to the sender for flow control purposes. In order to avoid confusion, we will call fc.WND the value that a standard RFC793bis TCP receiver calculates to set in the receive window for flow control purposes. We call rl.WND the window value calculated by rLEDBAT algorithm and we call RCV.WND the value actually included in the Receive Window field of the TCP header. For a RFC793bis receiver, RCV.WND == fc.WND. In the case of rLEDBAT receiver, the rLEDBAT receiver sets the RCV.WND to the minimum of rl.WND and fc.WND, honoring both. When using rLEDBAT, two congestion controllers are in action in the flow of data from the sender to the receiver, namely, the congestion control algorithm of TCP in the sender side and the rLEDBAT congestion control algorithm executed in the receiver and conveyed to the sender through the RCV.WND. In the normal TCP operation, the sender uses the minimum of the congestion window cwnd and the receiver window RCV.WND to calculate the sender's window SND.WND. This is also true for rLEDBAT, as the sender is a regular TCP sender. Because rLEDBAT is designed to react earlier and more aggressively to congestion than regular TCP congestion control, the rl.WND contained Bagnulo, et al. Expires January 9, 2020 [Page 5] Internet-Draft rLEDBAT July 2019 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. Moreover, this also guarantees that even if the queuing delay is mis-estimated, the flow will never transmit more aggressively than a TCP flow, as the sender's congestion window limits the sending rate. In summary, the sender's window is: SND.WND = min(cwnd, rl.WND, fc.WND) 4.1.1. Avoiding window shrinking The rLEDBAT algorithm increases or decreases the rl.WND according to congestion signals (variations on the estimations of the queueing delay and packet loss). If the new congestion window is smaller than the current one and there is the possibility that directly announcing it in the RCV.WND may result in shrinking the window, i.e., moving the right window edge to the left. Shrinking the window is discouraged as per [I-D.ietf-tcpm-rfc793bis], as it may cause unnecessary packet loss and performance penalty. In order to avoid window shrinking, upon the reception of a data packet, the announced window can be reduced in the number of bytes contained in the packet at most. This may not always be enough to honor the new calculated value of the rl.WND. So, in order to reduce the window as dictated by the rLEDBAT algorithm, the receiver will progressively reduce the advertised RCV.WND, always honoring that the reduction is less or equal than the received bytes, until the target window determined by the rLEDBAT algorithm is reached. Because the rLEDBAT algorithm only allows to perform at most one multiplicative decrease per RTT, this allows the receiver to drain enough packets from the packets in-flight to reach the reduced window resulting form the rLEDBAT algorithm without need for resorting to shrinking the receiver window. 4.1.2. Window Scale Option The Window Scale (WS) option [RFC7323] is a mean to increase the maximum window size permitted by the Receive Window. The use of the WS option implies that the changes in the window are expressed in the units resulting of the WS option used in the TCP connection. This means that the rLEDBAT client will have to accumulate the increases resulting from the different received packets, and only convey a change in the window when the accumulated sum of increases is equal or higher than one unit used to express the receive window according to the WS option in place for the TCP connection. Bagnulo, et al. Expires January 9, 2020 [Page 6] Internet-Draft rLEDBAT July 2019 Changes in the receive window that are smaller than 1 MSS are unlikely to have any immediate impact on the sender's rate, as usual TCP segmentation practice results in sending full segments (i.e., segments of size equal to the MSS). So, accumulating changes in the receive window until completing a full MSS in the sender or in the receiver makes little difference. Current WS option specification [RFC7323] defines that allowed values for the WS option are between 0 and 14. Assuming a MSS around 1500 bytes, WS option values between 0 and 11 result in the receive window being expressed in units that are about 1 MSS or smaller. So, WS option values between 0 and 11 have no impact in rLEDBAT. WS option values higher than 11 can affect the dynamics of rLEDBAT, since control may become too coarse (e.g., with WS of 14, a change in one unit of the receive window implies a change of 10 MSS in the effective window). For the above reasons, we recommend that when rLEDBAT is used, the rLEDBAT client should set WS option values lower than 12. Additional experimentation is required to explore the impact of larger WS values in rLEDBAT dynamics. Note that the recommendation for rLEDBAT to set the WS option value to lower values does not precludes the communication with servers that set the WS option values to larger values, since the WS option value used is set independently for each direction of the TCP connection. 4.2. Using the RTT to estimate the queueing delay rLEDBAT uses the round trip time (RTT) instead of the one-way delay to estimate the queueing delay. In order to estimate the queueing delay using the RTT, the rLEDBAT receiver estimates the base RTT (i.e., the constant components of the RTT) and also measures the current RTT. By subtracting these two values, we obtain the queuing delay to be used by the rLEDBAT controller. rLEDBAT discovers the base RTT (RTTb) by taking the minimum value of the measured RTTs over a period of time. The current RTT (RTTc) is estimated using a number of recent samples and applying a filter, such as the minimum (or the mean) of the last k samples. Using the RTT to estimate the queueing delay has a number of shortcomings and difficulties that we discuss next. The queuing delay measured using the RTT includes also the queueing delay experienced by the return packets in the direction from the rLEDBAT receiver to the sender. This is a fundamental limitation of Bagnulo, et al. Expires January 9, 2020 [Page 7] Internet-Draft rLEDBAT July 2019 this approach. The impact of this error is that the rLEDBAT controller will also react to congestion in the reverse path direction which results in an even more conservative mechanism. In order to measure the RTT, rLEDBAT relies on the Time Stamp (TS) option [RFC7323]. By matching the TSVal value carried in outgoing packets with the TSecr value observed in incoming packets, it is possible to measure the RTT. This allows the rLEDBAT receiver to measure the RTT even if it is acting as a pure receiver. In a pure receiver there is no data flowing from the rLEDBAT receiver to the sender, making impossible to match data packets with acknowledgements packets to measure the RTT, as it is usually done in TCP for other purposes. Several issues must be addressed when using this approach in order to avoid an artificial increase of the observed RTT. Consider a TCP communication involving two hosts, host A, which is the legacy server, and host B, the rLEDBAT client, which is a pure receiver i.e. it has no data to send. Following the proposed method for estimating the RTT, host B will include a TSVal value in a TS option when sending packets to A. Since we are assuming that B has no data to send, the TS option will be carried in pure Acknowledgment packets. Upon the reception to the TS Option, host A 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 B. However, there are two reasons why A may not send a packet immediately back to B, 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 sender has no data to send when it receives the pure Acknowledgement carrying the TSVal to be echoed is rare in the expected rLEDBAT use cases.rLEDBAT will be used mostly for background file transfers so the sender will have data to send throughout the lifetime of the communication. If the file is structured in blocks of data, it may be the case that seldom, the sender will have to wait until the next block is available to proceed with the data transfer. We propose to address this situation by using a minimum filter of the last k samples when measuring the current RTT to discard the (rare) artificially bloated samples. The limitation of available sender's window to send more packets can come either from the congestion window in host A or from the announced receive window from the rLEDBAT in host B. Normally, the receive window will be the one to limit the sender's transmission rate, since rLEDBAT is designed to be more restrictive on the sender's rate than standard-TCP. In any case, if the limiting factor Bagnulo, et al. Expires January 9, 2020 [Page 8] Internet-Draft rLEDBAT July 2019 is the congestion window in the sender, it is irrelevant if rLEDBAT further reduces the receive window due to a bloated RTT measurement, since the rLEDBAT is not actively controlling the sender's rate. To address the case in which the limiting factor is the receive window announced by rLEDBAT, the receiver should discard the RTT measurements done while reducing the window and avoid including bloated samples in the queueing delay estimation. The rLEDBAT receiver is aware whether a given TSVal value was sent in a packet where the window was reduced, and if so, it can discard the corresponding RTT measurement. In the proposed algorithm, the affected samples are used for the current RTT estimation, but are not used for updating the rl.WND, as rl.WND remain unchanged for one RTT after a decrease episode. Finally, depending on the frequency of the local clock used to generate the values included in the TS option, several packets may carry the same TSVal value. If that happens, the rLEDBAT receiver will be unable to match the different outgoing packets carrying the same TSVal value with the different incoming packets carrying also the same TSecr value. However, it is not necessary for rLEDBAT to use all packets to estimate the RTT and sampling a subset of in- flight packets per RTT is enough to properly assess the queueing delay. rLEDBAT mitigates this issue by using a minimum filter in the last sampled RTT values to estimate the current RTT. 4.3. Inter-rLEDBAT fairness The use of an additive increase/multiplicative decrease (AIMD) algorithm provides inter-rLEDBAT fairness. When using AIMD, the congestion control algorithm causes the larger flow to reduce its rate more aggressively and leave room for the new flow to grow, resulting in the well-known AIMD fairness property. Moreover, in the case of LEDBAT, after a multiplicative decrease, the buffer is drained and the base RTT can be more accurately estimated. In rLEDBAT the congestion window is decreased by a multiplicative factor betad when the measured queueing delay is larger than the target T. 4.4. Reacting to packet loss The rLEDBAT receiver is capable of detecting retransmitted packets in the following way. We call RCV.HGH the highest sequence number correspondent to a received byte of data (not assuming that all bytes with smaller sequence numbers have been received already, there may be holes) and we call TSV.HGH the TSVal value corresponding to the segment in which that byte was carried. SEG.SEQ stands for the Bagnulo, et al. Expires January 9, 2020 [Page 9] Internet-Draft rLEDBAT July 2019 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. rLEDBAT reduces the rl.WND by a factor betal when detects a retransmission. rLEDBAT reacts to retransmitted packets at most once per RTT. If the sender has detected the packet loss via a timeout, the standard-TCP sender reduces its congestion window to 1 MSS and enters in slow start/exponential increase mode. During the exponential growth, the connection rate will be determined by standard-TCP congestion control at the sender, until the congestion window reaches the receive window announced by rLEDBAT, at which point rLEDBAT takes the control back from the TCP sender. rLEDBAT has two different multiplicative decrease factors, betal and betad. betad is the multiplicative decrease factor used for decreasing the window when the measured queueing exceeds the target T and betal is the one used when packet loss are detected. These two parameters may have different values. 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 taking over the control of the sending rate, until slow start ramps up and catches the current value of the rLEDBAT window. 4.5. Bootstrapping rLEDBAT uses a additive increase mechanism to grow the window. While this algorithm works well in steady-state, it performs poorly for bootstrapping, as it takes significant time to increase the sending rate. In order to ramp-up to the available capacity faster, rLEDBAT uses the initial window used by the flow control algorithm. This implies that when a flow starts, the rLEDBAT algorithm starts with a large window and the sending rate is in fact limited by the slow start algorithm of the sender's TCP. This means that while the Bagnulo, et al. Expires January 9, 2020 [Page 10] Internet-Draft rLEDBAT July 2019 queueing delay is no larger than the target T, rLEDBAT increases its sending rate in the same way as standard-TCP, but if the queueing delay exceeds the target, rLEDBAT takes over. 4.6. Reaction to path changes rLEDBAT adopts the mechanism defined by LEDBAT to deal with path changes. The LEDBAT algorithm [RFC6817] estimates the base delay by calculating the minimum observed delay in a n minute window. The historical data older than n minutes is not taken into account to estimate the base delay. The reason for this is to react when path changes occur. If the new path has a larger base delay, LEDBAT will keep on using the base delay of the former path and will impose a queueing delay that is larger than the target T. LEDBAT addresses this issue by limiting historical data to n minutes. If there is path change, LEDBAT will use the outdated base delay estimation for a maximum time of n minutes. After that, all the historical data used for the base delay estimation will be of the new path. 5. rLEDBAT algorithm 5.1. Data structures Parameters: T: Target delay betal: multiplicative decrease factor in case of packet loss betad: multiplicative decrease factor in case of RTT exceeds T alpha: additive increase factor. Variables: current_RTTs is an array with the last k measured RTTs base_RTTs is an array with the minimum observed RTTs in the last n minutes RCV.SEQ is the sequence number of the last byte that was received and acknowledged RCV.HGH is the highest sequence number of a received byte (which may not have been acknowledged yet) TSE.HGH is the TSecr value contained in the segment containing the byte with sequence number RCV.HGH Bagnulo, et al. Expires January 9, 2020 [Page 11] Internet-Draft rLEDBAT July 2019 SEG.SEQ is the sequence number of the incoming segment SEG.TSE is the TSecr value of the incoming segment SEG.time is the local time at which the incoming segment was received SEG.RTT is the latest sample of the RTT QD latest estimation of the queueing delay rl.WND window calculated by rLEDBAT without taking into account the window shrinking avoidance constraints rl.WND.WS window calculated by rLEDBAT after taking into account the window shrinking avoidance constrains DRAINED.BYTES number of bytes drained from the flight-size since the last packet sent fc.WND window calculated by standard TCP receiver end.reduction.time auxiliary variable used to prevent rl.WND from being updated after a window reduction 5.2. Algorithm on initialization DRAINED.BYTES = 0 base_RTTs set to maximum value current_RTTs set to maximum value rl.WND set to max value end.reduction.time = 0 Bagnulo, et al. Expires January 9, 2020 [Page 12] Internet-Draft rLEDBAT July 2019 on packet arrival DRAINED.BYTES = DRAINED.BYTES + SEG.LEN RTT calculation SEG.RTT = SEG.Time - SEG.TSE (the new sample of the RTT is the time of arrival of the segment minus the time at which the segment containing the TSVal value was issued) Update current_RTTs with SEG.RTT (substitute the oldest RTT sample in the current_RTTs array by SEG.RTT) Update base_RTTs with SEG.RTT (store SEG.RTT in the current current minute position, if SEG.RTT is smaller than the value in that position) QD = min(current_RTTs) - min(base_RTTs) If local.time > end.reduction.time then If SEG.SEQ < RCV.HGH AND SEG.TSE > TSE.HGH then rl.WND = max(rl.WND*betal, 1) end.reduction.time = local.time + min(current_RTTs) else If QD < T, then rl.WND = rl.WND+ alpha*MSS/rl.WND else QD > T, then rl.WND(t1) = max(rl.WND*beta1, 1) on sending a packet if rl.WND > rl.WND.WS or (rl.WND.WS - rl.WND) < DRAINED.BYTES then rl.WND.WS = rl.WND else rl.WND.WS = rl.WND.WS - DRAINED.BYTES DRAINED.BYTES = 0 RCV.WND = min(fc.WND, rl.WND.WS) The presented algorithm assumes WS option is not being used. The algorithm also assumes that the precision of the clock used to populate the TS option is fine grained enough for this purpose (e.g. 1 ms). If this is not the case, then the receiver should store the local time at which a packet carrying each TSVal value was issued and at which time the same value was received int he TSecr and calculate the RTT subtracting these two values. 5.3. rLEDBAT parameters 6. Security Considerations 7. IANA Considerations Bagnulo, et al. Expires January 9, 2020 [Page 13] Internet-Draft rLEDBAT July 2019 8. Acknowledgements This work was supported by the EU through the H2020 5G-RANGE project and by the Spanish Ministry of Economy and Competitiveness through the 5G-City project (TEC2016-76795-C6-3-R). 9. Informative References [I-D.ietf-tcpm-rfc793bis] Eddy, W., "Transmission Control Protocol Specification", draft-ietf-tcpm-rfc793bis-13 (work in progress), June 2019. [RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind, "Low Extra Delay Background Transport (LEDBAT)", RFC 6817, DOI 10.17487/RFC6817, December 2012, . [RFC7323] Borman, D., Braden, B., Jacobson, V., and R. Scheffenegger, Ed., "TCP Extensions for High Performance", RFC 7323, DOI 10.17487/RFC7323, September 2014, . [RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", RFC 8312, DOI 10.17487/RFC8312, February 2018, . Authors' Addresses Marcelo Bagnulo UC3M Email: marcelo@it.uc3m.es Alberto Garcia-Martinez UC3M Email: alberto@it.uc3m.es Gabriel Montenegro Microsoft Email: Gabriel.Montenegro@microsoft.com Bagnulo, et al. Expires January 9, 2020 [Page 14] Internet-Draft rLEDBAT July 2019 Praveen Balasubramanian Microsoft Email: pravb@microsoft.com Bagnulo, et al. Expires January 9, 2020 [Page 15]