TCPM L. Xu Internet-Draft UNL Obsoletes: 8312 (if approved) S. Ha Intended status: Standards Track Colorado Expires: 6 August 2021 I. Rhee Bowery V. Goel Apple Inc. L. Eggert, Ed. NetApp 2 February 2021 CUBIC for Fast Long-Distance Networks draft-eggert-tcpm-rfc8312bis-01 Abstract CUBIC is an extension to the current TCP standards. It differs from the current TCP standards only in the congestion control algorithm on the sender side. In particular, it uses a cubic function instead of a linear window increase function of the current TCP standards to improve scalability and stability under fast and long-distance networks. CUBIC and its predecessor algorithm have been adopted as defaults by Linux and have been used for many years. This document provides a specification of CUBIC to enable third-party implementations and to solicit community feedback through experimentation on the performance of CUBIC. This documents obsoletes [RFC8312], updating the specification of CUBIC to conform to the current Linux version. Note to Readers Discussion of this draft takes place on the TCPM working group mailing list (mailto:tcpm@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tcpm/. Working Group information can be found at https://datatracker.ietf.org/wg/tcpm/; source code and issues list for this draft can be found at https://github.com/NTAP/rfc8312bis. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Xu, et al. Expires 6 August 2021 [Page 1] Internet-Draft CUBIC February 2021 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 2021. Copyright Notice Copyright (c) 2021 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Design Principles of CUBIC . . . . . . . . . . . . . . . . . 4 4. CUBIC Congestion Control . . . . . . . . . . . . . . . . . . 6 4.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6 4.1.1. Constants of Interest . . . . . . . . . . . . . . . . 6 4.1.2. Variables of Interest . . . . . . . . . . . . . . . . 7 4.2. Window Increase Function . . . . . . . . . . . . . . . . 8 4.3. TCP-Friendly Region . . . . . . . . . . . . . . . . . . . 9 4.4. Concave Region . . . . . . . . . . . . . . . . . . . . . 11 4.5. Convex Region . . . . . . . . . . . . . . . . . . . . . . 11 4.6. Multiplicative Decrease . . . . . . . . . . . . . . . . . 12 4.7. Fast Convergence . . . . . . . . . . . . . . . . . . . . 12 4.8. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.9. Spurious Congestion Events . . . . . . . . . . . . . . . 13 4.10. Slow Start . . . . . . . . . . . . . . . . . . . . . . . 15 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 15 5.1. Fairness to Standard TCP . . . . . . . . . . . . . . . . 16 5.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 18 5.3. Difficult Environments . . . . . . . . . . . . . . . . . 19 Xu, et al. Expires 6 August 2021 [Page 2] Internet-Draft CUBIC February 2021 5.4. Investigating a Range of Environments . . . . . . . . . . 19 5.5. Protection against Congestion Collapse . . . . . . . . . 19 5.6. Fairness within the Alternative Congestion Control Algorithm . . . . . . . . . . . . . . . . . . . . . . . 19 5.7. Performance with Misbehaving Nodes and Outside Attackers . . . . . . . . . . . . . . . . . . . . . . . 19 5.8. Behavior for Application-Limited Flows . . . . . . . . . 19 5.9. Responses to Sudden or Transient Events . . . . . . . . . 20 5.10. Incremental Deployment . . . . . . . . . . . . . . . . . 20 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 8.1. Normative References . . . . . . . . . . . . . . . . . . 20 8.2. Informative References . . . . . . . . . . . . . . . . . 21 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 23 Appendix B. Evolution of CUBIC . . . . . . . . . . . . . . . . . 23 B.1. Since draft-eggert-tcpm-rfc8312bis-00 . . . . . . . . . . 23 B.2. Since RFC8312 . . . . . . . . . . . . . . . . . . . . . . 24 B.3. Since the Original Paper . . . . . . . . . . . . . . . . 24 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25 1. Introduction The low utilization problem of TCP in fast long-distance networks is well documented in [K03] and [RFC3649]. This problem arises from a slow increase of the congestion window following a congestion event in a network with a large bandwidth-delay product (BDP). [HKLRX06] indicates that this problem is frequently observed even in the range of congestion window sizes over several hundreds of packets. This problem is equally applicable to all Reno-style TCP standards and their variants, including TCP-Reno [RFC5681], TCP-NewReno [RFC6582][RFC6675], SCTP [RFC4960], and TFRC [RFC5348], which use the same linear increase function for window growth, which we refer to collectively as "Standard TCP" below. CUBIC, originally proposed in [HRX08], is a modification to the congestion control algorithm of Standard TCP to remedy this problem. This document describes the most recent specification of CUBIC. Specifically, CUBIC uses a cubic function instead of a linear window increase function of Standard TCP to improve scalability and stability under fast and long-distance networks. Binary Increase Congestion Control (BIC-TCP) [XHR04], a predecessor of CUBIC, was selected as the default TCP congestion control algorithm by Linux in the year 2005 and has been used for several years by the Internet community at large. CUBIC uses a similar window increase function as BIC-TCP and is designed to be less aggressive and fairer to Standard TCP in bandwidth usage than BIC-TCP Xu, et al. Expires 6 August 2021 [Page 3] Internet-Draft CUBIC February 2021 while maintaining the strengths of BIC-TCP such as stability, window scalability, and RTT fairness. CUBIC has already replaced BIC-TCP as the default TCP congestion control algorithm in Linux and has been deployed globally by Linux. Through extensive testing in various Internet scenarios, we believe that CUBIC is safe for testing and deployment in the global Internet. In the following sections, we first briefly explain the design principles of CUBIC, then provide the exact specification of CUBIC, and finally discuss the safety features of CUBIC following the guidelines specified in [RFC5033]. 2. Conventions 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Design Principles of CUBIC CUBIC is designed according to the following design principles: Principle 1: For better network utilization and stability, CUBIC uses both the concave and convex profiles of a cubic function to increase the congestion window size, instead of using just a convex function. Principle 2: To be TCP-friendly, CUBIC is designed to behave like Standard TCP in networks with short RTTs and small bandwidth where Standard TCP performs well. Principle 3: For RTT-fairness, CUBIC is designed to achieve linear bandwidth sharing among flows with different RTTs. Principle 4: CUBIC appropriately sets its multiplicative window decrease factor in order to balance between the scalability and convergence speed. Principle 1: For better network utilization and stability, CUBIC [HRX08] uses a cubic window increase function in terms of the elapsed time from the last congestion event. While most alternative congestion control algorithms to Standard TCP increase the congestion window using convex functions, CUBIC uses both the concave and convex profiles of a cubic function for window growth. After a window reduction in response to a congestion event is detected by duplicate ACKs or Explicit Congestion Notification-Echo (ECN-Echo) ACKs Xu, et al. Expires 6 August 2021 [Page 4] Internet-Draft CUBIC February 2021 [RFC3168], CUBIC registers the congestion window size where it got the congestion event as _W_(max)_ and performs a multiplicative decrease of congestion window. After it enters into congestion avoidance, it starts to increase the congestion window using the concave profile of the cubic function. The cubic function is set to have its plateau at _W_(max)_ so that the concave window increase continues until the window size becomes _W_(max)_. After that, the cubic function turns into a convex profile and the convex window increase begins. This style of window adjustment (concave and then convex) improves the algorithm stability while maintaining high network utilization [CEHRX07]. This is because the window size remains almost constant, forming a plateau around _W_(max)_ where network utilization is deemed highest. Under steady state, most window size samples of CUBIC are close to _W_(max)_, thus promoting high network utilization and stability. Note that those congestion control algorithms using only convex functions to increase the congestion window size have the maximum increments around _W_(max)_, and thus introduce a large number of packet bursts around the saturation point of the network, likely causing frequent global loss synchronizations. Principle 2: CUBIC promotes per-flow fairness to Standard TCP. Note that Standard TCP performs well under short RTT and small bandwidth (or small BDP) networks. There is only a scalability problem in networks with long RTTs and large bandwidth (or large BDP). An alternative congestion control algorithm to Standard TCP designed to be friendly to Standard TCP on a per-flow basis must operate to increase its congestion window less aggressively in small BDP networks than in large BDP networks. The aggressiveness of CUBIC mainly depends on the maximum window size before a window reduction, which is smaller in small BDP networks than in large BDP networks. Thus, CUBIC increases its congestion window less aggressively in small BDP networks than in large BDP networks. Furthermore, in cases when the cubic function of CUBIC increases its congestion window less aggressively than Standard TCP, CUBIC simply follows the window size of Standard TCP to ensure that CUBIC achieves at least the same throughput as Standard TCP in small BDP networks. We call this region where CUBIC behaves like Standard TCP, the "TCP-friendly region". Principle 3: Two CUBIC flows with different RTTs have their throughput ratio linearly proportional to the inverse of their RTT ratio, where the throughput of a flow is approximately the size of its congestion window divided by its RTT. Specifically, CUBIC maintains a window increase rate independent of RTTs outside of the TCP-friendly region, and thus flows with different RTTs have similar congestion window sizes under steady state when they operate outside the TCP-friendly region. This notion of a linear throughput ratio is Xu, et al. Expires 6 August 2021 [Page 5] Internet-Draft CUBIC February 2021 similar to that of Standard TCP under high statistical multiplexing environments where packet losses are independent of individual flow rates. However, under low statistical multiplexing environments, the throughput ratio of Standard TCP flows with different RTTs is quadratically proportional to the inverse of their RTT ratio [XHR04]. CUBIC always ensures the linear throughput ratio independent of the levels of statistical multiplexing. This is an improvement over Standard TCP. While there is no consensus on particular throughput ratios of different RTT flows, we believe that under wired Internet, use of a linear throughput ratio seems more reasonable than equal throughputs (i.e., the same throughput for flows with different RTTs) or a higher-order throughput ratio (e.g., a quadratical throughput ratio of Standard TCP under low statistical multiplexing environments). Principle 4: To balance between the scalability and convergence speed, CUBIC sets the multiplicative window decrease factor to 0.7 while Standard TCP uses 0.5. While this improves the scalability of CUBIC, a side effect of this decision is slower convergence, especially under low statistical multiplexing environments. This design choice is following the observation that the author of HighSpeed TCP (HSTCP) [RFC3649] has made along with other researchers (e.g., [GV02]): the current Internet becomes more asynchronous with less frequent loss synchronizations with high statistical multiplexing. Under this environment, even strict Multiplicative- Increase Multiplicative-Decrease (MIMD) can converge. CUBIC flows with the same RTT always converge to the same throughput independent of statistical multiplexing, thus achieving intra-algorithm fairness. We also find that under the environments with sufficient statistical multiplexing, the convergence speed of CUBIC flows is reasonable. 4. CUBIC Congestion Control In this section, we discuss how the congestion window is updated during the different stages of the CUBIC congestion controller. 4.1. Definitions The unit of all window sizes in this document is segments of the maximum segment size (MSS), and the unit of all times is seconds. 4.1.1. Constants of Interest β__(cubic)_: CUBIC multiplication decrease factor as described in Section 4.6. Xu, et al. Expires 6 August 2021 [Page 6] Internet-Draft CUBIC February 2021 _C_: constant that determines the aggressiveness of CUBIC in competing with other congestion control algorithms in high BDP networks. Please see Section 5 for more explanation on how it is set. The unit for _C_ is segment ------- 3 second 4.1.2. Variables of Interest Variables required to implement CUBIC are described in this section. _RTT_: Smoothed round-trip time in seconds calculated as described in [RFC6298]. _cwnd_: Current congestion window in segments. _ssthresh_: Current slow start threshold in segments. _W_(max)_: Size of _cwnd_ in segments just before _cwnd_ is reduced in the last congestion event. _K_: The time period in seconds it takes to increase the congestion window size at the beginning of the current congestion avoidance stage to _W_(max)_. _current_time_: Current time of the system in seconds. _epoch_(start)_: The time in seconds at which the current congestion avoidance stage starts. _cwnd_(start)_: The _cwnd_ at the beginning of the current congestion avoidance stage, i.e., at time _epoch_(start)_. W_(cubic)(_t_): Target value of the congestion window in segments at time t in seconds based on the cubic increase function as described in Section 4.2. _target_: Target value of congestion window in segments after the next _RTT_, that is, W_(cubic)(_t_ + _RTT_) as described in Section 4.2. _W_(est)_: An estimate for the congestion window in segments in the TCP-friendly region, that is, an estimate for the congestion window using the AIMD approach similar to TCP-NewReno congestion controller. Xu, et al. Expires 6 August 2021 [Page 7] Internet-Draft CUBIC February 2021 4.2. Window Increase Function CUBIC maintains the acknowledgment (ACK) clocking of Standard TCP by increasing the congestion window only at the reception of an ACK. It does not make any change to the fast recovery and retransmit of TCP, such as TCP-NewReno [RFC6582][RFC6675]. During congestion avoidance after a congestion event where a packet loss is detected by duplicate ACKs or a network congestion is detected by ACKs with ECN-Echo flags [RFC3168], CUBIC changes the window increase function of Standard TCP. CUBIC uses the following window increase function: 3 W (t) = C * (t - K) + W cubic max Figure 1 where t is the elapsed time in seconds from the beginning of the current congestion avoidance stage, that is, t = current_time - epoch start and where _epoch_(start)_ is the time at which the current congestion avoidance stage starts. _K_ is the time period that the above function takes to increase the congestion window size at the beginning of the current congestion avoidance stage to _W_(max)_ if there are no further congestion events and is calculated using the following equation: ________________ /W - cwnd 3 / max start K = | / ---------------- |/ C Figure 2 where _cwnd_(start)_ is the congestion window at the beginning of the current congestion avoidance stage. _cwnd_(start)_ is calculated as described in Section 4.6 when a congestion event is detected, although implementations can further adjust _cwnd_(start)_ based on other fast recovery mechanisms. In special cases, if _cwnd_(start)_ is greater than _W_(max)_, _K_ is set to 0. Xu, et al. Expires 6 August 2021 [Page 8] Internet-Draft CUBIC February 2021 Upon receiving an ACK during congestion avoidance, CUBIC computes the _target_ congestion window size after the next _RTT_ using Figure 1 as follows, where _RTT_ is the smoothed round-trip time. The lower and upper bounds below ensure that CUBIC's congestion window increase rate is non-decreasing and is less than the increase rate of slow start. / | if W (t + RTT) < cwnd |cwnd cubic | | | target = < if W (t + RTT) > 1.5 * cwnd |1.5 * cwnd cubic | | |W (t + RTT) | cubic otherwise \ Depending on the value of the current congestion window size _cwnd_, CUBIC runs in three different modes. 1. The TCP-friendly region, which ensures that CUBIC achieves at least the same throughput as Standard TCP. 2. The concave region, if CUBIC is not in the TCP-friendly region and _cwnd_ is less than _W_(max)_. 3. The convex region, if CUBIC is not in the TCP-friendly region and _cwnd_ is greater than _W_(max)_. Below, we describe the exact actions taken by CUBIC in each region. 4.3. TCP-Friendly Region Standard TCP performs well in certain types of networks, for example, under short RTT and small bandwidth (or small BDP) networks. In these networks, we use the TCP-friendly region to ensure that CUBIC achieves at least the same throughput as Standard TCP. Xu, et al. Expires 6 August 2021 [Page 9] Internet-Draft CUBIC February 2021 The TCP-friendly region is designed according to the analysis described in [FHP00]. The analysis studies the performance of an Additive Increase and Multiplicative Decrease (AIMD) algorithm with an additive factor of α__(aimd)_ (segments per _RTT_) and a multiplicative factor of β__(aimd)_, denoted by AIMD(α__(aimd)_, β__(aimd)_). Specifically, the average congestion window size of AIMD(α__(aimd)_, β__(aimd)_) can be calculated using Figure 3. The analysis shows that AIMD(α__(aimd)_, β__(aimd)_) with 1 - β cubic α = 3 * ---------- aimd 1 + β cubic achieves the same average window size as Standard TCP that uses AIMD(1, 0.5). ___________________ /α * (1 + β ) / aimd aimd AVG_AIMD(α , β ) = | / ------------------- aimd aimd | / 2 * (1 - β ) * p |/ aimd Figure 3 Based on the above analysis, CUBIC uses Figure 4 to estimate the window size _W_(est)_ of AIMD(α__(aimd)_, β__(aimd)_) with 1 - β cubic α = 3 * ---------- aimd 1 + β cubic β = β aimd cubic which achieves the same average window size as Standard TCP. When receiving an ACK in congestion avoidance (_cwnd_ could be greater than or less than _W_(max)_), CUBIC checks whether W_(cubic)(_t_) is less than _W_(est)_. If so, CUBIC is in the TCP-friendly region and _cwnd_ SHOULD be set to _W_(est)_ at each reception of an ACK. _W_(est)_ is set equal to _cwnd_ at the start of the congestion avoidance stage. After that, on every ACK, _W_(est)_ is updated using Figure 4. Xu, et al. Expires 6 August 2021 [Page 10] Internet-Draft CUBIC February 2021 segments_acked W = W + α * -------------- est est aimd cwnd Figure 4 Note that once _W_(est)_ reaches _W_(max)_, that is, _W_(est)_ >= _W_(max)_, α__(aimd)_ SHOULD be set to 1 to achieve the same congestion window size as standard TCP that uses AIMD. 4.4. Concave Region When receiving an ACK in congestion avoidance, if CUBIC is not in the TCP-friendly region and _cwnd_ is less than _W_(max)_, then CUBIC is in the concave region. In this region, _cwnd_ MUST be incremented by target - cwnd ------------- cwnd for each received ACK, where _target_ is calculated as described in Section 4.2. 4.5. Convex Region When receiving an ACK in congestion avoidance, if CUBIC is not in the TCP-friendly region and _cwnd_ is larger than or equal to _W_(max)_, then CUBIC is in the convex region. The convex region indicates that the network conditions might have been perturbed since the last congestion event, possibly implying more available bandwidth after some flow departures. Since the Internet is highly asynchronous, some amount of perturbation is always possible without causing a major change in available bandwidth. In this region, CUBIC is being very careful by very slowly increasing its window size. The convex profile ensures that the window increases very slowly at the beginning and gradually increases its increase rate. We also call this region the "maximum probing phase" since CUBIC is searching for a new _W_(max)_. In this region, _cwnd_ MUST be incremented by target - cwnd ------------- cwnd for each received ACK, where _target_ is calculated as described in Section 4.2. Xu, et al. Expires 6 August 2021 [Page 11] Internet-Draft CUBIC February 2021 4.6. Multiplicative Decrease When a packet loss is detected by duplicate ACKs or a network congestion is detected by receiving packets marked with ECN-Echo (ECE), CUBIC updates its _W_(max)_ and reduces its _cwnd_ and _ssthresh_ immediately as below. For both packet loss and congestion detection through ECN, the sender MAY employ a fast recovery algorithm to gradually adjust the congestion window to its new reduced value. Parameter β__(cubic)_ SHOULD be set to 0.7. ssthresh = cwnd * β // new slow-start threshold cubic ssthresh = max(ssthresh, 2) // threshold is at least 2 MSS // window reduction cwnd = ssthresh A side effect of setting β__(cubic)_ to a value bigger than 0.5 is slower convergence. We believe that while a more adaptive setting of β__(cubic)_ could result in faster convergence, it will make the analysis of CUBIC much harder. This adaptive adjustment of β__(cubic)_ is an item for the next version of CUBIC. 4.7. Fast Convergence To improve the convergence speed of CUBIC, we add a heuristic in CUBIC. When a new flow joins the network, existing flows in the network need to give up some of their bandwidth to allow the new flow some room for growth if the existing flows have been using all the bandwidth of the network. To speed up this bandwidth release by existing flows, the following mechanism called "fast convergence" SHOULD be implemented. With fast convergence, when a congestion event occurs, we update _W_(max)_ as follows before the window reduction as described in Section 4.6. / 1 + β | cubic if cwnd < W , further reduce W |W * ---------- max max W = < max 2 max | | otherwise, remember cwnd before reduction \cwnd Xu, et al. Expires 6 August 2021 [Page 12] Internet-Draft CUBIC February 2021 At a congestion event, if the current _cwnd_ is less than _W_(max)_, this indicates that the saturation point experienced by this flow is getting reduced because of the change in available bandwidth. Then we allow this flow to release more bandwidth by reducing _W_(max)_ further. This action effectively lengthens the time for this flow to increase its congestion window because the reduced _W_(max)_ forces the flow to have the plateau earlier. This allows more time for the new flow to catch up to its congestion window size. The fast convergence is designed for network environments with multiple CUBIC flows. In network environments with only a single CUBIC flow and without any other traffic, the fast convergence SHOULD be disabled. 4.8. Timeout In case of timeout, CUBIC follows Standard TCP to reduce _cwnd_ [RFC5681], but sets _ssthresh_ using β__(cubic)_ (same as in Section 4.6) that is different from Standard TCP [RFC5681]. During the first congestion avoidance after a timeout, CUBIC increases its congestion window size using Figure 1, where t is the elapsed time since the beginning of the current congestion avoidance, _K_ is set to 0, and _W_(max)_ is set to the congestion window size at the beginning of the current congestion avoidance. In addition, for the tcp-friendliness region, _W_(est)_ should be set to the congestion window size at the beginning of the current congestion avoidance. 4.9. Spurious Congestion Events For the case where CUBIC reduces its congestion window in response to detection of packet loss via duplicate ACKs or timeout, there is a possibility that the missing ACK would arrive after the congestion window reduction and the corresponding packet retransmission. For example, packet reordering which is common in networks could trigger this behavior. A high degree of packet reordering could cause multiple events of congestion window reduction where spurious losses are incorrectly interpreted as congestion signals, thus degrading CUBIC's performance significantly. When there is a congestion event, a CUBIC implementation SHOULD save the current value of the following variables before the congestion window reduction. Xu, et al. Expires 6 August 2021 [Page 13] Internet-Draft CUBIC February 2021 prior_cwnd = cwnd prior_ssthresh = ssthresh prior_W = W max max prior_K = K prior_epoch = epoch start start prior_W_{est} = W est CUBIC MAY implement an algorithm to detect spurious retransmissions, such as DSACK [RFC3708], Forward RTO-Recovery [RFC5682] or Eifel [RFC3522]. Once a spurious congestion event is detected, CUBIC SHOULD restore the original values of above mentioned variables as follows if the current _cwnd_ is lower than _prior_cwnd_. Restoring to the original values ensures that CUBIC's performance is similar to what it would be if there were no spurious losses. \ cwnd = prior_cwnd | | ssthresh = prior_ssthresh | | W = prior_W | max max | >if cwnd < prior_cwnd K = prior_K | | epoch = prior_epoch | start start| | W = prior_W | est est / In rare cases, when the detection happens long after a spurious loss event and the current _cwnd_ is already higher than the _prior_cwnd_, CUBIC SHOULD continue to use the current and the most recent values of these variables. Xu, et al. Expires 6 August 2021 [Page 14] Internet-Draft CUBIC February 2021 4.10. Slow Start CUBIC MUST employ a slow-start algorithm, when _cwnd_ is no more than _ssthresh_. Among the slow-start algorithms, CUBIC MAY choose the standard TCP slow start [RFC5681] in general networks, or the limited slow start [RFC3742] or hybrid slow start [HR08] for fast and long- distance networks. In the case when CUBIC runs the hybrid slow start [HR08], it may exit the first slow start without incurring any packet loss and thus _W_(max)_ is undefined. In this special case, CUBIC switches to congestion avoidance and increases its congestion window size using Figure 1, where t is the elapsed time since the beginning of the current congestion avoidance, _K_ is set to 0, and _W_(max)_ is set to the congestion window size at the beginning of the current congestion avoidance. 5. Discussion In this section, we further discuss the safety features of CUBIC following the guidelines specified in [RFC5033]. With a deterministic loss model where the number of packets between two successive packet losses is always _1/p_, CUBIC always operates with the concave window profile, which greatly simplifies the performance analysis of CUBIC. The average window size of CUBIC can be obtained by the following function: ________________ ____ /C * (3 + β ) 3 / 4 4 / cubic |/ RTT AVG_W = | / ---------------- * ------- cubic | / 4 * (1 - β ) __ |/ cubic 3 / 4 |/ p Figure 5 With β__(cubic)_ set to 0.7, the above formula is reduced to: ____ _______ 3 / 4 4 /C * 3.7 |/ RTT AVG_W = | / ------- * ------- cubic |/ 1.2 __ 3 / 4 |/ p Xu, et al. Expires 6 August 2021 [Page 15] Internet-Draft CUBIC February 2021 Figure 6 We will determine the value of _C_ in the following subsection using Figure 6. 5.1. Fairness to Standard TCP In environments where Standard TCP is able to make reasonable use of the available bandwidth, CUBIC does not significantly change this state. Standard TCP performs well in the following two types of networks: 1. networks with a small bandwidth-delay product (BDP) 2. networks with a short RTTs, but not necessarily a small BDP CUBIC is designed to behave very similarly to Standard TCP in the above two types of networks. The following two tables show the average window sizes of Standard TCP, HSTCP, and CUBIC. The average window sizes of Standard TCP and HSTCP are from [RFC3649]. The average window size of CUBIC is calculated using Figure 6 and the CUBIC TCP-friendly region for three different values of _C_. +=============+=======+========+================+=========+========+ | Loss Rate P | TCP | HSTCP | CUBIC (C=0.04) | CUBIC | CUBIC | | | | | | (C=0.4) | (C=4) | +=============+=======+========+================+=========+========+ | 1.0e-02 | 12 | 12 | 12 | 12 | 12 | +-------------+-------+--------+----------------+---------+--------+ | 1.0e-03 | 38 | 38 | 38 | 38 | 59 | +-------------+-------+--------+----------------+---------+--------+ | 1.0e-04 | 120 | 263 | 120 | 187 | 333 | +-------------+-------+--------+----------------+---------+--------+ | 1.0e-05 | 379 | 1795 | 593 | 1054 | 1874 | +-------------+-------+--------+----------------+---------+--------+ | 1.0e-06 | 1200 | 12280 | 3332 | 5926 | 10538 | +-------------+-------+--------+----------------+---------+--------+ | 1.0e-07 | 3795 | 83981 | 18740 | 33325 | 59261 | +-------------+-------+--------+----------------+---------+--------+ | 1.0e-08 | 12000 | 574356 | 105383 | 187400 | 333250 | +-------------+-------+--------+----------------+---------+--------+ Table 1: Standard TCP, HSTCP, and CUBIC with RTT = 0.1 seconds Table 1 describes the response function of Standard TCP, HSTCP, and CUBIC in networks with _RTT_ = 0.1 seconds. The average window size is in MSS-sized segments. Xu, et al. Expires 6 August 2021 [Page 16] Internet-Draft CUBIC February 2021 +=============+=======+========+================+=========+=======+ | Loss Rate P | TCP | HSTCP | CUBIC (C=0.04) | CUBIC | CUBIC | | | | | | (C=0.4) | (C=4) | +=============+=======+========+================+=========+=======+ | 1.0e-02 | 12 | 12 | 12 | 12 | 12 | +-------------+-------+--------+----------------+---------+-------+ | 1.0e-03 | 38 | 38 | 38 | 38 | 38 | +-------------+-------+--------+----------------+---------+-------+ | 1.0e-04 | 120 | 263 | 120 | 120 | 120 | +-------------+-------+--------+----------------+---------+-------+ | 1.0e-05 | 379 | 1795 | 379 | 379 | 379 | +-------------+-------+--------+----------------+---------+-------+ | 1.0e-06 | 1200 | 12280 | 1200 | 1200 | 1874 | +-------------+-------+--------+----------------+---------+-------+ | 1.0e-07 | 3795 | 83981 | 3795 | 5926 | 10538 | +-------------+-------+--------+----------------+---------+-------+ | 1.0e-08 | 12000 | 574356 | 18740 | 33325 | 59261 | +-------------+-------+--------+----------------+---------+-------+ Table 2: Standard TCP, HSTCP, and CUBIC with RTT = 0.01 seconds Table 2 describes the response function of Standard TCP, HSTCP, and CUBIC in networks with _RTT_ = 0.01 seconds. The average window size is in MSS-sized segments. Both tables show that CUBIC with any of these three _C_ values is more friendly to TCP than HSTCP, especially in networks with a short _RTT_ where TCP performs reasonably well. For example, in a network with _RTT_ = 0.01 seconds and p=10^-6, TCP has an average window of 1200 packets. If the packet size is 1500 bytes, then TCP can achieve an average rate of 1.44 Gbps. In this case, CUBIC with _C_=0.04 or _C_=0.4 achieves exactly the same rate as Standard TCP, whereas HSTCP is about ten times more aggressive than Standard TCP. We can see that _C_ determines the aggressiveness of CUBIC in competing with other congestion control algorithms for bandwidth. CUBIC is more friendly to Standard TCP, if the value of _C_ is lower. However, we do not recommend setting _C_ to a very low value like 0.04, since CUBIC with a low _C_ cannot efficiently use the bandwidth in long-_RTT_ and high-bandwidth networks. Based on these observations and our experiments, we find _C_=0.4 gives a good balance between TCP- friendliness and aggressiveness of window increase. Therefore, _C_ SHOULD be set to 0.4. With _C_ set to 0.4, Figure 6 is reduced to: Xu, et al. Expires 6 August 2021 [Page 17] Internet-Draft CUBIC February 2021 ____ 3 / 4 |/ RTT AVG_W = 1.054 * ------- cubic __ 3 / 4 |/ p Figure 7 Figure 7 is then used in the next subsection to show the scalability of CUBIC. 5.2. Using Spare Capacity CUBIC uses a more aggressive window increase function than Standard TCP under long-_RTT_ and high-bandwidth networks. The following table shows that to achieve the 10 Gbps rate, Standard TCP requires a packet loss rate of 2.0e-10, while CUBIC requires a packet loss rate of 2.9e-8. +===================+===========+=========+=========+=========+ | Throughput (Mbps) | Average W | TCP P | HSTCP P | CUBIC P | +===================+===========+=========+=========+=========+ | 1 | 8.3 | 2.0e-2 | 2.0e-2 | 2.0e-2 | +-------------------+-----------+---------+---------+---------+ | 10 | 83.3 | 2.0e-4 | 3.9e-4 | 2.9e-4 | +-------------------+-----------+---------+---------+---------+ | 100 | 833.3 | 2.0e-6 | 2.5e-5 | 1.4e-5 | +-------------------+-----------+---------+---------+---------+ | 1000 | 8333.3 | 2.0e-8 | 1.5e-6 | 6.3e-7 | +-------------------+-----------+---------+---------+---------+ | 10000 | 83333.3 | 2.0e-10 | 1.0e-7 | 2.9e-8 | +-------------------+-----------+---------+---------+---------+ Table 3: Required packet loss rate for Standard TCP, HSTCP, and CUBIC to achieve a certain throughput Table 3 describes the required packet loss rate for Standard TCP, HSTCP, and CUBIC to achieve a certain throughput. We use 1500-byte packets and an _RTT_ of 0.1 seconds. Our test results in [HKLRX06] indicate that CUBIC uses the spare bandwidth left unused by existing Standard TCP flows in the same bottleneck link without taking away much bandwidth from the existing flows. Xu, et al. Expires 6 August 2021 [Page 18] Internet-Draft CUBIC February 2021 5.3. Difficult Environments CUBIC is designed to remedy the poor performance of TCP in fast and long-distance networks. 5.4. Investigating a Range of Environments CUBIC has been extensively studied by using both NS-2 simulation and test-bed experiments covering a wide range of network environments. More information can be found in [HKLRX06]. Same as Standard TCP, CUBIC is a loss-based congestion control algorithm. Because CUBIC is designed to be more aggressive (due to a faster window increase function and bigger multiplicative decrease factor) than Standard TCP in fast and long-distance networks, it can fill large drop-tail buffers more quickly than Standard TCP and increase the risk of a standing queue [RFC8511]. In this case, proper queue sizing and management [RFC7567] could be used to reduce the packet queuing delay. 5.5. Protection against Congestion Collapse With regard to the potential of causing congestion collapse, CUBIC behaves like Standard TCP since CUBIC modifies only the window adjustment algorithm of TCP. Thus, it does not modify the ACK clocking and Timeout behaviors of Standard TCP. 5.6. Fairness within the Alternative Congestion Control Algorithm CUBIC ensures convergence of competing CUBIC flows with the same _RTT_ in the same bottleneck links to an equal throughput. When competing flows have different _RTT_ values, their throughput ratio is linearly proportional to the inverse of their _RTT_ ratios. This is true independent of the level of statistical multiplexing in the link. 5.7. Performance with Misbehaving Nodes and Outside Attackers This is not considered in the current CUBIC. 5.8. Behavior for Application-Limited Flows CUBIC does not raise its congestion window size if the flow is currently limited by the application instead of the congestion window. In case of long periods when _cwnd_ has not been updated due to the application rate limit, such as idle periods, t in Figure 1 MUST NOT include these periods; otherwise, W_(cubic)(_t_) might be very high after restarting from these periods. Xu, et al. Expires 6 August 2021 [Page 19] Internet-Draft CUBIC February 2021 5.9. Responses to Sudden or Transient Events If there is a sudden congestion, a routing change, or a mobility event, CUBIC behaves the same as Standard TCP. 5.10. Incremental Deployment CUBIC requires only the change of TCP senders, and it does not make any changes to TCP receivers. That is, a CUBIC sender works correctly with the Standard TCP receivers. In addition, CUBIC does not require any changes to the routers and does not require any assistance from the routers. 6. Security Considerations This proposal makes no changes to the underlying security of TCP. More information about TCP security concerns can be found in [RFC5681]. 7. IANA Considerations This document does not require any IANA actions. 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001, . [RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion Control Algorithms", BCP 133, RFC 5033, DOI 10.17487/RFC5033, August 2007, . [RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP Friendly Rate Control (TFRC): Protocol Specification", RFC 5348, DOI 10.17487/RFC5348, September 2008, . Xu, et al. Expires 6 August 2021 [Page 20] Internet-Draft CUBIC February 2021 [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, . [RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent, "Computing TCP's Retransmission Timer", RFC 6298, DOI 10.17487/RFC6298, June 2011, . [RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The NewReno Modification to TCP's Fast Recovery Algorithm", RFC 6582, DOI 10.17487/RFC6582, April 2012, . [RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M., and Y. Nishida, "A Conservative Loss Recovery Algorithm Based on Selective Acknowledgment (SACK) for TCP", RFC 6675, DOI 10.17487/RFC6675, August 2012, . [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF Recommendations Regarding Active Queue Management", BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 8.2. Informative References [CEHRX07] Cai, H., Eun, D., Ha, S., Rhee, I., and L. Xu, "Stochastic Ordering for Internet Congestion Control and its Applications", IEEE INFOCOM 2007 - 26th IEEE International Conference on Computer Communications, DOI 10.1109/infcom.2007.111, 2007, . [FHP00] Floyd, S., Handley, M., and J. Padhye, "A Comparison of Equation-Based and AIMD Congestion Control", May 2000, . [GV02] Gorinsky, S. and H. Vin, "Extended Analysis of Binary Adjustment Algorithms", Technical Report TR2002-29, Department of Computer Sciences, The University of Texas at Austin, 11 August 2002, . Xu, et al. Expires 6 August 2021 [Page 21] Internet-Draft CUBIC February 2021 [HKLRX06] Ha, S., Kim, Y., Le, L., Rhee, I., and L. Xu, "A Step toward Realistic Performance Evaluation of High-Speed TCP Variants", International Workshop on Protocols for Fast Long-Distance Networks, February 2006, . [HR08] Ha, S. and I. Rhee, "Hybrid Slow Start for High-Bandwidth and Long-Distance Networks", International Workshop on Protocols for Fast Long-Distance Networks, March 2008, . [HRX08] Ha, S., Rhee, I., and L. Xu, "CUBIC: a new TCP-friendly high-speed TCP variant", ACM SIGOPS Operating Systems Review Vol. 42, pp. 64-74, DOI 10.1145/1400097.1400105, July 2008, . [K03] Kelly, T., "Scalable TCP: improving performance in highspeed wide area networks", ACM SIGCOMM Computer Communication Review Vol. 33, pp. 83-91, DOI 10.1145/956981.956989, April 2003, . [RFC3522] Ludwig, R. and M. Meyer, "The Eifel Detection Algorithm for TCP", RFC 3522, DOI 10.17487/RFC3522, April 2003, . [RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows", RFC 3649, DOI 10.17487/RFC3649, December 2003, . [RFC3708] Blanton, E. and M. Allman, "Using TCP Duplicate Selective Acknowledgement (DSACKs) and Stream Control Transmission Protocol (SCTP) Duplicate Transmission Sequence Numbers (TSNs) to Detect Spurious Retransmissions", RFC 3708, DOI 10.17487/RFC3708, February 2004, . [RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March 2004, . [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, . Xu, et al. Expires 6 August 2021 [Page 22] Internet-Draft CUBIC February 2021 [RFC5682] Sarolahti, P., Kojo, M., Yamamoto, K., and M. Hata, "Forward RTO-Recovery (F-RTO): An Algorithm for Detecting Spurious Retransmission Timeouts with TCP", RFC 5682, DOI 10.17487/RFC5682, September 2009, . [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, . [RFC8511] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst, "TCP Alternative Backoff with ECN (ABE)", RFC 8511, DOI 10.17487/RFC8511, December 2018, . [SXEZ19] Sun, W., Xu, L., Elbaum, S., and D. Zhao, "Model-Agnostic and Efficient Exploration of Numerical State Space of Real-World TCP Congestion Control Implementations", USENIX NSDI 2019, February 2019, . [XHR04] Xu, L., Harfoush, K., and I. Rhee, "Binary Increase Congestion Control (BIC) for Fast Long-Distance Networks", IEEE INFOCOM 2004, DOI 10.1109/infcom.2004.1354672, March 2004, . Appendix A. Acknowledgements Richard Scheffenegger and Alexander Zimmermann originally co-authored [RFC8312]. Appendix B. Evolution of CUBIC B.1. Since draft-eggert-tcpm-rfc8312bis-00 * acknowledge former co-authors (#15 (https://github.com/NTAP/rfc8312bis/issues/15)) * prevent _cwnd_ from becoming less than two (#7 (https://github.com/NTAP/rfc8312bis/issues/7)) * add list of variables and constants (#5 (https://github.com/NTAP/rfc8312bis/issues/5), #6 (https://github.com/NTAP/rfc8312bis/issues/5)) Xu, et al. Expires 6 August 2021 [Page 23] Internet-Draft CUBIC February 2021 * update _K_'s definition and add bounds for CUBIC _target_ _cwnd_ [SXEZ19] (#1 (https://github.com/NTAP/rfc8312bis/issues/1), #14 (https://github.com/NTAP/rfc8312bis/issues/14)) * update _W_(est)_ to use AIMD approach (#20 (https://github.com/NTAP/rfc8312bis/issues/20)) * set alpha__(aimd)_ to 1 once _W_(est)_ reaches _W_(max)_ (#2 (https://github.com/NTAP/rfc8312bis/issues/2)) * add Vidhi as co-author * (#17 (https://github.com/NTAP/rfc8312bis/issues/17)) * note for fast recovery during _cwnd_ decrease due to congestion event (#11 (https://github.com/NTAP/rfc8312bis11/issues/11)) * add section for spurious congestion events (#23 (https://github.com/NTAP/rfc8312bis/issues/23)) * initialize _W_(est)_ after timeout and remove variable _W_(last_max)_ (#28 (https://github.com/NTAP/rfc8312bis/ issues/28)) B.2. Since RFC8312 * converted to Markdown and xml2rfc v3 * updated references (as part of the conversion) * updated author information * various formatting changes * move to Standards Track B.3. Since the Original Paper CUBIC has gone through a few changes since the initial release [HRX08] of its algorithm and implementation. Below we highlight the differences between its original paper and [RFC8312]. * The original paper [HRX08] includes the pseudocode of CUBIC implementation using Linux's pluggable congestion control framework, which excludes system-specific optimizations. The simplified pseudocode might be a good source to start with and understand CUBIC. Xu, et al. Expires 6 August 2021 [Page 24] Internet-Draft CUBIC February 2021 * [HRX08] also includes experimental results showing its performance {{and fairness. * The definition of beta__(cubic)_ constant was changed in [RFC8312]. For example, beta__(cubic)_ in the original paper was the window decrease constant while [RFC8312] changed it to CUBIC multiplication decrease factor. With this change, the current congestion window size after a congestion event in [RFC8312] was beta__(cubic)_ * _W_(max)_ while it was (1-beta__(cubic)_) * _W_(max)_ in the original paper. * Its pseudocode used _W_(last_max)_ while [RFC8312] used _W_(max)_. * Its TCP friendly window was W_(tcp) while [RFC8312] used _W_(est)_. Authors' Addresses Lisong Xu University of Nebraska-Lincoln Department of Computer Science and Engineering Lincoln, NE 68588-0115 United States of America Email: xu@unl.edu URI: https://cse.unl.edu/~xu/ Sangtae Ha University of Colorado at Boulder Department of Computer Science Boulder, CO 80309-0430 United States of America Email: sangtae.ha@colorado.edu URI: https://netstech.org/sangtaeha/ Injong Rhee Bowery Farming 151 W 26TH Street, 12TH Floor New York, NY 10001 United States of America Email: injongrhee@gmail.com Xu, et al. Expires 6 August 2021 [Page 25] Internet-Draft CUBIC February 2021 Vidhi Goel Apple Inc. One Apple Park Way Cupertino, California 95014 United States of America Email: vidhi_goel@apple.com Lars Eggert (editor) NetApp Stenbergintie 12 B FI-02700 Kauniainen Finland Email: lars@eggert.org URI: https://eggert.org/ Xu, et al. Expires 6 August 2021 [Page 26]