Internet Engineering Task Force (IETF) I. Rhee Request for Comments: 8312 NCSU Category: Informational L. Xu ISSN: 2070-1721 UNL S. Ha Colorado A. Zimmermann L. Eggert R. Scheffenegger NetApp February 2018 CUBIC for Fast Long-Distance Networks
AbstractCUBIC 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. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8312.
Copyright Notice Copyright (c) 2018 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. 1. Introduction ....................................................3 2. Conventions .....................................................3 3. Design Principles of CUBIC ......................................4 4. CUBIC Congestion Control ........................................6 4.1. Window Increase Function ...................................6 4.2. TCP-Friendly Region ........................................7 4.3. Concave Region .............................................8 4.4. Convex Region ..............................................8 4.5. Multiplicative Decrease ....................................8 4.6. Fast Convergence ...........................................9 4.7. Timeout ...................................................10 4.8. Slow Start ................................................10 5. Discussion .....................................................10 5.1. Fairness to Standard TCP ..................................11 5.2. Using Spare Capacity ......................................13 5.3. Difficult Environments ....................................13 5.4. Investigating a Range of Environments .....................13 5.5. Protection against Congestion Collapse ....................14 5.6. Fairness within the Alternative Congestion Control Algorithm .................................................14 5.7. Performance with Misbehaving Nodes and Outside Attackers ..14 5.8. Behavior for Application-Limited Flows ....................14 5.9. Responses to Sudden or Transient Events ...................14 5.10. Incremental Deployment ...................................14 6. Security Considerations ........................................15 7. IANA Considerations ............................................15 8. References .....................................................15 8.1. Normative References ......................................15 8.2. Informative References ....................................16 Acknowledgements ..................................................17 Authors' Addresses ................................................18
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 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]. RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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 [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 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. 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. Suppose that W_max is the window size just before the window is reduced in the last congestion event. CUBIC uses the following window increase function: W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1) where C is a constant fixed to determine the aggressiveness of window increase in high BDP networks, t is the elapsed time from the beginning of the current congestion avoidance, and K is the time period that the above function takes to increase the current window size to W_max if there are no further congestion events and is calculated using the following equation: K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2) where beta_cubic is the CUBIC multiplication decrease factor, that is, when a congestion event is detected, CUBIC reduces its cwnd to W_cubic(0)=W_max*beta_cubic. We discuss how we set beta_cubic in Section 4.5 and how we set C in Section 5.
Upon receiving an ACK during congestion avoidance, CUBIC computes the window increase rate during the next RTT period using Eq. 1. It sets W_cubic(t+RTT) as the candidate target value of the congestion window, where RTT is the weighted average RTT calculated by Standard TCP. 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. FHP00]. The analysis studies the performance of an Additive Increase and Multiplicative Decrease (AIMD) algorithm with an additive factor of alpha_aimd (segments per RTT) and a multiplicative factor of beta_aimd, denoted by AIMD(alpha_aimd, beta_aimd). Specifically, the average congestion window size of AIMD(alpha_aimd, beta_aimd) can be calculated using Eq. 3. The analysis shows that AIMD(alpha_aimd, beta_aimd) with alpha_aimd=3*(1-beta_aimd)/(1+beta_aimd) achieves the same average window size as Standard TCP that uses AIMD(1, 0.5). AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) / (2*(1-beta_aimd)*p) ]^0.5 (Eq. 3) Based on the above analysis, CUBIC uses Eq. 4 to estimate the window size W_est of AIMD(alpha_aimd, beta_aimd) with alpha_aimd=3*(1-beta_cubic)/(1+beta_cubic) and beta_aimd=beta_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(t). If so, CUBIC is in the TCP-friendly region and cwnd SHOULD be set to W_est(t) at each reception of an ACK. W_est(t) = W_max*beta_cubic + [3*(1-beta_cubic)/(1+beta_cubic)] * (t/RTT) (Eq. 4)
A side effect of setting beta_cubic to a value bigger than 0.5 is slower convergence. We believe that while a more adaptive setting of beta_cubic could result in faster convergence, it will make the analysis of CUBIC much harder. This adaptive adjustment of beta_cubic is an item for the next version of CUBIC.
RFC5681], but sets ssthresh using beta_cubic (same as in Section 4.5) that is different from Standard TCP [RFC5681]. During the first congestion avoidance after a timeout, CUBIC increases its congestion window size using Eq. 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. 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 Eq. 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. 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: AVG_W_cubic = [C*(3+beta_cubic)/(4*(1-beta_cubic))]^0.25 * (RTT^0.75) / (p^0.75) (Eq. 5) With beta_cubic set to 0.7, the above formula is reduced to: AVG_W_cubic = (C*3.7/1.2)^0.25 * (RTT^0.75) / (p^0.75) (Eq. 6) We will determine the value of C in the following subsection using Eq. 6.
RFC3649]. The average window size of CUBIC is calculated using Eq. 6 and the CUBIC TCP-friendly region for three different values of C. +--------+----------+-----------+------------+-----------+----------+ | Loss | Average | Average | CUBIC | CUBIC | CUBIC | | Rate P | TCP W | HSTCP W | (C=0.04) | (C=0.4) | (C=4) | +--------+----------+-----------+------------+-----------+----------+ | 10^-2 | 12 | 12 | 12 | 12 | 12 | | 10^-3 | 38 | 38 | 38 | 38 | 59 | | 10^-4 | 120 | 263 | 120 | 187 | 333 | | 10^-5 | 379 | 1795 | 593 | 1054 | 1874 | | 10^-6 | 1200 | 12279 | 3332 | 5926 | 10538 | | 10^-7 | 3795 | 83981 | 18740 | 33325 | 59261 | | 10^-8 | 12000 | 574356 | 105383 | 187400 | 333250 | +--------+----------+-----------+------------+-----------+----------+ Table 1 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.
+--------+-----------+-----------+------------+-----------+---------+ | Loss | Average | Average | CUBIC | CUBIC | CUBIC | | Rate P | TCP W | HSTCP W | (C=0.04) | (C=0.4) | (C=4) | +--------+-----------+-----------+------------+-----------+---------+ | 10^-2 | 12 | 12 | 12 | 12 | 12 | | 10^-3 | 38 | 38 | 38 | 38 | 38 | | 10^-4 | 120 | 263 | 120 | 120 | 120 | | 10^-5 | 379 | 1795 | 379 | 379 | 379 | | 10^-6 | 1200 | 12279 | 1200 | 1200 | 1874 | | 10^-7 | 3795 | 83981 | 3795 | 5926 | 10538 | | 10^-8 | 12000 | 574356 | 18740 | 33325 | 59261 | +--------+-----------+-----------+------------+-----------+---------+ Table 2 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, Eq. 6 is reduced to: AVG_W_cubic = 1.054 * (RTT^0.75) / (p^0.75) (Eq. 7) Eq. 7 is then used in the next subsection to show the scalability of CUBIC.
+------------------+-----------+---------+---------+---------+ | 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 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. 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
RFC5681]. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [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, <https://www.rfc-editor.org/info/rfc3168>. [RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows", RFC 3649, DOI 10.17487/RFC3649, December 2003, <https://www.rfc-editor.org/info/rfc3649>. [RFC3742] Floyd, S., "Limited Slow-Start for TCP with Large Congestion Windows", RFC 3742, DOI 10.17487/RFC3742, March 2004, <https://www.rfc-editor.org/info/rfc3742>. [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007, <https://www.rfc-editor.org/info/rfc4960>. [RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion Control Algorithms", BCP 133, RFC 5033, DOI 10.17487/RFC5033, August 2007, <https://www.rfc-editor.org/info/rfc5033>. [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, <https://www.rfc-editor.org/info/rfc5348>.
[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>. [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, <https://www.rfc-editor.org/info/rfc6582>. [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, <https://www.rfc-editor.org/info/rfc6675>. [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF Recommendations Regarding Active Queue Management", BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015, <https://www.rfc-editor.org/info/rfc7567>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [CEHRX07] Cai, H., Eun, D., Ha, S., Rhee, I., and L. Xu, "Stochastic Ordering for Internet Congestion Control and its Applications", In Proceedings of IEEE INFOCOM, DOI 10.1109/INFCOM.2007.111, May 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, August 2002. [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. [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.
[HRX08] Ha, S., Rhee, I., and L. Xu, "CUBIC: A New TCP-Friendly High-Speed TCP Variant", ACM SIGOPS Operating System Review, DOI 10.1145/1400097.1400105, July 2008. [K03] Kelly, T., "Scalable TCP: Improving Performance in HighSpeed Wide Area Networks", ACM SIGCOMM Computer Communication Review, DOI 10.1145/956981.956989, April 2003. [KWAF17] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst, "TCP Alternative Backoff with ECN (ABE)", Work in Progress, draft-ietf-tcpm-alternativebackoff-ecn-05, December 2017. [XHR04] Xu, L., Harfoush, K., and I. Rhee, "Binary Increase Congestion Control for Fast, Long Distance Networks", In Proceedings of IEEE INFOCOM, DOI 10.1109/INFCOM.2004.1354672, March 2004.