draft-ietf-tcpm-cubic-07.txt   rfc8312.txt 
TCP Maintenance and Minor Extensions (TCPM) WG I. Rhee Internet Engineering Task Force (IETF) I. Rhee
Internet-Draft NCSU Request for Comments: 8312 NCSU
Intended status: Informational L. Xu Category: Informational L. Xu
Expires: May 17, 2018 UNL ISSN: 2070-1721 UNL
S. Ha S. Ha
Colorado Colorado
A. Zimmermann A. Zimmermann
L. Eggert L. Eggert
NetApp
R. Scheffenegger R. Scheffenegger
November 13, 2017 NetApp
February 2018
CUBIC for Fast Long-Distance Networks CUBIC for Fast Long-Distance Networks
draft-ietf-tcpm-cubic-07
Abstract Abstract
CUBIC is an extension to the current TCP standards. It differs from CUBIC is an extension to the current TCP standards. It differs from
the current TCP standards only in the congestion control algorithm in the current TCP standards only in the congestion control algorithm on
the sender side. In particular, it uses a cubic function instead of the sender side. In particular, it uses a cubic function instead of
a linear window increase function of the current TCP standards to a linear window increase function of the current TCP standards to
improve scalability and stability under fast and long distance improve scalability and stability under fast and long-distance
networks. CUBIC and its predecessor algorithm have been adopted as networks. CUBIC and its predecessor algorithm have been adopted as
default by Linux and have been used for many years. This document defaults by Linux and have been used for many years. This document
provides a specification of CUBIC to enable third party provides a specification of CUBIC to enable third-party
implementations and to solicit the community feedback through implementations and to solicit community feedback through
experimentation on the performance of CUBIC. experimentation on the performance of CUBIC.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for informational purposes.
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-
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." 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.
This Internet-Draft will expire on May 17, 2018. 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 Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction ....................................................3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions .....................................................3
3. Design principles of CUBIC . . . . . . . . . . . . . . . . . 4 3. Design Principles of CUBIC ......................................4
4. CUBIC Congestion Control . . . . . . . . . . . . . . . . . . 6 4. CUBIC Congestion Control ........................................6
4.1. Window increase function . . . . . . . . . . . . . . . . 6 4.1. Window Increase Function ...................................6
4.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 7 4.2. TCP-Friendly Region ........................................7
4.3. Concave region . . . . . . . . . . . . . . . . . . . . . 8 4.3. Concave Region .............................................8
4.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 8 4.4. Convex Region ..............................................8
4.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 8 4.5. Multiplicative Decrease ....................................8
4.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 9 4.6. Fast Convergence ...........................................9
4.7. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.7. Timeout ...................................................10
4.8. Slowstart . . . . . . . . . . . . . . . . . . . . . . . . 10 4.8. Slow Start ................................................10
5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 10 5. Discussion .....................................................10
5.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 10 5.1. Fairness to Standard TCP ..................................11
5.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 12 5.2. Using Spare Capacity ......................................13
5.3. Difficult Environments . . . . . . . . . . . . . . . . . 13 5.3. Difficult Environments ....................................13
5.4. Investigating a Range of Environments . . . . . . . . . . 13 5.4. Investigating a Range of Environments .....................13
5.5. Protection against Congestion Collapse . . . . . . . . . 13 5.5. Protection against Congestion Collapse ....................14
5.6. Fairness within the Alternative Congestion Control 5.6. Fairness within the Alternative Congestion Control
Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 13 Algorithm .................................................14
5.7. Performance with Misbehaving Nodes and Outside Attackers 13 5.7. Performance with Misbehaving Nodes and Outside Attackers ..14
5.8. Behavior for Application-Limited Flows . . . . . . . . . 13 5.8. Behavior for Application-Limited Flows ....................14
5.9. Responses to Sudden or Transient Events . . . . . . . . . 14 5.9. Responses to Sudden or Transient Events ...................14
5.10. Incremental Deployment . . . . . . . . . . . . . . . . . 14 5.10. Incremental Deployment ...................................14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14 6. Security Considerations ........................................15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 7. IANA Considerations ............................................15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14 8. References .....................................................15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 8.1. Normative References ......................................15
9.1. Normative References . . . . . . . . . . . . . . . . . . 14 8.2. Informative References ....................................16
9.2. Informative References . . . . . . . . . . . . . . . . . 15 Acknowledgements ..................................................17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 Authors' Addresses ................................................18
1. Introduction 1. Introduction
The low utilization problem of TCP in fast long-distance networks is The low utilization problem of TCP in fast long-distance networks is
well documented in [K03] [RFC3649]. This problem arises from a slow well documented in [K03] and [RFC3649]. This problem arises from a
increase of congestion window following a congestion event in a slow increase of the congestion window following a congestion event
network with a large bandwidth delay product (BDP). Experience in a network with a large bandwidth-delay product (BDP). [HKLRX06]
[HKLRX06] indicates that this problem is frequently observed even in indicates that this problem is frequently observed even in the range
the range of congestion window sizes over several hundreds of packets of congestion window sizes over several hundreds of packets. This
especially under a network path with over 100ms round-trip times problem is equally applicable to all Reno-style TCP standards and
(RTTs). This problem is equally applicable to all Reno style TCP their variants, including TCP-RENO [RFC5681], TCP-NewReno [RFC6582]
standards and their variants, including TCP-RENO [RFC5681], TCP- [RFC6675], SCTP [RFC4960], and TFRC [RFC5348], which use the same
NewReno [RFC6582] [RFC6675], SCTP [RFC4960], TFRC [RFC5348] that use linear increase function for window growth, which we refer to
the same linear increase function for window growth, which we refer collectively as "Standard TCP" below.
to collectively as Standard TCP below.
CUBIC, originally proposed in [HRX08], is a modification to the CUBIC, originally proposed in [HRX08], is a modification to the
congestion control algorithm of Standard TCP to remedy this problem. congestion control algorithm of Standard TCP to remedy this problem.
This document describes the most recent specification of CUBIC. This document describes the most recent specification of CUBIC.
Specifically, CUBIC uses a cubic function instead of a linear window Specifically, CUBIC uses a cubic function instead of a linear window
increase function of Standard TCP to improve scalability and increase function of Standard TCP to improve scalability and
stability under fast and long distance networks. stability under fast and long-distance networks.
BIC-TCP [XHR04], a predecessor of CUBIC, has been selected as the Binary Increase Congestion Control (BIC-TCP) [XHR04], a predecessor
default TCP congestion control algorithm by Linux in the year 2005 of CUBIC, was selected as the default TCP congestion control
and been used for several years by the Internet community at large. algorithm by Linux in the year 2005 and has been used for several
CUBIC uses a similar window increase function as BIC-TCP and is years by the Internet community at large. CUBIC uses a similar
designed to be less aggressive and fairer to Standard TCP in window increase function as BIC-TCP and is designed to be less
bandwidth usage than BIC-TCP while maintaining the strengths of BIC- aggressive and fairer to Standard TCP in bandwidth usage than BIC-TCP
TCP such as stability, window scalability and RTT fairness. CUBIC while maintaining the strengths of BIC-TCP such as stability, window
has already replaced BIC-TCP as the default TCP congestion control scalability, and RTT fairness. CUBIC has already replaced BIC-TCP as
algorithm in Linux and has been deployed globally by Linux. Through the default TCP congestion control algorithm in Linux and has been
extensive testing in various Internet scenarios, we believe that deployed globally by Linux. Through extensive testing in various
CUBIC is safe for testing and deployment in the global Internet. 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 In the following sections, we first briefly explain the design
principles of CUBIC, then provide the exact specification of CUBIC, principles of CUBIC, then provide the exact specification of CUBIC,
and finally discuss the safety features of CUBIC following the and finally discuss the safety features of CUBIC following the
guidelines specified in [RFC5033]. guidelines specified in [RFC5033].
2. Conventions 2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "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 3. Design Principles of CUBIC
CUBIC is designed according to the following design principles. CUBIC is designed according to the following design principles:
Principle 1: For better network utilization and stability, CUBIC Principle 1: For better network utilization and stability, CUBIC
uses both the concave and convex profiles of a cubic function to uses both the concave and convex profiles of a cubic function to
increase the congestion window size, instead of using just a increase the congestion window size, instead of using just a
convex function. convex function.
Principle 2: To be TCP-friendly, CUBIC is designed to behave like 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 in networks with short RTTs and small bandwidth where
Standard TCP performs well. Standard TCP performs well.
Principle 3: For RTT-fairness, CUBIC is designed to achieve linear Principle 3: For RTT-fairness, CUBIC is designed to achieve linear
bandwidth share among flows with different RTTs. bandwidth sharing among flows with different RTTs.
Principle 4: CUBIC appropriately sets its multiplicative window Principle 4: CUBIC appropriately sets its multiplicative window
decrease factor, in order to balance between the scalability and decrease factor in order to balance between the scalability and
convergence speed. convergence speed.
Principle 1: For better network utilization and stability, CUBIC Principle 1: For better network utilization and stability, CUBIC
[HRX08] uses a cubic window increase function in terms of the elapsed [HRX08] uses a cubic window increase function in terms of the elapsed
time from the last congestion event. While most alternative time from the last congestion event. While most alternative
congestion control algorithms to Standard TCP increase the congestion congestion control algorithms to Standard TCP increase the congestion
window using convex functions, CUBIC uses both the concave and convex window using convex functions, CUBIC uses both the concave and convex
profiles of a cubic function for window growth. After a window profiles of a cubic function for window growth. After a window
reduction in response to a congestion event detected by duplicate reduction in response to a congestion event is detected by duplicate
ACKs or ECN-Echo ACKs[RFC3168], CUBIC registers the congestion window ACKs or Explicit Congestion Notification-Echo (ECN-Echo) ACKs
size where it got the congestion event as W_max and performs a [RFC3168], CUBIC registers the congestion window size where it got
multiplicative decrease of congestion window. After it enters into the congestion event as W_max and performs a multiplicative decrease
congestion avoidance, it starts to increase the congestion window of congestion window. After it enters into congestion avoidance, it
using the concave profile of the cubic function. The cubic function starts to increase the congestion window using the concave profile of
is set to have its plateau at W_max so that the concave window the cubic function. The cubic function is set to have its plateau at
increase continues until the window size becomes W_max. After that, W_max so that the concave window increase continues until the window
the cubic function turns into a convex profile and the convex window size becomes W_max. After that, the cubic function turns into a
increase begins. This style of window adjustment (concave and then convex profile and the convex window increase begins. This style of
convex) improves the algorithm stability while maintaining high window adjustment (concave and then convex) improves the algorithm
network utilization [CEHRX07]. This is because the window size stability while maintaining high network utilization [CEHRX07]. This
remains almost constant, forming a plateau around W_max where network is because the window size remains almost constant, forming a plateau
utilization is deemed highest. Under steady state, most window size around W_max where network utilization is deemed highest. Under
samples of CUBIC are close to W_max, thus promoting high network steady state, most window size samples of CUBIC are close to W_max,
utilization and stability. Note that those congestion control thus promoting high network utilization and stability. Note that
algorithms using only convex functions to increase the congestion those congestion control algorithms using only convex functions to
window size have the maximum increments around W_max and thus increase the congestion window size have the maximum increments
introduce a large number of packet bursts around the saturation point around W_max, and thus introduce a large number of packet bursts
of the network, likely causing frequent global loss synchronizations. around the saturation point of the network, likely causing frequent
global loss synchronizations.
Principle 2: CUBIC promotes per-flow fairness to Standard TCP. Note Principle 2: CUBIC promotes per-flow fairness to Standard TCP. Note
that Standard TCP performs well under short RTT and small bandwidth that Standard TCP performs well under short RTT and small bandwidth
(or small BDP) networks. Only in long RTT and large bandwidth (or (or small BDP) networks. There is only a scalability problem in
large BDP) networks, it has the scalability problem. An alternative networks with long RTTs and large bandwidth (or large BDP). An
congestion control algorithm to Standard TCP designed to be friendly alternative congestion control algorithm to Standard TCP designed to
to Standard TCP at a per-flow basis must operate to increase its be friendly to Standard TCP on a per-flow basis must operate to
congestion window less aggressively in small BDP networks than in increase its congestion window less aggressively in small BDP
large BDP networks. The aggressiveness of CUBIC mainly depends on networks than in large BDP networks. The aggressiveness of CUBIC
the maximum window size before a window reduction, which is smaller mainly depends on the maximum window size before a window reduction,
in small BDP networks than in large BDP networks. Thus, CUBIC which is smaller in small BDP networks than in large BDP networks.
increases its congestion window less aggressively in small BDP Thus, CUBIC increases its congestion window less aggressively in
networks than in large BDP networks. Furthermore, in cases when the small BDP networks than in large BDP networks. Furthermore, in cases
cubic function of CUBIC increases its congestion window less when the cubic function of CUBIC increases its congestion window less
aggressively than Standard TCP, CUBIC simply follows the window size aggressively than Standard TCP, CUBIC simply follows the window size
of Standard TCP to ensure that CUBIC achieves at least the same of Standard TCP to ensure that CUBIC achieves at least the same
throughput as Standard TCP in small BDP networks. We call this throughput as Standard TCP in small BDP networks. We call this
region where CUBIC behaves like Standard TCP, the TCP-friendly region where CUBIC behaves like Standard TCP, the "TCP-friendly
region. region".
Principle 3: Two CUBIC flows with different RTTs have their Principle 3: Two CUBIC flows with different RTTs have their
throughput ratio linearly proportional to the inverse of their RTT throughput ratio linearly proportional to the inverse of their RTT
ratio, where the throughput of a flow is approximately its congestion ratio, where the throughput of a flow is approximately the size of
window size divided by its RTT. Specifically, CUBIC maintains a its congestion window divided by its RTT. Specifically, CUBIC
window increase rate independent of RTTs outside of the TCP-friendly maintains a window increase rate independent of RTTs outside of the
region, and thus flows with different RTTs have similar congestion TCP-friendly region, and thus flows with different RTTs have similar
window sizes under steady state when they operate outside the TCP- congestion window sizes under steady state when they operate outside
friendly region. This notion of a linear throughput ratio is similar the TCP-friendly region. This notion of a linear throughput ratio is
to that of Standard TCP under high statistical multiplexing similar to that of Standard TCP under high statistical multiplexing
environments where packet losses are independent of individual flow environments where packet losses are independent of individual flow
rates. However, under low statistical multiplexing environments, the rates. However, under low statistical multiplexing environments, the
throughput ratio of Standard TCP flows with different RTTs is throughput ratio of Standard TCP flows with different RTTs is
quadratically proportional to the inverse of their RTT ratio [XHR04]. quadratically proportional to the inverse of their RTT ratio [XHR04].
CUBIC always ensures the linear throughput ratio independent of the CUBIC always ensures the linear throughput ratio independent of the
levels of statistical multiplexing. This is an improvement over levels of statistical multiplexing. This is an improvement over
Standard TCP. While there is no consensus on particular throughput Standard TCP. While there is no consensus on particular throughput
ratios of different RTT flows, we believe that under wired Internet, ratios of different RTT flows, we believe that under wired Internet,
use of a linear throughput ratio seems more reasonable than equal use of a linear throughput ratio seems more reasonable than equal
throughputs (i.e., same throughput for flows with different RTTs) or throughputs (i.e., the same throughput for flows with different RTTs)
a higher order throughput ratio (e.g., a quadratical throughput ratio or a higher-order throughput ratio (e.g., a quadratical throughput
of Standard TCP under low statistical multiplexing environments). ratio of Standard TCP under low statistical multiplexing
environments).
Principle 4: To balance between the scalability and convergence Principle 4: To balance between the scalability and convergence
speed, CUBIC sets the multiplicative window decrease factor to 0.7 speed, CUBIC sets the multiplicative window decrease factor to 0.7
while Standard TCP uses 0.5. While this improves the scalability of while Standard TCP uses 0.5. While this improves the scalability of
CUBIC, a side effect of this decision is slower convergence CUBIC, a side effect of this decision is slower convergence,
especially under low statistical multiplexing environments. This especially under low statistical multiplexing environments. This
design choice is following the observation that the author of HSTCP design choice is following the observation that the author of
[RFC3649] has made along with other researchers (e.g., [GV02]): the HighSpeed TCP (HSTCP) [RFC3649] has made along with other researchers
current Internet becomes more asynchronous with less frequent loss (e.g., [GV02]): the current Internet becomes more asynchronous with
synchronizations with high statistical multiplexing. Under this less frequent loss synchronizations with high statistical
environment, even strict Multiplicative-Increase Multiplicative- multiplexing. Under this environment, even strict Multiplicative-
Decrease (MIMD) can converge. CUBIC flows with the same RTT always Increase Multiplicative-Decrease (MIMD) can converge. CUBIC flows
converge to the same throughput independent of statistical with the same RTT always converge to the same throughput independent
multiplexing, thus achieving intra-algorithm fairness. We also find of statistical multiplexing, thus achieving intra-algorithm fairness.
that under the environments with sufficient statistical multiplexing, We also find that under the environments with sufficient statistical
the convergence speed of CUBIC flows is reasonable. multiplexing, the convergence speed of CUBIC flows is reasonable.
4. CUBIC Congestion Control 4. CUBIC Congestion Control
The unit of all window sizes in this document is segments of the 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. maximum segment size (MSS), and the unit of all times is seconds.
Let cwnd denote the congestion window size of a flow, and ssthresh Let cwnd denote the congestion window size of a flow, and ssthresh
denote the slow start threshold. denote the slow-start threshold.
4.1. Window increase function 4.1. Window Increase Function
CUBIC maintains the acknowledgment (ACK) clocking of Standard TCP by CUBIC maintains the acknowledgment (ACK) clocking of Standard TCP by
increasing congestion window only at the reception of ACK. It does increasing the congestion window only at the reception of an ACK. It
not make any change to the fast recovery and retransmit of TCP, such does not make any change to the fast recovery and retransmit of TCP,
as TCP-NewReno [RFC6582] [RFC6675]. During congestion avoidance such as TCP-NewReno [RFC6582] [RFC6675]. During congestion avoidance
after a congestion event where a packet loss is detected by duplicate 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 ACKs or a network congestion is detected by ACKs with ECN-Echo flags
[RFC3168], CUBIC changes the window increase function of Standard [RFC3168], CUBIC changes the window increase function of Standard
TCP. Suppose that W_max is the window size just before the window is TCP. Suppose that W_max is the window size just before the window is
reduced in the last congestion event. reduced in the last congestion event.
CUBIC uses the following window increase function: CUBIC uses the following window increase function:
W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1) W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1)
skipping to change at page 7, line 4 skipping to change at page 7, line 7
K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2) K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2)
where beta_cubic is the CUBIC multiplication decrease factor, that where beta_cubic is the CUBIC multiplication decrease factor, that
is, when a congestion event is detected, CUBIC reduces its cwnd to 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 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. Section 4.5 and how we set C in Section 5.
Upon receiving an ACK during congestion avoidance, CUBIC computes the Upon receiving an ACK during congestion avoidance, CUBIC computes the
window increase rate during the next RTT period using Eq. 1. It sets window increase rate during the next RTT period using Eq. 1. It sets
W_cubic(t+RTT) as the candidate target value of congestion window, W_cubic(t+RTT) as the candidate target value of the congestion
where RTT is the weighted average RTT calculated by Standard TCP. window, where RTT is the weighted average RTT calculated by Standard
TCP.
Depending on the value of the current congestion window size cwnd, Depending on the value of the current congestion window size cwnd,
CUBIC runs in three different modes. CUBIC runs in three different modes.
1) The TCP-friendly region, which ensures that CUBIC achieves at 1. The TCP-friendly region, which ensures that CUBIC achieves at
least the same throughput as Standard TCP. least the same throughput as Standard TCP.
2) The concave region, if CUBIC is not in the TCP-friendly region 2. The concave region, if CUBIC is not in the TCP-friendly region
and cwnd is less than W_max. and cwnd is less than W_max.
3) The convex region, if CUBIC is not in the TCP-friendly region 3. The convex region, if CUBIC is not in the TCP-friendly region and
and cwnd is greater than W_max. cwnd is greater than W_max.
Below, we describe the exact actions taken by CUBIC in each region. Below, we describe the exact actions taken by CUBIC in each region.
4.2. TCP-friendly region 4.2. TCP-Friendly Region
Standard TCP performs well in certain types of networks, for example, Standard TCP performs well in certain types of networks, for example,
under short RTT and small bandwidth (or small BDP) networks. In under short RTT and small bandwidth (or small BDP) networks. In
these networks, we use the TCP-friendly region to ensure that CUBIC these networks, we use the TCP-friendly region to ensure that CUBIC
achieves at least the same throughput as Standard TCP. achieves at least the same throughput as Standard TCP.
The TCP-friendly region is designed according to the analysis The TCP-friendly region is designed according to the analysis
described in [FHP00]. The analysis studies the performance of an described in [FHP00]. The analysis studies the performance of an
Additive Increase and Multiplicative Decrease (AIMD) algorithm with Additive Increase and Multiplicative Decrease (AIMD) algorithm with
an additive factor of alpha_aimd (segments per RTT) and a an additive factor of alpha_aimd (segments per RTT) and a
skipping to change at page 7, line 49 skipping to change at page 8, line 6
AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) / AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) /
(2*(1-beta_aimd)*p) ]^0.5 (Eq. 3) (2*(1-beta_aimd)*p) ]^0.5 (Eq. 3)
Based on the above analysis, CUBIC uses Eq. 4 to estimate the window Based on the above analysis, CUBIC uses Eq. 4 to estimate the window
size W_est of AIMD(alpha_aimd, beta_aimd) with size W_est of AIMD(alpha_aimd, beta_aimd) with
alpha_aimd=3*(1-beta_cubic)/(1+beta_cubic) and beta_aimd=beta_cubic, 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 which achieves the same average window size as Standard TCP. When
receiving an ACK in congestion avoidance (cwnd could be greater than 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 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 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 ACK. be set to W_est(t) at each reception of an ACK.
W_est(t) = W_max*beta_cubic + W_est(t) = W_max*beta_cubic +
[3*(1-beta_cubic)/(1+beta_cubic)] * (t/RTT) (Eq. 4) [3*(1-beta_cubic)/(1+beta_cubic)] * (t/RTT) (Eq. 4)
4.3. Concave region 4.3. Concave Region
When receiving an ACK in congestion avoidance, if CUBIC is not in the 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 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 concave region. In this region, cwnd MUST be incremented by
(W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where
W_cubic(t+RTT) is calculated using Eq. 1. W_cubic(t+RTT) is calculated using Eq. 1.
4.4. Convex region 4.4. Convex Region
When receiving an ACK in congestion avoidance, if CUBIC is not in the 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 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 CUBIC is in the convex region. The convex region indicates that the
network conditions might have been perturbed since the last network conditions might have been perturbed since the last
congestion event, possibly implying more available bandwidth after congestion event, possibly implying more available bandwidth after
some flow departures. Since the Internet is highly asynchronous, some flow departures. Since the Internet is highly asynchronous,
some amount of perturbation is always possible without causing a some amount of perturbation is always possible without causing a
major change in available bandwidth. In this region, CUBIC is being major change in available bandwidth. In this region, CUBIC is being
very careful by very slowly increasing its window size. The convex very careful by very slowly increasing its window size. The convex
profile ensures that the window increases very slowly at the profile ensures that the window increases very slowly at the
beginning and gradually increases its increase rate. We also call beginning and gradually increases its increase rate. We also call
this region as the maximum probing phase since CUBIC is searching for this region the "maximum probing phase" since CUBIC is searching for
a new W_max. In this region, cwnd MUST be incremented by a new W_max. In this region, cwnd MUST be incremented by
(W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where
W_cubic(t+RTT) is calculated using Eq. 1. W_cubic(t+RTT) is calculated using Eq. 1.
4.5. Multiplicative decrease 4.5. Multiplicative Decrease
When a packet loss is detected by duplicate ACKs or a network When a packet loss is detected by duplicate ACKs or a network
congestion is detected by ECN-Echo ACKs, CUBIC updates its W_max, congestion is detected by ECN-Echo ACKs, CUBIC updates its W_max,
cwnd, and ssthresh (slow start threshold) as follows. Parameter cwnd, and ssthresh as follows. Parameter beta_cubic SHOULD be set to
beta_cubic SHOULD be set to 0.7. 0.7.
W_max = cwnd; // save window size before reduction W_max = cwnd; // save window size before reduction
ssthresh = cwnd * beta_cubic; // new slow start threshold ssthresh = cwnd * beta_cubic; // new slow-start threshold
ssthresh = max(ssthresh, 2); // threshold is at least 2 MSS ssthresh = max(ssthresh, 2); // threshold is at least 2 MSS
cwnd = cwnd * beta_cubic; // window reduction cwnd = cwnd * beta_cubic; // window reduction
A side effect of setting beta_cubic to a bigger value than 0.5 is 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 slower convergence. We believe that while a more adaptive setting of
beta_cubic could result in faster convergence, it will make the beta_cubic could result in faster convergence, it will make the
analysis of CUBIC much harder. This adaptive adjustment of analysis of CUBIC much harder. This adaptive adjustment of
beta_cubic is an item for the next version of CUBIC. beta_cubic is an item for the next version of CUBIC.
4.6. Fast convergence 4.6. Fast Convergence
To improve the convergence speed of CUBIC, we add a heuristic in To improve the convergence speed of CUBIC, we add a heuristic in
CUBIC. When a new flow joins the network, existing flows in the 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 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 some room for growth if the existing flows have been using all the
bandwidth of the network. To speed up this bandwidth release by bandwidth of the network. To speed up this bandwidth release by
existing flows, the following mechanism called fast convergence existing flows, the following mechanism called "fast convergence"
SHOULD be implemented. SHOULD be implemented.
With fast convergence, when a congestion event occurs, before the With fast convergence, when a congestion event occurs, before the
window reduction of the congestion window, a flow remembers the last window reduction of the congestion window, a flow remembers the last
value of W_max before it updates W_max for the current congestion value of W_max before it updates W_max for the current congestion
event. Let us call the last value of W_max to be W_last_max. event. Let us call the last value of W_max to be W_last_max.
if (W_max < W_last_max){ // should we make room for others if (W_max < W_last_max){ // should we make room for others
W_last_max = W_max; // remember the last W_max W_last_max = W_max; // remember the last W_max
W_max = W_max*(1.0+beta_cubic)/2.0; // further reduce W_max W_max = W_max*(1.0+beta_cubic)/2.0; // further reduce W_max
skipping to change at page 9, line 34 skipping to change at page 9, line 40
W_last_max = W_max // remember the last W_max W_last_max = W_max // remember the last W_max
} }
At a congestion event, if the current value of W_max is less than At a congestion event, if the current value of W_max is less than
W_last_max, this indicates that the saturation point experienced by W_last_max, this indicates that the saturation point experienced by
this flow is getting reduced because of the change in available this flow is getting reduced because of the change in available
bandwidth. Then we allow this flow to release more bandwidth by bandwidth. Then we allow this flow to release more bandwidth by
reducing W_max further. This action effectively lengthens the time reducing W_max further. This action effectively lengthens the time
for this flow to increase its congestion window because the reduced for this flow to increase its congestion window because the reduced
W_max forces the flow to have the plateau earlier. This allows more W_max forces the flow to have the plateau earlier. This allows more
time for the new flow to catch up its congestion window size time for the new flow to catch up to its congestion window size.
The fast convergence is designed for network environments with The fast convergence is designed for network environments with
multiple CUBIC flows. In network environments with only a single multiple CUBIC flows. In network environments with only a single
CUBIC flow and without any other traffic, the fast convergence SHOULD CUBIC flow and without any other traffic, the fast convergence SHOULD
be disabled. be disabled.
4.7. Timeout 4.7. Timeout
In case of timeout, CUBIC follows Standard TCP to reduce cwnd In case of timeout, CUBIC follows Standard TCP to reduce cwnd
[RFC5681], but sets ssthresh using beta_cubic (same as in [RFC5681], but sets ssthresh using beta_cubic (same as in
Section 4.5) that is different from Standard TCP [RFC5681]. Section 4.5) that is different from Standard TCP [RFC5681].
During the first congestion avoidance after a timeout, CUBIC During the first congestion avoidance after a timeout, CUBIC
increases its congestion window size using Eq. 1, where t is the increases its congestion window size using Eq. 1, where t is the
elapsed time since the beginning of the current congestion avoidance, 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 K is set to 0, and W_max is set to the congestion window size at the
beginning of the current congestion avoidance. beginning of the current congestion avoidance.
4.8. Slowstart 4.8. Slow Start
CUBIC MUST employ a slow start algorithm, when the cwnd is no more CUBIC MUST employ a slow-start algorithm, when the cwnd is no more
than ssthresh. Among the slow start algorithms, CUBIC MAY choose the than ssthresh. Among the slow-start algorithms, CUBIC MAY choose the
standard TCP slow start [RFC5681] in general networks, or the limited standard TCP slow start [RFC5681] in general networks, or the limited
slow start [RFC3742] or hybrid slow start [HR08] for fast and long- slow start [RFC3742] or hybrid slow start [HR08] for fast and long-
distance networks. distance networks.
In the case when CUBIC runs the hybrid slow start [HR08], it may exit 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 the first slow start without incurring any packet loss and thus W_max
is undefined. In this special case, CUBIC switches to congestion is undefined. In this special case, CUBIC switches to congestion
avoidance and increases its congestion window size using Eq. 1, where avoidance and increases its congestion window size using Eq. 1, where
t is the elapsed time since the beginning of the current congestion 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 avoidance, K is set to 0, and W_max is set to the congestion window
size at the beginning of the current congestion avoidance. size at the beginning of the current congestion avoidance.
5. Discussion 5. Discussion
In this section, we further discuss the safety features of CUBIC In this section, we further discuss the safety features of CUBIC
following the guidelines specified in [RFC5033]. following the guidelines specified in [RFC5033].
With a deterministic loss model where the number of packets between With a deterministic loss model where the number of packets between
two successive packet losses is always 1/p, CUBIC always operates two successive packet losses is always 1/p, CUBIC always operates
with the concave window profile which greatly simplifies the with the concave window profile, which greatly simplifies the
performance analysis of CUBIC. The average window size of CUBIC can performance analysis of CUBIC. The average window size of CUBIC can
be obtained by the following function: be obtained by the following function:
AVG_W_cubic = [C*(3+beta_cubic)/(4*(1-beta_cubic))]^0.25 * AVG_W_cubic = [C*(3+beta_cubic)/(4*(1-beta_cubic))]^0.25 *
(RTT^0.75) / (p^0.75) (Eq. 5) (RTT^0.75) / (p^0.75) (Eq. 5)
With beta_cubic set to 0.7, the above formula is reduced to: 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) 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 We will determine the value of C in the following subsection using
Eq. 6. Eq. 6.
5.1. Fairness to standard TCP 5.1. Fairness to Standard TCP
In environments where Standard TCP is able to make reasonable use of In environments where Standard TCP is able to make reasonable use of
the available bandwidth, CUBIC does not significantly change this the available bandwidth, CUBIC does not significantly change this
state. state.
Standard TCP performs well in the following two types of networks: Standard TCP performs well in the following two types of networks:
1. networks with a small bandwidth-delay product (BDP) 1. networks with a small bandwidth-delay product (BDP)
2. networks with a short RTT, but not necessarily a small 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 CUBIC is designed to behave very similarly to Standard TCP in the
above two types of networks. The following two tables show the above two types of networks. The following two tables show the
average window sizes of Standard TCP, HSTCP, and CUBIC. The average average window sizes of Standard TCP, HSTCP, and CUBIC. The average
window sizes of Standard TCP and HSTCP are from [RFC3649]. The window sizes of Standard TCP and HSTCP are from [RFC3649]. The
average window size of CUBIC is calculated using Eq. 6 and the CUBIC average window size of CUBIC is calculated using Eq. 6 and the CUBIC
TCP friendly region for three different values of C. TCP-friendly region for three different values of C.
+--------+----------+-----------+------------+-----------+----------+ +--------+----------+-----------+------------+-----------+----------+
| Loss | Average | Average | CUBIC | CUBIC | CUBIC | | Loss | Average | Average | CUBIC | CUBIC | CUBIC |
| Rate P | TCP W | HSTCP W | (C=0.04) | (C=0.4) | (C=4) | | Rate P | TCP W | HSTCP W | (C=0.04) | (C=0.4) | (C=4) |
+--------+----------+-----------+------------+-----------+----------+ +--------+----------+-----------+------------+-----------+----------+
| 10^-2 | 12 | 12 | 12 | 12 | 12 | | 10^-2 | 12 | 12 | 12 | 12 | 12 |
| 10^-3 | 38 | 38 | 38 | 38 | 59 | | 10^-3 | 38 | 38 | 38 | 38 | 59 |
| 10^-4 | 120 | 263 | 120 | 187 | 333 | | 10^-4 | 120 | 263 | 120 | 187 | 333 |
| 10^-5 | 379 | 1795 | 593 | 1054 | 1874 | | 10^-5 | 379 | 1795 | 593 | 1054 | 1874 |
| 10^-6 | 1200 | 12279 | 3332 | 5926 | 10538 | | 10^-6 | 1200 | 12279 | 3332 | 5926 | 10538 |
| 10^-7 | 3795 | 83981 | 18740 | 33325 | 59261 | | 10^-7 | 3795 | 83981 | 18740 | 33325 | 59261 |
| 10^-8 | 12000 | 574356 | 105383 | 187400 | 333250 | | 10^-8 | 12000 | 574356 | 105383 | 187400 | 333250 |
+--------+----------+-----------+------------+-----------+----------+ +--------+----------+-----------+------------+-----------+----------+
Response function of Standard TCP, HSTCP, and CUBIC in networks with
RTT = 0.1 seconds. The average window size is in MSS-sized segments.
Table 1 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 | | Loss | Average | Average | CUBIC | CUBIC | CUBIC |
| Rate P | TCP W | HSTCP W | (C=0.04) | (C=0.4) | (C=4) | | Rate P | TCP W | HSTCP W | (C=0.04) | (C=0.4) | (C=4) |
+--------+-----------+-----------+------------+-----------+---------+ +--------+-----------+-----------+------------+-----------+---------+
| 10^-2 | 12 | 12 | 12 | 12 | 12 | | 10^-2 | 12 | 12 | 12 | 12 | 12 |
| 10^-3 | 38 | 38 | 38 | 38 | 38 | | 10^-3 | 38 | 38 | 38 | 38 | 38 |
| 10^-4 | 120 | 263 | 120 | 120 | 120 | | 10^-4 | 120 | 263 | 120 | 120 | 120 |
| 10^-5 | 379 | 1795 | 379 | 379 | 379 | | 10^-5 | 379 | 1795 | 379 | 379 | 379 |
| 10^-6 | 1200 | 12279 | 1200 | 1200 | 1874 | | 10^-6 | 1200 | 12279 | 1200 | 1200 | 1874 |
| 10^-7 | 3795 | 83981 | 3795 | 5926 | 10538 | | 10^-7 | 3795 | 83981 | 3795 | 5926 | 10538 |
| 10^-8 | 12000 | 574356 | 18740 | 33325 | 59261 | | 10^-8 | 12000 | 574356 | 18740 | 33325 | 59261 |
+--------+-----------+-----------+------------+-----------+---------+ +--------+-----------+-----------+------------+-----------+---------+
Response function of Standard TCP, HSTCP, and CUBIC in networks with
RTT = 0.01 seconds. The average window size is in MSS-sized
segments.
Table 2 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 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 friendly to TCP than HSTCP, especially in networks with a short RTT
where TCP performs reasonably well. For example, in a network with 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 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 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 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 achieves exactly the same rate as Standard TCP, whereas HSTCP is
about ten times more aggressive than Standard TCP. about ten times more aggressive than Standard TCP.
We can see that C determines the aggressiveness of CUBIC in competing We can see that C determines the aggressiveness of CUBIC in competing
with other congestion control algorithms for the bandwidth. CUBIC is with other congestion control algorithms for bandwidth. CUBIC is
more friendly to the Standard TCP, if the value of C is lower. more friendly to Standard TCP, if the value of C is lower. However,
However, we do not recommend to set C to a very low value like 0.04, we do not recommend setting C to a very low value like 0.04, since
since CUBIC with a low C cannot efficiently use the bandwidth in long CUBIC with a low C cannot efficiently use the bandwidth in long RTT
RTT and high bandwidth networks. Based on these observations and our and high-bandwidth networks. Based on these observations and our
experiments, we find C=0.4 gives a good balance between TCP- experiments, we find C=0.4 gives a good balance between TCP-
friendliness and aggressiveness of window increase. Therefore, C 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: 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) 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 Eq. 7 is then used in the next subsection to show the scalability of
CUBIC. CUBIC.
5.2. Using Spare Capacity 5.2. Using Spare Capacity
CUBIC uses a more aggressive window increase function than Standard CUBIC uses a more aggressive window increase function than Standard
TCP under long RTT and high bandwidth networks. TCP under long RTT and high-bandwidth networks.
The following table shows that to achieve the 10Gbps rate, Standard 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 TCP requires a packet loss rate of 2.0e-10, while CUBIC requires a
packet loss rate of 2.9e-8. packet loss rate of 2.9e-8.
+------------------+-----------+---------+---------+---------+ +------------------+-----------+---------+---------+---------+
| Throughput(Mbps) | Average W | TCP P | HSTCP P | CUBIC P | | Throughput(Mbps) | Average W | TCP P | HSTCP P | CUBIC P |
+------------------+-----------+---------+---------+---------+ +------------------+-----------+---------+---------+---------+
| 1 | 8.3 | 2.0e-2 | 2.0e-2 | 2.0e-2 | | 1 | 8.3 | 2.0e-2 | 2.0e-2 | 2.0e-2 |
| 10 | 83.3 | 2.0e-4 | 3.9e-4 | 2.9e-4 | | 10 | 83.3 | 2.0e-4 | 3.9e-4 | 2.9e-4 |
| 100 | 833.3 | 2.0e-6 | 2.5e-5 | 1.4e-5 | | 100 | 833.3 | 2.0e-6 | 2.5e-5 | 1.4e-5 |
| 1000 | 8333.3 | 2.0e-8 | 1.5e-6 | 6.3e-7 | | 1000 | 8333.3 | 2.0e-8 | 1.5e-6 | 6.3e-7 |
| 10000 | 83333.3 | 2.0e-10 | 1.0e-7 | 2.9e-8 | | 10000 | 83333.3 | 2.0e-10 | 1.0e-7 | 2.9e-8 |
+------------------+-----------+---------+---------+---------+ +------------------+-----------+---------+---------+---------+
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.
Table 3 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 Our test results in [HKLRX06] indicate that CUBIC uses the spare
bandwidth left unused by existing Standard TCP flows in the same bandwidth left unused by existing Standard TCP flows in the same
bottleneck link without taking away much bandwidth from the existing bottleneck link without taking away much bandwidth from the existing
flows. flows.
5.3. Difficult Environments 5.3. Difficult Environments
CUBIC is designed to remedy the poor performance of TCP in fast and CUBIC is designed to remedy the poor performance of TCP in fast and
long-distance networks. long-distance networks.
5.4. Investigating a Range of Environments 5.4. Investigating a Range of Environments
CUBIC has been extensively studied by using both NS-2 simulation and CUBIC has been extensively studied by using both NS-2 simulation and
test-bed experiments covering a wide range of network environments. test-bed experiments covering a wide range of network environments.
More information can be found in [HKLRX06]. More information can be found in [HKLRX06].
Same as Standard TCP, CUBIC is a loss-based congestion control Same as Standard TCP, CUBIC is a loss-based congestion control
algorithm. Because CUBIC is designed to be more aggressive (due to algorithm. Because CUBIC is designed to be more aggressive (due to a
faster window increase function and bigger multiplicative decrease faster window increase function and bigger multiplicative decrease
factor) than Standard TCP in fast and long distance networks, it can factor) than Standard TCP in fast and long-distance networks, it can
fill large drop-tail buffers more quickly than Standard TCP and fill large drop-tail buffers more quickly than Standard TCP and
increase the risk of a standing queue[KWAF16]. In this case, proper increase the risk of a standing queue [KWAF17]. In this case, proper
queue sizing and management [RFC7567] could be used to reduce the queue sizing and management [RFC7567] could be used to reduce the
packet queueing delay. packet queuing delay.
5.5. Protection against Congestion Collapse 5.5. Protection against Congestion Collapse
With regard to the potential of causing congestion collapse, CUBIC With regard to the potential of causing congestion collapse, CUBIC
behaves like Standard TCP since CUBIC modifies only the window behaves like Standard TCP since CUBIC modifies only the window
adjustment algorithm of TCP. Thus, it does not modify the ACK adjustment algorithm of TCP. Thus, it does not modify the ACK
clocking and Timeout behaviors of Standard TCP. clocking and Timeout behaviors of Standard TCP.
5.6. Fairness within the Alternative Congestion Control Algorithm. 5.6. Fairness within the Alternative Congestion Control Algorithm
CUBIC ensures convergence of competing CUBIC flows with the same RTT CUBIC ensures convergence of competing CUBIC flows with the same RTT
in the same bottleneck links to an equal throughput. When competing in the same bottleneck links to an equal throughput. When competing
flows have different RTTs, their throughput ratio is linearly flows have different RTTs, their throughput ratio is linearly
proportional to the inverse of their RTT ratios. This is true proportional to the inverse of their RTT ratios. This is true
independent of the level of statistical multiplexing in the link. independent of the level of statistical multiplexing in the link.
5.7. Performance with Misbehaving Nodes and Outside Attackers 5.7. Performance with Misbehaving Nodes and Outside Attackers
This is not considered in the current CUBIC. This is not considered in the current CUBIC.
skipping to change at page 14, line 9 skipping to change at page 14, line 38
CUBIC does not raise its congestion window size if the flow is CUBIC does not raise its congestion window size if the flow is
currently limited by the application instead of the congestion currently limited by the application instead of the congestion
window. In case of long periods when cwnd has not been updated due window. In case of long periods when cwnd has not been updated due
to the application rate limit, such as idle periods, t in Eq. 1 MUST to the application rate limit, such as idle periods, t in Eq. 1 MUST
NOT include these periods; otherwise, W_cubic(t) might be very high NOT include these periods; otherwise, W_cubic(t) might be very high
after restarting from these periods. after restarting from these periods.
5.9. Responses to Sudden or Transient Events 5.9. Responses to Sudden or Transient Events
In case that there is a sudden congestion, a routing change, or a If there is a sudden congestion, a routing change, or a mobility
mobility event, CUBIC behaves the same as Standard TCP. event, CUBIC behaves the same as Standard TCP.
5.10. Incremental Deployment 5.10. Incremental Deployment
CUBIC requires only the change of TCP senders, and it does not make CUBIC requires only the change of TCP senders, and it does not make
any changes to TCP receivers. That is, a CUBIC sender works any changes to TCP receivers. That is, a CUBIC sender works
correctly with the Standard TCP receivers. In addition, CUBIC does correctly with the Standard TCP receivers. In addition, CUBIC does
not require any changes to the routers, and does not require any not require any changes to the routers and does not require any
assistant from the routers. assistance from the routers.
6. Security Considerations 6. Security Considerations
This proposal makes no changes to the underlying security of TCP. This proposal makes no changes to the underlying security of TCP.
More information about TCP security concerns can be found in More information about TCP security concerns can be found in
[RFC5681]. [RFC5681].
7. IANA Considerations 7. IANA Considerations
There are no IANA considerations regarding this document. This document does not require any IANA actions.
8. Acknowledgements
Alexander Zimmermann and Lars Eggert have received funding from the
European Union's Horizon 2020 research and innovation program
2014-2018 under grant agreement No. 644866 (SSICLOPS). This document
reflects only the authors' views and the European Commission is not
responsible for any use that may be made of the information it
contains.
9. References 8. References
9.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001, RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>. <https://www.rfc-editor.org/info/rfc3168>.
skipping to change at page 15, line 47 skipping to change at page 16, line 25
and Y. Nishida, "A Conservative Loss Recovery Algorithm and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP", Based on Selective Acknowledgment (SACK) for TCP",
RFC 6675, DOI 10.17487/RFC6675, August 2012, RFC 6675, DOI 10.17487/RFC6675, August 2012,
<https://www.rfc-editor.org/info/rfc6675>. <https://www.rfc-editor.org/info/rfc6675>.
[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management", Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015, BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>. <https://www.rfc-editor.org/info/rfc7567>.
9.2. Informative References [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>.
8.2. Informative References
[CEHRX07] Cai, H., Eun, D., Ha, S., Rhee, I., and L. Xu, "Stochastic [CEHRX07] Cai, H., Eun, D., Ha, S., Rhee, I., and L. Xu, "Stochastic
Ordering for Internet Congestion Control and its Ordering for Internet Congestion Control and its
Applications", In Proceedings of IEEE INFOCOM , May 2007. 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 [FHP00] Floyd, S., Handley, M., and J. Padhye, "A Comparison of
Equation-Based and AIMD Congestion Control", May 2000. Equation-Based and AIMD Congestion Control", May 2000.
[GV02] Gorinsky, S. and H. Vin, "Extended Analysis of Binary [GV02] Gorinsky, S. and H. Vin, "Extended Analysis of Binary
Adjustment Algorithms", Technical Report TR2002-29, Adjustment Algorithms", Technical Report TR2002-29,
Department of Computer Sciences , The University of Texas Department of Computer Sciences, The University of
at Austin , August 2002. Texas at Austin, August 2002.
[HKLRX06] Ha, S., Kim, Y., Le, L., Rhee, I., and L. Xu, "A Step [HKLRX06] Ha, S., Kim, Y., Le, L., Rhee, I., and L. Xu, "A Step
toward Realistic Performance Evaluation of High-Speed TCP toward Realistic Performance Evaluation of High-Speed TCP
Variants", International Workshop on Protocols for Fast Variants", International Workshop on Protocols for Fast
Long-Distance Networks , February 2006. Long-Distance Networks.
[HR08] Ha, S. and I. Rhee, "Hybrid Slow Start for High-Bandwidth [HR08] Ha, S. and I. Rhee, "Hybrid Slow Start for High-Bandwidth
and Long-Distance Networks", International Workshop on and Long-Distance Networks", International Workshop on
Protocols for Fast Long-Distance Networks , 2008. Protocols for Fast Long-Distance Networks.
[HRX08] Ha, S., Rhee, I., and L. Xu, "CUBIC: A New TCP-Friendly [HRX08] Ha, S., Rhee, I., and L. Xu, "CUBIC: A New TCP-Friendly
High-Speed TCP Variant", ACM SIGOPS Operating System High-Speed TCP Variant", ACM SIGOPS Operating System
Review , 2008. Review, DOI 10.1145/1400097.1400105, July 2008.
[K03] Kelly, T., "Scalable TCP: Improving Performance in [K03] Kelly, T., "Scalable TCP: Improving Performance in
HighSpeed Wide Area Networks", ACM SIGCOMM Computer HighSpeed Wide Area Networks", ACM SIGCOMM
Communication Review , April 2003. Computer Communication Review, DOI 10.1145/956981.956989,
April 2003.
[KWAF16] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst, [KWAF17] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", Internet-draft, "TCP Alternative Backoff with ECN (ABE)", Work in
IETF work-in-progress draft-khademi-tcpm- Progress, draft-ietf-tcpm-alternativebackoff-ecn-05,
alternativebackoff-ecn-01 , October 2016. December 2017.
[XHR04] Xu, L., Harfoush, K., and I. Rhee, "Binary Increase [XHR04] Xu, L., Harfoush, K., and I. Rhee, "Binary Increase
Congestion Control for Fast, Long Distance Networks", In Congestion Control for Fast, Long Distance Networks", In
Proceedings of IEEE INFOCOM , March 2004. Proceedings of IEEE INFOCOM,
DOI 10.1109/INFCOM.2004.1354672, March 2004.
Acknowledgements
Alexander Zimmermann and Lars Eggert have received funding from the
European Union's Horizon 2020 research and innovation program
2014-2018 under grant agreement No. 644866 (SSICLOPS). This document
reflects only the authors' views and the European Commission is not
responsible for any use that may be made of the information it
contains.
The work of Lisong Xu was partially supported by the National Science
Foundation (NSF) under Grant No. 1526253. Any opinions, findings,
and conclusions or recommendations expressed in this material are
those of the authors and do not necessarily reflect the views of the
NSF.
Authors' Addresses Authors' Addresses
Injong Rhee Injong Rhee
North Carolina State University North Carolina State University
Department of Computer Science Department of Computer Science
Raleigh, NC 27695-7534 Raleigh, NC 27695-7534
US United States of America
Email: rhee@ncsu.edu Email: rhee@ncsu.edu
Lisong Xu Lisong Xu
University of Nebraska-Lincoln University of Nebraska-Lincoln
Department of Computer Science and Engineering Department of Computer Science and Engineering
Lincoln, NE 68588-0115 Lincoln, NE 68588-0115
US United States of America
Email: xu@unl.edu Email: xu@unl.edu
Sangtae Ha Sangtae Ha
University of Colorado at Boulder University of Colorado at Boulder
Department of Computer Science Department of Computer Science
Boulder, CO 80309-0430 Boulder, CO 80309-0430
US United States of America
Email: sangtae.ha@colorado.edu Email: sangtae.ha@colorado.edu
Alexander Zimmermann Alexander Zimmermann
Phone: +49 175 5766838 Phone: +49 175 5766838
Email: alexander.zimmermann@rwth-aachen.de Email: alexander.zimmermann@rwth-aachen.de
Lars Eggert Lars Eggert
NetApp NetApp
Sonnenallee 1 Sonnenallee 1
Kirchheim 85551 Kirchheim 85551
Germany Germany
Phone: +49 151 12055791 Phone: +49 151 12055791
Email: lars@netapp.com Email: lars@netapp.com
Richard Scheffenegger Richard Scheffenegger
NetApp
Email: rscheff@gmx.at Am Europlatz 2
Vienna 1120
Austria
Email: rs.ietf@gmx.at
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