Internet Engineering Task Force (IETF) I. Rhee

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

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.

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.

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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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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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 .....................................................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

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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
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.

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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
[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.

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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

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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

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.
Let cwnd denote the congestion window size of a flow, and ssthresh
denote the slow-start threshold.

4.1. 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. 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.

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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.

4.2. 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.

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 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

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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)

4.3. 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
(W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where
W_cubic(t+RTT) is calculated using Eq. 1.

4.4. 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
(W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where
W_cubic(t+RTT) is calculated using Eq. 1.

4.5. Multiplicative Decrease

When a packet loss is detected by duplicate ACKs or a network
congestion is detected by ECN-Echo ACKs, CUBIC updates its W_max,
cwnd, and ssthresh as follows. Parameter beta_cubic SHOULD be set to
0.7.

W_max = cwnd; // save window size before reduction
ssthresh = cwnd * beta_cubic; // new slow-start threshold
ssthresh = max(ssthresh, 2); // threshold is at least 2 MSS
cwnd = cwnd * beta_cubic; // window reduction

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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.

4.6. 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, before the
window reduction of the congestion window, a flow remembers the last
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.

if (W_max < W_last_max){ // should we make room for others
W_last_max = W_max; // remember the last W_max
W_max = W_max*(1.0+beta_cubic)/2.0; // further reduce W_max
} else {
W_last_max = W_max // remember the last W_max
}

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
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.

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4.7. Timeout

In case of timeout, CUBIC follows Standard TCP to reduce cwnd
[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.

4.8. Slow Start

CUBIC MUST employ a slow-start algorithm, when the 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 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.

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:

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.

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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 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.

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+--------+-----------+-----------+------------+-----------+---------+
| 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.

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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

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.

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

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increase the risk of a standing queue [KWAF17]. 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 RTTs, 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 Eq. 1 MUST
NOT include these periods; otherwise, W_cubic(t) might be very high
after restarting from these periods.

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.

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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,
<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>.

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RFC 8312 CUBIC February 2018

[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>.

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", 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.

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RFC 8312 CUBIC February 2018

[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.

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.

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Authors' Addresses

Injong Rhee
North Carolina State University
Department of Computer Science
Raleigh, NC 27695-7534
United States of America
Email: [email protected]

Lisong Xu
University of Nebraska-Lincoln
Department of Computer Science and Engineering
Lincoln, NE 68588-0115
United States of America

Email: [email protected]

Sangtae Ha
University of Colorado at Boulder
Department of Computer Science
Boulder, CO 80309-0430
United States of America
Email: [email protected]

Alexander Zimmermann
Phone: +49 175 5766838
Email: [email protected]

Lars Eggert
NetApp
Sonnenallee 1
Kirchheim 85551
Germany
Phone: +49 151 12055791
Email: [email protected]

Richard Scheffenegger
NetApp
Am Europlatz 2
Vienna 1120
Austria
Email: [email protected]

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