draft-ietf-tcpm-cubic-00.txt   draft-ietf-tcpm-cubic-01.txt 
TCP Maintenance and Minor Extensions (TCPM) WG I. Rhee TCP Maintenance and Minor Extensions (TCPM) WG I. Rhee
Internet-Draft NCSU Internet-Draft NCSU
Intended status: Informational L. Xu Intended status: Informational L. Xu
Expires: December 20, 2015 UNL Expires: July 21, 2016 UNL
S. Ha S. Ha
NCSU Colorado
A. Zimmermann A. Zimmermann
L. Eggert L. Eggert
R. Scheffenegger R. Scheffenegger
NetApp NetApp
June 18, 2015 January 18, 2016
CUBIC for Fast Long-Distance Networks CUBIC for Fast Long-Distance Networks
draft-ietf-tcpm-cubic-00 draft-ietf-tcpm-cubic-01
Abstract Abstract
CUBIC is an extension to the current TCP standards. The protocol CUBIC is an extension to the current TCP standards. The protocol
differs from the current TCP standards only in the congestion window differs from the current TCP standards only in the congestion window
adjustment function in the sender side. In particular, it uses a adjustment function in the sender side. In particular, it uses a
cubic function instead of a linear window increase of the current TCP cubic function instead of a linear window increase of the current TCP
standards to improve scalability and stability under fast and long standards to improve scalability and stability under fast and long
distance networks. BIC-TCP, a predecessor of CUBIC, has been a distance networks. BIC-TCP, a predecessor of CUBIC, has been a
default TCP adopted by Linux since year 2005 and has already been default TCP adopted by Linux since year 2005 and has already been
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This Internet-Draft will expire on December 20, 2015. This Internet-Draft will expire on July 21, 2016.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. CUBIC Congestion Control . . . . . . . . . . . . . . . . . . 5 3. CUBIC Congestion Control . . . . . . . . . . . . . . . . . . 5
3.1. Window growth function . . . . . . . . . . . . . . . . . 5 3.1. Window growth function . . . . . . . . . . . . . . . . . 5
3.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 6 3.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 6
3.3. Concave region . . . . . . . . . . . . . . . . . . . . . 6 3.3. Concave region . . . . . . . . . . . . . . . . . . . . . 7
3.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 7 3.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 7 3.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 7
3.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 7 3.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 7
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 8 4.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 8
4.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 10 4.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 10
4.3. Difficult Environments . . . . . . . . . . . . . . . . . 10 4.3. Difficult Environments . . . . . . . . . . . . . . . . . 11
4.4. Investigating a Range of Environments . . . . . . . . . . 11 4.4. Investigating a Range of Environments . . . . . . . . . . 11
4.5. Protection against Congestion Collapse . . . . . . . . . 11 4.5. Protection against Congestion Collapse . . . . . . . . . 11
4.6. Fairness within the Alternative Congestion Control 4.6. Fairness within the Alternative Congestion Control
Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 11 Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 11
4.7. Performance with Misbehaving Nodes and Outside Attackers 11 4.7. Performance with Misbehaving Nodes and Outside Attackers 11
4.8. Responses to Sudden or Transient Events . . . . . . . . . 11 4.8. Responses to Sudden or Transient Events . . . . . . . . . 11
4.9. Incremental Deployment . . . . . . . . . . . . . . . . . 11 4.9. Incremental Deployment . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12 8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 12 8.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. ToDo List . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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][RFC3649]. This problem arises from a slow
increase of congestion window following a congestion event in a increase of congestion window following a congestion event in a
network with a large bandwidth delay product (BDP). Our experience network with a large bandwidth delay product (BDP). Our experience
[HKLRX06] indicates that this problem is frequently observed even in [HKLRX06] indicates that this problem is frequently observed even in
the range of congestion window sizes over several hundreds of packets the range of congestion window sizes over several hundreds of packets
(each packet is sized around 1000 bytes) especially under a network (each packet is sized around 1000 bytes) especially under a network
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synchronizations. synchronizations.
Another notable feature of CUBIC is that its window increase rate is Another notable feature of CUBIC is that its window increase rate is
mostly independent of RTT, and follows a (cubic) function of the mostly independent of RTT, and follows a (cubic) function of the
elapsed time since the last loss event. This feature promotes per- elapsed time since the last loss event. This feature promotes per-
flow fairness to Standard TCP as well as RTT-fairness. Note that flow fairness to Standard TCP as well as RTT-fairness. Note that
Standard TCP performs well under short RTT and small bandwidth (or Standard TCP performs well under short RTT and small bandwidth (or
small BDP) networks. Only in a large long RTT and large bandwidth small BDP) networks. Only in a large long RTT and large bandwidth
(or large BDP) networks, it has the scalability problem. An (or large BDP) networks, it has the scalability problem. An
alternative protocol to Standard TCP designed to be friendly to alternative protocol to Standard TCP designed to be friendly to
Standard TCP at a per-flow basis must operate must increase its Standard TCP at a per-flow basis must operate to increase its window
window much less aggressively in small BDP networks than in large BDP much less aggressively in small BDP networks than in large BDP
networks. In CUBIC, its window growth rate is slowest around the networks. In CUBIC, its window growth rate is slowest around the
inflection point of the cubic function and this function does not inflection point of the cubic function and this function does not
depend on RTT. In a smaller BDP network where Standard TCP flows are depend on RTT. In a smaller BDP network where Standard TCP flows are
working well, the absolute amount of the window decrease at a loss working well, the absolute amount of the window decrease at a loss
event is always smaller because of the multiplicative decrease. event is always smaller because of the multiplicative decrease.
Therefore, in CUBIC, the starting window size after a loss event from Therefore, in CUBIC, the starting window size after a loss event from
which the window starts to increase, is smaller in a smaller BDP which the window starts to increase, is smaller in a smaller BDP
network, thus falling nearer to the plateau of the cubic function network, thus falling nearer to the plateau of the cubic function
where the growth rate is slowest. By setting appropriate values of where the growth rate is slowest. By setting appropriate values of
the cubic function parameters, CUBIC sets its growth rate always no the cubic function parameters, CUBIC sets its growth rate always no
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multiplexing environments, the bandwidth share ratio of Standard TCP multiplexing environments, the bandwidth share ratio of Standard TCP
flows with different RTTs is squarely proportional to the inverse of flows with different RTTs is squarely proportional to the inverse of
their RTT ratio [XHR04]. CUBIC always ensures the linear ratio their RTT ratio [XHR04]. CUBIC always ensures the linear ratio
independent of the levels of statistical multiplexing. This is an independent of the levels of statistical multiplexing. This is an
improvement over Standard TCP. While there is no consensus on a improvement over Standard TCP. While there is no consensus on a
particular bandwidth share ratios of different RTT flows, we believe particular bandwidth share ratios of different RTT flows, we believe
that under wired Internet, use of the linear share notion seems more that under wired Internet, use of the linear share notion seems more
reasonable than equal share or a higher order shares. HTCP [LS08] reasonable than equal share or a higher order shares. HTCP [LS08]
currently uses the equal share. currently uses the equal share.
CUBIC sets the multiplicative window decrease factor to 0.2 while CUBIC sets the multiplicative window decrease factor to 0.7 while
Standard TCP uses 0.5. While this improves the scalability of the Standard TCP uses 0.5. While this improves the scalability of the
protocol, a side effect of this decision is slower convergence protocol, 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 HSTCP
[RFC3649] has made along with other researchers (e.g., [GV02]): the [RFC3649] has made along with other researchers (e.g., [GV02]): the
current Internet becomes more asynchronous with less frequent loss current Internet becomes more asynchronous with less frequent loss
synchronizations with high statistical multiplexing. Under this synchronizations with high statistical multiplexing. Under this
environment, even strict MIMD can converge. CUBIC flows with the environment, even strict MIMD can converge. CUBIC flows with the
same RTT always converge to the same share of bandwidth independent same RTT always converge to the same share of bandwidth independent
of statistical multiplexing, thus achieving intra-protocol fairness. of statistical multiplexing, thus achieving intra-protocol fairness.
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specified in [RFC5033]. 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. CUBIC Congestion Control 3. 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.
3.1. Window growth function 3.1. Window growth 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. The increasing congestion window only at the reception of ACK. The
protocol does not make any change to the fast recovery and retransmit protocol does not make any change to the fast recovery and retransmit
of TCP-NewReno [RFC6582] and TCP-SACK [RFC2018]. During congestion of TCP-NewReno [RFC6582] and TCP-SACK [RFC2018]. During congestion
avoidance after fast recovery, CUBIC changes the window update avoidance after fast recovery, CUBIC changes the window update
algorithm of Standard TCP. Suppose that W_max is the window size algorithm of Standard TCP. Suppose that W_max is the window size
before the window is reduced in the last fast retransmit and before the window is reduced in the last fast retransmit and
recovery. recovery.
The window growth function of CUBIC uses the following function: The window growth function of CUBIC uses the following function:
W(t) = C*(t-K)^3 + W_max (Eq. 1) W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1)
where C is a constant fixed to determine the aggressiveness of window where C is a constant fixed to determine the aggressiveness of window
growth in high BDP networks, t is the elapsed time from the last growth in high BDP networks, t is the elapsed time from the last
window reduction,and K is the time period that the above function window reduction,and K is the time period that the above function
takes to increase W to W_max when there is no further loss event and takes to increase the current window size to W_max when there is no
is calculated by using the following equation: further loss event and is calculated by using the following equation:
K = cubic_root(W_max*beta/C) (Eq. 2) K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2)
where beta is the multiplication decrease factor. We discuss how we where beta_cubic is the CUBIC multiplication decrease factor, that
set C in the next Section in more details. is, when a packet loss occurs, CUBIC reduces its current window cwnd
to cwnd*beta_cubic. We discuss how we set C in the next Section in
more details.
Upon receiving an ACK during congestion avoidance, CUBIC computes the Upon receiving an ACK during congestion avoidance, CUBIC computes the
window growth rate during the next RTT period using Eq. 1. It sets window growth rate during the next RTT period using Eq. 1. It sets
W(t+RTT) as the candidate target value of congestion window. Suppose W_cubic(t+RTT) as the candidate target value of congestion window.
that the current window size is cwnd. Depending on the value of
cwnd, CUBIC runs in three different modes. First, if cwnd is less Depending on the value of the current window size cwnd, CUBIC runs in
than the window size that Standard TCP would reach at time t after three different modes. First, if cwnd is less than the window size
the last loss event, then CUBIC is in the TCP friendly region (we that Standard TCP would reach at time t after the last loss event,
describe below how to determine this window size of Standard TCP in then CUBIC is in the TCP friendly region (we describe below how to
term of time t). Otherwise, if cwnd is less than W_max, then CUBIC determine this window size of Standard TCP in term of time t).
is the concave region, and if cwnd is larger than W_max, CUBIC is in Otherwise, if cwnd is less than W_max, then CUBIC is the concave
the convex region. Below, we describe the exact actions taken by region, and if cwnd is larger than W_max, CUBIC is in the convex
CUBIC in each region. region. Below, we describe the exact actions taken by CUBIC in each
region.
3.2. TCP-friendly region 3.2. TCP-friendly region
When receiving an ACK in congestion avoidance, we first check whether When receiving an ACK in congestion avoidance, we first check whether
the protocol is in the TCP region or not. This is done as follows. the protocol is in the TCP region or not. This is done as follows.
We can analyze the window size of Standard TCP in terms of the We can analyze the window size of a TCP-friendly AIMD in terms of the
elapsed time t. Using a simple analysis in [FHP00], we can analyze elapsed time t. Using a simple analysis in [FHP00], we can analyze
the average window size of additive increase and multiplicative the average window size of additive increase and multiplicative
decrease (AIMD) with an additive factor alpha and a multiplicative decrease (AIMD) with an additive factor alpha_aimd and a
factor beta to be the following function: multiplicative factor beta_aimd with the following function:
(alpha/2 * (2-beta)/beta * 1/p)^0.5 (Eq. 3) AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) /
(2*(1-beta_aimd)*p) ]^0.5 (Eq. 3)
By the same analysis, the average window size of Standard TCP with By the same analysis, the average window size of Standard TCP is
alpha 1 and beta 0.5 is (3/2 *1/p)^0.5. Thus, for Eq. 3 to be the (1.5/p)^0.5, as the Standard TCP is a special case of AIMD with
same as that of Standard TCP, alpha must be equal to 3*beta/(2-beta). alpha_aimd=1 and beta_aimd=0.5. Thus, for Eq. 3 to be the same as
As Standard TCP increases its window by alpha per RTT, we can get the that of Standard TCP, alpha_aimd must be equal to
window size of Standard TCP in terms of the elapsed time t as 3*(1-beta_aimd)/(1+beta_aimd). As AIMD increases its window by
follows: alpha_aimd per RTT, we can get the window size of AIMD in terms of
the elapsed time t as follows:
W_tcp(t) = W_max*(1-beta) + 3*beta/(2-beta)* t/RTT (Eq. 4) W_aimd(t) = W_max*beta_aimd +
[3*(1-beta_aimd)/(1+beta_aimd)] * (t/RTT) (Eq. 4)
If cwnd is less than W_tcp(t), then the protocol is in the TCP If W_cubic(t) is less than W_aimd(t), then the protocol is in the TCP
friendly region and cwnd SHOULD be set to W_tcp(t) at each reception friendly region and cwnd SHOULD be set to W_aimd(t) at each reception
of ACK. of ACK.
3.3. Concave region 3.3. Concave region
When receiving an ACK in congestion avoidance, if the protocol is not When receiving an ACK in congestion avoidance, if the protocol is not
in the TCP-friendly region and cwnd is less than W_max, then the in the TCP-friendly region and cwnd is less than W_max, then the
protocol is in the concave region. In this region, cwnd MUST be protocol is in the concave region. In this region, cwnd MUST be
incremented by (W(t+RTT) - cwnd)/cwnd. incremented by (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK.
3.4. Convex region 3.4. Convex region
When the window size of CUBIC is larger than W_max, it passes the When the current window size of CUBIC is larger than W_max, it passes
plateau of the cubic function after which CUBIC follows the convex the plateau of the cubic function after which CUBIC follows the
profile of the cubic function. Since cwnd is larger than the convex profile of the cubic function. Since cwnd is larger than the
previous saturation point W_max, this indicates that the network previous saturation point W_max, this indicates that the network
conditions might have been perturbed since the last loss event, conditions might have been perturbed since the last loss event,
possibly implying more available bandwidth after some flow possibly implying more available bandwidth after some flow
departures. Since the Internet is highly asynchronous, some amount departures. Since the Internet is highly asynchronous, some amount
of perturbation is always possible without causing a major change in of perturbation is always possible without causing a major change in
available bandwidth. In this phase, CUBIC is being very careful by available bandwidth. In this phase, CUBIC is being very careful by
very slowly increasing its window size. The convex profile ensures very slowly increasing its window size. The convex profile ensures
that the window increases very slowly at the beginning and gradually that the window increases very slowly at the beginning and gradually
increases its growth rate. We also call this phase as the maximum increases its growth rate. We also call this phase as the maximum
probing phase since CUBIC is searching for a new W_max. In this probing phase since CUBIC is searching for a new W_max. In this
region, cwnd MUST be incremented by (W(t+RTT) - cwnd)/cwnd for each region, cwnd MUST be incremented by (W_cubic(t+RTT) - cwnd)/cwnd for
received ACK. each received ACK.
3.5. Multiplicative decrease 3.5. Multiplicative decrease
When a packet loss occurs, CUBIC reduces its window size by a factor When a packet loss occurs, CUBIC reduces its window size by a factor
of beta. Parameter beta SHOULD be set to 0.2. of beta. Parameter beta_cubic SHOULD be set to 0.7.
W_max = cwnd; // save window size before reduction W_max = cwnd; // save window size before reduction
cwnd = cwnd * (1-beta); // window reduction cwnd = cwnd * beta_cubic; // window reduction
A side effect of setting beta to a smaller value than 0.5 is slower A side effect of setting beta_cubic to a bigger value than 0.5 is
convergence. We believe that while a more adaptive setting of beta slower convergence. We believe that while a more adaptive setting of
could result in faster convergence, it will make the analysis of the beta_cubic could result in faster convergence, it will make the
protocol much harder. This adaptive adjustment of beta is an item analysis of the protocol much harder. This adaptive adjustment of
for the next version of CUBIC. beta_cubic is an item for the next version of CUBIC.
3.6. Fast convergence 3.6. Fast convergence
To improve the convergence speed of CUBIC, we add a heuristic in the To improve the convergence speed of CUBIC, we add a heuristic in the
protocol. When a new flow joins the network, existing flows in the protocol. When a new flow joins the network, existing flows in the
network need to give up their bandwidth shares to allow the flow some network need to give up their bandwidth shares to allow the flow some
room for growth if the existing flows have been using all the room for growth if the existing flows have been using all the
bandwidth of the network. To increase this release of bandwidth by bandwidth of the network. To increase this release of bandwidth 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 loss event occurs, before a window With fast convergence, when a loss event occurs, before a window
reduction of congestion window, a flow remembers the last value of reduction of congestion window, a flow remembers the last value of
W_max before it updates W_max for the current loss event. Let us W_max before it updates W_max for the current loss event. Let us
call the last value of W_max to be W_last_max. call the last value of W_max to be W_last_max.
if (W_max < W_last_max){ // check downward trend if (W_max < W_last_max){ // check downward trend
W_last_max = W_max; // remember the last W_max W_last_max = W_max; // remember the last W_max
W_max = W_max*(2-beta)/2; // further reduce W_max W_max = W_max*(1+beta_cubic)/2; // further reduce W_max
} else { // check upward trend } else { // check upward trend
W_last_max = W_max // remember the last W_max W_last_max = W_max // remember the last W_max
} }
This allows W_max to be slightly less than the original W_max. Since This allows W_max to be slightly less than the original W_max. Since
flows spend most of time around their W_max, flows with larger flows spend most of time around their W_max, flows with larger
bandwidth shares tend to spend more time around the plateau allowing bandwidth shares tend to spend more time around the plateau allowing
more time for flows with smaller shares to increase their windows. more time for flows with smaller shares to increase their windows.
4. Discussion 4. Discussion
With a deterministic loss model where the number of packets between With a deterministic loss model where the number of packets between
two successive lost events is always 1/p, CUBIC always operates with two successive lost events is always 1/p, CUBIC always operates with
the concave window profile which greatly simplifies the performance the concave window profile which greatly simplifies the performance
analysis of CUBIC. The average window size of CUBIC can be obtained analysis of CUBIC. The average window size of CUBIC can be obtained
by the following function: by the following function:
(C*(4-beta)/4/beta)^0.25 * RTT^0.75 / p^0.75 (Eq. 5) AVG_W_cubic = [C*(3+beta_cubic)/(4*(1-beta_cubic))]^0.25 *
(RTT^0.75) / (p^0.75) (Eq. 5)
With beta set to 0.2, the above formula is reduced to: With beta_cubic set to 0.7, the above formula is reduced to:
(C*3.8/0.8)^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.
4.1. Fairness to standard TCP 4.1. 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.
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average window size of standard TCP, HSTCP, and CUBIC. The average average window size of standard TCP, HSTCP, and CUBIC. The average
window size of standard TCP and HSTCP is from [RFC3649]. The average window size of standard TCP and HSTCP is from [RFC3649]. The average
window size of CUBIC is calculated by using Eq. 6 and CUBIC TCP window size of CUBIC is calculated by using Eq. 6 and CUBIC TCP
friendly mode for three different values of C. friendly mode for three different values of C.
+----------+-------+--------+-------------+-------------+-----------+ +----------+-------+--------+-------------+-------------+-----------+
| Loss | TCP | HSTCP | CUBIC | CUBIC | CUBIC | | Loss | TCP | HSTCP | CUBIC | CUBIC | CUBIC |
| Rate P | | | (C=0.04) | (C=0.4) | (C=4) | | Rate P | | | (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 | 66 | | 10^-3 | 38 | 38 | 38 | 38 | 59 |
| 10^-4 | 120 | 263 | 120 | 209 | 371 | | 10^-4 | 120 | 263 | 120 | 187 | 333 |
| 10^-5 | 379 | 1795 | 660 | 1174 | 2087 | | 10^-5 | 379 | 1795 | 593 | 1054 | 1874 |
| 10^-6 | 1200 | 12279 | 3713 | 6602 | 11740 | | 10^-6 | 1200 | 12279 | 3332 | 5926 | 10538 |
| 10^-7 | 3795 | 83981 | 20878 | 37126 | 66022 | | 10^-7 | 3795 | 83981 | 18740 | 33325 | 59261 |
| 10^-8 | 12000 | 574356 | 117405 | 208780 | 371269 | | 10^-8 | 12000 | 574356 | 105383 | 187400 | 333250 |
+----------+-------+--------+-------------+-------------+-----------+ +----------+-------+--------+-------------+-------------+-----------+
Response function of standard TCP, HSTCP, and CUBIC in networks with Response function of standard TCP, HSTCP, and CUBIC in networks with
RTT = 100ms. The average window size W is in MSS-sized segments. RTT = 0.1 seconds. The average window size is in MSS-sized segments.
Table 1 Table 1
+--------+-----------+-----------+------------+-----------+---------+ +--------+-----------+-----------+------------+-----------+---------+
| 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 | 2087 | | 10^-6 | 1200 | 12279 | 1200 | 1200 | 1874 |
| 10^-7 | 3795 | 83981 | 3795 | 6603 | 11740 | | 10^-7 | 3795 | 83981 | 3795 | 5926 | 10538 |
| 10^-8 | 12000 | 574356 | 20878 | 37126 | 66022 | | 10^-8 | 12000 | 574356 | 18740 | 33325 | 59261 |
+--------+-----------+-----------+------------+-----------+---------+ +--------+-----------+-----------+------------+-----------+---------+
Response function of standard TCP, HSTCP, and CUBIC in networks with Response function of standard TCP, HSTCP, and CUBIC in networks with
RTT = 10ms. The average window size W is in MSS-sized segments. RTT = 0.01 seconds. The average window size is in MSS-sized
segments.
Table 2 Table 2
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 = 10ms and p=10^-6, TCP has an average window of 1200 packets. RTT = 0.01 seconds and p=10^-6, TCP has an average window of 1200
If the packet size is 1500 bytes, then TCP can achieve an average packets. If the packet size is 1500 bytes, then TCP can achieve an
rate of 1.44 Gbps. In this case, CUBIC with C=0.04 or C=0.4 achieves average rate of 1.44 Gbps. In this case, CUBIC with C=0.04 or C=0.4
exactly the same rate as Standard TCP, whereas HSTCP is about ten achieves exactly the same rate as Standard TCP, whereas HSTCP is
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 protocols for the bandwidth. CUBIC is more friendly to with other protocols for the bandwidth. CUBIC is more friendly to
the Standard TCP, if the value of C is lower. However, we do not the Standard TCP, if the value of C is lower. However, we do not
recommend to set C to a very low value like 0.04, since CUBIC with a recommend to set 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 low C cannot efficiently use the bandwidth in long RTT and high
bandwidth networks. Based on these observations, we find C=0.4 gives bandwidth networks. Based on these observations, we find C=0.4 gives
a good balance between TCP-friendliness and aggressiveness of window a good balance between TCP-friendliness and aggressiveness of window
growth. Therefore, C SHOULD be set to 0.4. With C set to 0.4, Eq. 6 growth. Therefore, C SHOULD be set to 0.4. With C set to 0.4, Eq. 6
is reduced to: is reduced to:
1.17 * 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.
4.2. Using Spare Capacity 4.2. Using Spare Capacity
CUBIC uses a more aggressive window growth function than Standard TCP CUBIC uses a more aggressive window growth function than Standard TCP
under long RTT and high bandwidth networks. under long RTT and high bandwidth networks.
The following table shows that to achieve 10Gbps rate, standard TCP The following table shows that to achieve 10Gbps rate, standard TCP
requires a packet loss rate of 2.0e-10, while CUBIC requires a packet requires a packet loss rate of 2.0e-10, while CUBIC requires a packet
loss rate of 3.4e-8. 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 | 3.3e-4 | | 10 | 83.3 | 2.0e-4 | 3.9e-4 | 2.9e-4 |
| 100 | 833.3 | 2.0e-6 | 2.5e-5 | 1.6e-5 | | 100 | 833.3 | 2.0e-6 | 2.5e-5 | 1.4e-5 |
| 1000 | 8333.3 | 2.0e-8 | 1.5e-6 | 7.3e-7 | | 1000 | 8333.3 | 2.0e-8 | 1.5e-6 | 6.3e-7 |
| 10000 | 83333.3 | 2.0e-10 | 1.0e-7 | 3.4e-8 | | 10000 | 83333.3 | 2.0e-10 | 1.0e-7 | 2.9e-8 |
+------------------+-----------+---------+---------+---------+ +------------------+-----------+---------+---------+---------+
Required packet loss rate for Standard TCP, HSTCP, and CUBIC to Required packet loss rate for Standard TCP, HSTCP, and CUBIC to
achieve a certain throughput. We use 1500-byte packets and an RTT of achieve a certain throughput. We use 1500-byte packets and an RTT of
0.1 seconds. 0.1 seconds.
Table 3 Table 3
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
skipping to change at page 12, line 19 skipping to change at page 12, line 27
2014-2018 under grant agreement No. 644866 (SSICLOPS). This document 2014-2018 under grant agreement No. 644866 (SSICLOPS). This document
reflects only the authors' views and the European Commission is not reflects only the authors' views and the European Commission is not
responsible for any use that may be made of the information it responsible for any use that may be made of the information it
contains. contains.
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP [RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996. Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<http://www.rfc-editor.org/info/rfc2018>.
[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, March 1997. Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows", [RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, December 2003. RFC 3649, DOI 10.17487/RFC3649, December 2003,
<http://www.rfc-editor.org/info/rfc3649>.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
4960, September 2007. RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion [RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033, August 2007. Control Algorithms", BCP 133, RFC 5033,
DOI 10.17487/RFC5033, August 2007,
<http://www.rfc-editor.org/info/rfc5033>.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP [RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification", RFC Friendly Rate Control (TFRC): Protocol Specification",
5348, September 2008. RFC 5348, DOI 10.17487/RFC5348, September 2008,
<http://www.rfc-editor.org/info/rfc5348>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, September 2009. Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.
[RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The [RFC6582] Henderson, T., Floyd, S., Gurtov, A., and Y. Nishida, "The
NewReno Modification to TCP's Fast Recovery Algorithm", NewReno Modification to TCP's Fast Recovery Algorithm",
RFC 6582, April 2012. RFC 6582, DOI 10.17487/RFC6582, April 2012,
<http://www.rfc-editor.org/info/rfc6582>.
8.2. Informative References 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 , 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.
skipping to change at page 13, line 31 skipping to change at page 14, line 5
Communication Review , April 2003. Communication Review , April 2003.
[LS08] Leith, D. and R. Shorten, "H-TCP: TCP Congestion Control [LS08] Leith, D. and R. Shorten, "H-TCP: TCP Congestion Control
for High Bandwidth-Delay Product Paths", Internet-draft for High Bandwidth-Delay Product Paths", Internet-draft
draft-leith-tcp-htcp-06 , April 2008. draft-leith-tcp-htcp-06 , April 2008.
[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 , March 2004.
Appendix A. ToDo List
o Incorporate ICCRG's feedback (see
http://trac.tools.ietf.org/group/irtf/trac/wiki/ICCRG_cubic)
o ncorporate feedback from Neal Cardwell (see http://www.ietf.org/
mail-archive/web/tcpm/current/msg09508.html)
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 US
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 68588-01150 Lincoln, NE 68588-01150
US US
Email: xu@unl.edu Email: xu@unl.edu
Sangtae Ha Sangtae Ha
University of Colorado at Boulder University of Colorado at Boulder
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