draft-ietf-tcpm-cubic-03.txt   draft-ietf-tcpm-cubic-04.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: June 5, 2017 UNL Expires: August 8, 2017 UNL
S. Ha S. Ha
Colorado Colorado
A. Zimmermann A. Zimmermann
L. Eggert L. Eggert
NetApp NetApp
R. Scheffenegger R. Scheffenegger
December 2, 2016 February 4, 2017
CUBIC for Fast Long-Distance Networks CUBIC for Fast Long-Distance Networks
draft-ietf-tcpm-cubic-03 draft-ietf-tcpm-cubic-04
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 function of the
standards to improve scalability and stability under fast and long current TCP standards to improve scalability and stability under fast
distance networks. BIC-TCP, a predecessor of CUBIC, has been a and long distance networks. CUBIC and its predecessor algorithm have
default TCP adopted by Linux since year 2005 and has already been been adopted as default by Linux and have been used for many years.
deployed globally and in use for several years by the Internet This document provides a specification of CUBIC to enable third party
community at large. CUBIC is using a similar window growth function implementation and to solicit the community feedback through
as BIC-TCP and is designed to be less aggressive and fairer to TCP in experimentation on the performance of CUBIC.
bandwidth usage than BIC-TCP while maintaining the strengths of BIC-
TCP such as stability, window scalability and RTT fairness. Through
extensive testing in various Internet scenarios, we believe that
CUBIC is safe for deployment and testing in the global Internet. The
intent of this document is to provide the protocol specification of
CUBIC for a third party implementation and solicit the community
feedback through experimentation on the performance of CUBIC.
Status of This Memo Status of This Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. CUBIC Congestion Control . . . . . . . . . . . . . . . . . . 5 3. Design principle of CUBIC . . . . . . . . . . . . . . . . . . 4
3.1. Window growth function . . . . . . . . . . . . . . . . . 5 4. CUBIC Congestion Control . . . . . . . . . . . . . . . . . . 5
3.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 6 4.1. Window growth function . . . . . . . . . . . . . . . . . 6
3.3. Concave region . . . . . . . . . . . . . . . . . . . . . 7 4.2. TCP-friendly region . . . . . . . . . . . . . . . . . . . 7
3.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 7 4.3. Concave region . . . . . . . . . . . . . . . . . . . . . 7
3.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 7 4.4. Convex region . . . . . . . . . . . . . . . . . . . . . . 7
3.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 8 4.5. Multiplicative decrease . . . . . . . . . . . . . . . . . 8
3.7. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 8
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.7. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 9 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 10 5.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 9
4.3. Difficult Environments . . . . . . . . . . . . . . . . . 11 5.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 11
4.4. Investigating a Range of Environments . . . . . . . . . . 11 5.3. Difficult Environments . . . . . . . . . . . . . . . . . 12
4.5. Protection against Congestion Collapse . . . . . . . . . 11 5.4. Investigating a Range of Environments . . . . . . . . . . 12
4.6. Fairness within the Alternative Congestion Control 5.5. Protection against Congestion Collapse . . . . . . . . . 12
Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 11 5.6. Fairness within the Alternative Congestion Control
4.7. Performance with Misbehaving Nodes and Outside Attackers 12 Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 12
4.8. Behavior for Application-Limited Flows . . . . . . . . . 12 5.7. Performance with Misbehaving Nodes and Outside Attackers 12
4.9. Responses to Sudden or Transient Events . . . . . . . . . 12 5.8. Behavior for Application-Limited Flows . . . . . . . . . 12
4.10. Incremental Deployment . . . . . . . . . . . . . . . . . 12 5.9. Responses to Sudden or Transient Events . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 5.10. Incremental Deployment . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . 13 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . 13 9.1. Normative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 9.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
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
path with over 100ms round-trip times (RTTs). This problem is path with over 100ms round-trip times (RTTs). This problem is
equally applicable to all Reno style TCP standards and their equally applicable to all Reno style TCP standards and their
variants, including TCP-RENO [RFC5681], TCP-NewReno [RFC6582], TCP- variants, including TCP-RENO [RFC5681], TCP-NewReno [RFC6582], TCP-
SACK [RFC2018], SCTP [RFC4960], TFRC [RFC5348] that use the same SACK [RFC2018], SCTP [RFC4960], TFRC [RFC5348] that use the same
linear increase function for window growth, which we refer to linear increase function for window growth, which we refer to
collectively as Standard TCP below. collectively as Standard TCP below.
CUBIC [HRX08] is a modification to the congestion control mechanism CUBIC [HRX08] is a modification to the congestion control mechanism
of Standard TCP, in particular, to the window increase function of of Standard TCP, in particular, to the window increase function of
Standard TCP senders, to remedy this problem. It uses a cubic Standard TCP senders, to remedy this problem. Specifically, it uses
increase function in terms of the elapsed time from the last a cubic function instead of a linear window increase function of the
congestion event. While most alternative algorithms to Standard TCP Standrad TCP to improve scalability and stability under fast and long
uses a convex increase function where during congestion avoidance the distance networks.
window increment is always increasing, CUBIC uses both the concave
and convex profiles of a cubic function for window increase. After a BIC-TCP, a predecessor of CUBIC, has been selected as default TCP
window reduction following a loss event detected by duplicate ACKs, congestion control algorithm by Linux in the year 2005 and been used
it registers the window size where it got the loss event as W_max and for several years by the Internet community at large. CUBIC uses a
performs a multiplicative decrease of congestion window and the similar window growth function as BIC-TCP and is designed to be less
regular fast recovery and retransmit of Standard TCP. After it aggressive and fairer to TCP in bandwidth usage than BIC-TCP while
enters into congestion avoidance from fast recovery, it starts to maintaining the strengths of BIC-TCP such as stability, window
increase the window using the concave profile of the cubic function. scalability and RTT fairness. CUBIC has already been deployed
The cubic function is set to have its plateau at W_max so the concave globally by Linux. Through extensive testing in various Internet
growth continues until the window size becomes W_max. After that, scenarios, we believe that CUBIC is safe for testing and deployment
the cubic function turns into a convex profile and the convex window in the global Internet.
growth begins. This style of window adjustment (concave and then
convex) improves protocol and network stability while maintaining In the ensuing sections, we first brefly explain the design principle
high network utilization [CEHRX07]. This is because the window size 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Design principle of CUBIC
CUBIC [HRX08] uses a cubic window increase function in terms of the
elapsed time from the last congestion event. While most alternative
algorithms to Standard TCP uses a convex increase function where
during congestion avoidance the window increment is always
increasing, CUBIC uses both the concave and convex profiles of a
cubic function for window increase. After a window reduction
following a loss event detected by duplicate ACKs, it registers the
window size where it got the loss event as W_max and performs a
multiplicative decrease of congestion window and the regular fast
recovery and retransmit of Standard TCP. After it enters into
congestion avoidance from fast recovery, it starts to increase the
window using the concave profile of the cubic function. The cubic
function is set to have its plateau at W_max so the concave growth
continues until the window size becomes W_max. After that, the cubic
function turns into a convex profile and the convex window growth
begins. This style of window adjustment (concave and then convex)
improves protocol and network stability while maintaining high
network utilization [CEHRX07]. This is because the window size
remains almost constant, forming a plateau around W_max where network remains almost constant, forming a plateau around W_max where network
utilization is deemed highest and under steady state, most window utilization is deemed highest and under steady state, most window
size samples of CUBIC are close to W_max, thus promoting high network size samples of CUBIC are close to W_max, thus promoting high network
utilization and protocol stability. Note that protocols with convex utilization and protocol stability. Note that protocols with convex
increase functions have the maximum increments around W_max and increase functions have the maximum increments around W_max and
introduces a large number of packet bursts around the saturation introduces a large number of packet bursts around the saturation
point of the network, likely causing frequent global loss point of the network, likely causing frequent global loss
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
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higher order shares. HTCP [LS08] currently uses the equal share. higher order shares. HTCP [LS08] currently uses the equal share.
CUBIC sets the multiplicative window decrease factor to 0.7 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 Multiplicative-Increase Multiplicative-
same RTT always converge to the same share of bandwidth independent Decrease (MIMD) can converge. CUBIC flows with the same RTT always
of statistical multiplexing, thus achieving intra-protocol fairness. converge to the same share of bandwidth independent of statistical
We also find that under the environments with sufficient statistical multiplexing, thus achieving intra-protocol fairness. We also find
multiplexing, the convergence speed of CUBIC flows is reasonable. that under the environments with sufficient statistical multiplexing,
the convergence speed of CUBIC flows is reasonable.
In the ensuing sections, we provide the exact specification of CUBIC
and 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. 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.
3.1. Window growth function 4.1. 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, such as TCP-NewReno [RFC6582] and TCP-SACK [RFC2018]. During of TCP, such as TCP-NewReno [RFC6582] and TCP-SACK [RFC2018]. During
congestion avoidance after fast recovery, CUBIC changes the window congestion avoidance after fast recovery, CUBIC changes the window
update algorithm of Standard TCP. Suppose that W_max is the window update algorithm of Standard TCP. Suppose that W_max is the window
size before the window is reduced in the last fast retransmit and size 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_cubic(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 (measured right after the fast recovery), and K is window reduction that is measured right after the fast recovery in
the time period that the above function takes to increase the current response to duplicate ACKs or after the congestion window reduction
window size to W_max if there is no further loss event and is in response to ECN-Echo ACKs, and K is the time period that the above
calculated by using the following equation: function takes to increase the current window size to W_max if there
is no further loss event and is calculated by using the following
equation:
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 packet loss (detected by duplicate ACKs) occurs, CUBIC is, when a packet loss detected by duplicate ACKs or a network
reduces its current window cwnd to W_cubic(0)=W_max*beta_cubic. We congestion detected by ECN-Echo ACKs occurs, CUBIC reduces its
discuss how we set C in the next Section in more details. current window cwnd to W_cubic(0)=W_max*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_cubic(t+RTT) as the candidate target value of congestion window, W_cubic(t+RTT) as the candidate target value of congestion window,
where RTT is the weithed average RTT calculated by the standard TCP. where RTT is the weithed average RTT calculated by the standard TCP.
Depending on the value of the current window size cwnd, CUBIC runs in Depending on the value of the current window size cwnd, CUBIC runs in
three different modes. First, if cwnd is less than the window size three different modes. First, if cwnd is less than the window size
that Standard TCP would reach at time t after the last loss event, that Standard TCP would reach at time t after the last loss event,
then CUBIC is in the TCP friendly region (we describe below how to then CUBIC is in the TCP friendly region (we describe below how to
determine this window size of Standard TCP in term of time t). determine this window size of Standard TCP in term of time t).
Otherwise, if cwnd is less than W_max, then CUBIC is the concave Otherwise, if cwnd is less than W_max, then CUBIC is the concave
region, and if cwnd is larger than W_max, CUBIC is in the convex region, and if cwnd is larger than W_max, CUBIC is in the convex
region. Below, we describe the exact actions taken by CUBIC in each region. Below, we describe the exact actions taken by CUBIC in each
region. region.
3.2. 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 the standard TCP. achieves at least the same throughput as the standard TCP.
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 by estimating the protocol is in the TCP region or not. This is done by estimating
the average rate of the Standard TCP using a simple analysis the average rate of the Standard TCP using a simple analysis
described in [FHP00]. It considers the Standard TCP as a special described in [FHP00]. It considers the Standard TCP as a special
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alpha_aimd per RTT, we can get the window size of AIMD in terms of alpha_aimd per RTT, we can get the window size of AIMD in terms of
the elapsed time t as follows: the elapsed time t as follows:
W_aimd(t) = W_max*beta_aimd + W_aimd(t) = W_max*beta_aimd +
[3*(1-beta_aimd)/(1+beta_aimd)] * (t/RTT) (Eq. 4) [3*(1-beta_aimd)/(1+beta_aimd)] * (t/RTT) (Eq. 4)
If W_cubic(t) is less than W_aimd(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_aimd(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 4.3. 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_cubic(t+RTT) - cwnd)/cwnd for each received ACK, incremented by (W_cubic(t+RTT) - cwnd)/cwnd for each received ACK,
where W_cubic(t+RTT) is calculated using Eq. 1. where W_cubic(t+RTT) is calculated using Eq. 1.
3.4. Convex region 4.4. Convex region
When the current window size of CUBIC is larger than W_max, it passes When the current window size of CUBIC is larger than W_max, it passes
the plateau of the cubic function after which CUBIC follows the the plateau of the cubic function after which CUBIC follows the
convex 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_cubic(t+RTT) - cwnd)/cwnd for 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. each received ACK, where W_cubic(t+RTT) is calculated using Eq. 1.
3.5. Multiplicative decrease 4.5. Multiplicative decrease
When a packet loss (detected by duplicate ACKs) occurs, CUBIC updates When a packet loss detected by duplicate ACKs or a network congestion
its W_max, cwnd, and ssthresh (slow start threshold) as follows. detected by ECN-Echo ACKs occurs, CUBIC updates its W_max, cwnd, and
Parameter beta_cubic SHOULD be set to 0.7. ssthresh (slow start threshold) as follows. 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
ssthresh = cwnd * beta_cubic; // new slow start threshold ssthresh = cwnd * beta_cubic; // new slow start threshold
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 bigger value 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 the protocol much harder. This adaptive adjustment of analysis of the protocol 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.
3.6. Fast convergence 4.6. 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
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W_max = W_max*(1+beta_cubic)/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.
3.7. Timeout 4.7. Timeout
In case of timeout, CUBIC follows the standard TCP to reduce cwnd, In case of timeout, CUBIC follows the standard TCP to reduce cwnd,
but sets ssthresh using beta_cubic (same as in Section 3.5). but sets ssthresh using beta_cubic (same as in Section 4.5).
4. Discussion 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 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:
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.
4.1. 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 RTT, 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 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.
+----------+-------+--------+-------------+-------------+-----------+ +----------+-------+--------+-------------+-------------+-----------+
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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:
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.
4.2. Using Spare Capacity 5.2. 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 2.9e-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 |
skipping to change at page 11, line 30 skipping to change at page 12, line 10
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
bottleneck link without taking away much bandwidth from the existing bottleneck link without taking away much bandwidth from the existing
flows. flows.
4.3. Difficult Environments 5.3. Difficult Environments
CUBIC is designed to remedy the poor performance of TCP in fast long- CUBIC is designed to remedy the poor performance of TCP in fast long-
distance networks. distance networks.
4.4. 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].
4.5. Protection against Congestion Collapse Same as Standard TCP, CUBIC is a loss-based congestion control
algorithm. Therefore, CUBIC, which is designed to be more aggressive
than Standard TCP in fast and long distance networks, can fill large
drop-tail buffers more quickly than Standard TCP. In this case,
proper queue sizing and management [RFC7567] could be used to reduce
the packet queueing delay.
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.
4.6. 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 bandwidth share. When in the same bottleneck links to an equal bandwidth share. When
competing flows have different RTTs, their bandwidth shares are competing flows have different RTTs, their bandwidth shares are
linearly proportional to the inverse of their RTT ratios. This is linearly proportional to the inverse of their RTT ratios. This is
true independent of the level of statistical multiplexing in the true independent of the level of statistical multiplexing in the
link. link.
4.7. 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.
4.8. Behavior for Application-Limited Flows 5.8. Behavior for Application-Limited Flows
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 cases of idle periods, t in Eq. 1 MUST NOT include the window. In cases of idle periods, t in Eq. 1 MUST NOT include the
idle time; otherwise, W_cubic(t) might be very high after restarting idle time; otherwise, W_cubic(t) might be very high after restarting
from a long idle time. from a long idle time.
4.9. 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 In case that there is a sudden congestion, a routing change, or a
mobility event, CUBIC behaves the same as Standard TCP. mobility event, CUBIC behaves the same as Standard TCP.
4.10. Incremental Deployment 5.10. Incremental Deployment
CUBIC requires only the change of TCP senders, and does not require CUBIC requires only the change of TCP senders, and does not require
any assistant of routers. any assistant of routers.
5. 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.
6. IANA Considerations 7. IANA Considerations
There are no IANA considerations regarding this document. There are no IANA considerations regarding this document.
7. Acknowledgements 8. Acknowledgements
Alexander Zimmermann and Lars Eggert have received funding from the Alexander Zimmermann and Lars Eggert have received funding from the
European Union's Horizon 2020 research and innovation program European Union's Horizon 2020 research and innovation program
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 9. References
8.1. Normative References
9.1. 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, Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996, DOI 10.17487/RFC2018, October 1996,
<http://www.rfc-editor.org/info/rfc2018>. <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, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
skipping to change at page 13, line 43 skipping to change at page 14, line 28
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>. <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, DOI 10.17487/RFC6582, April 2012, RFC 6582, DOI 10.17487/RFC6582, April 2012,
<http://www.rfc-editor.org/info/rfc6582>. <http://www.rfc-editor.org/info/rfc6582>.
8.2. Informative References [RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<http://www.rfc-editor.org/info/rfc7567>.
9.2. 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.
[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,
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