draft-ietf-tcpm-cubic-02.txt   draft-ietf-tcpm-cubic-03.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: February 6, 2017 UNL Expires: June 5, 2017 UNL
S. Ha S. Ha
Colorado Colorado
A. Zimmermann A. Zimmermann
L. Eggert L. Eggert
R. Scheffenegger
NetApp NetApp
August 5, 2016 R. Scheffenegger
December 2, 2016
CUBIC for Fast Long-Distance Networks CUBIC for Fast Long-Distance Networks
draft-ietf-tcpm-cubic-02 draft-ietf-tcpm-cubic-03
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
skipping to change at page 2, line 7 skipping to change at page 2, line 10
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 6, 2017. This Internet-Draft will expire on June 5, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 34 skipping to change at page 2, line 37
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 . . . . . . . . . . . . . . . . . . . . . 7 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 . . . . . . . . . . . . . . . . . . . . 8
3.7. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 8 4.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 9
4.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 10 4.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 10
4.3. Difficult Environments . . . . . . . . . . . . . . . . . 11 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 12
4.8. Behavior for Application-Limited Flows . . . . . . . . . 11 4.8. Behavior for Application-Limited Flows . . . . . . . . . 12
4.9. Responses to Sudden or Transient Events . . . . . . . . . 11 4.9. Responses to Sudden or Transient Events . . . . . . . . . 12
4.10. Incremental Deployment . . . . . . . . . . . . . . . . . 12 4.10. Incremental Deployment . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 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 . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . 13 8.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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
skipping to change at page 3, line 28 skipping to change at page 3, line 31
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. It uses a cubic
increase function in terms of the elapsed time from the last increase function in terms of the elapsed time from the last
congestion event. While most alternative algorithms to Standard TCP congestion event. While most alternative algorithms to Standard TCP
uses a convex increase function where after a loss event, the window uses a convex increase function where during congestion avoidance the
increment is always increasing, CUBIC uses both the concave and window increment is always increasing, CUBIC uses both the concave
convex profiles of a cubic function for window increase. After a and convex profiles of a cubic function for window increase. After a
window reduction following a loss event, it registers the window size window reduction following a loss event detected by duplicate ACKs,
where it got the loss event as W_max and performs a multiplicative it registers the window size where it got the loss event as W_max and
decrease of congestion window and the regular fast recovery and performs a multiplicative decrease of congestion window and the
retransmit of Standard TCP. After it enters into congestion regular fast recovery and retransmit of Standard TCP. After it
avoidance from fast recovery, it starts to increase the window using enters into congestion avoidance from fast recovery, it starts to
the concave profile of the cubic function. The cubic function is set increase the window using the concave profile of the cubic function.
to have its plateau at W_max so the concave growth continues until The cubic function is set to have its plateau at W_max so the concave
the window size becomes W_max. After that, the cubic function turns growth continues until the window size becomes W_max. After that,
into a convex profile and the convex window growth begins. This the cubic function turns into a convex profile and the convex window
style of window adjustment (concave and then convex) improves growth begins. This style of window adjustment (concave and then
protocol and network stability while maintaining high network convex) improves protocol and network stability while maintaining
utilization [CEHRX07]. This is because the window size remains high network utilization [CEHRX07]. This is because the window size
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
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 from the beginning of congestion avoidance. This
flow fairness to Standard TCP as well as RTT-fairness. Note that feature promotes per-flow fairness to Standard TCP as well as RTT-
Standard TCP performs well under short RTT and small bandwidth (or fairness. Note that Standard TCP performs well under short RTT and
small BDP) networks. Only in a large long RTT and large bandwidth small bandwidth (or small BDP) networks. Only in a large long RTT
(or large BDP) networks, it has the scalability problem. An and large bandwidth (or large BDP) networks, it has the scalability
alternative protocol to Standard TCP designed to be friendly to problem. An alternative protocol to Standard TCP designed to be
Standard TCP at a per-flow basis must operate to increase its window friendly to Standard TCP at a per-flow basis must operate to increase
much less aggressively in small BDP networks than in large BDP its window much less aggressively in small BDP networks than in large
networks. In CUBIC, its window growth rate is slowest around the BDP 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
faster than Standard TCP around its inflection point. When the cubic faster than Standard TCP around its inflection point. When the cubic
function grows slower than the window of Standard TCP, CUBIC simply function grows slower than the window of Standard TCP, CUBIC simply
follows the window size of Standard TCP to ensure fairness to follows the window size of Standard TCP to ensure fairness to
Standard TCP in a small BDP network. We call this region where CUBIC Standard TCP in a small BDP network. We call this region where CUBIC
behaves like Standard TCP, the TCP-friendly region. behaves like Standard TCP, the TCP-friendly region.
CUBIC maintains the same window growth rate independent of RTTs CUBIC maintains the same window growth rate independent of RTTs
outside of the TCP-friendly region, and flows with different RTTs outside of the TCP-friendly region, and flows with different RTTs
have the similar window sizes under steady state when they operate have the similar window sizes under steady state when they operate
outside the TCP-friendly region. This ensures CUBIC flows with outside the TCP-friendly region. This ensures CUBIC flows with
different RTTs to have their bandwidth shares linearly proportional different RTTs to have their bandwidth shares (approximately, window/
to the inverse of their RTT ratio (the longer RTT, the smaller the RTT) linearly proportional to the inverse of their RTT ratio (the
share). This behavior is the same as that of Standard TCP under high longer RTT, the smaller the share). This behavior is the same as
statistical multiplexing environments where packet losses are that of Standard TCP under high statistical multiplexing environments
independent of individual flow rates. However, under low statistical where packet losses are independent of individual flow rates.
multiplexing environments, the bandwidth share ratio of Standard TCP However, under low statistical multiplexing environments, the
flows with different RTTs is squarely proportional to the inverse of bandwidth share ratio of Standard TCP flows with different RTTs is
their RTT ratio [XHR04]. CUBIC always ensures the linear ratio squarely proportional to the inverse of their RTT ratio [XHR04].
independent of the levels of statistical multiplexing. This is an CUBIC always ensures the linear ratio independent of the levels of
improvement over Standard TCP. While there is no consensus on a statistical multiplexing. This is an improvement over Standard TCP.
particular bandwidth share ratios of different RTT flows, we believe While there is no consensus on a particular bandwidth share ratios of
that under wired Internet, use of the linear share notion seems more different RTT flows, we believe that under wired Internet, use of the
reasonable than equal share or a higher order shares. HTCP [LS08] linear share notion seems more reasonable than equal share or a
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 MIMD can converge. CUBIC flows with the
skipping to change at page 5, line 43 skipping to change at page 5, line 47
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 (measured right after the fast recovery), and K is
the time period that the above function takes to increase the current the time period that the above function takes to increase the current
window size to W_max if there is no further loss event and is window size to W_max if there is no further loss event and is
calculated by using the following equation: 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 occurs, CUBIC reduces its current window cwnd is, when a packet loss (detected by duplicate ACKs) occurs, CUBIC
to cwnd*beta_cubic. We discuss how we set C in the next Section in reduces its current window cwnd to W_cubic(0)=W_max*beta_cubic. We
more details. 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.
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 3.2. 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 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
case of an Additive Increase and Multiplicative Decrease algorithm case of an Additive Increase and Multiplicative Decrease algorithm
(AIMD), which has an additive factor alpha_aimd and a multiplicative (AIMD), which has an additive factor alpha_aimd and a multiplicative
factor beta_aimd with the following function: factor beta_aimd with the following function:
AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) / AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) /
(2*(1-beta_aimd)*p) ]^0.5 (Eq. 3) (2*(1-beta_aimd)*p) ]^0.5 (Eq. 3)
skipping to change at page 7, line 10 skipping to change at page 7, line 19
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 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_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.
3.4. Convex region 3.4. 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. each received ACK, where W_cubic(t+RTT) is calculated using Eq. 1.
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 (detected by duplicate ACKs) occurs, CUBIC updates
of beta. Parameter beta_cubic SHOULD be set to 0.7. its W_max, cwnd, and 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
cwnd = cwnd * beta_cubic; // window reduction ssthresh = cwnd * beta_cubic; // new slow start threshold
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 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
skipping to change at page 8, line 22 skipping to change at page 8, line 35
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
In case of timeout, CUBIC follows the standard TCP to reduce cwnd,
but sets ssthresh using beta_cubic (same as in Section 3.5).
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:
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 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.
skipping to change at page 11, line 10 skipping to change at page 11, line 33
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 4.3. 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. It is not designed for wireless networks. distance networks.
4.4. Investigating a Range of Environments 4.4. 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 4.5. Protection against Congestion Collapse
In case that there is congestion collapse, CUBIC behaves likely With regard to the potential of causing congestion collapse, CUBIC
standard TCP since CUBIC modifies only the window adjustment behaves like standard TCP since CUBIC modifies only the window
algorithm of TCP. Thus, it does not modify the ACK clocking and adjustment algorithm of TCP. Thus, it does not modify the ACK
Timeout behaviors of Standard TCP. clocking and Timeout behaviors of Standard TCP.
4.6. Fairness within the Alternative Congestion Control Algorithm. 4.6. 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 4.7. 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 4.8. 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 should 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 4.9. 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 4.10. Incremental Deployment
skipping to change at page 14, line 18 skipping to change at page 14, line 44
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 68588-0115
US US
Email: xu@unl.edu Email: xu@unl.edu
Sangtae Ha Sangtae Ha
University of Colorado at Boulder University of Colorado at Boulder
Department of Computer Science Department of Computer Science
Boulder, CO 80309-0430 Boulder, CO 80309-0430
US US
Email: sangtae.ha@colorado.edu Email: sangtae.ha@colorado.edu
Alexander Zimmermann Alexander Zimmermann
NetApp
Sonnenallee 1
Kirchheim 85551
Germany
Phone: +49 89 900594712 Phone: +49 175 5766838
Email: alexander.zimmermann@netapp.com Email: alexander.zimmermann@rwth-aachen.de
Lars Eggert Lars Eggert
NetApp NetApp
Sonnenallee 1 Sonnenallee 1
Kirchheim 85551 Kirchheim 85551
Germany Germany
Phone: +49 151 12055791 Phone: +49 151 12055791
Email: lars@netapp.com Email: lars@netapp.com
Richard Scheffenegger Richard Scheffenegger
NetApp
Am Euro Platz 2
Vienna 1120
Austria
Phone: +43 1 3676811 3146 Email: rscheff@gmx.at
Email: rs@netapp.com
 End of changes. 34 change blocks. 
81 lines changed or deleted 89 lines changed or added

This html diff was produced by rfcdiff 1.45. The latest version is available from http://tools.ietf.org/tools/rfcdiff/