CUBIC for Fast Long-Distance Networks
University of Nebraska-Lincoln
Department of Computer Science and Engineering
Lincoln
NE
68588-0115
USA
xu@unl.edu
https://cse.unl.edu/~xu/
University of Colorado at Boulder
Department of Computer Science
Boulder
CO
80309-0430
USA
sangtae.ha@colorado.edu
https://netstech.org/sangtaeha/
NetApp
Stenbergintie 12 B
Kauniainen
02700
FI
lars@eggert.org
https://eggert.org/
Transport
TCPM
CUBIC is an extension to the current TCP standards. It differs from
the current TCP standards only in the congestion control algorithm on
the sender side. In particular, it uses a cubic function instead of
a linear window increase function of the current TCP standards to
improve scalability and stability under fast and long-distance
networks. CUBIC and its predecessor algorithm have been adopted as
defaults by Linux and have been used for many years. This document
provides a specification of CUBIC to enable third-party
implementations and to solicit community feedback through
experimentation on the performance of CUBIC.
This documents obsoletes , updating the specification of
CUBIC to conform to the current Linux version.
Note to Readers
Discussion of this draft takes place on the TCPM working group mailing
list, which is archived at
.
Working Group information can be found at
; source code and issues list for this
draft can be found at .
Introduction
The low utilization problem of TCP in fast long-distance networks is
well documented in and . This problem arises from a
slow increase of the congestion window following a congestion event
in a network with a large bandwidth-delay product (BDP).
indicates that this problem is frequently observed even in the range
of congestion window sizes over several hundreds of packets. This
problem is equally applicable to all Reno-style TCP standards and
their variants, including TCP-Reno , TCP-NewReno
, SCTP , and TFRC , which
use the same linear increase function for window growth, which we refer to
collectively as "Standard TCP" below.
CUBIC, originally proposed in , is a modification to the
congestion control algorithm of Standard TCP to remedy this problem.
This document describes the most recent specification of CUBIC.
Specifically, CUBIC uses a cubic function instead of a linear window
increase function of Standard TCP to improve scalability and
stability under fast and long-distance networks.
Binary Increase Congestion Control (BIC-TCP) , a predecessor
of CUBIC, was selected as the default TCP congestion control
algorithm by Linux in the year 2005 and has been used for several
years by the Internet community at large. CUBIC uses a similar
window increase function as BIC-TCP and is designed to be less
aggressive and fairer to Standard TCP in bandwidth usage than BIC-TCP
while maintaining the strengths of BIC-TCP such as stability, window
scalability, and RTT fairness. CUBIC has already replaced BIC-TCP as
the default TCP congestion control algorithm in Linux and has been
deployed globally by Linux. Through extensive testing in various
Internet scenarios, we believe that CUBIC is safe for testing and
deployment in the global Internet.
In the following sections, we first briefly explain the design
principles of CUBIC, then provide the exact specification of CUBIC,
and finally discuss the safety features of CUBIC following the
guidelines specified in .
Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14 when, and only when, they
appear in all capitals, as shown here.
Design Principles of CUBIC
CUBIC is designed according to the following design principles:
- Principle 1:
-
For better network utilization and stability, CUBIC
uses both the concave and convex profiles of a cubic function to
increase the congestion window size, instead of using just a
convex function.
- Principle 2:
-
To be TCP-friendly, CUBIC is designed to behave like
Standard TCP in networks with short RTTs and small bandwidth where
Standard TCP performs well.
- Principle 3:
-
For RTT-fairness, CUBIC is designed to achieve linear
bandwidth sharing among flows with different RTTs.
- Principle 4:
-
CUBIC appropriately sets its multiplicative window
decrease factor in order to balance between the scalability and
convergence speed.
Principle 1: For better network utilization and stability, CUBIC
uses a cubic window increase function in terms of the elapsed
time from the last congestion event. While most alternative
congestion control algorithms to Standard TCP increase the congestion
window using convex functions, CUBIC uses both the concave and convex
profiles of a cubic function for window growth. After a window
reduction in response to a congestion event is detected by duplicate
ACKs or Explicit Congestion Notification-Echo (ECN-Echo) ACKs
, CUBIC registers the congestion window size where it got
the congestion event as W_max and performs a multiplicative decrease
of congestion window. After it enters into congestion avoidance, it
starts to increase the congestion window using the concave profile of
the cubic function. The cubic function is set to have its plateau at
W_max so that the concave window increase continues until the window
size becomes W_max. After that, the cubic function turns into a
convex profile and the convex window increase begins. This style of
window adjustment (concave and then convex) improves the algorithm
stability while maintaining high network utilization . This
is because the window size remains almost constant, forming a plateau
around W_max where network utilization is deemed highest. Under
steady state, most window size samples of CUBIC are close to W_max,
thus promoting high network utilization and stability. Note that
those congestion control algorithms using only convex functions to
increase the congestion window size have the maximum increments
around W_max, and thus introduce a large number of packet bursts
around the saturation point of the network, likely causing frequent
global loss synchronizations.
Principle 2: CUBIC promotes per-flow fairness to Standard TCP. Note
that Standard TCP performs well under short RTT and small bandwidth
(or small BDP) networks. There is only a scalability problem in
networks with long RTTs and large bandwidth (or large BDP). An
alternative congestion control algorithm to Standard TCP designed to
be friendly to Standard TCP on a per-flow basis must operate to
increase its congestion window less aggressively in small BDP
networks than in large BDP networks. The aggressiveness of CUBIC
mainly depends on the maximum window size before a window reduction,
which is smaller in small BDP networks than in large BDP networks.
Thus, CUBIC increases its congestion window less aggressively in
small BDP networks than in large BDP networks. Furthermore, in cases
when the cubic function of CUBIC increases its congestion window less
aggressively than Standard TCP, CUBIC simply follows the window size
of Standard TCP to ensure that CUBIC achieves at least the same
throughput as Standard TCP in small BDP networks. We call this
region where CUBIC behaves like Standard TCP, the "TCP-friendly
region".
Principle 3: Two CUBIC flows with different RTTs have their
throughput ratio linearly proportional to the inverse of their RTT
ratio, where the throughput of a flow is approximately the size of
its congestion window divided by its RTT. Specifically, CUBIC
maintains a window increase rate independent of RTTs outside of the
TCP-friendly region, and thus flows with different RTTs have similar
congestion window sizes under steady state when they operate outside
the TCP-friendly region. This notion of a linear throughput ratio is
similar to that of Standard TCP under high statistical multiplexing
environments where packet losses are independent of individual flow
rates. However, under low statistical multiplexing environments, the
throughput ratio of Standard TCP flows with different RTTs is
quadratically proportional to the inverse of their RTT ratio .
CUBIC always ensures the linear throughput ratio independent of the
levels of statistical multiplexing. This is an improvement over
Standard TCP. While there is no consensus on particular throughput
ratios of different RTT flows, we believe that under wired Internet,
use of a linear throughput ratio seems more reasonable than equal
throughputs (i.e., the same throughput for flows with different RTTs)
or a higher-order throughput ratio (e.g., a quadratical throughput
ratio of Standard TCP under low statistical multiplexing
environments).
Principle 4: To balance between the scalability and convergence
speed, CUBIC sets the multiplicative window decrease factor to 0.7
while Standard TCP uses 0.5. While this improves the scalability of
CUBIC, a side effect of this decision is slower convergence,
especially under low statistical multiplexing environments. This
design choice is following the observation that the author of
HighSpeed TCP (HSTCP) has made along with other researchers
(e.g., ): the current Internet becomes more asynchronous with
less frequent loss synchronizations with high statistical
multiplexing. Under this environment, even strict Multiplicative-Increase
Multiplicative-Decrease (MIMD) can converge. CUBIC flows
with the same RTT always converge to the same throughput independent
of statistical multiplexing, thus achieving intra-algorithm fairness.
We also find that under the environments with sufficient statistical
multiplexing, the convergence speed of CUBIC flows is reasonable.
CUBIC Congestion Control
The unit of all window sizes in this document is segments of the
maximum segment size (MSS), and the unit of all times is seconds.
Let cwnd denote the congestion window size of a flow, and ssthresh
denote the slow-start threshold.
Window Increase Function
CUBIC maintains the acknowledgment (ACK) clocking of Standard TCP by
increasing the congestion window only at the reception of an ACK. It
does not make any change to the fast recovery and retransmit of TCP,
such as TCP-NewReno . During congestion avoidance
after a congestion event where a packet loss is detected by duplicate
ACKs or a network congestion is detected by ACKs with ECN-Echo flags
, CUBIC changes the window increase function of Standard
TCP. Suppose that W_max is the window size just before the window is
reduced in the last congestion event.
CUBIC uses the following window increase function:
where C is a constant fixed to determine the aggressiveness of window
increase in high BDP networks, t is the elapsed time from the
beginning of the current congestion avoidance, and K is the time
period that the above function takes to increase the current window
size to W_max if there are no further congestion events and is
calculated using the following equation:
where beta_cubic is the CUBIC multiplication decrease factor, that
is, when a congestion event is detected, CUBIC reduces its cwnd to
W_cubic(0)=W_max*beta_cubic. We discuss how we set beta_cubic in
and how we set C in .
Upon receiving an ACK during congestion avoidance, CUBIC computes the
window increase rate during the next RTT period using Eq. 1. It sets
W_cubic(t+RTT) as the candidate target value of the congestion
window, where RTT is the weighted average RTT calculated by Standard
TCP.
Depending on the value of the current congestion window size cwnd,
CUBIC runs in three different modes.
- The TCP-friendly region, which ensures that CUBIC achieves at
least the same throughput as Standard TCP.
- The concave region, if CUBIC is not in the TCP-friendly region
and cwnd is less than W_max.
- The convex region, if CUBIC is not in the TCP-friendly region and
cwnd is greater than W_max.
Below, we describe the exact actions taken by CUBIC in each region.
TCP-Friendly Region
Standard TCP performs well in certain types of networks, for example,
under short RTT and small bandwidth (or small BDP) networks. In
these networks, we use the TCP-friendly region to ensure that CUBIC
achieves at least the same throughput as Standard TCP.
The TCP-friendly region is designed according to the analysis
described in . The analysis studies the performance of an
Additive Increase and Multiplicative Decrease (AIMD) algorithm with
an additive factor of alpha_aimd (segments per RTT) and a
multiplicative factor of beta_aimd, denoted by AIMD(alpha_aimd,
beta_aimd). Specifically, the average congestion window size of
AIMD(alpha_aimd, beta_aimd) can be calculated using Eq. 3. The
analysis shows that AIMD(alpha_aimd, beta_aimd) with
alpha_aimd=3*(1-beta_aimd)/(1+beta_aimd) achieves the same average
window size as Standard TCP that uses AIMD(1, 0.5).
Based on the above analysis, CUBIC uses Eq. 4 to estimate the window
size W_est of AIMD(alpha_aimd, beta_aimd) with
alpha_aimd=3*(1-beta_cubic)/(1+beta_cubic) and beta_aimd=beta_cubic,
which achieves the same average window size as Standard TCP. When
receiving an ACK in congestion avoidance (cwnd could be greater than
or less than W_max), CUBIC checks whether W_cubic(t) is less than
W_est(t). If so, CUBIC is in the TCP-friendly region and cwnd SHOULD
be set to W_est(t) at each reception of an ACK.
Concave Region
When receiving an ACK in congestion avoidance, if CUBIC is not in the
TCP-friendly region and cwnd is less than W_max, then CUBIC is in the
concave region. In this region, cwnd MUST be incremented by
(W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where
W_cubic(t+RTT) is calculated using Eq. 1.
Convex Region
When receiving an ACK in congestion avoidance, if CUBIC is not in the
TCP-friendly region and cwnd is larger than or equal to W_max, then
CUBIC is in the convex region. The convex region indicates that the
network conditions might have been perturbed since the last
congestion event, possibly implying more available bandwidth after
some flow departures. Since the Internet is highly asynchronous,
some amount of perturbation is always possible without causing a
major change in available bandwidth. In this region, CUBIC is being
very careful by very slowly increasing its window size. The convex
profile ensures that the window increases very slowly at the
beginning and gradually increases its increase rate. We also call
this region the "maximum probing phase" since CUBIC is searching for
a new W_max. In this region, cwnd MUST be incremented by
(W_cubic(t+RTT) - cwnd)/cwnd for each received ACK, where
W_cubic(t+RTT) is calculated using Eq. 1.
Multiplicative Decrease
When a packet loss is detected by duplicate ACKs or a network
congestion is detected by ECN-Echo ACKs, CUBIC updates its W_max,
cwnd, and ssthresh as follows. Parameter beta_cubic SHOULD be set to
0.7.
A side effect of setting beta_cubic to a value bigger than 0.5 is
slower convergence. We believe that while a more adaptive setting of
beta_cubic could result in faster convergence, it will make the
analysis of CUBIC much harder. This adaptive adjustment of
beta_cubic is an item for the next version of CUBIC.
Fast Convergence
To improve the convergence speed of CUBIC, we add a heuristic in
CUBIC. When a new flow joins the network, existing flows in the
network need to give up some of their bandwidth to allow the new flow
some room for growth if the existing flows have been using all the
bandwidth of the network. To speed up this bandwidth release by
existing flows, the following mechanism called "fast convergence"
SHOULD be implemented.
With fast convergence, when a congestion event occurs, before the
window reduction of the congestion window, a flow remembers the last
value of W_max before it updates W_max for the current congestion
event. Let us call the last value of W_max to be W_last_max.
At a congestion event, if the current value of W_max is less than
W_last_max, this indicates that the saturation point experienced by
this flow is getting reduced because of the change in available
bandwidth. Then we allow this flow to release more bandwidth by
reducing W_max further. This action effectively lengthens the time
for this flow to increase its congestion window because the reduced
W_max forces the flow to have the plateau earlier. This allows more
time for the new flow to catch up to its congestion window size.
The fast convergence is designed for network environments with
multiple CUBIC flows. In network environments with only a single
CUBIC flow and without any other traffic, the fast convergence SHOULD
be disabled.
Timeout
In case of timeout, CUBIC follows Standard TCP to reduce cwnd
, but sets ssthresh using beta_cubic (same as in
) that is different from Standard TCP .
During the first congestion avoidance after a timeout, CUBIC
increases its congestion window size using Eq. 1, where t is the
elapsed time since the beginning of the current congestion avoidance,
K is set to 0, and W_max is set to the congestion window size at the
beginning of the current congestion avoidance.
Slow Start
CUBIC MUST employ a slow-start algorithm, when the cwnd is no more
than ssthresh. Among the slow-start algorithms, CUBIC MAY choose the
standard TCP slow start in general networks, or the limited
slow start or hybrid slow start for fast and long-
distance networks.
In the case when CUBIC runs the hybrid slow start , it may exit
the first slow start without incurring any packet loss and thus W_max
is undefined. In this special case, CUBIC switches to congestion
avoidance and increases its congestion window size using Eq. 1, where
t is the elapsed time since the beginning of the current congestion
avoidance, K is set to 0, and W_max is set to the congestion window
size at the beginning of the current congestion avoidance.
Discussion
In this section, we further discuss the safety features of CUBIC
following the guidelines specified in .
With a deterministic loss model where the number of packets between
two successive packet losses is always 1/p, CUBIC always operates
with the concave window profile, which greatly simplifies the
performance analysis of CUBIC. The average window size of CUBIC can
be obtained by the following function:
With beta_cubic set to 0.7, the above formula is reduced to:
We will determine the value of C in the following subsection using
Eq. 6.
Fairness to Standard TCP
In environments where Standard TCP is able to make reasonable use of
the available bandwidth, CUBIC does not significantly change this
state.
Standard TCP performs well in the following two types of networks:
- networks with a small bandwidth-delay product (BDP)
- networks with a short RTTs, but not necessarily a small BDP
CUBIC is designed to behave very similarly to Standard TCP in the
above two types of networks. The following two tables show the
average window sizes of Standard TCP, HSTCP, and CUBIC. The average
window sizes of Standard TCP and HSTCP are from . The
average window size of CUBIC is calculated using Eq. 6 and the CUBIC
TCP-friendly region for three different values of C.
Table 1 describes the response function of Standard TCP, HSTCP, and
CUBIC in networks with RTT = 0.1 seconds. The average window size is
in MSS-sized segments.
Table 2 describes the response function of Standard TCP, HSTCP, and
CUBIC in networks with RTT = 0.01 seconds. The average window size
is in MSS-sized segments.
Both tables show that CUBIC with any of these three C values is more
friendly to TCP than HSTCP, especially in networks with a short RTT
where TCP performs reasonably well. For example, in a network with
RTT = 0.01 seconds and p=10^-6, TCP has an average window of 1200
packets. If the packet size is 1500 bytes, then TCP can achieve an
average rate of 1.44 Gbps. In this case, CUBIC with C=0.04 or C=0.4
achieves exactly the same rate as Standard TCP, whereas HSTCP is
about ten times more aggressive than Standard TCP.
We can see that C determines the aggressiveness of CUBIC in competing
with other congestion control algorithms for bandwidth. CUBIC is
more friendly to Standard TCP, if the value of C is lower. However,
we do not recommend setting C to a very low value like 0.04, since
CUBIC with a low C cannot efficiently use the bandwidth in long RTT
and high-bandwidth networks. Based on these observations and our
experiments, we find C=0.4 gives a good balance between TCP-
friendliness and aggressiveness of window increase. Therefore, C
SHOULD be set to 0.4. With C set to 0.4, Eq. 6 is reduced to:
Eq. 7 is then used in the next subsection to show the scalability of
CUBIC.
Using Spare Capacity
CUBIC uses a more aggressive window increase function than Standard
TCP under long RTT and high-bandwidth networks.
The following table shows that to achieve the 10 Gbps rate, Standard
TCP requires a packet loss rate of 2.0e-10, while CUBIC requires a
packet loss rate of 2.9e-8.
Table 3 describes the required packet loss rate for Standard TCP,
HSTCP, and CUBIC to achieve a certain throughput. We use 1500-byte
packets and an RTT of 0.1 seconds.
Our test results in indicate that CUBIC uses the spare
bandwidth left unused by existing Standard TCP flows in the same
bottleneck link without taking away much bandwidth from the existing
flows.
Difficult Environments
CUBIC is designed to remedy the poor performance of TCP in fast and
long-distance networks.
Investigating a Range of Environments
CUBIC has been extensively studied by using both NS-2 simulation and
test-bed experiments covering a wide range of network environments.
More information can be found in .
Same as Standard TCP, CUBIC is a loss-based congestion control
algorithm. Because CUBIC is designed to be more aggressive (due to a
faster window increase function and bigger multiplicative decrease
factor) than Standard TCP in fast and long-distance networks, it can
fill large drop-tail buffers more quickly than Standard TCP and
increase the risk of a standing queue . In this case, proper
queue sizing and management could be used to reduce the
packet queuing delay.
Protection against Congestion Collapse
With regard to the potential of causing congestion collapse, CUBIC
behaves like Standard TCP since CUBIC modifies only the window
adjustment algorithm of TCP. Thus, it does not modify the ACK
clocking and Timeout behaviors of Standard TCP.
Fairness within the Alternative Congestion Control Algorithm
CUBIC ensures convergence of competing CUBIC flows with the same RTT
in the same bottleneck links to an equal throughput. When competing
flows have different RTTs, their throughput ratio is linearly
proportional to the inverse of their RTT ratios. This is true
independent of the level of statistical multiplexing in the link.
Performance with Misbehaving Nodes and Outside Attackers
This is not considered in the current CUBIC.
Behavior for Application-Limited Flows
CUBIC does not raise its congestion window size if the flow is
currently limited by the application instead of the congestion
window. In case of long periods when cwnd has not been updated due
to the application rate limit, such as idle periods, t in Eq. 1 MUST
NOT include these periods; otherwise, W_cubic(t) might be very high
after restarting from these periods.
Responses to Sudden or Transient Events
If there is a sudden congestion, a routing change, or a mobility
event, CUBIC behaves the same as Standard TCP.
Incremental Deployment
CUBIC requires only the change of TCP senders, and it does not make
any changes to TCP receivers. That is, a CUBIC sender works
correctly with the Standard TCP receivers. In addition, CUBIC does
not require any changes to the routers and does not require any
assistance from the routers.
Security Considerations
This proposal makes no changes to the underlying security of TCP.
More information about TCP security concerns can be found in
.
IANA Considerations
This document does not require any IANA actions.
References
Normative References
TCP Congestion Control
This document defines TCP's four intertwined congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. In addition, the document specifies how TCP should begin transmission after a relatively long idle period, as well as discussing various acknowledgment generation methods. This document obsoletes RFC 2581. [STANDARDS-TRACK]
The NewReno Modification to TCP's Fast Recovery Algorithm
RFC 5681 documents the following four intertwined TCP congestion control algorithms: slow start, congestion avoidance, fast retransmit, and fast recovery. RFC 5681 explicitly allows certain modifications of these algorithms, including modifications that use the TCP Selective Acknowledgment (SACK) option (RFC 2883), and modifications that respond to "partial acknowledgments" (ACKs that cover new data, but not all the data outstanding when loss was detected) in the absence of SACK. This document describes a specific algorithm for responding to partial acknowledgments, referred to as "NewReno". This response to partial acknowledgments was first proposed by Janey Hoe. This document obsoletes RFC 3782. [STANDARDS-TRACK]
A Conservative Loss Recovery Algorithm Based on Selective Acknowledgment (SACK) for TCP
This document presents a conservative loss recovery algorithm for TCP that is based on the use of the selective acknowledgment (SACK) TCP option. The algorithm presented in this document conforms to the spirit of the current congestion control specification (RFC 5681), but allows TCP senders to recover more effectively when multiple segments are lost from a single flight of data. This document obsoletes RFC 3517 and describes changes from it. [STANDARDS-TRACK]
TCP Friendly Rate Control (TFRC): Protocol Specification
This document specifies TCP Friendly Rate Control (TFRC). TFRC is a congestion control mechanism for unicast flows operating in a best-effort Internet environment. It is reasonably fair when competing for bandwidth with TCP flows, but has a much lower variation of throughput over time compared with TCP, making it more suitable for applications such as streaming media where a relatively smooth sending rate is of importance.
This document obsoletes RFC 3448 and updates RFC 4342. [STANDARDS-TRACK]
Specifying New Congestion Control Algorithms
The IETF's standard congestion control schemes have been widely shown to be inadequate for various environments (e.g., high-speed networks). Recent research has yielded many alternate congestion control schemes that significantly differ from the IETF's congestion control principles. Using these new congestion control schemes in the global Internet has possible ramifications to both the traffic using the new congestion control and to traffic using the currently standardized congestion control. Therefore, the IETF must proceed with caution when dealing with alternate congestion control proposals. The goal of this document is to provide guidance for considering alternate congestion control algorithms within the IETF. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.
Key words for use in RFCs to Indicate Requirement Levels
In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.
The Addition of Explicit Congestion Notification (ECN) to IP
This memo specifies the incorporation of ECN (Explicit Congestion Notification) to TCP and IP, including ECN's use of two bits in the IP header. [STANDARDS-TRACK]
IETF Recommendations Regarding Active Queue Management
This memo presents recommendations to the Internet community concerning measures to improve and preserve Internet performance. It presents a strong recommendation for testing, standardization, and widespread deployment of active queue management (AQM) in network devices to improve the performance of today's Internet. It also urges a concerted effort of research, measurement, and ultimate deployment of AQM mechanisms to protect the Internet from flows that are not sufficiently responsive to congestion notification.
Based on 15 years of experience and new research, this document replaces the recommendations of RFC 2309.
Informative References
A Comparison of Equation-Based and AIMD Congestion Control
Extended Analysis of Binary Adjustment Algorithms
A Step toward Realistic Performance Evaluation of High-Speed TCP Variants
Hybrid Slow Start for High-Bandwidth and Long-Distance Networks
Binary Increase Congestion Control (BIC) for Fast Long-Distance Networks
Stochastic Ordering for Internet Congestion Control and its Applications
CUBIC: a new TCP-friendly high-speed TCP variant
Scalable TCP: improving performance in highspeed wide area networks
CUBIC for Fast Long-Distance Networks
CUBIC is an extension to the current TCP standards. It differs from the current TCP standards only in the congestion control algorithm on the sender side. In particular, it uses a cubic function instead of a linear window increase function of the current TCP standards to improve scalability and stability under fast and long-distance networks. CUBIC and its predecessor algorithm have been adopted as defaults by Linux and have been used for many years. This document provides a specification of CUBIC to enable third-party implementations and to solicit community feedback through experimentation on the performance of CUBIC.
HighSpeed TCP for Large Congestion Windows
The proposals in this document are experimental. While they may be deployed in the current Internet, they do not represent a consensus that this is the best method for high-speed congestion control. In particular, we note that alternative experimental proposals are likely to be forthcoming, and it is not well understood how the proposals in this document will interact with such alternative proposals. This document proposes HighSpeed TCP, a modification to TCP's congestion control mechanism for use with TCP connections with large congestion windows. The congestion control mechanisms of the current Standard TCP constrains the congestion windows that can be achieved by TCP in realistic environments. For example, for a Standard TCP connection with 1500-byte packets and a 100 ms round-trip time, achieving a steady-state throughput of 10 Gbps would require an average congestion window of 83,333 segments, and a packet drop rate of at most one congestion event every 5,000,000,000 packets (or equivalently, at most one congestion event every 1 2/3 hours). This is widely acknowledged as an unrealistic constraint. To address his limitation of TCP, this document proposes HighSpeed TCP, and solicits experimentation and feedback from the wider community.
Stream Control Transmission Protocol
This document obsoletes RFC 2960 and RFC 3309. It describes the Stream Control Transmission Protocol (SCTP). SCTP is designed to transport Public Switched Telephone Network (PSTN) signaling messages over IP networks, but is capable of broader applications.
SCTP is a reliable transport protocol operating on top of a connectionless packet network such as IP. It offers the following services to its users:
-- acknowledged error-free non-duplicated transfer of user data,
-- data fragmentation to conform to discovered path MTU size,
-- sequenced delivery of user messages within multiple streams, with an option for order-of-arrival delivery of individual user messages,
-- optional bundling of multiple user messages into a single SCTP packet, and
-- network-level fault tolerance through supporting of multi-homing at either or both ends of an association.
The design of SCTP includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks. [STANDARDS-TRACK]
Limited Slow-Start for TCP with Large Congestion Windows
This document describes an optional modification for TCP's slow-start for use with TCP connections with large congestion windows. For TCP connections that are able to use congestion windows of thousands (or tens of thousands) of MSS-sized segments (for MSS the sender's MAXIMUM SEGMENT SIZE), the current slow-start procedure can result in increasing the congestion window by thousands of segments in a single round-trip time. Such an increase can easily result in thousands of packets being dropped in one round-trip time. This is often counter-productive for the TCP flow itself, and is also hard on the rest of the traffic sharing the congested link. This note describes Limited Slow-Start as an optional mechanism for limiting the number of segments by which the congestion window is increased for one window of data during slow-start, in order to improve performance for TCP connections with large congestion windows. This memo defines an Experimental Protocol for the Internet community.
TCP Alternative Backoff with ECN (ABE)
Active Queue Management (AQM) mechanisms allow for burst tolerance while enforcing short queues to minimise the time that packets spend enqueued at a bottleneck. This can cause noticeable performance degradation for TCP connections traversing such a bottleneck, especially if there are only a few flows or their bandwidth-delay product (BDP) is large. The reception of a Congestion Experienced (CE) Explicit Congestion Notification (ECN) mark indicates that an AQM mechanism is used at the bottleneck, and the bottleneck network queue is therefore likely to be short. Feedback of this signal allows the TCP sender-side ECN reaction in congestion avoidance to reduce the Congestion Window (cwnd) by a smaller amount than the congestion control algorithm's reaction to inferred packet loss. Therefore, this specification defines an experimental change to the TCP reaction specified in RFC 3168, as permitted by RFC 8311.
Changes from RFC8312
Since RFC8312
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