Transmission Control Protocol
The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is common ...
(TCP) uses a
network congestion-avoidance algorithm that includes various aspects of an
additive increase/multiplicative decrease
The additive-increase/multiplicative-decrease (AIMD) algorithm is a feedback control algorithm best known for its use in TCP congestion control. AIMD combines linear growth of the congestion window when there is no congestion with an exponential re ...
(AIMD) scheme, along with other schemes including
slow start and
congestion window
Transmission Control Protocol (TCP) uses a network congestion-avoidance algorithm that includes various aspects of an additive increase/multiplicative decrease (AIMD) scheme, along with other schemes including slow start and congestion windo ...
(CWND), to achieve congestion avoidance. The TCP congestion-avoidance algorithm is the primary basis for
congestion control
Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...
in the Internet.
Per the
end-to-end principle
The end-to-end principle is a design framework in computer networking. In networks designed according to this principle, guaranteeing certain application-specific features, such as reliability and security, requires that they reside in the commu ...
, congestion control is largely a function of
internet host
A network host is a computer or other device connected to a computer network. A host may work as a server offering information resources, services, and applications to users or other hosts on the network. Hosts are assigned at least one network a ...
s, not the network itself. There are several variations and versions of the algorithm implemented in
protocol stack
The protocol stack or network stack is an implementation of a computer networking protocol suite or protocol family. Some of these terms are used interchangeably but strictly speaking, the ''suite'' is the definition of the communication protoco ...
s of
operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...
s of computers that connect to the
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a '' network of networks'' that consists of private, pub ...
.
To avoid
congestive collapse, TCP uses multi-faceted congestion-control strategy. For each connection, TCP maintains a CWND, limiting the total number of unacknowledged packets that may be in transit end-to-end. This is somewhat analogous to TCP's
sliding window
A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the data link layer ( OSI layer 2) as well as in the Tran ...
used for
flow control.
Additive increase/multiplicative decrease
The
additive increase/multiplicative decrease
The additive-increase/multiplicative-decrease (AIMD) algorithm is a feedback control algorithm best known for its use in TCP congestion control. AIMD combines linear growth of the congestion window when there is no congestion with an exponential re ...
(AIMD) algorithm is a
closed-loop control algorithm. AIMD combines linear growth of the congestion window with an exponential reduction when a congestion takes place. Multiple flows using AIMD congestion control will eventually converge to use equal amounts of a contended link.
This is the algorithm that is described in for the "congestion avoidance" state.
Congestion window
In TCP, the congestion window (CWND) is one of the factors that determines the number of bytes that can be sent out at any time. The congestion window is maintained by the sender and is a means of stopping ''a link'' between the sender and the receiver from becoming overloaded with too much traffic. This should not be confused with the sliding window maintained by the sender which exists to prevent ''the receiver'' from becoming overloaded. The congestion window is calculated by estimating how much congestion there is on the link.
When a connection is set up, the congestion window, a value maintained independently at each host, is set to a small multiple of the
maximum segment size The maximum segment size (MSS) is a parameter of the ''options'' field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment. It does not coun ...
(MSS) allowed on that connection. Further variance in the congestion window is dictated by an
additive increase/multiplicative decrease
The additive-increase/multiplicative-decrease (AIMD) algorithm is a feedback control algorithm best known for its use in TCP congestion control. AIMD combines linear growth of the congestion window when there is no congestion with an exponential re ...
(AIMD) approach. This means that if all segments are received and the acknowledgments reach the sender on time, some constant is added to the window size. It will follow different algorithms.
A
system administrator
A system administrator, or sysadmin, or admin is a person who is responsible for the upkeep, configuration, and reliable operation of computer systems, especially multi-user computers, such as servers. The system administrator seeks to en ...
may adjust the maximum window size limit, or adjust the constant added during additive increase, as part of
TCP tuning.
The flow of data over a TCP connection is also controlled by the use of the ''
receive window
TCP tuning techniques adjust the network congestion avoidance parameters of Transmission Control Protocol (TCP) connections over high- bandwidth, high- latency networks. Well-tuned networks can perform up to 10 times faster in some cases. Howe ...
'' advertised by the receiver. A sender can send data less than its own congestion window and the ''receive window''.
Slow start
Slow start, defined by .
is part of the
congestion control
Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...
strategy used by TCP in conjunction with other
algorithm
In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...
s to avoid sending more data than the network is capable of forwarding, that is, to avoid causing network congestion.
Slow start begins initially with a congestion window size (CWND) of 1, 2, 4 or 10 MSS.
The value for the congestion window size can be increased by 1 MSS with each
acknowledgement (ACK) received, effectively doubling the window size each
RTT. Or less
The transmission rate will be increased by the slow-start algorithm until either a packet loss is detected, or the receiver's advertised window (rwnd) is the limiting factor.
Or ''slow start threshold (ssthresh) is reached,which is used to determine whether the slow start or congestion avoidance algorithm is used, which is a value set to limit slow start''
If the CWND reaches ''ssthresh'', TCP changes to congestion avoidance algorithm. It should be increased by up to 1 MSS for each RTT.
A common formula is that ''each new ACK'' increases the CWND by MSS * ' It increases almost linearly and provides an acceptable approximation.
If a loss event occurs, TCP assumes that it is due to network congestion and takes steps to reduce the offered load on the network. These measurements depend on the exact TCP congestion avoidance algorithm used.
When a TCP sender detects segment loss using the retransmission timer and the given segment has not yet been resent by way of the retransmission timer, the value of ssthresh must be set to no more than
half of the amount of data that has been sent but not yet cumulatively acknowledged. Or 2 * MSS
;TCP Tahoe
: When a loss occurs, retransmit is sent, half of the current CWND is saved as ''ssthresh'' and slow start begins again from its initial CWND.
; TCP Reno
: A
fast retransmit
Transmission Control Protocol (TCP) uses a network congestion-avoidance algorithm that includes various aspects of an additive increase/multiplicative decrease (AIMD) scheme, along with other schemes including slow start and congestion window ...
is sent, half of the current CWND is saved as ''ssthresh'' and as new CWND, thus skipping slow start and going directly to the congestion avoidance algorithm. The overall algorithm here is called .
Slow start assumes that unacknowledged segments are due to network congestion. While this is an acceptable assumption for many networks, segments may be lost for other reasons, such as poor
data link layer
The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer ...
transmission quality. Thus, slow start can perform poorly in situations with poor reception, such as
wireless networks
A wireless network is a computer network that uses wireless data connections between network nodes.
Wireless networking is a method by which homes, telecommunications networks and business installations avoid the costly process of introducing c ...
.
The slow start protocol also performs badly for short-lived connections. Older
web browsers
A web browser is application software for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's screen. Browsers are used on ...
would create many consecutive short-lived connections to the web server, and would open and close the connection for each file requested. This kept most connections in the slow start mode, which resulted in poor response time. To avoid this problem, modern browsers either open multiple connections simultaneously or
reuse one connection for all files requested from a particular web server. Connections, however, cannot be reused for the multiple third-party servers used by web sites to implement
web advertising
Online advertising, also known as online marketing, Internet advertising, digital advertising or web advertising, is a form of marketing and advertising which uses the Internet to promote products and services to audiences and platform users. ...
,
sharing features of
social networking services, and
counter scripts of web analytics.
Fast retransmit
Fast retransmit is an enhancement to
TCP that reduces the time a sender waits before retransmitting a lost segment. A TCP sender normally uses a simple timer to recognize lost segments. If an acknowledgement is not received for a particular segment within a specified time (a function of the estimated
round-trip delay time
In telecommunications, round-trip delay (RTD) or round-trip time (RTT) is the amount of time it takes for a signal to be sent ''plus'' the amount of time it takes for acknowledgement of that signal having been received. This time delay includes pr ...
), the sender will assume the segment was lost in the network, and will retransmit the segment.
Duplicate acknowledgement is the basis for the fast retransmit mechanism. After receiving a packet an acknowledgement is sent for the last in-order byte of data received. For an in-order packet, this is effectively the last packet's sequence number plus the current packet's payload length. If the next packet in the sequence is lost but a third packet in the sequence is received, then the receiver can only acknowledge the last in-order byte of data, which is the same value as was acknowledged for the first packet. The second packet is lost and the third packet is not in order, so the last in-order byte of data remains the same as before. Thus a ''Duplicate acknowledgement'' occurs. The sender continues to send packets, and a fourth and fifth packet are received by the receiver. Again, the second packet is missing from the sequence, so the last in-order byte has not changed. Duplicate acknowledgements are sent for both of these packets.
When a sender receives three duplicate acknowledgements, it can be reasonably confident that the segment carrying the data that followed the last in-order byte specified in the acknowledgment was lost. A sender with fast retransmit will then retransmit this packet immediately without waiting for its timeout. On receipt of the retransmitted segment, the receiver can acknowledge the last in-order byte of data received. In the above example, this would acknowledge to the end of the payload of the fifth packet. There is no need to acknowledge intermediate packets, since TCP uses cumulative acknowledgements by default.
Algorithms
The naming convention for congestion control algorithms (CCAs) may have originated in a 1996 paper by Kevin Fall and Sally Floyd.
The following is one possible classification according to the following properties:
# the type and amount of feedback received from the network
# incremental deployability on the current Internet
# the aspect of performance it aims to improve: high
bandwidth-delay product
In data communications, the bandwidth-delay product is the product of a data link's capacity (in bits per second) and its round-trip delay time (in seconds). The result, an amount of data measured in bits (or bytes), is equivalent to the maxim ...
networks (B); lossy links (L); fairness (F); advantage to short flows (S); variable-rate links (V);
speed of convergence (C)
# the fairness criterion it uses
Some well-known congestion avoidance mechanisms are classified by this scheme as follows:
TCP Tahoe and Reno
TCP Tahoe and Reno algorithms were retrospectively named after the versions or flavours of the
4.3BSD The History of the Berkeley Software Distribution begins in the 1970s.
1BSD (PDP-11)
The earliest distributions of Unix from Bell Labs in the 1970s included the source code to the operating system, allowing researchers at universities to modify an ...
operating system in which each first appeared (which were themselves named after
Lake Tahoe and the nearby city of
Reno, Nevada
Reno ( ) is a city in the northwest section of the U.S. state of Nevada, along the Nevada-California border, about north from Lake Tahoe, known as "The Biggest Little City in the World". Known for its casino and tourism industry, Reno is the ...
). The Tahoe algorithm first appeared in 4.3BSD-Tahoe (which was made to support the
CCI Power 6/32 "Tahoe" minicomputer), and was later made available to non-AT&T licensees as part of the 4.3BSD Networking Release 1; this ensured its wide distribution and implementation. Improvements were made in 4.3BSD-Reno and subsequently released to the public as Networking Release 2 and later 4.4BSD-Lite.
While both consider retransmission timeout (RTO) and duplicate ACKs as packet loss events, the behavior of Tahoe and Reno differ primarily in how they react to duplicate ACKs:
* Tahoe: if three duplicate ACKs are received (i.e. four ACKs acknowledging the same packet, which are not piggybacked on data and do not change the receiver's advertised window), Tahoe performs a fast retransmit, sets the slow start threshold to half of the current congestion window, reduces the congestion window to 1 MSS, and resets to slow start state.
* Reno: if three duplicate ACKs are received, Reno will perform a fast retransmit and skip the slow start phase by instead halving the congestion window (instead of setting it to 1 MSS like Tahoe), setting the ssthresh equal to the new congestion window, and enter a phase called ''fast recovery''.
In both Tahoe and Reno, if an ACK times out (RTO timeout), slow start is used, and both algorithms reduce congestion window to 1 MSS.
TCP New Reno
TCP New Reno, defined by (which obsolesces previous definitions in and ), improves retransmission during the fast-recovery phase of TCP Reno.
During fast recovery, to keep the transmit window full, for every duplicate ACK that is returned, a new unsent packet from the end of the congestion window is sent.
The difference from Reno is that New Reno does not halve ssthresh immediately which may reduce window too much if multiple packet losses occur. It doesn't exit fast-recovery and reset ssthresh until acknowledges all of the data.
After retransmission, newly acknowledged data have two cases:
* Full acknowledgments: The ACK acknowledges all the intermediate segments sent, the ssthresh can be not changed, cwnd can be set to ssthresh
* Partial acknowledgments: The ACK does not acknowledge all data. It means another loss may occur, retransmit the first unacknowledged segment if permitted
It uses a variable called "recover" to record how much data needs to be recovered. After a retransmit timeout, it records the highest sequence number transmitted in the recover variable and exits the fast recovery procedure. If this sequence number is acknowledged, TCP returns to the congestion avoidance state.
A problem occurs with New Reno when there are no packet losses but instead, packets are reordered by more than 3 packet sequence numbers. In this case, New Reno mistakenly enters fast recovery. When the reordered packet is delivered, duplicate and needless retransmissions are immediately sent.
New Reno performs as well as SACK at low packet error rates, and substantially outperforms Reno at high error rates.
TCP Vegas
Until the mid-1990s, all of TCP's set timeouts and measured round-trip delays were based upon only the last transmitted packet in the transmit buffer.
University of Arizona
The University of Arizona (Arizona, U of A, UArizona, or UA) is a public land-grant research university in Tucson, Arizona. Founded in 1885 by the 13th Arizona Territorial Legislature, it was the first university in the Arizona Territory.
T ...
researchers Larry Peterson and
Lawrence Brakmo introduced TCP Vegas in which timeouts were set and round-trip delays were measured for every packet in the transmit buffer. In addition, TCP Vegas uses additive increases in the congestion window. In a comparison study of various TCP s, TCP Vegas appeared to be the smoothest followed by TCP CUBIC.
TCP Vegas was not widely deployed outside Peterson's laboratory but was selected as the default congestion control method for
DD-WRT
DD-WRT is Linux-based firmware for wireless routers and access points. Originally designed for the Linksys WRT54G series, it now runs on a wide variety of models. DD-WRT is one of a handful of third-party firmware projects designed to replac ...
firmware v24 SP2.
TCP Hybla
TCP Hybla aims to eliminate penalties to TCP connections that incorporate a high-latency terrestrial or satellite radio links. Hybla improvements are based on analytical evaluation of the congestion window dynamics.
TCP BIC
Binary Increase Congestion control (BIC) is a TCP implementation with an optimized CCA for high speed networks with high latency, known as
long fat network
In data communications, the bandwidth-delay product is the product of a data link's capacity (in bits per second) and its round-trip delay time (in seconds). The result, an amount of data measured in bits (or bytes), is equivalent to the maximu ...
s (LFNs). BIC is used by default in
Linux kernels 2.6.8 through 2.6.18.
TCP CUBIC
CUBIC is a less aggressive and more systematic derivative of BIC, in which the window is a cubic function of time since the last congestion event, with the inflection point set to the window prior to the event. CUBIC is used by default in
Linux kernels since version 2.6.19.
Agile-SD TCP
Agile-SD is a Linux-based CCA which is designed for the real Linux kernel. It is a receiver-side algorithm that employs a loss-based approach using a novel mechanism, called ''agility factor'' (AF). to increase the bandwidth utilization over high-speed and short-distance networks (low-BDP networks) such as local area networks or fiber-optic network, especially when the applied buffer size is small.
It has been evaluated by comparing its performance to Compound TCP (the default CCA in MS Windows) and CUBIC (the default of Linux) using NS-2 simulator. It improves the total performance up to 55% in term of average throughput.
TCP Westwood+
Westwood+ is a sender-only modification of TCP Reno that optimizes the performance of TCP congestion control over both wired and
wireless network
A wireless network is a computer network that uses wireless data connections between network nodes.
Wireless networking is a method by which homes, telecommunications networks and business installations avoid the costly process of introducing ...
s. TCP Westwood+ is based on end-to-end
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
estimation to set the congestion window and slow-start threshold after a congestion episode, that is, after three duplicate acknowledgments or a timeout. The bandwidth is estimated by averaging the rate of returning acknowledgment packets. In contrast with TCP Reno, which blindly halves the congestion window after three duplicate ACKs, TCP Westwood+ adaptively sets a slow-start threshold and a congestion window which takes into account an estimate of bandwidth available at the time congestion is experienced. Compared to Reno and New Reno, Westwood+ significantly increases throughput over wireless links and improves fairness in wired networks.
Compound TCP
Compound TCP is a
Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washin ...
implementation of TCP which maintains two different congestion windows simultaneously, with the goal of achieving good performance on LFNs while not impairing
fairness
Fairness or being fair can refer to:
* Justice
* The character in the award-nominated musical comedy '' A Theory of Justice: The Musical.''
* Equity (law), a legal principle allowing for the use of discretion and fairness when applying justice ...
. It has been widely deployed in Windows versions since Microsoft
Windows Vista
Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...
and
Windows Server 2008
Windows Server 2008 is the fourth release of the Windows Server operating system produced by Microsoft as part of the Windows NT family of the operating systems. It was released to manufacturing on February 4, 2008, and generally to retail on F ...
and has been ported to older Microsoft Windows versions as well as
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...
.
TCP Proportional Rate Reduction
TCP Proportional Rate Reduction (PRR) is an algorithm designed to improve the accuracy of data sent during recovery. The algorithm ensures that the window size after recovery is as close as possible to the slow start threshold. In tests performed by
Google
Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
, PRR resulted in a 3–10% reduction in average latency and recovery timeouts were reduced by 5%. PRR is available in
Linux kernels since version 3.2.
TCP BBR
Bottleneck Bandwidth and Round-trip propagation time (BBR) is a CCA developed at Google in 2016.
While most CCAs are loss-based, in that they rely on packet loss to detect congestion and lower rates of transmission, BBR, like
TCP Vegas, is model-based. The algorithm uses the maximum bandwidth and round-trip time at which the network delivered the most recent flight of outbound data packets to build a model of the network. Each cumulative or selective acknowledgment of packet delivery produces a rate sample which records the amount of data delivered over the time interval between the transmission of a data packet and the acknowledgment of that packet. As network interface controllers evolve from megabit per second to gigabit per second performance, the latency associated with
bufferbloat
Bufferbloat is a cause of high latency and jitter in packet-switched networks caused by excess buffering of packets. Bufferbloat can also cause packet delay variation (also known as jitter), as well as reduce the overall network throughput. ...
instead of packet loss becomes a more reliable marker of the maximum throughput, making model-based CCAs which provide higher throughput and lower latency, such as BBR, a more reliable alternative to more popular loss-based algorithms like
TCP CUBIC.
BBR has been available for Linux TCP since Linux 4.9. It is also available for
QUIC
QUIC (pronounced "quick") is a general-purpose transport layer network protocol initially designed by Jim Roskind at Google, implemented, and deployed in 2012, announced publicly in 2013 as experimentation broadened, and described at an IETF meet ...
.
BBR version 1 (BBRv1) is efficient and fast, but its fairness to non-BBR streams is disputed. When implemented at
YouTube
YouTube is a global online video sharing and social media platform headquartered in San Bruno, California. It was launched on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim. It is owned by Google, and is the second mo ...
, BBRv1 yielded an average of 4% higher network throughput and up to 14% in some countries. While Google's presentation shows BBRv1 co-existing well with CUBIC,
[ researchers like Geoff Huston and Hock, Bless and Zitterbart finds it unfair to other streams and not scalable. Hock et al. also found "some severe inherent issues such as increased queuing delays, unfairness, and massive packet loss" in the BBR implementation of Linux 4.9.
Soheil Abbasloo et al. (authors of C2TCP) show that BBRv1 doesn't perform well in dynamic environments such as cellular networks.] They have also shown that BBR has an unfairness issue. For instance, when a CUBIC flow (which is the default TCP implementation in Linux, Android, and MacOS) coexists with a BBR flow in the network, the BBR flow can dominate the CUBIC flow and get the whole link bandwidth from it (see figure 16 in ).
Version 2 attempts to deal with the issue of unfairness when operating alongside loss-based congestion management such as CUBIC. In BBRv2 the model used by BBRv1 is augmented to include information about packet loss and information from Explicit Congestion Notification
Explicit Congestion Notification (ECN) is an extension to the Internet Protocol and to the Transmission Control Protocol and is defined in RFC 3168 (2001). ECN allows end-to-end notification of network congestion without dropping packets. ECN is ...
(ECN). Whilst BBRv2 may at times have lower throughput than BBRv1 it is generally considered to have better goodput
In computer networks, goodput (a portmanteau of good and throughput) is the application-level throughput of a communication; i.e. the number of useful information bits delivered by the network to a certain destination per unit of time. The amou ...
.
C2TCP
Cellular Controlled Delay TCP (C2TCP) was motivated by the lack of a flexible end-to-end TCP approach that can satisfy various QoS requirements for different applications without requiring any changes in the network devices. C2TCP aims to satisfy ultra-low latency and high-bandwidth requirements of applications such as virtual reality
Virtual reality (VR) is a simulated experience that employs pose tracking and 3D near-eye displays to give the user an immersive feel of a virtual world. Applications of virtual reality include entertainment (particularly video games), e ...
, video conferencing
Videotelephony, also known as videoconferencing and video teleconferencing, is the two-way or multipoint reception and transmission of audio signal, audio and video signals by people in different locations for Real-time, real time communication. ...
, online gaming, vehicular communication systems
Vehicular communication systems are computer networks in which vehicles and roadside units are the communicating nodes, providing each other with information, such as safety warnings and traffic information. They can be effective in avoiding accid ...
, etc. in a highly dynamic environment such as current LTE and future 5G cellular networks. C2TCP works as an add-on on top of loss-based TCP (e.g. Reno, NewReno, CUBIC, BIC, ...), it is only required to be installed on the server-side and makes the average delay of packets bounded to the desired delays set by the applications.
Researchers at NYU
New York University (NYU) is a private research university in New York City. Chartered in 1831 by the New York State Legislature, NYU was founded by a group of New Yorkers led by then-Secretary of the Treasury Albert Gallatin.
In 1832, the ...
showed that C2TCP outperforms the delay and delay-variation performance of various state-of-the-art TCP schemes. For instance, they showed that compared to BBR, CUBIC, and Westwood on average, C2TCP decreases the average delay of packets by about 250%, 900%, and 700% respectively on various cellular network environments.
Elastic-TCP
Elastic-TCP was proposed in February 2019 to increase the bandwidth utilization over high-BDP networks in support of cloud computing. It is a Linux-based CCA that is designed for the Linux kernel. It is a receiver-side algorithm that employs a loss-delay-based approach using a novel mechanism called a window-correlated weighting function (WWF). It has a high level of elasticity to deal with different network characteristics without the need for human tuning. It has been evaluated by comparing its performance to Compound TCP (the default CCA in MS Windows), CUBIC (the default for Linux) and TCP-BBR (the default of Linux 4.9 used by Google) using the NS-2 simulator and testbed. Elastic-TCP significantly improves the total performance in terms of average throughput, loss ratio, and delay.
NATCP
Soheil Abbasloo et al. proposed NATCP (Network-Assisted TCP) a TCP design targeting multi-access edge computing
Multi-access edge computing (MEC), formerly mobile edge computing, is an ETSI-defined network architecture concept that enables cloud computing capabilities and an IT service environment at the edge of the cellular network and, more in general at t ...
(MEC). The key idea of NATCP is that if the characteristics of the network were known beforehand, TCP would have been designed differently. Therefore, NATCP employs the available features and properties in the current MEC-based cellular architectures to push the performance of TCP close to the optimal performance. NATCP uses an out-of-band feedback from the network to the servers located nearby. The feedback from the network, which includes the capacity of the cellular access link and the minimum RTT of the network, guides the servers to adjust their sending rates. As preliminary results show, NATCP outperforms the state-of-the-art TCP schemes.
Other TCP congestion avoidance algorithms
* FAST TCP
FAST TCP (also written FastTCP) is a TCP congestion avoidance algorithm especially targeted at long-distance, high latency links, developed at the Netlab, California Institute of Technology and now being commercialized by FastSoft. FastSoft was a ...
* Generalized FAST TCP
* H-TCP
H-TCP is another implementation of TCP with an optimized congestion control algorithm for high speed networks with high latency (LFN: Long Fat Networks). It was created by researchers at the Hamilton Institute in Ireland.
H-TCP is an option ...
* Data Center TCP
* High Speed TCP
* HSTCP-LP
* TCP-Illinois
* TCP-LP
* TCP SACK
* Scalable TCP
* TCP Veno
* Westwood
* XCP
* YeAH-TCP
* TCP-FIT
* Congestion Avoidance with Normalized Interval of Time (CANIT)
* Non-linear neural network congestion control based on genetic algorithm for TCP/IP networks
*D-TCP
*NexGen D-TCP
* Copa
TCP New Reno
Transmission Control Protocol (TCP) uses a network congestion-avoidance algorithm that includes various aspects of an additive increase/multiplicative decrease (AIMD) scheme, along with other schemes including slow start and congestion windo ...
was the most commonly implemented algorithm, SACK support is very common and is an extension to Reno/New Reno. Most others are competing proposals that still need evaluation. Starting with 2.6.8 the Linux kernel switched the default implementation from New Reno to BIC. The default implementation was again changed to CUBIC in the 2.6.19 version. FreeBSD uses New Reno as the default algorithm. However, it supports a number of other choices.
When the per-flow product of bandwidth and latency increases, regardless of the queuing scheme, TCP becomes inefficient and prone to instability. This becomes increasingly important as the Internet evolves to incorporate very high-bandwidth optical links.
TCP Interactive (iTCP) allows applications to subscribe to TCP events and respond accordingly enabling various functional extensions to TCP from outside TCP layer. Most TCP congestion schemes work internally. iTCP additionally enables advanced applications to directly participate in congestion control such as to control the source generation rate.
Zeta-TCP detects congestion from both latency and loss rate measures. To maximize the goodput
In computer networks, goodput (a portmanteau of good and throughput) is the application-level throughput of a communication; i.e. the number of useful information bits delivered by the network to a certain destination per unit of time. The amou ...
Zeta-TCP and applies different congestion window backoff strategies based on the likelihood of congestion. It also has other improvements to accurately detect packet losses, avoiding retransmission timeout retransmission; and accelerate and control the inbound (download) traffic.
Classification by network awareness
CCAs may be classified in relation to network awareness, meaning the extent to which these algorithms are aware of the state of the network. This consist of three primary categories: black box, grey box, and green box.
Black box algorithms offer blind methods of congestion control. They operate only on the binary feedback received upon congestion and do not assume any knowledge concerning the state of the networks which they manage.
Grey box algorithms use in order to obtain measurements and estimations of bandwidth, flow contention, and other knowledge of network conditions.
Green box algorithms offer bimodal methods of congestion control which measures the fair-share of total bandwidth which should be allocated for each flow, at any point, during the system's execution.
Black box
* Highspeed-TCP
* BIC TCP
BIC TCP (Binary Increase Congestion control) is one of the congestion control algorithms that can be used for Transmission Control Protocol (TCP). BIC is optimized for high speed networks with high latency: so-called "long fat networks". For t ...
(Binary Increase Congestion Control Protocol) uses a concave increase of the sources rate after each congestion event until the window is equal to that before the event, in order to maximise the time that the network is fully utilised. After that, it probes aggressively.
* CUBIC TCP
CUBIC is a network congestion avoidance algorithm for TCP which can achieve high bandwidth connections over networks more quickly and reliably in the face of high latency than earlier algorithms. It helps optimize long fat networks.
In 2006, ...
– a less aggressive and more systematic derivative of BIC, in which the window is a cubic function of time since the last congestion event, with the inflection point set to the window prior to the event.
* AIMD-FC (additive increase multiplicative decrease with fast convergence), an improvement of AIMD.
* Binomial Mechanisms
* SIMD Protocol
* GAIMD
Grey box
* TCP Vegas – estimates the queuing delay, and linearly increases or decreases the window so that a constant number of packets per flow are queued in the network. Vegas implements proportional fairness.
* FAST TCP
FAST TCP (also written FastTCP) is a TCP congestion avoidance algorithm especially targeted at long-distance, high latency links, developed at the Netlab, California Institute of Technology and now being commercialized by FastSoft. FastSoft was a ...
– achieves the same equilibrium as Vegas, but uses proportional control
Proportional control, in engineering and process control, is a type of linear feedback control system in which a correction is applied to the controlled variable, and the size of the correction is proportional to the difference between the desi ...
instead of linear increase, and intentionally scales the gain down as the bandwidth increases with the aim of ensuring stability.
* TCP BBR – estimates the queuing delay but uses exponential increase. Intentionally slows down periodically for fairness and decreased delay.
* TCP-Westwood (TCPW) – a loss causes the window to be reset to the sender's estimate of the bandwidth-delay product (the smallest measured RTT multiplied by the observed rate of receiving ACKs).
*C2TCP
* TFRC
Transferrin receptor protein 1 (TfR1), also known as Cluster of Differentiation 71 (CD71), is a protein that in humans is encoded by the ''TFRC'' gene. TfR1 is required for iron import from transferrin into cells by endocytosis.
Structure and f ...
* TCP-Real
* TCP-Jersey
Green box
* Bimodal Mechanism – Bimodal Congestion Avoidance and Control
In statistics, a multimodal distribution is a probability distribution with more than one mode. These appear as distinct peaks (local maxima) in the probability density function, as shown in Figures 1 and 2. Categorical, continuous, and d ...
mechanism.
* Signalling methods implemented by routers
** Random Early Detection
Random early detection (RED), also known as random early discard or random early drop is a queuing discipline for a network scheduler suited for congestion avoidance.
In the conventional tail drop algorithm, a router or other network component ...
(RED) randomly drops packets in proportion to the router's queue size, triggering multiplicative decrease in some flows.
** Explicit Congestion Notification
Explicit Congestion Notification (ECN) is an extension to the Internet Protocol and to the Transmission Control Protocol and is defined in RFC 3168 (2001). ECN allows end-to-end notification of network congestion without dropping packets. ECN is ...
(ECN)
* Network-Assisted Congestion Control
** NATCP - Network-Assisted TCP uses out-of-band explicit feedback indicating minimum RTT of the network and capacity of the cellular access link.
**The variable-structure congestion control protocol (VCP) uses two ECN bits to explicitly feedback the network state of congestion. It includes an end host side algorithm as well.
The following algorithms require custom fields to be added to the TCP packet structure:
* Explicit Control Protocol (XCP) – XCP packets carry a congestion header with a feedback field, indicating the increase or decrease of the sender's congestion window. XCP routers set the feedback value explicitly for efficiency and fairness.
* MaxNet – Uses a single header field, which carries the maximum congestion level of any router on a flow's path. The rate is set as a function of this maximum congestion, resulting in max-min fairness
In communication networks, multiplexing and the division of scarce resources, max-min fairness is said to be achieved by an allocation if and only if the allocation is feasible and an attempt to increase the allocation of any participant necessari ...
.
* JetMax, like MaxNet, responds only to the maximum congestion signal, but also carries other overhead fields.
Linux usage
* BIC is used by default in Linux kernels 2.6.8 through 2.6.18. (August 2004 – September 2006)
* CUBIC is used by default in Linux kernels since version 2.6.19. (November 2006)
* PRR is incorporated in Linux kernels to improve loss recovery since version 3.2. (January 2012)
* BBR is incorporated in Linux kernels to enable model-based congestion control since version 4.9. (December 2016)
See also
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*
* Low Extra Delay Background Transport
Low Extra Delay Background Transport (LEDBAT) is a way to transfer data on the Internet quickly without clogging the network. LEDBAT was invented by Stanislav Shalunov and is used by Apple for software updates, by BitTorrent for most of its transf ...
(LEDBAT)
Notes
References
Sources
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External links
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Papers in Congestion Control
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{snd The TCP/IP Guide
Flow control (data)
Network performance