FAST TCP
Encyclopedia
FAST TCP 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. It is compatible with existing TCP algorithms, requiring modification only to the computer
which is sending data
.
for FAST AQM Scalable TCP, where AQM stands for Active Queue Management, and TCP stands for Transmission Control Protocol.
and the rate at which other users are transmitting. Like TCP Vegas
, FAST TCP
uses queueing delay instead of loss probability
as a congestion signal.
Most current congestion control algorithms detect congestion and slow down when they discover that packets are being dropped, so that the average sending rate depends on the loss probability. This has two drawbacks. First, low loss probabilities are required to sustain high data rates; in the case of TCP Reno, very low loss probabilities are required, but even new congestion avoidance algorithms such as H-TCP
, BIC TCP
and HSTCP
require loss rates lower than those provided by most wireless wide area network
s. Moreover, packet loss only provides a single bit of information about the congestion level, whereas delay is a continuous quantity and in principle provides more information about the network.
A FAST TCP flow seeks to maintain a constant number of packets in queues throughout the network. The number of packets in queues is estimated by measuring the difference between the observed round trip time
(RTT) and the base RTT, defined as the round trip time when there is no queueing. The base RTT is estimated as the minimum observed RTT for the connection. If too few packets are queued, the sending rate is increased, while if too many are queued, the rate is decreased. In this respect, it is a direct descendant of TCP Vegas.
The difference between TCP Vegas and FAST TCP lies in the way in which the rate is adjusted when the number of packets stored is too small or large. TCP Vegas makes fixed size adjustments to the rate, independent of how far the current rate is from the target rate. FAST TCP makes larger steps when the system is further from equilibrium and smaller steps near equilibrium. This improves the speed of convergence and the stability.
Delay measurements are also subject to jitter as a result of operating system
scheduling, or bus
contention.
Whether the strengths or weaknesses prevail is not clear, and depends in large part on the particular scenario.
TCP congestion avoidance algorithm
Transmission Control Protocol uses a network congestion avoidance algorithm that includes various aspects of an additive increase/multiplicative decrease scheme, with other schemes such as slow-start in order to achieve congestion avoidance....
especially targeted at long-distance, high latency links, developed at the Netlab, California Institute of Technology
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...
and now being commercialized by FastSoft. It is compatible with existing TCP algorithms, requiring modification only to the computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
which is sending data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
.
Name
The name FAST is a recursive acronymRecursive acronym
A recursive acronym is an acronym or initialism that refers to itself in the expression for which it stands...
for FAST AQM Scalable TCP, where AQM stands for Active Queue Management, and TCP stands for Transmission Control Protocol.
Principles of operation
The role of congestion control is to moderate the rate at which data is transmitted, according to the capacity of the networkTelecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
and the rate at which other users are transmitting. Like TCP Vegas
TCP Vegas
TCP Vegas is a TCP congestion avoidance algorithm that emphasizes packet delay, rather than packet loss, as a signal to help determine the rate at which to send packets. It was developed at the University of Arizona by Lawrence Brakmo and Larry L...
, FAST TCP
uses queueing delay instead of loss probability
Packet loss
Packet loss occurs when one or more packets of data travelling across a computer network fail to reach their destination. Packet loss is distinguished as one of the three main error types encountered in digital communications; the other two being bit error and spurious packets caused due to noise.-...
as a congestion signal.
Most current congestion control algorithms detect congestion and slow down when they discover that packets are being dropped, so that the average sending rate depends on the loss probability. This has two drawbacks. First, low loss probabilities are required to sustain high data rates; in the case of TCP Reno, very low loss probabilities are required, but even new congestion avoidance algorithms such as H-TCP
H-TCP
H-TCP is another implementation of TCP with an optimized congestion control algorithm for high speed networks with high latency...
, BIC TCP
BIC TCP
BIC TCP is an implementation of TCP with an optimized congestion control algorithm for high speed networks with high latency: so-called "long fat networks".BIC has a unique congestion window algorithm...
and HSTCP
HSTCP
HighSpeed TCP is a new congestion control algorithm protocol defined in RFC 3649 for TCP. Standard TCP performs poorly in networks with a large bandwidth delay product. It is unable to fully utilize available bandwidth...
require loss rates lower than those provided by most wireless wide area network
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
s. Moreover, packet loss only provides a single bit of information about the congestion level, whereas delay is a continuous quantity and in principle provides more information about the network.
A FAST TCP flow seeks to maintain a constant number of packets in queues throughout the network. The number of packets in queues is estimated by measuring the difference between the observed round trip time
Round-trip delay time
In telecommunications, the round-trip delay time or round-trip time is the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgment of that signal to be received...
(RTT) and the base RTT, defined as the round trip time when there is no queueing. The base RTT is estimated as the minimum observed RTT for the connection. If too few packets are queued, the sending rate is increased, while if too many are queued, the rate is decreased. In this respect, it is a direct descendant of TCP Vegas.
The difference between TCP Vegas and FAST TCP lies in the way in which the rate is adjusted when the number of packets stored is too small or large. TCP Vegas makes fixed size adjustments to the rate, independent of how far the current rate is from the target rate. FAST TCP makes larger steps when the system is further from equilibrium and smaller steps near equilibrium. This improves the speed of convergence and the stability.
Strengths and weaknesses
Delay-based algorithms can, in principle, maintain a constant window size, avoiding the oscillations inherent in loss-based algorithms. However, they also detect congestion earlier than loss-based algorithms, since delay corresponds to partially filled buffers, while loss results from totally filled buffers. This can be either a strength or a weakness. If the only protocol used in a network is delay-based, then the inefficiency of loss can be avoided; however, if loss-based and delay-based protocols share the network, then delay-based algorithms tend to be less aggressive. This can be overcome by suitable choice of parameters, leading to complex interactions studied by Tang et al.Delay measurements are also subject to jitter as a result of operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
scheduling, or bus
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
contention.
Whether the strengths or weaknesses prevail is not clear, and depends in large part on the particular scenario.
Intellectual property
Unlike most TCP congestion avoidance algorithms, FAST TCP is protected by several patents. Instead of seeking standardization by the IETF, the inventors of FAST, notably Steven Low and Cheng Jin, are seeking to commercialize it through the company FastSoft. Currently FastSoft sells a 1-Unit rack appliance which can be deployed at the sender-side with no other software or hardware modifications needed on either end.See also
- TCP congestion avoidance algorithmTCP congestion avoidance algorithmTransmission Control Protocol uses a network congestion avoidance algorithm that includes various aspects of an additive increase/multiplicative decrease scheme, with other schemes such as slow-start in order to achieve congestion avoidance....
- Transmission Control Protocol#Development