FAST TCP (also written FastTCP) is a
TCP congestion avoidance algorithm
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, slow start and #Conge ...
especially targeted at long-distance, high latency links, developed at the Netlab,
California Institute of Technology
The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...
and now being commercialized by FastSoft. FastSoft was acquired by
Akamai Technologies
Akamai Technologies, Inc. is an American content delivery networkJ. Dilley, B. Maggs, J. Parikh, H. Prokop, R. Sitaraman, and B. Weihl. (CDN), cybersecurity, and cloud service company, providing web and Internet security services. Akamai's Inte ...
in 2012.
FastTCP is compatible with existing TCP algorithms, requiring modification only to the
computer which is sending
data
In the pursuit of knowledge, data (; ) is a collection of discrete Value_(semiotics), values that convey information, describing quantity, qualitative property, quality, fact, statistics, other basic units of meaning, or simply sequences of sy ...
.
Name
The name FAST is a
recursive acronym
A recursive acronym is an acronym that refers to itself, and appears most frequently in computer programming. The term was first used in print in 1979 in Douglas Hofstadter's book '' Gödel, Escher, Bach: An Eternal Golden Braid'', in which Hofs ...
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, "congestion", according to the capacity of the
network
Network, networking and networked may refer to:
Science and technology
* Network theory, the study of graphs as a representation of relations between discrete objects
* Network science, an academic field that studies complex networks
Mathematics ...
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
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 ...
,
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 ...
and
HSTCP
HighSpeed TCP (HSTCP) is a congestion control algorithm protocol defined in RFC 3649 for Transport Control Protocol (TCP). Standard TCP performs poorly in networks with a large bandwidth-delay product. It is unable to fully utilize available b ...
require loss rates lower than those provided by most wireless
wide area network
A wide area network (WAN) is a telecommunications network that extends over a large geographic area. Wide area networks are often established with leased telecommunication circuits.
Businesses, as well as schools and government entities, u ...
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
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 ...
(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
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 ...
scheduling, or
bus
A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
contention.
Whether the strengths or weaknesses prevail is not clear, and depends in large part on the particular scenario.
Propagation delay is used in the FAST window control algorithm. In a clean network, the queueing delay maintained by existing FAST flows may be mistaken as part of the propagation delay by new flows that join later, as shown in ns-2 simulations in.
[L. Tan, C. Yuan, and M. Zukerman, “FAST TCP: fairness and queuing issues,” IEEE Commun. Lett., vol. 9, no. 8, pp. 762–764, Aug. 2005.] The effect of this estimation error is equivalent to modifying the underlying utility functions to favor new flows over existing flows. Method to eliminate this error is suggested in.
Generalized FAST TCP
FAST TCP has been shown to be promising in terms of system stability, throughput and fairness. However,
it requires buffering which increases linearly with the number of flows bottlenecked at a link. The paper
proposes a new TCP algorithm that extends FAST TCP to achieve (''α'', ''n'')-proportional fairness in steady
state, yielding buffer requirements which grow only as the nth power of the number of flows. The authors call
the new algorithm Generalized FAST TCP. They prove stability for the case of a single bottleneck link with
homogeneous sources in the absence of feedback delay. Simulation results verify that the new scheme is
stable in the presence of feedback delay, and that its buffering requirements can be made to scale significantly
better than standard FAST TCP.
Intellectual property
Unlike most TCP congestion avoidance algorithms, FAST TCP is protected by several patents.
Instead of seeking standardization by the
IETF
The Internet Engineering Task Force (IETF) is a standards organization for the Internet and is responsible for the technical standards that make up the Internet protocol suite (TCP/IP). It has no formal membership roster or requirements and a ...
, the inventors of FAST, notably
Steven H. 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 algorithm
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, slow start and #Conge ...
*
Transmission Control Protocol, Development
References
External links
FASTHome Page.
FastSoft home page
{{DEFAULTSORT:Fast Tcp
Internet Standards
TCP congestion control