Slow-start
Encyclopedia
Slow-start is part of the congestion control strategy used by TCP
, the data transmission protocol
used by many Internet
applications. Slow-start is used in conjunction with other algorithms to avoid sending more data than the network is capable of transmitting, that is, to avoid causing network congestion
.
phase.
During the exponential growth phase, slow-start works by increasing the TCP congestion window
each time the acknowledgment is received. It increases the window size by the number of segments acknowledged. This happens until either an acknowledgment is not received for some segment or a predetermined threshold value is reached. 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. Once a loss event has occurred or the threshold has been reached, TCP enters the linear growth (congestion avoidance
) phase. At this point, the window is increased by 1 segment for each RTT
. This happens until a loss event occurs.
Although the strategy is referred to as "slow-start", its congestion window growth is quite aggressive, more aggressive than the congestion avoidance phase (Jacobson
, 1988). Before "slow start" was introduced in TCP, the initial pre-congestion avoidance phase was even faster.
When a loss occurs half of the current cwnd is saved as a Slow Start Threshold (SSThresh) and slow start begins again from its initial cwnd. Once the cwnd reaches the SSThresh TCP goes into congestion avoidance mode where each ACK increases the cwnd by SS*SS/cwnd. This results in a linear increase of the cwnd.
SeeRFC 2001 .
followed by congestion avoidance. In the fast recovery algorithm
, during congestion avoidance mode, when packets (detected through 3 duplicate ACKs) are not received, the congestion window
size is reduced to the slow-start threshold, rather than the smaller initial value.
transmission quality. Thus, slow-start can perform poorly in situations with poor reception, such as wireless networks
.
The slow-start protocol performs badly for short-lived connections. Older web browsers 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.
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
, the data transmission protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
used by many Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
applications. Slow-start is used in conjunction with other algorithms to avoid sending more data than the network is capable of transmitting, that is, to avoid causing network congestion
Network congestion
In data networking and queueing theory, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections...
.
Algorithm
Slow-start is one of the algorithms that TCP uses to control congestion inside the network. It is also known as the exponential growthExponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...
phase.
During the exponential growth phase, slow-start works by increasing the TCP congestion window
Congestion window
In Transmission Control Protocol , the congestion window is one of the factors that determines the number of bytes that can be outstanding at any time. This is not to be confused with the TCP window size which is maintained by the receiver. This is a means of stopping the link between two places...
each time the acknowledgment is received. It increases the window size by the number of segments acknowledged. This happens until either an acknowledgment is not received for some segment or a predetermined threshold value is reached. 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. Once a loss event has occurred or the threshold has been reached, TCP enters the linear growth (congestion avoidance
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....
) phase. At this point, the window is increased by 1 segment for each RTT
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...
. This happens until a loss event occurs.
Although the strategy is referred to as "slow-start", its congestion window growth is quite aggressive, more aggressive than the congestion avoidance phase (Jacobson
Van Jacobson
Van Jacobson is one of the primary contributors to the TCP/IP protocol stack which is the technological foundation of today’s Internet. He is renowned for his pioneering achievements in network performance and scaling....
, 1988). Before "slow start" was introduced in TCP, the initial pre-congestion avoidance phase was even faster.
Basic slow-start
The algorithm begins in the exponential growth phase initially with a congestion window size (cwnd) of 1 or 2 segments and increases it by 1 Segment Size (SS) for each ACK received. Since the receiver typically sends an ACK for every two segments, this behavior effectively doubles the window size each round trip of the network. This behavior continues until the congestion window size (cwnd) reaches the size of the receiver's advertised window or until a loss occurs.When a loss occurs half of the current cwnd is saved as a Slow Start Threshold (SSThresh) and slow start begins again from its initial cwnd. Once the cwnd reaches the SSThresh TCP goes into congestion avoidance mode where each ACK increases the cwnd by SS*SS/cwnd. This results in a linear increase of the cwnd.
See
Fast recovery
There is a variation to the slow-start algorithm known as fast recovery, which uses fast retransmitFast retransmit
Fast Retransmit is an enhancement to TCP which reduces the time a sender waits before retransmitting a lost segment.A TCP sender uses a timer to recognize lost segments...
followed by congestion avoidance. In the fast recovery algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
, during congestion avoidance mode, when packets (detected through 3 duplicate ACKs) are not received, the congestion window
Congestion window
In Transmission Control Protocol , the congestion window is one of the factors that determines the number of bytes that can be outstanding at any time. This is not to be confused with the TCP window size which is maintained by the receiver. This is a means of stopping the link between two places...
size is reduced to the slow-start threshold, rather than the smaller initial value.
Problems
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 layerData link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
transmission quality. Thus, slow-start can perform poorly in situations with poor reception, such as wireless networks
Wireless LAN
A wireless local area network links two or more devices using some wireless distribution method , and usually providing a connection through an access point to the wider internet. This gives users the mobility to move around within a local coverage area and still be connected to the network...
.
The slow-start protocol performs badly for short-lived connections. Older web browsers 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.