Congestion window
Encyclopedia
In Transmission Control Protocol (TCP)
, 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 from getting overloaded with too much traffic. The size of this window is calculated by estimating how much congestion there is between the two places. The sender maintains the congestion window.
When a connection is set up, the congestion window is set to the maximum segment size
(MSS) allowed on that connection. Further variance in the collision window is dictated by an Additive Increase/Multiplicative Decrease
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. The window keeps growing exponentially until a timeout occurs or the receiver reaches its limit (a threshold value "ssthresh"). After this the congestion window increases linearly at the rate of 1/(congestion window)packets on each new acknowledgement received.
On timeout:
A system administrator
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 receiver advertised TCP Receive Window. By comparing its own congestion window with the receive window of the receiver, a sender can determine how much data it may send at any given time.
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 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 from getting overloaded with too much traffic. The size of this window is calculated by estimating how much congestion there is between the two places. The sender maintains the congestion window.
When a connection is set up, the congestion window is set to the maximum segment size
Maximum segment size
The maximum segment size is a parameter of the TCP protocol that specifies the largest amount of data, specified in octets, that a computer or communications device can receive in a single TCP segment, and therefore in a single IP datagram. It does not count the TCP header or the IP header...
(MSS) allowed on that connection. Further variance in the collision window is dictated by an Additive Increase/Multiplicative Decrease
AIMD
The additive increase/multiplicative-decrease algorithm is a feedback control algorithm best known for its use in TCP Congestion Avoidance. AIMD combines linear growth of the congestion window with an exponential reduction when a congestion takes place. Multiple flows using AIMD congestion control...
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. The window keeps growing exponentially until a timeout occurs or the receiver reaches its limit (a threshold value "ssthresh"). After this the congestion window increases linearly at the rate of 1/(congestion window)packets on each new acknowledgement received.
On timeout:
- Congestion window is reset to 1 MSS
- "ssthresh" is set to half the window size before packet loss started
- "slow start"Slow-startSlow-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...
is initiated.
A system administrator
System administrator
A system administrator, IT systems administrator, systems administrator, or sysadmin is a person employed to maintain and operate a computer system and/or network...
may adjust the maximum window size limit, or adjust the constant added during additive increase, as part of TCP tuning
TCP tuning
TCP tuning techniques adjust the network congestion avoidance parameters of TCP connections over high-bandwidth, high-latency networks. Well-tuned networks can perform up to 10 times faster in some cases.- Bandwidth-delay product :...
.
The flow of data over a TCP connection is also controlled by the use of the receiver advertised TCP Receive Window. By comparing its own congestion window with the receive window of the receiver, a sender can determine how much data it may send at any given time.
See also
- TCP congestion control
- 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....
s