Slow-start

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. The algorithm is specified by RFC 5681.

Algorithm

Slow-start is one of the algorithms that TCP uses to control congestion inside the network. It is also known as the exponential growth 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. 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, 2 or 10 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 RFC 2001.

Fast recovery

There is a variation to the slow-start algorithm known as fast recovery, which uses fast retransmit 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.

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 layer 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. However, connections cannot be reused for the multiple third-party servers used by web sites to implement web advertising, sharing features of social networking services, and counter scripts of web analytics.

See also

• Congestive collapse
• TCP congestion avoidance algorithm

External links

• TCP Slow Start Sequence Diagram