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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.


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.

Slow-start begins initially with a congestion window Size (cwnd) of 1, 2 or 10.[1] The value of the Congestion Window will be increased with each acknowledgement (ACK) received, effectively doubling the window size each round trip time ("although it is not exactly exponential because the receiver may delay its ACKs, typically sending one ACK for every two segments that it receives"[2]). The transmission rate will be increased with slow-start algorithm until either a loss is detected, or the receiver's advertised window (rwnd) is the limiting factor, or the slow start threshold (ssthresh) 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. These measurements depend on the used TCP congestion avoidance algorithm. Once ssthresh is reached, TCP changes from slow-start algorithm to the linear growth (congestion avoidance) algorithm. At this point, the window is increased by 1 segment for each RTT.

Although the strategy is referred to as "Slow-Start", its congestion window growth is quite aggressive, more aggressive than the congestion avoidance phase.[3] Before slow-start was introduced in TCP, the initial pre-congestion avoidance phase was even faster.

Behaviour upon packet loss[edit]

The behavior upon packet loss depends on the TCP congestion avoidance algorithm that is used.

TCP Tahoe
In TCP Tahoe, when a loss occurs, fast retransmit is sent, 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 changes to congestion avoidance algorithm where each new ACK increases the CWND by SS + SS / CWND. This results in a linear increase of the CWND.
TCP Reno
TCP Reno implements an algorithm called Fast recovery. A fast retransmit is sent, half of the current CWND is saved as Slow Start Threshold (SSThresh) and as new CWND, thus skipping slow-start and going directly to Congestion Avoidance algorithm


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,[4] and counter scripts of web analytics.

See also[edit]


  1. ^ Corbet, Jonathan. "Increasing the TCP initial congestion window". LWN. Retrieved 10 October 2012. 
  2. ^ "TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms". January 1997. 
  3. ^ Jacobson, Van; Karels, MJ (1988). "Congestion avoidance and control" (PDF). ACM SIGCOMM Computer Communication Review 18 (4): 314–329. doi:10.1145/52325.52356. 
  4. ^ Nick O'Neill. "What's Making Your Site Go Slow? Could Be The Like Button". AllFacebook, November 10, 2010. Retrieved on September 12, 2012.

External links[edit]