---
title: "TCP slow start - Glossary | MDN"
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source: "https://developer.mozilla.org/en-US/docs/Glossary/TCP_slow_start"
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date_saved: "2026-05-05T05:46:42.707428+00:00"
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  1. [Glossary](https://developer.mozilla.org/en-US/docs/Glossary)
  2. [TCP slow start](https://developer.mozilla.org/en-US/docs/Glossary/TCP_slow_start)

# TCP slow start
[TCP](https://developer.mozilla.org/en-US/docs/Glossary/TCP) slow start is an algorithm that helps discover the available network bandwidth for packet transfer, and balance transmission speed according to the network's capabilities. The mechanism rapidly increases the volume of information sent from a very low level to a threshold level. If congestion is detected the threshold is reset to a much reduced level and the slow start process starts again. This prevents congestion when a network's capabilities are unknown or exceeded — at the beginning of a connection, after a period of inactivity, or after detecting congestion — without itself causing congestion.
Network congestion occurs when more data is sent than the network is able to transfer between nodes at any given time. When a network is congested, machines on the network experience degraded quality of service, including slow data transfers and dropped packets.
The slow start algorithm is a major part of TCP congestion control, as it ensures that new connections progressively increase data rates and that those rates don't overwhelm other nodes.
## In this article
  * [See also](https://developer.mozilla.org/en-US/docs/Glossary/TCP_slow_start#see_also)

##  [See also](https://developer.mozilla.org/en-US/docs/Glossary/TCP_slow_start#see_also)
  * [Populating the page: how browsers work](https://developer.mozilla.org/en-US/docs/Web/Performance/Guides/How_browsers_work)
  * [Overview of HTTP](https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/Overview)