Starting Carefully
When a TCP connection opens, the sender does not know how much the path can carry. Slow start makes it begin with a small amount of unacknowledged data and grow as success is confirmed.
How It Grows
The sender tracks a congestion window, the number of bytes it may have in flight without an acknowledgment.
- It begins with a small initial window of a few segments.
- For each acknowledgment, the window grows, roughly doubling every round trip.
- Growth continues until a loss appears or a threshold is reached.
This exponential climb means early transfers are limited by round trips, not by bandwidth.
Why It Matters
A short transfer such as a small web asset may finish before slow start ever reaches full speed. That is why latency and the initial window strongly shape how quickly small responses arrive.
Key idea
Slow start opens a connection with a small congestion window that doubles each round trip, so short transfers are often bounded by round trips rather than by the available bandwidth.