Networking
Why?
Why TCP Starts Slow on Purpose
Why does TCP send only a little data when a connection starts?
Quick check · tap an answer to continue
Lesson notes
Key points
- Because the sender cannot see the network, so it does not know how much spare capacity the path between hosts has.
- Because sending too much at once overflows router buffers, causing drops and retransmissions that worsen congestion for everyone.
- Because of that, TCP begins with a small window and roughly doubles it each round trip while acknowledgements keep arriving.
Definitions
- Congestion window
- Sender-side limit on how many unacknowledged bytes TCP may have in flight at once.
Takeaway: So TCP probes capacity safely, growing fast until loss signals a limit.
Quick check: During TCP slow start, how does the congestion window change each round trip?
- It roughly doubles
- It stays fixed
- It halves
Each acknowledged segment grows the window, so it doubles per round trip.
Unknown capacityCongestion collapseExponential growth
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