HTTP 1.1 introduced the use of persistent connections for downloading web objects. Explain why this improves performance over a connection using TCP congestion control (i.e. AIMD as used in TCP Reno and Tahoe)
Try an answer before revealing the guidance below.
Key Concepts
- Persistent HTTP
- TCP handshake
- Slow start
- Congestion window
- AIMD
Answer Approach
- Compare one TCP connection per object with one reused connection.
- Count the setup and teardown cost of each new connection.
- Ask what state the congestion window is in on a brand-new connection.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
With non-persistent HTTP every object needs a new TCP connection: an extra RTT for the handshake and a fresh congestion window starting in slow start at about 1 MSS. A persistent connection avoids repeated handshakes and keeps the congestion window that has already grown, so later objects are sent at a higher rate instead of restarting slow start for each one. This reduces total page load time.
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