Cambridge A-Level Computer Science (9618) · lesson 7 of 15 · 30 min
The TCP/IP Model
TCP vs UDP
TCP Three-Way Handshake
Before TCP data transfer, a connection is established:
- SYN: Client sends SYN (synchronise) packet with initial sequence number.
- SYN-ACK: Server responds with SYN-ACK, acknowledging client’s SYN and sending its own sequence number.
- ACK: Client acknowledges server’s SYN. Connection established.
Port Numbers
Port numbers identify specific services on a host. Well-known ports (0–1023) are assigned by IANA:
Which protocol provides reliable, ordered delivery of data?
- UDP
- IP
- TCP
- DNS
Answer: TCP
TCP provides guaranteed, ordered delivery with error checking and retransmission.
Which port does HTTPS use?
- 80
- 21
- 443
- 25
Answer: 443
HTTPS uses port 443.
UDP is preferred over TCP for video streaming because:
- UDP is more reliable
- UDP has lower overhead and latency; occasional packet loss is acceptable
- UDP supports more connections
- UDP encrypts data automatically
- Answer: 1
For real-time video, low latency matters more than perfect reliability — a dropped frame is better than a stall.
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