4.1.6 - Network and email protocols
One web page or email can use several protocols, with each set of rules doing a different job. In specification point 4.1.6, you will learn how Ethernet, Wi-Fi, TCP/IP, HTTP, HTTPS, FTP, POP3, SMTP and IMAP cooperate, then choose and justify the right protocol for a situation.
Shared rules and local links
Imagine two devices trying to exchange data without agreeing what a message looks like, when it begins, or how to respond. Even if the physical connection works, each device could interpret the same bits differently. Shared rules make communication predictable and allow devices and software made by different organisations to work together.
Communication protocol
An agreed set of rules that governs how data is transmitted between devices.
A protocol can specify matters such as the format and order of messages and what a device should do when it receives one. A protocol is not a cable, an app or a network device: those things use or implement protocols.
Two protocol families govern how data travels over a local network connection:
| Protocol family | Connection | Role |
|---|---|---|
| Ethernet | Wired, commonly through copper cable or fibre | Provides rules for transmitting data over a wired local-network link. |
| Wi-Fi | Wireless, using radio signals | Provides rules for transmitting data over a wireless local-network link. |
Ethernet does not mean the internet, and Wi-Fi does not mean a web page. They handle the local part of a transfer. For example, a laptop may use Wi-Fi to reach the school network while the web request it sends uses HTTP or HTTPS. A desktop connected by cable may make the same request using Ethernet for its local connection.
Worked identification
- A desktop is plugged into the classroom network: Ethernet carries data over the local wired link.
- A tablet joins the classroom network without a cable: Wi-Fi carries data over the local wireless link.
- Both can communicate with other devices because they follow agreed protocol rules.
TCP/IP: reliable internet delivery
TCP/IP is the name used for the suite of protocols that supports communication across interconnected networks. TCP and IP cooperate, but they do not perform the same job.
TCP and IP
Transmission Control Protocol (TCP) divides an application's stream of data into numbered segments and provides reliable, in-order delivery. Internet Protocol (IP) places those segments into addressed packets and carries the packets towards the destination across interconnected networks.
A simplified transfer works like this:
- The sending application supplies data.
- TCP splits the data into segments. Sequence numbers show their correct order.
- IP provides source and destination addressing so each packet can be carried towards the correct device.
- At the destination, TCP detects missing or damaged data, causes it to be retransmitted where needed, restores the correct order and rebuilds the data stream for the receiving application.
At GCSE level, all the transmitted units are often called packets. More precisely, TCP creates a segment and IP carries that segment inside an IP packet. The important distinction is the job: TCP provides reliability and order; IP provides addressed delivery.
Worked transfer
TCP divides a small document into segments 1, 2, 3 and 4. The receiver gets 1, 3 and 2, but not 4.
- The IP information has helped the packets reach the addressed destination.
- TCP uses the sequence numbers to place
2before3. - TCP detects that
4is missing and retransmission supplies it. - Only then does TCP present the complete ordered data to the application.
Do not write that IP guarantees delivery. TCP supplies the reliable, ordered service; IP carries addressed packets towards their destination.
Web and file protocols
Once TCP/IP can carry data between devices, another protocol defines the particular service the application needs. Browsing the web and transferring files use different rules.
| Protocol | Full name | Main role | Key distinction |
|---|---|---|---|
| HTTP | Hypertext Transfer Protocol | A client sends requests and a web server returns web resources or other responses. | The HTTP communication is not protected by the extra TLS security used by HTTPS. |
| HTTPS | Hypertext Transfer Protocol Secure | Performs HTTP requests and responses over a connection secured using TLS. | The communication is encrypted for confidentiality and protected against undetected alteration; the server's identity is authenticated. |
| FTP | File Transfer Protocol | Uploads files to or downloads files from a remote computer or server. | Its purpose is file transfer rather than requesting and returning web resources. |
HTTPS protects the communication in transit. It does not prove that every claim on the page is true or that every file offered by the site is harmless. This is why the address uses HTTPS, so all its content must be safe is not a valid conclusion.
Worked protocol selection
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