Glossary
What is a communication protocol? | ExpressVPN Glossary

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Communication protocol

What is a communication protocol?

A communication protocol is a set of rules that define how data is exchanged between devices or systems. It establishes a common standard so that different technologies can communicate and interpret information consistently and reliably.

How does a communication protocol work?

A communication protocol typically includes:

Data formatting: Defines how information is structured for transmission, so both sides interpret the data consistently.
Addressing and routing: Specifies how senders and receivers are identified and how data is directed across networks to reach the correct destination.
Error handling: Uses mechanisms to detect corruption or loss and ensure reliable delivery.
Flow and timing control: Regulates transmission speed, sequencing, and synchronization to prevent congestion and maintain correct data order.
Security rules: May define encryption, authentication, and integrity protections to prevent unauthorized access, tampering, or interception.

Types of communication protocols

Communication protocols serve different purposes. Common categories include:

Network protocols: Manage routing and transport between devices. For example, the Internet Protocol (IP) handles packet addressing and routing, the Transmission Control Protocol (TCP) provides reliable data transport, and the User Datagram Protocol (UDP) transmits data with minimal overhead.
Web protocols: Enable communication between browsers and servers. Hypertext Transfer Protocol (HTTP) transfers web content, HTTPS adds encryption using Transport Layer Security (TLS), and WebSocket supports persistent, real-time communication.
Name resolution protocols: Translate domain names into IP addresses. The Domain Name System (DNS) performs this function, while DNS over QUIC (DoQ), DNS over HTTPS (DoH), and DNS over TLS (DoT) encrypt DNS requests.
Security protocols: Protect data in transit. TLS encrypts web traffic, Secure Shell (SSH) secures remote access, and Internet Protocol Security (IPSec) encrypts network-layer communications.
Wireless and Internet of Things (IoT) protocols: Support communication between connected devices. Examples include Wi-Fi, Bluetooth, and ZigBee.

Protocols can also differ in how they encode data. Some use human-readable text formats, while others use compact binary formats designed for efficient machine processing.

Where are communication protocols used?

Communication protocols are used wherever devices or systems exchange data over a network. They operate across both public and private infrastructures, including web services, email systems, file transfers, cloud and enterprise networks, and wireless and IoT environments.

They also function at multiple layers of networking, from physical data transmission to application-level services.

Why are communication protocols important?

Communication protocols are essential because they enable interoperability between different devices, operating systems, and networks. Without standardized rules, different systems can’t exchange data reliably. In addition, standardized protocols improve reliability and support the scalability of the internet.

Protocols also create abstraction. Systems can communicate without understanding each other’s internal systems or implementations.

Risks and privacy concerns

Not all communication protocols provide the same level of security. Some older or poorly configured protocols transmit data without encryption, making it possible for attackers to intercept or read sensitive information.

Even secure protocols can be vulnerable if outdated versions are used. Legacy versions may contain known weaknesses that expose systems to attacks. In some cases, attackers attempt downgrade attacks, which force a connection to use a less secure protocol version.

Protocols can also expose metadata, like IP addresses or domain requests, even when content is encrypted. In addition, misconfigured services may unintentionally leave systems accessible to unauthorized users.

FAQ

What’s the difference between TCP and UDP?

Transmission Control Protocol (TCP) provides reliable, ordered delivery and retransmits lost data. User Datagram Protocol (UDP) prioritizes speed, sending data without confirming delivery.

Why is HTTPS safer than HTTP?

Hypertext Transfer Protocol Secure (HTTPS) encrypts data using Transport Layer Security (TLS), while HTTP sends data in plain text. HTTPS also verifies website identity through digital certificates.

What are DNS over HTTPS (DoH) and DNS over TLS (DoT)?

DoH and DoT are protocols that encrypt Domain Name System (DNS) queries. Traditional DNS requests are unencrypted; DoH and DoT add encryption to protect domain lookup requests from monitoring or modification.

How do protocols affect VPN security?

Virtual private networks (VPNs) rely on tunneling and encryption protocols to secure data in transit. Security depends partly on how the protocol handles encryption, authentication, and key exchange. Modern VPN protocols are designed to provide stronger security and improved performance.

What is a “secure” protocol version?

A secure protocol version is one that addresses known vulnerabilities found in earlier releases. Over time, outdated versions are deprecated because they no longer meet current security standards. Using the latest stable version of a protocol helps reduce exposure to known weaknesses and compatibility issues.