Network protocols list: The language of the digital world explained

Network protocols are a lot like those made-up languages from your childhood, except they allow billions of devices to communicate with one another on the internet reliably instead of keeping your parents out of the loop. Protocols are the secret languages of the digital realm, the ones responsible for loading web pages, sending emails, and streaming your favorite shows.

The crucial difference is that network protocols follow a standardized set of rules so that manufacturers' devices running different operating systems can all understand one another. Protocols are like good diplomats in the networking world, forging compromises and alliances to ensure everyone gets along and plays by the same rules.

We're going to crack the secret code of the internet. In this post, we'll decode its secret language, point out the protocols you care about most, and show you how to monitor them in practice.

The foundation: TCP/IP and the protocol stack

Before we take a deeper look at individual protocols, we first need to cover some basics on how the world of network protocols is organized. The TCP/IP model, which is not really a model but a conceptual framework, breaks protocols up into groups that are responsible for different aspects of networking.

This concept is often represented as a stack with several distinct layers, each with its own protocols:

The lowest layer is the Network Access Layer (also known as the Link Layer), which covers the physical connection between devices on a local network.

The next layer up is the Internet Layer, which takes care of routing packets between different networks.

Third up is the Transport Layer, responsible for the reliable end-to-end data delivery between communication endpoints.

At the very top sits the Application Layer, where the protocols are directly used by your software applications.

The layering approach here is all about managing the complexity of networking. It allows individual protocols to change and evolve without impacting others. For example, if web browsers decide to upgrade how they talk to web servers, it should not require changes to your home router. Protocols are like an apartment building, where a renovation to a single floor or apartment doesn't mean you need to tear down and rebuild the whole thing.

Transport layer protocols: The reliable messengers

The transport layer is all about moving data reliably between two endpoints. Two protocols rule this layer: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). They're like two different types of delivery services, each with its pros and cons.

TCP is the workhorse of internet communication. It sets up a connection before any data is transmitted, sends acknowledgments to confirm successful deliveries, and retransmits lost packets. It's a connection-oriented protocol that guarantees data will be delivered complete and in order.

TCP takes care of the downloading of files, banking statement verification, and email sending. It's a methodical process, but that ensures not a single byte will be lost in transit.

UDP, on the other hand, is the opposite of TCP. It's a connectionless protocol with no delivery guarantees or packet ordering. You can think of it like tossing messages in bottles out to sea and hoping for the best - you don't wait around for acknowledgments.

For applications where speed is more important than perfect reliability, UDP is ideal. Real-time voice and video calls, online games, and live video streams prefer UDP, as it causes less delay than the packet verification and acknowledgment overhead of TCP. Small data losses in these use cases are unnoticeable to the user.

Internet layer protocols: The traffic directors

The internet layer, as you probably guessed, is in charge of traffic between different networks, which may be located on the other side of the world.

IP is the universal addressing system of the internet. Every device needs an IP address, and it's the job of the Internet Protocol to assign and manage those addresses, and direct the routing of data packets between networks.

IP comes in two versions:

IPv4 is the original version of IP, which uses 32-bit addresses (look at your computer's IP address, and you'll see one like 192.168.1.1). IPv4 is still dominant but fast running out of available addresses.

IPv6 is the long-awaited upgrade that expanded the address space to 128 bits, meaning it can now support an almost inconceivably large number of unique addresses. Adoption of IPv6 has been slow but is gradually accelerating.

ICMP is the network messenger that primarily reports errors and provides information about network device status. Pinging a remote server to see if it is available for connection is one of its primary use cases.

Network administrators heavily rely on ICMP as a troubleshooting tool. It's like having an inbuilt diagnostic protocol that sends you alerts to let you know exactly when and where something goes wrong in the packet delivery process.

Application layer protocols: The specialized workers

The application layer is where you will find most of the protocols you have likely already heard of. This is where application-specific worker bees live, each handling a narrow range of duties like web browsing, email, and file transfers.

HTTP and HTTPS: The web communicators

HTTP (Hypertext Transfer Protocol) is the foundation of the World Wide Web. It's the messenger that defines how web browsers request pages from web servers and how those servers respond with the requested information. Nearly every web page starts with the "http://" URL prefix for a reason.

HTTPS is just the same as HTTP with the addition of TLS (Transport Layer Security, SSL's successor) encryption, to prevent data from being eavesdropped on during transit. This is vital for any online banking, shopping, or any website that handles sensitive information.

DNS: The internet's phonebook

DNS (Domain Name System) is your computer's contact list for the internet. It translates human-friendly domain names (like paessler.com) into computer-readable IP addresses.

Imagine if every time you wanted to visit a website you had to remember a numeric IP address instead of a short, easy-to-remember domain name. You would be running to your phone constantly to look up numbers like a certain someone you know. Fortunately, DNS servers do all the lookup work for us behind the scenes.

DNS is another protocol where a failure will be glaringly obvious, as it means your web browsers will display errors rather than web pages.

SMTP, POP3, and IMAP: The postal workers

Sending and receiving email relies on several protocols working in harmony. The mail gets sent by SMTP (Simple Mail Transfer Protocol), which transfers your message between your mail client and the mail server.

POP3 (Post Office Protocol version 3) and IMAP (Internet Message Access Protocol) are two protocols responsible for retrieving mail from your mail server and downloading it to your device. IMAP is more advanced and can sync email across several of your devices while keeping messages on the server.

FTP and SFTP: The file movers

FTP (File Transfer Protocol) is a veteran, having been around since 1971, but is still useful today, as it provides a means of uploading and downloading files to and from computers. It's commonly used for uploading files to web servers or downloading large datasets to a local machine.

SFTP (Secure File Transfer Protocol) is just like FTP but with the added encryption of SSH. It is important to use SFTP when moving sensitive files over public networks.

DHCP: The address assigner

DHCP (Dynamic Host Configuration Protocol) automatically gives IP addresses to devices when they connect to your network. This frees network administrators from the need to manually assign each new device unique IP addresses and other information like subnet masks, default gateways, and DNS servers.

Without DHCP, users would have to manually configure every network-connected device on their network – no DHCP is a nightmare for network administrators and makes the network setup much more user-unfriendly.

DHCP makes networks dramatically more manageable, as users simply plug in a new device and have it automatically assigned all the network settings it needs without manual configuration.

Specialized protocols for specific needs

Beyond the well-known protocols covered above, there are several specialized protocols that cater to specific industries or use cases.

SNMP: The network monitor

SNMP (Simple Network Management Protocol) is used by network management systems to monitor the health and performance of networked devices. SNMP allows administrators to retrieve device information (like CPU and memory usage), change device settings, and detect and troubleshoot problems.

SNMP is the protocol that makes network monitoring tools like PRTG Network Monitor possible.

BGP: The internet's traffic director

BGP (Border Gateway Protocol) is the protocol that controls traffic between the large autonomous systems (like ISPs) that make up the internet. BGP is responsible for finding the best paths for data to take between these different systems and is a key part of making the global internet possible.

SIP: The call connector

SIP (Session Initiation Protocol) is the protocol responsible for establishing real-time voice and video calls. SIP controls the setup and teardown of these calls, and as such is a critical part of VoIP phone systems and video conferencing software.

Monitoring protocols in the real world

We have covered the most important network protocols, but understanding the theory only gets you so far in real-world IT environments. The challenge for network administrators is keeping everything working correctly 24 hours a day, 7 days a week, 365 days a year.

Protocol monitoring is essential to getting this done, but it's not a case of just monitoring every single protocol in existence. You have to focus on the right protocols in the right context. This is where good network monitoring tools make all the difference.

Protocol monitoring scenarios

To see what this looks like in practice, let's take a look at some real-world protocol monitoring scenarios you might encounter.

Scenario 1: The disappearing website

Users complain that they cannot access your company website. Is it a DNS issue, a web server problem, or something else entirely?

In PRTG, you can use multiple protocol sensors working together to determine where the problem is. In this case, you should run:

• HTTP sensors that check if the web server is responding to requests
• DNS sensors to see if domain name resolution is working
• TCP sensors to check if the server is accepting connections
• ICMP sensors to determine if the server is reachable at all

This example shows how monitoring multiple protocols together can let you identify exactly where failures occur in the communication chain. Protocols depend on one another, and problems with one can cause issues with others.

Scenario 2: The sluggish email

Your users are complaining that sending and receiving email has become unbearably slow. It works, but messages take an age to get to their destination.

To get to the bottom of the issue, you can use PRTG to monitor:

• SMTP response times to check performance of outgoing mail delivery
• POP3/IMAP response times for incoming mail delivery
• DNS resolution for the mail server
• The path between the clients and the mail server itself

By monitoring a range of different protocols, you can work out whether the problem is in your mail server, your network, or with an outside service.

Scenario 3: The mysterious bandwidth consumer

Your internet connection is saturated with usage, but no one seems to be willing to accept responsibility for excessive usage. Who or what is consuming all of your bandwidth?

PRTG can help you pinpoint the responsible culprit by monitoring:

• Which protocols are using the most bandwidth
• Which devices are generating the most traffic
• If any unexpected protocols are active on your network
• Potential security risks like unauthorized file sharing

Protocol-specific traffic visibility allows you to see exactly how your network bandwidth is being used and take action.

FAQ: The not-so-obvious questions about network protocols

How do protocols adapt to changing network conditions?

The fixed rulebook image that most people have of network protocols is misleading. Modern protocols are often adaptive and adjust their behavior dynamically based on conditions. TCP, for example, uses congestion control algorithms that automatically throttle back data transmission rates in response to packet loss. This helps avoid overwhelming the network.

As the network load eases, TCP automatically increases its speed again. The underlying concept of TCP's congestion control, called Additive Increase Multiplicative Decrease (AIMD), has been proven to work well but is also used in more modern protocols. HTTP/2 and HTTP/3 have new multiplexing, header compression, and server push features added on top of their predecessors to address earlier limitations.

SIP can negotiate codec changes mid-call to adapt to changing bandwidth conditions. The most sophisticated network protocols today have near-biological qualities in the way they sense their environment and change their behavior in response.

What happens when protocols conflict with each other?

Protocol conflicts are surprisingly common but can create pernicious and subtle problems. When two protocols fight over limited resources like bandwidth, QoS (Quality of Service) techniques are needed to prioritize traffic. Without QoS, real-time voice and video calls might suffer from jitter and latency while bulk file transfers consume all available bandwidth.

Security protocols can also conflict with network monitoring tools and processes. Deep packet inspection may be blocked by encryption, resulting in blind spots in network visibility. This is why modern network monitoring solutions like PRTG use a multi-pronged approach for gathering performance data, not just packet inspection.

Protocol conflicts can also happen when unrelated protocols interact in unexpected ways. IPv6 and IPv4 are fully compatible and can run side by side on the same device in dual-stack mode. Unfortunately, this can also create unforeseen routing issues if not properly configured, as applications sometimes have inexplicable protocol preferences that break in mysterious ways.

How are protocols evolving for IoT and 5G networks?

Traditional internet protocols like TCP/IP, HTTP, and SMTP were not designed with billions of small, power-constrained IoT devices in mind. This has led to a set of lightweight new protocols for IoT environments.

MQTT and CoAP are similar to HTTP but with far less overhead and much lower power consumption. These protocols are designed to keep bandwidth and power use to an absolute minimum. They are also more reliable, as IoT devices often sleep most of the time and only wake briefly to send small amounts of data.

5G is also driving changes in the network protocols landscape with its network slicing technology. Network slices are essentially different protocol stacks that can be separately optimized for different use cases – one might prioritize ultra-low-latency and ultra-reliable communications for autonomous vehicles, while another slice might maximize bandwidth for video streaming, and another could be optimized for massive IoT deployments.

The future of network protocols will not be about new universal protocols to replace existing ones, but rather new specialized protocol environments that are optimized for different types of applications and devices.

Bringing it all together with unified monitoring

The increasing complexity of modern networks means dozens of protocols often running simultaneously. Comprehensive monitoring that covers all protocols is therefore critical. With its vast array of specialized sensors, PRTG Network Monitor offers visibility into nearly every protocol running on your network.

Specialized protocol sensors understand each protocol's intricacies, allowing you to monitor its traffic and status at a level of detail that is not possible with tools that just try to capture as much information about packets as possible.

Monitoring protocols at every layer of the stack, from TCP/UDP up through application protocols like HTTP, DNS, and SMTP, gives you a complete view of your network's health and performance.

The value in this all comes when you are troubleshooting, as most network problems are related to multiple protocols. A user might complain that "the internet is slow", for example, but your job is to figure out if the problem is with DNS, TCP, HTTP, the web server itself, your router, or something else.

The protocol-focused approach that PRTG takes to monitoring provides the visibility you need to never fly blind, even as networks become more and more complex each year.

Try PRTG free for 30 days and discover how unified network monitoring can take your experience to the next level.