Which Protocol Header Would a Layer 4 Device Read and Process?

This commodity explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in apparently English.

The OSI model is a conceptual framework that is used to describe how a network functions. In evidently English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit similar learning a language - there are lots of standards and and so some exceptions. Therefore, information technology'southward important to actually understand that the OSI model is not a set of rules. Information technology is a tool for understanding how networks function.

One time you learn the OSI model, you volition exist able to further empathize and appreciate this glorious entity we call the Internet, too as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Cyberspace!

Prerequisites

Yous don't need any prior programming or networking feel to understand this commodity. However, y'all will need:

• Basic familiarity with common networking terms (explained beneath)
• A marvel nigh how things piece of work :)

Learning Objectives

Over the grade of this article, you lot will learn:

1. What the OSI model is
2. The purpose of each of the 7 layers
3. The problems that can happen at each of the seven layers
4. The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should exist familiar with to get the most out of this commodity. I'll use these terms when I talk nearly OSI layers side by side.

Nodes

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If fix up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up upwards next to one other, wherein Node A tin can connect directly to Node B, or there may exist an intermediate node, similar a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Net and switches operate within a network to facilitate intra-network communication. Learn more almost hub vs. switch vs. router.

Here's an example:

Source

For the nitpicky among united states (aye, I see you), host is another term that y'all will encounter in networking. I will ascertain a host as a type of node that requires an IP accost. All hosts are nodes, but non all nodes are hosts. Please Tweet angrily at me if y'all disagree.

Links

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either exist indicate-to-indicate, where Node A is connected to Node B, or multipoint, where Node A is continued to Node B and Node C.

When we're talking near information existence transmitted, this may as well be described equally a i-to-one vs. a one-to-many relationship.

Protocol

A protocol is a mutually agreed upon set of rules that allows ii nodes on a network to exchange information.

"A protocol defines the rules governing the syntax (what can be communicated), semantics (how information technology can exist communicated), and synchronization (when and at what speed it can exist communicated) of the communications process. Protocols can be implemented on hardware, software, or a combination of both. Protocols can exist created by anyone, but the almost widely adopted protocols are based on standards." - The Illustrated Network.

Both wired and cable-gratuitous links can take protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such equally the Internet Engineering Chore Forcefulness (IETF).

Networks

A network is a general term for a group of computers, printers, or any other device that wants to share information.

Network types include LAN, HAN, CAN, Human, WAN, BAN, or VPN. Recollect I'm simply randomly rhyming things with the word tin? I can't say I am - these are all real network types. Learn more here.

Topology

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Hither are some common network topology types:

Source + learn more most network topologies here

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complication networks become to, you lot will understand what'southward happening in all calculator networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what's a layer?

Source

Ooo, lair.

No, a layer - non a lair. Hither there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the about tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality abroad until past the time you lot become to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

• Please | Physical Layer
• Do | Data Link Layer
• Not | Network Layer
• Tell (the) | Transport Layer
• Clandestine | Session Layer
• Countersign (to) | Presentation Layer
• Anyone | Awarding Layer

Go on in mind that while certain technologies, like protocols, may logically "belong to" one layer more than than another, non all technologies fit neatly into a unmarried layer in the OSI model. For example, Ethernet, 802.xi (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >ane layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer i

Layer 1 is the concrete layer. There's a lot of technology in Layer i - everything from concrete network devices, cabling, to how the cables claw up to the devices. Plus if we don't need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer one, I've created broader categories for these technologies. I encourage readers to acquire more almost each of these categories:

• Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and then on. Hardware components that live within of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
• Device interface mechanics. How and where does a cablevision connect to a device (cablevision connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pivot is active or inactive?
• Functional and procedural logic. What is the function of each pivot in the connector - transport or receive? What procedural logic dictates the sequence of events so a node can start to communicate with some other node on Layer 2?
• Cabling protocols and specifications. Ethernet (Cat), USB, Digital Subscriber Line (DSL), and more. Specifications include maximum cablevision length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
• Cablevision types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and and then on. Learn more about cablevision types here.
• Indicate blazon. Baseband is a single bit stream at a time, like a railway track - one-way merely. Broadband consists of multiple bit streams at the same fourth dimension, similar a bi-directional highway.
• Signal manual method (may be wired or cable-free). Options include electrical (Ethernet), lite (optical networks, fiber optics), radio waves (802.xi WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, and then also consider frequency: two.5 GHz vs. 5 GHz. If it's cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the chip.

A bit the smallest unit of measurement of transmittable digital information. $.25 are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to correspond single characters, like a alphabetic character, numeral, or symbol.

Bits are sent to and from hardware devices in accord with the supported data charge per unit (transmission charge per unit, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is chosen bit synchronization). The way bits are transmitted depends on the betoken transmission method.

Nodes can send, receive, or send and receive bits. If they can just practise one, then the node uses a simplex mode. If they can do both, so the node uses a duplex way. If a node tin ship and receive at the same time, it's full-duplex – if non, information technology'southward just one-half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an selection now, given the right equipment.

How to Troubleshoot OSI Layer one Problems

Here are some Layer 1 problems to watch out for:

• Defunct cables, for example damaged wires or broken connectors
• Cleaved hardware network devices, for example damaged circuits
• Stuff existence unplugged (...we've all been there)

If there are issues in Layer 1, anything beyond Layer one will not role properly.

TL;DR

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: abyssal communications cables transmit data effectually the globe. This map will blow your mind: https://www.submarinecablemap.com/

And because yous fabricated it this far, here's a koala:

Source

OSI Layer 2

Layer 2 is the data link layer. Layer 2 defines how information is formatted for transmission, how much data can flow between nodes, for how long, and what to practise when errors are detected in this flow.

In more official tech terms:

• Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
• Menstruation control. How much data should be transmitted?
• Fault control - detection and correction. All information transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the arrangement administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with mistake detection, not mistake correction. (Source)

In that location are 2 singled-out sublayers within Layer 2:

• Media Access Command (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should take the same MAC accost. The MAC address is assigned at the betoken of manufacturing. Information technology is automatically recognized by almost networks. MAC addresses live on Network Interface Cards (NICs). Switches keep rail of all MAC addresses on a network. Learn more than most MAC addresses on PC Magazine and in this article. Larn more about network switches here.
• Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for instance Ethernet or Wifi.

The data unit on Layer 2 is a frame.

Each frame contains a frame header, body, and a frame trailer:

• Header: typically includes MAC addresses for the source and destination nodes.
• Trunk: consists of the bits beingness transmitted.
• Trailer: includes fault detection data. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the fault may be reported upward to higher layers for further error correction. Examples of mistake detection mechanisms: Cyclic Redundancy Cheque (CRC) and Frame Check Sequence (FCS). Acquire more than about mistake detection techniques here.

Source

Typically there is a maximum frame size limit, called an Maximum Transmission Unit of measurement, MTU. Colossal frames exceed the standard MTU, learn more nearly jumbo frames here.

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to picket out for:

• All the problems that can occur on Layer 1
• Unsuccessful connections (sessions) between two nodes
• Sessions that are successfully established but intermittently neglect
• Frame collisions

TL;DR

The Data Link Layer allows nodes to communicate with each other within a local surface area network. The foundations of line field of study, menstruum control, and error control are established in this layer.

OSI Layer three

Layer 3 is the network layer. This is where we send data betwixt and beyond networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn't have Layer 3 without them. They move data packets beyond multiple networks.

Not simply do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep runway of what'south on its network (retrieve that switches keep track of all MAC addresses on a network), what other networks information technology's continued to, and the different paths for routing data packets across these networks.

Routers shop all of this addressing and routing information in routing tables.

Hither's a simple example of a routing table:

Source + larn more nearly routing tables hither

The data unit of measurement on Layer 3 is the data packet. Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer three addressing data.

The information being transmitted in a packet is as well sometimes called the payload. While each packet has everything information technology needs to get to its destination, whether or non information technology makes it there is another story.

Layer three transmissions are connectionless, or best endeavor - they don't do anything just ship the traffic where it'southward supposed to go. More on information transport protocols on Layer 4.

Once a node is continued to the Internet, information technology is assigned an Cyberspace Protocol (IP) address, which looks either like 172.16. 254.one (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node's MAC accost via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node'southward respective IP address.

ARP is conventionally considered office of Layer 2, but since IP addresses don't be until Layer iii, it's also part of Layer 3.

How to Troubleshoot OSI Layer 3 Issues

Here are some Layer 3 bug to sentinel out for:

• All the problems that tin crop up on previous layers :)
• Faulty or non-functional router or other node
• IP address is incorrectly configured

Many answers to Layer three questions will require the employ of command-line tools like ping, trace, bear witness ip route, or bear witness ip protocols. Acquire more than about troubleshooting on layer ane-3 here.

TL;DR

The Network Layer allows nodes to connect to the Internet and transport information across unlike networks.

OSI Layer iv

Layer four is the transport layer. This where we swoop into the nitty gritty specifics of the connectedness between two nodes and how data is transmitted between them. It builds on the functions of Layer ii - line discipline, period control, and error control.

This layer is also responsible for data package sectionalisation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not only the contents of each private data package. With this agreement, Layer 4 is able to manage network congestion by non sending all the packets at in one case.

The data units of Layer 4 get by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I'll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connectedness with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does non receive all of the data, TCP volition ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct lodge. Learn more than almost TCP here.

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it'south chosen connectionless.

Because UDP doesn't have to expect for this acknowledgement, it can ship information at a faster charge per unit, but not all of the data may exist successfully transmitted and nosotros'd never know.

If information is divide into multiple datagrams, unless those datagrams comprise a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP hither.

TCP and UDP both ship data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here.

Acquire more about the differences and similarities between these two protocols here.

How to Troubleshoot OSI Layer iv Problems

Here are some Layer 4 problems to watch out for:

• All the problems that can ingather up on previous layers :)
• Blocked ports - check your Access Command Lists (ACL) & firewalls
• Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they tin really muck things up. Learn more near QoS hither.

TL;DR

The Transport Layer provides end-to-end manual of a message past segmenting a bulletin into multiple data packets; the layer supports connectedness-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer. This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established betwixt 2 network applications. Not two nodes! Nope, we've moved on from nodes. They were then Layer four.

Just kidding, we still have nodes, but Layer 5 doesn't need to retain the concept of a node because that's been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are ii of import concepts to consider hither:

• Customer and server model: the application requesting the information is called the customer, and the awarding that has the requested information is called the server.
• Request and response model: while a session is being established and during a session, there is a abiding back-and-forth of requests for data and responses containing that data or "hey, I don't have what you lot're requesting."

Sessions may be open for a very short amount of fourth dimension or a long corporeality of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or total-duplex modes.

Examples of protocols on Layer five include Network Basic Input Output Organization (NetBIOS) and Remote Procedure Phone call Protocol (RPC), and many others.

From here on out (layer five and upward), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer five Problems

Here are some Layer v problems to sentinel out for:

• Servers are unavailable
• Servers are incorrectly configured, for example Apache or PHP configs
• Session failure - disconnect, timeout, then on.

TL;DR

The Session Layer initiates, maintains, and terminates connections between ii end-user applications. It responds to requests from the presentation layer and bug requests to the transport layer.

OSI Layer half dozen

Layer 6 is the presentation layer. This layer is responsible for information formatting, such equally grapheme encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer seven can successfully swallow data and, of course, eventually display it.

There are three information formatting methods to be aware of:

• American Standard Lawmaking for Information Interchange (ASCII): this vii-bit encoding technique is the most widely used standard for character encoding. Ane superset is ISO-8859-i, which provides most of the characters necessary for languages spoken in Western Europe.
• Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
• Unicode: grapheme encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more well-nigh character encoding methods in this article, and besides here.

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols assistance ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer six problems to watch out for:

• Not-real or corrupted drivers
• Incorrect Bone user access level

TL;DR

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer seven is the application layer.

Truthful to its name, this is the layer that is ultimately responsible for supporting services used by cease-user applications. Applications include software programs that are installed on the operating system, similar Internet browsers (for example, Firefox) or word processing programs (for instance, Microsoft Word).

Applications tin can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for instance, are specifically created to run over a network and employ networking functionality, such equally email protocols, which fall nether Layer vii.

Applications will besides control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and and then on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Vanquish (SSH), Uncomplicated Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of data. (Source)

How to Troubleshoot OSI Layer 7 Bug

Here are some Layer 7 problems to watch out for:

• All issues on previous layers
• Incorrectly configured software applications
• User error (... nosotros've all been there)

TL;DR

The Awarding Layer owns the services and functions that end-user applications need to piece of work. It does non include the applications themselves.

Determination

Our Layer i koala is all grown upwardly.

Learning check - tin you employ makeup to a koala?

Don't accept a koala?

Well - answer these questions instead. It's the adjacent best affair, I promise.

• What is the OSI model?
• What are each of the layers?
• How could I utilise this information to troubleshoot networking problems?

Congratulations - you've taken i step farther to understanding the glorious entity nosotros call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out whatever O'Reilly-published books about the subject or about network applied science in general.

Here are some resource I used when writing this article:

• The Illustrated Network, 2nd Edition
• Protocol Information Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
• Troubleshooting Forth the OSI Model: https://www.pearsonitcertification.com/manufactures/article.aspx?p=1730891
• The OSI Model Demystified: https://www.youtube.com/sentry?five=HEEnLZV2wGI
• OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

About Me

Chloe Tucker is an creative person and estimator science enthusiast based in Portland, Oregon. As a erstwhile educator, she'southward continuously searching for the intersection of learning and teaching, or technology and fine art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev.

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