In order to write eBPF programs that interact with the Linux Networking stack, we need to understand how the internet works. What is better than building something from scratch and understand how it works.

If we design our own internet from scratch, solving one problem at a time, improvising, how do you think it will unfold? How different will it be from what we already have?

In this article we are at the core, no big words, just you encountering problems, and implementing solutions for them. Let's get started

Ground Rules

Before we even start, let's set some ground rules to begin with.

Our internet should be:

• Fast
• Scalable
• Efficient
• Dynamic

Why? Because the Old internet has all of these things, and the new Internet won't be of any use if it is not as good or better than the old one.

What we want

We want to send the data from one device to another in a Fast, efficient and scalable way. So, how do we do that? Let's design our own internet, which would be "interconnection of multiple devices" talking to each other.

Basic Communication and Need of addresses

Physical World vs Digital World

Let's say you want to send a message to your friend, who lives two houses away from you. You would need their address, so that you can write the information on let's say a piece of paper and tell your younger brother to give that piece of paper to the people living in that house.

You would also attach the name of the person on top of it, because you wouldn't want someone other than your friend to read your messages right? But let's think about this later, we just want the message to somehow reach your friend's house.

This is the physical way of sharing information, but since we are creating our own internet, we would need a "digital way". Luckily you and your friend both have a computer to work with.

In the Physical world, your younger brother is the messenger who carries your message. But in the Digital World we need a medium that can carry information in a way computers understand. This medium could be a wire, like an Ethernet cable, or even wireless signals like Wi-Fi.

Similarly, computers can't read handwritten notes. They need the message in a special format (binary). Think of it as translating your note into a secret code that only computers can understand. This code is called "binary", made up of 0s and 1s.

So, you and your friend decided to buy an Ethernet cable as your medium, that carries binary information in the form of Electrical pulses, these pulses travel from your computer to your friend's. As for converting your messages into binary, let's say for simplicity's sake, your computer has a messenger built in-a magical helper called a driver. This driver knows how to convert your messages into bits and pass it to the Ethernet Cable.

Comparison with Old internet

In our Old Internet , the work of converting your messages into bits is done by NIC(Network Interface Card), we will touch upon it in later sections.

Need of Addresses

This was working amazing for a few weeks, but turns out, your other 2 friends (C and D) found out about this new mode of communication you and your friend came up with. They want in on this.

So, you decided that the task is fairly easy, just buy more Ethernet Cables, connect it to each other's computers and start sharing messages. But there are a few problems.

• There are too many cables now! Each computer has 3 cables connected to it, which is causing clutter and your Mom is not liking it. Also your computer has limited Ethernet sockets. Less than the number of friends you have.

• Since there are 4 computers now (A, B, C and D) and if A wants to send messages to B, how would A uniquely identify which cable is connected to B?

Let's solve the problems one at a time.

Too many Cables

To solve this problem, one of your friend came up with a solution. He had an extra computer (let's call it X) sitting idle at home, so he thought instead of computers being directly connected to each other, let's have X has the common connection between all the other computers, so now each computer (A, B, C and D) have just one cable connected to them, which connects them to X.

If A wants to send a message to B, the message goes from A -> X and X sends the message from X -> B.

But this has one common problem as the previous solution.

How to uniquely identify each computer

Our last solution made sure that irrespective of who is sending message to who, they just have one Ethernet cable connected to them, which takes message to X. But now X has 4 cables attached to it, connecting it to (A, B, C and D) respectively. On receiving a message how does X know which cable to write that message on? We need Unique addresses!

So, you decided to give each computer a hard-coded number, A:1, B:2, C:3 and D:4. And you added a table (let's call it lookup table ) in the computer X, that has a mapping of which cable sends message to which unique address.

Now if A wants to send message to B, it wraps the message into something like an envelope, with B's unique address on it and sends it to X. Then X reads that envelope and knows that this message needs to be sent to B, therefore it checks the lookup table to know which out of the 4 cables connects X to B, and send the message on it.

Comparison with Old internet

In Old Internet, this unique address assigned to each computer is called MAC Addrress, which is assigned to each NIC by the Manufacturer. It is also considered the Physical address of the Device.

This lookup table is called MAC address lookup table and we have multiple protocols like ARP that make it possible . This common device X is a Hub/Switch, Finally, this interconnection of 4 computers you just built is called a LAN(Local Area Network) .

From Local to Global

This section is about understanding how you would send messages over longer distances, efficiently.

You and your friends are enjoying the new internet you built, your parents are amazed and therefore telling your distant relatives about it. Your favorite cousin heard and wants to be a part of it.

You realize that this is something that would keep growing, because then your cousin's friends would want to use it as well, even though they might want to have an isolated part of it. After all, they are not your friends and would want their own separate LAN.

So, if we break down the problem, here is what we want:

• Your cousin's group should have a separate Local Area Network (LAN) where they can talk about their personal things.
• Your cousin wants to talk to you as well, so you need to figure out a way to make these two LANs talk to each other.

Setting Up a New LAN

The first problem is fairly easy to solve: just setting up a new Local Area Network of interconnected devices that have their own unique addresses and names. Let’s say your cousin sets up a network with devices labeled E, F, and so on, just like you set up for you and your friends. The common computer’s (Switch) name here is Y.

Making Two LANs Talk to Each Other

For simplicity, let's say Ethernet cables are free and abundant. You take a cable long enough to connect your LAN's switch (Computer X) to your cousin's switch (Computer Y).

Now, by adding the entries of all the computers and their unique addresses in both X and Y, we can ensure that communication between the computers happen.

When your brother sends a message to you, your computer's address is attached on top. It goes from his computer to Y, from Y to X and from X to your computer.

But, there are still many problems.

• Both X and Y need to constantly be aware of every computer’s physical location (which cable connects to which computer). What if there are tens of thousands of Computers? X and Y both store the same information and every time anyone new joins the Network, the entire table in both X and Y needs to be updated. System is not Scalable enough.
• Another problem is the current process of addressing is based on physical presence instead of a logical hierarchy i.e. Switch X needs to know exact address and path of every computer in the world. So if there are multiple Switch between you and your brother's computer, still each Switch needs to know the exact path to be followed in order for the message to reach from one place to another. Longer the distance, larger the path to remember.

You need a way to make the system scalable. Instead of having Switch X and Switch Y know about every device and always needing updates, you want a system where X only needs to know, “If I send this message on this cable, it will eventually reach Y and then Y can handle the logic of sending the information to computer.

Scalable Addressing Scheme

Let's imagine a new system. What if there could be a device (let's call it R) on top of every LAN. With an address attached to itself, this device is responsible only for sending messages between two LANs, and can guess the direction of other R type devices.

Also, each devices connected to R will have a new type of address as well, which is dynamic and logical, not hard coded in the system, so it can keep changing. This address is assigned to computers in a specific way. If 2 computers are connected to R1, they will have address 1.1 and 1.2. Similarly computers connected to R3 will have addresses in 3.x format.

This forms a hierarchy telling which computer is within the LAN and which one is out of it. For R1, computer 1.1 is internal since it's address starts with 1. And 2.1 (but internal for R2) is external.

R1 has two types of tables now, one for addresses outside it's LAN and other for internal devices. If the destination address of a message is internal, our older system still works. R1 just has to send the message to the Physical Address (MAC address) of the destination device.

If the destination is external let's say 3.1 , R1 doesn't have to know the exact path the message has to follow to reach 3.1, it just needs to know which direction should I pass the message so it eventually reaches R3. So it passes the message to the next closest device to it, R2 and R2 does the same thing and passes it to R3. Since the destination addresses 3.x are internal for R3, therefore our previous system will work again.

You might be thinking, why? What are the benefits of this? Let's try to understand that.

• The older system doesn't change, it is still responsible for sending message from one device to it's immediate neighbour another. We just built a higher level system so that the older one still works, but the new one makes it dynamic

Dynamic

Initially if a new device joined your internet, the entire Physical address lookup table of every device in the entire internet needed to be updated, which is a massive overhead if the system is huge (Old internet is huge).

This new mechanism doesn't update the entire system. If a new device gets connected to R1, it will just increase entries in the two (Physical and Logical) tables of R1, the Physical address table will store the MAC address of the new device and similarly the Logical address table will assign a new address (Logical address) to it in the format of 1.x and save it. The entire system is unaware of this change.

Now let's say our system looks like the diagram below.

If R2 has to send a message with destination address of format 1.x, it just knows that it is a local address for the device R1. R2 doesn't know the exact path to be followed by this message to reach R1. It just knows that addresses of the format 1.x and 5.x are somewhere to the left and 4.x are somewhere to the right (how? We will check it out later) .

It checks the Physical address table to get the Physical address of the device to it's immediate left, which in our example case is R5, and just sends the message to R5. On receiving the message, R5 does the same, but turns out the immediate left of R5 is the address family 1.x. R5 gives the message to R1.

Since 1.1 format is internal for R1, there is no need of logical addresses anymore. It just get's the Physical address of the computer whose Logical address is 1.1 which is unique for the LAN. Finally sends the message to that Physical address and message is delivered to the right destination computer.

In case your brother came to visit you in summer vacations.

Nothing changes. All addresses related to him (Physical and Logical) are removed from his R level device(let's say R1).

When he comes to your home, he just connects his device to your LAN's R level device (let's say R2). R2 stores the physical address of your brother's computer and also assigns it a logical address of the format 2.x. The routing process still remains the same. And the entire system except R1 and R2 is unaware of any changes.

Comparison to the Old Internet

This R level device is called Router. This device is responsible to route message between any two LANs. The Logical addresses assigned by the Routers is called IP addresses.

The process of assigning logical addresses to newly connected devices, is called IP address assignment and the protocol that does this is called DHCP.

Routers don't guess the right direction to pass the message to, there are many router protocols like OSPF, BPG etc. that keep the logical address table of routers updated. Mapping specific IP ranges to neighbouring routers. Hence routers can tell which IP range is on the right and which one is on the Left.

The process of sending message from one device to it's immediate neighbour using MAC (Physical) addresses is called Hop to Hop delivery.

The process of sending message from sending device's IP address to receiving device's IP address is called End to End Delivery.

This process of building a higher level system like IP addressing over lower level systems like MAC addressing is called Abstraction. Here the higher level system relies on lower level system for Hop to Hop delivery while ensuring End to End delivery through it's own addressing system.

There is a pattern now, even if we are trying to build a new internet from scratch, it still looks extremely like the older one! I guess the "forefathers" knew what they were doing.

I can't remember the computer addresses

Your new internet idea has sky-rocketed, almost everyone in your family is using it, even their friends, turns out that there are more than thousands of people, currently actively using the internet you just created. But that has raised a new issue, there are too many computer addresses to remember, Because the sender is the one who is deciding who to send the message to so they are the people who is putting in the destination address. There are only so many addresses a human can remember right?

Now there is a need of a system that can turn these computer addresses into something easy to remember. But internally your routes and switches will need an address to finally get the message to it's destination.

In the old internet, you just type in google.com and it takes you to the right place, it's easy to remember names as compared to computer addresses, and as mentioned earlier, these computer addresses keep changing right? Your cousin's computer has a different address when he is at his home, and a different one when he comes to your home for vacation.

Let's get back to imagining a solution for this. What if there was a system, that can take the name of your friend and convert it to the current IP address of his computer? This system needs to be real-time, so doesn't matter from where your brother connects to the internet, your messages should reach him and the system needs to know the "current" IP address of his computer.

You just ask, "what is the current IP address of my Bother's computer" and the system responds with it, extremely fast. Then you put that address as "destination address" and send it off to the internet for it to Sail.

This article is long enough to begin with, and adding this section will be too much to read. This system I am talking about already exists, it's name is DNS (Domain Name System). Therefore we will continue this journey of making our own internet in the upcoming articles, but the current system works as it is, now we are trying to make it User friendly.

Key Takeaways

• We started with basic communication: Just like sending a letter in the physical world, we explored how computers can send messages to each other through a digital "medium" (like Ethernet cables or Wi-Fi) and how that message is translated into binary.

• The need for unique addresses: As more devices join the network, we faced the problem of how to uniquely identify each device. The solution? Assigning unique hard-coded physical addresses (MAC addresses) to each device, allowing switches to know where to send data.

• The first network expansion: To manage multiple devices, we set up a LAN with one central switch connecting all computers. However, we quickly realized that as the network grows, each device can't know about every other device. This led us to a more scalable approach.

• The move to routers and IP addresses: As the network expands globally, we need a system that scales beyond a single LAN. We introduced routers to route data between LANs, and IP addresses to uniquely identify devices in a hierarchical, dynamic way—allowing us to manage large networks without excessive overhead.

• Dynamic addressing: Instead of updating every device on the Internet when a new one joins, we adopted a dynamic addressing system. Routers update only their own tables, so the rest of the Internet doesn't need to be aware of every change.

• Comparison with the old Internet: The design of our new Internet mirrors key concepts from the old one, such as MAC addresses for local communication and IP addresses for global communication. We also explored how routing protocols (like OSPF and BGP) help manage data across networks.

• The problem of remembering addresses: As more devices join the network, remembering IP addresses becomes impractical. This is where the Domain Name System (DNS) comes in—translating easy-to-remember names like "google.com" into the current IP addresses of devices, making the system user-friendly.

• Scalable and dynamic design: We’ve created a system where new devices can join without disrupting the entire network, and where the routing system remains efficient even as the Internet continues to grow.

Future Scope

In the upcoming articles we will talk about how to make this new internet we built, user friendly. In the old internet (the one you are using right now), everything is already very convenient. Your just have to click a few times, connect to your closest router and everything works. There are many systems working in the background to make this possible.

Every message you send from your devices, follow a large number of steps to reach their destination and all of it happens in a few seconds. Hard to imagine right? Everything you work with today, listening to music, watching videos online, talking to your friends, playing games (real time) while voice chatting, doing video calls etc. It all happens through this marvel, named Internet which we humans built from thin air.

In the future we will talk about how every moving part of this system works. Topics including OSI/TCP-IP model, how this Networking System is built into the Linux kernel. We will go as deep as it goes, from kernel buffers to C structs handling each packet (your message). Eventually we will even write those Magical Drivers we talked about above.

So, keep an eye on this and if there are any questions, please reach out.