RST, ACK Packet vs. RST Packet: Difference?

Here’s the difference between RST, ACK packets and RST packets, and what they mean:

An RST, ACK packet is a packet in a TCP connection that is flagged to tell the system that the packet was received and the transmission is done accepting requests. 

This flag can show up in many different instances, but a common one is with DDoS attacks. 

A large number of RST, ACK flags indicates such an attack.

So if you want to learn all about the difference between RST, ACK packets and RST packets, and their meanings, then this article is for you.

Keep reading!

RST, ACK Packet vs. RST Packet: Difference? (Meaning)

What Is RST?

RST is known as the reset flag in transmission control protocol (TCP). 

To explain this, I’m going to have to take you through more than a few technical terms and ideas but stay with me. I’ll hit them all.

Before we go down the rabbit hole, RST is a specific designation for a packet. 

The idea of having a flag on any given packet is that it tells the computer system how to assess the information that is sent along a pipeline. 

Ultimately, the RST flag can be used to communicate a lot of different things. 

We’re going to be focusing on what it means when it is paired with an ACK flag.

Before we can get to all of that, we’ll have to understand the ACK flag and what packets are in general. 

I’ll also have to take you through TCP. So, let’s get started by looking at ACK.

What About ACK?

The ACK flack means “acknowledge.” 

The computer scientists, who came up with this, decided that “ACK” is a good shortening of the word acknowledge. It’s neither here nor there.

The point is that this is another flag that can ultimately tell a computer system a lot about what is happening when information is shared.

Here’s the gist of how ACK works. 

Let’s say we have two computers that want to share some data. 

In order to do that, they need some type of network connection. 

It doesn’t really matter how they’re connected in this example, only that they are.

Before the data transfer begins, the computers link up and determine that a connection exists in the first place. 

So, one computer will send a packet of data of a specific length. 

When doing that, the sending computer will generate an acknowledgment number for the data. 

Basically, this number is saying how much information is in the packet that is being sent.

When the second computer receives the packet, it will check the length of data and generate an acknowledgment number. 

It will then send that number back to the first computer. 

If the acknowledgment numbers match, then both computers know that the transmission was successful and no data was lost. 

If the acknowledgment numbers don’t match, then the systems know that there is a problem.

What Is a Packet?

Hopefully, this is making a little bit of sense, but there are still some terms that need clarifying. 

For instance, what is the packet that goes back and forth between computers?

Basically, this is how computers break down data for sending it across a network. 

Let’s say you want to send an email. It’s a short, simple email that you typed up in a few minutes. 

In order for your computer to actually upload that email to a server or send it along the internet, it has to send digital signals along networking infrastructure to a recipient computer.

The full path of an email can be pretty long and complicated, but what matters right now is that your computer is sending data to another device. 

The thing is, even a basic email can have a lot of information packed into it. 

Try to send that as a single signal doesn’t really work. 

The chain of 1s and 0s involved would be too complicated.

So, your computer breaks the email up into small little pieces of data. 

Each of these pieces is of a size that can be transmitted in a single signal (or signal package), and such a chunk of data is called a packet. 

Any file you send on a network is ultimately broken into a bunch of packets. 

The packets are sent one at a time (for the most part), and the receiving computer then reassembles the file by putting the packets back together.

You can almost think of it as mailing a jigsaw puzzle to someone one piece at a time, only this is being done at the insanely fast pace of a modern computer. 

As long as all of the pieces are there, the computer can very reliably assemble the information back into its original form.

What Is a Flag?


We’ve talked about two different flags and the idea of a packet. 

Let’s start to put some of the concepts together. 

Whenever a packet is sent, it can have any number of flags attached to it. 

So, the ACK flag is attached to a packet when communication is being established. 

This tells the receiving computer that it needs to send the acknowledgment number back to the sending computer so the system can confirm that the packet transfer was successful.

Other flags tell the system that the connection has been initiated (SYN) or that the connection is being terminated (FIN). 

RST and ACK are just two of several flags that exist in total.

On top of there being different flags, combinations of flags can tell the system very specific things. 

That’s where the RST, ACK packet flag comes into play. It’s describing more than either flag can alone.

I will get into that, but there’s one more piece of information needed before this will fully makes sense.

What Is TCP?

Here’s the last missing element that ties all of the information together. 

TCP is a protocol that allows computers to communicate via the internet. There are different protocols, but TCP is one of the most prolific. 

For the most part, all computers can communicate via TCP, and data transmissions with this protocol are well-established in universal and accessible ways.

In other words, TCP is how your computer (or phone, or smartwatch, etc.) talks to other devices on the internet. 

A TCP connection is established in order to send packets. 

So, when we’re talking about flags and packets, it’s all part of the transmission control protocol. 

This is the master set of rules and processes that makes everything work.

So, What Is an RST, ACK Packet?

So, when a packet has the RST, ACK flag, this is telling the TCP connection something specific. 

Essentially, this combination flag is telling the system that the last packet was received, and the connection is now closing. 

It’s saying that the last packet is being treated as the end of a transmission train, and to send more, the connection has to reopen.

This might sound more extreme than it really is. 

Let’s say you want to send five separate files from one computer to another. 

Each file has to be broken into packets, but the system wants a separator to distinguish packets in one file from packets in another file.

So, the computers will establish their TCP connection, and the packets for the first file will be sent. 

When that process finishes, an RST, ACK flag can let the first computer know that the last packet was sent, and the job is done. 

The sending computer will then reopen the connection to start on the next file. It’s all automated, and it all happens very quickly.

It’s also worth noting that this isn’t the only way computers can send a series of files. 

It’s one possible way it would turn out, depending on very long lists of possibilities and potential complications. 

Digital data transfers are not simple exchanges, and yet I’m trying to break this down on a very simplified level. 

So, the example you just read can happen, but it’s not the way things always happen.

Why Do RST, ACK Packets Matter?

We just covered a lot of technical jargon and the basic process of establishing and flagging TCP packets. 


Why should anyone who isn’t a computer scientist care about this?

Natural curiosity aside, here’s where this does mean something to an average computer user. 

It’s possible for RST, ACK flags to show up way more often than they are supposed to. 

When this is the case, connections can sever or fall apart, and you might even see an error code relating to RST, ACK flags.

If that happens, then it’s telling you that there is a significant problem. 

Now, you can try to contact tech support and get help with it. 

But, if you want to really understand the problem, this is your chance.

An excess of RST, ACK flags can be an unfortunate circumstance that you can’t do a whole lot about. 

It can also be a sign of a DDoS attack, and that’s the real point behind this long conversation.

DDoS Attacks

DDoS attack is a special way for people with malicious intent to bring down a server or a computer system. It basically works like this. 

If a single system gets too many requests to establish a TCP connection, it can get overwhelmed. 

It will try to sort through the requests, but if they keep coming in too fast, then the system becomes inaccessible.

With a DDoS attack, someone can use a computer (or group of computers) to spam a system with endless TCP requests. 

It overwhelms the system and keeps it overwhelmed so that regular communication stops working.

It’s a way to make websites, servers, and other remote devices completely inaccessible for their intended purposes. 

So, if a hacker wanted to take down a government server for some reason, a DDoS attack could do it.

Let’s bring this back to the RST, ACK flag. 

That flag pops up with DDoS attacks (depending on how the attack was structured). 

Basically, the attacking device is sending tons and tons of packets with those flags on them. 

So, the attacked system is constantly opening, confirming, and then closing TCP connections, and it can’t keep up with the request rate. 

This specific flag helps IT professionals identify DDoS attacks.

And in case you’re concerned, there are very effective defenses against DDoS attacks

So, if the flags ever reveal that an attack has happened to something of yours, you can invest in DDoS protection, and you’ll be fine moving forward.


  • Theresa McDonough

    Tech entrepreneur and founder of Tech Medic, who has become a prominent advocate for the Right to Repair movement. She has testified before the US Federal Trade Commission and been featured on CBS Sunday Morning, helping influence change within the tech industry.