Packet Capture – TCP Flags

In the context of networking and the TCP (Transmission Control Protocol), RST (Reset) and ACK (Acknowledgment) are flags within the TCP header that are used for different purposes. Let’s discuss these flags and the typical data flow when an RST, ACK flag combination is captured.

1. Establishment of Connection:

Normally, before any RST or ACK flag is seen, there’s a 3-way handshake to establish a TCP connection. This is done using the SYN (synchronize) and ACK flags. The client sends a SYN packet, the server responds with a SYN, ACK packet, and finally, the client sends an ACK packet.

2. Normal Data Transfer:

Once the connection is established, data is exchanged between the client and server using TCP segments with the ACK flag set.

3. RST, ACK Captured:

The RST flag is used to reset the connection. When a packet has the RST flag set, it indicates that something has gone wrong and the connection should be terminated immediately.

The combination of RST and ACK flags is usually seen when a party wants to both acknowledge the receipt of data and reset the connection. 

This may occur in scenarios where:

  • The application at one end of the connection has been closed or has crashed.
  • There’s a problem with the TCP stack.
  • A packet was received that wasn’t expected.
  • An attempt to connect to a port that is closed.

4. Connection Termination:

When the other party receives the RST, ACK packet, it knows that something has gone wrong. It terminates the connection and no further data is exchanged.

5. Analysis (if captured for that purpose):

It’s important to note that the RST, ACK packet is not part of the normal graceful TCP connection teardown, which involves using FIN flags. The RST flag is a more abrupt way of closing a connection and often indicates an error or issue with the connection.



More examples of scenarios where a RST, ACK might be captured include:

1. Unexpected Data: If an endpoint receives unexpected data on a connection, it may send a RST, ACK to immediately terminate the connection. This can be because of desynchronization in the state of the connection between the two endpoints.

2. Firewall/Security Device Intervention: Sometimes, a firewall or another security device in the network might send a RST, ACK packet to forcibly close a connection it deems unauthorized or harmful.

3. Resource Constraints: When a system is under heavy load, it might not be able to handle all incoming connections, and as a result, it might use RST, ACK packets to close connections forcibly.

4. Refusal of Connection: If a client tries to connect to a server on a port that is not open, the server will respond with a RST, ACK to indicate that the connection cannot be established.

5. Keepalive Timeout: Some TCP implementations use keepalive packets to determine if a peer is still available. If it does not receive a response within a specific time frame, it might decide that the peer is unreachable and send an RST, ACK to tear down the connection.

6. Application Layer Error: If an error occurs at the application layer (e.g. HTTP, FTP) and the application decides it can’t recover from the error, it might instruct the TCP layer to send a RST, ACK to quickly terminate the connection.



Regarding a graceful TCP connection teardown, it involves the following steps:

1. Initiation: One end of the connection (let’s say the client) decides to close the connection. It sends a TCP packet with the FIN flag set, indicating it is finished sending data.

2. Acknowledgment of the FIN: The server receives the FIN packet. It sends back an acknowledgment for the FIN by sending a packet with the ACK flag set.

3. Server’s FIN: The server will then also decide to close the connection and sends a TCP packet with the FIN flag set.

4. Acknowledgment of the Server’s FIN: The client receives the server’s FIN packet. It sends back an acknowledgment for the FIN by sending a packet with the ACK flag set.

This sequence of steps is known as the “FIN, ACK” sequence and is a graceful connection teardown because both ends of the connection are aware that the connection is being closed and have an opportunity to ensure all data is transmitted before the teardown is complete. This is in contrast to the more abrupt termination of a connection when an RST packet is used.



Yes, a RST, ACK sent from a client on a Windows 11 PC back to the server could still be network-related, but it could also be due to various other factors. Here are some possibilities:

1. Application Layer: The application running on the client might have encountered an error or terminated unexpectedly, and as a result, the underlying TCP stack sends a RST, ACK to the server.

2. TCP/IP Stack: The TCP/IP stack on the Windows 11 PC might have received data that it did not expect or that was not in the correct state, and it responded by resetting the connection with an RST, ACK.

3. Security Software: Security software on the Windows 11 PC, such as a firewall or antivirus, might have detected something in the connection that it considers a threat or violation of policy, and forcibly closed the connection with a RST, ACK.

4. Resource Exhaustion: The Windows 11 PC might be running out of resources (memory, CPU, etc.) and the TCP/IP stack could send an RST, ACK as it can’t handle the connection at that time.

5. Configuration or Policies: There might be specific configurations or group policies applied to the Windows 11 PC that are causing the connection to be reset under certain conditions.

6. Network Issues: There could be network issues, such as latency, jitter, or packet loss, that are causing the client to believe the connection is no longer viable and to send an RST, ACK.

7. Port Unavailability: If for some reason the client believes that the port it is connected on is no longer available or should not be connected to, it can send an RST, ACK.

Troubleshooting a TCP RST, ACK can be complex as it might involve analyzing logs from the application, the system, and potentially capturing and analyzing network traffic to determine the exact cause. Additionally, it’s important to consider any recent changes that might have affected the behavior of the connection.



Example 1: Normal TCP Connection (Establishment and Graceful Termination)
1. Client -> Server (SYN)
2. Server -> Client (SYN, ACK)
3. Client -> Server (ACK)
… Data exchange happens here with ACKs going back and forth …
4. Client -> Server (FIN, ACK)
5. Server -> Client (ACK)
6. Server -> Client (FIN, ACK)
7. Client -> Server (ACK)

Steps 1-3 are the 3-way handshake for connection establishment.
Steps 4-7 are the normal connection termination. The client initiates the close, the server acknowledges this, and then the server also closes the connection which is acknowledged by the client.

Example 2: TCP Connection with RST, ACK Interruption
1. Client -> Server (SYN)
2. Server -> Client (SYN, ACK)
3. Client -> Server (ACK)
… Data exchange happens here …
4. Client -> Server (RST, ACK) or Server -> Client (RST, ACK)

Steps 1-3 are the 3-way handshake for connection establishment.
At step 4, either the client or the server sends an RST, ACK packet to abruptly close the connection due to some error or issue. This could be due to one of the reasons mentioned in the previous responses (e.g., unexpected data, firewall intervention, resource constraints, etc.). The connection is terminated immediately upon receipt of the RST, ACK without the normal graceful close process.

Please note that these examples are simplified representations of the flags in TCP headers during a connection’s lifecycle. In an actual packet capture, you would see more details such as IP addresses, port numbers, sequence numbers, and so on. Tools like Wireshark can be used to capture and analyze network traffic in detail.