/ˌɑːr-ɛs-ˈtiː/
n. “The TCP reset signal that ends a connection abruptly.”
RST, short for Reset, is a flag in the TCP (Transmission Control Protocol) header that indicates an immediate termination of a TCP connection. When a device sends a packet with the RST flag set, it signals that something went wrong or that the connection should be closed immediately without following the usual graceful teardown process.
Key characteristics of RST include:
- Immediate Termination: Forces the connection to close without completing the normal FIN/ACK handshake.
- Error Handling: Often sent when a connection attempt is made to a closed port, or if one side receives unexpected data.
- No Data Delivery Guarantee: Any in-flight data may be lost because the connection is aborted immediately.
- Part of TCP Flags: Alongside SYN, ACK, FIN, PSH, URG, and ECE, the RST flag is used to control and manage TCP connections.
A simple conceptual example of when an RST might occur:
Client: SYN → Server (wants to open connection)
Server: RST ← Server refuses connection (port closed)Conceptually, RST is like slamming the phone down mid-call — the connection ends immediately, signaling an error or refusal to continue.
In essence, RST is a critical TCP mechanism used to handle unexpected or invalid connections, providing a way for devices to quickly abort communication when necessary.