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The question is fairly simple, and maybe a bit remedial to most reading this, but I've been searching the web for something definitive about this point.

I understand that the RST flag is intended to terminate a TCP connection immediately, and that so far as the sending host is concerned, the conversation is over the moment the RST or RST/ACK packet is sent. But do hosts that receive that RST or RST/ACK packet "honor" it on the spot by not sending any acknowledgment of the RST?

In other words, do hosts immediately drop the conversation upon receipt of a RST or RST/ACK packet? Any exceptions?

  • The answer to "Any exceptions?" for almost every situation is "Yes", perhaps you want to word that part a bit differently. – tlund Apr 4 '16 at 22:28
  • @Ron Maupin has provided the RFC excerpt below but also note that it's really down to the individual OS or network stack within the OS as to how they implement the TCP RFC. For example, nmap uses the scanning of TCP port 0 to help identify which OS an IP address might be assigned to since different operating systems respond different to connection requests to TCP port 0, as the vendors implement their own interpretation of the TCP RFC. – jwbensley Apr 5 '16 at 11:58
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RFC 793, TRANSMISSION CONTROL PROTOCOL explains, in detail, how TCP works:

Reset Processing

In all states except SYN-SENT, all reset (RST) segments are validated by checking their SEQ-fields. A reset is valid if its sequence number is in the window. In the SYN-SENT state (a RST received in response to an initial SYN), the RST is acceptable if the ACK field acknowledges the SYN.

The receiver of a RST first validates it, then changes state. If the receiver was in the LISTEN state, it ignores it. If the receiver was in SYN-RECEIVED state and had previously been in the LISTEN state, then the receiver returns to the LISTEN state, otherwise the receiver aborts the connection and goes to the CLOSED state. If the receiver was in any other state, it aborts the connection and advises the user and goes to the CLOSED state.

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