TCP session refused immediately after accepting it

robocat

8/24/24, 1:53 PM

Host_A tries to send some data to Host_B over TCP. Host_B is listening on port 4712. Host_A is a remote PC connected on a dedicated WAN. Host_B is a local embedded device based on embOS OS, connected to the WAN throuth a pair of redundant (HSRP) CISCO routers.

When Host A tries to establish a TCP session, even through a simple telnet command, Host B sends initially an [ACK,SYN] followed by a [RST].

Host A (client) --> Host B (server) SYN
Host B (server) --> Host A (client) SYN-ACK
Host B (server) --> Host A (client) RST-ACK
Host A (client) --> Host B (server) ACK

This happens about 8 times out of ten. The rest of the times, instead, the session opens and protocol starts.

I am wondering under what circumstance does a TCP listener send [RST,ACK] after sending [SYN,ACK] in response to a [SYN]?

Note: this happens only when connecting through redundant CISCO routers (virtual MAC and virtual gateway IP), otherwise everything works 100% fine all the time.

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tcp

rst

session

hsrp

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EN: TCP session refused immediately after accepting it

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