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[lwip-users] TCP state machine problem? LWIP 1.4.1

From: Fabian Koch
Subject: [lwip-users] TCP state machine problem? LWIP 1.4.1
Date: Fri, 8 Mar 2019 14:24:41 +0000

Hey all,


we had some weird behavior with a TCP connection on LWIP 1.4.1 when the peer (non-LWIP) has a cable disconnect:


  • LWIP has an established TCP connection #1 running fine
  • Peer has a cable disconnect
  • Our application on top of LWIP runs into a receive timeout and closes the socket (500ms)
  • Peer reconnects cable
  • Our application opens a new connection #2 which again is established and running fine
  • The FINACK+PSHACK re-sends of connection #1 also reaches the peer which answers RSTACK
  • This keeps on looping until we restart the whole machine with LWIP


Also, I have a sort of “netstat” implemented on top of the LWIP socket API which runs over all possible sockets we have and if it finds a valid conn pointer there, prints infos (local addr, remot addr, port, TCP state and such). And connection #1 does not show up anymore in this view!


In my mind, the TCP state machine should be in FIN_WAIT_1 while the peer cable is disconnected?

And it should just jump to either CLOSED or TIME_WAIT when receiving the RSTs upon cable reconnect?


I attached a clipped pcap with only connection #1 shown and the problem starting at packet #19. Image the final exchange going on forever to understand the problem ;o)


Any comments or debugging ideas appreciated.


Kind regards



Attachment: LWIP_1_4_1_TCP_state.pcapng
Description: LWIP_1_4_1_TCP_state.pcapng

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