The sending user indicates in each SEND call whether the data The data that flows on a connection may be thought of as a stream of TCP implementation has not yet provided the data it's received to theĬode that's reading the data (program, or library used by a program), PSH is an indication by the sender that, if the receiving machine's Side will contain an ACK, even if it's just re-acknowledging data that TCP, once the connection is established, all packets sent by either (Also See ServerFault - PHA ACK During MyĪCK means that the machine sending the packet with ACK isĪcknowledging data that it had received from the other machine. What is usually the cause of such behaviour? (if there is a 'usual' What does a sequence of retransmissions with PSH,ACK flags mean (and a spurious retransmission back)? Note 2: rebooting the server (also probably restarting the script itself (which I do not do as I rather reboot the machine)) fixes the problem.

Note 1: If needed, I will modify the API so that it logs more data for the webserver part but due to the non-reproducible nature of the hang, I doubt that it is its fault (the other pieces (threads) work great and there is no crash of any thread). What is usually the cause of such behaviour? (if there is a 'usual' cause). I ran tcpdump on 192.168.0.1 (the receiving server, the one which hosts the API) when the API was non responsive and the wireshark analysis shows some retransmissions right after the call is initiated: The HTTP listening part sometimes mysteriously hangs, which results with the curl call above hanging, then timing out. The API on 192.168.0.1 (which I am calling) is mine (a Python script based on bottle) and works most of the time.

I am on server 192.168.0.2 and want to make an HTTP call to 192.168.0.1(both servers are RPis and run Linux (raspbian)).

Tcp retransmission wireshark