![]() ![]() Receiving TCP sees the PUSH flag, it must not wait for more data from Send that data in segments at its own convenience, until the pushįunction is signaled, then it must send all unsent data. Through to the receiving user by the setting of the PUSH flag.Ī sending TCP is allowed to collect data from the sending user and to In that call (and any preceeding calls) should be immediately pushed ![]() 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. If the recipient should empty its receive buffers at all (in other words, the application makes even a partial pickup), it will announce the new “space available” with a TCP Window Update.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)). Also, it might be that the application does not pick up the packets in a timely fashion from the TCP buffer. Or it could be that there is an error in the TCP receiver. It could be that the machine is running too many processes at that moment, and its processor is maxed. This means that the machine is not able to receive further information at the moment, and the TCP transmission should be halted until it can process the information that is pending in its buffer. TCP Zero Window is when the Window size in a machine remains at zero for a specified amount of time. ![]() If you want to filter on TCP duplicates use this Wireshark filter: These are called fast retransmissions.Ĭonnections with more latency between the client and server will typically have more duplicate acknowledgment packets when a segment is lost. In most cases, once the sender receives three duplicate acknowledgments, it will immediately retransmit the missing packet instead of waiting for a timer to expire. They are a common symptom of packet loss. Typically, duplicate acknowledgments mean that one or more packets have been lost in the stream and the connection is attempting to recover. Most packet analyzers will indicate a duplicate acknowledgment condition when two ACK packets are detected with the same ACK numbers. If you want to filter on TCP transmissions use this Wireshark filter: Above you can see that after more than 1s a frame get’s sent again. ![]()
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