How can communication continue even after RST,ACK (ECONNABORTED)?

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Hello,

I'm troubleshooting an issue with our application's ftp client, which are having timeout issues on the FTP control channel while transferring large files. We have tried to setup a keep-alive mechanism which sends NOOP messages on the control channel periodically, but this seem to cause other issues with FTP servers that ignores messages on the control channel while there is an active transfer on the data channel.

We know that FileZilla manages to upload these large files, so what I did was to monitor traffic using WireShark while uploading a file using FileZilla. The results are a bit strange:

After 1 hour of transfer the server sends a RST,ACK message, closing the control channel. I can even see in FileZilla that it says: "Disconnected from server: ECONNABORTED - Connection aborted". There is no response from the client on this message though. Then about 1 hour later when the transfer finishes it reads the 226 Transfer complete message from the server - on the same control channel, without reconnecting. How does it manage to do that?

You can also see that FileZilla seem to use SO_KEEPALIVE to keep the control channel active during the transfer, and not the NOOP method.

Excerpt from Wireshark, filtering on port 21:

67  38.382093   192.168.1.103   xxxxxxxxx   FTP 60  Request: PASV
68  38.403053   xxxxxxxxx   192.168.1.103   FTP 107 Response: 227 Entering Passive Mode (xxxxxxxxx).
69  38.403466   192.168.1.103   xxxxxxxxx   FTP 69  Request: STOR test.bin
70  38.424603   xxxxxxxxx   192.168.1.103   FTP 96  Response: 150 Opening BINARY mode data connection.
71  38.465021   192.168.1.103   xxxxxxxxx   TCP 54  58981 → 21 [ACK] Seq=118 Ack=408 Win=262144 Len=0

[...]

84  3627.210719 192.168.1.103   xxxxxxxxx   TCP 55  [TCP Keep-Alive] 59549 → 21 [ACK] Seq=168 Ack=823
85  3627.231802 xxxxxxxxx   192.168.1.103   TCP 66  [TCP Keep-Alive ACK] 21 → 59549 [ACK] Seq=823 Ack=169

[...]

86  3634.081473 xxxxxxxxx   192.168.1.103   TCP 60  21 → 59549 [RST, ACK] Seq=823 Ack=169 Win=0 Len=0
[FileZilla: Disconnected from server: ECONNABORTED - Connection aborted]

87  3638.504643 192.168.1.103   xxxxxxxxx   TCP 55  [TCP Keep-Alive] 58981 → 21 [ACK] Seq=117 Ack=408
88  3638.525476 xxxxxxxxx   192.168.1.103   TCP 66  [TCP Keep-Alive ACK] 21 → 58981 [ACK] Seq=408 Ack=118

[...]

95  6478.177713 xxxxxxxxx   192.168.1.103   FTP 78  Response: 226 Transfer complete.
96  6478.206762 192.168.1.103   xxxxxxxxx   FTP 60  Request: PASV
97  6478.227939 xxxxxxxxx   192.168.1.103   FTP 107 Response: 227 Entering Passive Mode (xxxxxxxxx).
98  6478.228310 192.168.1.103   xxxxxxxxx   FTP 60  Request: LIST
99  6478.249480 xxxxxxxxx   192.168.1.103   FTP 96  Response: 150 Opening BINARY mode data connection.
100 6478.267052 xxxxxxxxx   192.168.1.103   FTP 78  Response: 226 Transfer complete.