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From Jayanga Dissanayake <jsdjaya...@gmail.com>
Subject Connection termination issue (sometimes connections are not closed after receiving [FIN, ACK] from server)
Date Thu, 09 Oct 2014 11:13:17 GMT
Hi All,

We are using WSO2ESB [1], and we want to communicate with a web server via
HTTPS. While we were trying to communicate with the web service, we
observed some weired behavior in the client side, when we analyzed the
tcpdump, found that, client sometimes try to send application data after
the [FIN, ACK] is received from the server, and server sends a [RST] after
we send application data. (Here, in this case, my end point is a HIS server)

As I know, according to the TCP spec, its allowed to send data via a half
closed connection. But in this case I want to stop sending data,
immediately after detecting the half closed connection. So, tried to put
debug points at endOfInput(), in all the implementation classes found in
synapse.transport and apache.http,  and see whether I can detect the
connection close from the server end. The break points were hit at the
times when the connection termination happens properly. But not at the
times when application messages were sent after the [FIN, ACK].

It seems like the connection termination from the remote end is not
detected. Please read the below for more information.

Once we create the connection with the back end service, response header
looks,

[2014-10-09 13:52:09,516] DEBUG - wire >> "HTTP/1.1 200 OK[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "Date: Thu, 09 Oct 2014 08:22:07
GMT[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "X-Powered-By:
Servlet/3.0[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "SOAPAction: ""[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "Accept: text/xml[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "Keep-Alive: timeout=10,
max=100[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "Connection: Keep-Alive[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "Transfer-Encoding:
chunked[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "Content-Type: text/xml[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "Content-Language: en-US[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "[\r][\n]"
[2014-10-09 13:52:09,516] DEBUG - wire >> "20b[\r][\n]"

As indicated in the Keep-Alive field, connection termination initiated from
the server end, after idle time of 10 seconds,

1206 72.626148 [BackEndServerIP] [MyIP] TCP 60 https > 59949 [FIN, ACK]
Seq=5230 Ack=2024 Win=19024 Len=0

As a response it sent an ACK to the server,

1207 72.665189 [MyIP] [BackEndServerIP] TCP 54 59949 > https [ACK] Seq=2024
Ack=5231 Win=24840 Len=0

After that it sends application data to the server,

1940 120.137122 [MyIP] [BackEndServerIP] TLSv1 443 Application Data
1941 120.137419 [MyIP] [BackEndServerIP] TLSv1 1275 Application Data

Then server sends a RST,

1949 120.182599 [BackEndServerIP] [MyIP] TCP 60 https > 59949 [RST]
Seq=5231 Win=0 Len=0

Note: there is a considerable time gap between ACK and first application
data packet (72.665189 – 120.137122)

1. My first questions is, why sometimes the client end sends just a ACK for
the server side [FIN, ACK]. If client can send a [FIN, ACK] and gracefully
terminate the connection, my problem is solved.
2. As we have problem (1), Is there anyway to detect partially closed
connection, so that we can call close() or shutdown() to close the client
end.

Appreciate if someone can answer my questions.

[1] http://wso2.com/products/enterprise-service-bus/

Thanks,
Jayanga.

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