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https://issues.apache.org/jira/browse/NET-258?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12998359#comment-12998359
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Bogdan Drozdowski commented on NET-258:
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The timeouts don't have to be the same order - if the control timeout is 30
seconds, but the data timeout is 10 minutes, we have the same problem. Yes,
it's not something like 1ms vs. 100 minutes, but generally if the control
timeout is less than the data timeout, there is a problem. Not only with the
socket timeout itself, but the server-side timeout (the higher-level timeout),
too.
I don't see how synchronizing a command and its reply would prevent the NOOPs
from being sent, assuming everything is working fine. The server sends a reply
immediately, so we leave the synchronized method as soon as we get the reply
(which is usually soon). Even for RETR/STOR/LIST/... the server first sends a
150 (which makes us leave __getReply() and sendCommand()) and after the
transfer the server sends a 226. Util.copyStream() (the long-lasting operation)
is called outside of the synchronized block (synchronization is in
_openDataConnection_()). CompletePendingCommand() is left unsynchronized right
now, however.
I'm not saying "no" to sending NOOPs only during transfers. On the contrary,
it's a good solution, because we have everything under control. But the
user-settable and server timeouts make me think that we need something working
independently of the transfer-handling thread.
> Implement A Keepalive Mechanism
> -------------------------------
>
> Key: NET-258
> URL: https://issues.apache.org/jira/browse/NET-258
> Project: Commons Net
> Issue Type: Improvement
> Affects Versions: 2.0
> Reporter: Rory Winston
> Assignee: Rory Winston
> Attachments: ftp-keepalive.diff
>
>
> For routers/firewalls that terminate idle connections, a separate heartbeat
> mechanism may need to be implemented to keep the control connection active.
> Some potential issues:
> * Synchronization between a heartbeat write and a __getReply() on an active
> control connection command;
> * Should use the NOOP command as a heartbeat signal;
> * Make the timeout configurable;
> * Default SocketImpl::setKeepAlive() wont do here.
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