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https://issues.apache.org/jira/browse/NET-258?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12998429#comment-12998429
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Sebb commented on NET-258:
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I've been looking at the code in more detail - unfortunately, __getReply() is
not thread-safe, because shared variables are updated without any
synchronisation.
This would need addressing if a separate thread was to be used.
However, during testing of the listener approach I came across another issue,
which is that not all FTP servers support concurrent access to the control port
during data transfer. They are supposed to, but not all do. If one sends a NOOP
(or STAT, which is intended for use during transfer, see RFC 1123 / 4.1.3.3),
it will be sent, but the reply won't arrive until the end of the transfer, and
in the case of the listener approach this means the transfer will hang. Even
using a separate thread won't help here, as the thread will hang waiting for
the first reply. I don't think it's sensible to send the NOOP/STAT without
waiting for a reply.
So I think a different approach needs to be used - something that tells the
router that the control connection is active, without needing a response from
the server.
> 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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