Hi ymnk,
If I use JSch to open a reverse tunnel (session.setPortForwardingR) to a
Windows VNC server, when I connect the VNC client to the forwarded port,
the performance is very choppy. The same setup with a normal forward
(session.setPortForwardingL) performs fine. When I use the OpenSSH
command line client to setup the same reverse tunnel (ssh
-R5900:localhost:5900 ...), the performance is also very good.
After investigating this issue, I discovered that the performance was
good with the native SSH client because it was allocating a pseudo
terminal (pty) to display the remote command shell. When I instructed
the SSH client NOT to allocate a pty (ssh -N -R5900:localhost:5900 ...),
I was able to duplicate the same poor performance I saw with JSch.
Apparently, if there is no pty assigned to the remote session, the SSH
server will buffer and "chunk" the data over all reverse forwards on
that session. To test this theory, I hacked up my application with the
following code prior to setting up the reverse forward:
Channel hackChannel = session.openChannel( "shell" );
hackChannel.setInputStream( new ByteArrayInputStream( new byte[ 100
] ) );
hackChannel.setOutputStream( System.out );
hackChannel.connect( );
Thread.sleep( 2000 );
hackChannel.disconnect( );
Lo and behold, my VNC performance was then excellent. Obviously, this is
not an acceptable solution for my application. I am wondering if you
have any ideas on a cleaner, more permanent solution. Do you know if
there is a way I can programmatically instruct the SSH server to disable
buffering over these channels?
I appreciate any input you would have on this issue.
Thanks.
- Scott Smith
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