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Subject: receive-window collapse on small reads

receive-window collapse on small reads

From: Nathan Myers <ncm_at_cantrip.org>
Date: Wed, 16 Jan 2013 17:31:44 -0800

I have code calling libssh2_channel_read_ex()
<http://www.libssh2.org//libssh2_channel_read_ex.html>, requesting small
reads,
typically < 100 bytes. I notice that the receive window size collapses
very quickly, resulting in timeouts when on a high-latency/high-loss
connection, and otherwise running much more slowly than expected.

The attached patch to examples/ssh2_exec.c illustrates the problem.
Manually updating the window, as shown in the #if 0 block, seems
to stabilize the window size, providing a larger window for worse
connections.

Am I using it right? This code in libssh2_channel_read_ex (src/channel.c)
seems to be at fault:

     if(buflen > recv_window) {
         BLOCK_ADJUST(rc, channel->session,
                      _libssh2_channel_receive_window_adjust(channel,
buflen,
                                                             1, NULL));
     }

When buflen is small, recv_window can never get large enough to be useful,
even when libssh2's (256K) input buffer is empty. Is there a recommended
way to keep the window from collapsing, or should clients not be doing
small reads?

This failure did not occur using libssh2-1.2.5. Running the attached code
(altered just enough to be) linked with 1.2.5 does not exhibit the problem.

Nathan Myers
ncm_at_cantrip.org

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Received on 2013-01-17

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