Re: SFTP packet size limited to 16k

jussi.mononen-asdf@xxxxxxxxxxxxxxxx wrote:
Hi,

I've been wondering why I can not receive larger packets
than 16 kbytes at a time, even though I have set my
applications window size to 137072 bytes and max packet
size to 32768 bytes.

My application is using libssh2 0.12 and I'm interacting
with OpenSSH_4.2p1, OpenSSL 0.9.7f 22 Mar 2005

OpenSSH agrees with my window size:

debug1: server_input_channel_open: ctype session rchan 0 win 131072 max 32768

but I still get only 16k packets:

debug1: subsystem: exec() /usr/libexec/openssh/sftp-server
debug2: fd 8 setting O_NONBLOCK
debug3: fd 8 is O_NONBLOCK
debug2: channel 0: rcvd adjust 16195
debug2: channel 0: rcvd adjust 16128
...

Hi again,

all tips directing to new directions would be greatly
appreciated. I've spent numerous hours debugging OpenSSH
ans libssh2 without any luck. There doesn't seem to be
any suspicious code concerning packet size in either of
them.

I've added lots of debug prints while trying to find a
place where the packet could be splitted. Sftp-server
fills the output queue properly with 32k packets and it
even calls the send_data -function with proper length.

My application sends SSH_FX_READ msg's with a request of
32k bytes, it is honored by the sftp-server as my debug
shows, but someone splits the packet after sftp-server
has dealt with it (I can see that sftp-server writes 32k
to its stdout).

Is it possible that something is restricting the pipeline
between sshd and sftp-server (sounds very unlikely as the
write size to stdout is the buffer size)?

Can the channel make conscious decisions of the packet
size? Or where could this packet split happen?

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