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SCP is slow in our company network. For testing, I am copying a file from a remote server which is connected via IPsec between the routers.

When using scp -vv I get lots of messages like this:

debug2: channel 0: window 1966080 sent adjust 131072
debug2: channel 0: window 1966080 sent adjust 131072
debug2: channel 0: window 1966080 sent adjust 131072
debug2: channel 0: window 1966080 sent adjust 131072

I couldn’t find out the meaning of this message. Mostly, people report this message appearing while SCP stalls. In my case, I am not experiencing scp stalls. It simply is very slow (350kb/s down) when there should be 1000 to 2000 kb/s down, according to the other network‘s operator.

What do these messages mean? Do they have anything to do with my poor SCP performance?

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fqxp
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    Possible duplicate of [What does limit performance of scp?](http://serverfault.com/questions/207164/what-does-limit-performance-of-scp) – Jakuje Feb 25 '16 at 10:18
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    @Jakuje: No, at least the accepted solution to that question doesn’t solve my problem, the original poster doesn’t use IPsec and doesn’t mention the above messages. – fqxp Feb 25 '16 at 10:33
  • he does not mention above messages because he does not run `scp` with `-vv` options. `scp` is slow especially on long latency lines. It is simple tool. If you need more performance, you should give a try `sftp`, `rsync` or some non-encrypted connection (FTP?), if the line inside IPSec is trusted. – Jakuje Feb 25 '16 at 10:36
  • @Jakuje, sftp is exactly as slow as scp, as it also uses an SSH tunnel. Thanks for the hint about high latency, hadn‘t checked this before. Edited my question: also asking for the meaning of the above messages in this context. – fqxp Feb 25 '16 at 11:01

1 Answers1

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The problem you see is in underlying network. Not in scp itself.

  1. It can be related to your network link quality, may be you're using wifi? Check signal quality.

  2. It can be related to your IPSec connection, for example, MTU problems. Try to lower the MTU setting of your VPN connection to 1450 or even 1400.

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