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Re: Bad transfer performance with FTP on high latency links

Thanks for your post.
WinSCP sets SIO_IDEAL_SEND_BACKLOG_QUERY. Which should actually fix this problem. So it has no effect in your system?

Please attach a full session log file showing the problem (using the latest version of WinSCP).

To generate the session log file, enable logging, log in to your server and do the operation and only the operation that causes the error. Submit the log with your post as an attachment. Note that passwords and passphrases not stored in the log. You may want to remove other data you consider sensitive though, such as host names, IP addresses, account names or file names (unless they are relevant to the problem). If you do not want to post the log publicly, you can mark the attachment as private.

Bad transfer performance with FTP on high latency links


We are using WinSCP as FTP-client and found it very useful. Thank you.

However, some of the server to which we upload are connected with large bandwidth (>100mbit/s) links, but which have high latency (>100ms). This is generally known as "long fat pipes" [1].

TCP has had multiple updates to accommodate this kind of links: new congestion mechanisms, window scaling, selective ACK, etc. New operating systems (e.g. Windows Vista and later) automatically use all these when CTCP is enabled (netsh interface tcp set global congestionprovider=ctcp), but also the application must have large TCP-buffers in use to fully benefit from this.

Would it be possible to have the TCP buffer sizes configurable, or at least hard coded to sufficiently large: 2 or 4 megabytes. I have actually patched the winscp.exe binary, to use larger TCP-buffers and fixed everything.

The patching is equivalent to modifying filezilla/TransferSocket.cpp, there's two parts that do SetSockOpt(SO_SNDBUF). However, the buffer size you pass there (2*BUFSIZE)=(2*16384)=32kB. However, that is far too low for "long fat pipes" such us ours. The buffer should be 2-4 megabytes.

Kind Regards,