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multiple concurrent connections

Posted on 2011-09-21
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Last Modified: 2012-05-12
When transferring with the ftp protocol, why is downloading multiple files, faster than downloading a singe file. What is this limitation called and is it possible to change it? Is it caused by TCP windowing?

Example:

Downloading one 4.7GB file transfers at 1.2MB per second

Downloading twenty 40 meg files transfers at 48MB per second
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Question by:stratech
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6 Comments
 
LVL 4

Expert Comment

by:AnthonyHamon
ID: 36574277
I experienced a similar problem, which was caused by an incorrect speed/duplex setting between the FTP server and the switchport it connects to OR the FTP client and the switchport it connects to.  I suggest that you check this first.
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Accepted Solution

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AlexPace earned 250 total points
ID: 36574325
Some FTP clients will automatically spawn additional FTP sessions when you initiate a multi-file download.  This is great if the bandwidth of each single connection is throttled and also if the remote server is configured to support it.

These clients may report the aggregate data transferred as a single large rate instead of 20 smaller rates.  Also, if the client is using inline compression, remember that some types of files are more compressible than others... for example image formats that are already compressed won't compress as well as plain text log files.  The FTP client will almost certainly claim the rate of the inflated file rather than the number of compressed bytes actually transferred over the wire.
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LVL 12

Expert Comment

by:Rant32
ID: 36574351
Sounds to me like per-connection rate limiting at the end of the FTP server (or even a client setting, if you're using a graphical FTP client).

If your connection can handle 48 MB/s then there is no other obvious reason for a single transfer to be that much slower.
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Author Comment

by:stratech
ID: 36574813
Both endpoints have no firewall. The switching has no QOS enabled. Do most ISP's use rate limiting to control saturation? This is something I've observed in every situation were I have allot of bandwidth.
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LVL 16

Expert Comment

by:AlexPace
ID: 36576613
Some ISPs promote the "peak" bandwidth but that is only available for short bursts and long transfers are throttled.  If you really MUST know what is happing and your FTP client software does not produce a sufficient log file then you could use a packet sniffer like WireShark to see if the FTP client is actually opening multiple parallel sessions.
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Author Comment

by:stratech
ID: 36583782
I know what is happening to my FTP client... Its being rate limited per connection. If I use an FTP client that makes multiple connections it bypasses the rate limiting... I am interested in not being rate limited. Would MPLS help me with this situation (If I own both endpoints?)
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