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Slow transfer of large files via VPN


I have 2 sites one in the US and one in the UK.

Both sites have Gigabit Internet connection.

Firewalls are ASA devices.

We have a VPN connections between the 2 sites.

The ping turnaround time between 2 servers are 100 - 110 msec.

We do log file replication between 2 windows 2003 servers, one at each site, using Robocopy.

Problem is that the actual transfer speed cannot exceed 800 kbps.

Any ideas what I should look for ?




Re: Slow transfer of large files via VPN

Hello Erik,

From a application layer perspective, are you seeing TCP retransmissions?

From the network layer perspective, are you seeing any abnormalties in the logging of your firewall. For instance MSS exceeds.

What ASA model do you have in place. Can you give more details on the topology.



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Re: Slow transfer of large files via VPN

The ASA's are 5520 in a failover configuration. LAN are dualized switches. Unfortunately I cannot include any drawings.

We will look for any information in the log files.



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Re: Slow transfer of large files via VPN

Personally, I would look for out of order packet receipt. We've seen this increase steadily over the past two years with Internet traffic, in particular with international links.

The issue is that packets are being delivered with the order mixed up. Packets 1 through 10 are being received 2 6 4 1 5 7 8 3 10 9 (example). This causes a problem with TCP flows, and will normally choke transfer speeds to practically nothing (similar to what you are seeing). If you run a sniffer on the traffic to and from your servers, you should see the packets being received out of order, and large numbers of TCP Fast Retransmits being issued from the the receiving system.

I don't know if the carriers are doing this deliberately, or there is some other explanation, but it's a difficult situation to correct for, and Cisco doesn't have a product that will do it. Other companies do have products which will reorder packets on the receiver end.

One benefit of this is that it chokes Bittorrent down to nothing.

You -will- need a sniffer to properly diagnose this issue. If you don't have a professional network sniffer, Ethereal does the job in a pinch.


Re: Slow transfer of large files via VPN


there are two things I would look for first. One could be MTU related issues, which would result in retransmissions. on a Cisco router those can be sorted out with the command "ip tcp adjust-mss 1300".

The second thing I would look for are TCP related problems, besides reordering mentioned above, also window size.

TCP throughput is approx. (TCP window size)/(round trip time). Given the numbers from your post RTT=0.11 sec and throughput 800000 bps one would get a window size of about close to 16 kByte, which is default on some Microsoft operating systems. Make sure you get at least 64 kByte or more to allow for higher throughput. The calculation only holds, when there are no dropped packets. Retransmissions lead to a reduced window size (half of previous value) and therefore reduced throughput.

Regards, Martin

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