Wan speed vs Latency

[jjjgroot]

Hi all,

I have a question about WAN internet speed connections.

My Dutch company has a 10Mbit WAN connection with Hong Kong.

Ping time <-> 206ms. But my copy speed HKserver2NLserver is only around 47Kbit/s

I now it will not be 10Mb/s but only 47Kb is a bit slow, so the question:

_What speed should be normal?

_Aldo the latency is high, this should not slow down an 800Mbytes copy speed, or does it?

Thx, Joop

[Steve Fuller]

Hi,

TCP performance is very much affected by network latency, in that with all other things being equal, the higher the latency the lower the throughput.

What you're experiencing here is known as a Long Fat Network i.e., a network with high bandwidth and high latency. When you transfer data using TCP over this type of network you'll generally see poorer performance than you might expect.

To maximise performance over this type of network you have to consider the amount of data in that' "in flight" at any point in time which is known as the Bandwidth-Delay Product. This is discussed at length on various sites with the Enabling High Performance Data Transfers at the Pittsburgh Supercomputing Center being a good place to start.

There are things that can be done to improve the situation such as tuning the TCP buffer sizes etc. Again you can find details of this for various operating systems at the Detailed procedures for system tuning under various operating systems page at PSC.

If you want to get an idea of the throughput you should achieve for some specific RTT and TCP buffer size, the TCP throughput calculator at Switch is useful.

I just put the numbers you quoted through and it reports a TCP buffer size of 266Kbyte is required to achieve 10Mbps. A number of standard OS only have 64KByte buffer, and if that's what you have the throughput could be as low as 2.4Mbps.

BDP (10 Mbit/sec, 208.0 ms) = 0.26 MByte

Required tcp buffer to reach 10 Mbps with RTT of 208.0 ms >= 266.2 KByte

Maximum throughput with a TCP window of 64 KByte and RTT of 208.0 ms <= 2.46 Mbit/sec.

Regards

[jjjgroot]

Thx Steve,

For explaining and the info links!

[Joseph W. Doherty]

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Posting

I just put the numbers you quoted through and it reports a TCP buffer size of 266Kbyte is required to achieve 10Mbps. A number of standard OS only have 64KByte buffer, and if that's what you have the throughput could be as low as 2.4Mbps.

BTW, some older TCP stacks not only might not support TCP buffers beyond 64KB but often don't default to even that large.

As Jeff notes in his posts, if using Microsoft CIFS/SMB, it too has its own struggles with LFNs.

Insuring buffers support BDP will often allow large file transfers to utilize full link bandwidth, but another issue with distance based latency, it also slows how fast flows can "speed up".  Small data transfers might never take advantage of the possible bandwidth.

The two solutions to dealing with large distance based latency are either provide/use local resources (i.e. avoid the problem) or use a WAN accelerator (again mentioned by Jeff) that will "spoof" ACKs.

[Jeff Van Houten]

If you're copying with cifs/smb I'd also try FTP and see if it performs better.

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[Jeff Van Houten]

You might also want to look at wan accelerators.

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