WAN throughput issues

[jonathan johnson]

Hi,

We have a customer who connects to us via metro lan service.  The service they have is 60Mb however when we check the throughput via a local speed test server we find the customer is onl able to get around 14-15Mb.  We've ruled out the CPE and have moved the customer's sub-int at the core side onto different 7200's however the issue remains.

What is strange though is when I run multiple speed tests at the same time I can get the service to operate at full capacity (60Mb).  I'm not confident that the speed test server is at fault as other customers can test against it and get full throughput.

I've removed all policing, checked the mtu (no fragmentation on a 1500 byte packet) checked for duplex and speed mismatches etc.

Does anyone have any ideas that will steer me in the right direction on this one?  I'm a little stumped by this...

Thanks,

Jono

[Joe Lentine]

Any acls or dynamic protocols running?

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[jonathan johnson]

thanks for the reply,

Yes, there is an ACL applied.  I have removed it and tested, but the result remains the same. 

I have ospf and BGP running on the core - I have ospf running only on the inerfaces that require it (about 2 or 3)  I have around 30 or so BGP sessions running on the router where the service connects to. 

In the process of fault finding, I moved the sub-ont where the customer's service hits our network on to another router which is also running ospf and BGP but has almost no load but the outcome is the same.

Thanks,

Jono

[patrick.preuss]

Hi

Are The Tests tcp based? What is The rtt?

What do The routers Tell about queing and packet drops .

You can use a packet caputre to See what is happening.

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[jonathan johnson]

Thanks for the reply Patrick.

Yes, the tests are TCP based - one is a http transfer and the other is FTP (I ran two tests)

We don't see dropped packets and I have increased the queues to 4096 on the router's physical interface.

I was going to do a packet capture, but I wasn't sure what I'd be looking for.  I'll run one as soon as I can to see if anything looks unusual.

Thanks,

Jono

[patrick.preuss]

Hi Jonathan

you can have a look at the "RTT" the ack rtt, the windows scaling and also to the normal throuput graph.

Depending on many factors this can be the Maximum speed per session.

HTH

Patrick

[jonathan johnson]

Hi Partick,

The RTT is around 25ms and is quite consistant.

I did some iperf testing with 64Kb window sizes and the maximum throughput was about 11Mb.  When I increase the window size extreamly high I get better throughput.

I have also put the iperf server on the same vlan as the customer metro LAN service which has effectively removed our router from the data path yet there hasn't been any improvement.  The data path now is:

1. Testing PC running windows

2. Customer CPE

3. Metro LAN network

4. Our switch (2960)

5. iperf server

When doing a speed test to an external site (public speed test server) we see much better throughput... which I find  very curious.

Thanks,

Jono

[Joseph W. Doherty]

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(see: http://www.switch.ch/network/tools/tcp_throughput/index.html?do+new+calculation=do+new+calculation)

Bandwidth-delay Product and buffer size

BDP (60 Mbit/sec, 25.0 ms) = 0.19 MByte

required tcp buffer to reach 60 Mbps with  RTT of  25.0 ms >= 192.0 KByte

maximum throughput with a TCP window of 64 KByte and RTT of 25.0 ms  <= 20.00 Mbit/sec.