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New Member

Data Transfer Rate Limited to about 12M over a full DS3 link

I have a full DS3 at both locations (Corp HQ and DR Site). I recently moved the DR servers to DR location in Dallas Tx and noticed the data transfer rate between the two locations is limited to only about 12M per second per session between hosts at both sites where I have a full DS3 between the two sites. And I can saturate the WAN link by creating multiple sessions between the two sites but the transfer rate for each session can only go up to about 12M per second.

Can someone help ? or has anybody ran into this issue before ?

Any helps/inputs will be greatly appreciated !!!

Thanks

Danny

8 REPLIES
Silver

Re: Data Transfer Rate Limited to about 12M over a full DS3 link

Router platform plays a major role in determining the bandwidth.For example 2650 platforms is not really good to handle full rate DS3.Cisco 3661 series and above supports full rate bandwidth.Refer the following URL for configuration section

http://www.cisco.com/en/US/products/hw/switches/ps718/products_configuration_guide_chapter09186a00800f01b5.html#wp1063406.

Gold

Re: Data Transfer Rate Limited to about 12M over a full DS3 link

Since you already know that creating 2 sessions fixes this you are already ahead of many people that see this issue.

I suspect your latency has increased from the old site to the new site. Since you can transfer 12m/sec your latencey still must be pretty good.

Your issue is what is called tcpwindowsize. This is the amount of data you can send in a tcp session without a acknoledgement.

It is much more complex since the windowsize will change dynamically. Lets assume you have the maximum window size of 32kbytes and a latency of 1 second (1000ms) For simplicity sake assume that you send this whole window as 1 packet. You then must wait 1 second to get your ack. This limits you to 256kbits/second. Now lets say the latency was 100ms. You would transfer 10 of these 32k windows in 1 second or 2.5m/second.

The maximum you can transfer with a single tcp session can be calulated by the formula

(windowsize in bytes)*8/latency in seconds

To a point you can adjust the window size in the TCP stack but even at maximum of 64kbytes even moderate latency will prevent you from fully using a ds3 with a single user. To fully use a ds3 with a 64k windowsize you would need a 11ms response time.

New Member

Re: Data Transfer Rate Limited to about 12M over a full DS3 link

So basically you're saying the speed of the throughput rate is depend on the window size and the window size is dynamically changed base on the latency between the two sites and changing tcp window size could help increase the speed of the throughput rate correct ?

Do you have any procedure or document on tcp window size and how to change tcp window size ?

Thank you and I really appreciate your inputs !!!!

Danny

Hall of Fame Super Silver

Re: Data Transfer Rate Limited to about 12M over a full DS3 link

What Tim refers to as tcp window size is what some other refer to as bandwidth * delay product and it certainly is the primary limiting factor on throughput of tcp sessions. RFC 1323 discusses the problems inherent in bandwidth * delay product and makes some suggestions for enhancing performance.

For a copy of that RFC you can use this link:

ftp://ftp.rfc-editor.org/in-notes/rfc1323.txt

The bottom line is that the further you go the more your throughput is limited. When your DR site was closer the throughput was better and when it is further away the throughput is worse. And the larger you can get the window size the better your performance will be.

HTH

Rick

New Member

Re: Data Transfer Rate Limited to about 12M over a full DS3 link

Thanks Rick ! I appreciate your response !

Danny

New Member

Re: Data Transfer Rate Limited to about 12M over a full DS3 link

Hi Rick ! Can I change the TCP window size during business hours ? is there any negative affect ? I have a full mesh MPLS network with full DS3 at Corporate site and DR site and T1 at all remote sites. I have never done this before and just want to make sure I understand the effects when making changes to production network.

Thanks Rick !! I appreciate your help !!

Danny

Gold

Re: Data Transfer Rate Limited to about 12M over a full DS3 link

window size is set on the end client machines not in the routers. You may be thinking of MTU size which is a packet limitation. A window can contain multiple packets.

Not being a PC guy I can only tell you that to change the windows size you must add a field to the registry in a microsoft based operating system. There are a number of documents on microsoft's site on this topic.

New Member

Re: Data Transfer Rate Limited to about 12M over a full DS3 link

Throughput tests should be done using UDP traffic generators; that way TCP window size is not an issue end to end.

Get Solarwinds engineer suite (google that) it comes with "WAN KILLER". Wan Killer can be set to generate a DS-3's worth of udp junk data to see how many packets a link can really push. Just make sure you set the MTU to like 1200 in wan killer so you wont risk 1500 + MPLS/IPSEC overhead causing fragmentation slow downs in router cpu's along the way...

Joe

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