Understand the bandwidth usage on production.

[martin.germano]

  Hi all, i try to understand the bandwidth usage on production time on the network. I took one C3560G with all device are conected to 1000Mb/s.

I make: show int gi0/1

  5 minute input rate 320000 bits/sec, 555 packets/sec

  5 minute output rate 9787000 bits/sec, 821 packets/sec

  The total is: 10.1 Mb/sec (on this port)

In all active port the summary is: 47 Mb/s (input rate+output rate in all ports)

So, i used 4,7% of the total of the bandwidth. And in cacti (by snmp) software i see one device transfer data with a maximun of 24Mb/s. Why don't use more bandwidth if is available?

Some can give me a hand to understand this?

Thanks!

[Joseph W. Doherty]

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Posting

Two common issues:

First, if you're measuring bandwidth utilization over multi-minute periods, you'll need a large sustained data transfer to push a peak measurement up.

Second, if the data transfers are using TCP, many older TCP stacks may not provide, especially by default, a large enough RWIN to allow TCP to fully utilize the available bandwidth. (Research bandwidth-delay-product [BDP] for more info.)

[martin.germano]

Hi Joseph, thanks for reply.

I had to refresh some concepts to understand what you are saying.

For example, one real case: Host1(1Gb) copy FTP data to host2(1Gb) and it takes 4,5 hours to finish. In 1 hour used less than 2Mb/s but in the rest 3.5 hours make a peak and use 10Mb/s, but no more (the windows size not increase at all in 3.5h)

Why don't increase the RWIN in 3,5 hours and use more bandwidth?

So, the bandwidth is limited for the windows size. How i manualy set the RWIN? in a LAN is posible?

thanks!

[Joseph W. Doherty · Accepted solution]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

So, the bandwidth is limited for the windows size. How i manualy set the RWIN? in a LAN is posible?

Yes, it's often possible.  How depends on the host.  Search the Internet for BDP and your host IOS, and you'll likely find how to adjust.

PS:

BTW, too large a RWIN can cause a sender to send too many packets at one time which can result in drops.  Have you checked your transit interfaces for drops?

Ideally, RWIN should be sized for BDP; not smaller and not larger.

[martin.germano]

Thanks again for reply. I understand more what happen with the bandwidth (30mbp/s in an gigabit network is ok). Maybe if a realy need to modify RWIN i'll do it.

I'll monitor bandwidth using UDP and see what happend.

Martín.