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From: HANK@VM.BIU.AC.IL (Hank Nussbacher)
Newsgroups: comp.protocols.tcp-ip
Subject: When is a link saturated?
Message-ID: <9101132129.AA06161@ucbvax.Berkeley.EDU>
Date: 13 Jan 91 21:29:31 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: Hank Nussbacher <hank%vm.tau.ac.il@CUNYVM.CUNY.EDU>
Organization: The Internet
Lines: 34
X-Unparsable-Date: Sun, 13 Jan 91 15:30:00 IST

I have recently started to monitor my links with SNMP on an hourly
basis and have seen that for my 64kb lines - the typical line utilization
is around 25%-30%.  I compute maximum link thruput for a 64kb line
as 28.1Mb/hour.  But I know that achieving 28.1Mb/hour is close to
impossible due to protocol overhead.  In addition, the lines I am
analyzing are routing IP, Decnet and Appletalk at the same time.

I previously believed that a 65% upper limit was a rational limit to
use for such a line.  That translates into 18Mb/hour (for a 64kb line)
and I figured that I may have a spike here and there above 18Mb/hour
but not anything that could be sustained.  This past week, I had 2 links
that maintained a 23Mb/hour and 25Mb/hour sustained rate for over 5
hours.  That is 88% link capacity.  This turned out to be almost all
VMNET (NJE/IP) traffic due to a reload of a 2 tapes that were restored
to the NJE spool system for dispersion throughout our network.  This
taught me that VMNET can drive a link to 88% of its total capacity, even
while other protocols are running in parallel as well as other
applications (Telnet and FTP).

This led me to check further and I found that our 9.6kb IP link to the
USA (which is hopelessly overloaded and scheduled to be upgraded to
64kb on January 14th), has been running at 70% of capacity (maximum
capacity is 101.2Mbytes per day) on average for the past 4 months.
This link is a strict IP link with just FTP, Telnet and IRC traffic.

I now have all the numbers but I am missing one crucial number.  At
what percentage of capacity should a link be upgraded?  Is it 25%?
40%? 65%?  I'd like to hear what "rules of thumb" others use in order
to determine when a link is saturated or near saturation and needs to
be upgraded.

Thanks,
Hank Nussbacher
Israel Network Information Center