Posts Tagged ‘test’

Two of the most common network characteristics we look at when investigating network-related concerns in the NOC are speed and throughput. You may have experienced the following scenario yourself: You just provisioned a new bad-boy server with a gigabit connection in a data center on the opposite side of the globe. You begin to upload your data and to your shock, you see “Time Remaining: 10 Hours.” “What’s wrong with the network?” you wonder. The traceroute and MTR look fine, but where’s the performance and bandwidth I’m paying for?

This issue is all too common and it has nothing to do with the network, but in fact, the culprits are none other than TCP and the laws of physics.

In data transmission, TCP sends a certain amount of data then pauses. To ensure proper delivery of data, it doesn’t send more until it receives an acknowledgement from the remote host that all data was received. This is called the “TCP Window.” Data travels at the speed of light, and typically, most hosts are fairly close together. This “windowing” happens so fast we don’t even notice it. But as the distance between two hosts increases, the speed of light remains constant. Thus, the further away the two hosts, the longer it takes for the sender to receive the acknowledgement from the remote host, reducing overall throughput. This effect is called “Bandwidth Delay Product,” or BDP.

We can overcome BDP to some degree by sending more data at a time. We do this by adjusting the “TCP Window” – telling TCP to send more data per flow than the default parameters. Each OS is different and the default values will vary, but most all operating systems allow tweaking of the TCP stack and/or using parallel data streams. So what is iPerf and how does it fit into all of this?

What is iPerf?

iPerf is simple, open-source, command-line, network diagnostic tool that can run on Linux, BSD, or Windows platforms which you install on two endpoints. One side runs in a ‘server’ mode listening for requests; the other end runs ‘client’ mode that sends data. When activated, it tries to send as much data down your pipe as it can, spitting out transfer statistics as it does. What’s so cool about iPerf is you can test in real time any number of TCP window settings, even using parallel streams. There’s even a Java based GUI you can install that runs on top of it called, JPerf (JPerf is beyond the scope of this article, but I recommend looking into it). What’s even cooler is that because iPerf resides in memory, there are no files to clean up.

Click to see how you can use and tweak iPerf with amazing results. »

I’ve been a Linux user for many years, but for various reasons I never bothered to get a certification even though it’s a fantastic validation of Linux skills. When I moved up in the world by joining SoftLayer, my attitude quickly changed.

As a new Systems Administrator at SoftLayer, one of the first challenges I was presented with was to try for my LPIC-1 certification. True to SoftLayer’s motto of “Challenging, but not Overwhelming,” I was given 3 months, a practice environment and reimbursement for my fees if I passed the tests. With an offer like that, it was impossible to refuse.

The LPIC-1 tests are not easy, and it took a lot of work to pass them, but if you’re interested all you need to succeed is a solid background in Linux and the time to dedicate to preparation. Here are some of the things I learned along the way:

1. Don’t attempt the LPIC-1 exam unless you have at least a couple of years’ worth of hands-on Linux experience. Seriously, it’s not for newbies.
2. Acquire at least two test-prep books, and read one of them every day. I used O’Reilly’s LPIC-1 Certification in a Nutshell and LPIC-1 In Depth by Michael Jang. Both are easy to read, have good explanations of concepts you need to understand, and provide valuable tips in addition to practice exams.
3. Set up a practice environment. It’s essential for reviewing commands you may not be familiar with.
4. When you think you are ready for the first exam, take a few free practice tests online. There are a number of them available.
5. I didn’t buy any test-prep software, but I did download a couple of trial versions as they offered some free practice questions.
6. Take all of the practice exams available to you several times each. You’ll get more comfortable with the format of the test questions and will also learn which areas you need to revisit before the actual test.

After earning the LPIC-1 certification I received a nice surprise in my mailbox along with my certificate. Apparently Novell and the Linux Professional Institute have a partnership: By earning the LPIC-1 I had also satisfied the requirements for Novell’s Certified Linux Administrator (CLA) certification, so now I can enjoy the benefits of having two IT certifications for the price of one and I have SoftLayer to thank for it!

-Todd