06 Oct

Behind the Scenes: Troubleshooting Slow Download Speeds

by Tom Fite, Engineering Manager at ExaVault

We here at ExaVault are obsessed with making sure that our customers get the best transfer speed possible. You paid good money for that 50 megabit connection, so we better make the most of it! Recently we found a problem that affected download speeds across the board, especially for customers with fast connections.

This is the story of how we found and fixed the problem.

Warning: this post is about to get really detailed and nerdy. I’m sorry, I can’t help it. As a dev-ops guy, it’s my job to dive into the weeds.

The first step is admitting you have a problem

How did we know that there was an issue in the first place? Diagnosing network transfer speeds can be very tricky, because networks are immensely complicated. There are a million different places where things can go wrong, but they generally boil down to these, which nicely follows the path the data travels from us to a customer.

  • The physical hardware running the application
  • The software running the application
  • The internal network for the service
  • The network path over the Internet from us to the customer
  • The customer’s connection
  • The customer’s computer

That’s a pretty long list and that’s just the 30,000 foot view.

We ruled out customer problems, because we heard from people with many different operating systems, configurations, and locations. Plus, I observed slow download speeds on my connection at home, and I have an awesome connection and computer. There’s no way it wasn’t up for the task.

We have a php script that tests our upload and download speeds and it’s installed on virtual servers located around the world. The aggregated results from these tests showed that we clearly had a problem with our download speeds:

Slow Download Speeds

This graph shows two weeks worth of data, the blue line is Christmas week 2013 and the red line is the first week in January 2014, broken down by hour. It shows us two things:

1. Download speeds overall were better during the Christmas week.

2. Download speeds were best in the early morning and late evening.

This indicated that we had an issue with traffic load. The more traffic on the network, the slower the download speed. Historically, Christmas is the week with the lowest traffic. It would only make sense that we’d see lower download speeds with increased connections if we were maxing out our outbound bandwidth, however, our firewall showed that we were far from max utilization.

Digging deeper

If you want to see what’s actually happening when you transfer a file over the internet, you need to look at the packets that make up the traffic. Wireshark is an excellent tool for doing this. Looking at the traffic for a download from ExaVault, I noticed that we had many error packets that looked similar to this:

810417.387064000192.168.1.667.208.64.240TCP94[TCP Dup ACK 8020#42] 49213 > 61198 [ACK] Seq=1 Ack=6739718 Win=4194304 Len=0 TSval=981006868 TSecr=604385392 SLE=6764334 SRE=6806326 SLE=6755646 SRE=6762886 SLE=6741166 SRE=6754198

The “TCP Dup ACK” means that somewhere along the way, one of the packets didn’t arrive at the destination, which caused the other packets to be received out of order. When this happens, the network algorithm assumes that it’s due to network congestion, and throttles the connection’s speed down.

Check out the speed graph of a solid connection on the left, and a download from ExaVault on the right. I used a great network monitoring tool called Rubbernet to produce this graph.

A comparison of a good transfer speed and one affected by the download speed issues

Note how the download on the left reaches a stable point, maxing out the available bandwidth on my connection. The download on the right speed varies drastically. During this test, thousands of TCP Dup ACK packets were received.

Finding the culprit

At this point I knew we were dropping TCP packets somewhere along the network path from our application server to the connecting client. I ran sftp to copy data to my computer from each point in the network travelling outward. I found that downloading files from the firewall was much faster than the storage box and I didn’t see the TCP Dup Ack packets during the download.

The firewall connects directly to the outbound link provided by our ISP, while our storage node connects to the firewall via a switch. The culprit had to be either our storage box or our switch. Transfers from one storage node to another storage node were fast, therefore, there had to be something wrong with the way the switch was communicating with the firewall.

A look at the stats command on the switch confirmed this:

# walkmib ifOutDiscards
ifOutDiscards.1 = 0
ifOutDiscards.2 = 12
ifOutDiscards.3 = 5,277,709 ← firewall port
ifOutDiscards.4 = 0
ifOutDiscards.5 = 0

Port 3 on the switch connects to the firewall, and the ifOutDiscards counter showed how many packets were dropped. For some reason, the switch itself was the cause of dropped packets.

After extensive Googling, I found a post from someone who had a similar issue who said the problems were caused by the lack of per-port buffer memory on their switch. Enabling flow control on both ends of the connection fixed their issue.

I configured our switch to enable flow control on the firewall port and retested -- this time I was able to pull data down at the same rate across every box on our network.

Later, we confirmed from our speed testing boxes that speed was consistent across all hours, shown as the purple line in the graph below.

A graph showing great speeds after the fix

Wrapping up and take-aways

After making these changes, our testing servers saw a 1.5x to 2.0x increase in download speeds. Quite a marked improvement! We fixed this issue back in February, so if you’re an ExaVault customer hopefully you’re enjoying the speed boost.

Troubleshooting network issues can be a huge pain and take a really long time to figure out. Although the path to fixing our issue might seem pretty straightforward after the fact, the steps we took to fix this problem were done over the course of a few months. For anyone else who might face a similar problem, here’s my advice:

  • Break your network traffic down from the point at which it is generated to the point at which it is delivered. Include applications, computers, firewalls, switches, routers, gateways, even cabling!
  • Test from outside of your firewall. Bring up virtual servers in as many geologically different areas as you can, and point them at your service.
  • Set up automated testing with these servers and look at data in aggregate for at least a week to smooth out any abnormailites.
  • tracerouteiperf, and mtr are your friends. Learn how to use them.
  • Get down to the lowest level possible. Sniff those packets!

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