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Network Troubleshooting: Latency Vs. Retransmission

When troubleshooting network performance problems, most analysts find themselves chasing one of two issues: latency or retransmissions. Both scenarios result in performance degradation, but have very different root causes and solutions.

I've been involved in many troubleshooting exercises where I get a totally different perspective by changing my test point. In this video, I'll show you how the same problem can look like a latency problem or retransmission-related issue by simply changing your location in the network.

Here are some additional tips when trying to identify if an issue is latency or retransmission related:

  • Try to monitor from the sender's perspective. If the sender is not physically close, then make yourself the sender by uploading a file or running iperf.
  • Pay attention to your protocols. This information presented here is helpful when dealing with TCP base protocols. UDP is a totally different animal.
  • Try to leverage operating system commands like netstat –s to identify retransmissions.
  • Understand what your tools are reporting. For example Wireshark might note a retransmission, spurious retransmission, fast retransmission, or other notes.
  • Look for TCP-specific hints like Selective Right Edge (SRE) or Selective Left Edge (SLE) to possibly indicate packet loss.

There is no guaranteed tip or trick when troubleshooting packet loss or latency issues. But awareness of tool placement will help you when troubleshooting.

>> See Tony discuss this issue further and answer audience questions at Love My Tool.

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