I don’t know if there is any way to measure this, but I’ll bet this is the most asked question since mobile phones became ubiquitous. It’s a frustrating situation; you can tell that you can hear the other person, and you can see how many reception bars your phone has, but you have no way of knowing what is going on at the other end of the call. Does the other person have good reception? Did their battery just go dead? In other words, what is the bidirectional quality of the mobile phone connection?

 

This problem is not just restricted to mobile phone connections. Today’s data networks are comprised of hundreds of individual connections supporting scores of applications. Typically the device testing the availability, quality and performance of the network is in just one location, as shown in the following diagram:

IP SLA graph.png

 

The quality tester at Site 1 has the ability to test the WAN connections A, B, and C that are directly connected to Site 1. The tester can test the combined WAN quality of links A and E, as well as the combined WAN quality of links C and D. Here is the problem; can this tester determine if users at Site 3 accessing servers at site 4 are experiencing the "Can you hear me now?" problem? Can these users open TCP connections to Site 4 servers? Can they reach web servers in Site 4? Our tester in Site 1 has no way of testing this. Enter the proxy test agent! The general term for this technology is proxy performance management. The Cisco specific implementation of this is called IP SLA.


By configuring an IP SLA agent on the Site 3 router, we can run WAN quality tests directly between Sites 3 and 4. The Site 3 router can then send the test results to the Quality Tester at Site 1. And the Site 3 router can be configured to perform tests over link D to determine the quality of that link.

 

Cisco's IP SLA

IP SLA capability is built into Cisco IOS, all you need do is configure it. So, what can you test using IP SLA?

  • VoIP quality (Jitter, delay, MOS)
  • TCP port open
  • UDP Port Open
  • DNS requests
  • DHCP requests
  • FTP file transfer capability and performance
  • ICMP Echo
  • ICMP path echo
  • HTTP access (includes DNS time and TCP port open time)
  • UDP Echo
  • Path Jitter
  • VoIP Jitter

 

IP SLA could be thought as a distributed "can you hear me now" process for the network. If you have not tried IP SLA on your network, the easiest way to implement it and see what it can do for you is to download either the SolarWinds IP SLA Free Tool or the IP SLA Manager and give it a try. These tools automate the router configuration process and provide graphical results of the tests.