Tuesday, November 11, 2014

Using cisco IP SLA for udp-jitter and MOS computing

In this post, we will look at a way for crafting a udp-jitter check.

This allows for link measurements of one-way traffic and with a computed MOS scoring. With cisco IP SLA udp-jitter we can compute the  delay in ms and select the codec type to mimic what traffic you are expecting over the path.

( Background )

UDP is used due to the fact that  voice  and most realtime data is udp-based.  MOS scores are computed from 5 to 1, with 5 being the best or pure to a DS0 and 56 bits ( 8bits times  8000 samples in a second )
note: one bit is used by legacy TDM for signalling.

The factors  that determines MOS scores and call quality will be  one-way latency, jitter, plos ( packet lost ),  and code type. We can do certain activities to eliminated  or reduce plos or jitter with circuit design and packet scheduler but latency is typically out of our control.

Now, let's look at the configuration that we will deploy for measurement of our udp-jitter.


( siteA)

config t

ip sla 301
 udp-jitter 16500 source-ip codec g711alaw
 tos 184
 threshold 10
 timeout 2000
 frequency 5
 history hours-of-statistics-kept 24


( siteB)

config t

  ip sla responder


And lastly we execute the IP SLA probe on siteA

config t

 ip sla schedule 301 life forever start-time now


Yeap it's really that simple that even a caveman can do it.


For IOS-XR it's very similar


config t

 operation 301
  type udp jitter
   source address
   destination address   statistics hourly
    buckets 24
   tos 184
   destination port 16500
   frequency 10
   statistics interval 3600
 schedule operation 301
  start-time now
  life forever

show commit 


config t
  type udp ipv4 address port 16500

show commit 

Yeap it's really that simple that even a caveman can do it.

After you have set the  probe you can use the following command on the siteA device;

show ip sla statistics 301 < details >

At this point we can now  review and monitor the  path between sender and responder and witness the  udp-jitter measurements.

One other issues, make sure you understand that different IOS code and models supports different IP SLA probes. Here's a snapshot of various devices with IP SLA  applications. You can find these by issuing the following;

IOS         show ip sla application
IOS-XR  show ipsla application

key points to  take away
  • the cisco ip sla  probes allows for quick checks & is easy to deploy

  • IP SLA diagnostics are built into most cisco routers/switches depending on code
  • SLA probes allows for a "hands-off" monitoring & reporting

  • you must enable the responder on one side for udptcp-jitter a few other checks

  • you can run the  SLA probe for a set duration or a lifetime of "fore-ever"

  • ip sla is available on most switches with some the nexus lineup not supporting  IP SLA

  • most current models of  routers supports  IP SLA

  • Make sure the ip sla responder supports the probe that you are  trying to use

  • for problems with one-way jitter, you need time sync between devices for proper measurements
For data statistics, you can use the following commands;

IOS show ip sla stat

IOS-XR show ipsla stat

Ken Felix
NSE ( network security expert) and Route/Switching Engineer
kfelix  -----a----t---- socpuppets ---dot---com
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