IP SLA ICMP Path Jitter - Strange results

RegisHumeau · ‎03-23-2012

Hi all,

I'm an intern in a network company, in France. I'm working on the IP SLAs fonctions of Cisco IOS.

I've got some equipments to make some tests :

I've configured the ICMP Path Jitter operation on R1 (a 2800 router), which has the IOS Software C2800NM-ADVIPSERVICESK9-M, 15.1(1)T (fc1) :

ip sla 2

path-jitter 192.168.2.2 source-ip 192.168.1.254

tag ICMP JITTER R1-PC2

frequency 30

ip sla schedule 2 life 150 start-time now

Then, i use a software called PCATTCP to send UDP datagrams from PC2 to PC1. When PC2 send UDP at 2Mb/s or more, the results look like :

IPSLA operation id: 2

Latest RTT: 319 milliseconds

Latest operation start time: 16:08:11.458 GMT+1 Thu Mar 22 2012

Latest operation return code: OK

---- Path Jitter Statistics ----

Hop IP 10.0.0.2:

Round Trip Time milliseconds:

Latest RTT: 326 ms

Number of RTT: 10

RTT Min/Avg/Max: 319/294/333 ms

Jitter time milliseconds:

Number of jitter: 7

Jitter Min/Avg/Max: 2/5/9 ms

Packet Values:

Packet Loss (Timeouts): 0

Out of Sequence: 1

Discarded Samples: 1

Hop IP 192.168.2.2:

Round Trip Time milliseconds:

Latest RTT: 319 ms

Number of RTT: 10

RTT Min/Avg/Max: 315/255/322 ms

Jitter time milliseconds:

Number of jitter: 5

Jitter Min/Avg/Max: 3/4/7 ms

Packet Values:

Packet Loss (Timeouts): 0

Out of Sequence: 2

Discarded Samples: 2

Operation time to live: 134 sec

How is it possible to get a minimum value greater than the average ??? I don't understand :/

Thanks.

Regis

Michel Hegeraat · ‎03-27-2012

15.1(1)T (fc1) maybe try to move to the latest IOS in that same 15.1()T train..

Or try the same with a GD (Genral Deployment) image for these routers.

Cheers,

Michel

David Stanford · ‎03-27-2012

Looks like a bug I saw before:

CSCtr14722

IP SLA - Incorrect RTT on C2801 15.1(x)T1 with 1500 bits payload

This should be fixed in  15.1(01)T3.6  or later

RegisHumeau · ‎03-28-2012

Thanks for your answers, I'll try to change the IOS.

I noticed that the problem happens only when there are Out of Sequence packets (ICMP reply not received).

I've done again the tests, and and captured the packets with Wireshark between the two routers. I calculated the average with the values in Wireshark by adding 0 for each Out of Sequence packet, then dividing by the Number of RTT. And I got the average value displayed.

*** For example ***

10 ICMP requests are send.

9 received (ms) : 337; 334; 328; 337; 334; 328; 323; 333; 327. [values obtained with Wireshark]

--> The IP SLA results are :

Round Trip Time milliseconds:

Number of RTT: 10

RTT Min/Avg/Max: 324/298/338 ms

Packet Values:

Packet Loss (Timeouts): 0

Out of Sequence: 1

Discarded Samples: 1

Then, let's calculate the average value :

Avg = (337+334+328+337+334+328+323+333+327+0)/10 = 298.1

And that works every time.

Regis

RegisHumeau · ‎03-28-2012

If somebody has the same problem, I also tried the ICMP Path Jitter operation with the other router, a 2851 with an IOS C2800NM-ADVENTERPRISEK9-M, 12.4(22)YB8 (fc1), and I got the same problem. (The first used was a 2821)

I'll try to get a newer IOS, but not sure my company has one...

Edit March 29, 2012

Well, I tried the experience on :

-a 2821 router, with IOS C2800NM-ADVIPSERVICESK9-M, Version 15.1(2)T2 (not the last version because of 64 MB flash)

-a 2851 router, with IOS C2800NM-ADVENTERPRISEK9-M, Version 15.1(3)T3

... and it failed on both of them.

Where can I report a such bug please ?

Thank you,

Régis