perfsonar development work

[Wenji Wu <>]

Hi, Joe,

Just based on the one-way delay, it might be hard to differentiate the load
balancing and route flapping if those paths have similar one-way delays. And
the queuing delay might easily hide the differences. We might need to
combine with other measurement.

Route flap might not happen so frequent because most routers would limit the
frequency to update the routing tables or limit the rate to broadcast the
route updates. For example, BGP router has the MRAI timer to limit the route
updates. 

However, load balancing could happen at the packet level.

It sees to me that the best way to differentiate the route flap and load
balancing might be traceroute.

With traceroute, we could generate the sequence of router along the path
from the src to dest. For Route flaps, the path might not change so
frequently. Most traceroute will generate the paths within the intervals of
route flaps.

For load balancing, the paths might change at the packet levels. And
traceroute will generate some weird results. For example, if we have the
topology like:
    c-d-e
   /       \            
a-b       I
   \     / 
    f-g-h

node b will do the load balancing. We run traceroute from a to I. Then
traceroute might generate no-existent links. Let's say, the third ICMP goes
to C, and the fourth ICMP goes to F. then non-existent link C-F might be
generated!

So if it is the load balancing case, you run traceroute at different times,
various results will be generated. It depends on the crossing traffic at the
load-balancing nodes.


wenji

-----Original Message-----
From: Joe Metzger 
[mailto:]
 
Sent: Friday, July 20, 2007 4:23 PM
To: 

Cc: 

Subject: Question about interpreting One Way Delay data.

Hi,
An interesting question came up in the course of debugging a  
performance problem today.
It is not real important from an operational perspective, but it is  
intriguing from
an academic point of view.

Is there a way to analyze one way latency measurements between  
endpoints over long
slow congested paths and differentiate between route-flapping and  
load balancing?

If so, do you need access to raw data, or can you do it from the type of
summarization that the DFN and I2 tools are typically doing today?

Thoughts?

--Joe