perfsonar development work

[Wenji Wu <>]

> Some OWD tools report the TTL. Is this sufficent?

TTL just tells us that there are route changes. Still we need to 
differentiate whether it is route flaps or load balancing cause the route 
changes.


> There are several known causes for persistent route flapping on short
> timescales. There are techniques to dampen announcements caused by these
> flaps but they are not all universally deployed.

That is true. Some routers do not have the MRAI configured. 

I just think about another hint:  route flaps usually cause instantaneous 
disconnectivity, and lead to packet drops. If there are no other reasons, 
load balancing should not lead to packet drops. So we might combine the 
packet drops to differentiate the route flaps and load balancing. 

> It is not just dependent on cross traffic. For example the Juniper  
> load balancing algorithm can be configured to balance on various parts of 
> the packet  
> header including IP address and TCP/UDP port numbers.  But in reality it 
> doesn't look  
> at the IP addresses and port numbers, it looks at specific offsets in the 
> header. The TCP  
> destination port and the ICMP checksum bytes are at the same offset. So 
> ICMP based  
> traceroutes generate confusing results.

here, my understanding is this: if the algorithm is configured to balance on 
various parts of the packet header including IP address and TCP/UDP port 
number, the load balancing is inter-flow based. for specific flow (some src, 
dest, port), there is no load balance at intra-flow level (or packet level) 
at all. 


weji
> >
> > -----Original Message-----
> > From: Joe Metzger [
> > 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
> >
> >
> >
> >