[e2e] Extracting No. of packets or bytes in a router buffer

[Fred Baker]

On Dec 14, 2006, at 12:05 PM, Alexandre Grojsgold wrote:
> Of course, it´s ephemeral.

I point that out because it is a fundamental criterion that the SNMP  
community has been using for a while. It is one thing to enable an  
NMS to read the configuration of a device (largely static) or read a  
counter (monotonically increasing, so that subsequent reads tell you  
what happened between the reads). ifOutQLen is a gauge, which is to  
say that it looks a lot like a random number in this context. In such  
a case, the SNMP community will generally suggest that the number is  
not all that meaningful.

> I am really impressed to know it is so difficult to grab this kind  
> of information, since router manufacturers claim they can do magic  
> with  queue managing, like diffser, traffic shaping, priority  
> queueing, etc... all of this looking at the queues and making  
> tricks with them.

Do I detect a note of sarcasm?

The point is what is known by whom at a particular time. A bit of  
code looking at a choice of queuing something locally or handing it  
to the next widget makes a pretty simple determination - when it  
tries to hand the datagram off the next widget accepts it or not, and  
if not, it does the local thing. "accepts" can have various meanings  
- it may actively reject it, or (more probably) has given permission  
to send some quantum and the quantum is used up. Looking at  
individual queues, one can do a lot of things such as you mention.

The hard part is in a distributed system (a system that has  
functionality on a variety of cards managed by a variety of  
communicating processes) to have a single overall view of the entire  
state of the process at exactly the time one wants to find the answer  
to the overall question.