[j-nsp] Re: verify QoS operation

[Guy Davies]

Hi Rainer,

> -----Original Message-----
> From: juniper-nsp-bounces at puck.nether.net 
> [mailto:juniper-nsp-bounces at puck.nether.net] On Behalf Of 
> Rainer Clasen
> 
> Erdem Sener wrote:
> >  show interfaces <interface-name> extensive --> where you 
> can see how 
> > much traffic is transmitted/dropped for each queue.
> 
> These are outgoing stats. I'm looking for incoming Stats - in 
> case I can't look at the output counters on the neighboring device.

A receiving device has no way of knowing how the transmitting device
implemented classification, queueing, scheduling and congestion
avoidance for each class of traffic.  This decision is made at each hop
and can be based on different models at each hop, particularly if your
devices support different queueing, scheduling and congestion avoidance
algorithms.  Assuming that the packets are marked at the ingress to the
network, you can see the number of packets arriving marked with a
particular class based on a particular marking (although I don't think
JUNOS actually records those stats).  However, this gives absolutely no
indication of whether the transmitting device is actually queueing and
scheduling packets appropriately (for some value of "appropriate") since
you have no idea of the rate at which packets for each class are
arriving at the transmitting device.  Each device is responsible for the
QoS of outbound packets.  There is little benefit to implementing QoS
inbound since the resource has already been utilized by the time you get
to make a choice whether or not to drop a packet.

In general, the packet passing through the device has to go through
several stages.

1. Packet received
2. Packet classified (based on existing marking or arbitrary header info
or port of ingress)
3. Egress Queue identified (based on classification)
4. Congestion avoidance measures (WRED, WTD)
5. Scheduling (MDRR, WFQ, SPQ, etc)
6. Rewrite markings.
7. Packet transmitted.

I'm not 100% sure that is the exact order (I'm sure that receive is
first and transmit is last ;-) but it's pretty close.  Certainly,
congestion avoidance, scheduling and rewriting all occur in the egress
half of the process.

Rgds,

Guy

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