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

Fred Baker fred at cisco.com
Wed Dec 13 12:16:41 PST 2006

You're talking about ifOutQLen. It was originally proposed in RFC  
1066 (1988) and deprecated in the Interfaces Group MIB (RFC 1573  
1994). The reason it was deprecated is not documented, but the  
fundamental issue is that it is non-trivial to calculate and is very  

The big issue in calculating it is that it is rarely exactly one  
queue. Consider a simple case on simple hardware available in 1994.

    +----------+ |
    |          | |
    |  CPU     +-+
    |          | |
    +----------+ | BUS
    +----------+ | +---------+
    |          | +-+ LANCE   |
    |          | | +---------+
    |  DRAM    +-+
    |          | | +---------+
    |          | +-+ LANCE   |
    +----------+ | +---------+

I'm using the term "bus" in the most general possible sense - some  
way for the various devices to get to the common memory. This gets  
implemented many ways.

The AMD 7990 LANCE chip was and is a common Ethernet implementation.  
It has in front of it a ring in which one can describe up to 2^N  
messages (0 <= N <= 7) awaiting transmission. The LANCE has no idea  
at any given time how many messages are waiting - it only knows  
whether it is working on one right now or is idle, and when switching  
from message to message it knows whether the next slot it considers  
contains a message. So it can't keep such a counter. The device  
driver similarly has a limited view; it might know how many it has  
put in and how many it has taken out again, but it doesn't know  
whether the LANCE has perhaps completed some of the messages it  
hasn't taken out yet. So in the sense of the definition ("The length  
of the output packet queue (in packets)."), it doesn't know how many  
are still waiting. In addition, it is common for such queues or rings  
to be configured pretty small, with excess going into a diffserv- 
described set of software queues.

There are far more general problems. Cisco has a fast forwarding  
technology that we use on some of our midrange products that  
calculates when messages should be sent and schedules them in a  
common calendar queue. Every mumble time units, the traffic that  
should be sent during THIS time interval are picked up and dispersed  
to the various interfaces they need to go out. Hence, there isn't a  
single "output queue", but rather a commingled output schedule that  
shifts traffic to other output queues at various times - which in  
turn do something akin to what I described above.

Also, in modern equipment one often has forwarders and drivers on NIC  
cards rather than having some central processor do that. For  
management purposes, the drivers maintain their counts locally and  
periodically (perhaps once a second) upload the contents of those  
counters to a place where management can see them.

So when you ask "what is the current queue depth", I have to ask what  
the hardware has, what of that has already been spent but isn't  
cleaned up yet, what is in how many software queues, how they are  
organized, and whether that number has been put somewhere that  
management can see it.

Oh - did I mention encrypt/decrypt units, compressors, and other  
inline services that might have their own queues associated with them?

Yes, there is a definition on the books. I don't know that it answers  
the question.

On Dec 13, 2006, at 10:54 AM, Craig Partridge wrote:

> Queue sizes are standard SNMP variables and thus could be sampled at
> these intervals.  But it looks as if you want the queues on a per host
> basis?
> Craig
> In message <Pine.LNX. 
> 4.44.0612130958100.28208-100000 at cmm2.cmmacs.ernet.in>, V A
> nil Kumar writes:
>> We are searching for any known techniques to continuously sample  
>> (say at
>> every 100 msec interval) the buffer occupancy of router  
>> interfaces. The
>> requirement is to extract or estimate the instantaneous value of the
>> number of packets or bytes in the router buffer from another  
>> machine in
>> the network, and not the maximum possible router buffer size.
>> Any suggestion, advice or pointer to literature on this?
>> Thanks in advance.
>> Anil

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