[j-nsp] delay-buffer in Juniper

[Stefan Fouant]

On Oct 13, 2012, at 1:52 PM, Huan Pham <drie.huanpham at gmail.com> wrote:

> [HP] I think you can transmit at rate higher than the configured value if theres no congestion (e.g. if there's no traffic waiting to be transmitted in other queues).
> 
> Here's the quote from Doc:
> 
> [Juniper Doc]
> rate-limit—(Optional) Limit the transmission rate to the rate-controlled amount during congestion. In contrast to the exact option, when there is no congestion, the scheduler with the rate-limit option shares unused bandwidth above the rate-controlled amount.
> 
> http://www.juniper.net/techpubs/en_US/junos9.5/topics/reference/configuration-statement/transmit-rate-edit-dynamic-profiles.html
> 
> 
> [HP] Not mentioned in the Doc but I think in case of congestion, traffic is is rate limited by being buffered, as opposed to being policed. I think it is only dropped if its queue is full (or subject to RED). In other words, they CAN BE queued if there is congestion and contention from different queues for the bandwidth!
> 
> Do you mean the queue buffer is always 0 when we use "rate limit" option?

Hi Huan,

Honestly, that's 9.5 documentation. Perhaps something changed at some point, but in current versions of code this is not how it works.  I have taught this class for a few years now and have also worked w/ QoS on Junos for some time and the rate-limit option is used to limit the transmission rate to the specified amount. Take a look at the latest docs and you will see. I would say that your analogy of the queue buffer being 0 when using a transmit-rate w/ the rate-limit option is spot on. There is no buffering taking place. In other words, if you configured a transmit-rate of 0% with the rate-limit knob, all packets belonging to that queue would be dropped.

Stefan Fouant
JNCIE-SEC, JNCIE-SP, JNCIE-ENT, JNCI
Technical Trainer, Juniper Networks

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