[j-nsp] delay-buffer in Juniper

[Huan Pham]

Hi Stefan,

Thanks for your post. It helps clarify things that are not always well documented, or hard to understand by just reading the doc

I partly disaggree with some points you said though.



[SF] "If you configure a transmit-rate with a keyword of rate-limit it will never buffer, it will simply drop any packets which are in excess of the configured transmit-rate."

[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?


[SF]
"If you configure a transmit-rate with a keyword of exact, it will buffer the traffic in excess of the configured transmit-rate, regardless of whether there is excess capacity available. It will buffer this traffic until it falls below the configured transmit-rate and then it will send this buffered traffic."

[HP] Totally aggree







On 14/10/2012, at 4:02 AM, Stefan Fouant <sfouant at shortestpathfirst.net> wrote:

> It depends on the options you've got configured with your transmit-rate.
> 
> If you configure a transmit-rate with a keyword of rate-limit it will never buffer, it will simply drop any packets which are in excess of the configured transmit-rate.
> 
> If you configure a transmit-rate with a keyword of exact, it will buffer the traffic in excess of the configured transmit-rate, regardless of whether there is excess capacity available. It will buffer this traffic until it falls below the configured transmit-rate and then it will send this buffered traffic.
> 
> If you configure a transmit-rate without either of the two options above, it will allow the queue to use excess capacity if it is available (whereby it will not be buffered), or if excess capacity is not available, then the traffic in excess of the configured transmit-rate will be buffered.
> 
> HTHs,
> 
> Stefan Fouant
> JNCIE-SEC, JNCIE-SP, JNCIE-ENT, JNCI
> Technical Trainer, Juniper Networks
> 
> Follow us on Twitter @JuniperEducate
> 
> On Oct 13, 2012, at 12:39 PM, tim tiriche <tim.tiriche at gmail.com> wrote:
> 
>> Hi,
>> 
>> I have a BRONZE queue configured with 20% TX rate/low priority on a
>> 10G interface.
>> When does juniper start buffering traffic.  If it exceeds the 20% TX
>> rate or if the 10G interface is oversubscribed?
>> 
>> -Tim
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