QoS Token buckets

[lerdalt]

Maybe I'm just dumb today, but I just read through the section for the ONT exam on Token Buckets, and it didn't make sense to me.

I understand your token bucket has a set size limit (let's say 700bytes). I think my confusion is if the bucket isn't completely full (only 200bytes), do those packets just sit in the bucket until it is full? Or is it that it will only transmit at a rate equal to the size of the bucket (700bytes in my example).

Thanks

 

[GM2005]

The token bucket is a buffer and so; as an analogy; will fill from the bottom up so will not 'hold data', but refill with fresh, queued packets. Another way of looking at it is that it is temporary store for data that can not be transmitted because there are not currently the resources. If you have a token you can 'go on standby' in the bucket. First come, first served.

If the bucket fills up the action that is taken by the router (with both packets in the bucket and those trying to get into the full bucket) is dependent on how the policy is configured. It may drop the packets, assign them to a lower class of traffic or even have an excess bucket that is a premium service offered.

In practice if the bucket is always full and too may packets are dropping the policy will be reviewed as it is either not correct, or it will spotlight a traffic issue in the network that will need addressing.

The rate of transmission of data is not dependent on the bucket at all.

 

[lerdalt]

Thanks..thats much clearer than how I read it in the book.

 

[GM2005]

wikipaedia do a good clean explanation believe it or not.

I meant to add to this that if the bucket is full and the resources are not there to empty it, newly arrived packes will drop.