LARTC - Oct 2006 - QoS HTB burst and cburst parameters-FLEX

All:

Does anyone know what the burst and cburst parameter do?  
	
My understanding so far:

*		I see a lot of different definitions on the web.  It
seems like burst is the number of bytes sent before serving other
queues/classes.  So if burst was 1000 bytes and class rate was 100kibit
per second.  It would send 1000 bytes each time the scheduler service
that queue to a rate of 100 kbit per second?    

Also does anyone know how the burst and cburst parameters are configured
by default?

*		Looking for a formula and all the parts to come up with
the automatically configrued number that is show with the below command
*		"tc -s -d class show dev eth1"

Thanks

Jon Flechsenhaar
Boeing WNW Team
Network Services
(714)-762-1231
202-E7

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Greetings Jon,

 : Does anyone know what the burst and cburst parameter do?  

Consider the burst parameter the bucket used until an HTB class is 
transmitting at its rate.

Consider the cburst parameter the bucket used when an HTB class is 
transmitting at or above rate, but below ceil.

 : * I see a lot of different definitions on the web.  It seems like 
 :   burst is the number of bytes sent before serving other 
 :   queues/classes.  So if burst was 1000 bytes and class rate was 
 :   100kibit per second.  It would send 1000 bytes each time the 
 :   scheduler service that queue to a rate of 100 kbit per second?

Here''s how I would succinctly describe the interrelationships 
between burst, quantum, cburst and the scheduling algorithm:

  A given leaf class is transmitting below rate 
  ============================================  Each time our leaf class has the
opportunity to dequeue
  packets, it will dequeue as many packets as possible until
  it reaches burst.
  
  A given leaf class is transmitting above rate
  ============================================  Each time our leaf class has the
opportunity to dequeue
  packets, it will dequeue quantum packets and yield its turn
  to the next class.  This prevents a single class from
  starving its sibling classes for borrowing from the parent.

 : Also does anyone know how the burst and cburst parameters are 
 : configured by default?

This, I cannot answer for you.  You may find my longer description 
of the borrowing model and HTB in general useful [0], and in 
particular, the diagram may be helpful for visualizing the system, 
however, for your needs I would recommend that you study the results 
that Stef Coene posted several years ago on the use of burst and 
cburst [2].

Best of luck,

- -Martin

 [0] http://tldp.org/HOWTO/Traffic-Control-HOWTO/classful-qdiscs.html#qc-htb
 [1] http://linux-ip.net/traffic-control/htb-class.png
     http://linux-ip.net/traffic-control/htb-class.pdf
 [2] http://www.docum.org/docum.org/tests/htb/burst/

- -- 
Martin A. Brown
http://linux-ip.net/
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