no bug in tcng; tc kbit v. tcng kbps [LARTC] was tcng error

no bug in tcng; tc kbit v. tcng kbps [LARTC] was tcng error

[Martin A. Brown]

Hello Charles,

 : after slicing and dicing, i found that i had cut and pasted bad syntax,
 : so i have solved the problem posted in my first message.

Where did you find the original (I'm hoping it's not one of mine).  If so,
let me know, and I'll fix it.

  [ example snipped ]

 : notice that the UPLINK of 512 kbps (arguably 524288 bps) has been
 : incorrectly calculated as 64000 bps

You have not actually found a bug, but rather a historical strangeness
about the Linux traffic control system.  For reasons of which I'm
ignorant, the syntax for the "tc" command uses bps for bytes/second.  So,
64000 bytes/second is actually 512 kilobits/second ("512 kbps" in common
usage), but is 512 kbit to the "tc" tool.  Here's a brief chart:

                      tc syntax       tcng syntax
                  +----------------+----------------+
     bytes/second |      bps       |      Bps       |
      bits/second |      bit       |      bps       |
 kilobytes/second |     kbps       |     kBps       |
  kilobits/second |     kbit       |     kbps       |
                  +----------------+----------------+

Note that the tcng syntax is exactly the same sort of syntax we use in
general when discussing speed of WAN links.  "It's a 512 kbps line" means
it's 512 kilobits per second, but this would be 64000 bytes per second if
we were writing a "tc" command line.

  [ another example snipped ]

I expect that you'll understand exactly what was happening in your second
example now that you are looking at this chart.

 : have i missed something, or indeed there is a problem??

I'm guessing that you just missed this oddity of command line tc behaviour
but that the tcng syntax is doing exactly what you desire.

Best of luck,

-Martin

-- 
Martin A. Brown --- SecurePipe, Inc. --- mabrown@securepipe.com

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Re: no bug in tcng; tc kbit v. tcng kbps [LARTC] was tcng error

[lartc]

hi martin,


On Thu, 2003-11-27 at 20:58, Martin A. Brown wrote:
> Hello Charles,
> 
>  : after slicing and dicing, i found that i had cut and pasted bad syntax,
>  : so i have solved the problem posted in my first message.
> 
> Where did you find the original (I'm hoping it's not one of mine).  If so,
> let me know, and I'll fix it.
indeed, i used some of your examples as well as those included in the
tcng/examples-ng directory, and wshaper.htb to create the following
(i'll post it now as it is untested but functioning less or more)

#define DEVICE eth0
#define DOWNLINK 1024 
#define UPLINK 512 

#include "fields.tc"
#include "ports.tc"

dev DEVICE {
    ingress {
        $policer = SLB( cir DOWNLINK kBps, cbs 60kB, mpu 0b );
        class (<>) if SLB_ok($policer);
        drop if 1;
    }

    egress {
        class (<$high>) if tcp_ACK || 
                           ip_proto = IPPROTO_ICMP ||
                           ip_tos = 0x10 ||
                           tcp_dport = PORT_SSH ||
                           tcp_dport = 8080 ||
                           tcp_dport = 18082 ||
                           tcp_dport = 18083;


        class (<$medium>) if tcp_dport = PORT_HTTP ||
                             tcp_dport = PORT_SMTP ; 

        class (<$low>) if 1;

        htb () {
            class (rate UPLINK bps, burst 6kB) {
                $high = class (prio 1, rate UPLINK kBps)
                    { sfq (perturb 10 sec); };
                $medium = class (prio 2, rate (0.9*UPLINK)kBps)
                    { sfq (perturb 10 sec); };
                $low = class (prio 3, rate (0.8*UPLINK) kBps)
                    { sfq (perturb 10 sec); };
            }
        }
    }
}


>   [ example snipped ]
> 
>  : notice that the UPLINK of 512 kbps (arguably 524288 bps) has been
>  : incorrectly calculated as 64000 bps
> 
> You have not actually found a bug, but rather a historical strangeness
> about the Linux traffic control system.  For reasons of which I'm
> ignorant, the syntax for the "tc" command uses bps for bytes/second.  So,
> 64000 bytes/second is actually 512 kilobits/second ("512 kbps" in common
> usage), but is 512 kbit to the "tc" tool.  Here's a brief chart:
> 
>                       tc syntax       tcng syntax
>                   +----------------+----------------+
>      bytes/second |      bps       |      Bps       |
>       bits/second |      bit       |      bps       |
>  kilobytes/second |     kbps       |     kBps       |
>   kilobits/second |     kbit       |     kbps       |
>                   +----------------+----------------+
> 
> Note that the tcng syntax is exactly the same sort of syntax we use in
> general when discussing speed of WAN links.  "It's a 512 kbps line" means
> it's 512 kilobits per second, but this would be 64000 bytes per second if
> we were writing a "tc" command line.
ah ha -- thanks for this!!!! much clearer now ... perhaps this table is
worthy of inclusion in the howto or a compatibility option in tcng?

curious also on your experience with ingress -- i noticed that using a
Single Leaky Bucket, and playing with the cbs parameter can dramatically
(obviously) affect the ingress policing. is there a general rule of
thumb in calculating the cbs size based on the cir?

cheers

charles




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Re: no bug in tcng; tc kbit v. tcng kbps [LARTC] was tcng error

[Martin A. Brown]

Charles,

 : #include "fields.tc"
 : #include "ports.tc"

[ You don't really need to explicitly include fields.tc and ports.tc,
  because they are implicitly included by tcc. ]

[ some snipped stuff ]

 : ah ha -- thanks for this!!!! much clearer now ...

Most certainly.  I think this is one of the more confusing aspects of
Linux traffic control.

 : perhaps this table is worthy of inclusion in the howto or a
 : compatibility option in tcng?

It's (now*) written in several places [0], [1], [2]*.  Which HOWTO were
you consulting?  The LARTC HOWTO?  It doesn't (yet) cover tcng, to my
knowledge.

 : curious also on your experience with ingress -- i noticed that using a
 : Single Leaky Bucket, and playing with the cbs parameter can
 : dramatically (obviously) affect the ingress policing. is there a
 : general rule of thumb in calculating the cbs size based on the cir?

I don't have a quick and easy answer....perhaps one of Stef's graphs [3]
will help you choose a bucket size.  Because a larger bucket supports
burstier traffic, it may be your choice in many situations.  Others may
want to prevent any bursting, and may deliberately wish to use a small
bucket.

This is also where some experimentation will lend a hand.  Better to try a
few bucket sizes, and see where you/your users are comfortable.

-Martin

 [0] http://linux-ip.net/gl/tcng/node21.html
 [1] http://tldp.org/HOWTO/Traffic-Control-tcng-HTB-HOWTO/misc.html
*[2] http://www.docum.org/stef.coene/qos/faq/cache/74.html
 [3] http://docum.org/

   * added (at your suggestion) to the LARTC FAQ-o-Matic

-- 
Martin A. Brown --- SecurePipe, Inc. --- mabrown@securepipe.com

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