Subject: RE: Significance of Minimum Policed Unit?

From: Kam Lee <[email-masked]>
Date: Tue, 16 Feb 1999 11:13:49 -0800

My understanding (based on RFC 2210) is that: given a payload bandwidth, say
X, (here payload includes app data, transport and IP headers), a router will
use the minimum policed unit (m) to "estimate" the (payload + link-layer
header) bandwidth required for admission control purpose. 

For a flow (e.g. coded video stream) that uses a wide range of packet sizes,
its Y will be much much greater than its X, if "m" is equated to the minimum
packet size of the flow. This increases the risk of admission failure. One
workaround is to set "m" to a more reasonable value, (e.g. setting "m" =
mean packet size - standard-deviation). But this has the side-effect of
slightly discounting the guaranteed payload bandwidth since all packets
smaller than "m" will consume "m" units of tokens regardless of their actual
sizes.

I am just wondering if this fudging of "m" has any other side-effects?

thanks,
kam

-----Original Message-----
From: Steven Berson [mailto:[email-masked]]
Sent: Tuesday, February 16, 1999 10:34 AM
To: Kam Lee
Cc: [email-masked]'; [email-masked]'
Subject: Re: Significance of Minimum Policed Unit?

> In RFC 2210 (see excerpt below), the Minimum Policed Unit (m) is meant to
be
> used by intermediatenetwork nodes to determine the amount of bandwidth
> compensation for link-level header overhead. 
> 
> Are there any other uses of this parameter elsewhere under the IntServ
> framework?

Kam,

I am not sure I understand what you mean.  Basically the minimally
policed unit is used to try to guarantee payload bandwidth rather than
(payload+headers) bandwidth.  So, when used properly, you can get at
least the payload bandwidth you reserve.

Steve

---

From: John Wroclawski <[email-masked]>
Date: Tue, 16 Feb 1999 15:59:10 -0500

>I am just wondering if this fudging of "m" has any other side-effects?

Kam,
In the context of the framework, the use for 'm' described in your 
first paragraph is the main one, and is the thing that caused 'm' to 
exist.

Of course, any box that has some other use for the info can use it. 
In that context, it's important for the box designer to understand 
the point made in your second paragraph - that there are tradeoffs 
that might lead to picking different values of 'm' for the same 
stream, and that the box might in fact legitimately see packets 
smaller than 'm' in the stream.

So basically I think you understand it perfectly :-)

Cheers,
John