Subject: why 4 AF classes?

From: "xizheng shi" <[email-masked]>
Date: Thu, 7 Oct 1999 23:40:29 -0700

Hello, diffservers

Can anyone enlighten me about following maybe naive questions:
Q1: Why do we need exactly 4 AF classes in AF PHB rather than 1, 2 or even
5...

Q2: Why "DS node MUST NOT aggregate two or more AF classes together"?
I am not sure the meaning of aggregate, does it mean putting them into one
queue or something else?

Thanks in advance.

Xizheng Shi
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From: John Schnizlein <[email-masked]>
Date: Fri, 08 Oct 1999 09:29:44 -0400

At 11:40 PM 10/07/1999 -0700, xizheng shi wrote:
>Q1: Why do we need exactly 4 AF classes in AF PHB 
>    rather than 1, 2 or even 5...

Exactly 4 was not decided, just that if you do AF, you should do 
at least 4. "Why" is not easily answered for consensus results.
The discussion wound around attempts to combine these 4 classes 
with the (partially backward compatible) class-selector codepoints.
All 8 of these were not seen as necessary, but more than 2 were.
Those constraints, and preference for powers of 2, led to 4.
After the AF PHBs were separated from the ordering requirement
of "probability of timely forwarding", nobody asked for the number
to change from 4 classes to anything else. Since each AF class
consumes 3 DSCP values, the result of 12 was more than any other set.

>    I am not sure the meaning of aggregate, does it mean putting 
>    them into one queue or something else?

It really means not putting them into the same queue, but we worked
hard to avoid saying anything as implementation-specific as "queue".