[c-nsp] DSCP or IP Precendence for CBWFQ

[Tim Franklin]

> I wanted to know (or understand) whether DSCP markings and IP 
> Prec. markings 
> are interchangeable - i.e. if you mark according to DSCP, 
> does IP Prec. based 
> classes (for queuing) also apply.
> 
> Likewise if you mark for IP Prec. do DSCP based classes (for 
> queuing) also 
> apply?

Sort of.

DSCP fills the same bits that are used for IP-precedence, as well as the
bits used for ToS.

So if you explicitly set the DSCP, you'll be able to guarantee what the
corresponding IP-precedence value is, as you know what the bit-pattern is -
and they do map in quite a sensible fashion, all DSCP AFx* to the same the
IP-Precedence y, and the like.

The reverse is not true.  If you set the IP-precedence, unless you also
explicitly set the ToS bits (or can guarantee that the packets are all
arriving with a given ToS), you don't know for sure what the DSCP value on
the packets is going to be.

Regards,
Tim.

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