[c-nsp] ToS equivalent to IP Precedence?

[Tim Franklin]

Hi Dave,

cisco-nsp-bounces at puck.nether.net wrote:
> Noting that the range of ToS bits is 0-255, is there a way to directly
> equate a ToS bit of say, 8, to an IP precedence?  My understanding is
> that a ToS of 8 is equal to precedence 1?  Is this correct?  I may be
> way off...
>
> I have a situation where I can reflect the ToS bit into a tunneled
> packet but I'm trying to figure out how to set the ToS bit so that I
> can do things prec-based.

It's easiest if you draw it:

"Old-world" QoS is:

P2 | P1 | P0 | T3 | T2 | T1 | T0 | NA

P2-P0 are IP precedence, T3-T0 are the old-style "ToS" (least-cost,
low-latency, high-bandwidth etc), NA is not used.

DSCP QoS is:

D5 | D4 | D3 | D2 | D1 | D0 | NA | NA

D5-D0 are DSCP values, NA is not used.  (Actually, I seem to remember
someone pointed out last time this came up that there *is* now an RFC
defining a use for the two LSBs, but I don't recall what it is, and by the
sound of things it's not important in your case.)

If you fill in the bit patterns for the items you want, it's fairly easy to
swap about between IP-prec, "TOS byte" and DSCP.

Regards,
Tim.

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____________   Tim Franklin                 e: tim at colt.net
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