Re: [tsvwg] COMMENT PLEASE: Which DSCP value should we use for LE PHB?

[Brian E Carpenter <[e-mail]>]

On 01/08/2017 20:52, Bless, Roland (TM) wrote:
> Hi,
> 
> Am 01.08.2017 um 10:22 schrieb [e-mail]:> GF> There
> seems evidence that there are still even now deployed routers
>> that have not adjusted their remarking behaviour after the IETF 
>> deprecated ToS Precedence bleaching. 
>>
>> [RG2] True. I prefer a core-router to be able to re-mark DSCPs by bit-mask at  
>> interconnection interfaces rather than DSCP by DSCP. The config effort to customize 
>> re-marking on a DSCP by DSCP basis is high. An easy to configure alternative is to classify 
>> by bit-mask and re-mark to a single DSCP. Gorry, do you have any preference here?
> 
> Hmm, RFC2474 states that the DSCP is unstructured:
>   "The DSCP field is defined as an
>    unstructured field to facilitate the definition of future per-hop
>    behaviors.
> 
>    With some exceptions noted below, the mapping of codepoints to PHBs
>    MUST be configurable.  A DS-compliant node MUST support the logical
>    equivalent of a configurable mapping table from codepoints to PHBs"

This is the fundamental point (and it is entirely the result of ISP
people who were active in the diffserv WG that we have this rule, by the
way). If an operator doesn't support this, anything can happen.

DSCPs are not end-to-end. DSCP to behaviour mappings are not end-to-end.
If you ignore this, things *will* go wrong. Trying to arrange things so
that some arbitrary bit-masking of DCSP values will produce predictable
results *will* fail in some cases. I think it's futile.

Fundamentally I don't care which arbitrary bit pattern is recommended
for LE, as long as it doesn't clash with a CS code point. But if
you encroach on pool 3 I think that requires a formal update to RFC2474
or at least some very careful wording of the IANA considerations.

> IMHO it shouldn't be difficult to have a mapping table
> from each DSCP to a PHB. The mapping table should
> initially only contain mappings to the default PHB
> (no need to remark anything then).
> Configuring DSCP remarking by using bitmasks is IMHO
> not RFC2474 compliant and a broken concept causing
> the trouble we just ran into.

And you shouldn't fix broken products by fiddling with a standard;
that rewards the makers of broken products.

Regards
    Brian

> 
> Regards,
>  Roland
> 
> .
>