Re: [tsvwg] draft diffserv-intercon: Handling of a scavenger class / CS1

[<[e-mail]>]

Hi Mikael,

thanks for your proposal. Indeed, transport of LE(Scavenger) by a DSCP 000xx? at DiffServ-Intercon interfaces was the only suitable solution I imagined myself.

Brian quotes RFC3662 below. Now if an AF class is used to implement LE within a domain, that's still not resulting in a globally non ambiguous DSCP. This may justify a re-write to 000xx0 at a DiffServ-Intercon interface if the latter is enhanced by text at least reserving these DSCPs for LE. 

There should be no unknown DSCPs with AF like syntax, if DSCPs 000xx0 indicate LE at DiffServ-Intercon interfaces and unknown DSCPs face a rewrite of DSCP bits 0-2 to zero (bleaching). Then every unknown AF like DSCP will be made LE. 
I just note that then bleaching removes the necessity to rewrite the AF_DSCPs_used_for_LE at a sending interconnection interface.

Regards,

Ruediger


-----Ursprüngliche Nachricht-----
Von: tsvwg [mailto:[e-mail]] Im Auftrag von Mikael Abrahamsson
Gesendet: Freitag, 29. Mai 2015 23:11
An: Brian E Carpenter
Cc: [e-mail]
Betreff: Re: [tsvwg] draft diffserv-intercon: Handling of a scavenger class / CS1

On Sat, 30 May 2015, Brian E Carpenter wrote:

> Firstly, note that RFC 3662 is very intentionally not standards-track 
> and does not recommend a DSCP value. To me, this discussion seems to 
> mix up two orthogonal concepts:
>
> 1. What the PHB for a scavenger class actually is (or in Mikael's 
> suggestion to AQM, what the PHB set is, since mention of priority 
> implies several members of a PHB set, like AFxy).
>
> 2. What DSCP values happen to be used for this PHB set. Despite the 
> confusion introduced by RFC 3662, those values don't have to ones 
> already used for other purposes, especially not CSx. Actually the text 
> makes it clear that the DSCPs recommended for AFxy should be 
> considered:
>
>   If a CS PHB is used, note that this configuration will violate the
>   "SHOULD" of section 4.2.2.2 of RFC 2474 [RFC2474] since CS1 will have
>   a less timely forwarding than CS0.  An operator's goal of providing
>   an LE PDB is sufficient cause for violating the SHOULD.  If an AF PHB
>   is used, it must be configured and a DSCP assigned such that it does
>   not violate the "MUST" of paragraph three of section 2 of RFC 2597
>   [RFC2597] which provides for a "minimum amount of forwarding
>   resources".

I was only talking about what the 6 DSCP bits should be for this to ever start to work widely across the Internet. My suggestion is 000xx0 and the recommendation is to only bleach the first three bits at ISP interconnects instead of all 6 bits (which is the most commonly done at this time). This is backwards compatible with CS0/BE and it's also backwards compatible with default queuing behaviour when doing automatic IP precedence -> 802.1p and the recommended 4 queue ethernet prioritization (where CS1 and
CS2 is lower than CS0 and CS3).

-- 
Mikael Abrahamsson    email: [e-mail]