Re: [tsvwg] residential broadband BCP PHB and CP treatment Re: CC/bleaching thoughts for draft-ietf-tsvwg-le-phb-04

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

Hi Mikael,

On 20/04/2018 18:28, Mikael Abrahamsson wrote:
> On Fri, 20 Apr 2018, Brian E Carpenter wrote:
> 
>> Mikael,
>>
>>> If CP == 1 goto queueLE;
>>
>> Sure, if IANA assigns the code point that the draft requests.
> 
> Of course.
> 
>>> I would
>>> like whatever we come up with to be RFC8325 compatible and at least not
>>> conflicting with it.
>>
>> If you mean 8325 *as updated by draft-ietf-tsvwg-le-phb* we are fine.
>> But it will be very important to get that update into everybody's head.
> 
> Can you please enlighten me why you think it's ok to put CS1 into the same 
> queue as LE (background, as per 8325), but it's not ok to remap CP 
> CS1->LE?

Because RFC2474 clearly states that CS1 gets precedence over DF (=CS0).
(Which in fact goes back to RFC791 and RFC1349.)

"PHBs selected by a Class Selector Codepoint
 SHOULD give packets a probability of timely forwarding that is not
 lower than that given to packets marked with a Class Selector
 codepoint of lower relative order..." (RFC2474 section 4.2.2.2)

None of the other RFCs that use the CS1 codepoint for LE have formally
updated RFC2474; in other words they all violate the SHOULD above.
Specifically, RFC4594 did not formally update RFC2474.

A user who sets CS1 because they want better treatment than default
is doing exactly what an IETF standard says. So if you see CS1 on
incoming traffic, you have no idea whether the user wanted Precedence 1
or LE, unless you have a specific understanding with that user.

To me that means that the correct default behaviour is to map CS1 to
DF, as a compromise. Of course, if you *know* whether the user is
relying on RFC2474 or on RFC4594, you can configure the correct
mapping. But that's always true of diffserv.

   Brian

> 
> Because my proposal for the second scenario in my earlier email would 
> otherwise have been (adding remap of CS1 -> LE):
> 
> If CP == 1 then pass; # allow LE
> If CP == 8 then set CP = 1; # remap CS1 to LE
> If CP => 2 then set CP = 0; # set 2-63 to 0
> 
> I think this is an important distinction and we need to decide how to 
> handle this case. Is CS1 more likely to be equivalent to BE or LE? Because 
> in this scenario, it's going into one of these two. What's the least harm?
>