Subject: [tsvwg] Two problems with draft-geib-tsvwg-diffserv-intercon-03

From: "Black, David" <[email-masked]>
Date: Sun, 28 Jul 2013 08:13:52 -0400

<WG chair hat on>

There are two items that I believe need to be corrected in
draft-geib-tsvwg-diffserv-intercon-03 before it's reasonable to consider that
draft for adoption as an official tsvwg WG draft.

(1) The draft confuses the Class Selector Codepoints (see RFC 2474) with service
classes (see RFC 4594).  This paragraph in the introduction illustrates the
problems which occur in numerous places in the draft:

   IP Precedence has been deprecated when DiffServ was standardised.  It
   is common practice today however to copy the DSCPs Bits 0-2 (called
   Class Selector Codepoints in the following) into MPLS TC or Ethernet
   P-Bits.  This is also reflected by the DiffServ codepoint definitions
   of AF and EF.  The Class Selector Codepoints shouldn't be used for
   backward compatibility only.  Class based PHBs may be applied in core
   network sections rather than then DSCP based PHBs. 

a) The class selector codepoints are *not* DSCP bits 0-2, see RFC 2474.  The
	attempt to redefine that term in Section 2 is not acceptable.  I realize
	that this draft is defining the class select*ion* codepoints in contrast
	to the class select*or* codepoints in RFC 2474, but I suspect that the
	latter were intended, and if not, those two terms are too close and
	easily confusable.
b) The class selector codepoints are used for more than backwards compatibility,
	see RFC 4594.
c) "Class based PHBs" means Service Classes, see RFC 4594.  PHBs are not "based
	on codepoints" - see the specifications of EF and AF for examples or
	RFC 4594.

(2) The discussion of CS6 and routing traffic at the end of Section 4 is
problematic.  This draft sets out to define 4 interconnect classes, and then
effectively adds a fifth based on CS6, but is rather unclear about it, weakening
the entire draft.

I believe we've received enough input from the Routing Area on the importance
of CS6 for BGP at carrier interconnects that it needs to be dealt with directly
in contrast to this draft's approach of admitting that it might be necessary
to use CS6, but discouraging its use.

</WG chair hat on>

<WG chair hat off>

I would also suggest revising the discussion about abstract classes at the end
of Section 2 and connecting it to RFC 4594's discussion of service classes as
much of this has already been covered there.

</WG chair hat off>

Thanks,
--David
----------------------------------------------------
David L. Black, Distinguished Engineer
EMC Corporation, 176 South St., Hopkinton, MA  01748
+1 (508) 293-7953             FAX: +1 (508) 293-7786
[email-masked]        Mobile: +1 (978) 394-7754
----------------------------------------------------

---

From: Brian E Carpenter <[email-masked]>
Date: Mon, 29 Jul 2013 08:07:13 +1200

On 29/07/2013 00:13, Black, David wrote:
>    of AF and EF.

It's a fact that the CS PHBs (with mandatory 6-bit DSCPs) and the EF
and AF PHBs (with  recommended 6-bit DSCPs) were artfully chosen such that
if they were interpreted by a legacy router supporting 3-bit IP Precedence,
something reasonable would happen. An unintended side-effect of that
is that if bits 0-2 are mapped into a 3-bit priority field in another
technology, something reasonable will happen. But *that's all*. It's
a logical error to assert anything else. I fully agree with David's
comments below.

>    The Class Selector Codepoints shouldn't be used for

Absolutely. That's been true for 20+ years, too.

   Brian