Re: [tsvwg] Update to Position Statement on ECT(1)

["Holland, Jake" <[e-mail]>]

Hi Sebastian,

On 5/28/20, 12:57 AM, "Sebastian Moeller" <[e-mail]> wrote:
> I have a question regarding tunneling, that IMHO is left unanswered
> so far. Why is the rule for ECN and tunneling not simply: copy the
> inner to outer on encapsulation and the outer to inner on
> decapsulation. Period.

Please see RFC 6040.  Section 7 gives the design principles,
and Section 4 describes the current standard rules.  The Intro
also reviews some of the other considerations that went into it.

Of course there are some important considerations toward ensuring
that CE is not erased on the inner or outer packet, regardless
of whatever network condition led to that occurring, if it
occurs, which seemed to get a bit more attention than the ECT(1)
overwriting.  But as far as I can tell, the overall reasoning is
more or less laid out there.

If you're looking for a deeper "why", I guess there's also the
comments in the writeup and final reviews:
https://datatracker.ietf.org/doc/rfc6040/writeup/
https://datatracker.ietf.org/doc/rfc6040/ballot/

Probably also there's some discussion in the list archives around
that time if you care to search for it.

But it seems the consensus at the time landed on a copy of outer
to inner for ECT(1) overwriting ECT(0) on decapsulation, but not
for ECT(0) overwriting ECT(1) (which I think is the only change I'd
want in order to enable the 2-signal proposal, to the extent it
seems possibly worth doing).  To me it's an understandable decision,
especially considering RFC 6660, which seems essentially "SCE but
only applicable inside DSCP", and would prefer not to lose that pre-
congestion signal if something weird happens while it's tunneled.

As an aside, I think it's a fair claim that Bob made, that he's the
one who has been trying to fit this jigsaw together for over a decade.
Not to endorse his conclusion that the best choice today is to do
something that seems to break regular ECN, but between his authorship
of those specs and his long list of other publications on the topic,
it's a good bet that Bob has given the whole space a larger quantity
of thought than anyone else alive, which is certainly worth something.

Best regards,
Jake