Re: [tsvwg] L4S DSCP (was: L4S drafts: Next Steps)

[Jonathan Morton <[e-mail]>]

> On 24 Mar, 2021, at 10:28 am, Gorry Fairhurst <[e-mail]> wrote:
> 
> This really  is not a new topic, and the WG should only be revisiting this if there is a new requirement - the design to allow the traffic to be DSCP-marked was not taken without thought and  visibility.

I agree that it's not a new topic.  However, it has renewed relevance with the introduction of the l4s-ops draft, since L4S has failed to adequately address the open issues which prevent its being deployed as an Internet-wide experiment (which is the predicate for many of the arguments against DSCP in B.4).

Since David Black brought up RFC-4774 (a remarkably prescient document), I will note here that it divides alternate-ECN proposals into three possible categories:

Option 1: cannot be deployed on the Internet.  This clearly describes DCTCP, for example.

Option 2: can reliably detect whether it is receiving RFC-3168 or "alternate" signals and switch modes gracefully.

Option 3: is inherently compatible with RFC-3168 traffic and infrastructure.

There is no Option 4 contemplated; certainly none where harm to or required action by innocent bystanders is accepted.

I appreciate that L4S intended to improve DCTCP sufficiently to merit an Option 2 classification.  However, it has clearly failed to do so to date, despite man-years of effort from its proponents and many hours of WG meeting time expounding its presumed benefits (while ignoring warnings of its downsides).  This leaves L4S as an Option 1 proposal, though with less restricted applicability than DCTCP since it does at least carry an identifier; it can only safely be deployed on networks that have been prepared for it.

This means a containment mechanism is required, and DSCP is perfect for the job.  The only alternative I see is to write l4s-ops - or include language in the other drafts - such that it requires deployment only in similarly isolated circumstances as DCTCP.  The latter option would be unpalatable, I'm sure, but people really have painted themselves into a corner on this issue.

 - Jonathan Morton