Re: [tsvwg] Question about proper node behavior -ECN

[Greg White <[e-mail]>]

RFC3168 also includes a statement which allows manipulation of the ECN field:

“When the router's buffer
   is not yet full and the router is prepared to drop a packet to inform
   end nodes of incipient congestion, the router should first check to
   see if the ECT codepoint is set in that packet's IP header.  If so,
   then instead of dropping the packet, the router MAY instead set the
   CE codepoint in the IP header.”

And, it includes a section 18 which talks about possible changes of the ECN field in the network, and specifically 18.1.4 “Falsely Indicating ECN-Capability” which describes the transition of Not-ECT to ECT0 or ECT1 (and by extension CE) as being “incorrect”.

RFC9331 section 5.1 discusses L4S network node behavior and includes the statement: “An L4S AQM treatment follows similar codepoint transition rules to those in [RFC3168<https://www.rfc-editor.org/rfc/rfc9331.html#RFC3168>].” But, I haven’t found those rules stated very explicitly in RFC3168.

-Greg

From: tsvwg <[e-mail]> on behalf of Gorry Fairhurst <[e-mail]>
Organization: UNIVERSITY OF ABERDEEN
Date: Wednesday, May 17, 2023 at 9:15 AM
To: Ingemar Johansson S <ingemar.s.johansson=[e-mail]>, "[e-mail]" <[e-mail]>
Subject: Re: [tsvwg] Question about proper node behavior -ECN

On 17/05/2023 15:57, Ingemar Johansson S wrote:
Hi
A very short question.. I am trying to find the proper refrenence that says that nodes (routers, 5G gNB, WiFi AP...) should not remark Not-ECT to any of the other 3 code points. Where do I find it

Regards
/Ingemar

Hi Ingemar,

A start is the statment in RFC3168 that says this in section 22:



   Prior to RFC 2474<https://datatracker.ietf.org/doc/html/rfc2474>, routers were not permitted to modify bits in

   either the DSCP or ECN field of packets forwarded through them, and

   hence routers that comply only with RFCs prior to 2474 should have no

   effect on ECN.



Hence, if a router is not permitted to modify the field, unless it supports a spec that implements ECN.



Section 5 then states this is updated as:



The not-ECT codepoint '00' indicates a packet that is not using ECN.



I am sure someone will tell us if there is a clearer reference text somewhere else.



Best wishes,

Gorry