That bit is already there. L4S changes the meaning of the bit to allow a more ac...

fragmede · 2023-12-11T09:15:38 1702286138

More particularly, L4S is an advancement to the existing ECN (Explicit Congestion Notification) extension to TCP/IP, allowing for more advanced algorithms to cut down latency further.

xorcist · 2023-12-11T13:09:27 1702300167

The main problem with ECN was the remarkably widespread behaviour by middleboxes that either cleared that bit or straight up dropped the packets. Maybe that situation has improved now?

jlivingood · 2023-12-11T14:58:11 1702306691

It has definitely changed. Looking at measurements on this it seems there is only 1 major transit network doing that, and they are working to fix it.

toomim · 2023-12-11T09:11:35 1702285895

Yes, thanks for the clarification. IIRC was explained to me as "we put the last unused bit in IP packets to use, and get this great feature from it."

ajb · 2023-12-11T09:20:54 1702286454

Yeah. It being the last bit (really the last codepoint in a 2 bit field) there was a big argument over it:

https://datatracker.ietf.org/meeting/interim-2020-tsvwg-01/s...

https://mailarchive.ietf.org/arch/msg/tsvwg/rXWRHAyGOuu_qOGM...