Re: [L4s-discuss] [EXTERNAL] Accurate ECN & TCP_CONGESTION & connect()

[rjmcmahon <[e-mail]>]

On 2024-01-05 06:45, Overcash, Michael (CCI-Atlanta) wrote:
> The ECR bits are part of the IP header and are not concerned with the
> TCP 3-way handshake. This is also why UDP protocols can utilize L4S.
> 
> If L4S is enabled after the TCP session is established, then the TCP
> handshake packets will end up in the Classic Queue but subsequent
> packets will go to the AQM queue.
> 
What about accurate ECN? I assume this can only be enabled per the 3WHS.

https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-accurate-ecn

During the TCP handshake at the start of a connection, to request more 
accurate ECN feedback the TCP Client (host A) MUST set the TCP flags 
(AE,CWR,ECE) = (1,1,1) in the initial SYN segment.

If a TCP Server (B) that is AccECN-enabled receives a SYN with the above 
three flags set, it MUST set both its half connections into AccECN mode. 
Then it MUST set the AE, CWR and ECE TCP flags on the SYN/ACK to the 
combination in the top block of Table 2 that feeds back the IP-ECN field 
that arrived on the SYN. This applies whether or not the Server itself 
supports setting the IP-ECN field on a SYN or SYN/ACK (see Section 2.5 
for rationale).

When the TCP Server returns any of the 4 combinations in the top block 
of Table 2, it confirms that it supports AccECN. The TCP Server MUST NOT 
set one of these 4 combination of flags on the SYN/ACK unless the 
preceding SYN requested support for AccECN as above.

Once a TCP Client (A) has sent the above SYN to declare that it supports 
AccECN, and once it has received the above SYN/ACK segment that confirms 
that the TCP Server supports AccECN, the TCP Client MUST set both its 
half connections into AccECN mode. The TCP Client MUST NOT enter AccECN 
mode (or any feedback mode) before it has received the first SYN/ACK.

Bob