Re: [tcpm] A possible simplification for AccECN servers

[Mirja Kuehlewind <[e-mail]>]

Hi Jonathan,

I don’t think we can or should derive a normative requirement from this table. The table rather explains which mode you are in depending on the handshake that has happened. So it says: when client A is ECN capable and requests classic ECN support in the handshake and server B is AccECN capable and negotiated classic ECN support in the handshake then you are in classic ECN mode. 

Even if you implement classic ECN or AccECN, you can of course always configure your machine to not use ECN. So for me that is an interface question which we don’t address in the draft.

Mirja



> On 25. Nov 2019, at 17:35, Jonathan Morton <[e-mail]> wrote:
> 
>> On 25 Nov, 2019, at 5:46 pm, Mirja Kuehlewind <[e-mail]> wrote:
>> 
>> I need to double-check what the exact context of the sentence Bob mentions below is in the accECN draft but probably this should not be a normative requirement in this draft at all. I’m actually further not sure what exactly is meant with “support” in that sentence, but the question if a server replies to a RFC3168 SYN with an RFC3168 SYN-ACK is solely a question for RFC3168 and not AccECN.
> 
> As far as I can tell, the requirement is not worded precisely as Bob put it, but is implied in Section 3.1.1:
> 
>   +--------+--------+------------+-------------+----------------------+
>   | A      | B      |  SYN A->B  |   SYN/ACK   | Feedback Mode        |
>   |        |        |            |     B->A    |                      |
>   +--------+--------+------------+-------------+----------------------+
>   |        |        | AE CWR ECE |  AE CWR ECE |                      |
> <snip>
>   | Nonce  | AccECN | 0   1   1  |  0   0   1  | classic ECN          |
>   | ECN    | AccECN | 0   1   1  |  0   0   1  | classic ECN          |
>   | No ECN | AccECN | 0   0   0  |  0   0   0  | Not ECN              |
> 
> [...]
> 
>   3.  The third block shows the cases where the TCP server (B) supports
>       AccECN but the TCP client (A) supports some earlier variant of
>       TCP feedback, indicated in its SYN.  Therefore, as soon as an
>       AccECN-enabled TCP server (B) receives the SYN shown, it MUST set
>       both its half connections into the feedback mode shown in the
>       rightmost column.