netdev - Re: [PATCH v12 net-next 06/15] tcp: accecn: AccECN negotiation

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Message-ID: <dc03363a-36d0-4a56-9247-baa75d516620@redhat.com>
Date: Mon, 14 Jul 2025 15:10:45 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: chia-yu.chang@...ia-bell-labs.com, edumazet@...gle.com,
 linux-doc@...r.kernel.org, corbet@....net, horms@...nel.org,
 dsahern@...nel.org, kuniyu@...zon.com, bpf@...r.kernel.org,
 netdev@...r.kernel.org, dave.taht@...il.com, jhs@...atatu.com,
 kuba@...nel.org, stephen@...workplumber.org, xiyou.wangcong@...il.com,
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 shuah@...nel.org, linux-kselftest@...r.kernel.org, ij@...nel.org,
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Cc: Olivier Tilmans <olivier.tilmans@...ia.com>
Subject: Re: [PATCH v12 net-next 06/15] tcp: accecn: AccECN negotiation

On 7/4/25 10:53 AM, chia-yu.chang@...ia-bell-labs.com wrote:
> @@ -375,7 +379,8 @@ struct tcp_sock {
>  	u8	compressed_ack;
>  	u8	dup_ack_counter:2,
>  		tlp_retrans:1,	/* TLP is a retransmission */
> -		unused:5;
> +		syn_ect_snt:2,	/* AccECN ECT memory, only */
> +		syn_ect_rcv:2;	/* ... needed durign 3WHS + first seqno */

Minor nit: typo above: durign -> during

> +/* Infer the ECT value our SYN arrived with from the echoed ACE field */
> +static inline int tcp_accecn_extract_syn_ect(u8 ace)
>  {
> -	tp->received_ce = 0;
> -	tp->received_ce_pending = 0;
> +	/* Below is an excerpt from Tables 2 of the AccECN spec:
> +	 * +================================+========================+
> +	 * |        Echoed ACE field        | Received ECT values of |
> +	 * |      AE      CWR      ECE      |  our SYN arrived with  |
> +	 * +================================+========================+
> +	 * |       0         1         0    |         Not-ECT        |
> +	 * |       0         1         1    |         ECT(1)         |
> +	 * |       1         0         0    |         ECT(0)         |
> +	 * |       1         1         0    |           CE           |
> +	 * +================================+========================+
> +	 */
> +	if (ace & 0x1)
> +		return INET_ECN_ECT_1;
> +	if (!(ace & 0x2))
> +		return INET_ECN_ECT_0;
> +	if (ace & 0x4)
> +		return INET_ECN_CE;
> +	return INET_ECN_NOT_ECT;

Nit: implementing the above with a static array lookup would probably
make the code simpler/more clear

> +/* Used to form the ACE flags for SYN/ACK */
> +static inline u16 tcp_accecn_reflector_flags(u8 ect)
> +{
> +	/* TCP ACE flags of SYN/ACK are set based on IP-ECN codepoint received
> +	 * from SYN. Below is an excerpt from Table 2 of the AccECN spec:
> +	 * +====================+====================================+
> +	 * |  IP-ECN codepoint  |  Respective ACE falgs on SYN/ACK   |
> +	 * |   received on SYN  |       AE       CWR       ECE       |
> +	 * +====================+====================================+
> +	 * |      Not-ECT       |       0         1         0        |
> +	 * |      ECT(1)        |       0         1         1        |
> +	 * |      ECT(0)        |       1         0         0        |
> +	 * |        CE          |       1         1         0        |
> +	 * +====================+====================================+
> +	 */
> +	u32 flags = ect + 2;
> +
> +	if (ect == 3)
> +		flags++;
> +	return FIELD_PREP(TCPHDR_ACE, flags);

Same here.

> +}
>  
> -	wire_ace = tp->received_ce + TCP_ACCECN_CEP_INIT_OFFSET;
> -	th->ece = !!(wire_ace & 0x1);
> -	th->cwr = !!(wire_ace & 0x2);
> -	th->ae = !!(wire_ace & 0x4);
> +/* AccECN specificaiton, 3.1.2: If a TCP server that implements AccECN

Minor nit: typo above: specificaiton -> specification

Otherwise LGTM,

Acked-by: Paolo Abeni <pabeni@...hat.com>