Re: [PATCH mptcp-next RFC v4 2/2] mptcp: send MP_FAIL and infinite mapping on the same ACK

Hi Chenguang,

On 18/08/2026 11:14, Chenguang Zhao wrote:
> From: Chenguang Zhao <[email protected]>
> 
> On a single subflow, accepting MP_FAIL must leave MPTCP mode at once
> (RFC8684 §3.7). Emit the infinite mapping on the MP_FAIL reply ACK,
> then call mptcp_try_fallback() immediately after.

I think Sashiko is right about the issue of lacking option space to have
both the MP_FAIL and the infinite mapping sent together.

Please also note this from RFC 8684 §3.7.11 [1]:

> This infinite mapping will be a DSS option (Section 3.3) on the first
> new packet, containing a Data Sequence Mapping that acts retroactively

The "on the first new packet" seems to suggest the infinite mapping
shouldn't be attached to the MP_FAIL.

[1] https://datatracker.ietf.org/doc/html/rfc8684#section-3.7-11


Maybe a fallback for the single subflow without csum is enough? Or maybe
nothing needs to be done? Or maybe this fallback is just an
optimisation, and can be done in net-next (and both commits can be
combined)? WDYT?

> Fixes: 1e39e5a32ad7 ("mptcp: infinite mapping sending")
> Signed-off-by: Chenguang Zhao <[email protected]>
> ---
>  net/mptcp/options.c  | 33 ++++++++++++++++++++++++++++++++-
>  net/mptcp/pm.c       |  5 +++++
>  net/mptcp/protocol.c |  8 +-------
>  3 files changed, 38 insertions(+), 8 deletions(-)
> 
> diff --git a/net/mptcp/options.c b/net/mptcp/options.c
> index 97da22668dbe..ecf77e77d2de 100644
> --- a/net/mptcp/options.c
> +++ b/net/mptcp/options.c
> @@ -638,7 +638,29 @@ static bool mptcp_established_options_dss(struct sock *sk, struct sk_buff *skb,
>  	opts->csum_reqd = READ_ONCE(msk->csum_enabled);
>  	mpext = skb ? mptcp_get_ext(skb) : NULL;
>  
> -	if (!skb || (mpext && mpext->use_map) || snd_data_fin_enable) {
> +	if (unlikely(subflow->send_infinite_map)) {
> +		unsigned int map_size = TCPOLEN_MPTCP_DSS_BASE + TCPOLEN_MPTCP_DSS_MAP64;
> +
> +		if (opts->csum_reqd)
> +			map_size += TCPOLEN_MPTCP_DSS_CHECKSUM;
> +
> +		if (mpext) {
> +			opts->ext_copy = *mpext;
> +			opts->ext_copy.infinite_map = 1;
> +			opts->ext_copy.data_len = 0;
> +		} else {
> +			opts->ext_copy.use_map = 1;
> +			opts->ext_copy.dsn64 = 1;> +			opts->ext_copy.infinite_map = 1;
> +			opts->ext_copy.data_len = 0;
> +			opts->ext_copy.data_seq = READ_ONCE(msk->snd_nxt);
> +			opts->ext_copy.subflow_seq = subflow->rel_write_seq;
> +		}

You probably also need to reset "csum". Note that common fields
(infinite_map and data_len) can also be set once here.

(...)

Cheers,
Matt
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