Re: [PATCH RFC mptcp] net: tcp: prevent transition to FIN_WAIT2 when mptcp has unacked data_fin

[Matthieu Baerts <matthieu.baerts@tessares.net>]

Hi Florian,

On 18/08/2022 18:21, Florian Westphal wrote:
> When an mptcp connection closes down, a MPTCP-level DATA_FIN (flag in mp tcp
> options) is sent.
> 
> We should wait until we can be sure the peer has received the mptcp-level ack of
> that DATA_FIN.  Else, we can end up with the last mptcp subflow in FIN_WAIT2 state.
> 
> In that state, TCP won't respond to incoming ACK anymore, because no
> progress happens from TCP point of view.

Thank you for looking at that!

(...)

> Its possible to either change tw_sk to have enough mptcp context to be
> able to send a packet back

Do you think changing tw_sk would be accepted upstream?

That would be good for MPTCP from a perf point of view I suppose. Or
maybe not worth it?
If it is worth it, could we have a 'struct mptcp_timewait_sock' for
MPTCP case, extending 'struct tcp_timewait_sock'? But then I guess the
"annoying" thing for upstream is that we might have to modify or split
functions from tcp_minisocks.c if we cannot add more indirections.

> or we can supress/delay the tw_sock
> transition.

All the time for MPTCP sockets?
Do you think it the impact will just be minor?

> This is what classic TCP is doing when it receives another
> FIN while in FIN1 state.
> 
> This change makes the test case work (another dss ack is sent), but
> there may be other corner cases where we need to delay the sk ->
> tw_sk transition.

We didn't hear anything about other cases so far, we can also see case
by case for this situation (corner cases at the end of a connection) :)

> If the general idea looks ok, perhaps its better to replace
> 
> tcp_time_wait(sk, skb, ..
> 
> with a mptcp aware helper, e.g.  tcp_time_wait_check(sk, skb, ..
> and place the option parsing for mptcp-subflows there?

Would it be better because we would put this in tcp_minisocks.c? Or just
to isolate MPTCP specific function in a small and dedicated function?
I don't have any strong opinion about one or the other.

Cheers,
Matt
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