netdev - Re: [PATCH net-next] tcp: mptcp: use mptcp receive buffer space to select rcv window

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Message-Id: <20200425.203815.652543937267054453.davem@davemloft.net>
Date:   Sat, 25 Apr 2020 20:38:15 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     fw@...len.de
Cc:     netdev@...r.kernel.org, edumazet@...gle.com,
        mathew.j.martineau@...ux.intel.com, matthieu.baerts@...sares.net,
        pabeni@...hat.com
Subject: Re: [PATCH net-next] tcp: mptcp: use mptcp receive buffer space to
 select rcv window

From: Florian Westphal <fw@...len.de>
Date: Fri, 24 Apr 2020 12:31:50 +0200

> In MPTCP, the receive window is shared across all subflows, because it
> refers to the mptcp-level sequence space.
> 
> MPTCP receivers already place incoming packets on the mptcp socket
> receive queue and will charge it to the mptcp socket rcvbuf until
> userspace consumes the data.
> 
> Update __tcp_select_window to use the occupancy of the parent/mptcp
> socket instead of the subflow socket in case the tcp socket is part
> of a logical mptcp connection.
> 
> This commit doesn't change choice of initial window for passive or active
> connections.
> While it would be possible to change those as well, this adds complexity
> (especially when handling MP_JOIN requests).  Furthermore, the MPTCP RFC
> specifically says that a MPTCP sender 'MUST NOT use the RCV.WND field
> of a TCP segment at the connection level if it does not also carry a DSS
> option with a Data ACK field.'
> 
> SYN/SYNACK packets do not carry a DSS option with a Data ACK field.
> 
> Signed-off-by: Florian Westphal <fw@...len.de>

Applied.