tcp: mptcp: use mptcp receive buffer space to select rcv window

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@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 2 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 2f45cde168c4..ba4482130f08 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2772,8 +2772,12 @@ u32 __tcp_select_window(struct sock *sk)
 	int mss = icsk->icsk_ack.rcv_mss;
 	int free_space = tcp_space(sk);
 	int allowed_space = tcp_full_space(sk);
-	int full_space = min_t(int, tp->window_clamp, allowed_space);
-	int window;
+	int full_space, window;
+
+	if (sk_is_mptcp(sk))
+		mptcp_space(sk, &free_space, &allowed_space);
+
+	full_space = min_t(int, tp->window_clamp, allowed_space);
 
 	if (unlikely(mss > full_space)) {
 		mss = full_space;