Re: [PATCH net-next] net: tcp: Updating MSS, when the sending window is smaller than MSS.

On Thu, Jun 10, 2021 at 8:00 AM Shuyi Cheng
[off-list ref] wrote:

Thank you very much for your reply!

Maybe it's not clear enough that I described it. The scenario where the
above problem occurs is precisely because the tcp server sets the size
of RCVBUFF to be smaller after the connection is established. Here is a
sample code that caused the problem.

# default tcp_rmem is 87380

Except that this value is overridden at connection establishment.

tcp_rmem[1] is only a floor value, say if you want a reasonable value
even if MSS == 100

tcpServerSocket= socket.socket(socket.AF_INET, socket.SOCK_STREAM)
tcpServerSocket.bind(server_addr)
tcpServerSocket.listen()
while True:
     connection,client_addr = tcpServerSocket.accept()
     # Shrink rmem
     connection.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, 16*1024)

Therefore, when the developer calls the sock_setsockopt function to
reset RCVBUF, we can use sock to determine the TCP state. When in the
connected state, it is not allowed to set RCVBUF smaller than mss.

Sure, but the application can _also_ set SO_RCVBUF before listen() or connect()

We can not have assumptions about SO_RCVBUF values and socket states.
Otherwise we would have to add some sk_rcvbuf adjustments every time
the socket state is changed.