tcp: accept RST for rcv_nxt - 1 after receiving a FIN
Using a Mac OSX box as a client connecting to a Linux server, we have found that when certain applications (such as 'ab'), are abruptly terminated (via ^C), a FIN is sent followed by a RST packet on tcp connections. The FIN is accepted by the Linux stack but the RST is sent with the same sequence number as the FIN, and Linux responds with a challenge ACK per RFC 5961. The OSX client then sometimes (they are rate-limited) does not reply with any RST as would be expected on a closed socket. This results in sockets accumulating on the Linux server left mostly in the CLOSE_WAIT state, although LAST_ACK and CLOSING are also possible. This sequence of events can tie up a lot of resources on the Linux server since there may be a lot of data in write buffers at the time of the RST. Accepting a RST equal to rcv_nxt - 1, after we have already successfully processed a FIN, has made a significant difference for us in practice, by freeing up unneeded resources in a more expedient fashion. A packetdrill test demonstrating the behavior: // testing mac osx rst behavior // Establish a connection 0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 0.000 bind(3, ..., ...) = 0 0.000 listen(3, 1) = 0 0.100 < S 0:0(0) win 32768 <mss 1460,nop,wscale 10> 0.100 > S. 0:0(0) ack 1 <mss 1460,nop,wscale 5> 0.200 < . 1:1(0) ack 1 win 32768 0.200 accept(3, ..., ...) = 4 // Client closes the connection 0.300 < F. 1:1(0) ack 1 win 32768 // now send rst with same sequence 0.300 < R. 1:1(0) ack 1 win 32768 // make sure we are in TCP_CLOSE 0.400 %{ assert tcpi_state == 7 }% Signed-off-by:Jason Baron <jbaron@akamai.com> Cc: Eric Dumazet <edumazet@google.com> Acked-by:
Eric Dumazet <edumazet@google.com> Signed-off-by:
David S. Miller <davem@davemloft.net>
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