On Fri, Jan 31, 2020 at 1:12 PM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote: > > > > On 1/31/20 7:10 AM, Neal Cardwell wrote: > > On Fri, Jan 31, 2020 at 7:25 AM <sjpark@xxxxxxxxxx> wrote: > >> > >> From: SeongJae Park <sjpark@xxxxxxxxx> > >> > >> When closing a connection, the two acks that required to change closing > >> socket's status to FIN_WAIT_2 and then TIME_WAIT could be processed in > >> reverse order. This is possible in RSS disabled environments such as a > >> connection inside a host. > >> > >> For example, expected state transitions and required packets for the > >> disconnection will be similar to below flow. > >> > >> 00 (Process A) (Process B) > >> 01 ESTABLISHED ESTABLISHED > >> 02 close() > >> 03 FIN_WAIT_1 > >> 04 ---FIN--> > >> 05 CLOSE_WAIT > >> 06 <--ACK--- > >> 07 FIN_WAIT_2 > >> 08 <--FIN/ACK--- > >> 09 TIME_WAIT > >> 10 ---ACK--> > >> 11 LAST_ACK > >> 12 CLOSED CLOSED > > > > AFAICT this sequence is not quite what would happen, and that it would > > be different starting in line 8, and would unfold as follows: > > > > 08 close() > > 09 LAST_ACK > > 10 <--FIN/ACK--- > > 11 TIME_WAIT > > 12 ---ACK--> > > 13 CLOSED CLOSED > > > > > >> The acks in lines 6 and 8 are the acks. If the line 8 packet is > >> processed before the line 6 packet, it will be just ignored as it is not > >> a expected packet, > > > > AFAICT that is where the bug starts. > > > > AFAICT, from first principles, when process A receives the FIN/ACK it > > should move to TIME_WAIT even if it has not received a preceding ACK. > > That's because ACKs are cumulative. So receiving a later cumulative > > ACK conveys all the information in the previous ACKs. > > > > Also, consider the de facto standard state transition diagram from > > "TCP/IP Illustrated, Volume 2: The Implementation", by Wright and > > Stevens, e.g.: > > > > https://courses.cs.washington.edu/courses/cse461/19sp/lectures/TCPIP_State_Transition_Diagram.pdf > > > > This first-principles analysis agrees with the Wright/Stevens diagram, > > which says that a connection in FIN_WAIT_1 that receives a FIN/ACK > > should move to TIME_WAIT. > > > > This seems like a faster and more robust solution than installing > > special timers. > > > > Thoughts? > > > This is orthogonal I think. > > No matter how hard we fix the other side, we should improve the active side. > > Since we send a RST, sending the SYN a few ms after the RST seems way better > than waiting 1 second as if we received no packet at all. > > Receiving this ACK tells us something about networking health, no need > to be very cautious about the next attempt. Yes, all good points. Thanks! > Of course, if you have a fix for the passive side, that would be nice to review ! I looked into fixing this, but my quick reading of the Linux tcp_rcv_state_process() code is that it should behave correctly and that a connection in FIN_WAIT_1 that receives a FIN/ACK should move to TIME_WAIT. SeongJae, do you happen to have a tcpdump trace of the problematic sequence where the "process A" ends up in FIN_WAIT_2 when it should be in TIME_WAIT? If I have time I will try to construct a packetdrill case to verify the behavior in this case. thanks, neal > > >
