ref: 8152e9d075e507d36b6edb48ac1e15082fabeed5
parent: 63a0d519bcdb02b226023b1c07343bc52791a677
author: cinap_lenrek <cinap_lenrek@felloff.net>
date: Sun Jan 27 17:12:50 EST 2019
devip: tcp: Don't respond to FIN-less ACKs during TIME-WAIT (thanks Barret Rhoden) Under the normal close sequence, when we receive a FIN|ACK, we enter TIME-WAIT and respond to that LAST-ACK with an ACK. Our TCP stack would send an ACK in response to *any* ACK, which included FIN|ACK but also included regular ACKs. (Or PSH|ACKs, which is what we were actually getting/sending). That was more ACKs than is necessary and results in an endless ACK storm if we were under the simultaneous close sequence. In that scenario, both sides of a connection are in TIME-WAIT. Both sides receive FIN|ACK, and both respond with an ACK. Then both sides receive *those* ACKs, and respond again. This continues until the TIME-WAIT wait period elapses and each side's TCP timers (in the Plan 9 / Akaros case) shut down. The fix for this is to only respond to a FIN|ACK when we are in TIME-WAIT.
--- a/sys/src/9/ip/tcp.c
+++ b/sys/src/9/ip/tcp.c
@@ -2399,7 +2399,8 @@
goto raise;
}
case Time_wait:
- tcb->flags |= FORCE;
+ if(seg.flags & FIN)
+ tcb->flags |= FORCE;
if(tcb->timer.state != TcptimerON)
tcpgo(tpriv, &tcb->timer);
}