[lwip-users] Duplicate FINs and CLOSE_WAIT state

[Kelvin Lawson]

Hi,

Jan's earlier message reminded me of a problem I've seen in tcp_in.c:

When we receive a FIN, we do a +1 and send an ack as we should, and go 
into CLOSE_WAIT state. If we receive a duplicate FIN, however, we do 
another +1, and send another ack with the new ackno. This can lead to 
sticky situations - From memory, I think the effect I saw was a storm of 
network traffic, where both ends are acking each other constantly, with 
the LWIP end erroneously upping the ackno every time.

The attached patch prevents the additional +1 if we receive duplicate FINs.

Cheers,
Kelvin.

Index: tcp_in.c
===================================================================
--- tcp_in.c    (revision 29)
+++ tcp_in.c    (working copy)
@@ -1023,7 +1023,13 @@
 
         tcplen = TCP_TCPLEN(&inseg);
 
-        pcb->rcv_nxt += tcplen;
+        /* FINs from the other end will be acked +1, but be careful
+           not to do this if we receive any duplicate FINs. For 
+           duplicate FINs we will already be in CLOSE_WAIT state
+           and will already have performed the +1. */
+        if (pcb->state != CLOSE_WAIT) {
+          pcb->rcv_nxt += tcplen;
+        }
 
         /* Update the receiver's (our) window. */
         if (pcb->rcv_wnd < tcplen) {