TCP congestion window pinned by integer arithmetic

[Brian Brombacher]

Hi,

RFC 5681 Section 3.1 has an Implementation Note that covers the bug fixed by 
the following patch.  I ran into this bug testing on a high latency link.  My 
congestion window was pinned to a specific value and could not open further, 
causing a lack of bandwidth utilization.

I chose if/assign rather than max(9) for clarity with the RFC; however, both 
work fine.

Cheers,
Brian


Index: tcp_input.c
===================================================================
RCS file: /home/brian/cvs/src/sys/netinet/tcp_input.c,v
retrieving revision 1.364
diff -u -r1.364 tcp_input.c
--- tcp_input.c 6 Dec 2019 14:43:14 -0000       1.364
+++ tcp_input.c 1 Jun 2020 21:16:26 -0000
@@ -1707,8 +1707,11 @@
                u_int cw = tp->snd_cwnd;
                u_int incr = tp->t_maxseg;
 
-               if (cw > tp->snd_ssthresh)
+               if (cw > tp->snd_ssthresh) {
                        incr = incr * incr / cw;
+                       if (incr == 0)
+                               incr = 1;
+               }
                if (tp->t_dupacks < tcprexmtthresh)
                        tp->snd_cwnd = ulmin(cw + incr,
                            TCP_MAXWIN << tp->snd_scale);