[Midnightbsd-cvs] src: tcp_syncache.c: Pick the smallest possible TCP window scaling

[laffer1 at midnightbsd.org]

Log Message:
-----------
Pick the smallest possible TCP window scaling factor that will still allow us ot scale up to sb_max aka
kern.ipc.maxsockbuf.

Anything larger will cause window scaling corruption on some firewalls.  (other end will think unscaled)

Obtained from: FreeBSD  revision 1.131

Modified Files:
--------------
    src/sys/netinet:
        tcp_syncache.c (r1.2 -> r1.3)

-------------- next part --------------
Index: tcp_syncache.c
===================================================================
RCS file: /home/cvs/src/sys/netinet/tcp_syncache.c,v
retrieving revision 1.2
retrieving revision 1.3
diff -L sys/netinet/tcp_syncache.c -L sys/netinet/tcp_syncache.c -u -r1.2 -r1.3
--- sys/netinet/tcp_syncache.c
+++ sys/netinet/tcp_syncache.c
@@ -966,14 +966,28 @@
 			int wscale = 0;
 
 			/*
-			 * Compute proper scaling value from buffer space:
-			 * Place window scale into sweet spot where it is
-			 * about one fourth of the MSS.  This allows us to
-			 * scale the receive buffer over a wide range while
-			 * not losing any efficiency or fine granularity.
+			 * Pick the smallest possible scaling factor that
+			 * will still allow us to scale up to sb_max, aka
+			 * kern.ipc.maxsockbuf.
+			 *
+			 * We do this because there are broken firewalls that
+			 * will corrupt the window scale option, leading to
+			 * the other endpoint believing that our advertised
+			 * window is unscaled.  At scale factors larger than
+			 * 5 the unscaled window will drop below 1500 bytes,
+			 * leading to serious problems when traversing these
+			 * broken firewalls.
+			 *
+			 * With the default maxsockbuf of 256K, a scale factor
+			 * of 3 will be chosen by this algorithm.  Those who
+			 * choose a larger maxsockbuf should watch out
+			 * for the compatiblity problems mentioned above.
+ 			 *
+ 			 * RFC1323: The Window field in a SYN (i.e., a <SYN>
+ 			 * or <SYN,ACK>) segment itself is never scaled.
 			 */
 			while (wscale < TCP_MAX_WINSHIFT &&
-			    (0x1 << wscale) < tcp_minmss)
+			    (TCP_MAXWIN << wscale) < sb_max)
 				wscale++;
 			sc->sc_request_r_scale = wscale;
 			sc->sc_requested_s_scale = to->to_requested_s_scale;