If the sender uses data clumping (including Nagle's algorithm) for
Silly Window Syndrome (SWS) avoidance, advertising less than a MSS
means the sender might stop sending altogether, and window updates
after a low window condition are just as important as they are in
a zero-window condition.
For simplicity, approximate that limit to zero, as we have an
implementation forcing window updates after zero-sized windows.
Signed-off-by: Stefano Brivio
---
tcp.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/tcp.c b/tcp.c
index fbf97a0..2220059 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1140,6 +1140,18 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn,
else
limit = SNDBUF_GET(conn) - (int)sendq;
+ /* If the sender uses Nagle's algorithm to prevent Silly Window
+ * Syndrome (SWS, RFC 813 Section 3) it's critical that, should
+ * the window ever become less than the MSS, we advertise a new
+ * value once it increases again to be above it.
+ *
+ * To this end, for simplicity, approximate a window value below
+ * the MSS to zero, as we already have mechanisms in place to
+ * force updates after the window becomes zero.
+ */
+ if (limit < MSS_GET(conn))
+ limit = 0;
+
new_wnd_to_tap = MIN((int)tinfo->tcpi_snd_wnd, limit);
}
--
2.43.0