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.
This matches the suggestion from RFC 813, section 4.
Signed-off-by: Stefano Brivio
Reviewed-by: David Gibson
---
tcp.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/tcp.c b/tcp.c
index 533c8a7..3c046a5 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1155,6 +1155,23 @@ 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 mechanisms to prevent Silly Window
+ * Syndrome (SWS, described in 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.
+ *
+ * The mechanism to avoid SWS in the kernel is, implicitly,
+ * implemented by Nagle's algorithm (which was proposed after
+ * RFC 813).
+ *
+ * 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. This matches the
+ * suggestion from RFC 813, Section 4.
+ */
+ if (limit < MSS_GET(conn))
+ limit = 0;
+
new_wnd_to_tap = MIN((int)tinfo->tcpi_snd_wnd, limit);
}
--
2.43.0