From cf1925fb7b777d1b4dae4816f30af1fa5c6ebfa5 Mon Sep 17 00:00:00 2001 From: Stefano Brivio Date: Thu, 4 Dec 2025 07:12:54 +0100 Subject: tcp: Don't limit window to less-than-MSS values, use zero instead 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 4080a1e..8bf1b51 100644 --- a/tcp.c +++ b/tcp.c @@ -1158,6 +1158,23 @@ 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); } -- cgit v1.2.3