From mboxrd@z Thu Jan 1 00:00:00 1970 Received: by passt.top (Postfix, from userid 1000) id 4B5835A0657; Mon, 08 Dec 2025 08:20:24 +0100 (CET) From: Stefano Brivio To: passt-dev@passt.top Subject: [PATCH v3 07/10] tcp: Don't limit window to less-than-MSS values, use zero instead Date: Mon, 8 Dec 2025 08:20:20 +0100 Message-ID: <20251208072024.3884137-8-sbrivio@redhat.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251208072024.3884137-1-sbrivio@redhat.com> References: <20251208072024.3884137-1-sbrivio@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Message-ID-Hash: E6VLI5NFBWO7RJJ5FSLZAWQ7UKIWP2KY X-Message-ID-Hash: E6VLI5NFBWO7RJJ5FSLZAWQ7UKIWP2KY X-MailFrom: sbrivio@passt.top X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: Max Chernoff , David Gibson X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: 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 923c1f2..6218f7c 100644 --- a/tcp.c +++ b/tcp.c @@ -1157,6 +1157,23 @@ 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