Re: [PATCH 5/8] tcp: Don't limit window to less-than-MSS values, use zero instead

On Thu, Dec 04, 2025 at 08:45:38AM +0100, Stefano Brivio wrote:

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

The logic change looks good to me, so, Reviewed-by: David Gibson However, a couple of points about the description (both commit message and comment). * Nagle's algorithm is certainly related, but it's not clear to me it's quite the same thing as the sender-side SWS avoidance algorithm - Nagle's exists for a different purpose, certainly. RFC 813 doesn't name Nagle's algorithm anywhere, although that could because the name wasn't as established at the time. * Since you're referencing RFC 813 anyway, it seems relevant that what you're doing here is pretty similar to the receiver-side SWS avoidance algorithm described in section 4.

--- 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

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson