Re: [PATCH v2 6/9] tcp: Don't limit window to less-than-MSS values, use zero instead

On Mon, 8 Dec 2025 17:43:14 +1100 David Gibson wrote:

On Mon, Dec 08, 2025 at 01:22:14AM +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. This matches the suggestion from RFC 813, section 4.

Signed-off-by: Stefano Brivio Reviewed-by: David Gibson

Looking at this again, I'm worrying if it might allow a pathalogical case here: unlikely to hit, but very bad if it did.

Suppose we have: 1. A receiver that wants to consume its input in fixed largish (~64kiB) records 2. The receiver has locked its SO_RCVBUF to that record length, or only slightly more 3. The receive buf is near full - but not quite a full record's worth

The receiver doesn't consume anything, because it doesn't have a full record. Its rcvbuf is near full, so its kernel advertises only a small window. We approximate that to zero, so the sender can't send anything. So, the record never gets completed and we stall completely.

I don't think it can be a problem because the receiver shouldn't advertise less than a MSS in that case anyway, but I need to look up normative references for this. -- Stefano