On Fri, Jan 09, 2026 at 03:14:43PM +0100, Stefano Brivio wrote:
In general, we approximate the advertised window to zero if we would otherwise advertise less than a MSS worth, and the reasoning behind that is explained in cf1925fb7b77 ("tcp: Don't limit window to less-than-MSS values, use zero instead").
Then, in commit b40f5cd8c8e1 ("tcp: Use less-than-MSS window on no queued data, or no data sent recently"), I introduced some conditions under which we won't do that, including a check on whether any data was sent recently.
As an arbitrary but probably reasonable threshold, we consider data to have recently been sent if that occurred less than ten times the round-trip time (RTT) ago.
The time elapsed since the last data transmission is reported by the kernel in milliseconds, in the tcpi_last_data_sent field of struct tcp_info, and the RTT is reported in microseconds instead, in tcpi_rtt.
To avoid the risk of overflow in a simple way, for the purpose of this comparison, I converted tcpi_rtt to milliseconds first, but this means that the check will always be false (and we'll never approximate the window to zero) if the RTT is below one millisecond.
This, in turn, reintroduces nasty delay issues in transfers in non-local connections which have however almost-local (low) latency.
Given that we want to use ten times the RTT as an arbitrary "long enough" upper bound, round the RTT up while converting it to milliseconds.
As an alternative, we could perform the comparison in microseconds, but we would need a slightly more complicated implementation to exclude overflows, and it's definitely not worth it given the nature of this threshold.
Fixes: b40f5cd8c8e1 ("tcp: Use less-than-MSS window on no queued data, or no data sent recently") Signed-off-by: Stefano Brivio
Reviewed-by: David Gibson
--- tcp.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/tcp.c b/tcp.c index e7fa85f..9b7f505 100644 --- a/tcp.c +++ b/tcp.c @@ -1182,6 +1182,7 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn, if ((conn->flags & LOCAL) || tcp_rtt_dst_low(conn)) { new_wnd_to_tap = tinfo->tcpi_snd_wnd; } else { + unsigned rtt_ms_ceiling = DIV_ROUND_UP(tinfo->tcpi_rtt, 1000); uint32_t sendq; int limit;
@@ -1225,7 +1226,7 @@ int tcp_update_seqack_wnd(const struct ctx *c, struct tcp_tap_conn *conn, * with pending data in the outbound queue */ if (limit < MSS_GET(conn) && sendq && - tinfo->tcpi_last_data_sent < tinfo->tcpi_rtt / 1000 * 10) + tinfo->tcpi_last_data_sent < rtt_ms_ceiling * 10) 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