Re: [PATCH v4 5/8] tcp: Don't try to transmit right after the peer shrank the window to zero

On 2025-09-09 14:16, Stefano Brivio wrote:

If the peer shrinks the window to zero, we'll skip storing the new window, as a convenient

Is this really convenient? It looks more like an inconsistency with potential for future trouble to me. Wouldn't it be better with just a SEND_WIN_PROBE flag or similar to be reset as soon as the window goes non-zero again?

way to cause window probes (which exceed any zero-sized window, strictly speaking) if we don't get window updates in a while.

As we do so, though, we need to ensure we don't try to queue more data from the socket right after we process this window update, as the entire point of a zero-window advertisement is to keep us from sending more data.

Signed-off-by: Stefano Brivio Reviewed-by: David Gibson --- tcp.c | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/tcp.c b/tcp.c index b83510b..9c70a25 100644 --- a/tcp.c +++ b/tcp.c @@ -1271,8 +1271,10 @@ static void tcp_get_tap_ws(struct tcp_tap_conn *conn, * @c: Execution context * @conn: Connection pointer * @wnd: Window value, host order, unscaled + * + * Return: false on zero window (not stored to wnd_from_tap), true otherwise */ -static void tcp_tap_window_update(const struct ctx *c, +static bool tcp_tap_window_update(const struct ctx *c, struct tcp_tap_conn *conn, unsigned wnd) { wnd = MIN(MAX_WINDOW, wnd << conn->ws_from_tap); @@ -1285,13 +1287,14 @@ static void tcp_tap_window_update(const struct ctx *c, */ if (!wnd && SEQ_LT(conn->seq_ack_from_tap, conn->seq_to_tap)) { tcp_rewind_seq(c, conn); - return; + return false; }

conn->wnd_from_tap = MIN(wnd >> conn->ws_from_tap, USHRT_MAX);

/* FIXME: reflect the tap-side receiver's window back to the sock-side * sender by adjusting SO_RCVBUF? */

Not so sure. That sender will stop in due time anyway, with no harm done. Starting fiddling with SO_RCVBUF sounds like something to avoid.

+ return true; }

/** @@ -2101,9 +2104,8 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af, if (!th->ack) goto reset;

- tcp_tap_window_update(c, conn, ntohs(th->window)); - - tcp_data_from_sock(c, conn); + if (tcp_tap_window_update(c, conn, ntohs(th->window))) + tcp_data_from_sock(c, conn);

if (p->count - idx == 1) return 1; @@ -2113,8 +2115,8 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af, if (conn->events & TAP_FIN_RCVD) { tcp_sock_consume(conn, ntohl(th->ack_seq)); tcp_update_seqack_from_tap(c, conn, ntohl(th->ack_seq)); - tcp_tap_window_update(c, conn, ntohs(th->window)); - tcp_data_from_sock(c, conn); + if (tcp_tap_window_update(c, conn, ntohs(th->window))) + tcp_data_from_sock(c, conn);

if (conn->seq_ack_from_tap == conn->seq_to_tap) { if (th->ack && conn->events & TAP_FIN_SENT)