From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: passt.top; dmarc=none (p=none dis=none) header.from=gibson.dropbear.id.au Authentication-Results: passt.top; dkim=pass (2048-bit key; secure) header.d=gibson.dropbear.id.au header.i=@gibson.dropbear.id.au header.a=rsa-sha256 header.s=202510 header.b=bZlBtnG4; dkim-atps=neutral Received: from mail.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by passt.top (Postfix) with ESMTPS id B23E05A026F for ; Mon, 03 Nov 2025 02:37:59 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202510; t=1762133877; bh=X2K/Z/I12MXoARthUPiZN9UO9mlu4/F3mblcpJjnLiU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=bZlBtnG4NTU53SU8WeWEsZTWSRLvX+PqmDLmSf2WF6MBLvFOpPGiMkH2MY22jVCgC WkdPxXZxO4a1zrrjC6HEudouNXkz0ti69IA/anFF21iQWWOnswjSe+IBtYrbwjf+bs 4RPMH1nm0tH7/aqk4iwrYo+r7SNTS0ak+tVQpVm1/d1fJumEnjm1OcRnFmtoBqxcNd nARx0bKTY/ztkNv0QkGto2RKe1mWe7Q7sqllH9zxopgZYLFvSxjgzldnGo+ukgscqi vVd3UVAwhpZI1H71KXDBlsHezZo0reuI0yk8Nxq+aoM4UAqztTazUaNvs4W8Dkcfgr dnW6zgA/sD3uw== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4d0Djx3DyHz4wCk; Mon, 03 Nov 2025 12:37:57 +1100 (AEDT) Date: Mon, 3 Nov 2025 12:18:27 +1100 From: David Gibson To: Yumei Huang Subject: Re: [PATCH v7 4/5] tcp: Update data retransmission timeout Message-ID: References: <20251031054242.7334-1-yuhuang@redhat.com> <20251031054242.7334-5-yuhuang@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="9mpg6yqik4JjfwDr" Content-Disposition: inline In-Reply-To: <20251031054242.7334-5-yuhuang@redhat.com> Message-ID-Hash: BV6LYTONWIQS4KQR57L2RTEGONRUCM5B X-Message-ID-Hash: BV6LYTONWIQS4KQR57L2RTEGONRUCM5B X-MailFrom: dgibson@gandalf.ozlabs.org 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: passt-dev@passt.top, sbrivio@redhat.com 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: --9mpg6yqik4JjfwDr Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Oct 31, 2025 at 01:42:41PM +0800, Yumei Huang wrote: > Use an exponential backoff timeout for data retransmission according > to RFC 2988 and RFC 6298. Set the initial RTO to one second as discussed > in Appendix A of RFC 6298. >=20 > Also combine the macros defining the initial RTO for both SYN and ACK. >=20 > Signed-off-by: Yumei Huang > Reviewed-by: David Gibson As reported, the carried over R-b was a minor mistake, since the code has changed, but here's a new one: Reviewed-by: David Gibson Small comment below, though. > --- > tcp.c | 30 ++++++++++++------------------ > 1 file changed, 12 insertions(+), 18 deletions(-) >=20 > diff --git a/tcp.c b/tcp.c > index bada88a..96ee56a 100644 > --- a/tcp.c > +++ b/tcp.c > @@ -179,16 +179,13 @@ > * > * Timeouts are implemented by means of timerfd timers, set based on fla= gs: > * > - * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest during hands= hake > - * (flag ACK_FROM_TAP_DUE without ESTABLISHED event) within this time,= resend > - * SYN. It's the starting timeout for the first SYN retry. Retry for > - * TCP_MAX_RETRIES or (tcp_syn_retries + tcp_syn_linear_timeouts) time= s, > - * reset the connection > - * > - * - ACK_TIMEOUT: if no ACK segment was received from tap/guest, after s= ending > - * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-send data f= rom the > - * socket and reset sequence to what was acknowledged. If this persist= s for > - * more than TCP_MAX_RETRIES times in a row, reset the connection > + * - RTO_INIT: if no ACK segment was received from tap/guest, either dur= ing > + * handshake (flag ACK_FROM_TAP_DUE without ESTABLISHED event) or after > + * sending data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re-sen= d data > + * from the socket and reset sequence to what was acknowledged. This i= s the > + * timeout for the first retry, in seconds. Retry for TCP_MAX_RETRIES = times > + * for established connections, or (tcp_syn_retries + > + * tcp_syn_linear_timeouts) times during the handshake, reset the conn= ection > * > * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_= TAP_DUE > * with TAP_FIN_SENT event), and no ACK is received within this time, = reset > @@ -342,8 +339,7 @@ enum { > #define WINDOW_DEFAULT 14600 /* RFC 6928 */ > =20 > #define ACK_INTERVAL 10 /* ms */ > -#define SYN_TIMEOUT_INIT 1 /* s, RFC 6928 */ > -#define ACK_TIMEOUT 2 > +#define RTO_INIT 1 /* s, RFC 6298 */ > #define FIN_TIMEOUT 60 > #define ACT_TIMEOUT 7200 > =20 > @@ -589,12 +585,10 @@ static void tcp_timer_ctl(const struct ctx *c, stru= ct tcp_tap_conn *conn) > if (conn->flags & ACK_TO_TAP_DUE) { > it.it_value.tv_nsec =3D (long)ACK_INTERVAL * 1000 * 1000; > } else if (conn->flags & ACK_FROM_TAP_DUE) { > - if (!(conn->events & ESTABLISHED)) { > - int exp =3D conn->retries - c->tcp.syn_linear_timeouts; I didn't spot it in the previous patch, but this is (theoretically) buggy. conn->retries is unsigned, so the subtraction will be performed unsigned and only then cast to signed. I think that will probably do the right thing in practice, but I don't think that's guaranteed by the C standard (and might even be UB). > - it.it_value.tv_sec =3D SYN_TIMEOUT_INIT << MAX(exp, 0); > - } > - else > - it.it_value.tv_sec =3D ACK_TIMEOUT; > + int exp =3D conn->retries; This change fixes it, by forcing the cast to a signed int before the subtraction. It also removes the minor style error I noted in the previous patch. Given that, I don't think we need to worry about either of them. > + if (!(conn->events & ESTABLISHED)) > + exp -=3D c->tcp.syn_linear_timeouts; > + it.it_value.tv_sec =3D RTO_INIT << MAX(exp, 0); > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > it.it_value.tv_sec =3D FIN_TIMEOUT; > } else { > --=20 > 2.49.0 >=20 --=20 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 --9mpg6yqik4JjfwDr Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmkIAuIACgkQzQJF27ox 2GcdbA/+I74IB5+kBf+BuuEohTJxQim2W+MgmS7CWEpxC8Wz73VfMb0eb1c1GpAJ ysFjVKjdWTVfHiSq+srMyG82Axil7qi8xPwX+OFecufnbCgdpLAKm9paubT00nIk v8KmYlKFrfBBiMPeydpaFwjpOtxwFUgqhqfs0C4lIzXGM6qdh0QrqapiaI6GAFkX SMe6O8I+NmuPXetqllIFGQxd+n/8spG/kymYpOvv7BHOaFhbTDaBdUPzDDNY7RfF NtE9+/9YvEPb+z38o4FXg2XGnZXoowWIzk8/aT9ORE4qP6jOcDvuJ7RNp+BN15Uq QlIBaEXFJwOBNNqQvcYy9P0MHpHE9Cq9EzNkX0RMZQKoI5A6/yjjfZdhndJ48rjm 86vyP5tS9EHpRXsPgDdkfOUFTL5ri/n49nUC+8b2rYBBHpaaQq9CTbWqGzZwZIUi 7t377MBTmBPTEmUCAY5G789F2NT3jaf8EblA8JzMigfk6TxAYqBAMYqX+9hoRQ6s 781tlBH517rgpk0wlBxtOw1r/oFGy/sZ0yw9XLfIDUUuzn6wvJb2IZoGmrV5E4SK dCd/VS5+893I3gwFbqeThzhHJuZk6y+ddOM/quzSOnriiWD5lIKVJ7wc2Bv1ZJum gyHGt/EzNKFVICXrnOsVsE3ZNDeLDOQQO0XVepMD+cg3Y8hdIYY= =gr/k -----END PGP SIGNATURE----- --9mpg6yqik4JjfwDr--