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=wVcAKP/J; dkim-atps=neutral Received: from mail.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) by passt.top (Postfix) with ESMTPS id 190CF5A0BC2 for ; Mon, 10 Nov 2025 11:56:47 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gibson.dropbear.id.au; s=202510; t=1762772204; bh=XTE2VARA9Ye75HoYtEZZjuQ+5HcL+8YqAPRl1STcEPU=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=wVcAKP/JBGw6QVqfGY34+NR5uF+roEf+PvovcqoLLdLBrmzHUrMj2owF7S11Hqygx Swe2YQ8FWgvX/eNWjgzyo1KA43qmhuYoUBwLaS7aXCvdszidbYxqDecA6k7TrBaK/U xT956SCklubh2VQSEuqFA7jMv3B4zC+M5tAQH7D9hhIhSIdQRvOQPdMMm/216TYg1I 63dk8q8I3c3uuV5JIbBOtbZhDt/UXtBSNb8Ct3R7CLsnMFzAcvT9qHUst0fBSkGyEh Fg+STy0MAkSfvjuFdsF9x73Qujixr4dxNduGbS7a/kVPsrpmHgzHU9SEHL++Sw+yWr 8YxRWYfKaTFmA== Received: by gandalf.ozlabs.org (Postfix, from userid 1007) id 4d4mnS0Hz9z4wCZ; Mon, 10 Nov 2025 21:56:44 +1100 (AEDT) Date: Mon, 10 Nov 2025 21:56:39 +1100 From: David Gibson To: Yumei Huang Subject: Re: [PATCH v8 6/6] tcp: Clamp the retry timeout Message-ID: References: <20251110093137.87705-7-yuhuang@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="umYXoOP4t0mxmy5k" Content-Disposition: inline In-Reply-To: <20251110093137.87705-7-yuhuang@redhat.com> Message-ID-Hash: 4UOANQKRYWYCKH6PA5GALQIL3EDIOV43 X-Message-ID-Hash: 4UOANQKRYWYCKH6PA5GALQIL3EDIOV43 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: --umYXoOP4t0mxmy5k Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Mon, Nov 10, 2025 at 05:31:37PM +0800, Yumei Huang wrote: > Clamp the TCP retry timeout as Linux kernel does. If a retry occurs > during the handshake and the RTO is below 3 seconds, re-initialise > it to 3 seconds for data retransmissions according to RFC 6298. >=20 > Suggested-by: Stefano Brivio > Signed-off-by: Yumei Huang Looks correct, so Reviewed-by: David Gibson A few possible improvements (mostly comments/names) below. > --- > tcp.c | 25 ++++++++++++++++++++----- > tcp.h | 2 ++ > tcp_conn.h | 1 + > 3 files changed, 23 insertions(+), 5 deletions(-) >=20 > diff --git a/tcp.c b/tcp.c > index ee111e0..b015658 100644 > --- a/tcp.c > +++ b/tcp.c > @@ -187,6 +187,9 @@ > * for established connections, or (syn_retries + syn_linear_timeouts)= times > * during the handshake, reset the connection > * > + * - RTO_INIT_ACK: if the RTO is less than this, re-initialise RTO to th= is for > + * data retransmissions > + * Now that this is conditional on SYN being retried, the description doesn't look entirely accurate any more. > * - 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 > * the connection > @@ -340,6 +343,7 @@ enum { > =20 > #define ACK_INTERVAL 10 /* ms */ > #define RTO_INIT 1 /* s, RFC 6298 */ > +#define RTO_INIT_ACK 3 /* s, RFC 6298 */ The relevance of "_ACK" in the name is not obvious to me. > #define FIN_TIMEOUT 60 > #define ACT_TIMEOUT 7200 > =20 -> @@ -365,9 +369,11 @@ uint8_t tcp_migrate_rcv_queue [TCP_MIGRATE_RCV_QUE= UE_MAX]; > =20 > #define SYN_RETRIES "/proc/sys/net/ipv4/tcp_syn_retries" > #define SYN_LINEAR_TIMEOUTS "/proc/sys/net/ipv4/tcp_syn_linear_timeouts" > +#define RTO_MAX_MS "/proc/sys/net/ipv4/tcp_rto_max_ms" > =20 > #define SYN_RETRIES_DEFAULT 6 > #define SYN_LINEAR_TIMEOUTS_DEFAULT 4 > +#define RTO_MAX_MS_DEFAULT 120000 > #define MAX_SYNCNT 127 /* derived from kernel's limit */ > =20 > /* "Extended" data (not stored in the flow table) for TCP flow migration= */ > @@ -392,7 +398,7 @@ static const char *tcp_state_str[] __attribute((__unu= sed__)) =3D { > =20 > static const char *tcp_flag_str[] __attribute((__unused__)) =3D { > "STALLED", "LOCAL", "ACTIVE_CLOSE", "ACK_TO_TAP_DUE", > - "ACK_FROM_TAP_DUE", "ACK_FROM_TAP_BLOCKS", > + "ACK_FROM_TAP_DUE", "ACK_FROM_TAP_BLOCKS", "SYN_RETRIED", > }; > =20 > /* Listening sockets, used for automatic port forwarding in pasta mode o= nly */ > @@ -590,10 +596,13 @@ 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) { > - int exp =3D conn->retries; > + int exp =3D conn->retries, timeout =3D RTO_INIT; > 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->flags & SYN_RETRIED) > + timeout =3D MAX(timeout, RTO_INIT_ACK); > + timeout <<=3D MAX(exp, 0); > + it.it_value.tv_sec =3D MIN(timeout, c->tcp.rto_max); > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > it.it_value.tv_sec =3D FIN_TIMEOUT; > } else { > @@ -2440,6 +2449,7 @@ void tcp_timer_handler(const struct ctx *c, union e= poll_ref ref) > flow_trace(conn, "SYN timeout, retry"); > tcp_send_flag(c, conn, SYN); > conn->retries++; > + conn_flag(c, conn, SYN_RETRIED); > tcp_timer_ctl(c, conn); > } > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED)) { > @@ -2811,10 +2821,15 @@ void tcp_get_rto_params(struct ctx *c) > v =3D read_file_integer(SYN_LINEAR_TIMEOUTS, SYN_LINEAR_TIMEOUTS_DEFAUL= T); > c->tcp.syn_linear_timeouts =3D MIN(v, MAX_SYNCNT); > =20 > + v =3D read_file_integer(RTO_MAX_MS, RTO_MAX_MS_DEFAULT); > + c->tcp.rto_max =3D MIN(DIV_ROUND_CLOSEST(v, 1000), INT_MAX); Possibly we should verify this is =3D> RTO_INIT. > + > debug("Read sysctl values syn_retries: %"PRIu8 > - ", syn_linear_timeouts: %"PRIu8, > + ", syn_linear_timeouts: %"PRIu8 > + ", rto_max: %d", > c->tcp.syn_retries, > - c->tcp.syn_linear_timeouts); > + c->tcp.syn_linear_timeouts, > + c->tcp.rto_max); > } > =20 > /** > diff --git a/tcp.h b/tcp.h > index 37d7758..c4945c3 100644 > --- a/tcp.h > +++ b/tcp.h > @@ -60,6 +60,7 @@ union tcp_listen_epoll_ref { > * @fwd_out: Port forwarding configuration for outbound packets > * @timer_run: Timestamp of most recent timer run > * @pipe_size: Size of pipes for spliced connections > + * @rto_max: Maximal retry timeout (in s) > * @syn_retries: SYN retries using exponential backoff timeout > * @syn_linear_timeouts: SYN retries before using exponential backoff ti= meout > */ > @@ -68,6 +69,7 @@ struct tcp_ctx { > struct fwd_ports fwd_out; > struct timespec timer_run; > size_t pipe_size; > + int rto_max; > uint8_t syn_retries; > uint8_t syn_linear_timeouts; > }; > diff --git a/tcp_conn.h b/tcp_conn.h > index 923af36..e36910c 100644 > --- a/tcp_conn.h > +++ b/tcp_conn.h > @@ -77,6 +77,7 @@ struct tcp_tap_conn { > #define ACK_TO_TAP_DUE BIT(3) > #define ACK_FROM_TAP_DUE BIT(4) > #define ACK_FROM_TAP_BLOCKS BIT(5) > +#define SYN_RETRIED BIT(6) > =20 > #define SNDBUF_BITS 24 > unsigned int sndbuf :SNDBUF_BITS; > --=20 > 2.51.0 >=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 --umYXoOP4t0mxmy5k Content-Type: application/pgp-signature; name=signature.asc -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEO+dNsU4E3yXUXRK2zQJF27ox2GcFAmkRxOYACgkQzQJF27ox 2GegpQ/+JnFl+ypPQlsJBhg0GKL6K9BFktB280Od85BGXmiglF6XAspIeiSzzK/3 yF5FYmu8ZKZM5oURHTmjEcVlvU0Ixsj/WVOYzI/UgSwpNZiimBjk0hbjrLJxM3EB mVswU/7Jpy3zbZ/nmsEWh9ibxdjnycMrxyrSX3YiFq+rkCadZzwoGIXkeYSJrna8 4LlOjfm2g1h1D+yfy3tPQPnpJtQfeKdEd9c0EpNPAq2vEGuIqjl7O65c5LE5ICHb SgkE8V3WpAXQju4tOdrDPURC0h+BMI3utgoGpXYS/Mto1gEbtl6s+OgdVO8Gzg1C 3mlmESwA9OxesAt27KME0THGCOymhesFglL4IDWTutyifr2XZBPMw2/0RK6Ccplv YpVILdRJkATmz+q7VXriPs6J1XLA2WmtmveBd3NvdNYBPy5+n0rpxzTm7hSGOjeH LKZym97Yxyn86RurH3Jg8sgP0Bb52bP7EAu8mXdJ2c25l7rJY2nvcDFEfoEViI7G MXTU+6LLOlYFtgd9UgxmOqEB/gTEI1luYdCA2xOHPSD8njEalROi1uBICZkps6gp dOKxzARl5gnRTix+MIj1mikQQJKxkyY2ND599yjNni6UfWHUIXG4FXf3BAtPcbTV COqjVcZWRtxnIe8pYzEopQi030NTg8lYQKoWqV32ULJcjmsd/n0= =L6R5 -----END PGP SIGNATURE----- --umYXoOP4t0mxmy5k--