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[176.103.220.4]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-429952cbb2bsm24986623f8f.13.2025.10.29.00.09.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Oct 2025 00:09:55 -0700 (PDT) Date: Wed, 29 Oct 2025 08:09:53 +0100 From: Stefano Brivio To: Yumei Huang Subject: Re: [PATCH v6 4/4] tcp: Update data retransmission timeout Message-ID: <20251029080953.4f671f56@elisabeth> In-Reply-To: References: <20251017062838.21041-1-yuhuang@redhat.com> <20251017062838.21041-5-yuhuang@redhat.com> <20251024010438.16e19757@elisabeth> <20251028124426.534fd236@elisabeth> <20251029053804.711716aa@elisabeth> Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.49; x86_64-pc-linux-gnu) MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: l0BNjBglpVe6PR2ij1eliFXPK_bNC7py1EWjgm0DXzE_1761721796 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Message-ID-Hash: S5TGESEHRYQHYD6Z5H6ZCRSIUNVJHSST X-Message-ID-Hash: S5TGESEHRYQHYD6Z5H6ZCRSIUNVJHSST X-MailFrom: sbrivio@redhat.com 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, david@gibson.dropbear.id.au 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: On Wed, 29 Oct 2025 13:11:48 +0800 Yumei Huang wrote: > On Wed, Oct 29, 2025 at 12:38=E2=80=AFPM Stefano Brivio wrote: > > > > On Wed, 29 Oct 2025 11:06:44 +0800 > > Yumei Huang wrote: > > =20 > > > On Tue, Oct 28, 2025 at 7:44=E2=80=AFPM Stefano Brivio wrote: =20 > > > > > > > > On Tue, 28 Oct 2025 16:09:03 +0800 > > > > Yumei Huang wrote: > > > > =20 > > > > > On Fri, Oct 24, 2025 at 7:04=E2=80=AFAM Stefano Brivio wrote: =20 > > > > > > > > > > > > On Fri, 17 Oct 2025 14:28:38 +0800 > > > > > > Yumei Huang wrote: > > > > > > =20 > > > > > > > Use an exponential backoff timeout for data retransmission ac= cording > > > > > > > to RFC 2988 and RFC 6298. Set the initial RTO to one second a= s discussed > > > > > > > in Appendix A of RFC 6298. > > > > > > > > > > > > > > Also combine the macros defining the initial RTO for both SYN= and ACK. > > > > > > > > > > > > > > Signed-off-by: Yumei Huang > > > > > > > Reviewed-by: David Gibson > > > > > > > --- > > > > > > > tcp.c | 27 ++++++++++++--------------- > > > > > > > 1 file changed, 12 insertions(+), 15 deletions(-) > > > > > > > > > > > > > > diff --git a/tcp.c b/tcp.c > > > > > > > index 9385132..dc0ec6c 100644 > > > > > > > --- a/tcp.c > > > > > > > +++ b/tcp.c > > > > > > > @@ -179,16 +179,14 @@ > > > > > > > * > > > > > > > * Timeouts are implemented by means of timerfd timers, set = based on flags: > > > > > > > * > > > > > > > - * - SYN_TIMEOUT_INIT: if no ACK is received from tap/guest = during handshake > > > > > > > - * (flag ACK_FROM_TAP_DUE without ESTABLISHED event) withi= n this time, resend > > > > > > > - * SYN. It's the starting timeout for the first SYN retry.= If this persists > > > > > > > - * for more than TCP_MAX_RETRIES or (tcp_syn_retries + > > > > > > > - * tcp_syn_linear_timeouts) times in a row, reset the conn= ection > > > > > > > - * > > > > > > > - * - ACK_TIMEOUT: if no ACK segment was received from tap/gu= est, after sending > > > > > > > - * data (flag ACK_FROM_TAP_DUE with ESTABLISHED event), re= -send data from the > > > > > > > - * socket and reset sequence to what was acknowledged. If = this persists for > > > > > > > - * more than TCP_MAX_RETRIES times in a row, reset the con= nection > > > > > > > + * - RTO_INIT: if no ACK segment was received from tap/guest= , either during > > > > > > > + * handshake (flag ACK_FROM_TAP_DUE without ESTABLISHED ev= ent) or after > > > > > > > + * sending data (flag ACK_FROM_TAP_DUE with ESTABLISHED ev= ent), re-send data > > > > > > > + * from the socket and reset sequence to what was acknowle= dged. This is the > > > > > > > + * timeout for the first retry, in seconds. If this persis= ts too many times > > > > > > > + * in a row, reset the connection: TCP_MAX_RETRIES for est= ablished > > > > > > > + * connections, or (tcp_syn_retries + tcp_syn_linear_timeo= uts) during the > > > > > > > + * handshake. > > > > > > > * > > > > > > > * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (fl= ag ACK_FROM_TAP_DUE > > > > > > > * with TAP_FIN_SENT event), and no ACK is received within= this time, reset > > > > > > > @@ -342,8 +340,7 @@ enum { > > > > > > > #define WINDOW_DEFAULT 14600 = /* RFC 6928 */ > > > > > > > > > > > > > > #define ACK_INTERVAL 10 /* ms *= / > > > > > > > -#define SYN_TIMEOUT_INIT 1 /* s */ > > > > > > > -#define ACK_TIMEOUT 2 > > > > > > > +#define RTO_INIT 1 /* s, R= FC 6298 */ > > > > > > > #define FIN_TIMEOUT 60 > > > > > > > #define ACT_TIMEOUT 7200 > > > > > > > > > > > > > > @@ -588,13 +585,13 @@ static void tcp_timer_ctl(const struct = ctx *c, struct tcp_tap_conn *conn) > > > > > > > } else if (conn->flags & ACK_FROM_TAP_DUE) { > > > > > > > if (!(conn->events & ESTABLISHED)) { > > > > > > > if (conn->retries < c->tcp.syn_linear_t= imeouts) > > > > > > > - it.it_value.tv_sec =3D SYN_TIME= OUT_INIT; > > > > > > > + it.it_value.tv_sec =3D RTO_INIT= ; > > > > > > > else > > > > > > > - it.it_value.tv_sec =3D SYN_TIME= OUT_INIT << > > > > > > > + it.it_value.tv_sec =3D RTO_INIT= << > > > > > > > (conn->retries - c->tcp= .syn_linear_timeouts); > > > > > > > } > > > > > > > else > > > > > > > - it.it_value.tv_sec =3D ACK_TIMEOUT; > > > > > > > + it.it_value.tv_sec =3D RTO_INIT << conn= ->retries; =20 > > > > > > > > > > > > Same as on 3/4, but here it's clearly more convenient: just ass= ign > > > > > > RTO_INIT, and multiply as needed in the if / else clauses. =20 > > > > > > > > > > I guess we can't just assign RTO_INIT. Maybe assign it only when > > > > > retries=3D=3D0, otherwise multiply as it.it_value.tv_sec <<=3D1. = =20 > > > > > > > > Why can't you do that? Say: > > > > > > > > it.it_value.tv_sec =3D RTO_INIT > > > > if (!(conn->events & ESTABLISHED)) { > > > > if (conn->retries >=3D c->tcp.syn_linear_timeouts) > > > > it.it_value.tv_sec <<=3D (conn->retries - > > > > c->tcp.syn_linear_t= imeouts); > > > > > > > > but anyway, see below. > > > > =20 > > > > > But it seems more complicated. What do you think? =20 > > > > > > > > Or maybe, building on my latest comment to 3/4: > > > > > > > > int factor =3D conn->retries; > > > > > > > > if (!(conn->events & ESTABLISHED)) > > > > factor -=3D c->tcp.syn_linear_timeouts; > > > > > > > > it.it_value.tv_sec =3D RTO_INIT << MAX(factor, 0); > > > > > > > > ? =20 > > > > > > Yeah, I understand this part now. > > > =20 > > > > =20 > > > > > > =20 > > > > > > > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED= )) { > > > > > > > it.it_value.tv_sec =3D FIN_TIMEOUT; > > > > > > > } else { =20 > > > > > > > > > > > > The rest of the series looks good to me. > > > > > > > > > > > > It might be slightly more practical to factor in directly the R= TO > > > > > > clamp, and I don't think it's complicated now that you have the= helper > > > > > > from 2/4, but it's not a strong preference from my side, as the= series > > > > > > makes sense in any case. =20 > > > > > > > > > > Reading tcp_rto_max_ms can be easy with the helper. My concern is > > > > > about the way we get the total time for retries. > > > > > > > > > > I used to do it like this in v2, > > > > > https://archives.passt.top/passt-dev/20251010074700.22177-4-yuhua= ng@redhat.com/: > > > > > > > > > > +#define RETRY_ELAPSED(timeout_init, retries) \ > > > > > + ((timeout_init) * ((1 << ((retries) + 1)) - 2)) > > > > > > > > > > Though the formula is not quite right, we could refine it as belo= w: > > > > > > > > > > #define RETRY_ELAPSED(retries) ((RTO_INIT) * ((1 << ((retries) + = 1)) - 1)) > > > > > > > > > > Does it make sense to get the time this way? =20 > > > > > > > > Well, it also depends on c->tcp.syn_linear_timeouts, right? =20 > > > > > > Not really, it's only used for data retransmission, so > > > syn_linear_timeouts is not relevant. =20 > > > > Hmm, no, why? RFC 6298 covers SYN retries as well, and that's the one > > stating: =20 >=20 > I meant RETRY_ELAPSED was only used for data retransmission, which > uses exponential backoff timeout directly, so syn_linear_timeouts was > not relevant. Ah, okay. > > > > (2.5) A maximum value MAY be placed on RTO provided it is at least 6= 0 > > seconds. =20 >=20 > For SYN retries, as we used linear backoff + exponential backoff, and > also limited by TCP_MAX_RETRIES, the possible max RTO is far less than > 60s. So we didn't clamp it. Do you think we need to clamp it as well? If syn_linear_timeouts is 0 and tcp_syn_retries is 7, I guess we'll reach 2^0 + 2^1 + 2^2 + 2^3 + 2^4 + 2^5 + 2^6 + 2^7 =3D 247 seconds? Or just up to ... + 2^6, that is, 119 seconds? In any case, what is the difference compared to data retransmissions? Don't we have 3 bits to store the retry count as well, so we're limited by TCP_MAX_RETRIES anyway? Looking at patch 1/4 I'd say it's the same counter. > > ...the only thing that I don't see implemented in this version of the > > patch is paragraph 5.7: > > > > (5.7) If the timer expires awaiting the ACK of a SYN segment and the > > TCP implementation is using an RTO less than 3 seconds, the RT= O > > MUST be re-initialized to 3 seconds when data transmission > > begins (i.e., after the three-way handshake completes). > > > > I missed that while reviewing earlier versions. I guess we need to use > > a MAX(x, 3) clamp if (c->conn->events & ESTABLISHED). I think it's > > simpler than re-introducing separate starting values (one second and > > three seconds). =20 >=20 > I'm not sure I understand here. If the timer expires, didn't we reset > the connection directly? We would never get to the data transmission > phase? In the RFC, "the timer expires" indicates one iteration of the timeout algorithm, that is, it's the same as our timer (tcp_timer_handler()) triggering. The paragraph says "after the three-way handshake completes", so it looks like the connection wasn't reset. > > > Probably I should name it more clearly. > > > =20 > > > > > > > > But in any case, do you really need to calculate that explicitly? I= was > > > > thinking that you can just clamp the value when you use it in > > > > tcp_timer_ctl(). > > > > > > > > If c->tcp.rto_max is DIV_ROUND_CLOSEST(x, 1000), where 'x' is the v= alue > > > > you read from the kernel, then I guess it's just: > > > > > > > > --- > > > > it.it_value.tv_sec =3D MIN(it.it_value.tv_sec, c->t= cp.rto_max); =20 > > > > > > After reading the comments in v3 when tcp_rto_max_ms was first > > > mentioned again, I realized I got something wrong again. I thought it > > > was for the total timeout for all retries, so I need to calculate tha= t > > > and decide to reset the connection as in v2. =20 > > > > I think it actually applies to all the retries. > > =20 > > > Anyway, you are right. We don't need to do that. Thanks for your pati= ence. > > > =20 > > > > } ... > > > > > > > > if (timerfd_settime(conn->timer, 0, &it, NULL)) > > > > flow_perror(conn, "failed to set timer"); > > > > --- =20 --=20 Stefano