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[2a10:fc81:a806:d6a9::1]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4771e202182sm53555425e9.10.2025.10.29.05.18.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Oct 2025 05:18:09 -0700 (PDT) Date: Wed, 29 Oct 2025 13:18:08 +0100 From: Stefano Brivio To: Yumei Huang Subject: Re: [PATCH v6 4/4] tcp: Update data retransmission timeout Message-ID: <20251029131808.3e2a70c9@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> <20251029080953.4f671f56@elisabeth> <20251029083929.0dc52bc6@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: V_NP3o-U6mpMVUWBjw9D-TOtCohZ1LfwoI4uMAMezXU_1761740292 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Message-ID-Hash: 6T46L4I6GNSW425HEAW6VTM4HYGLGCBG X-Message-ID-Hash: 6T46L4I6GNSW425HEAW6VTM4HYGLGCBG 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 16:59:51 +0800 Yumei Huang wrote: > On Wed, Oct 29, 2025 at 3:39=E2=80=AFPM Stefano Brivio wrote: > > > > On Wed, 29 Oct 2025 15:32:33 +0800 > > Yumei Huang wrote: > > =20 > > > On Wed, Oct 29, 2025 at 3:10=E2=80=AFPM Stefano Brivio wrote: =20 > > > > > > > > On Wed, 29 Oct 2025 13:11:48 +0800 > > > > Yumei Huang wrote: > > > > =20 > > > > > On Wed, Oct 29, 2025 at 12:38=E2=80=AFPM Stefano Brivio wrote: =20 > > > > > > > > > > > > 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 retransmi= ssion according > > > > > > > > > > > to RFC 2988 and RFC 6298. Set the initial RTO to one = second as 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 time= rs, set based on flags: > > > > > > > > > > > * > > > > > > > > > > > - * - SYN_TIMEOUT_INIT: if no ACK is received from ta= p/guest during handshake > > > > > > > > > > > - * (flag ACK_FROM_TAP_DUE without ESTABLISHED even= t) within this time, resend > > > > > > > > > > > - * SYN. It's the starting timeout for the first SY= N retry. If this persists > > > > > > > > > > > - * for more than TCP_MAX_RETRIES or (tcp_syn_retri= es + > > > > > > > > > > > - * tcp_syn_linear_timeouts) times in a row, reset = the connection > > > > > > > > > > > - * > > > > > > > > > > > - * - ACK_TIMEOUT: if no ACK segment was received fro= m tap/guest, after sending > > > > > > > > > > > - * data (flag ACK_FROM_TAP_DUE with ESTABLISHED ev= ent), re-send data from the > > > > > > > > > > > - * socket and reset sequence to what was acknowled= ged. If this persists for > > > > > > > > > > > - * more than TCP_MAX_RETRIES times in a row, reset= the connection > > > > > > > > > > > + * - RTO_INIT: if no ACK segment was received from t= ap/guest, either during > > > > > > > > > > > + * handshake (flag ACK_FROM_TAP_DUE without ESTABL= ISHED event) or after > > > > > > > > > > > + * sending data (flag ACK_FROM_TAP_DUE with ESTABL= ISHED event), re-send data > > > > > > > > > > > + * from the socket and reset sequence to what was = acknowledged. This is the > > > > > > > > > > > + * timeout for the first retry, in seconds. If thi= s persists too many times > > > > > > > > > > > + * in a row, reset the connection: TCP_MAX_RETRIES= for established > > > > > > > > > > > + * connections, or (tcp_syn_retries + tcp_syn_line= ar_timeouts) during the > > > > > > > > > > > + * handshake. > > > > > > > > > > > * > > > > > > > > > > > * - FIN_TIMEOUT: if a FIN segment was sent to tap/g= uest (flag ACK_FROM_TAP_DUE > > > > > > > > > > > * with TAP_FIN_SENT event), and no ACK is receive= d 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, RFC 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_timeouts) > > > > > > > > > > > - it.it_value.tv_sec =3D = SYN_TIMEOUT_INIT; > > > > > > > > > > > + it.it_value.tv_sec =3D = RTO_INIT; > > > > > > > > > > > else > > > > > > > > > > > - it.it_value.tv_sec =3D = SYN_TIMEOUT_INIT << > > > > > > > > > > > + it.it_value.tv_sec =3D = RTO_INIT << > > > > > > > > > > > (conn->retries = - c->tcp.syn_linear_timeouts); > > > > > > > > > > > } > > > > > > > > > > > else > > > > > > > > > > > - it.it_value.tv_sec =3D ACK_TIME= OUT; > > > > > > > > > > > + it.it_value.tv_sec =3D RTO_INIT= << conn->retries; =20 > > > > > > > > > > > > > > > > > > > > Same as on 3/4, but here it's clearly more convenient: = just assign > > > > > > > > > > RTO_INIT, and multiply as needed in the if / else claus= es. =20 > > > > > > > > > > > > > > > > > > I guess we can't just assign RTO_INIT. Maybe assign it o= nly 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_ti= meouts) > > > > > > > > it.it_value.tv_sec <<=3D (conn->ret= ries - > > > > > > > > c->tcp.syn_= linear_timeouts); > > > > > > > > > > > > > > > > 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_timeo= uts; > > > > > > > > > > > > > > > > it.it_value.tv_sec =3D RTO_INIT << MAX(fact= or, 0); > > > > > > > > > > > > > > > > ? =20 > > > > > > > > > > > > > > Yeah, I understand this part now. > > > > > > > =20 > > > > > > > > =20 > > > > > > > > > > =20 > > > > > > > > > > > } else if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_F= IN_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 direct= ly the RTO > > > > > > > > > > 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 co= ncern 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-yuhuang@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 below: > > > > > > > > > > > > > > > > > > #define RETRY_ELAPSED(retries) ((RTO_INIT) * ((1 << ((ret= ries) + 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 t= he one > > > > > > stating: =20 > > > > > > > > > > I meant RETRY_ELAPSED was only used for data retransmission, whic= h > > > > > uses exponential backoff timeout directly, so syn_linear_timeouts= was > > > > > not relevant. =20 > > > > > > > > Ah, okay. > > > > =20 > > > > > > > > > > > > (2.5) A maximum value MAY be placed on RTO provided it is at= least 60 > > > > > > seconds. =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 w= ell? =20 > > > > > > > > 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 retransmission= s? =20 > > > > > > You are right. I assumed wrongly that syn_linear_timeouts is always 4= . > > > When it's 0, it's the same as data retransmissions. =20 > > > > > > > > Don't we have 3 bits to store the retry count as well, so we're lim= ited > > > > by TCP_MAX_RETRIES anyway? Looking at patch 1/4 I'd say it's the sa= me > > > > counter. =20 > > > > > > Yes, it's the same counter. I guess you mean we should clamp it as we= ll. =20 > > > > I didn't check this part, I thought we already did, but if we don't, we > > should do that, yes. > > =20 > > > > > > ...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 RTO > > > > > > MUST be re-initialized to 3 seconds when data transmis= sion > > > > > > 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 i= t's > > > > > > simpler than re-introducing separate starting values (one secon= d and > > > > > > three seconds). =20 >=20 > Sorry I'd like to confirm one more thing. By "use MAX(x, 3) clamp if > (c->conn->events & ESTABLISHED)", I guess you mean: >=20 > it.it_value.tv_sec =3D MAX(it.it_value.tv_sec, 3); Right. > Then the code would be: >=20 > it.it_value.tv_sec =3D RTO_INIT; > if (!(conn->events & ESTABLISHED)) { > int exp =3D conn->retries - c->tcp.syn_linear_timeouts; > it.it_value.tv_sec <<=3D MAX(exp, 0); > } else { > it.it_value.tv_sec =3D MAX(it.it_value.tv_sec, 3); > it.it_value.tv_sec <<=3D conn->retries; Hmm what's wrong with my previous suggestion of initialising 'exp' with conn->retries? Then it becomes (equivalent to your code snippet, but shorter): > } > it.it_value.tv_sec =3D MIN( > it.it_value.tv_sec, c->tcp.tcp_rto_max); something like: =09=09int exp =3D conn->retries, min =3D 0, max =3D c->tcp.tcp_rto_max; =09=09if (!(conn->events & ESTABLISHED)) =09=09=09exp -=3D c->tcp.syn_linear_timeouts; =09=09else =09=09=09min =3D /* add a constant for this */ 3; =09=09it.it_value.tv_sec =3D RTO_INIT << MAX(exp, 0); =09=09it.it_value.tv_sec =3D CLAMP(it.it_value.tv_sec, min, max); ...we don't have a CLAMP() macro at the moment, just MIN() and MAX() in util.h, but now that we need one, I would add it, similar to the one from the Linux kernel but without type checking (as it's not really practical here). Inspired from include/linux/kernel.h (current Linux kernel tree): #define clamp(val, lo, hi) min((typeof(val))max(val, lo), hi) we can do something similar with two differences: 1. all our macros are uppercase (we don't have as many as the kernel, and it's nice to know that they are macros from the name) and 2. as I mentioned we don't need/want type checking, so, say: #define CLAMP(x, min, max) MIN(MAX((x), (min)), (max)) ...or keep val / lo / hi if it's clearer, I don't really have a preference. I added parentheses around the arguments by the way because I think it's good practice, even though not needed in this case. It's the PRE02-C recommendation in CERT C (we generally stick to those recommendations whenever practical): https://wiki.sei.cmu.edu/confluence/display/c/PRE02-C.+Macro+replacement+= lists+should+be+parenthesized https://clang.llvm.org/extra/clang-tidy/checks/bugprone/macro-parentheses= .html Note that the Linux kernel has a compatible license (it's the same, actually, GPLv2+), and, regardless of that, the implementation is trivial and the idea is obvious, so I don't think we need to give explicit attribution in this case. But in other cases we do, or I guess it's fair to the author anyway, see for example siphash.h. > we basically set the starting value to 3 for data retransmissions no > matter if we have retried SYN. And the max total timeout would be > 3+6+12+24+48+96+120+120 =3D 429s. Is it what we want? Ah, yes. In the discussion I previously assumed that the default clamp value was 60 seconds, but it's actually 120 seconds. It looks correct to me. > Besides, I guess I > need to define a macro for "3" as well, like "ACK_TIMEOUT_INIT"? Or RTO_INIT_ACK? Maybe RTO_INIT_DATA? I'm thinking that you already have a RTO_INIT at this point, and this constant is very closely related to that one. > > > > > > > > > > I'm not sure I understand here. If the timer expires, didn't we r= eset > > > > > the connection directly? We would never get to the data transmiss= ion > > > > > phase? =20 > > > > > > > > In the RFC, "the timer expires" indicates one iteration of the time= out > > > > algorithm, that is, it's the same as our timer (tcp_timer_handler()= ) > > > > triggering. =20 > > > > > > I see. > > > > > > If you agree, I can add a new patch in this series to address the cla= mping. =20 > > > > I don't really have a preference, I guess it could be directly in 4/4 o= r > > in a separate patch, neither option should complicate things too much. > > =20 > > > > The paragraph says "after the three-way handshake completes", so it > > > > looks like the connection wasn't reset. > > > > =20 > > > > > > > Probably I should name it more clearly. > > > > > > > =20 > > > > > > > > > > > > > > > > But in any case, do you really need to calculate that expli= citly? 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 value > > > > > > > > you read from the kernel, then I guess it's just: > > > > > > > > > > > > > > > > --- > > > > > > > > it.it_value.tv_sec =3D MIN(it.it_value.tv_s= ec, c->tcp.rto_max); =20 > > > > > > > > > > > > > > After reading the comments in v3 when tcp_rto_max_ms was firs= t > > > > > > > mentioned again, I realized I got something wrong again. I th= ought it > > > > > > > was for the total timeout for all retries, so I need to calcu= late that > > > > > > > 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 y= our patience. > > > > > > > =20 > > > > > > > > } ... > > > > > > > > > > > > > > > > if (timerfd_settime(conn->timer, 0, &it, NULL)) > > > > > > > > flow_perror(conn, "failed to set timer"); > > > > > > > > --- =20 --=20 Stefano