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Message-ID: <CADVnQyn7dqO8m4UjjQvujH4z8HFYOm0_mb5xNpNhgTdpG8L_PA@mail.gmail.com>
Date: Sun, 18 Jan 2026 19:05:37 -0500
From: Neal Cardwell <ncardwell@...gle.com>
To: "Chia-Yu Chang (Nokia)" <chia-yu.chang@...ia-bell-labs.com>
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Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 1/1] selftests/net: Add packetdrill packetdrill cases
On Sun, Jan 18, 2026 at 5:56 PM Chia-Yu Chang (Nokia)
<chia-yu.chang@...ia-bell-labs.com> wrote:
>
>
> > -----Original Message-----
> > From: Neal Cardwell <ncardwell@...gle.com>
> > Sent: Sunday, January 18, 2026 5:11 PM
> > To: Chia-Yu Chang (Nokia) <chia-yu.chang@...ia-bell-labs.com>
> > Cc: pabeni@...hat.com; edumazet@...gle.com; parav@...dia.com; linux-doc@...r.kernel.org; corbet@....net; horms@...nel.org; dsahern@...nel.org; kuniyu@...gle.com; bpf@...r.kernel.org; netdev@...r.kernel.org; dave.taht@...il.com; jhs@...atatu.com; kuba@...nel.org; stephen@...workplumber.org; xiyou.wangcong@...il.com; jiri@...nulli.us; davem@...emloft.net; andrew+netdev@...n.ch; donald.hunter@...il.com; ast@...erby.net; liuhangbin@...il.com; shuah@...nel.org; linux-kselftest@...r.kernel.org; ij@...nel.org; Koen De Schepper (Nokia) <koen.de_schepper@...ia-bell-labs.com>; g.white@...lelabs.com; ingemar.s.johansson@...csson.com; mirja.kuehlewind@...csson.com; cheshire <cheshire@...le.com>; rs.ietf@....at; Jason_Livingood@...cast.com; Vidhi Goel <vidhi_goel@...le.com>; Willem de Bruijn <willemb@...gle.com>
> > Subject: Re: [PATCH net-next 1/1] selftests/net: Add packetdrill packetdrill cases
> >
> >
> > CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.
> >
> >
> >
> > On Thu, Jan 8, 2026 at 5:46 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> > >
> > > On Thu, Jan 8, 2026 at 10:58 AM <chia-yu.chang@...ia-bell-labs.com> wrote:
> > > >
> > > > From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> > > >
> > > > Linux Accurate ECN test sets using ACE counters and AccECN options
> > > > to cover several scenarios: Connection teardown, different ACK
> > > > conditions, counter wrapping, SACK space grabbing, fallback schemes,
> > > > negotiation retransmission/reorder/loss, AccECN option drop/loss,
> > > > different handshake reflectors, data with marking, and different sysctl values.
> > > >
> > > > Co-developed-by: Ilpo Järvinen <ij@...nel.org>
> > > > Signed-off-by: Ilpo Järvinen <ij@...nel.org>
> > > > Co-developed-by: Neal Cardwell <ncardwell@...gle.com>
> > > > Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> > > > ---
> > >
> > > Chia-Yu, thank you for posting the packetdrill tests.
> > >
> > > A couple thoughts:
> > >
> > > (1) These tests are using the experimental AccECN packetdrill support
> > > that is not in mainline packetdrill yet. Can you please share the
> > > github URL for the version of packetdrill you used? I will work on
> > > merging the appropriate experimental AccECN packetdrill support into
> > > the Google packetdrill mainline branch.
> >
> > An update on the 3 patches at:
> >
> > https://github.com/google/packetdrill/pull/96
> >
> > (1) I have merged the following patch into the google packetdrill repo to facilitate testing of the AccECN patch series:
> >
> > "net-test: packetdrill: add Accurate ECN (AccECN) option support"
> > https://github.com/google/packetdrill/pull/96/changes/f6861f888bc7f1e08026de4825519a95504d1047
> >
> > (2) The following patch I did not yet merge, because it proposes to add an odd number of u32 fields to tcp_info, so AFAICT leaves a 4-byte padding hole at the end of tcp_info:
> >
> > net-test: packetdrill: Support AccECN counters through tcpi
> > https://github.com/google/packetdrill/pull/96/changes/f43649c87a2aa79a33a78111d3d7e5f027d13a7f
> >
> > I think we'll need to tweak the AccECN kernel patch series so that it does not leave a 4-byte padding hole at the end of tcp_info, then update this packetdrill patch to match the kernel patch.
> >
> > Let's come up with another useful u32 field we can add to the tcp_info struct, so that the kernel patch doesn't add a padding hole at the end of tcp_info.
> >
> > One idea would be to add another field to represent newer options and connection features that are enabled. AFAICT all 8 bits of the tcpi_options field have been used, so we can't use more bits in that field. I'd suggest we add a u32 tcpi_more_options field before the tcpi_received_ce field, so we can encode other useful info, like:
> >
> > + 1 bit to indicate whether AccECN was negotiated (this can go in a
> > separate patch)
> >
> > + 1 bit to indicate whether TCP_NODELAY was set (since forgetting to
> > use TCP_NODELAY is a classic cause of performance problems; again this can go in a separate patch)
> >
> > (And there will be future bits of info we want to add...)
> >
> > Also, regarding the comment in this line:
> > __u32 tcpi_received_ce; /* # of CE marks received */
> >
> > That comment is ambiguous, since it doesn't indicate whether it's counting (potentially LRO/GRO) skbs or TCP segments. I would suggest clarifying that this is counting segments:
> >
> > __u32 tcpi_received_ce; /* # of CE marked segments received */
> >
>
> Hi Neal,
>
> Related to these 32-bit hole, two extra entries are added into b40671b5ee588c8a61b2d0eacbad32ffc57e9a8f of net-next, and one straightforward way is to apply these changes also in tcp.h of packetdrill (This is my miss).
>
> + __u16 tcpi_accecn_fail_mode;
> + __u16 tcpi_accecn_opt_seen;
>
> But I would prefer to update this, because tcpi_accecn_fail_mode and tcpi_accecn_opt_seen overall just needs 8 bits (i.e., 4 bits for tcpi_accecn_fail_mode and 2 bits for tcpi_accecn_opt_seen).
>
> So, maybe we could add u16 tcpi_more_options before tcpi_received_ce and change tcpi_accecn_fail_mode and tcpi_accecn_opt_seen both into u8.
> Within tcpi_more_options, add one bit related to TCP_NODELAY as you said.
> And within tcpi_accecn_opt_seen, add one bit related to whether AccECN was negotiated as you said, then we can leave more unused bits in tcpi_more_options.
>
> Another thought is to use a single u32 before tcpi_received_ce, in which 4 bits for tcpi_accecn_fail_mode, 2 bits for tcpi_accecn_opt_seen, 26 bits for tcpi_more_options.
>
> What do you think?
I would suggest something like your last suggestion, where there is a
u32 before tcpi_received_ce, with bit fields for tcpi_accecn_fail_mode
(4 bits) and tcpi_accecn_opt_seen (2 bits), and a tcpi_options2 for
the remaining unused bits in the u32.
I am leaning toward tcpi_options2 rather than tcpi_more_options,
because I guess in the future we might want yet another options bit
field, in which case it would be better to have {tcpi_options,
tcpi_options2, and tcpi_options3}, rather than having {tcpi_options,
tcpi_more_options, and tcpi_yet_more_options}. :-)
And rather than a single bit indicating whether AccECN was negotiated,
it occurs to me that it would probably be better to have a 2-bit enum
with 4 values, corresponding to the modes in tcp_ecn.h:
tcp_ecn_disabled(), tcp_ecn_mode_rfc3168(), tcp_ecn_mode_accecn(), and
tcp_ecn_mode_pending().
We also need to keep in mind that since the tcpi_accecn_fail_mode (4
bits) and tcpi_accecn_opt_seen (2 bits) enums are exported to
user-space, they will become part of the kernel API to userspace, so
should be moved out of tcp_ecn.h and instead be declared in
include/uapi/linux/tcp.h. We declare constant values exported to user
space in that file: (a) to make it easier for maintainers to remember
not to change the values for these, so kernel changes don't break
user-space apps; (b) to make it easier for application developers to
find the #define values they need to decode the values exported in
struct tcp_info. :-)
So how about something like:
--- in include/uapi/linux/tcp.h:
/* Values for tcpi_ecn_mode */
#define TCPI_ECN_DISABLED 0x0
#define TCPI_ECN_RFC3168 0x1
#define TCPI_ECN_ACCECN 0x2
#define TCPI_ECN_PENDING 0x3
/* Values for tcpi_accecn_opt_seen */
#define TCP_ACCECN_OPT_NOT_SEEN 0x0
#define TCP_ACCECN_OPT_EMPTY_SEEN 0x1
#define TCP_ACCECN_OPT_COUNTER_SEEN 0x2
#define TCP_ACCECN_OPT_FAIL_SEEN 0x3
/* Values for tcpi_accecn_fail_mode */
#define TCP_ACCECN_ACE_FAIL_SEND BIT(0)
#define TCP_ACCECN_ACE_FAIL_RECV BIT(1)
#define TCP_ACCECN_OPT_FAIL_SEND BIT(2)
#define TCP_ACCECN_OPT_FAIL_RECV BIT(3)
...
__u32 tcpi_ecn_mode:2,
tcpi_accecn_opt_seen: 2,
tcpi_accecn_fail_mode: 4,
tcpi_options2:24;
__u32 tcpi_received_ce; /* # of CE marked segments received */
...
--- in tcp_get_info() in net/ipv4/tcp.c:
if (tcp_ecn_disabled(tp))
info-> tcpi_ecn_mode = TCPI_ECN_DISABLED;
else if (tcp_ecn_mode_rfc3168(tp))
info-> tcpi_ecn_mode = TCPI_ECN_RFC3168;
else if (tcp_ecn_mode_accecn(tp))
info-> tcpi_ecn_mode = TCPI_ECN_ACCECN;
else if (tcp_ecn_mode_pending(tp))
info-> tcpi_ecn_mode = TCPI_ECN_PENDING;
WDYT?
> And I will update the comment of tcpi_received_ce, thanks for the comments.
Great. Thanks!
neal
> Chia-Yu
>
> > (3) The following patch I did not merge, because I'd like to migrate to having all packetdrill tests for the Linux kernel reside in one place, in the Linux kernel source tree (not the Google packetdrill
> > repo):
> >
> > net-test: add TCP Accurate ECN cases
> > https://github.com/google/packetdrill/pull/96/changes/fe4c7293ea640a4c81178b6c88744d7a5d209fd6
> >
> > Thanks!
> > neal
> Chia-Yu
>
> -----Original Message-----
> From: Neal Cardwell <ncardwell@...gle.com>
> Sent: Sunday, January 18, 2026 5:11 PM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@...ia-bell-labs.com>
> Cc: pabeni@...hat.com; edumazet@...gle.com; parav@...dia.com; linux-doc@...r.kernel.org; corbet@....net; horms@...nel.org; dsahern@...nel.org; kuniyu@...gle.com; bpf@...r.kernel.org; netdev@...r.kernel.org; dave.taht@...il.com; jhs@...atatu.com; kuba@...nel.org; stephen@...workplumber.org; xiyou.wangcong@...il.com; jiri@...nulli.us; davem@...emloft.net; andrew+netdev@...n.ch; donald.hunter@...il.com; ast@...erby.net; liuhangbin@...il.com; shuah@...nel.org; linux-kselftest@...r.kernel.org; ij@...nel.org; Koen De Schepper (Nokia) <koen.de_schepper@...ia-bell-labs.com>; g.white@...lelabs.com; ingemar.s.johansson@...csson.com; mirja.kuehlewind@...csson.com; cheshire <cheshire@...le.com>; rs.ietf@....at; Jason_Livingood@...cast.com; Vidhi Goel <vidhi_goel@...le.com>; Willem de Bruijn <willemb@...gle.com>
> Subject: Re: [PATCH net-next 1/1] selftests/net: Add packetdrill packetdrill cases
>
>
> CAUTION: This is an external email. Please be very careful when clicking links or opening attachments. See the URL nok.it/ext for additional information.
>
>
>
> On Thu, Jan 8, 2026 at 5:46 PM Neal Cardwell <ncardwell@...gle.com> wrote:
> >
> > On Thu, Jan 8, 2026 at 10:58 AM <chia-yu.chang@...ia-bell-labs.com> wrote:
> > >
> > > From: Chia-Yu Chang <chia-yu.chang@...ia-bell-labs.com>
> > >
> > > Linux Accurate ECN test sets using ACE counters and AccECN options
> > > to cover several scenarios: Connection teardown, different ACK
> > > conditions, counter wrapping, SACK space grabbing, fallback schemes,
> > > negotiation retransmission/reorder/loss, AccECN option drop/loss,
> > > different handshake reflectors, data with marking, and different sysctl values.
> > >
> > > Co-developed-by: Ilpo Järvinen <ij@...nel.org>
> > > Signed-off-by: Ilpo Järvinen <ij@...nel.org>
> > > Co-developed-by: Neal Cardwell <ncardwell@...gle.com>
> > > Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> > > ---
> >
> > Chia-Yu, thank you for posting the packetdrill tests.
> >
> > A couple thoughts:
> >
> > (1) These tests are using the experimental AccECN packetdrill support
> > that is not in mainline packetdrill yet. Can you please share the
> > github URL for the version of packetdrill you used? I will work on
> > merging the appropriate experimental AccECN packetdrill support into
> > the Google packetdrill mainline branch.
>
> An update on the 3 patches at:
>
> https://github.com/google/packetdrill/pull/96
>
> (1) I have merged the following patch into the google packetdrill repo to facilitate testing of the AccECN patch series:
>
> "net-test: packetdrill: add Accurate ECN (AccECN) option support"
> https://github.com/google/packetdrill/pull/96/changes/f6861f888bc7f1e08026de4825519a95504d1047
>
> (2) The following patch I did not yet merge, because it proposes to add an odd number of u32 fields to tcp_info, so AFAICT leaves a 4-byte padding hole at the end of tcp_info:
>
> net-test: packetdrill: Support AccECN counters through tcpi
> https://github.com/google/packetdrill/pull/96/changes/f43649c87a2aa79a33a78111d3d7e5f027d13a7f
>
> I think we'll need to tweak the AccECN kernel patch series so that it does not leave a 4-byte padding hole at the end of tcp_info, then update this packetdrill patch to match the kernel patch.
>
> Let's come up with another useful u32 field we can add to the tcp_info struct, so that the kernel patch doesn't add a padding hole at the end of tcp_info.
>
> One idea would be to add another field to represent newer options and connection features that are enabled. AFAICT all 8 bits of the tcpi_options field have been used, so we can't use more bits in that field. I'd suggest we add a u32 tcpi_more_options field before the tcpi_received_ce field, so we can encode other useful info, like:
>
> + 1 bit to indicate whether AccECN was negotiated (this can go in a
> separate patch)
>
> + 1 bit to indicate whether TCP_NODELAY was set (since forgetting to
> use TCP_NODELAY is a classic cause of performance problems; again this can go in a separate patch)
>
> (And there will be future bits of info we want to add...)
>
> Also, regarding the comment in this line:
> __u32 tcpi_received_ce; /* # of CE marks received */
>
> That comment is ambiguous, since it doesn't indicate whether it's counting (potentially LRO/GRO) skbs or TCP segments. I would suggest clarifying that this is counting segments:
>
> __u32 tcpi_received_ce; /* # of CE marked segments received */
>
> (3) The following patch I did not merge, because I'd like to migrate to having all packetdrill tests for the Linux kernel reside in one place, in the Linux kernel source tree (not the Google packetdrill
> repo):
>
> net-test: add TCP Accurate ECN cases
> https://github.com/google/packetdrill/pull/96/changes/fe4c7293ea640a4c81178b6c88744d7a5d209fd6
>
> Thanks!
> neal
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