Linux currently disable ECN for incoming connections when the SYN
requests ECN and the IP header has ECT(0)/ECT(1) set, as some
networks were reportedly mangling the ToS byte, hence could later
trigger false congestion notifications.
RFC8311 §4.3 relaxes RFC3168's requirements such that ECT can be set
one TCP control packets (including SYNs). The main benefit of this
is the decreased probability of losing a SYN in a congested
ECN-capable network (i.e., it avoids the initial 1s timeout).
Additionally, this allows the development of newer TCP extensions,
such as AccECN.
This patch relaxes the previous check, by enabling ECN on incoming
connections using SYN+ECT if at least one bit of the reserved flags
of the TCP header is set. Such bit would indicate that the sender of
the SYN is using a newer TCP feature than what the host implements,
such as AccECN, and is thus implementing RFC8311. This enables
end-hosts not supporting such extensions to still negociate ECN, and
to have some of the benefits of using ECN on control packets.
Signed-off-by: Olivier Tilmans <[email protected]>
Suggested-by: Bob Briscoe <[email protected]>
Cc: Koen De Schepper <[email protected]>
---
net/ipv4/tcp_input.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 5dfbc333e79a..6660ce2a7333 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6263,6 +6263,11 @@ static inline void pr_drop_req(struct request_sock *req, __u16 port, int family)
* congestion control: Linux DCTCP asserts ECT on all packets,
* including SYN, which is most optimal solution; however,
* others, such as FreeBSD do not.
+ *
+ * Exception: At least one of the reserved bits of the TCP header (th->res1) is
+ * set, indicating the use of a future TCP extension (such as AccECN). See
+ * RFC8311 §4.3 which updates RFC3168 to allow the development of such
+ * extensions.
*/
static void tcp_ecn_create_request(struct request_sock *req,
const struct sk_buff *skb,
@@ -6282,7 +6287,7 @@ static void tcp_ecn_create_request(struct request_sock *req,
ecn_ok_dst = dst_feature(dst, DST_FEATURE_ECN_MASK);
ecn_ok = net->ipv4.sysctl_tcp_ecn || ecn_ok_dst;
- if ((!ect && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
+ if (((!ect || th->res1) && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
(ecn_ok_dst & DST_FEATURE_ECN_CA) ||
tcp_bpf_ca_needs_ecn((struct sock *)req))
inet_rsk(req)->ecn_ok = 1;
--
2.21.0
On Wed, Apr 3, 2019 at 6:49 AM Tilmans, Olivier (Nokia - BE/Antwerp)
<[email protected]> wrote:
>
> Linux currently disable ECN for incoming connections when the SYN
> requests ECN and the IP header has ECT(0)/ECT(1) set, as some
> networks were reportedly mangling the ToS byte, hence could later
> trigger false congestion notifications.
>
> RFC8311 §4.3 relaxes RFC3168's requirements such that ECT can be set
> one TCP control packets (including SYNs). The main benefit of this
> is the decreased probability of losing a SYN in a congested
> ECN-capable network (i.e., it avoids the initial 1s timeout).
> Additionally, this allows the development of newer TCP extensions,
> such as AccECN.
>
> This patch relaxes the previous check, by enabling ECN on incoming
> connections using SYN+ECT if at least one bit of the reserved flags
> of the TCP header is set. Such bit would indicate that the sender of
> the SYN is using a newer TCP feature than what the host implements,
> such as AccECN, and is thus implementing RFC8311. This enables
> end-hosts not supporting such extensions to still negociate ECN, and
nit : negotiate
> to have some of the benefits of using ECN on control packets.
>
> Signed-off-by: Olivier Tilmans <[email protected]>
> Suggested-by: Bob Briscoe <[email protected]>
> Cc: Koen De Schepper <[email protected]>
> ---
LGTM, thank you.
Signed-off-by: Eric Dumazet <[email protected]>
On Wed, Apr 3, 2019 at 9:50 AM Tilmans, Olivier (Nokia - BE/Antwerp)
<[email protected]> wrote:
>
> Linux currently disable ECN for incoming connections when the SYN
> requests ECN and the IP header has ECT(0)/ECT(1) set, as some
> networks were reportedly mangling the ToS byte, hence could later
> trigger false congestion notifications.
>
> RFC8311 §4.3 relaxes RFC3168's requirements such that ECT can be set
> one TCP control packets (including SYNs). The main benefit of this
> is the decreased probability of losing a SYN in a congested
> ECN-capable network (i.e., it avoids the initial 1s timeout).
> Additionally, this allows the development of newer TCP extensions,
> such as AccECN.
>
> This patch relaxes the previous check, by enabling ECN on incoming
> connections using SYN+ECT if at least one bit of the reserved flags
> of the TCP header is set. Such bit would indicate that the sender of
> the SYN is using a newer TCP feature than what the host implements,
> such as AccECN, and is thus implementing RFC8311. This enables
> end-hosts not supporting such extensions to still negociate ECN, and
> to have some of the benefits of using ECN on control packets.
>
> Signed-off-by: Olivier Tilmans <[email protected]>
> Suggested-by: Bob Briscoe <[email protected]>
> Cc: Koen De Schepper <[email protected]>
Acked-by: Neal Cardwell <[email protected]>
Thank you for this patch, Olivier and Bob.
thanks,
neal
On Wed, Apr 3, 2019 at 8:39 AM Neal Cardwell <[email protected]> wrote:
>
> On Wed, Apr 3, 2019 at 9:50 AM Tilmans, Olivier (Nokia - BE/Antwerp)
> <[email protected]> wrote:
> >
> > Linux currently disable ECN for incoming connections when the SYN
> > requests ECN and the IP header has ECT(0)/ECT(1) set, as some
> > networks were reportedly mangling the ToS byte, hence could later
> > trigger false congestion notifications.
> >
> > RFC8311 §4.3 relaxes RFC3168's requirements such that ECT can be set
> > one TCP control packets (including SYNs). The main benefit of this
> > is the decreased probability of losing a SYN in a congested
> > ECN-capable network (i.e., it avoids the initial 1s timeout).
> > Additionally, this allows the development of newer TCP extensions,
> > such as AccECN.
> >
> > This patch relaxes the previous check, by enabling ECN on incoming
> > connections using SYN+ECT if at least one bit of the reserved flags
> > of the TCP header is set. Such bit would indicate that the sender of
> > the SYN is using a newer TCP feature than what the host implements,
> > such as AccECN, and is thus implementing RFC8311. This enables
> > end-hosts not supporting such extensions to still negociate ECN, and
> > to have some of the benefits of using ECN on control packets.
> >
> > Signed-off-by: Olivier Tilmans <[email protected]>
> > Suggested-by: Bob Briscoe <[email protected]>
> > Cc: Koen De Schepper <[email protected]>
>
> Acked-by: Neal Cardwell <[email protected]>
Acked-by: Yuchung Cheng <[email protected]>
>
> Thank you for this patch, Olivier and Bob.
>
> thanks,
> neal
From: "Tilmans, Olivier (Nokia - BE/Antwerp)" <[email protected]>
Date: Wed, 3 Apr 2019 13:49:42 +0000
> Linux currently disable ECN for incoming connections when the SYN
> requests ECN and the IP header has ECT(0)/ECT(1) set, as some
> networks were reportedly mangling the ToS byte, hence could later
> trigger false congestion notifications.
>
> RFC8311 ?4.3 relaxes RFC3168's requirements such that ECT can be set
> one TCP control packets (including SYNs). The main benefit of this
> is the decreased probability of losing a SYN in a congested
> ECN-capable network (i.e., it avoids the initial 1s timeout).
> Additionally, this allows the development of newer TCP extensions,
> such as AccECN.
>
> This patch relaxes the previous check, by enabling ECN on incoming
> connections using SYN+ECT if at least one bit of the reserved flags
> of the TCP header is set. Such bit would indicate that the sender of
> the SYN is using a newer TCP feature than what the host implements,
> such as AccECN, and is thus implementing RFC8311. This enables
> end-hosts not supporting such extensions to still negociate ECN, and
> to have some of the benefits of using ECN on control packets.
>
> Signed-off-by: Olivier Tilmans <[email protected]>
> Suggested-by: Bob Briscoe <[email protected]>
Applied, thanks.