[PATCH v6 0/2] Update ECN and Add AccECN feature

[PATCH v6 0/2] Update ECN and Add AccECN feature

[chia-yu.chang]

From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

Hello,

  Please find ECN and AccECN feature patch v6:

v6 (14-Apr-2025)
- Shrink descriptions and make more reference to RFC3168 ECN and AccECN

v5 (06-Mar-2025)
- Update feature bits of VIRTIO_NET_F_HOST_ACCECN(69) and VIRTIO_NET_F_GUEST_ACCECN(70)

v4 (05-Mar-2025)
- Update commit message of AccECN patch
- Add reference to RFC3168 ECN and Accurate ECN
- Simplify feature bit descriptinos to avoid duplication of text

v3 (21-Feb-2025)
- Separate the original patch into 2 different patches
- Fix several reabase issues in v2

v2 (17-Feb-2025)
- Rebase the patch on top of v1.4 virtio-spec

Best Regards,
Chia-Yu

Chia-Yu Chang (2):
  virtio-net: Fix ECN feature descritpion
  virtio-net: define Accurate ECN feature in virtio-spec

 device-types/net/description.tex | 77 ++++++++++++++++++++++----------
 introduction.tex                 |  6 +++
 2 files changed, 60 insertions(+), 23 deletions(-)

-- 
2.34.1

[PATCH v6 1/2] virtio-net: Fix ECN feature descritpion

[chia-yu.chang]

From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

Clarify that the VIRTIO_NET_HDR_GSO_ECN gso_type flag does not mean that
TCP has IP-ECN set, instead it identifies TCP CWR flag is set and will be
cleared from the second segment. This is used to offload TCP CWR flag in a
way that is compatible with RFC3168 ECN but is problematic for non-RFC3168
use of CWR flag.

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
---
 device-types/net/description.tex | 33 +++++++++++++++++---------------
 introduction.tex                 |  3 +++
 2 files changed, 21 insertions(+), 15 deletions(-)

diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index 1b6b54d..a2c9de8 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -54,7 +54,9 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
 
 \item[VIRTIO_NET_F_GUEST_TSO6 (8)] Driver can receive TSOv6.
 
-\item[VIRTIO_NET_F_GUEST_ECN (9)] Driver can receive TSO with ECN.
+\item[VIRTIO_NET_F_GUEST_ECN (9)] Driver can receive TSO with TCP CWR flag set
+    and follow the ACE bits handling approach mentioned in
+    \hyperref[intro:rfc3168]{[RFC3168]}.
 
 \item[VIRTIO_NET_F_GUEST_UFO (10)] Driver can receive UFO.
 
@@ -62,7 +64,9 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
 
 \item[VIRTIO_NET_F_HOST_TSO6 (12)] Device can receive TSOv6.
 
-\item[VIRTIO_NET_F_HOST_ECN (13)] Device can receive TSO with ECN.
+\item[VIRTIO_NET_F_HOST_ECN (13)] Device can receive TSO with TCP CWR flag set
+    and follow the ACE bits handling approach mentioend in
+    \hyperref[intro:rfc3168]{[RFC3168]}.
 
 \item[VIRTIO_NET_F_HOST_UFO (14)] Device can receive UFO.
 
@@ -695,8 +699,9 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
 
   \item If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature,
     the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}
-    indicates that the TCP packet has the ECN bit set\footnote{This case is not handled by some older hardware, so is called out
-specifically in the protocol.}.
+    indicates that the TCP packet has TCP CWR flag set and the flag will be handled differently to all segements of
+    an aggregated segment, as mentioned in \hyperref[intro:rfc3168]{[RFC3168]}
+    \footnote{This case is not handled by some older hardware, so is called out specifically in the protocol.}.
    \end{itemize}
 
 \item If the driver negotiated the VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO feature and the
@@ -788,10 +793,9 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
 \field{gso_type} to VIRTIO_NET_HDR_GSO_UDP_L4.
 
 The driver SHOULD NOT send to the device TCP packets requiring segmentation offload
-which have the Explicit Congestion Notification bit set, unless the
-VIRTIO_NET_F_HOST_ECN feature is negotiated, in which case the
-driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in
-\field{gso_type}.
+which have the TCP CWR flag set and require the flag be handled as mentioned in
+\hyperref[intro:rfc3168]{[RFC3168]}, unless the VIRTIO_NET_F_HOST_ECN feature is
+negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
 
 If VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO is negotiated, the driver MAY set
 VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or the VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit
@@ -1105,9 +1109,9 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
 \end{enumerate}
 
 Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP, UDP_TUNNEL
-and ECN features enable receive checksum, large receive offload and ECN
-support which are the input equivalents of the transmit checksum,
-transmit segmentation offloading and ECN features, as described
+and ECN features enable receive checksum, large receive offload and RFC3168
+ECN support which are the input equivalents of the transmit checksum,
+transmit segmentation offloading and RFC3168 ECN features, as described
 in \ref{sec:Device Types / Network Device / Device Operation /
 Packet Transmission}:
 \begin{enumerate}
@@ -1210,10 +1214,9 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
 the VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM bit in \field{flags}.
 
 The device SHOULD NOT send to the driver TCP packets requiring segmentation offload
-which have the Explicit Congestion Notification bit set, unless the
-VIRTIO_NET_F_GUEST_ECN feature is negotiated, in which case the
-device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in
-\field{gso_type}.
+which have the TCP CWR flag set and require the flag be handled as mentioned in
+\hyperref[intro:rfc3168]{[RFC3168]}, unless the VIRTIO_NET_F_GUEST_ECN feature is
+negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
 
 If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the
 device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in
diff --git a/introduction.tex b/introduction.tex
index e60298a..d52622e 100644
--- a/introduction.tex
+++ b/introduction.tex
@@ -168,6 +168,9 @@ \section{Normative References}\label{sec:Normative References}
     Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP
     14, RFC 8174, DOI 10.17487/RFC8174, May 2017
         \newline\url{http://www.ietf.org/rfc/rfc8174.txt}\\
+	\phantomsection\label{intro:rfc3168}\textbf{[RFC3168]} &
+    S. Floyd., ``The Addition of Explicit Congestion Notification (ECN) to IP'', September 2001.
+	\newline\url{http://www.ietf.org/rfc/rfc3168.txt}\\
 \end{longtable}
 
 \section{Non-Normative References}
-- 
2.34.1

[PATCH v6 2/2] virtio-net: define Accurate ECN feature in virtio-spec

[chia-yu.chang]

From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

This change implements Accurate ECN based on AccECN specifications:
  https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt

Unlike RFC 3168 ECN, Accurate ECN uses the CWR flag as part of the ACE
field to count new packets with CE mark; however, RFC 3168 ECN-aware TSO
will clean CWR flag from the 2nd segment of an aggregated segment.
Therefore, fallback shall be applied by setting NETIF_F_GSO_ACCECN to
ensure that the CWR flag should not be changed within a aggregated segment
(e.g., super-skb in Linux).

To apply it in virtio-spec, new feature bits for host and guest are added
for feature negotiation between driver and device. And the translation
of Accurate ECN GSO flag between virtio_net_hdr and skb header for
NETIF_F_GSO_ACCECN is also added to avoid CWR flag corruption due to
RFC3168 ECN TSO.

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
---
 device-types/net/description.tex | 50 +++++++++++++++++++++++++-------
 introduction.tex                 |  3 ++
 2 files changed, 42 insertions(+), 11 deletions(-)

diff --git a/device-types/net/description.tex b/device-types/net/description.tex
index a2c9de8..3b9247d 100644
--- a/device-types/net/description.tex
+++ b/device-types/net/description.tex
@@ -140,6 +140,14 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
 
 \item[VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO_CSUM (68)] Device handles packets
   carried by a UDP tunnel with partial csum for the outer header.
+
+\item[VIRTIO_NET_F_HOST_ACCECN (69)] Device can receive TSO with TCP CWR flag set
+  and follow the ACE bits handling approach mentioend in
+  \hyperref[intro:accecn]{[AccECN]}.
+
+\item[VIRTIO_NET_F_GUEST_ACCECN (70)] Driver can receive TSO with TCP CWR flag set
+  and follow the ACE bits handling approach mentioend in
+  \hyperref[intro:accecn]{[AccECN]}.
 \end{description}
 
 \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device / Feature bits / Feature bit requirements}
@@ -151,6 +159,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
 \item[VIRTIO_NET_F_GUEST_TSO4] Requires VIRTIO_NET_F_GUEST_CSUM.
 \item[VIRTIO_NET_F_GUEST_TSO6] Requires VIRTIO_NET_F_GUEST_CSUM.
 \item[VIRTIO_NET_F_GUEST_ECN] Requires VIRTIO_NET_F_GUEST_TSO4 or VIRTIO_NET_F_GUEST_TSO6.
+\item[VIRTIO_NET_F_GUEST_ACCECN] Requires VIRTIO_NET_F_GUEST_TSO4 or VIRTIO_NET_F_GUEST_TSO6.
 \item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM.
 \item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM.
 \item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM.
@@ -161,6 +170,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
 \item[VIRTIO_NET_F_HOST_TSO4] Requires VIRTIO_NET_F_CSUM.
 \item[VIRTIO_NET_F_HOST_TSO6] Requires VIRTIO_NET_F_CSUM.
 \item[VIRTIO_NET_F_HOST_ECN] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
+\item[VIRTIO_NET_F_HOST_ACCECN] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
 \item[VIRTIO_NET_F_HOST_UFO] Requires VIRTIO_NET_F_CSUM.
 \item[VIRTIO_NET_F_HOST_USO] Requires VIRTIO_NET_F_CSUM.
 \item[VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO] Requires VIRTIO_NET_F_HOST_TSO4, VIRTIO_NET_F_HOST_TSO6
@@ -284,11 +294,11 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
 The device MUST NOT modify \field{mtu} once it has been set.
 
 The device MUST NOT pass received packets that exceed \field{mtu} (plus low
-level ethernet header length) size with \field{gso_type} NONE or ECN
+level ethernet header length) size with \field{gso_type} NONE or ECN or ACCECN
 after VIRTIO_NET_F_MTU has been successfully negotiated.
 
 The device MUST forward transmitted packets of up to \field{mtu} (plus low
-level ethernet header length) size with \field{gso_type} NONE or ECN, and do
+level ethernet header length) size with \field{gso_type} NONE or ECN or ACCECN, and do
 so without fragmentation, after VIRTIO_NET_F_MTU has been successfully
 negotiated.
 
@@ -338,11 +348,11 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
 
 If the driver negotiates VIRTIO_NET_F_MTU, it MUST supply enough receive
 buffers to receive at least one receive packet of size \field{mtu} (plus low
-level ethernet header length) with \field{gso_type} NONE or ECN.
+level ethernet header length) with \field{gso_type} NONE or ECN or ACCECN.
 
 If the driver negotiates VIRTIO_NET_F_MTU, it MUST NOT transmit packets of
 size exceeding the value of \field{mtu} (plus low level ethernet header length)
-with \field{gso_type} NONE or ECN.
+with \field{gso_type} NONE or ECN or ACCECN.
 
 A driver SHOULD negotiate the VIRTIO_NET_F_STANDBY feature if the device offers it.
 
@@ -433,7 +443,7 @@ \subsection{Device Initialization}\label{sec:Device Types / Network Device / Dev
    The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially
   checksummed packets can be received, and if it can do that then
   the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
-  VIRTIO_NET_F_GUEST_UFO, VIRTIO_NET_F_GUEST_ECN, VIRTIO_NET_F_GUEST_USO4,
+  VIRTIO_NET_F_GUEST_UFO, VIRTIO_NET_F_GUEST_ECN, VIRTIO_NET_F_GUEST_ACCECN, VIRTIO_NET_F_GUEST_USO4,
   VIRTIO_NET_F_GUEST_USO6 VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO and
   VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM are the input equivalents of
   the features described above.
@@ -592,6 +602,7 @@ \subsection{Device Operation}\label{sec:Device Types / Network Device / Device O
 #define VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 0x20
 #define VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 0x40
 #define VIRTIO_NET_HDR_GSO_ECN      0x80
+#define VIRTIO_NET_HDR_GSO_ACCECN   0x10
         u8 gso_type;
         le16 hdr_len;
         le16 gso_size;
@@ -702,6 +713,12 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
     indicates that the TCP packet has TCP CWR flag set and the flag will be handled differently to all segements of
     an aggregated segment, as mentioned in \hyperref[intro:rfc3168]{[RFC3168]}
     \footnote{This case is not handled by some older hardware, so is called out specifically in the protocol.}.
+
+  \item If the driver negotiated the VIRTIO_NET_F_HOST_ACCECN feature,
+    the VIRTIO_NET_HDR_GSO_ACCECN bit in \field{gso_type}
+    indicates that the TCP packet has TCP CWR flag set and the flag will be applied to all segments of an aggregated
+    segment, as mentioend in \hyperref[intro:accecn]{[AccECN]}
+    \footnote{This case is not handled by some older hardware, so is called out specifically in the protocol.}.
    \end{itemize}
 
 \item If the driver negotiated the VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO feature and the
@@ -797,6 +814,11 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
 \hyperref[intro:rfc3168]{[RFC3168]}, unless the VIRTIO_NET_F_HOST_ECN feature is
 negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
 
+The driver SHOULD NOT send to the device TCP packets requiring segmentation offload
+which have the TCP CWR flag set and require the flag be applied as mentioend in
+\hyperref[intro:accecn]{[AccECN]}, unless the VIRTIO_NET_F_HOST_ACCECN feature is
+negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ACCECN bit in \field{gso_type}.
+
 If VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO is negotiated, the driver MAY set
 VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or the VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit
 in \field{gso_type} according to the inner network header protocol type
@@ -1108,12 +1130,12 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
   in case of tunnels) has been validated.
 \end{enumerate}
 
-Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP, UDP_TUNNEL
-and ECN features enable receive checksum, large receive offload and RFC3168
-ECN support which are the input equivalents of the transmit checksum,
-transmit segmentation offloading and RFC3168 ECN features, as described
-in \ref{sec:Device Types / Network Device / Device Operation /
-Packet Transmission}:
+Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP, UDP_TUNNEL, ECN and
+ACCECN features enable receive checksum, large receive offload, RFC3168 ECN
+and Accurate ECN support which are the input equivalents of the transmit
+checksum, transmit segmentation offloading, RFC3168 ECN and Accurate ECN
+features, as described in \ref{sec:Device Types / Network Device /
+Device Operation / Packet Transmission}:
 \begin{enumerate}
 \item If the VIRTIO_NET_F_GUEST_TSO4, TSO6, UFO, USO4 or USO6 options were
   negotiated, then \field{gso_type} MAY be something other than
@@ -1218,6 +1240,11 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
 \hyperref[intro:rfc3168]{[RFC3168]}, unless the VIRTIO_NET_F_GUEST_ECN feature is
 negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
 
+The device SHOULD NOT send to the driver TCP packets requiring segmentation offload
+which have the TCP CWR flag set and require the flag be handled as mentioned in
+\hyperref[intro:accecn]{[AccECN]}, unless the VIRTIO_NET_F_GUEST_ACCECN feature is
+negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ACCECN bit in \field{gso_type}.
+
 If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the
 device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in
 \field{flags}, if so:
@@ -2193,6 +2220,7 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
 #define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM 47
 #define VIRTIO_NET_F_GUEST_USO4       54
 #define VIRTIO_NET_F_GUEST_USO6       55
+#define VIRTIO_NET_F_GUEST_ACCECN     70
 
 #define VIRTIO_NET_CTRL_GUEST_OFFLOADS       5
  #define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET   0
diff --git a/introduction.tex b/introduction.tex
index d52622e..9320ca1 100644
--- a/introduction.tex
+++ b/introduction.tex
@@ -171,6 +171,9 @@ \section{Normative References}\label{sec:Normative References}
 	\phantomsection\label{intro:rfc3168}\textbf{[RFC3168]} &
     S. Floyd., ``The Addition of Explicit Congestion Notification (ECN) to IP'', September 2001.
 	\newline\url{http://www.ietf.org/rfc/rfc3168.txt}\\
+        \phantomsection\label{intro:accecn}\textbf{[AccECN]} &
+    B. Briscoe., ``More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP'', February 2025.
+        \newline\url{https://www.ietf.org/archive/id/draft-ietf-tcpm-accurate-ecn-33.txt}\\
 \end{longtable}
 
 \section{Non-Normative References}
-- 
2.34.1

Re: [PATCH v6 1/2] virtio-net: Fix ECN feature descritpion

[Michael S. Tsirkin]

On Mon, Apr 14, 2025 at 05:34:24PM +0200, chia-yu.chang@nokia-bell-labs.com wrote:
> From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> 
> Clarify that the VIRTIO_NET_HDR_GSO_ECN gso_type flag does not mean that
> TCP has IP-ECN set, instead it identifies TCP CWR flag is set and will be
> cleared from the second segment. This is used to offload TCP CWR flag in a
> way that is compatible with RFC3168 ECN but is problematic for non-RFC3168
> use of CWR flag.
> 
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> ---


if this is accepted, we might want to backport this to 1.3 maybe?


>  device-types/net/description.tex | 33 +++++++++++++++++---------------
>  introduction.tex                 |  3 +++
>  2 files changed, 21 insertions(+), 15 deletions(-)
> 
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index 1b6b54d..a2c9de8 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -54,7 +54,9 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
>  
>  \item[VIRTIO_NET_F_GUEST_TSO6 (8)] Driver can receive TSOv6.
>  
> -\item[VIRTIO_NET_F_GUEST_ECN (9)] Driver can receive TSO with ECN.
> +\item[VIRTIO_NET_F_GUEST_ECN (9)] Driver can receive TSO with TCP CWR flag set
> +    and follow the ACE bits handling approach mentioned in
> +    \hyperref[intro:rfc3168]{[RFC3168]}.
>  
>  \item[VIRTIO_NET_F_GUEST_UFO (10)] Driver can receive UFO.
>  
> @@ -62,7 +64,9 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
>  
>  \item[VIRTIO_NET_F_HOST_TSO6 (12)] Device can receive TSOv6.
>  
> -\item[VIRTIO_NET_F_HOST_ECN (13)] Device can receive TSO with ECN.
> +\item[VIRTIO_NET_F_HOST_ECN (13)] Device can receive TSO with TCP CWR flag set
> +    and follow the ACE bits handling approach mentioend in
> +    \hyperref[intro:rfc3168]{[RFC3168]}.
>  
>  \item[VIRTIO_NET_F_HOST_UFO (14)] Device can receive UFO.
>  
> @@ -695,8 +699,9 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
>  
>    \item If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature,
>      the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}
> -    indicates that the TCP packet has the ECN bit set\footnote{This case is not handled by some older hardware, so is called out
> -specifically in the protocol.}.
> +    indicates that the TCP packet has TCP CWR flag set and the flag will be handled differently to all segements of
> +    an aggregated segment, as mentioned in \hyperref[intro:rfc3168]{[RFC3168]}
> +    \footnote{This case is not handled by some older hardware, so is called out specifically in the protocol.}.
>     \end{itemize}
>  
>  \item If the driver negotiated the VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO feature and the
> @@ -788,10 +793,9 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
>  \field{gso_type} to VIRTIO_NET_HDR_GSO_UDP_L4.
>  
>  The driver SHOULD NOT send to the device TCP packets requiring segmentation offload
> -which have the Explicit Congestion Notification bit set, unless the
> -VIRTIO_NET_F_HOST_ECN feature is negotiated, in which case the
> -driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in
> -\field{gso_type}.
> +which have the TCP CWR flag set and require the flag be handled as mentioned in
> +\hyperref[intro:rfc3168]{[RFC3168]}, unless the VIRTIO_NET_F_HOST_ECN feature is
> +negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
>  
>  If VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO is negotiated, the driver MAY set
>  VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or the VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit
> @@ -1105,9 +1109,9 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
>  \end{enumerate}
>  
>  Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP, UDP_TUNNEL
> -and ECN features enable receive checksum, large receive offload and ECN
> -support which are the input equivalents of the transmit checksum,
> -transmit segmentation offloading and ECN features, as described
> +and ECN features enable receive checksum, large receive offload and RFC3168
> +ECN support which are the input equivalents of the transmit checksum,
> +transmit segmentation offloading and RFC3168 ECN features, as described
>  in \ref{sec:Device Types / Network Device / Device Operation /
>  Packet Transmission}:
>  \begin{enumerate}
> @@ -1210,10 +1214,9 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
>  the VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM bit in \field{flags}.
>  
>  The device SHOULD NOT send to the driver TCP packets requiring segmentation offload
> -which have the Explicit Congestion Notification bit set, unless the
> -VIRTIO_NET_F_GUEST_ECN feature is negotiated, in which case the
> -device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in
> -\field{gso_type}.
> +which have the TCP CWR flag set and require the flag be handled as mentioned in
> +\hyperref[intro:rfc3168]{[RFC3168]}, unless the VIRTIO_NET_F_GUEST_ECN feature is
> +negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
>  
>  If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the
>  device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in
> diff --git a/introduction.tex b/introduction.tex
> index e60298a..d52622e 100644
> --- a/introduction.tex
> +++ b/introduction.tex
> @@ -168,6 +168,9 @@ \section{Normative References}\label{sec:Normative References}
>      Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP
>      14, RFC 8174, DOI 10.17487/RFC8174, May 2017
>          \newline\url{http://www.ietf.org/rfc/rfc8174.txt}\\
> +	\phantomsection\label{intro:rfc3168}\textbf{[RFC3168]} &
> +    S. Floyd., ``The Addition of Explicit Congestion Notification (ECN) to IP'', September 2001.
> +	\newline\url{http://www.ietf.org/rfc/rfc3168.txt}\\
>  \end{longtable}
>  
>  \section{Non-Normative References}
> -- 
> 2.34.1

Re: [PATCH v6 2/2] virtio-net: define Accurate ECN feature in virtio-spec

[Michael S. Tsirkin]

On Mon, Apr 14, 2025 at 05:34:25PM +0200, chia-yu.chang@nokia-bell-labs.com wrote:
> From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> 
> This change implements Accurate ECN based on AccECN specifications:
>   https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt
> 
> Unlike RFC 3168 ECN, Accurate ECN uses the CWR flag as part of the ACE
> field to count new packets with CE mark; however, RFC 3168 ECN-aware TSO
> will clean CWR flag from the 2nd segment of an aggregated segment.
> Therefore, fallback shall be applied by setting NETIF_F_GSO_ACCECN to
> ensure that the CWR flag should not be changed within a aggregated segment
> (e.g., super-skb in Linux).
> 
> To apply it in virtio-spec, new feature bits for host and guest are added
> for feature negotiation between driver and device. And the translation
> of Accurate ECN GSO flag between virtio_net_hdr and skb header for
> NETIF_F_GSO_ACCECN is also added to avoid CWR flag corruption due to
> RFC3168 ECN TSO.
> 
> Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> ---
>  device-types/net/description.tex | 50 +++++++++++++++++++++++++-------
>  introduction.tex                 |  3 ++
>  2 files changed, 42 insertions(+), 11 deletions(-)
> 
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index a2c9de8..3b9247d 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -140,6 +140,14 @@ \subsection{Feature bits}\label{sec:Device Types / Network Device / Feature bits
>  
>  \item[VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO_CSUM (68)] Device handles packets
>    carried by a UDP tunnel with partial csum for the outer header.
> +
> +\item[VIRTIO_NET_F_HOST_ACCECN (69)] Device can receive TSO with TCP CWR flag set
> +  and follow the ACE bits handling approach mentioend in
> +  \hyperref[intro:accecn]{[AccECN]}.
> +
> +\item[VIRTIO_NET_F_GUEST_ACCECN (70)] Driver can receive TSO with TCP CWR flag set
> +  and follow the ACE bits handling approach mentioend in
> +  \hyperref[intro:accecn]{[AccECN]}.
>  \end{description}

typos



>  
>  \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device / Feature bits / Feature bit requirements}
> @@ -151,6 +159,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
>  \item[VIRTIO_NET_F_GUEST_TSO4] Requires VIRTIO_NET_F_GUEST_CSUM.
>  \item[VIRTIO_NET_F_GUEST_TSO6] Requires VIRTIO_NET_F_GUEST_CSUM.
>  \item[VIRTIO_NET_F_GUEST_ECN] Requires VIRTIO_NET_F_GUEST_TSO4 or VIRTIO_NET_F_GUEST_TSO6.
> +\item[VIRTIO_NET_F_GUEST_ACCECN] Requires VIRTIO_NET_F_GUEST_TSO4 or VIRTIO_NET_F_GUEST_TSO6.
>  \item[VIRTIO_NET_F_GUEST_UFO] Requires VIRTIO_NET_F_GUEST_CSUM.
>  \item[VIRTIO_NET_F_GUEST_USO4] Requires VIRTIO_NET_F_GUEST_CSUM.
>  \item[VIRTIO_NET_F_GUEST_USO6] Requires VIRTIO_NET_F_GUEST_CSUM.
> @@ -161,6 +170,7 @@ \subsubsection{Feature bit requirements}\label{sec:Device Types / Network Device
>  \item[VIRTIO_NET_F_HOST_TSO4] Requires VIRTIO_NET_F_CSUM.
>  \item[VIRTIO_NET_F_HOST_TSO6] Requires VIRTIO_NET_F_CSUM.
>  \item[VIRTIO_NET_F_HOST_ECN] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
> +\item[VIRTIO_NET_F_HOST_ACCECN] Requires VIRTIO_NET_F_HOST_TSO4 or VIRTIO_NET_F_HOST_TSO6.
>  \item[VIRTIO_NET_F_HOST_UFO] Requires VIRTIO_NET_F_CSUM.
>  \item[VIRTIO_NET_F_HOST_USO] Requires VIRTIO_NET_F_CSUM.
>  \item[VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO] Requires VIRTIO_NET_F_HOST_TSO4, VIRTIO_NET_F_HOST_TSO6
> @@ -284,11 +294,11 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
>  The device MUST NOT modify \field{mtu} once it has been set.
>  
>  The device MUST NOT pass received packets that exceed \field{mtu} (plus low
> -level ethernet header length) size with \field{gso_type} NONE or ECN
> +level ethernet header length) size with \field{gso_type} NONE or ECN or ACCECN
>  after VIRTIO_NET_F_MTU has been successfully negotiated.


NINE, ECN or ACCECN

>  
>  The device MUST forward transmitted packets of up to \field{mtu} (plus low
> -level ethernet header length) size with \field{gso_type} NONE or ECN, and do
> +level ethernet header length) size with \field{gso_type} NONE or ECN or ACCECN, and do


same

>  so without fragmentation, after VIRTIO_NET_F_MTU has been successfully
>  negotiated.
>  
> @@ -338,11 +348,11 @@ \subsection{Device configuration layout}\label{sec:Device Types / Network Device
>  
>  If the driver negotiates VIRTIO_NET_F_MTU, it MUST supply enough receive
>  buffers to receive at least one receive packet of size \field{mtu} (plus low
> -level ethernet header length) with \field{gso_type} NONE or ECN.
> +level ethernet header length) with \field{gso_type} NONE or ECN or ACCECN.

same

>  If the driver negotiates VIRTIO_NET_F_MTU, it MUST NOT transmit packets of
>  size exceeding the value of \field{mtu} (plus low level ethernet header length)
> -with \field{gso_type} NONE or ECN.
> +with \field{gso_type} NONE or ECN or ACCECN.
>  
>  A driver SHOULD negotiate the VIRTIO_NET_F_STANDBY feature if the device offers it.
>  
> @@ -433,7 +443,7 @@ \subsection{Device Initialization}\label{sec:Device Types / Network Device / Dev
>     The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially
>    checksummed packets can be received, and if it can do that then
>    the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6,
> -  VIRTIO_NET_F_GUEST_UFO, VIRTIO_NET_F_GUEST_ECN, VIRTIO_NET_F_GUEST_USO4,
> +  VIRTIO_NET_F_GUEST_UFO, VIRTIO_NET_F_GUEST_ECN, VIRTIO_NET_F_GUEST_ACCECN, VIRTIO_NET_F_GUEST_USO4,
>    VIRTIO_NET_F_GUEST_USO6 VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO and
>    VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM are the input equivalents of
>    the features described above.
> @@ -592,6 +602,7 @@ \subsection{Device Operation}\label{sec:Device Types / Network Device / Device O
>  #define VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 0x20
>  #define VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 0x40
>  #define VIRTIO_NET_HDR_GSO_ECN      0x80
> +#define VIRTIO_NET_HDR_GSO_ACCECN   0x10
>          u8 gso_type;
>          le16 hdr_len;
>          le16 gso_size;
> @@ -702,6 +713,12 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
>      indicates that the TCP packet has TCP CWR flag set and the flag will be handled differently to all segements of
>      an aggregated segment, as mentioned in \hyperref[intro:rfc3168]{[RFC3168]}
>      \footnote{This case is not handled by some older hardware, so is called out specifically in the protocol.}.
> +
> +  \item If the driver negotiated the VIRTIO_NET_F_HOST_ACCECN feature,
> +    the VIRTIO_NET_HDR_GSO_ACCECN bit in \field{gso_type}
> +    indicates that the TCP packet has TCP CWR flag set and the flag will be applied to all segments of an aggregated
> +    segment, as mentioend in \hyperref[intro:accecn]{[AccECN]}
> +    \footnote{This case is not handled by some older hardware, so is called out specifically in the protocol.}.
>     \end{itemize}
>  
>  \item If the driver negotiated the VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO feature and the
> @@ -797,6 +814,11 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
>  \hyperref[intro:rfc3168]{[RFC3168]}, unless the VIRTIO_NET_F_HOST_ECN feature is
>  negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
>  
> +The driver SHOULD NOT send to the device TCP packets requiring segmentation offload
> +which have the TCP CWR flag set and require the flag be applied as mentioend in
> +\hyperref[intro:accecn]{[AccECN]}, unless the VIRTIO_NET_F_HOST_ACCECN feature is
> +negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ACCECN bit in \field{gso_type}.
> +
>  If VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO is negotiated, the driver MAY set
>  VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or the VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit
>  in \field{gso_type} according to the inner network header protocol type
> @@ -1108,12 +1130,12 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
>    in case of tunnels) has been validated.
>  \end{enumerate}
>  
> -Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP, UDP_TUNNEL
> -and ECN features enable receive checksum, large receive offload and RFC3168
> -ECN support which are the input equivalents of the transmit checksum,
> -transmit segmentation offloading and RFC3168 ECN features, as described
> -in \ref{sec:Device Types / Network Device / Device Operation /
> -Packet Transmission}:
> +Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP, UDP_TUNNEL, ECN and
> +ACCECN features enable receive checksum, large receive offload, RFC3168 ECN
> +and Accurate ECN support which are the input equivalents of the transmit
> +checksum, transmit segmentation offloading, RFC3168 ECN and Accurate ECN
> +features, as described in \ref{sec:Device Types / Network Device /
> +Device Operation / Packet Transmission}:
>  \begin{enumerate}
>  \item If the VIRTIO_NET_F_GUEST_TSO4, TSO6, UFO, USO4 or USO6 options were
>    negotiated, then \field{gso_type} MAY be something other than
> @@ -1218,6 +1240,11 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
>  \hyperref[intro:rfc3168]{[RFC3168]}, unless the VIRTIO_NET_F_GUEST_ECN feature is
>  negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
>  
> +The device SHOULD NOT send to the driver TCP packets requiring segmentation offload
> +which have the TCP CWR flag set and require the flag be handled as mentioned in
> +\hyperref[intro:accecn]{[AccECN]}, unless the VIRTIO_NET_F_GUEST_ACCECN feature is
> +negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ACCECN bit in \field{gso_type}.
> +
>  If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the
>  device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in
>  \field{flags}, if so:
> @@ -2193,6 +2220,7 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  #define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM 47
>  #define VIRTIO_NET_F_GUEST_USO4       54
>  #define VIRTIO_NET_F_GUEST_USO6       55
> +#define VIRTIO_NET_F_GUEST_ACCECN     70
>  
>  #define VIRTIO_NET_CTRL_GUEST_OFFLOADS       5
>   #define VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET   0
> diff --git a/introduction.tex b/introduction.tex
> index d52622e..9320ca1 100644
> --- a/introduction.tex
> +++ b/introduction.tex
> @@ -171,6 +171,9 @@ \section{Normative References}\label{sec:Normative References}
>  	\phantomsection\label{intro:rfc3168}\textbf{[RFC3168]} &
>      S. Floyd., ``The Addition of Explicit Congestion Notification (ECN) to IP'', September 2001.
>  	\newline\url{http://www.ietf.org/rfc/rfc3168.txt}\\
> +        \phantomsection\label{intro:accecn}\textbf{[AccECN]} &
> +    B. Briscoe., ``More Accurate Explicit Congestion Notification (AccECN) Feedback in TCP'', February 2025.
> +        \newline\url{https://www.ietf.org/archive/id/draft-ietf-tcpm-accurate-ecn-33.txt}\\
>  \end{longtable}
>  
>  \section{Non-Normative References}
> -- 
> 2.34.1

RE: [PATCH v6 1/2] virtio-net: Fix ECN feature descritpion

[Chia-Yu Chang (Nokia)]

> -----Original Message-----
> From: Michael S. Tsirkin <mst@redhat.com> 
> Sent: Monday, April 14, 2025 8:59 PM
> To: Chia-Yu Chang (Nokia) <chia-yu.chang@nokia-bell-labs.com>
> Cc: virtio-comment@lists.linux.dev; cohuck@redhat.com; mvaralar@redhat.com; jasowang@redhat.com; xuanzhuo@linux.alibaba.com; eperezma@redhat.com; ij@kernel.org; ncardwell@google.com; Koen De Schepper (Nokia) <koen.de_schepper@nokia-bell-labs.com>; g.white <g.white@cablelabs.com>; vidhi_goel <vidhi_goel@apple.com>; ingemar.s.johansson@ericsson.com; mirja.kuehlewind@ericsson.com
> Subject: Re: [PATCH v6 1/2] virtio-net: Fix ECN feature descritpion
> 
> 
> 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 Mon, Apr 14, 2025 at 05:34:24PM +0200, chia-yu.chang@nokia-bell-labs.com wrote:
> > From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> >
> > Clarify that the VIRTIO_NET_HDR_GSO_ECN gso_type flag does not mean 
> > that TCP has IP-ECN set, instead it identifies TCP CWR flag is set and 
> > will be cleared from the second segment. This is used to offload TCP 
> > CWR flag in a way that is compatible with RFC3168 ECN but is 
> > problematic for non-RFC3168 use of CWR flag.
> >
> > Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> > ---
> 
> 
> if this is accepted, we might want to backport this to 1.3 maybe?

Thanks for feedback.
In my opinion, this fix shall be backported to clarify its relation to RFC3168 ECN.
Shall we make further patch, or you will do the merge once this patch is accepted?

Best regards,
Chia-Yu

> 
> 
> >  device-types/net/description.tex | 33 +++++++++++++++++---------------
> >  introduction.tex                 |  3 +++
> >  2 files changed, 21 insertions(+), 15 deletions(-)
> >
> > diff --git a/device-types/net/description.tex 
> > b/device-types/net/description.tex
> > index 1b6b54d..a2c9de8 100644
> > --- a/device-types/net/description.tex
> > +++ b/device-types/net/description.tex
> > @@ -54,7 +54,9 @@ \subsection{Feature bits}\label{sec:Device Types / 
> > Network Device / Feature bits
> >
> >  \item[VIRTIO_NET_F_GUEST_TSO6 (8)] Driver can receive TSOv6.
> >
> > -\item[VIRTIO_NET_F_GUEST_ECN (9)] Driver can receive TSO with ECN.
> > +\item[VIRTIO_NET_F_GUEST_ECN (9)] Driver can receive TSO with TCP CWR flag set
> > +    and follow the ACE bits handling approach mentioned in
> > +    \hyperref[intro:rfc3168]{[RFC3168]}.
> >
> >  \item[VIRTIO_NET_F_GUEST_UFO (10)] Driver can receive UFO.
> >
> > @@ -62,7 +64,9 @@ \subsection{Feature bits}\label{sec:Device Types / 
> > Network Device / Feature bits
> >
> >  \item[VIRTIO_NET_F_HOST_TSO6 (12)] Device can receive TSOv6.
> >
> > -\item[VIRTIO_NET_F_HOST_ECN (13)] Device can receive TSO with ECN.
> > +\item[VIRTIO_NET_F_HOST_ECN (13)] Device can receive TSO with TCP CWR flag set
> > +    and follow the ACE bits handling approach mentioend in
> > +    \hyperref[intro:rfc3168]{[RFC3168]}.
> >
> >  \item[VIRTIO_NET_F_HOST_UFO (14)] Device can receive UFO.
> >
> > @@ -695,8 +699,9 @@ \subsubsection{Packet 
> > Transmission}\label{sec:Device Types / Network Device / De
> >
> >    \item If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature,
> >      the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}
> > -    indicates that the TCP packet has the ECN bit set\footnote{This case is not handled by some older hardware, so is called out
> > -specifically in the protocol.}.
> > +    indicates that the TCP packet has TCP CWR flag set and the flag will be handled differently to all segements of
> > +    an aggregated segment, as mentioned in \hyperref[intro:rfc3168]{[RFC3168]}
> > +    \footnote{This case is not handled by some older hardware, so is called out specifically in the protocol.}.
> >     \end{itemize}
> >
> >  \item If the driver negotiated the VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO 
> > feature and the @@ -788,10 +793,9 @@ \subsubsection{Packet 
> > Transmission}\label{sec:Device Types / Network Device / De  \field{gso_type} to VIRTIO_NET_HDR_GSO_UDP_L4.
> >
> >  The driver SHOULD NOT send to the device TCP packets requiring 
> > segmentation offload -which have the Explicit Congestion Notification 
> > bit set, unless the -VIRTIO_NET_F_HOST_ECN feature is negotiated, in 
> > which case the -driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in 
> > -\field{gso_type}.
> > +which have the TCP CWR flag set and require the flag be handled as 
> > +mentioned in \hyperref[intro:rfc3168]{[RFC3168]}, unless the 
> > +VIRTIO_NET_F_HOST_ECN feature is negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
> >
> >  If VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO is negotiated, the driver MAY set
> >  VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or the 
> > VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit @@ -1105,9 +1109,9 @@ 
> > \subsubsection{Processing of Incoming Packets}\label{sec:Device Types 
> > / Network  \end{enumerate}
> >
> >  Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP, UDP_TUNNEL 
> > -and ECN features enable receive checksum, large receive offload and 
> > ECN -support which are the input equivalents of the transmit checksum, 
> > -transmit segmentation offloading and ECN features, as described
> > +and ECN features enable receive checksum, large receive offload and 
> > +RFC3168 ECN support which are the input equivalents of the transmit 
> > +checksum, transmit segmentation offloading and RFC3168 ECN features, 
> > +as described
> >  in \ref{sec:Device Types / Network Device / Device Operation /  
> > Packet Transmission}:
> >  \begin{enumerate}
> > @@ -1210,10 +1214,9 @@ \subsubsection{Processing of Incoming 
> > Packets}\label{sec:Device Types / Network  the VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM bit in \field{flags}.
> >
> >  The device SHOULD NOT send to the driver TCP packets requiring 
> > segmentation offload -which have the Explicit Congestion Notification 
> > bit set, unless the -VIRTIO_NET_F_GUEST_ECN feature is negotiated, in 
> > which case the -device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in 
> > -\field{gso_type}.
> > +which have the TCP CWR flag set and require the flag be handled as 
> > +mentioned in \hyperref[intro:rfc3168]{[RFC3168]}, unless the 
> > +VIRTIO_NET_F_GUEST_ECN feature is negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
> >
> >  If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the  
> > device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in diff --git 
> > a/introduction.tex b/introduction.tex index e60298a..d52622e 100644
> > --- a/introduction.tex
> > +++ b/introduction.tex
> > @@ -168,6 +168,9 @@ \section{Normative References}\label{sec:Normative References}
> >      Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP
> >      14, RFC 8174, DOI 10.17487/RFC8174, May 2017
> >          \newline\url{http://www.ietf.org/rfc/rfc8174.txt}\\
> > +     \phantomsection\label{intro:rfc3168}\textbf{[RFC3168]} &
> > +    S. Floyd., ``The Addition of Explicit Congestion Notification (ECN) to IP'', September 2001.
> > +     \newline\url{http://www.ietf.org/rfc/rfc3168.txt}\\
> >  \end{longtable}
> >
> >  \section{Non-Normative References}
> > --
> > 2.34.1

Re: [PATCH v6 1/2] virtio-net: Fix ECN feature descritpion

[Michael S. Tsirkin]

On Tue, Apr 15, 2025 at 01:33:45PM +0000, Chia-Yu Chang (Nokia) wrote:
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@redhat.com> 
> > Sent: Monday, April 14, 2025 8:59 PM
> > To: Chia-Yu Chang (Nokia) <chia-yu.chang@nokia-bell-labs.com>
> > Cc: virtio-comment@lists.linux.dev; cohuck@redhat.com; mvaralar@redhat.com; jasowang@redhat.com; xuanzhuo@linux.alibaba.com; eperezma@redhat.com; ij@kernel.org; ncardwell@google.com; Koen De Schepper (Nokia) <koen.de_schepper@nokia-bell-labs.com>; g.white <g.white@cablelabs.com>; vidhi_goel <vidhi_goel@apple.com>; ingemar.s.johansson@ericsson.com; mirja.kuehlewind@ericsson.com
> > Subject: Re: [PATCH v6 1/2] virtio-net: Fix ECN feature descritpion
> > 
> > 
> > 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 Mon, Apr 14, 2025 at 05:34:24PM +0200, chia-yu.chang@nokia-bell-labs.com wrote:
> > > From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> > >
> > > Clarify that the VIRTIO_NET_HDR_GSO_ECN gso_type flag does not mean 
> > > that TCP has IP-ECN set, instead it identifies TCP CWR flag is set and 
> > > will be cleared from the second segment. This is used to offload TCP 
> > > CWR flag in a way that is compatible with RFC3168 ECN but is 
> > > problematic for non-RFC3168 use of CWR flag.
> > >
> > > Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
> > > ---
> > 
> > 
> > if this is accepted, we might want to backport this to 1.3 maybe?
> 
> Thanks for feedback.
> In my opinion, this fix shall be backported to clarify its relation to RFC3168 ECN.
> Shall we make further patch, or you will do the merge once this patch is accepted?
> 
> Best regards,
> Chia-Yu

For the backport, you will need to open another github issue.


> > 
> > 
> > >  device-types/net/description.tex | 33 +++++++++++++++++---------------
> > >  introduction.tex                 |  3 +++
> > >  2 files changed, 21 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/device-types/net/description.tex 
> > > b/device-types/net/description.tex
> > > index 1b6b54d..a2c9de8 100644
> > > --- a/device-types/net/description.tex
> > > +++ b/device-types/net/description.tex
> > > @@ -54,7 +54,9 @@ \subsection{Feature bits}\label{sec:Device Types / 
> > > Network Device / Feature bits
> > >
> > >  \item[VIRTIO_NET_F_GUEST_TSO6 (8)] Driver can receive TSOv6.
> > >
> > > -\item[VIRTIO_NET_F_GUEST_ECN (9)] Driver can receive TSO with ECN.
> > > +\item[VIRTIO_NET_F_GUEST_ECN (9)] Driver can receive TSO with TCP CWR flag set
> > > +    and follow the ACE bits handling approach mentioned in
> > > +    \hyperref[intro:rfc3168]{[RFC3168]}.
> > >
> > >  \item[VIRTIO_NET_F_GUEST_UFO (10)] Driver can receive UFO.
> > >
> > > @@ -62,7 +64,9 @@ \subsection{Feature bits}\label{sec:Device Types / 
> > > Network Device / Feature bits
> > >
> > >  \item[VIRTIO_NET_F_HOST_TSO6 (12)] Device can receive TSOv6.
> > >
> > > -\item[VIRTIO_NET_F_HOST_ECN (13)] Device can receive TSO with ECN.
> > > +\item[VIRTIO_NET_F_HOST_ECN (13)] Device can receive TSO with TCP CWR flag set
> > > +    and follow the ACE bits handling approach mentioend in
> > > +    \hyperref[intro:rfc3168]{[RFC3168]}.
> > >
> > >  \item[VIRTIO_NET_F_HOST_UFO (14)] Device can receive UFO.
> > >
> > > @@ -695,8 +699,9 @@ \subsubsection{Packet 
> > > Transmission}\label{sec:Device Types / Network Device / De
> > >
> > >    \item If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature,
> > >      the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}
> > > -    indicates that the TCP packet has the ECN bit set\footnote{This case is not handled by some older hardware, so is called out
> > > -specifically in the protocol.}.
> > > +    indicates that the TCP packet has TCP CWR flag set and the flag will be handled differently to all segements of
> > > +    an aggregated segment, as mentioned in \hyperref[intro:rfc3168]{[RFC3168]}
> > > +    \footnote{This case is not handled by some older hardware, so is called out specifically in the protocol.}.
> > >     \end{itemize}
> > >
> > >  \item If the driver negotiated the VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO 
> > > feature and the @@ -788,10 +793,9 @@ \subsubsection{Packet 
> > > Transmission}\label{sec:Device Types / Network Device / De  \field{gso_type} to VIRTIO_NET_HDR_GSO_UDP_L4.
> > >
> > >  The driver SHOULD NOT send to the device TCP packets requiring 
> > > segmentation offload -which have the Explicit Congestion Notification 
> > > bit set, unless the -VIRTIO_NET_F_HOST_ECN feature is negotiated, in 
> > > which case the -driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in 
> > > -\field{gso_type}.
> > > +which have the TCP CWR flag set and require the flag be handled as 
> > > +mentioned in \hyperref[intro:rfc3168]{[RFC3168]}, unless the 
> > > +VIRTIO_NET_F_HOST_ECN feature is negotiated, in which case the driver MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
> > >
> > >  If VIRTIO_NET_F_HOST_UDP_TUNNEL_GSO is negotiated, the driver MAY set
> > >  VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or the 
> > > VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit @@ -1105,9 +1109,9 @@ 
> > > \subsubsection{Processing of Incoming Packets}\label{sec:Device Types 
> > > / Network  \end{enumerate}
> > >
> > >  Additionally, VIRTIO_NET_F_GUEST_CSUM, TSO4, TSO6, UDP, UDP_TUNNEL 
> > > -and ECN features enable receive checksum, large receive offload and 
> > > ECN -support which are the input equivalents of the transmit checksum, 
> > > -transmit segmentation offloading and ECN features, as described
> > > +and ECN features enable receive checksum, large receive offload and 
> > > +RFC3168 ECN support which are the input equivalents of the transmit 
> > > +checksum, transmit segmentation offloading and RFC3168 ECN features, 
> > > +as described
> > >  in \ref{sec:Device Types / Network Device / Device Operation /  
> > > Packet Transmission}:
> > >  \begin{enumerate}
> > > @@ -1210,10 +1214,9 @@ \subsubsection{Processing of Incoming 
> > > Packets}\label{sec:Device Types / Network  the VIRTIO_NET_HDR_F_UDP_TUNNEL_CSUM bit in \field{flags}.
> > >
> > >  The device SHOULD NOT send to the driver TCP packets requiring 
> > > segmentation offload -which have the Explicit Congestion Notification 
> > > bit set, unless the -VIRTIO_NET_F_GUEST_ECN feature is negotiated, in 
> > > which case the -device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in 
> > > -\field{gso_type}.
> > > +which have the TCP CWR flag set and require the flag be handled as 
> > > +mentioned in \hyperref[intro:rfc3168]{[RFC3168]}, unless the 
> > > +VIRTIO_NET_F_GUEST_ECN feature is negotiated, in which case the device MUST set the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type}.
> > >
> > >  If the VIRTIO_NET_F_GUEST_CSUM feature has been negotiated, the  
> > > device MAY set the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in diff --git 
> > > a/introduction.tex b/introduction.tex index e60298a..d52622e 100644
> > > --- a/introduction.tex
> > > +++ b/introduction.tex
> > > @@ -168,6 +168,9 @@ \section{Normative References}\label{sec:Normative References}
> > >      Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP
> > >      14, RFC 8174, DOI 10.17487/RFC8174, May 2017
> > >          \newline\url{http://www.ietf.org/rfc/rfc8174.txt}\\
> > > +     \phantomsection\label{intro:rfc3168}\textbf{[RFC3168]} &
> > > +    S. Floyd., ``The Addition of Explicit Congestion Notification (ECN) to IP'', September 2001.
> > > +     \newline\url{http://www.ietf.org/rfc/rfc3168.txt}\\
> > >  \end{longtable}
> > >
> > >  \section{Non-Normative References}
> > > --
> > > 2.34.1