[PATCH v7 0/2] Update ECN and Add AccECN feature

[PATCH v7 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 v7:

v7 (15-Apr-2025)
- Fix typos in AccECN patch (Michael S. Tsirkin <mst@redhat.com>)

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 v7 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 v7 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..61e2aac 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 mentioned 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 mentioned 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, 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, 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, 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, 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 v7 1/2] virtio-net: Fix ECN feature descritpion

[Michael S. Tsirkin]

On Thu, Apr 17, 2025 at 11:57:34PM +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>
> ---

even the subject has a typo.

>  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

what does require refer to? driver should not require? packets require?

> +\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}.

set for which packets?

>  
>  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

same question

> +\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}.

set for which packets?

>  
>  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