Re: [PATCH v11 1/2] virtio-net: Clarify ECN and GSO_ECN descriptions

On Wed, Feb 4, 2026 at 12:21 AM <[email protected]> wrote:
>
> From: Chia-Yu Chang <[email protected]>
>
> The existing specification that describing VIRTIO_NET_F_*_ECN and
> VIRTIO_NET_HDR_GSO_ECN can be misinterpreted as reffering to the IP ECN
> field. In reality, VIRTIO_NET_HDR_GSO_ECN corresponds to the TCP CWR flag
> and indicates that segment offload must follow the rules defined in
> RFC3168 Section 6.1.2: the first emitted segment retains the CWR flag,
> and all subsequent segments must clear it.
>
> This patch updates ECN feature descriptions and the transmit/receive
> requirements to accurately reflect the intended behavior explicitly
> specified in RFC3168.
>
> For detailed requirements, see RFC3168:
> https://datatracker.ietf.org/doc/html/rfc3168
>
> Signed-off-by: Chia-Yu Chang <[email protected]>
> ---
>  device-types/net/description.tex | 35 +++++++++++++++++---------------
>  introduction.tex                 |  3 +++
>  2 files changed, 22 insertions(+), 16 deletions(-)
>
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index da9ff17..0da8f9f 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 the CWR flag set
> +    and follow the requirements of the CWR bit described in
> +    ``Section 6.1.2. The TCP Sender'' of \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 the CWR flag set
> +    and follow the requirements of the CWR bit described in
> +    ``Section 6.1.2. The TCP Sender'' of \hyperref[intro:rfc3168]{[RFC3168]}.
>
>  \item[VIRTIO_NET_F_HOST_UFO (14)] Device can receive UFO.
>
> @@ -729,9 +733,10 @@ \subsubsection{Packet Transmission}\label{sec:Device Types / Network Device / De
>      header (ie. MSS).
>
>    \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.}.
> +    the VIRTIO_NET_HDR_GSO_ECN bit in \field{gso_type} indicates that the TCP packet
> +    has the CWR flag set and follows the requirements of the CWR flag described in
> +    ``Section 6.1.2. The TCP Sender'' of \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
> @@ -828,10 +833,9 @@ \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}.

I wonder if it's better to have some compatibility description here
instead of removing the old ECN completely.

> +which need to follow the CWR flag requirements described in ``Section 6.1.2. The TCP Sender''
> +of \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
> @@ -1168,9 +1172,9 @@ \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}
> @@ -1273,10 +1277,9 @@ \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 need to follow the CWR flag requirements described in ``Section 6.1.2. The TCP Sender''
> +of \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 VIRTIO_NET_F_OUT_NET_HEADER has been negotiated, the device MAY
>  set the \field{outer_nh_offset} to nonzero value to indicate outer network header offset,
> diff --git a/introduction.tex b/introduction.tex
> index 9a9cbde..ddbad93 100644
> --- a/introduction.tex
> +++ b/introduction.tex
> @@ -186,6 +186,9 @@ \section{Normative References}\label{sec:Normative References}
>         \phantomsection\label{intro:UTC-SLS}\textbf{[UTC-SLS]} &
>      UTC with Smoothed Leap Seconds (UTC-SLS)
>         \newline\url{https://www.cl.cam.ac.uk/~mgk25/time/utc-sls/}\\
> +       \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
>

Thanks