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

["Michael S. Tsirkin" <mst@redhat.com>]

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