From: Chia-Yu Chang <[email protected]>
Clarify that the VIRTIO_NET_HDR_GSO_ECN gso_type flag does not mean that
TCP has IP-ECN set; instead, it identifies that the TCP CWR flag is set
and will be cleared from the second segment of an aggregated segment.
This is used to offload the TCP CWR flag in a way that is compatible with
RFC3168 ECN but is problematic for non-RFC3168 use of the TCP CWR flag.
Foe detailed requirements, please refer to IETF 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}.
+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 80aa67a..1c4cb33 100644
--- a/introduction.tex
+++ b/introduction.tex
@@ -180,6 +180,9 @@ \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
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