[PATCH net-next 08/10] docs: net: add Rx notes to the checksum guide
From: Jakub Kicinski <kuba@kernel.org>
Date: 2026-05-26 16:02:11
Also in:
linux-doc
Subsystem:
documentation, networking [general], the rest · Maintainers:
Jonathan Corbet, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
The Rx checksum processing gives people pause. The two main questions in my experience are: - what to do with bad IPv4 checksum; and - what to do with packets with bad checksum. Folks often feel the urge to drop the latter, to "avoid overloading the host". Signed-off-by: Jakub Kicinski <kuba@kernel.org> --- Documentation/networking/checksum-offloads.rst | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-)
diff --git a/Documentation/networking/checksum-offloads.rst b/Documentation/networking/checksum-offloads.rst
index 907aed9f3a3b..d838fe5c1606 100644
--- a/Documentation/networking/checksum-offloads.rst
+++ b/Documentation/networking/checksum-offloads.rst@@ -19,7 +19,6 @@ take advantage of checksum offload capabilities of various NICs. Things that should be documented here but aren't yet: -* RX Checksum Offload * CHECKSUM_UNNECESSARY conversion
@@ -139,3 +138,19 @@ In Linux, RCO is implemented individually in each encapsulation protocol, and most tunnel types have flags controlling its use. For instance, VXLAN has the configuration flag VXLAN_F_REMCSUM_TX to indicate that RCO should be used when transmitting. + + +RX Checksum Offload +=================== + +RX checksum offload is controlled via NETIF_F_RXCSUM. When disabled the driver +must not set skb->ip_summed on ingress packets. As mentioned, IPv4 checksum +is not offloaded, the RXCSUM feature controls the offload of verification of +transport layer checksums. + +Note that packets with bad TCP/UDP checksums must still be passed +to the stack. skb->ip_summed of such packets can be set to ``CHECKSUM_COMPLETE`` +or left at ``CHECKSUM_NONE``. Drivers **must not discard** packets with +bad TCP/UDP checksum and must not configure the device to drop them. +Checksum validation is relatively inexpensive and having bad packets reflected +in SNMP counters is crucial for network monitoring.
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2.54.0