From: Jakub Kicinski Date: Tue, 26 May 2026 16:01:49 +0000 (-0700) Subject: docs: net: add Rx notes to the checksum guide X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=6686e7f03f42c9476770e06e0a030b7267018e71;p=thirdparty%2Fkernel%2Flinux.git docs: net: add Rx notes to the checksum guide 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". Reviewed-by: Willem de Bruijn Link: https://patch.msgid.link/20260526160151.2793354-9-kuba@kernel.org Signed-off-by: Jakub Kicinski --- diff --git a/Documentation/networking/checksum-offloads.rst b/Documentation/networking/checksum-offloads.rst index 907aed9f3a3b8..d838fe5c16066 100644 --- a/Documentation/networking/checksum-offloads.rst +++ b/Documentation/networking/checksum-offloads.rst @@ -19,7 +19,6 @@ The following technologies are described: 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.