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docs: net: add Rx notes to the checksum guide

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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 <willemb@google.com>
Link: https://patch.msgid.link/20260526160151.2793354-9-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Jakub Kicinski 2026-05-26 09:01:49 -07:00
parent f94874c388
commit 6686e7f03f
1 changed files with 16 additions and 1 deletions
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17
Documentation/networking/checksum-offloads.rst
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@ -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.