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+.. SPDX-License-Identifier: GPL-2.0
+Checksum Offloads
+This document describes a set of techniques in the Linux networking stack to
+take advantage of checksum offload capabilities of various NICs.
+The following technologies are described:
+* TX Checksum Offload
+* LCO: Local Checksum Offload
+* RCO: Remote Checksum Offload
+Things that should be documented here but aren't yet:
+* RX Checksum Offload
+TX Checksum Offload
+The interface for offloading a transmit checksum to a device is explained in
+detail in comments near the top of include/linux/skbuff.h.
+In brief, it allows to request the device fill in a single ones-complement
+checksum defined by the sk_buff fields skb->csum_start and skb->csum_offset.
+The device should compute the 16-bit ones-complement checksum (i.e. the
+'IP-style' checksum) from csum_start to the end of the packet, and fill in the
+result at (csum_start + csum_offset).
+Because csum_offset cannot be negative, this ensures that the previous value of
+the checksum field is included in the checksum computation, thus it can be used
+to supply any needed corrections to the checksum (such as the sum of the
+pseudo-header for UDP or TCP).
+This interface only allows a single checksum to be offloaded. Where
+encapsulation is used, the packet may have multiple checksum fields in
+different header layers, and the rest will have to be handled by another
+mechanism such as LCO or RCO.
+CRC32c can also be offloaded using this interface, by means of filling
+skb->csum_start and skb->csum_offset as described above, and setting
+skb->csum_not_inet: see skbuff.h comment (section 'D') for more details.
+No offloading of the IP header checksum is performed; it is always done in
+software. This is OK because when we build the IP header, we obviously have it
+in cache, so summing it isn't expensive. It's also rather short.
+The requirements for GSO are more complicated, because when segmenting an
+encapsulated packet both the inner and outer checksums may need to be edited or
+recomputed for each resulting segment. See the skbuff.h comment (section 'E')
+for more details.
+A driver declares its offload capabilities in netdev->hw_features; see
+Documentation/networking/netdev-features.txt for more. Note that a device
+which only advertises NETIF_F_IP[V6]_CSUM must still obey the csum_start and
+csum_offset given in the SKB; if it tries to deduce these itself in hardware
+(as some NICs do) the driver should check that the values in the SKB match
+those which the hardware will deduce, and if not, fall back to checksumming in
+software instead (with skb_csum_hwoffload_help() or one of the
+skb_checksum_help() / skb_crc32c_csum_help functions, as mentioned in
+The stack should, for the most part, assume that checksum offload is supported
+by the underlying device. The only place that should check is
+validate_xmit_skb(), and the functions it calls directly or indirectly. That
+function compares the offload features requested by the SKB (which may include
+other offloads besides TX Checksum Offload) and, if they are not supported or
+enabled on the device (determined by netdev->features), performs the
+corresponding offload in software. In the case of TX Checksum Offload, that
+means calling skb_csum_hwoffload_help(skb, features).
+LCO: Local Checksum Offload
+LCO is a technique for efficiently computing the outer checksum of an
+encapsulated datagram when the inner checksum is due to be offloaded.
+The ones-complement sum of a correctly checksummed TCP or UDP packet is equal
+to the complement of the sum of the pseudo header, because everything else gets
+'cancelled out' by the checksum field. This is because the sum was
+complemented before being written to the checksum field.
+More generally, this holds in any case where the 'IP-style' ones complement
+checksum is used, and thus any checksum that TX Checksum Offload supports.
+That is, if we have set up TX Checksum Offload with a start/offset pair, we
+know that after the device has filled in that checksum, the ones complement sum
+from csum_start to the end of the packet will be equal to the complement of
+whatever value we put in the checksum field beforehand. This allows us to
+compute the outer checksum without looking at the payload: we simply stop
+summing when we get to csum_start, then add the complement of the 16-bit word
+at (csum_start + csum_offset).
+Then, when the true inner checksum is filled in (either by hardware or by
+skb_checksum_help()), the outer checksum will become correct by virtue of the
+LCO is performed by the stack when constructing an outer UDP header for an
+encapsulation such as VXLAN or GENEVE, in udp_set_csum(). Similarly for the
+IPv6 equivalents, in udp6_set_csum().
+It is also performed when constructing an IPv4 GRE header, in
+net/ipv4/ip_gre.c:build_header(). It is *not* currently performed when
+constructing an IPv6 GRE header; the GRE checksum is computed over the whole
+packet in net/ipv6/ip6_gre.c:ip6gre_xmit2(), but it should be possible to use
+LCO here as IPv6 GRE still uses an IP-style checksum.
+All of the LCO implementations use a helper function lco_csum(), in
+LCO can safely be used for nested encapsulations; in this case, the outer
+encapsulation layer will sum over both its own header and the 'middle' header.
+This does mean that the 'middle' header will get summed multiple times, but
+there doesn't seem to be a way to avoid that without incurring bigger costs
+(e.g. in SKB bloat).
+RCO: Remote Checksum Offload
+RCO is a technique for eliding the inner checksum of an encapsulated datagram,
+allowing the outer checksum to be offloaded. It does, however, involve a
+change to the encapsulation protocols, which the receiver must also support.
+For this reason, it is disabled by default.
+RCO is detailed in the following Internet-Drafts:
+* https://tools.ietf.org/html/draft-herbert-remotecsumoffload-00
+* https://tools.ietf.org/html/draft-herbert-vxlan-rco-00
+In Linux, RCO is implemented individually in each encapsulation protocol, and
+most tunnel types have flags controlling its use. For instance, VXLAN has the
+flag VXLAN_F_REMCSUM_TX (per struct vxlan_rdst) to indicate that RCO should be
+used when transmitting to a given remote destination.