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-Checksum Offloads in the Linux Networking Stack
-
-
-Introduction
-============
-
-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
- * CHECKSUM_UNNECESSARY conversion
-
-
-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
- include/linux/skbuff.h).
-
-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 arithmetic.
-
-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
- include/linux/skbuff.h.
-
-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.