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diff --git a/Documentation/networking/checksum-offloads.txt b/Documentation/networking/checksum-offloads.txt deleted file mode 100644 index 27bc09cfcf6d..000000000000 --- a/Documentation/networking/checksum-offloads.txt +++ /dev/null @@ -1,122 +0,0 @@ -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. |