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