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doc: networking: prepare offload documents for conversion into RST
Add small number of markups which are sufficient for conversion into reStructuredText. Unfortunately there was necessary to restructure all sections in checksum-offloads.txt file and create paragraphs separated by newline. There also must not be a space at the beginning of paragpraph. There are no semantic changes. Signed-off-by: Otto Sabart <ottosabart@seberm.com> Acked-by: David S. Miller <davem@davemloft.net> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Otto Sabart
Jonathan Corbet
parent
9f63df26be
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.. SPDX-License-Identifier: GPL-2.0
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===============================================
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Checksum Offloads in the Linux Networking Stack
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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
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to take advantage of checksum offload capabilities of various NICs.
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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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* 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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* 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
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in detail in comments near the top of include/linux/skbuff.h.
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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
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skb->csum_offset. The device should compute the 16-bit ones-complement
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checksum (i.e. the 'IP-style' checksum) from csum_start to the end of the
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packet, and fill in the result at (csum_start + csum_offset).
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Because csum_offset cannot be negative, this ensures that the previous
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value of the checksum field is included in the checksum computation, thus
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it can be used to supply any needed corrections to the checksum (such as
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the sum of the pseudo-header for UDP or TCP).
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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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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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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
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have it in cache, so summing it isn't expensive. It's also rather short.
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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
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edited or recomputed for each resulting segment. See the skbuff.h comment
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(section 'E') for more details.
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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.txt for more. Note that a device
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which only advertises NETIF_F_IP[V6]_CSUM must still obey the csum_start
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and csum_offset given in the SKB; if it tries to deduce these itself in
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hardware (as some NICs do) the driver should check that the values in the
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SKB match those which the hardware will deduce, and if not, fall back to
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checksumming in software instead (with skb_csum_hwoffload_help() or one of
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the skb_checksum_help() / skb_crc32c_csum_help functions, as mentioned in
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include/linux/skbuff.h).
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Documentation/networking/netdev-features.txt 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
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supported 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.
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That function compares the offload features requested by the SKB (which
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may include other offloads besides TX Checksum Offload) and, if they are
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not supported or enabled on the device (determined by netdev->features),
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performs the corresponding offload in software. In the case of TX
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Checksum Offload, that means calling skb_csum_hwoffload_help(skb, features).
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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
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equal to the complement of the sum of the pseudo header, because everything
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else gets '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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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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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
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complement sum from csum_start to the end of the packet will be equal to
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the complement of whatever value we put in the checksum field beforehand.
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This allows us to compute the outer checksum without looking at the payload:
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we simply stop summing when we get to csum_start, then add the complement of
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the 16-bit word at (csum_start + csum_offset).
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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
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the arithmetic.
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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
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the IPv6 equivalents, in udp6_set_csum().
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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
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whole packet in net/ipv6/ip6_gre.c:ip6gre_xmit2(), but it should be
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possible to use LCO here as IPv6 GRE still uses an IP-style checksum.
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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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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'
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header. This does mean that the 'middle' header will get summed multiple
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times, but there doesn't seem to be a way to avoid that without incurring
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bigger costs (e.g. in SKB bloat).
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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
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datagram, allowing the outer checksum to be offloaded. It does, however,
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involve a change to the encapsulation protocols, which the receiver must
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also support. For this reason, it is disabled by default.
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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,
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and most tunnel types have flags controlling its use. For instance, VXLAN
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has the flag VXLAN_F_REMCSUM_TX (per struct vxlan_rdst) to indicate that
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RCO should be used when transmitting to a given remote destination.
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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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@ -1,4 +1,9 @@
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.. SPDX-License-Identifier: GPL-2.0
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===================================================
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Segmentation Offloads in the Linux Networking Stack
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===================================================
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Introduction
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============
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@ -15,6 +20,7 @@ The following technologies are described:
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* Partial Generic Segmentation Offload - GSO_PARTIAL
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* SCTP accelleration with GSO - GSO_BY_FRAGS
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TCP Segmentation Offload
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========================
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@ -42,6 +48,7 @@ NETIF_F_TSO_MANGLEID set then the IP ID can be ignored when performing TSO
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and we will either increment the IP ID for all frames, or leave it at a
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static value based on driver preference.
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UDP Fragmentation Offload
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=========================
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@ -54,6 +61,7 @@ UFO is deprecated: modern kernels will no longer generate UFO skbs, but can
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still receive them from tuntap and similar devices. Offload of UDP-based
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tunnel protocols is still supported.
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IPIP, SIT, GRE, UDP Tunnel, and Remote Checksum Offloads
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========================================================
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@ -71,17 +79,19 @@ refer to the tunnel headers as the outer headers, while the encapsulated
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data is normally referred to as the inner headers. Below is the list of
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calls to access the given headers:
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IPIP/SIT Tunnel:
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Outer Inner
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MAC skb_mac_header
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Network skb_network_header skb_inner_network_header
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Transport skb_transport_header
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IPIP/SIT Tunnel::
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UDP/GRE Tunnel:
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Outer Inner
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MAC skb_mac_header skb_inner_mac_header
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Network skb_network_header skb_inner_network_header
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Transport skb_transport_header skb_inner_transport_header
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Outer Inner
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MAC skb_mac_header
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Network skb_network_header skb_inner_network_header
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Transport skb_transport_header
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UDP/GRE Tunnel::
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Outer Inner
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MAC skb_mac_header skb_inner_mac_header
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Network skb_network_header skb_inner_network_header
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Transport skb_transport_header skb_inner_transport_header
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In addition to the above tunnel types there are also SKB_GSO_GRE_CSUM and
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SKB_GSO_UDP_TUNNEL_CSUM. These two additional tunnel types reflect the
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@ -93,6 +103,7 @@ header has requested a remote checksum offload. In this case the inner
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headers will be left with a partial checksum and only the outer header
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checksum will be computed.
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Generic Segmentation Offload
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============================
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@ -106,6 +117,7 @@ Before enabling any hardware segmentation offload a corresponding software
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offload is required in GSO. Otherwise it becomes possible for a frame to
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be re-routed between devices and end up being unable to be transmitted.
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Generic Receive Offload
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=======================
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@ -117,6 +129,7 @@ this is IPv4 ID in the case that the DF bit is set for a given IP header.
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If the value of the IPv4 ID is not sequentially incrementing it will be
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altered so that it is when a frame assembled via GRO is segmented via GSO.
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Partial Generic Segmentation Offload
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====================================
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@ -134,6 +147,7 @@ is the outer IPv4 ID field. It is up to the device drivers to guarantee
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that the IPv4 ID field is incremented in the case that a given header does
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not have the DF bit set.
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SCTP accelleration with GSO
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===========================
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@ -157,14 +171,14 @@ appropriately.
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There are some helpers to make this easier:
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- skb_is_gso(skb) && skb_is_gso_sctp(skb) is the best way to see if
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an skb is an SCTP GSO skb.
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- skb_is_gso(skb) && skb_is_gso_sctp(skb) is the best way to see if
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an skb is an SCTP GSO skb.
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- For size checks, the skb_gso_validate_*_len family of helpers correctly
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considers GSO_BY_FRAGS.
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- For size checks, the skb_gso_validate_*_len family of helpers correctly
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considers GSO_BY_FRAGS.
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- For manipulating packets, skb_increase_gso_size and skb_decrease_gso_size
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will check for GSO_BY_FRAGS and WARN if asked to manipulate these skbs.
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- For manipulating packets, skb_increase_gso_size and skb_decrease_gso_size
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will check for GSO_BY_FRAGS and WARN if asked to manipulate these skbs.
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This also affects drivers with the NETIF_F_FRAGLIST & NETIF_F_GSO_SCTP bits
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set. Note also that NETIF_F_GSO_SCTP is included in NETIF_F_GSO_SOFTWARE.
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