Documentation/networking/checksum-offloads.txt

   1 Checksum Offloads in the Linux Networking Stack
   2
   3
   4 Introduction
   5 ============
   6
   7 This document describes a set of techniques in the Linux networking stack
   8  to take advantage of checksum offload capabilities of various NICs.
   9
  10 The following technologies are described:
  11  * TX Checksum Offload
  12  * LCO: Local Checksum Offload
  13  * RCO: Remote Checksum Offload
  14
  15 Things that should be documented here but aren't yet:
  16  * RX Checksum Offload
  17  * CHECKSUM_UNNECESSARY conversion
  18
  19
  20 TX Checksum Offload
  21 ===================
  22
  23 The interface for offloading a transmit checksum to a device is explained
  24  in detail in comments near the top of include/linux/skbuff.h.
  25 In brief, it allows to request the device fill in a single ones-complement
  26  checksum defined by the sk_buff fields skb->csum_start and
  27  skb->csum_offset.  The device should compute the 16-bit ones-complement
  28  checksum (i.e. the 'IP-style' checksum) from csum_start to the end of the
  29  packet, and fill in the result at (csum_start + csum_offset).
  30 Because csum_offset cannot be negative, this ensures that the previous
  31  value of the checksum field is included in the checksum computation, thus
  32  it can be used to supply any needed corrections to the checksum (such as
  33  the sum of the pseudo-header for UDP or TCP).
  34 This interface only allows a single checksum to be offloaded.  Where
  35  encapsulation is used, the packet may have multiple checksum fields in
  36  different header layers, and the rest will have to be handled by another
  37  mechanism such as LCO or RCO.
  38 CRC32c can also be offloaded using this interface, by means of filling
  39  skb->csum_start and skb->csum_offset as described above, and setting
  40  skb->csum_not_inet: see skbuff.h comment (section 'D') for more details.
  41 No offloading of the IP header checksum is performed; it is always done in
  42  software.  This is OK because when we build the IP header, we obviously
  43  have it in cache, so summing it isn't expensive.  It's also rather short.
  44 The requirements for GSO are more complicated, because when segmenting an
  45  encapsulated packet both the inner and outer checksums may need to be
  46  edited or recomputed for each resulting segment.  See the skbuff.h comment
  47  (section 'E') for more details.
  48
  49 A driver declares its offload capabilities in netdev->hw_features; see
  50  Documentation/networking/netdev-features for more.  Note that a device
  51  which only advertises NETIF_F_IP[V6]_CSUM must still obey the csum_start
  52  and csum_offset given in the SKB; if it tries to deduce these itself in
  53  hardware (as some NICs do) the driver should check that the values in the
  54  SKB match those which the hardware will deduce, and if not, fall back to
  55  checksumming in software instead (with skb_csum_hwoffload_help() or one of
  56  the skb_checksum_help() / skb_crc32c_csum_help functions, as mentioned in
  57  include/linux/skbuff.h).
  58
  59 The stack should, for the most part, assume that checksum offload is
  60  supported by the underlying device.  The only place that should check is
  61  validate_xmit_skb(), and the functions it calls directly or indirectly.
  62  That function compares the offload features requested by the SKB (which
  63  may include other offloads besides TX Checksum Offload) and, if they are
  64  not supported or enabled on the device (determined by netdev->features),
  65  performs the corresponding offload in software.  In the case of TX
  66  Checksum Offload, that means calling skb_csum_hwoffload_help(skb, features).
  67
  68
  69 LCO: Local Checksum Offload
  70 ===========================
  71
  72 LCO is a technique for efficiently computing the outer checksum of an
  73  encapsulated datagram when the inner checksum is due to be offloaded.
  74 The ones-complement sum of a correctly checksummed TCP or UDP packet is
  75  equal to the complement of the sum of the pseudo header, because everything
  76  else gets 'cancelled out' by the checksum field.  This is because the sum was
  77  complemented before being written to the checksum field.
  78 More generally, this holds in any case where the 'IP-style' ones complement
  79  checksum is used, and thus any checksum that TX Checksum Offload supports.
  80 That is, if we have set up TX Checksum Offload with a start/offset pair, we
  81  know that after the device has filled in that checksum, the ones
  82  complement sum from csum_start to the end of the packet will be equal to
  83  the complement of whatever value we put in the checksum field beforehand.
  84  This allows us to compute the outer checksum without looking at the payload:
  85  we simply stop summing when we get to csum_start, then add the complement of
  86  the 16-bit word at (csum_start + csum_offset).
  87 Then, when the true inner checksum is filled in (either by hardware or by
  88  skb_checksum_help()), the outer checksum will become correct by virtue of
  89  the arithmetic.
  90
  91 LCO is performed by the stack when constructing an outer UDP header for an
  92  encapsulation such as VXLAN or GENEVE, in udp_set_csum().  Similarly for
  93  the IPv6 equivalents, in udp6_set_csum().
  94 It is also performed when constructing an IPv4 GRE header, in
  95  net/ipv4/ip_gre.c:build_header().  It is *not* currently performed when
  96  constructing an IPv6 GRE header; the GRE checksum is computed over the
  97  whole packet in net/ipv6/ip6_gre.c:ip6gre_xmit2(), but it should be
  98  possible to use LCO here as IPv6 GRE still uses an IP-style checksum.
  99 All of the LCO implementations use a helper function lco_csum(), in
 100  include/linux/skbuff.h.
 101
 102 LCO can safely be used for nested encapsulations; in this case, the outer
 103  encapsulation layer will sum over both its own header and the 'middle'
 104  header.  This does mean that the 'middle' header will get summed multiple
 105  times, but there doesn't seem to be a way to avoid that without incurring
 106  bigger costs (e.g. in SKB bloat).
 107
 108
 109 RCO: Remote Checksum Offload
 110 ============================
 111
 112 RCO is a technique for eliding the inner checksum of an encapsulated
 113  datagram, allowing the outer checksum to be offloaded.  It does, however,
 114  involve a change to the encapsulation protocols, which the receiver must
 115  also support.  For this reason, it is disabled by default.
 116 RCO is detailed in the following Internet-Drafts:
 117 https://tools.ietf.org/html/draft-herbert-remotecsumoffload-00
 118 https://tools.ietf.org/html/draft-herbert-vxlan-rco-00
 119 In Linux, RCO is implemented individually in each encapsulation protocol,
 120  and most tunnel types have flags controlling its use.  For instance, VXLAN
 121  has the flag VXLAN_F_REMCSUM_TX (per struct vxlan_rdst) to indicate that
 122  RCO should be used when transmitting to a given remote destination.