Based on kernel version 4.9. Page generated on 2016-12-21 14:36 EST.
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 No offloading of the IP header checksum is performed; it is always done in 39 software. This is OK because when we build the IP header, we obviously 40 have it in cache, so summing it isn't expensive. It's also rather short. 41 The requirements for GSO are more complicated, because when segmenting an 42 encapsulated packet both the inner and outer checksums may need to be 43 edited or recomputed for each resulting segment. See the skbuff.h comment 44 (section 'E') for more details. 45 46 A driver declares its offload capabilities in netdev->hw_features; see 47 Documentation/networking/netdev-features for more. Note that a device 48 which only advertises NETIF_F_IP[V6]_CSUM must still obey the csum_start 49 and csum_offset given in the SKB; if it tries to deduce these itself in 50 hardware (as some NICs do) the driver should check that the values in the 51 SKB match those which the hardware will deduce, and if not, fall back to 52 checksumming in software instead (with skb_checksum_help or one of the 53 skb_csum_off_chk* functions as mentioned in include/linux/skbuff.h). This 54 is a pain, but that's what you get when hardware tries to be clever. 55 56 The stack should, for the most part, assume that checksum offload is 57 supported by the underlying device. The only place that should check is 58 validate_xmit_skb(), and the functions it calls directly or indirectly. 59 That function compares the offload features requested by the SKB (which 60 may include other offloads besides TX Checksum Offload) and, if they are 61 not supported or enabled on the device (determined by netdev->features), 62 performs the corresponding offload in software. In the case of TX 63 Checksum Offload, that means calling skb_checksum_help(skb). 64 65 66 LCO: Local Checksum Offload 67 =========================== 68 69 LCO is a technique for efficiently computing the outer checksum of an 70 encapsulated datagram when the inner checksum is due to be offloaded. 71 The ones-complement sum of a correctly checksummed TCP or UDP packet is 72 equal to the complement of the sum of the pseudo header, because everything 73 else gets 'cancelled out' by the checksum field. This is because the sum was 74 complemented before being written to the checksum field. 75 More generally, this holds in any case where the 'IP-style' ones complement 76 checksum is used, and thus any checksum that TX Checksum Offload supports. 77 That is, if we have set up TX Checksum Offload with a start/offset pair, we 78 know that after the device has filled in that checksum, the ones 79 complement sum from csum_start to the end of the packet will be equal to 80 the complement of whatever value we put in the checksum field beforehand. 81 This allows us to compute the outer checksum without looking at the payload: 82 we simply stop summing when we get to csum_start, then add the complement of 83 the 16-bit word at (csum_start + csum_offset). 84 Then, when the true inner checksum is filled in (either by hardware or by 85 skb_checksum_help()), the outer checksum will become correct by virtue of 86 the arithmetic. 87 88 LCO is performed by the stack when constructing an outer UDP header for an 89 encapsulation such as VXLAN or GENEVE, in udp_set_csum(). Similarly for 90 the IPv6 equivalents, in udp6_set_csum(). 91 It is also performed when constructing an IPv4 GRE header, in 92 net/ipv4/ip_gre.c:build_header(). It is *not* currently performed when 93 constructing an IPv6 GRE header; the GRE checksum is computed over the 94 whole packet in net/ipv6/ip6_gre.c:ip6gre_xmit2(), but it should be 95 possible to use LCO here as IPv6 GRE still uses an IP-style checksum. 96 All of the LCO implementations use a helper function lco_csum(), in 97 include/linux/skbuff.h. 98 99 LCO can safely be used for nested encapsulations; in this case, the outer 100 encapsulation layer will sum over both its own header and the 'middle' 101 header. This does mean that the 'middle' header will get summed multiple 102 times, but there doesn't seem to be a way to avoid that without incurring 103 bigger costs (e.g. in SKB bloat). 104 105 106 RCO: Remote Checksum Offload 107 ============================ 108 109 RCO is a technique for eliding the inner checksum of an encapsulated 110 datagram, allowing the outer checksum to be offloaded. It does, however, 111 involve a change to the encapsulation protocols, which the receiver must 112 also support. For this reason, it is disabled by default. 113 RCO is detailed in the following Internet-Drafts: 114 https://tools.ietf.org/html/draft-herbert-remotecsumoffload-00 115 https://tools.ietf.org/html/draft-herbert-vxlan-rco-00 116 In Linux, RCO is implemented individually in each encapsulation protocol, 117 and most tunnel types have flags controlling its use. For instance, VXLAN 118 has the flag VXLAN_F_REMCSUM_TX (per struct vxlan_rdst) to indicate that 119 RCO should be used when transmitting to a given remote destination.