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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.
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