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Based on kernel version 4.9. Page generated on 2016-12-21 14:36 EST.

1	Segmentation Offloads in the Linux Networking Stack
2	
3	Introduction
4	============
5	
6	This document describes a set of techniques in the Linux networking stack
7	to take advantage of segmentation offload capabilities of various NICs.
8	
9	The following technologies are described:
10	 * TCP Segmentation Offload - TSO
11	 * UDP Fragmentation Offload - UFO
12	 * IPIP, SIT, GRE, and UDP Tunnel Offloads
13	 * Generic Segmentation Offload - GSO
14	 * Generic Receive Offload - GRO
15	 * Partial Generic Segmentation Offload - GSO_PARTIAL
16	
17	TCP Segmentation Offload
18	========================
19	
20	TCP segmentation allows a device to segment a single frame into multiple
21	frames with a data payload size specified in skb_shinfo()->gso_size.
22	When TCP segmentation requested the bit for either SKB_GSO_TCP or
23	SKB_GSO_TCP6 should be set in skb_shinfo()->gso_type and
24	skb_shinfo()->gso_size should be set to a non-zero value.
25	
26	TCP segmentation is dependent on support for the use of partial checksum
27	offload.  For this reason TSO is normally disabled if the Tx checksum
28	offload for a given device is disabled.
29	
30	In order to support TCP segmentation offload it is necessary to populate
31	the network and transport header offsets of the skbuff so that the device
32	drivers will be able determine the offsets of the IP or IPv6 header and the
33	TCP header.  In addition as CHECKSUM_PARTIAL is required csum_start should
34	also point to the TCP header of the packet.
35	
36	For IPv4 segmentation we support one of two types in terms of the IP ID.
37	The default behavior is to increment the IP ID with every segment.  If the
38	GSO type SKB_GSO_TCP_FIXEDID is specified then we will not increment the IP
39	ID and all segments will use the same IP ID.  If a device has
40	NETIF_F_TSO_MANGLEID set then the IP ID can be ignored when performing TSO
41	and we will either increment the IP ID for all frames, or leave it at a
42	static value based on driver preference.
43	
44	UDP Fragmentation Offload
45	=========================
46	
47	UDP fragmentation offload allows a device to fragment an oversized UDP
48	datagram into multiple IPv4 fragments.  Many of the requirements for UDP
49	fragmentation offload are the same as TSO.  However the IPv4 ID for
50	fragments should not increment as a single IPv4 datagram is fragmented.
51	
52	IPIP, SIT, GRE, UDP Tunnel, and Remote Checksum Offloads
53	========================================================
54	
55	In addition to the offloads described above it is possible for a frame to
56	contain additional headers such as an outer tunnel.  In order to account
57	for such instances an additional set of segmentation offload types were
58	introduced including SKB_GSO_IPIP, SKB_GSO_SIT, SKB_GSO_GRE, and
59	SKB_GSO_UDP_TUNNEL.  These extra segmentation types are used to identify
60	cases where there are more than just 1 set of headers.  For example in the
61	case of IPIP and SIT we should have the network and transport headers moved
62	from the standard list of headers to "inner" header offsets.
63	
64	Currently only two levels of headers are supported.  The convention is to
65	refer to the tunnel headers as the outer headers, while the encapsulated
66	data is normally referred to as the inner headers.  Below is the list of
67	calls to access the given headers:
68	
69	IPIP/SIT Tunnel:
70			Outer			Inner
71	MAC		skb_mac_header
72	Network		skb_network_header	skb_inner_network_header
73	Transport	skb_transport_header
74	
75	UDP/GRE Tunnel:
76			Outer			Inner
77	MAC		skb_mac_header		skb_inner_mac_header
78	Network		skb_network_header	skb_inner_network_header
79	Transport	skb_transport_header	skb_inner_transport_header
80	
81	In addition to the above tunnel types there are also SKB_GSO_GRE_CSUM and
82	SKB_GSO_UDP_TUNNEL_CSUM.  These two additional tunnel types reflect the
83	fact that the outer header also requests to have a non-zero checksum
84	included in the outer header.
85	
86	Finally there is SKB_GSO_REMCSUM which indicates that a given tunnel header
87	has requested a remote checksum offload.  In this case the inner headers
88	will be left with a partial checksum and only the outer header checksum
89	will be computed.
90	
91	Generic Segmentation Offload
92	============================
93	
94	Generic segmentation offload is a pure software offload that is meant to
95	deal with cases where device drivers cannot perform the offloads described
96	above.  What occurs in GSO is that a given skbuff will have its data broken
97	out over multiple skbuffs that have been resized to match the MSS provided
98	via skb_shinfo()->gso_size.
99	
100	Before enabling any hardware segmentation offload a corresponding software
101	offload is required in GSO.  Otherwise it becomes possible for a frame to
102	be re-routed between devices and end up being unable to be transmitted.
103	
104	Generic Receive Offload
105	=======================
106	
107	Generic receive offload is the complement to GSO.  Ideally any frame
108	assembled by GRO should be segmented to create an identical sequence of
109	frames using GSO, and any sequence of frames segmented by GSO should be
110	able to be reassembled back to the original by GRO.  The only exception to
111	this is IPv4 ID in the case that the DF bit is set for a given IP header.
112	If the value of the IPv4 ID is not sequentially incrementing it will be
113	altered so that it is when a frame assembled via GRO is segmented via GSO.
114	
115	Partial Generic Segmentation Offload
116	====================================
117	
118	Partial generic segmentation offload is a hybrid between TSO and GSO.  What
119	it effectively does is take advantage of certain traits of TCP and tunnels
120	so that instead of having to rewrite the packet headers for each segment
121	only the inner-most transport header and possibly the outer-most network
122	header need to be updated.  This allows devices that do not support tunnel
123	offloads or tunnel offloads with checksum to still make use of segmentation.
124	
125	With the partial offload what occurs is that all headers excluding the
126	inner transport header are updated such that they will contain the correct
127	values for if the header was simply duplicated.  The one exception to this
128	is the outer IPv4 ID field.  It is up to the device drivers to guarantee
129	that the IPv4 ID field is incremented in the case that a given header does
130	not have the DF bit set.
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