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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	Open vSwitch datapath developer documentation
2	=============================================
3	
4	The Open vSwitch kernel module allows flexible userspace control over
5	flow-level packet processing on selected network devices.  It can be
6	used to implement a plain Ethernet switch, network device bonding,
7	VLAN processing, network access control, flow-based network control,
8	and so on.
9	
10	The kernel module implements multiple "datapaths" (analogous to
11	bridges), each of which can have multiple "vports" (analogous to ports
12	within a bridge).  Each datapath also has associated with it a "flow
13	table" that userspace populates with "flows" that map from keys based
14	on packet headers and metadata to sets of actions.  The most common
15	action forwards the packet to another vport; other actions are also
16	implemented.
17	
18	When a packet arrives on a vport, the kernel module processes it by
19	extracting its flow key and looking it up in the flow table.  If there
20	is a matching flow, it executes the associated actions.  If there is
21	no match, it queues the packet to userspace for processing (as part of
22	its processing, userspace will likely set up a flow to handle further
23	packets of the same type entirely in-kernel).
24	
25	
26	Flow key compatibility
27	----------------------
28	
29	Network protocols evolve over time.  New protocols become important
30	and existing protocols lose their prominence.  For the Open vSwitch
31	kernel module to remain relevant, it must be possible for newer
32	versions to parse additional protocols as part of the flow key.  It
33	might even be desirable, someday, to drop support for parsing
34	protocols that have become obsolete.  Therefore, the Netlink interface
35	to Open vSwitch is designed to allow carefully written userspace
36	applications to work with any version of the flow key, past or future.
37	
38	To support this forward and backward compatibility, whenever the
39	kernel module passes a packet to userspace, it also passes along the
40	flow key that it parsed from the packet.  Userspace then extracts its
41	own notion of a flow key from the packet and compares it against the
42	kernel-provided version:
43	
44	    - If userspace's notion of the flow key for the packet matches the
45	      kernel's, then nothing special is necessary.
46	
47	    - If the kernel's flow key includes more fields than the userspace
48	      version of the flow key, for example if the kernel decoded IPv6
49	      headers but userspace stopped at the Ethernet type (because it
50	      does not understand IPv6), then again nothing special is
51	      necessary.  Userspace can still set up a flow in the usual way,
52	      as long as it uses the kernel-provided flow key to do it.
53	
54	    - If the userspace flow key includes more fields than the
55	      kernel's, for example if userspace decoded an IPv6 header but
56	      the kernel stopped at the Ethernet type, then userspace can
57	      forward the packet manually, without setting up a flow in the
58	      kernel.  This case is bad for performance because every packet
59	      that the kernel considers part of the flow must go to userspace,
60	      but the forwarding behavior is correct.  (If userspace can
61	      determine that the values of the extra fields would not affect
62	      forwarding behavior, then it could set up a flow anyway.)
63	
64	How flow keys evolve over time is important to making this work, so
65	the following sections go into detail.
66	
67	
68	Flow key format
69	---------------
70	
71	A flow key is passed over a Netlink socket as a sequence of Netlink
72	attributes.  Some attributes represent packet metadata, defined as any
73	information about a packet that cannot be extracted from the packet
74	itself, e.g. the vport on which the packet was received.  Most
75	attributes, however, are extracted from headers within the packet,
76	e.g. source and destination addresses from Ethernet, IP, or TCP
77	headers.
78	
79	The <linux/openvswitch.h> header file defines the exact format of the
80	flow key attributes.  For informal explanatory purposes here, we write
81	them as comma-separated strings, with parentheses indicating arguments
82	and nesting.  For example, the following could represent a flow key
83	corresponding to a TCP packet that arrived on vport 1:
84	
85	    in_port(1), eth(src=e0:91:f5:21:d0:b2, dst=00:02:e3:0f:80:a4),
86	    eth_type(0x0800), ipv4(src=172.16.0.20, dst=172.18.0.52, proto=17, tos=0,
87	    frag=no), tcp(src=49163, dst=80)
88	
89	Often we ellipsize arguments not important to the discussion, e.g.:
90	
91	    in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...)
92	
93	
94	Wildcarded flow key format
95	--------------------------
96	
97	A wildcarded flow is described with two sequences of Netlink attributes
98	passed over the Netlink socket. A flow key, exactly as described above, and an
99	optional corresponding flow mask.
100	
101	A wildcarded flow can represent a group of exact match flows. Each '1' bit
102	in the mask specifies a exact match with the corresponding bit in the flow key.
103	A '0' bit specifies a don't care bit, which will match either a '1' or '0' bit
104	of a incoming packet. Using wildcarded flow can improve the flow set up rate
105	by reduce the number of new flows need to be processed by the user space program.
106	
107	Support for the mask Netlink attribute is optional for both the kernel and user
108	space program. The kernel can ignore the mask attribute, installing an exact
109	match flow, or reduce the number of don't care bits in the kernel to less than
110	what was specified by the user space program. In this case, variations in bits
111	that the kernel does not implement will simply result in additional flow setups.
112	The kernel module will also work with user space programs that neither support
113	nor supply flow mask attributes.
114	
115	Since the kernel may ignore or modify wildcard bits, it can be difficult for
116	the userspace program to know exactly what matches are installed. There are
117	two possible approaches: reactively install flows as they miss the kernel
118	flow table (and therefore not attempt to determine wildcard changes at all)
119	or use the kernel's response messages to determine the installed wildcards.
120	
121	When interacting with userspace, the kernel should maintain the match portion
122	of the key exactly as originally installed. This will provides a handle to
123	identify the flow for all future operations. However, when reporting the
124	mask of an installed flow, the mask should include any restrictions imposed
125	by the kernel.
126	
127	The behavior when using overlapping wildcarded flows is undefined. It is the
128	responsibility of the user space program to ensure that any incoming packet
129	can match at most one flow, wildcarded or not. The current implementation
130	performs best-effort detection of overlapping wildcarded flows and may reject
131	some but not all of them. However, this behavior may change in future versions.
132	
133	
134	Basic rule for evolving flow keys
135	---------------------------------
136	
137	Some care is needed to really maintain forward and backward
138	compatibility for applications that follow the rules listed under
139	"Flow key compatibility" above.
140	
141	The basic rule is obvious:
142	
143	    ------------------------------------------------------------------
144	    New network protocol support must only supplement existing flow
145	    key attributes.  It must not change the meaning of already defined
146	    flow key attributes.
147	    ------------------------------------------------------------------
148	
149	This rule does have less-obvious consequences so it is worth working
150	through a few examples.  Suppose, for example, that the kernel module
151	did not already implement VLAN parsing.  Instead, it just interpreted
152	the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the
153	packet.  The flow key for any packet with an 802.1Q header would look
154	essentially like this, ignoring metadata:
155	
156	    eth(...), eth_type(0x8100)
157	
158	Naively, to add VLAN support, it makes sense to add a new "vlan" flow
159	key attribute to contain the VLAN tag, then continue to decode the
160	encapsulated headers beyond the VLAN tag using the existing field
161	definitions.  With this change, a TCP packet in VLAN 10 would have a
162	flow key much like this:
163	
164	    eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...)
165	
166	But this change would negatively affect a userspace application that
167	has not been updated to understand the new "vlan" flow key attribute.
168	The application could, following the flow compatibility rules above,
169	ignore the "vlan" attribute that it does not understand and therefore
170	assume that the flow contained IP packets.  This is a bad assumption
171	(the flow only contains IP packets if one parses and skips over the
172	802.1Q header) and it could cause the application's behavior to change
173	across kernel versions even though it follows the compatibility rules.
174	
175	The solution is to use a set of nested attributes.  This is, for
176	example, why 802.1Q support uses nested attributes.  A TCP packet in
177	VLAN 10 is actually expressed as:
178	
179	    eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800),
180	    ip(proto=6, ...), tcp(...)))
181	
182	Notice how the "eth_type", "ip", and "tcp" flow key attributes are
183	nested inside the "encap" attribute.  Thus, an application that does
184	not understand the "vlan" key will not see either of those attributes
185	and therefore will not misinterpret them.  (Also, the outer eth_type
186	is still 0x8100, not changed to 0x0800.)
187	
188	Handling malformed packets
189	--------------------------
190	
191	Don't drop packets in the kernel for malformed protocol headers, bad
192	checksums, etc.  This would prevent userspace from implementing a
193	simple Ethernet switch that forwards every packet.
194	
195	Instead, in such a case, include an attribute with "empty" content.
196	It doesn't matter if the empty content could be valid protocol values,
197	as long as those values are rarely seen in practice, because userspace
198	can always forward all packets with those values to userspace and
199	handle them individually.
200	
201	For example, consider a packet that contains an IP header that
202	indicates protocol 6 for TCP, but which is truncated just after the IP
203	header, so that the TCP header is missing.  The flow key for this
204	packet would include a tcp attribute with all-zero src and dst, like
205	this:
206	
207	    eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0)
208	
209	As another example, consider a packet with an Ethernet type of 0x8100,
210	indicating that a VLAN TCI should follow, but which is truncated just
211	after the Ethernet type.  The flow key for this packet would include
212	an all-zero-bits vlan and an empty encap attribute, like this:
213	
214	    eth(...), eth_type(0x8100), vlan(0), encap()
215	
216	Unlike a TCP packet with source and destination ports 0, an
217	all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka
218	VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan
219	attribute expressly to allow this situation to be distinguished.
220	Thus, the flow key in this second example unambiguously indicates a
221	missing or malformed VLAN TCI.
222	
223	Other rules
224	-----------
225	
226	The other rules for flow keys are much less subtle:
227	
228	    - Duplicate attributes are not allowed at a given nesting level.
229	
230	    - Ordering of attributes is not significant.
231	
232	    - When the kernel sends a given flow key to userspace, it always
233	      composes it the same way.  This allows userspace to hash and
234	      compare entire flow keys that it may not be able to fully
235	      interpret.
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