196 lines
8.7 KiB
Plaintext
196 lines
8.7 KiB
Plaintext
Open vSwitch datapath developer documentation
|
|
=============================================
|
|
|
|
The Open vSwitch kernel module allows flexible userspace control over
|
|
flow-level packet processing on selected network devices. It can be
|
|
used to implement a plain Ethernet switch, network device bonding,
|
|
VLAN processing, network access control, flow-based network control,
|
|
and so on.
|
|
|
|
The kernel module implements multiple "datapaths" (analogous to
|
|
bridges), each of which can have multiple "vports" (analogous to ports
|
|
within a bridge). Each datapath also has associated with it a "flow
|
|
table" that userspace populates with "flows" that map from keys based
|
|
on packet headers and metadata to sets of actions. The most common
|
|
action forwards the packet to another vport; other actions are also
|
|
implemented.
|
|
|
|
When a packet arrives on a vport, the kernel module processes it by
|
|
extracting its flow key and looking it up in the flow table. If there
|
|
is a matching flow, it executes the associated actions. If there is
|
|
no match, it queues the packet to userspace for processing (as part of
|
|
its processing, userspace will likely set up a flow to handle further
|
|
packets of the same type entirely in-kernel).
|
|
|
|
|
|
Flow key compatibility
|
|
----------------------
|
|
|
|
Network protocols evolve over time. New protocols become important
|
|
and existing protocols lose their prominence. For the Open vSwitch
|
|
kernel module to remain relevant, it must be possible for newer
|
|
versions to parse additional protocols as part of the flow key. It
|
|
might even be desirable, someday, to drop support for parsing
|
|
protocols that have become obsolete. Therefore, the Netlink interface
|
|
to Open vSwitch is designed to allow carefully written userspace
|
|
applications to work with any version of the flow key, past or future.
|
|
|
|
To support this forward and backward compatibility, whenever the
|
|
kernel module passes a packet to userspace, it also passes along the
|
|
flow key that it parsed from the packet. Userspace then extracts its
|
|
own notion of a flow key from the packet and compares it against the
|
|
kernel-provided version:
|
|
|
|
- If userspace's notion of the flow key for the packet matches the
|
|
kernel's, then nothing special is necessary.
|
|
|
|
- If the kernel's flow key includes more fields than the userspace
|
|
version of the flow key, for example if the kernel decoded IPv6
|
|
headers but userspace stopped at the Ethernet type (because it
|
|
does not understand IPv6), then again nothing special is
|
|
necessary. Userspace can still set up a flow in the usual way,
|
|
as long as it uses the kernel-provided flow key to do it.
|
|
|
|
- If the userspace flow key includes more fields than the
|
|
kernel's, for example if userspace decoded an IPv6 header but
|
|
the kernel stopped at the Ethernet type, then userspace can
|
|
forward the packet manually, without setting up a flow in the
|
|
kernel. This case is bad for performance because every packet
|
|
that the kernel considers part of the flow must go to userspace,
|
|
but the forwarding behavior is correct. (If userspace can
|
|
determine that the values of the extra fields would not affect
|
|
forwarding behavior, then it could set up a flow anyway.)
|
|
|
|
How flow keys evolve over time is important to making this work, so
|
|
the following sections go into detail.
|
|
|
|
|
|
Flow key format
|
|
---------------
|
|
|
|
A flow key is passed over a Netlink socket as a sequence of Netlink
|
|
attributes. Some attributes represent packet metadata, defined as any
|
|
information about a packet that cannot be extracted from the packet
|
|
itself, e.g. the vport on which the packet was received. Most
|
|
attributes, however, are extracted from headers within the packet,
|
|
e.g. source and destination addresses from Ethernet, IP, or TCP
|
|
headers.
|
|
|
|
The <linux/openvswitch.h> header file defines the exact format of the
|
|
flow key attributes. For informal explanatory purposes here, we write
|
|
them as comma-separated strings, with parentheses indicating arguments
|
|
and nesting. For example, the following could represent a flow key
|
|
corresponding to a TCP packet that arrived on vport 1:
|
|
|
|
in_port(1), eth(src=e0:91:f5:21:d0:b2, dst=00:02:e3:0f:80:a4),
|
|
eth_type(0x0800), ipv4(src=172.16.0.20, dst=172.18.0.52, proto=17, tos=0,
|
|
frag=no), tcp(src=49163, dst=80)
|
|
|
|
Often we ellipsize arguments not important to the discussion, e.g.:
|
|
|
|
in_port(1), eth(...), eth_type(0x0800), ipv4(...), tcp(...)
|
|
|
|
|
|
Basic rule for evolving flow keys
|
|
---------------------------------
|
|
|
|
Some care is needed to really maintain forward and backward
|
|
compatibility for applications that follow the rules listed under
|
|
"Flow key compatibility" above.
|
|
|
|
The basic rule is obvious:
|
|
|
|
------------------------------------------------------------------
|
|
New network protocol support must only supplement existing flow
|
|
key attributes. It must not change the meaning of already defined
|
|
flow key attributes.
|
|
------------------------------------------------------------------
|
|
|
|
This rule does have less-obvious consequences so it is worth working
|
|
through a few examples. Suppose, for example, that the kernel module
|
|
did not already implement VLAN parsing. Instead, it just interpreted
|
|
the 802.1Q TPID (0x8100) as the Ethertype then stopped parsing the
|
|
packet. The flow key for any packet with an 802.1Q header would look
|
|
essentially like this, ignoring metadata:
|
|
|
|
eth(...), eth_type(0x8100)
|
|
|
|
Naively, to add VLAN support, it makes sense to add a new "vlan" flow
|
|
key attribute to contain the VLAN tag, then continue to decode the
|
|
encapsulated headers beyond the VLAN tag using the existing field
|
|
definitions. With this change, a TCP packet in VLAN 10 would have a
|
|
flow key much like this:
|
|
|
|
eth(...), vlan(vid=10, pcp=0), eth_type(0x0800), ip(proto=6, ...), tcp(...)
|
|
|
|
But this change would negatively affect a userspace application that
|
|
has not been updated to understand the new "vlan" flow key attribute.
|
|
The application could, following the flow compatibility rules above,
|
|
ignore the "vlan" attribute that it does not understand and therefore
|
|
assume that the flow contained IP packets. This is a bad assumption
|
|
(the flow only contains IP packets if one parses and skips over the
|
|
802.1Q header) and it could cause the application's behavior to change
|
|
across kernel versions even though it follows the compatibility rules.
|
|
|
|
The solution is to use a set of nested attributes. This is, for
|
|
example, why 802.1Q support uses nested attributes. A TCP packet in
|
|
VLAN 10 is actually expressed as:
|
|
|
|
eth(...), eth_type(0x8100), vlan(vid=10, pcp=0), encap(eth_type(0x0800),
|
|
ip(proto=6, ...), tcp(...)))
|
|
|
|
Notice how the "eth_type", "ip", and "tcp" flow key attributes are
|
|
nested inside the "encap" attribute. Thus, an application that does
|
|
not understand the "vlan" key will not see either of those attributes
|
|
and therefore will not misinterpret them. (Also, the outer eth_type
|
|
is still 0x8100, not changed to 0x0800.)
|
|
|
|
Handling malformed packets
|
|
--------------------------
|
|
|
|
Don't drop packets in the kernel for malformed protocol headers, bad
|
|
checksums, etc. This would prevent userspace from implementing a
|
|
simple Ethernet switch that forwards every packet.
|
|
|
|
Instead, in such a case, include an attribute with "empty" content.
|
|
It doesn't matter if the empty content could be valid protocol values,
|
|
as long as those values are rarely seen in practice, because userspace
|
|
can always forward all packets with those values to userspace and
|
|
handle them individually.
|
|
|
|
For example, consider a packet that contains an IP header that
|
|
indicates protocol 6 for TCP, but which is truncated just after the IP
|
|
header, so that the TCP header is missing. The flow key for this
|
|
packet would include a tcp attribute with all-zero src and dst, like
|
|
this:
|
|
|
|
eth(...), eth_type(0x0800), ip(proto=6, ...), tcp(src=0, dst=0)
|
|
|
|
As another example, consider a packet with an Ethernet type of 0x8100,
|
|
indicating that a VLAN TCI should follow, but which is truncated just
|
|
after the Ethernet type. The flow key for this packet would include
|
|
an all-zero-bits vlan and an empty encap attribute, like this:
|
|
|
|
eth(...), eth_type(0x8100), vlan(0), encap()
|
|
|
|
Unlike a TCP packet with source and destination ports 0, an
|
|
all-zero-bits VLAN TCI is not that rare, so the CFI bit (aka
|
|
VLAN_TAG_PRESENT inside the kernel) is ordinarily set in a vlan
|
|
attribute expressly to allow this situation to be distinguished.
|
|
Thus, the flow key in this second example unambiguously indicates a
|
|
missing or malformed VLAN TCI.
|
|
|
|
Other rules
|
|
-----------
|
|
|
|
The other rules for flow keys are much less subtle:
|
|
|
|
- Duplicate attributes are not allowed at a given nesting level.
|
|
|
|
- Ordering of attributes is not significant.
|
|
|
|
- When the kernel sends a given flow key to userspace, it always
|
|
composes it the same way. This allows userspace to hash and
|
|
compare entire flow keys that it may not be able to fully
|
|
interpret.
|