WHY-OVS

                          Why Open vSwitch?
                          =================

Hypervisors need the ability to bridge traffic between VMs and with the
outside world.  On Linux-based hypervisors, this used to mean using the
built-in L2 switch (the Linux bridge), which is fast and reliable.  So,
it is reasonable to ask why Open vSwitch is used.

The answer is that Open vSwitch is targeted at multi-server
virtualization deployments, a landscape for which the previous stack is
not well suited.  These environments are often characterized by highly
dynamic end-points, the maintenance of logical abstractions, and
(sometimes) integration with or offloading to special purpose switching
hardware.

The following characteristics and design considerations help Open
vSwitch cope with the above requirements.

* The mobility of state: All network state associated with a network
  entity (say a virtual machine) should be easily identifiable and
  migratable between different hosts.  This may include traditional
  "soft state" (such as an entry in an L2 learning table), L3 forwarding
  state, policy routing state, ACLs, QoS policy, monitoring
  configuration (e.g. NetFlow, sFlow), etc.

  Open vSwitch has support for both configuring and migrating both slow
  (configuration) and fast network state between instances.  For
  example, if a VM migrates between end-hosts, it is possible to not
  only migrate associated configuration (SPAN rules, ACLs, QoS) but any
  live network state (including, for example, existing state which
  may be difficult to reconstruct).  Further, Open vSwitch state is
  typed and backed by a real data-model allowing for the development of
  structured automation systems.

* Responding to network dynamics: Virtual environments are often
  characterized by high-rates of change.  VMs coming and going, VMs
  moving backwards and forwards in time, changes to the logical network
  environments, and so forth.

  Open vSwitch supports a number of features that allow a network
  control system to respond and adapt as the environment changes.  This
  includes simple accounting and visibility support such as NetFlow and
  sFlow.  But perhaps more useful, Open vSwitch supports a network state
  database (OVSDB) that supports remote triggers.  Therefore, a piece of
  orchestration software can "watch" various aspects of the network and
  respond if/when they change.  This is used heavily today, for example,
  to respond to and track VM migrations.

  Open vSwitch also supports OpenFlow as a method of exporting remote
  access to control traffic.  There are a number of uses for this
  including global network discovery through inspection of discovery
  or link-state traffic (e.g. LLDP, CDP, OSPF, etc.).

* Maintenance of logical tags: Distributed virtual switches (such as
  VMware vDS and Cisco's Nexus 1000V) often maintain logical context
  within the network through appending or manipulating tags in network
  packets.  This can be used to uniquely identify a VM (in a manner
  resistant to hardware spoofing), or to hold some other context that
  is only relevant in the logical domain.  Much of the problem of
  building a distributed virtual switch is to efficiently and correctly
  manage these tags.

  Open vSwitch includes multiple methods for specifying and maintaining
  tagging rules, all of which are accessible to a remote process for
  orchestration.  Further, in many cases these tagging rules are stored
  in an optimized form so they don't have to be coupled with a
  heavyweight network device.  This allows, for example, thousands of
  tagging or address remapping rules to be configured, changed, and
  migrated.

  In a similar vein, Open vSwitch supports a GRE implementation that can
  handle thousands of simultaneous GRE tunnels and supports remote
  configuration for tunnel creation, configuration, and tear-down.
  This, for example, can be used to connect private VM networks in
  different data centers.

* Hardware integration: Open vSwitch's forwarding path (the in-kernel
  datapath) is designed to be amenable to "offloading" packet processing
  to hardware chipsets, whether housed in a classic hardware switch
  chassis or in an end-host NIC.  This allows for the Open vSwitch
  control path to be able to both control a pure software
  implementation or a hardware switch.

  There are many ongoing efforts to port Open vSwitch to hardware
  chipsets.  These include multiple merchant silicon chipsets (Broadcom
  and Marvell), as well as a number of vendor-specific platforms.  (The
  PORTING file discusses how one would go about making such a port.)

  The advantage of hardware integration is not only performance within
  virtualized environments.  If physical switches also expose the Open
  vSwitch control abstractions, both bare-metal and virtualized hosting
  environments can be managed using the same mechanism for automated
  network control.

In many ways, Open vSwitch targets a different point in the design space
than previous hypervisor networking stacks, focusing on the need for
automated and dynamic network control in large-scale Linux-based
virtualization environments.

The goal with Open vSwitch is to keep the in-kernel code as small as
possible (as is necessary for performance) and to re-use existing
subsystems when applicable (for example Open vSwitch uses the existing
QoS stack).  As of Linux 3.3, Open vSwitch is included as a part of the
kernel and packaging for the userspace utilities are available on most
popular distributions.