1. Introduction
  
  Linux distinguishes between administrative and operational state of an
  interface. Administrative state is the result of "ip link set dev
  <dev> up or down" and reflects whether the administrator wants to use
  the device for traffic.
  
  However, an interface is not usable just because the admin enabled it
  - ethernet requires to be plugged into the switch and, depending on
  a site's networking policy and configuration, an 802.1X authentication
  to be performed before user data can be transferred. Operational state
  shows the ability of an interface to transmit this user data.
  
  Thanks to 802.1X, userspace must be granted the possibility to
  influence operational state. To accommodate this, operational state is
  split into two parts: Two flags that can be set by the driver only, and
  a RFC2863 compatible state that is derived from these flags, a policy,
  and changeable from userspace under certain rules.
  
  
  2. Querying from userspace
  
  Both admin and operational state can be queried via the netlink
  operation RTM_GETLINK. It is also possible to subscribe to RTMGRP_LINK
  to be notified of updates. This is important for setting from userspace.
  
  These values contain interface state:
  
  ifinfomsg::if_flags & IFF_UP:
   Interface is admin up
  ifinfomsg::if_flags & IFF_RUNNING:
   Interface is in RFC2863 operational state UP or UNKNOWN. This is for
   backward compatibility, routing daemons, dhcp clients can use this
   flag to determine whether they should use the interface.
  ifinfomsg::if_flags & IFF_LOWER_UP:
   Driver has signaled netif_carrier_on()
  ifinfomsg::if_flags & IFF_DORMANT:
   Driver has signaled netif_dormant_on()
  
  TLV IFLA_OPERSTATE
  
  contains RFC2863 state of the interface in numeric representation:
  
  IF_OPER_UNKNOWN (0):
   Interface is in unknown state, neither driver nor userspace has set
   operational state. Interface must be considered for user data as
   setting operational state has not been implemented in every driver.
  IF_OPER_NOTPRESENT (1):
   Unused in current kernel (notpresent interfaces normally disappear),
   just a numerical placeholder.
  IF_OPER_DOWN (2):
   Interface is unable to transfer data on L1, f.e. ethernet is not
   plugged or interface is ADMIN down.
  IF_OPER_LOWERLAYERDOWN (3):
   Interfaces stacked on an interface that is IF_OPER_DOWN show this
   state (f.e. VLAN).
  IF_OPER_TESTING (4):
   Unused in current kernel.
  IF_OPER_DORMANT (5):
   Interface is L1 up, but waiting for an external event, f.e. for a
   protocol to establish. (802.1X)
  IF_OPER_UP (6):
   Interface is operational up and can be used.
  
  This TLV can also be queried via sysfs.
  
  TLV IFLA_LINKMODE
  
  contains link policy. This is needed for userspace interaction
  described below.
  
  This TLV can also be queried via sysfs.
  
  
  3. Kernel driver API
  
  Kernel drivers have access to two flags that map to IFF_LOWER_UP and
  IFF_DORMANT. These flags can be set from everywhere, even from
  interrupts. It is guaranteed that only the driver has write access,
  however, if different layers of the driver manipulate the same flag,
  the driver has to provide the synchronisation needed.
  
  __LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP:
  
  The driver uses netif_carrier_on() to clear and netif_carrier_off() to
  set this flag. On netif_carrier_off(), the scheduler stops sending
  packets. The name 'carrier' and the inversion are historical, think of
  it as lower layer.
  
  Note that for certain kind of soft-devices, which are not managing any
  real hardware, it is possible to set this bit from userspace.  One
  should use TVL IFLA_CARRIER to do so.
  
  netif_carrier_ok() can be used to query that bit.
  
  __LINK_STATE_DORMANT, maps to IFF_DORMANT:
  
  Set by the driver to express that the device cannot yet be used
  because some driver controlled protocol establishment has to
  complete. Corresponding functions are netif_dormant_on() to set the
  flag, netif_dormant_off() to clear it and netif_dormant() to query.
  
  On device allocation, networking core sets the flags equivalent to
  netif_carrier_ok() and !netif_dormant().
  
  
  Whenever the driver CHANGES one of these flags, a workqueue event is
  scheduled to translate the flag combination to IFLA_OPERSTATE as
  follows:
  
  !netif_carrier_ok():
   IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN
   otherwise. Kernel can recognise stacked interfaces because their
   ifindex != iflink.
  
  netif_carrier_ok() && netif_dormant():
   IF_OPER_DORMANT
  
  netif_carrier_ok() && !netif_dormant():
   IF_OPER_UP if userspace interaction is disabled. Otherwise
   IF_OPER_DORMANT with the possibility for userspace to initiate the
   IF_OPER_UP transition afterwards.
  
  
  4. Setting from userspace
  
  Applications have to use the netlink interface to influence the
  RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1
  via RTM_SETLINK instructs the kernel that an interface should go to
  IF_OPER_DORMANT instead of IF_OPER_UP when the combination
  netif_carrier_ok() && !netif_dormant() is set by the
  driver. Afterwards, the userspace application can set IFLA_OPERSTATE
  to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set
  netif_carrier_off() or netif_dormant_on(). Changes made by userspace
  are multicasted on the netlink group RTMGRP_LINK.
  
  So basically a 802.1X supplicant interacts with the kernel like this:
  
  -subscribe to RTMGRP_LINK
  -set IFLA_LINKMODE to 1 via RTM_SETLINK
  -query RTM_GETLINK once to get initial state
  -if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until
   netlink multicast signals this state
  -do 802.1X, eventually abort if flags go down again
  -send RTM_SETLINK to set operstate to IF_OPER_UP if authentication
   succeeds, IF_OPER_DORMANT otherwise
  -see how operstate and IFF_RUNNING is echoed via netlink multicast
  -set interface back to IF_OPER_DORMANT if 802.1X reauthentication
   fails
  -restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag
  
  if supplicant goes down, bring back IFLA_LINKMODE to 0 and
  IFLA_OPERSTATE to a sane value.
  
  A routing daemon or dhcp client just needs to care for IFF_RUNNING or
  waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before
  considering the interface / querying a DHCP address.
  
  
  For technical questions and/or comments please e-mail to Stefan Rompf
  (stefan at loplof.de).