linux/Documentation/networking/operstates.txt
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   21. Introduction
   3
   4Linux distinguishes between administrative and operational state of an
   5interface. Administrative state is the result of "ip link set dev
   6<dev> up or down" and reflects whether the administrator wants to use
   7the device for traffic.
   8
   9However, an interface is not usable just because the admin enabled it
  10- ethernet requires to be plugged into the switch and, depending on
  11a site's networking policy and configuration, an 802.1X authentication
  12to be performed before user data can be transferred. Operational state
  13shows the ability of an interface to transmit this user data.
  14
  15Thanks to 802.1X, userspace must be granted the possibility to
  16influence operational state. To accommodate this, operational state is
  17split into two parts: Two flags that can be set by the driver only, and
  18a RFC2863 compatible state that is derived from these flags, a policy,
  19and changeable from userspace under certain rules.
  20
  21
  222. Querying from userspace
  23
  24Both admin and operational state can be queried via the netlink
  25operation RTM_GETLINK. It is also possible to subscribe to RTMGRP_LINK
  26to be notified of updates. This is important for setting from userspace.
  27
  28These values contain interface state:
  29
  30ifinfomsg::if_flags & IFF_UP:
  31 Interface is admin up
  32ifinfomsg::if_flags & IFF_RUNNING:
  33 Interface is in RFC2863 operational state UP or UNKNOWN. This is for
  34 backward compatibility, routing daemons, dhcp clients can use this
  35 flag to determine whether they should use the interface.
  36ifinfomsg::if_flags & IFF_LOWER_UP:
  37 Driver has signaled netif_carrier_on()
  38ifinfomsg::if_flags & IFF_DORMANT:
  39 Driver has signaled netif_dormant_on()
  40
  41TLV IFLA_OPERSTATE
  42
  43contains RFC2863 state of the interface in numeric representation:
  44
  45IF_OPER_UNKNOWN (0):
  46 Interface is in unknown state, neither driver nor userspace has set
  47 operational state. Interface must be considered for user data as
  48 setting operational state has not been implemented in every driver.
  49IF_OPER_NOTPRESENT (1):
  50 Unused in current kernel (notpresent interfaces normally disappear),
  51 just a numerical placeholder.
  52IF_OPER_DOWN (2):
  53 Interface is unable to transfer data on L1, f.e. ethernet is not
  54 plugged or interface is ADMIN down.
  55IF_OPER_LOWERLAYERDOWN (3):
  56 Interfaces stacked on an interface that is IF_OPER_DOWN show this
  57 state (f.e. VLAN).
  58IF_OPER_TESTING (4):
  59 Unused in current kernel.
  60IF_OPER_DORMANT (5):
  61 Interface is L1 up, but waiting for an external event, f.e. for a
  62 protocol to establish. (802.1X)
  63IF_OPER_UP (6):
  64 Interface is operational up and can be used.
  65
  66This TLV can also be queried via sysfs.
  67
  68TLV IFLA_LINKMODE
  69
  70contains link policy. This is needed for userspace interaction
  71described below.
  72
  73This TLV can also be queried via sysfs.
  74
  75
  763. Kernel driver API
  77
  78Kernel drivers have access to two flags that map to IFF_LOWER_UP and
  79IFF_DORMANT. These flags can be set from everywhere, even from
  80interrupts. It is guaranteed that only the driver has write access,
  81however, if different layers of the driver manipulate the same flag,
  82the driver has to provide the synchronisation needed.
  83
  84__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP:
  85
  86The driver uses netif_carrier_on() to clear and netif_carrier_off() to
  87set this flag. On netif_carrier_off(), the scheduler stops sending
  88packets. The name 'carrier' and the inversion are historical, think of
  89it as lower layer.
  90
  91netif_carrier_ok() can be used to query that bit.
  92
  93__LINK_STATE_DORMANT, maps to IFF_DORMANT:
  94
  95Set by the driver to express that the device cannot yet be used
  96because some driver controlled protocol establishment has to
  97complete. Corresponding functions are netif_dormant_on() to set the
  98flag, netif_dormant_off() to clear it and netif_dormant() to query.
  99
 100On device allocation, networking core sets the flags equivalent to
 101netif_carrier_ok() and !netif_dormant().
 102
 103
 104Whenever the driver CHANGES one of these flags, a workqueue event is
 105scheduled to translate the flag combination to IFLA_OPERSTATE as
 106follows:
 107
 108!netif_carrier_ok():
 109 IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN
 110 otherwise. Kernel can recognise stacked interfaces because their
 111 ifindex != iflink.
 112
 113netif_carrier_ok() && netif_dormant():
 114 IF_OPER_DORMANT
 115
 116netif_carrier_ok() && !netif_dormant():
 117 IF_OPER_UP if userspace interaction is disabled. Otherwise
 118 IF_OPER_DORMANT with the possibility for userspace to initiate the
 119 IF_OPER_UP transition afterwards.
 120
 121
 1224. Setting from userspace
 123
 124Applications have to use the netlink interface to influence the
 125RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1
 126via RTM_SETLINK instructs the kernel that an interface should go to
 127IF_OPER_DORMANT instead of IF_OPER_UP when the combination
 128netif_carrier_ok() && !netif_dormant() is set by the
 129driver. Afterwards, the userspace application can set IFLA_OPERSTATE
 130to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set
 131netif_carrier_off() or netif_dormant_on(). Changes made by userspace
 132are multicasted on the netlink group RTMGRP_LINK.
 133
 134So basically a 802.1X supplicant interacts with the kernel like this:
 135
 136-subscribe to RTMGRP_LINK
 137-set IFLA_LINKMODE to 1 via RTM_SETLINK
 138-query RTM_GETLINK once to get initial state
 139-if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until
 140 netlink multicast signals this state
 141-do 802.1X, eventually abort if flags go down again
 142-send RTM_SETLINK to set operstate to IF_OPER_UP if authentication
 143 succeeds, IF_OPER_DORMANT otherwise
 144-see how operstate and IFF_RUNNING is echoed via netlink multicast
 145-set interface back to IF_OPER_DORMANT if 802.1X reauthentication
 146 fails
 147-restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag
 148
 149if supplicant goes down, bring back IFLA_LINKMODE to 0 and
 150IFLA_OPERSTATE to a sane value.
 151
 152A routing daemon or dhcp client just needs to care for IFF_RUNNING or
 153waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before
 154considering the interface / querying a DHCP address.
 155
 156
 157For technical questions and/or comments please e-mail to Stefan Rompf
 158(stefan at loplof.de).
 159
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