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-
-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.
-
-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).