1
  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 RTNLGRP_LINK
 26to be notified of updates while the interface is admin up. This is
 27important for setting from userspace.
 28
 29These values contain interface state:
 30
 31ifinfomsg::if_flags & IFF_UP:
 32 Interface is admin up
 33ifinfomsg::if_flags & IFF_RUNNING:
 34 Interface is in RFC2863 operational state UP or UNKNOWN. This is for
 35 backward compatibility, routing daemons, dhcp clients can use this
 36 flag to determine whether they should use the interface.
 37ifinfomsg::if_flags & IFF_LOWER_UP:
 38 Driver has signaled netif_carrier_on()
 39ifinfomsg::if_flags & IFF_DORMANT:
 40 Driver has signaled netif_dormant_on()
 41
 42TLV IFLA_OPERSTATE
 43
 44contains RFC2863 state of the interface in numeric representation:
 45
 46IF_OPER_UNKNOWN (0):
 47 Interface is in unknown state, neither driver nor userspace has set
 48 operational state. Interface must be considered for user data as
 49 setting operational state has not been implemented in every driver.
 50IF_OPER_NOTPRESENT (1):
 51 Unused in current kernel (notpresent interfaces normally disappear),
 52 just a numerical placeholder.
 53IF_OPER_DOWN (2):
 54 Interface is unable to transfer data on L1, f.e. ethernet is not
 55 plugged or interface is ADMIN down.
 56IF_OPER_LOWERLAYERDOWN (3):
 57 Interfaces stacked on an interface that is IF_OPER_DOWN show this
 58 state (f.e. VLAN).
 59IF_OPER_TESTING (4):
 60 Unused in current kernel.
 61IF_OPER_DORMANT (5):
 62 Interface is L1 up, but waiting for an external event, f.e. for a
 63 protocol to establish. (802.1X)
 64IF_OPER_UP (6):
 65 Interface is operational up and can be used.
 66
 67This TLV can also be queried via sysfs.
 68
 69TLV IFLA_LINKMODE
 70
 71contains link policy. This is needed for userspace interaction
 72described below.
 73
 74This TLV can also be queried via sysfs.
 75
 76
 773. Kernel driver API
 78
 79Kernel drivers have access to two flags that map to IFF_LOWER_UP and
 80IFF_DORMANT. These flags can be set from everywhere, even from
 81interrupts. It is guaranteed that only the driver has write access,
 82however, if different layers of the driver manipulate the same flag,
 83the driver has to provide the synchronisation needed.
 84
 85__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP:
 86
 87The driver uses netif_carrier_on() to clear and netif_carrier_off() to
 88set this flag. On netif_carrier_off(), the scheduler stops sending
 89packets. The name 'carrier' and the inversion are historical, think of
 90it as lower layer.
 91
 92Note that for certain kind of soft-devices, which are not managing any
 93real hardware, it is possible to set this bit from userspace.  One
 94should use TVL IFLA_CARRIER to do so.
 95
 96netif_carrier_ok() can be used to query that bit.
 97
 98__LINK_STATE_DORMANT, maps to IFF_DORMANT:
 99
100Set by the driver to express that the device cannot yet be used
101because some driver controlled protocol establishment has to
102complete. Corresponding functions are netif_dormant_on() to set the
103flag, netif_dormant_off() to clear it and netif_dormant() to query.
104
105On device allocation, both flags __LINK_STATE_NOCARRIER and
106__LINK_STATE_DORMANT are cleared, so the effective state is equivalent
107to netif_carrier_ok() and !netif_dormant().
108
109
110Whenever the driver CHANGES one of these flags, a workqueue event is
111scheduled to translate the flag combination to IFLA_OPERSTATE as
112follows:
113
114!netif_carrier_ok():
115 IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN
116 otherwise. Kernel can recognise stacked interfaces because their
117 ifindex != iflink.
118
119netif_carrier_ok() && netif_dormant():
120 IF_OPER_DORMANT
121
122netif_carrier_ok() && !netif_dormant():
123 IF_OPER_UP if userspace interaction is disabled. Otherwise
124 IF_OPER_DORMANT with the possibility for userspace to initiate the
125 IF_OPER_UP transition afterwards.
126
127
1284. Setting from userspace
129
130Applications have to use the netlink interface to influence the
131RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1
132via RTM_SETLINK instructs the kernel that an interface should go to
133IF_OPER_DORMANT instead of IF_OPER_UP when the combination
134netif_carrier_ok() && !netif_dormant() is set by the
135driver. Afterwards, the userspace application can set IFLA_OPERSTATE
136to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set
137netif_carrier_off() or netif_dormant_on(). Changes made by userspace
138are multicasted on the netlink group RTNLGRP_LINK.
139
140So basically a 802.1X supplicant interacts with the kernel like this:
141
142-subscribe to RTNLGRP_LINK
143-set IFLA_LINKMODE to 1 via RTM_SETLINK
144-query RTM_GETLINK once to get initial state
145-if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until
146 netlink multicast signals this state
147-do 802.1X, eventually abort if flags go down again
148-send RTM_SETLINK to set operstate to IF_OPER_UP if authentication
149 succeeds, IF_OPER_DORMANT otherwise
150-see how operstate and IFF_RUNNING is echoed via netlink multicast
151-set interface back to IF_OPER_DORMANT if 802.1X reauthentication
152 fails
153-restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag
154
155if supplicant goes down, bring back IFLA_LINKMODE to 0 and
156IFLA_OPERSTATE to a sane value.
157
158A routing daemon or dhcp client just needs to care for IFF_RUNNING or
159waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before
160considering the interface / querying a DHCP address.
161
162
163For technical questions and/or comments please e-mail to Stefan Rompf
164(stefan at loplof.de).