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1
21. Introduction
3
4Linux distinguishes between administrative and operational state of an
5interface. Admininstrative 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
41These interface flags can also be queried without netlink using the
42SIOCGIFFLAGS ioctl.
43
44TLV IFLA_OPERSTATE
45
46contains RFC2863 state of the interface in numeric representation:
47
48IF_OPER_UNKNOWN (0):
49 Interface is in unknown state, neither driver nor userspace has set
50 operational state. Interface must be considered for user data as
51 setting operational state has not been implemented in every driver.
52IF_OPER_NOTPRESENT (1):
53 Unused in current kernel (notpresent interfaces normally disappear),
54 just a numerical placeholder.
55IF_OPER_DOWN (2):
56 Interface is unable to transfer data on L1, f.e. ethernet is not
57 plugged or interface is ADMIN down.
58IF_OPER_LOWERLAYERDOWN (3):
59 Interfaces stacked on an interface that is IF_OPER_DOWN show this
60 state (f.e. VLAN).
61IF_OPER_TESTING (4):
62 Unused in current kernel.
63IF_OPER_DORMANT (5):
64 Interface is L1 up, but waiting for an external event, f.e. for a
65 protocol to establish. (802.1X)
66IF_OPER_UP (6):
67 Interface is operational up and can be used.
68
69This TLV can also be queried via sysfs.
70
71TLV IFLA_LINKMODE
72
73contains link policy. This is needed for userspace interaction
74described below.
75
76This TLV can also be queried via sysfs.
77
78
793. Kernel driver API
80
81Kernel drivers have access to two flags that map to IFF_LOWER_UP and
82IFF_DORMANT. These flags can be set from everywhere, even from
83interrupts. It is guaranteed that only the driver has write access,
84however, if different layers of the driver manipulate the same flag,
85the driver has to provide the synchronisation needed.
86
87__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP:
88
89The driver uses netif_carrier_on() to clear and netif_carrier_off() to
90set this flag. On netif_carrier_off(), the scheduler stops sending
91packets. The name 'carrier' and the inversion are historical, think of
92it as lower layer.
93
94netif_carrier_ok() can be used to query that bit.
95
96__LINK_STATE_DORMANT, maps to IFF_DORMANT:
97
98Set by the driver to express that the device cannot yet be used
99because some driver controlled protocol establishment has to
100complete. Corresponding functions are netif_dormant_on() to set the
101flag, netif_dormant_off() to clear it and netif_dormant() to query.
102
103On device allocation, networking core sets the flags equivalent to
104netif_carrier_ok() and !netif_dormant().
105
106
107Whenever the driver CHANGES one of these flags, a workqueue event is
108scheduled to translate the flag combination to IFLA_OPERSTATE as
109follows:
110
111!netif_carrier_ok():
112 IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN
113 otherwise. Kernel can recognise stacked interfaces because their
114 ifindex != iflink.
115
116netif_carrier_ok() && netif_dormant():
117 IF_OPER_DORMANT
118
119netif_carrier_ok() && !netif_dormant():
120 IF_OPER_UP if userspace interaction is disabled. Otherwise
121 IF_OPER_DORMANT with the possibility for userspace to initiate the
122 IF_OPER_UP transition afterwards.
123
124
1254. Setting from userspace
126
127Applications have to use the netlink interface to influence the
128RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1
129via RTM_SETLINK instructs the kernel that an interface should go to
130IF_OPER_DORMANT instead of IF_OPER_UP when the combination
131netif_carrier_ok() && !netif_dormant() is set by the
132driver. Afterwards, the userspace application can set IFLA_OPERSTATE
133to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set
134netif_carrier_off() or netif_dormant_on(). Changes made by userspace
135are multicasted on the netlink group RTMGRP_LINK.
136
137So basically a 802.1X supplicant interacts with the kernel like this:
138
139-subscribe to RTMGRP_LINK
140-set IFLA_LINKMODE to 1 via RTM_SETLINK
141-query RTM_GETLINK once to get initial state
142-if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until
143 netlink multicast signals this state
144-do 802.1X, eventually abort if flags go down again
145-send RTM_SETLINK to set operstate to IF_OPER_UP if authentication
146 succeeds, IF_OPER_DORMANT otherwise
147-see how operstate and IFF_RUNNING is echoed via netlink multicast
148-set interface back to IF_OPER_DORMANT if 802.1X reauthentication
149 fails
150-restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag
151
152if supplicant goes down, bring back IFLA_LINKMODE to 0 and
153IFLA_OPERSTATE to a sane value.
154
155A routing daemon or dhcp client just needs to care for IFF_RUNNING or
156waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before
157considering the interface / querying a DHCP address.
158
159
160For technical questions and/or comments please e-mail to Stefan Rompf
161(stefan at loplof.de).