1.. SPDX-License-Identifier: GPL-2.0
2
3==================
4Operational States
5==================
6
7
81. Introduction
9===============
10
11Linux distinguishes between administrative and operational state of an
12interface. Administrative state is the result of "ip link set dev
13<dev> up or down" and reflects whether the administrator wants to use
14the device for traffic.
15
16However, an interface is not usable just because the admin enabled it
17- ethernet requires to be plugged into the switch and, depending on
18a site's networking policy and configuration, an 802.1X authentication
19to be performed before user data can be transferred. Operational state
20shows the ability of an interface to transmit this user data.
21
22Thanks to 802.1X, userspace must be granted the possibility to
23influence operational state. To accommodate this, operational state is
24split into two parts: Two flags that can be set by the driver only, and
25a RFC2863 compatible state that is derived from these flags, a policy,
26and changeable from userspace under certain rules.
27
28
292. Querying from userspace
30==========================
31
32Both admin and operational state can be queried via the netlink
33operation RTM_GETLINK. It is also possible to subscribe to RTNLGRP_LINK
34to be notified of updates while the interface is admin up. This is
35important for setting from userspace.
36
37These values contain interface state:
38
39ifinfomsg::if_flags & IFF_UP:
40 Interface is admin up
41
42ifinfomsg::if_flags & IFF_RUNNING:
43 Interface is in RFC2863 operational state UP or UNKNOWN. This is for
44 backward compatibility, routing daemons, dhcp clients can use this
45 flag to determine whether they should use the interface.
46
47ifinfomsg::if_flags & IFF_LOWER_UP:
48 Driver has signaled netif_carrier_on()
49
50ifinfomsg::if_flags & IFF_DORMANT:
51 Driver has signaled netif_dormant_on()
52
53TLV IFLA_OPERSTATE
54------------------
55
56contains RFC2863 state of the interface in numeric representation:
57
58IF_OPER_UNKNOWN (0):
59 Interface is in unknown state, neither driver nor userspace has set
60 operational state. Interface must be considered for user data as
61 setting operational state has not been implemented in every driver.
62
63IF_OPER_NOTPRESENT (1):
64 Unused in current kernel (notpresent interfaces normally disappear),
65 just a numerical placeholder.
66
67IF_OPER_DOWN (2):
68 Interface is unable to transfer data on L1, f.e. ethernet is not
69 plugged or interface is ADMIN down.
70
71IF_OPER_LOWERLAYERDOWN (3):
72 Interfaces stacked on an interface that is IF_OPER_DOWN show this
73 state (f.e. VLAN).
74
75IF_OPER_TESTING (4):
76 Interface is in testing mode, for example executing driver self-tests
77 or media (cable) test. It can't be used for normal traffic until tests
78 complete.
79
80IF_OPER_DORMANT (5):
81 Interface is L1 up, but waiting for an external event, f.e. for a
82 protocol to establish. (802.1X)
83
84IF_OPER_UP (6):
85 Interface is operational up and can be used.
86
87This TLV can also be queried via sysfs.
88
89TLV IFLA_LINKMODE
90-----------------
91
92contains link policy. This is needed for userspace interaction
93described below.
94
95This TLV can also be queried via sysfs.
96
97
983. Kernel driver API
99====================
100
101Kernel drivers have access to two flags that map to IFF_LOWER_UP and
102IFF_DORMANT. These flags can be set from everywhere, even from
103interrupts. It is guaranteed that only the driver has write access,
104however, if different layers of the driver manipulate the same flag,
105the driver has to provide the synchronisation needed.
106
107__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP:
108
109The driver uses netif_carrier_on() to clear and netif_carrier_off() to
110set this flag. On netif_carrier_off(), the scheduler stops sending
111packets. The name 'carrier' and the inversion are historical, think of
112it as lower layer.
113
114Note that for certain kind of soft-devices, which are not managing any
115real hardware, it is possible to set this bit from userspace.  One
116should use TLV IFLA_CARRIER to do so.
117
118netif_carrier_ok() can be used to query that bit.
119
120__LINK_STATE_DORMANT, maps to IFF_DORMANT:
121
122Set by the driver to express that the device cannot yet be used
123because some driver controlled protocol establishment has to
124complete. Corresponding functions are netif_dormant_on() to set the
125flag, netif_dormant_off() to clear it and netif_dormant() to query.
126
127On device allocation, both flags __LINK_STATE_NOCARRIER and
128__LINK_STATE_DORMANT are cleared, so the effective state is equivalent
129to netif_carrier_ok() and !netif_dormant().
130
131
132Whenever the driver CHANGES one of these flags, a workqueue event is
133scheduled to translate the flag combination to IFLA_OPERSTATE as
134follows:
135
136!netif_carrier_ok():
137 IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN
138 otherwise. Kernel can recognise stacked interfaces because their
139 ifindex != iflink.
140
141netif_carrier_ok() && netif_dormant():
142 IF_OPER_DORMANT
143
144netif_carrier_ok() && !netif_dormant():
145 IF_OPER_UP if userspace interaction is disabled. Otherwise
146 IF_OPER_DORMANT with the possibility for userspace to initiate the
147 IF_OPER_UP transition afterwards.
148
149
1504. Setting from userspace
151=========================
152
153Applications have to use the netlink interface to influence the
154RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1
155via RTM_SETLINK instructs the kernel that an interface should go to
156IF_OPER_DORMANT instead of IF_OPER_UP when the combination
157netif_carrier_ok() && !netif_dormant() is set by the
158driver. Afterwards, the userspace application can set IFLA_OPERSTATE
159to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set
160netif_carrier_off() or netif_dormant_on(). Changes made by userspace
161are multicasted on the netlink group RTNLGRP_LINK.
162
163So basically a 802.1X supplicant interacts with the kernel like this:
164
165- subscribe to RTNLGRP_LINK
166- set IFLA_LINKMODE to 1 via RTM_SETLINK
167- query RTM_GETLINK once to get initial state
168- if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until
169  netlink multicast signals this state
170- do 802.1X, eventually abort if flags go down again
171- send RTM_SETLINK to set operstate to IF_OPER_UP if authentication
172  succeeds, IF_OPER_DORMANT otherwise
173- see how operstate and IFF_RUNNING is echoed via netlink multicast
174- set interface back to IF_OPER_DORMANT if 802.1X reauthentication
175  fails
176- restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag
177
178if supplicant goes down, bring back IFLA_LINKMODE to 0 and
179IFLA_OPERSTATE to a sane value.
180
181A routing daemon or dhcp client just needs to care for IFF_RUNNING or
182waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before
183considering the interface / querying a DHCP address.
184
185
186For technical questions and/or comments please e-mail to Stefan Rompf
187(stefan at loplof.de).
188