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Stefan Rompf3a01c1e2006-05-09 15:15:35 -07001
21. Introduction
3
4Linux distinguishes between administrative and operational state of an
Matt LaPlante3f6dee92006-10-03 22:45:33 +02005interface. Administrative state is the result of "ip link set dev
Stefan Rompf3a01c1e2006-05-09 15:15:35 -07006<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
Stefan Rompf3a01c1e2006-05-09 15:15:35 -070041TLV 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).