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Documentation / networking / operstates.txt




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Based on kernel version 3.2. Page generated on 2012-01-05 23:29 EST.

1	
2	1. Introduction
3	
4	Linux distinguishes between administrative and operational state of an
5	interface. Administrative state is the result of "ip link set dev
6	<dev> up or down" and reflects whether the administrator wants to use
7	the device for traffic.
8	
9	However, an interface is not usable just because the admin enabled it
10	- ethernet requires to be plugged into the switch and, depending on
11	a site's networking policy and configuration, an 802.1X authentication
12	to be performed before user data can be transferred. Operational state
13	shows the ability of an interface to transmit this user data.
14	
15	Thanks to 802.1X, userspace must be granted the possibility to
16	influence operational state. To accommodate this, operational state is
17	split into two parts: Two flags that can be set by the driver only, and
18	a RFC2863 compatible state that is derived from these flags, a policy,
19	and changeable from userspace under certain rules.
20	
21	
22	2. Querying from userspace
23	
24	Both admin and operational state can be queried via the netlink
25	operation RTM_GETLINK. It is also possible to subscribe to RTMGRP_LINK
26	to be notified of updates. This is important for setting from userspace.
27	
28	These values contain interface state:
29	
30	ifinfomsg::if_flags & IFF_UP:
31	 Interface is admin up
32	ifinfomsg::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.
36	ifinfomsg::if_flags & IFF_LOWER_UP:
37	 Driver has signaled netif_carrier_on()
38	ifinfomsg::if_flags & IFF_DORMANT:
39	 Driver has signaled netif_dormant_on()
40	
41	TLV IFLA_OPERSTATE
42	
43	contains RFC2863 state of the interface in numeric representation:
44	
45	IF_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.
49	IF_OPER_NOTPRESENT (1):
50	 Unused in current kernel (notpresent interfaces normally disappear),
51	 just a numerical placeholder.
52	IF_OPER_DOWN (2):
53	 Interface is unable to transfer data on L1, f.e. ethernet is not
54	 plugged or interface is ADMIN down.
55	IF_OPER_LOWERLAYERDOWN (3):
56	 Interfaces stacked on an interface that is IF_OPER_DOWN show this
57	 state (f.e. VLAN).
58	IF_OPER_TESTING (4):
59	 Unused in current kernel.
60	IF_OPER_DORMANT (5):
61	 Interface is L1 up, but waiting for an external event, f.e. for a
62	 protocol to establish. (802.1X)
63	IF_OPER_UP (6):
64	 Interface is operational up and can be used.
65	
66	This TLV can also be queried via sysfs.
67	
68	TLV IFLA_LINKMODE
69	
70	contains link policy. This is needed for userspace interaction
71	described below.
72	
73	This TLV can also be queried via sysfs.
74	
75	
76	3. Kernel driver API
77	
78	Kernel drivers have access to two flags that map to IFF_LOWER_UP and
79	IFF_DORMANT. These flags can be set from everywhere, even from
80	interrupts. It is guaranteed that only the driver has write access,
81	however, if different layers of the driver manipulate the same flag,
82	the driver has to provide the synchronisation needed.
83	
84	__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP:
85	
86	The driver uses netif_carrier_on() to clear and netif_carrier_off() to
87	set this flag. On netif_carrier_off(), the scheduler stops sending
88	packets. The name 'carrier' and the inversion are historical, think of
89	it as lower layer.
90	
91	netif_carrier_ok() can be used to query that bit.
92	
93	__LINK_STATE_DORMANT, maps to IFF_DORMANT:
94	
95	Set by the driver to express that the device cannot yet be used
96	because some driver controlled protocol establishment has to
97	complete. Corresponding functions are netif_dormant_on() to set the
98	flag, netif_dormant_off() to clear it and netif_dormant() to query.
99	
100	On device allocation, networking core sets the flags equivalent to
101	netif_carrier_ok() and !netif_dormant().
102	
103	
104	Whenever the driver CHANGES one of these flags, a workqueue event is
105	scheduled to translate the flag combination to IFLA_OPERSTATE as
106	follows:
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	
113	netif_carrier_ok() && netif_dormant():
114	 IF_OPER_DORMANT
115	
116	netif_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	
122	4. Setting from userspace
123	
124	Applications have to use the netlink interface to influence the
125	RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1
126	via RTM_SETLINK instructs the kernel that an interface should go to
127	IF_OPER_DORMANT instead of IF_OPER_UP when the combination
128	netif_carrier_ok() && !netif_dormant() is set by the
129	driver. Afterwards, the userspace application can set IFLA_OPERSTATE
130	to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set
131	netif_carrier_off() or netif_dormant_on(). Changes made by userspace
132	are multicasted on the netlink group RTMGRP_LINK.
133	
134	So 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	
149	if supplicant goes down, bring back IFLA_LINKMODE to 0 and
150	IFLA_OPERSTATE to a sane value.
151	
152	A routing daemon or dhcp client just needs to care for IFF_RUNNING or
153	waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before
154	considering the interface / querying a DHCP address.
155	
156	
157	For technical questions and/or comments please e-mail to Stefan Rompf
158	(stefan at loplof.de).
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