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Documentation / networking / operstates.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 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	Note that for certain kind of soft-devices, which are not managing any
92	real hardware, it is possible to set this bit from userspace.  One
93	should use TVL IFLA_CARRIER to do so.
94	
95	netif_carrier_ok() can be used to query that bit.
96	
97	__LINK_STATE_DORMANT, maps to IFF_DORMANT:
98	
99	Set by the driver to express that the device cannot yet be used
100	because some driver controlled protocol establishment has to
101	complete. Corresponding functions are netif_dormant_on() to set the
102	flag, netif_dormant_off() to clear it and netif_dormant() to query.
103	
104	On device allocation, networking core sets the flags equivalent to
105	netif_carrier_ok() and !netif_dormant().
106	
107	
108	Whenever the driver CHANGES one of these flags, a workqueue event is
109	scheduled to translate the flag combination to IFLA_OPERSTATE as
110	follows:
111	
112	!netif_carrier_ok():
113	 IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN
114	 otherwise. Kernel can recognise stacked interfaces because their
115	 ifindex != iflink.
116	
117	netif_carrier_ok() && netif_dormant():
118	 IF_OPER_DORMANT
119	
120	netif_carrier_ok() && !netif_dormant():
121	 IF_OPER_UP if userspace interaction is disabled. Otherwise
122	 IF_OPER_DORMANT with the possibility for userspace to initiate the
123	 IF_OPER_UP transition afterwards.
124	
125	
126	4. Setting from userspace
127	
128	Applications have to use the netlink interface to influence the
129	RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1
130	via RTM_SETLINK instructs the kernel that an interface should go to
131	IF_OPER_DORMANT instead of IF_OPER_UP when the combination
132	netif_carrier_ok() && !netif_dormant() is set by the
133	driver. Afterwards, the userspace application can set IFLA_OPERSTATE
134	to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set
135	netif_carrier_off() or netif_dormant_on(). Changes made by userspace
136	are multicasted on the netlink group RTMGRP_LINK.
137	
138	So basically a 802.1X supplicant interacts with the kernel like this:
139	
140	-subscribe to RTMGRP_LINK
141	-set IFLA_LINKMODE to 1 via RTM_SETLINK
142	-query RTM_GETLINK once to get initial state
143	-if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until
144	 netlink multicast signals this state
145	-do 802.1X, eventually abort if flags go down again
146	-send RTM_SETLINK to set operstate to IF_OPER_UP if authentication
147	 succeeds, IF_OPER_DORMANT otherwise
148	-see how operstate and IFF_RUNNING is echoed via netlink multicast
149	-set interface back to IF_OPER_DORMANT if 802.1X reauthentication
150	 fails
151	-restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag
152	
153	if supplicant goes down, bring back IFLA_LINKMODE to 0 and
154	IFLA_OPERSTATE to a sane value.
155	
156	A routing daemon or dhcp client just needs to care for IFF_RUNNING or
157	waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before
158	considering the interface / querying a DHCP address.
159	
160	
161	For technical questions and/or comments please e-mail to Stefan Rompf
162	(stefan at loplof.de).
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