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 Unused in current kernel. 77 78IF_OPER_DORMANT (5): 79 Interface is L1 up, but waiting for an external event, f.e. for a 80 protocol to establish. (802.1X) 81 82IF_OPER_UP (6): 83 Interface is operational up and can be used. 84 85This TLV can also be queried via sysfs. 86 87TLV IFLA_LINKMODE 88----------------- 89 90contains link policy. This is needed for userspace interaction 91described below. 92 93This TLV can also be queried via sysfs. 94 95 963. Kernel driver API 97==================== 98 99Kernel drivers have access to two flags that map to IFF_LOWER_UP and 100IFF_DORMANT. These flags can be set from everywhere, even from 101interrupts. It is guaranteed that only the driver has write access, 102however, if different layers of the driver manipulate the same flag, 103the driver has to provide the synchronisation needed. 104 105__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP: 106 107The driver uses netif_carrier_on() to clear and netif_carrier_off() to 108set this flag. On netif_carrier_off(), the scheduler stops sending 109packets. The name 'carrier' and the inversion are historical, think of 110it as lower layer. 111 112Note that for certain kind of soft-devices, which are not managing any 113real hardware, it is possible to set this bit from userspace. One 114should use TVL IFLA_CARRIER to do so. 115 116netif_carrier_ok() can be used to query that bit. 117 118__LINK_STATE_DORMANT, maps to IFF_DORMANT: 119 120Set by the driver to express that the device cannot yet be used 121because some driver controlled protocol establishment has to 122complete. Corresponding functions are netif_dormant_on() to set the 123flag, netif_dormant_off() to clear it and netif_dormant() to query. 124 125On device allocation, both flags __LINK_STATE_NOCARRIER and 126__LINK_STATE_DORMANT are cleared, so the effective state is equivalent 127to netif_carrier_ok() and !netif_dormant(). 128 129 130Whenever the driver CHANGES one of these flags, a workqueue event is 131scheduled to translate the flag combination to IFLA_OPERSTATE as 132follows: 133 134!netif_carrier_ok(): 135 IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN 136 otherwise. Kernel can recognise stacked interfaces because their 137 ifindex != iflink. 138 139netif_carrier_ok() && netif_dormant(): 140 IF_OPER_DORMANT 141 142netif_carrier_ok() && !netif_dormant(): 143 IF_OPER_UP if userspace interaction is disabled. Otherwise 144 IF_OPER_DORMANT with the possibility for userspace to initiate the 145 IF_OPER_UP transition afterwards. 146 147 1484. Setting from userspace 149========================= 150 151Applications have to use the netlink interface to influence the 152RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1 153via RTM_SETLINK instructs the kernel that an interface should go to 154IF_OPER_DORMANT instead of IF_OPER_UP when the combination 155netif_carrier_ok() && !netif_dormant() is set by the 156driver. Afterwards, the userspace application can set IFLA_OPERSTATE 157to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set 158netif_carrier_off() or netif_dormant_on(). Changes made by userspace 159are multicasted on the netlink group RTNLGRP_LINK. 160 161So basically a 802.1X supplicant interacts with the kernel like this: 162 163- subscribe to RTNLGRP_LINK 164- set IFLA_LINKMODE to 1 via RTM_SETLINK 165- query RTM_GETLINK once to get initial state 166- if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until 167 netlink multicast signals this state 168- do 802.1X, eventually abort if flags go down again 169- send RTM_SETLINK to set operstate to IF_OPER_UP if authentication 170 succeeds, IF_OPER_DORMANT otherwise 171- see how operstate and IFF_RUNNING is echoed via netlink multicast 172- set interface back to IF_OPER_DORMANT if 802.1X reauthentication 173 fails 174- restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag 175 176if supplicant goes down, bring back IFLA_LINKMODE to 0 and 177IFLA_OPERSTATE to a sane value. 178 179A routing daemon or dhcp client just needs to care for IFF_RUNNING or 180waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before 181considering the interface / querying a DHCP address. 182 183 184For technical questions and/or comments please e-mail to Stefan Rompf 185(stefan at loplof.de). 186