forked from Qortal/Brooklyn
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
187 lines
6.5 KiB
187 lines
6.5 KiB
.. SPDX-License-Identifier: GPL-2.0 |
|
|
|
================== |
|
Operational States |
|
================== |
|
|
|
|
|
1. Introduction |
|
=============== |
|
|
|
Linux distinguishes between administrative and operational state of an |
|
interface. Administrative state is the result of "ip link set dev |
|
<dev> up or down" and reflects whether the administrator wants to use |
|
the device for traffic. |
|
|
|
However, an interface is not usable just because the admin enabled it |
|
- ethernet requires to be plugged into the switch and, depending on |
|
a site's networking policy and configuration, an 802.1X authentication |
|
to be performed before user data can be transferred. Operational state |
|
shows the ability of an interface to transmit this user data. |
|
|
|
Thanks to 802.1X, userspace must be granted the possibility to |
|
influence operational state. To accommodate this, operational state is |
|
split into two parts: Two flags that can be set by the driver only, and |
|
a RFC2863 compatible state that is derived from these flags, a policy, |
|
and changeable from userspace under certain rules. |
|
|
|
|
|
2. Querying from userspace |
|
========================== |
|
|
|
Both admin and operational state can be queried via the netlink |
|
operation RTM_GETLINK. It is also possible to subscribe to RTNLGRP_LINK |
|
to be notified of updates while the interface is admin up. This is |
|
important for setting from userspace. |
|
|
|
These values contain interface state: |
|
|
|
ifinfomsg::if_flags & IFF_UP: |
|
Interface is admin up |
|
|
|
ifinfomsg::if_flags & IFF_RUNNING: |
|
Interface is in RFC2863 operational state UP or UNKNOWN. This is for |
|
backward compatibility, routing daemons, dhcp clients can use this |
|
flag to determine whether they should use the interface. |
|
|
|
ifinfomsg::if_flags & IFF_LOWER_UP: |
|
Driver has signaled netif_carrier_on() |
|
|
|
ifinfomsg::if_flags & IFF_DORMANT: |
|
Driver has signaled netif_dormant_on() |
|
|
|
TLV IFLA_OPERSTATE |
|
------------------ |
|
|
|
contains RFC2863 state of the interface in numeric representation: |
|
|
|
IF_OPER_UNKNOWN (0): |
|
Interface is in unknown state, neither driver nor userspace has set |
|
operational state. Interface must be considered for user data as |
|
setting operational state has not been implemented in every driver. |
|
|
|
IF_OPER_NOTPRESENT (1): |
|
Unused in current kernel (notpresent interfaces normally disappear), |
|
just a numerical placeholder. |
|
|
|
IF_OPER_DOWN (2): |
|
Interface is unable to transfer data on L1, f.e. ethernet is not |
|
plugged or interface is ADMIN down. |
|
|
|
IF_OPER_LOWERLAYERDOWN (3): |
|
Interfaces stacked on an interface that is IF_OPER_DOWN show this |
|
state (f.e. VLAN). |
|
|
|
IF_OPER_TESTING (4): |
|
Interface is in testing mode, for example executing driver self-tests |
|
or media (cable) test. It can't be used for normal traffic until tests |
|
complete. |
|
|
|
IF_OPER_DORMANT (5): |
|
Interface is L1 up, but waiting for an external event, f.e. for a |
|
protocol to establish. (802.1X) |
|
|
|
IF_OPER_UP (6): |
|
Interface is operational up and can be used. |
|
|
|
This TLV can also be queried via sysfs. |
|
|
|
TLV IFLA_LINKMODE |
|
----------------- |
|
|
|
contains link policy. This is needed for userspace interaction |
|
described below. |
|
|
|
This TLV can also be queried via sysfs. |
|
|
|
|
|
3. Kernel driver API |
|
==================== |
|
|
|
Kernel drivers have access to two flags that map to IFF_LOWER_UP and |
|
IFF_DORMANT. These flags can be set from everywhere, even from |
|
interrupts. It is guaranteed that only the driver has write access, |
|
however, if different layers of the driver manipulate the same flag, |
|
the driver has to provide the synchronisation needed. |
|
|
|
__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP: |
|
|
|
The driver uses netif_carrier_on() to clear and netif_carrier_off() to |
|
set this flag. On netif_carrier_off(), the scheduler stops sending |
|
packets. The name 'carrier' and the inversion are historical, think of |
|
it as lower layer. |
|
|
|
Note that for certain kind of soft-devices, which are not managing any |
|
real hardware, it is possible to set this bit from userspace. One |
|
should use TLV IFLA_CARRIER to do so. |
|
|
|
netif_carrier_ok() can be used to query that bit. |
|
|
|
__LINK_STATE_DORMANT, maps to IFF_DORMANT: |
|
|
|
Set by the driver to express that the device cannot yet be used |
|
because some driver controlled protocol establishment has to |
|
complete. Corresponding functions are netif_dormant_on() to set the |
|
flag, netif_dormant_off() to clear it and netif_dormant() to query. |
|
|
|
On device allocation, both flags __LINK_STATE_NOCARRIER and |
|
__LINK_STATE_DORMANT are cleared, so the effective state is equivalent |
|
to netif_carrier_ok() and !netif_dormant(). |
|
|
|
|
|
Whenever the driver CHANGES one of these flags, a workqueue event is |
|
scheduled to translate the flag combination to IFLA_OPERSTATE as |
|
follows: |
|
|
|
!netif_carrier_ok(): |
|
IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN |
|
otherwise. Kernel can recognise stacked interfaces because their |
|
ifindex != iflink. |
|
|
|
netif_carrier_ok() && netif_dormant(): |
|
IF_OPER_DORMANT |
|
|
|
netif_carrier_ok() && !netif_dormant(): |
|
IF_OPER_UP if userspace interaction is disabled. Otherwise |
|
IF_OPER_DORMANT with the possibility for userspace to initiate the |
|
IF_OPER_UP transition afterwards. |
|
|
|
|
|
4. Setting from userspace |
|
========================= |
|
|
|
Applications have to use the netlink interface to influence the |
|
RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1 |
|
via RTM_SETLINK instructs the kernel that an interface should go to |
|
IF_OPER_DORMANT instead of IF_OPER_UP when the combination |
|
netif_carrier_ok() && !netif_dormant() is set by the |
|
driver. Afterwards, the userspace application can set IFLA_OPERSTATE |
|
to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set |
|
netif_carrier_off() or netif_dormant_on(). Changes made by userspace |
|
are multicasted on the netlink group RTNLGRP_LINK. |
|
|
|
So basically a 802.1X supplicant interacts with the kernel like this: |
|
|
|
- subscribe to RTNLGRP_LINK |
|
- set IFLA_LINKMODE to 1 via RTM_SETLINK |
|
- query RTM_GETLINK once to get initial state |
|
- if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until |
|
netlink multicast signals this state |
|
- do 802.1X, eventually abort if flags go down again |
|
- send RTM_SETLINK to set operstate to IF_OPER_UP if authentication |
|
succeeds, IF_OPER_DORMANT otherwise |
|
- see how operstate and IFF_RUNNING is echoed via netlink multicast |
|
- set interface back to IF_OPER_DORMANT if 802.1X reauthentication |
|
fails |
|
- restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag |
|
|
|
if supplicant goes down, bring back IFLA_LINKMODE to 0 and |
|
IFLA_OPERSTATE to a sane value. |
|
|
|
A routing daemon or dhcp client just needs to care for IFF_RUNNING or |
|
waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before |
|
considering the interface / querying a DHCP address. |
|
|
|
|
|
For technical questions and/or comments please e-mail to Stefan Rompf |
|
(stefan at loplof.de).
|
|
|