[NET]: Add missing operstates documentation.
Signed-off-by: Stefan Rompf <stefan@loplof.de> Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Stefan Rompf
David S. Miller
parent
3cd73eedde
commit
3a01c1ef75
|
|
@ -0,0 +1,161 @@
|
|||
|
||||
1. Introduction
|
||||
|
||||
Linux distinguishes between administrative and operational state of an
|
||||
interface. Admininstrative 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 RTMGRP_LINK
|
||||
to be notified of updates. 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()
|
||||
|
||||
These interface flags can also be queried without netlink using the
|
||||
SIOCGIFFLAGS ioctl.
|
||||
|
||||
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):
|
||||
Unused in current kernel.
|
||||
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.
|
||||
|
||||
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, networking core sets the flags 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 RTMGRP_LINK.
|
||||
|
||||
So basically a 802.1X supplicant interacts with the kernel like this:
|
||||
|
||||
-subscribe to RTMGRP_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).
|
||||
Loading…
Reference in New Issue