Self-managed Cells for Ubiquitous Systems - Computer Network Security - page 3

watchdog is also responsible for cleanly removing and restarting core components
when a core component fails, or if a core component needs to be updated. Essentially
the cell watchdog has the responsibility to ensure the survivability of the core man-
agement components, and ideally should be in firmware and always alive.
2.2 Event Service
Management systems are essentially event-driven, as changes of states need to be
notified to several, potentially unknown management services. Examples of events
include: the discovery of a new device, a change in context (e.g. battery level low), an
intrusion alert. The event service provides at-most-once, persistent publish/subscribe
delivery and is used for both intra-cell and inter-cell management. The event service
supports event correlation for flexibility.
2.3 Discovery Service
The discovery service is responsible for detecting the presence of devices that come
into wireless range. These may be primitive devices that are managed by the cell,
devices that are managed by others cells, or devices that are not currently managed by
any cell. Once a device is discovered, the discovery service communicates with the
device to get further attributes (e.g. type, profile, services provided) and generates a
“new-device” event for other management components. The discovery service needs
to distinguish between transient failures, which are common in wireless communica-
tions, and when some device is really no longer available (e.g. out of range or
switched off).
2.4 Policy Service
The policy service is responsible for the execution of policies. Policies are rules that
govern the choices in behaviour of the cell. Two kinds of policy are currently sup-
ported. Obligation policies (event-condition-action rules), which define what actions
to carry out when specific events occur, and authorisation policies which define what
actions are permitted or not permitted, for what or for whom, and under what condi-
tions. Policies can be added, removed, enabled or disabled to change the behaviour of
a cell. See cell policy language (section 3).
2.5 Domain Service
The domain service provides a means of hierarchically grouping references to objects
(c.f a filesystem). Objects include devices, services (including core services), poli-
cies, neighbouring cells. For example, when a new device is discovered, a reference
to it, is normally added to the domain /dev as well as to application-specific do-
mains, for example, /music/headset/bluetooth . Domains are also used to
define authorisation policies in the cell policy language, e.g. objects within the subject
domain /players/mp3 are permitted to perform the action play on objects in the
target domain /headsets .