Handover mechanisms in next generation heterogeneous wireless ...

SECURITY FOR HANDOVER ACROSS HETEROGENEOUS WIRELESS NETWORKS
effectively reduce the number of roaming agreements required for interoperation from
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O ( N ) to O (N ) as stated in [53].
In Figure 3.5, three different types of AAA entities are presented: Foreign AAA server
(FAAA), Home AAA server (HAAA) and AAA proxy. An AAA server may play these
different roles in different contexts. For example, the AAA server in a network may
negotiate the identity verification with an external authority (e.g. HAAA) when dealing
with a roaming mobile user, and in this case, acts as a FAAA. When processing
authentication requests from its own subscriber residing in either its own network or a
foreign network, the same AAA server makes authorisation decision and acts as a
HAAA. When it has been interconnected with a series of AAA servers with relevant
trust associations in place, this AAA server may serve as an AAA proxy for relaying
AAA messages from/to other networks. RFC 2903 [54] proposed such a generic AAA
architecture that facilitates interoperability between peered AAA servers via a standard
AAA protocol.
Figure 3.5 indicates a security model that is applied to the presented AAA architecture.
It is important to identify all the trust relationships needed for such an AAA architecture.
With the concept borrowed from the social science literature, there is no clear consensus
on the definition of trust in distributed computer networks [55]. A trust relationship is
enforced on top of a number of security features that are enabled via a security
association between two entities. Therefore, it is interpreted as security association in
some literatures [53, 56, 57]. A security association between entities X and Y is defined
as the combination of the entities’ identity information (e.g. Network Access Identifier,
NAI), some forms of cryptographic keys (e.g., public keys, preshared symmetric keys),
and information on cryptographic algorithms to use in order to authenticate and/or
protect data in transit between X and Y [53].
Figure 3.6 shows all forms of trust relationship that need to be present on a generic
AAA architecture. First, the MH has a trust relationship TR MH , HAAA with its HAAA,
which means that the MH belongs to its home network. Second, the FAAA and HAAA
have to trust each other for service roaming; otherwise, they can not exchange AAA
messages to pass authentication requests. The trust relationship TR FAAA,
HAAA between the
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