Military Communications and Information Technology: A Trusted ...

254 Military Communications and Information Technology...
as IPTV solutions [15]. The reasons to use P2P instead of a centralized architecture
are always the same: The first and most important is load balancing. The second
reason is resilience against failures.
Many emerging applications such as social networks are based on an eventdriven
publish/subscribe model. In contrast to the traditional client/server model,
publish/subscribe is a one-to-many communication scheme and complements
the traditional one-to-one web communication. Publish/subscribe services and
applications are popular and work smoothly as they relieve the user from acquiring
information himself. The right information comes automatically to the user.
This work aims to contribute in bringing the two promising innovations
in network technology together: Peer-to-peer networks and publish/subscribe.
Not only has the way of information exchange changed in the last decade
but also the way how people communicate. Nowadays ubiquitous computing
becomes reality. Mobile computers have the size of a mobile phone while still
being able to use the full potential of online services. These new ways of information
exchange face new challenges. The cellular networks or wireless local area
networks do not offer the same quality of service for data transmission as wired
communication. User mobility, wireless transmission and battery lifetime pose
new challenges in terms of connection disruptions and device failures. From
the application’s point of view users disconnect and connect again. This process
of coming and going is called churn. Protocols developed for the Internet may
perform badly when faced with churn.
Peer-to-peer networks were originally designed to interconnect users in wired
networks. Surprisingly, some P2P systems cope quite well with the new challenges
because they already anticipate volatile user behavior and low data rate links.
The new challenges in mobile wireless networks are the main constraints to be
respected when considering publish/subscribe P2P systems.
II. Publish/subscribe in disadvantaged networks
The Publish/subscribe communication scheme has much in common with
multicast. While multicasting describes the process of information delivery to
the receivers, often the membership or group management is also considered a part
of it. The multicast management provides a method to start and stop the reception
of multicast messages. The receivers of multicast information form a multicast group.
The members of a multicast group are called the subscribers. Users can choose what
information they are interested in by subscribing to topics. This reduces the transmission
of undesired messages.
A multicast system with membership management is called a publish/subscribe
system in the following. Groups and topics form the very basic concepts in a publish/subscribe
system. Advanced concepts include group authorization, source
specific multicast, multi-stream multicast [4] as well as filtering and aggregation.