Military Communications and Information Technology: A Trusted ...

Chapter 3: Information Technology for Interoperability and Decision...
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The queried node looks up the bucket which shares the longest matching prefix with
the destination key. If there are less than k entries in the bucket, buckets with shorter
matching prefix length are searched. The k entries are returned to the querying
node. The newly learned nodes are then queried until no closer nodes are found.
The distance to the final destination is halved in each routing step if the tables
are reasonably filled. Routing to a node can be seen as increasing the matching prefix
length to the destination hop at least by one bit in each hop. The number of routing
hops in Kademlia is within O(log(n)). In existing implementations the number
of hops seldom exceeds 5. Routing to destination keys is used for publishing and
in a slightly modified form also for subscribing.
When a user of KadScribe wants to subscribe to a topic, it first generates
an identifier of the topic. The node which is closest to the topic location according
to Kademlia’s routing metric is called Rendezvous Point (RP). The subscriber,
the topic location, the RP and the P2P connectivity are shown in figure 2(a).
The edges depicted in the diagram are connections at the transport layer, e.g. UDP
connectivity. The underlying technologies like MANET hops or wireless connection
are abstracted. One hop in the P2P network may include multiple hops on lower
layers. As the P2P network is on top of other network technologies it is also called
an overlay network. The same mechanisms to derive the identifier as in a Distributed
Hash Table may be used, e.g. hashing a textual topic description with a hash
function. The method of generating a topic description is shared by all nodes. By
doing so, a range query or a search which searches for similar topics is not possible.
Other schemes ([16], [9], [19]) to determine topic identifiers or enable range
queries or more detailed search requests exist. After the key of the topic has been
calculated, a node is able to subscribe to a topic. The Kademlia routing table is
searched and the known nodes closest to the RP are returned. No messages have
been sent over the network so far. The subscriber then sends a subscribe message
to the nearest node of the topic. The contacted node becomes a forwarder and adds
the requesting node to a list. The forwarder sends a subscribe ack as an acknowledgment
back to the subscriber. The forwarder then uses the same procedure to
subscribe to the topic if it is not already subscribed. If a node is not able to send
a subscribe to a node which is closer to the topic than itself the algorithm terminates
and the node becomes RP for this topic. Any successive subscription from
a different node terminates at the first forwarder (Fig. 2(c)). The constructed tree
is a rooted tree with directed edges. The subscribers form the leaves of the tree.
For each topic a separate tree is built. For simplification purposes, only a single
topic is considered. When updated information is available for the topic, the publisher
first finds the RP by means of the Kademlia routing. The topic identifier
is used as the destination key. The root and the forwarders replicate the message
and send out copies to all their successors in the tree until all subscribers have
received the topic update.