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mechanism, a more robust and more reliable publish/subscribe functionality
is possible. In contrast to Pastry there are millions of active Kademlia nodes and
three major implementations which are actively developed and researched [11].
A performance analysis of KadScribe is presented in section IV. A description
of KadScribe follows.
Kademlia defines a distance metric and a mechanism to route to a position –
or equivalently – a key in the P2P network. The identifier is a key in the Kademlia
key space. The routing table of a Kademlia node is a binary tree (Fig. 1). Each
leaf contains a list of nodes, the so called buckets. A bucket holds a fixed number
k of references to reach other nodes. As the network may contain up to n nodes,
the routing table size has to be limited. In Kademlia the memory requirement
for the routing table is O(k . b), with b as the number of bits of an identifier.
A node carries a tag which defines which identifiers are contained in its subtree.
In the figure, b and k are assumed to be 4, the standard key length of Kademlia
is 160. A tag of 1xxx means that the most significant bit is 1 for the whole subtree
and the other bits are unknown. The right subtree of the root carries this tag.
The local node identifier is assumed to be 0000 in the depicted tree. If the local
identifier is not 0000 the local node may do an XOR operation with its own
identifier on all identifiers in the routing table. The table then looks exactly
as in the example. The transformation can to be undone by applying the XOR
operation again as the XOR operation is involutary. In comparison to the original
description of Kademlia where the tree is built according to the local identifier,
this transformation may simplify the implementation.
Figure 1. The Kademlia routing table
Routing to destination keys is done in Kademlia by iteratively querying nodes
which are increasingly closer to the final destination. To do so, responses from
queried nodes contain the k closest nodes to the destination they are aware of.