Building

• General Use:
  • Install Qt
  • Install Qt Cryptographic Architecture library
• Sample application (see below)

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Kademlia Client

An implementation of a distributed hash table (DHT) using the Kademlia algorithm.

API

For programs wishing to use a DHT to store files across a local area network (LAN), create a KademliaClientThread, which provides asynchronouse Find, Store, and Remove functionality through use of the following methods:

• QIODevice* Find(Qkey key)
• void Store(QKey key, QIODevice* device)
• void Remove(QKey key)

In order to make a synchronous find request, create a local QEventLoop and connect its quit() slot to the KademliaClientThread's FindRequestComplete() signal. The found value can be retrieved with the accessor, get_last_found_value(), which returns a pointer to the value coresponding to the last lookup, or NULL if the resource could not be found in the DHT.

Message Protocol

A basic packet contains the following fields:

• NodeId: of sending node
• RequestId: unique identifier for the request (not globally unique)
• Type: of request/reply

In addition, a packet might contain the following keys:

• Id (FIND_NODE): of requested node
• Key (FIND_VALUE): of requested resource
• Nodes (FIND_NODE_REPLY): of k-nodes with ids closest to requested resource (node or value)
• Key (STORE, FIND_VALUE_REPLY): of resource being stored/requested

The Type field may take one of the following values:

• JOIN, JOIN_REPLY
• PING, ACK
• STORE, READY_DOWNLOAD
• FIND_NODE, FIND_NODE_REPLY
• FIND_VALUE, FIND_VALUE_REPLY

The JOIN pair is an addition to the protocol that reflects the target use case for the client implementation, i.e. a LAN. Instead of invoking a FIND_NODE as specified by the original protocol, each client sends a broadcast to the LAN with its Node Id. All clients then respond with their own Node Ids, and the bootstrapping node fills its k-buckets with their responses.

READY_DOWNLOAD is simply an acknowledgement of the STORE RPC, and signifies that the target client is preparing to open a TCP connection with the sending client in order to transfer the given resource.

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Peer Cache

A subclass of QAbstractNetworkCache that can be used with a QNetworkAccessManager to cache network requests across a LAN. Built using the Kademlia client described above; a PeerCache object communicates with a KademliaClientThread through signals and slots, making synchronous requests to the client by creating a local QEventLoop.

API

For use as part of network application, see the documentation for QAbstractNetworkCache

Stand-Alone Use

To use as a basic interface for making HTTP Get requests, invoke the built program on the command line as follows:

cd src
qmake
make
peer_cache

Enter the requested url in the text box and hit enter to make a request. To switch into Cache-only mode (i.e. offline), click the button below the text box.