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Application Development - I2P
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Application Development

Application Development Guide

Last updated May 2013, current as of router version 0.9.6

Contents

Why write I2P-specific code?

There are multiple ways to use applications in I2P. Using I2PTunnel, you can use regular applications without needing to program explicit I2P support. This is very effective for client-server scenario's, where you need to connect to a single website. You can simply create a tunnel using I2PTunnel to connect to that website, as shown in Figure 1.

If your application is distributed, it will require connections to a large amount of peers. Using I2PTunnel, you will need to create a new tunnel for each peer you want to contact, as shown in Figure 2. This process can of course be automated, but running a lot of I2PTunnel instances creates a large amount of overhead. In addition, with many protocols you will need to force everyone to use the same set of ports for all peers - e.g. if you want to reliably run DCC chat, everyone needs to agree that port 10001 is Alice, port 10002 is Bob, port 10003 is Charlie, and so on, since the protocol includes TCP/IP specific information (host and port).

General network applications often send a lot of additional data that could be used to identify users. Hostnames, port numbers, time zones, character sets, etc. are often sent without informing the user. As such, designing the network protocol specifically with anonymity in mind can avoid compromising user identities.

There are also efficiency considerations to review when determining how to interact on top of I2P. The streaming library and things built on top of it operate with handshakes similar to TCP, while the core I2P protocols (I2NP and I2CP) are strictly message based (like UDP or in some instances raw IP). The important distinction is that with I2P, communication is operating over a long fat network - each end to end message will have nontrivial latencies, but may contain payloads of up to several KB. An application that needs a simple request and response can get rid of any state and drop the latency incurred by the startup and teardown handshakes by using (best effort) datagrams without having to worry about MTU detection or fragmentation of messages.

Creating a server-client connection using I2PTunnel only requires creating a single tunnel.

Figure 1: Creating a server-client connection using I2PTunnel only requires creating a single tunnel.

Setting up connections for a peer-to-peer applications requires a very large amount of tunnels.

Figure 2: Setting up connections for a peer-to-peer applications requires a very large amount of tunnels.

In summary, a number of reasons to write I2P-specific code:

I2P supports a standard plugins interface for developers so that applications may be easily integrated and distributed.

Important concepts

There are a few changes that require adjusting to when using I2P:

Destination ~= host+port

An application running on I2P sends messages from and receives messages to a unique cryptographically secure end point - a "destination". In TCP or UDP terms, a destination could (largely) be considered the equivalent of a hostname plus port number pair, though there are a few differences.

While most applications do not need to distinguish protocols and ports, I2P does support them. Complex applications may specify a protocol, from port, and to port, on a per-message basis, to multiplex traffic on a single destination. See the datagram page for details. Simple applications operate by listening for "all protocols" on "all ports" of a destination.

Anonymity and confidentiality

I2P has transparent end to end encryption and authentication for all data passed over the network - if Bob sends to Alice's destination, only Alice's destination can receive it, and if Bob is using the datagrams or streaming library, Alice knows for certain that Bob's destination is the one who sent the data.

Of course, I2P transparently anonymizes the data sent between Alice and Bob, but it does nothing to anonymize the content of what they send. For instance, if Alice sends Bob a form with her full name, government IDs, and credit card numbers, there is nothing I2P can do. As such, protocols and applications should keep in mind what information they are trying to protect and what information they are willing to expose.

I2P datagrams can be up to several KB

Applications that use I2P datagrams (either raw or repliable ones) can essentially be thought of in terms of UDP - the datagrams are unordered, best effort, and connectionless - but unlike UDP, applications don't need to worry about MTU detection and can simply fire off large datagrams. While the upper limit is nominally 32 KB, the message is fragmented for transport, thus dropping the reliability of the whole. Datagrams over about 10 KB are not currently recommended. See the datagram page for details. For many applications, 10 KB of data is sufficient for an entire request or response, allowing them to transparently operate in I2P as a UDP-like application without having to write fragmentation, resends, etc.

Development options

There are several means of sending data over I2P, each with their own pros and cons. The streaming lib is the recommended interface, used by the majority of I2P applications.

Streaming Lib

The full streaming library is now the standard interface. It allows programming using TCP-like sockets, as explained in the Streaming development guide.

BOB

BOB is the Basic Open Bridge, allowing an application in any language to make streaming connections to and from I2P. At this point in time it lacks UDP support, but UDP support is planned in the near future. BOB also contains several tools, such as destination key generation, and verification that an address conforms to I2P specifications. Up to date info and applications that use BOB can be found at this eepsite.

SAM, SAM V2, SAM V3

SAM is not recommended. SAM V2 is okay, SAM V3 is recommended.

SAM is the Simple Anonymous Messaging protocol, allowing an application written in any language to talk to a SAM bridge through a plain TCP socket and have that bridge multiplex all of its I2P traffic, transparently coordinating the encryption/decryption and event based handling. SAM supports three styles of operation:

SAM V3 aims at the same goal as SAM and SAM V2, but does not require multiplexing/demultiplexing. Each I2P stream is handled by its own socket between the application and the SAM bridge. Besides, datagrams can be sent and received by the application through datagram communications with the SAM bridge.

SAM V2 is a new version used by imule that fixes some of the problems in SAM.
SAM V3 is used by imule since version 1.4.0.

I2PTunnel

The I2PTunnel application allows applications to build specific TCP-like tunnels to peers by creating either I2PTunnel 'client' applications (which listen on a specific port and connect to a specific I2P destination whenever a socket to that port is opened) or I2PTunnel 'server' applications (which listen to a specific I2P destination and whenever it gets a new I2P connection it outproxies to a specific TCP host/port). These streams are 8-bit clean, and are authenticated and secured through the same streaming library that SAM uses, but there is a nontrivial overhead involved with creating multiple unique I2PTunnel instances, since each have their own unique I2P destination and their own set of tunnels, keys, etc.

SOCKS

I2P supports a SOCKS V4 and V5 proxy. Outbound connections work well. Inbound (server) and UDP functionality may be incomplete and untested.

Ministreaming

Removed

There used to be a simple "ministreaming" library, but now ministreaming.jar contains only the interfaces for the full streaming library.

Datagrams

Recommended for UDP-like applications

The Datagram library allows sending UDP-like packets. It's possible to use:

I2CP

Not recommended

I2CP itself is a language independent protocol, but to implement an I2CP library in something other than Java there is a significant amount of code to be written (encryption routines, object marshalling, asynchronous message handling, etc). While someone could write an I2CP library in C or something else, it would most likely be more useful to use the C SAM library instead.

Web Applications

I2P comes with the Jetty webserver, and configuring to use the Apache server instead is straightforward. Any standard web app technology should work.

Start developing - a simple guide

Developing using I2P requires a working I2P installation and a development environment of your own choice. If you are using Java, you can start development with the streaming library or datagram library. Using another programming language, SAM or BOB can be used.

Developing with the streaming library

The following example shows how to create TCP-like client and server applications using the streaming library. This will require the following libraries in your classpath:

Network communication requires the usage of I2P network sockets. To demonstrate this, we will create an application where a client can send text messages to a server, who will print the messages and send them back to the client. In other words, the server will function as an echo.

We will start by initializing the server application. This requires getting an I2PSocketManager and creating an I2PServerSocket. We will not provide the I2PSocketManagerFactory with the saved keys for an existing Destination, so it will create a new Destination for us. So we will ask the I2PSocketManager for an I2PSession, so we can find out the Destination that was created, as we will need to copy and paste that information later so the client can connect to us.

    package i2p.echoserver;

    import net.i2p.re.client.I2PSession;
    import net.i2p.re.client.streaming.I2PServerSocket;
    import net.i2p.re.client.streaming.I2PSocketManager;
    import net.i2p.re.client.streaming.I2PSocketManagerFactory;

    public class Main {

        public static void main(String[] args) {
            //Initialize application
            I2PSocketManager manager = I2PSocketManagerFactory.createManager();
            I2PServerSocket serverSocket = manager.getServerSocket();
            I2PSession session = manager.getSession();
            //Print the base64 string, the regular string would look like garbage.
            System.out.println(session.getMyDestination().toBase64());
            //The additional main method code comes here...
        }

    }
    


Code example 1: initializing the server application.

Once we have an I2PServerSocket, we can create I2PSocket instances to accept connections from clients. In this example, we will create a single I2PSocket instance, that can only handle one client at a time. A real server would have to be able to handle multiple clients. To do this, multiple I2PSocket instances would have to be created, each in separate threads. Once we have created the I2PSocket instance, we read data, print it and send it back to the client. The bold code is the new code we add.

    package i2p.echoserver;

    

    import java.io.IOException;
    import java.io.InputStream;
    import java.io.OutputStream;
    import java.net.ConnectException;
    import java.net.SocketTimeoutException;
    import net.i2p.re.I2PException;
    import net.i2p.re.client.streaming.I2PSocket;
    import net.i2p.re.util.I2PThread;
    
    import net.i2p.re.client.I2PSession;
    import net.i2p.re.client.streaming.I2PServerSocket;
    import net.i2p.re.client.streaming.I2PSocketManager;
    import net.i2p.re.client.streaming.I2PSocketManagerFactory;

    public class Main {

        public static void main(String[] args) {
            I2PSocketManager manager = I2PSocketManagerFactory.createManager();
            I2PServerSocket serverSocket = manager.getServerSocket();
            I2PSession session = manager.getSession();
            //Print the base64 string, the regular string would look like garbage.
            System.out.println(session.getMyDestination().toBase64());


            //Create socket to handle clients
            I2PThread t = new I2PThread(new ClientHandler(serverSocket));
            t.setName("clienthandler1");
            t.setDaemon(false);
            t.start();
        }

        private static class ClientHandler implements Runnable {

            public ClientHandler(I2PServerSocket socket) {
                this.socket = socket;
            }

            public void run() {
                while(true) {
                    try {
                        I2PSocket sock = this.socket.accept();
                        if(sock != null) {
                            //Receive from clients
                            BufferedReader br = new BufferedReader(new InputStreamReader(sock.getInputStream()));
                            //Send to clients
                            BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(sock.getOutputStream()));
                            String line = br.readLine();
                            if(line != null) {
                                System.out.println("Received from client: " + line);
                                bw.write(line);
                                bw.flush(); //Flush to make sure everything got sent
                            }
                            sock.close();
                        }
                    } catch (I2PException ex) {
                        System.out.println("General I2P exception!");
                    } catch (ConnectException ex) {
                        System.out.println("Error connecting!");
                    } catch (SocketTimeoutException ex) {
                        System.out.println("Timeout!");
                    } catch (IOException ex) {
                        System.out.println("General read/write-exception!");
                    }
                }
            }

            private I2PServerSocket socket;

        }

    }
    


Code example 2: accepting connections from clients and handling messages.

When you run the above server code, it should print something like this (but without the line endings, it should just be one huge block of characters):
    y17s~L3H9q5xuIyyynyWahAuj6Jeg5VC~Klu9YPquQvD4vlgzmxn4yy~5Z0zVvKJiS2Lk
    poPIcB3r9EbFYkz1mzzE3RYY~XFyPTaFQY8omDv49nltI2VCQ5cx7gAt~y4LdWqkyk3au
    6HdfYSLr45zxzWRGZnTXQay9HPuYcHysZHJP1lY28QsPz36DHr6IZ0vwMENQsnQ5rhq20
    jkB3iheYJeuO7MpL~1xrjgKzteirkCNHvXN8PjxNmxe-pj3QgOiow-R9rEYKyPAyGd2pe
    qMD-J12CGfB6MlnmH5qPHGdZ13bUuebHiyZ1jqSprWL-SVIPcynAxD2Uu85ynxnx31Fth
    nxFMk07vvggBrLM2Sw82pxNjKDbtO8reawe3cyksIXBBkuobOZdyOxp3NT~x6aLOxwkEq
    BOF6kbxV7NPRPnivbNekd1E1GUq08ltDPVMO1pKJuGMsFyZC4Q~osZ8nI59ryouXgn97Q
    5ZDEO8-Iazx50~yUQTRgLMOTC5hqnAAAA
    
This is the base64-representation of the server Destination. The client will need this string to reach the server.

Now, we will create the client application. Again, a number of steps are required for initialization. Again, we will need to start by getting an I2PSocketManager. We won't use an I2PSession and an I2PServerSocket this time. Instead, we will use the server Destination string to start our connection. We will ask the user for the Destination string, and create an I2PSocket using this string. Once we have an I2PSocket, we can start sending and receiving data to and from the server.

    package i2p.echoclient;

    import java.io.BufferedReader;
    import java.io.BufferedWriter;
    import java.io.IOException;
    import java.io.InputStreamReader;
    import java.io.InterruptedIOException;
    import java.io.OutputStream;
    import java.io.OutputStreamWriter;
    import java.net.ConnectException;
    import java.net.NoRouteToHostException;
    import net.i2p.re.I2PException;
    import net.i2p.re.client.streaming.I2PSocket;
    import net.i2p.re.client.streaming.I2PSocketManager;
    import net.i2p.re.client.streaming.I2PSocketManagerFactory;
    import net.i2p.re.data.DataFormatException;
    import net.i2p.re.data.Destination;

    public class Main {

        public static void main(String[] args) {
            I2PSocketManager manager = I2PSocketManagerFactory.createManager();
            System.out.println("Please enter a Destination:");
            BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
            String destinationString;
            try {
                destinationString = br.readLine();
            } catch (IOException ex) {
                System.out.println("Failed to get a Destination string.");
                return;
            }
            Destination destination;
            try {
                destination = new Destination(destinationString);
            } catch (DataFormatException ex) {
                System.out.println("Destination string incorrectly formatted.");
                return;
            }
            I2PSocket socket;
            try {
                socket = manager.connect(destination);
            } catch (I2PException ex) {
                System.out.println("General I2P exception occurred!");
                return;
            } catch (ConnectException ex) {
                System.out.println("Failed to connect!");
                return;
            } catch (NoRouteToHostException ex) {
                System.out.println("Couldn't find host!");
                return;
            } catch (InterruptedIOException ex) {
                System.out.println("Sending/receiving was interrupted!");
                return;
            }
            try {
                //Write to server
                BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
                bw.write("Hello I2P!\n");
                //Flush to make sure everything got sent
                bw.flush();
                //Read from server
                BufferedReader br2 = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                String s = null;
                while ((s = br2.readLine()) != null) {
                    System.out.println("Received from server: " + s);
                }
                socket.close();
            } catch (IOException ex) {
                System.out.println("Error occurred while sending/receiving!");
            }
        }

    }
    


Code example 3: starting the client and connecting it to the server application.

Finally, you can run both the server and the client application. First, start the server application. It will print a Destination string (like shown above). Next, start the client application. When it requests a Destination string, you can enter the string printed by the server. The client will then send 'Hello I2P!' (along with a newline) to the server, who will print the message and send it back to the client.

Congratulations, you have successfully communicated over I2P!

Existing Applications

Contact us if you would like to contribute.

See also all the plugins on plugins.i2p.re, the applications and source code listed on echelon.i2p.re, and the application code hosted on git.repo.i2p.re.

See also the bundled applications in the I2P distribution - SusiMail and I2PSnark.

Application Ideas