What is I2P?
The Invisible Internet Project provides software to download that connects you to the network. In addition to the network privacy benefits, I2P provides an application layer that allows people to use and create familiar apps for daily use. I2P provides its own unique DNS so that you can self host or mirror content on the network. You can create and own your own platform that you can add to the I2P directory or only invite your friends. The I2P network functions the same way the Internet does. When you download the I2P software, it includes everything you need to connect, share, and create privately.
An Overview of the Network
I2P uses cryptography to achieve a variety of properties for the tunnels it builds and the communications it transports. I2P tunnels use transports, NTCP2 and SSU, to hide the nature of the traffic being transported over it. Connections are encrypted from router-to-router, and from client-to-client(end-to-end). Forward-secrecy is provided for all connections. Because I2P is cryptographically addressed, I2P addresses are self-authenticating and only belong to the user who generated them.
I2P is a secure and traffic protecting Internet-like layer. The network is made up of peers ("routers") and unidirectional inbound and outbound virtual tunnels. Routers communicate with each other using protocols built on existing transport mechanisms (TCP, UDP, etc), passing messages. Client applications have their own cryptographic identifier ("Destination") which enables it to send and receive messages. These clients can connect to any router and authorize the temporary allocation ("lease") of some tunnels that will be used for sending and receiving messages through the network. I2P has its own internal network database (using a modification of the Kademlia DHT) for distributing routing and contact information securely.
About Decentralization and I2P
The I2P network is almost completely decentralized, with exception to what are called "Reseed Servers," which is how you first join the network. This is to deal with the DHT ( Distributed Hash Table ) bootstrap problem. Basically, there's not a good and reliable way to get out of running at least one permanent bootstrap node that non-network users can find to get started. Once you're connected to the network, you only discover peers by building "exploratory" tunnels, but to make your initial connection, you need to get a peer set from somewhere. The reseed servers, which you can see listed on http://127.0.0.1:7657/configreseed in the Java I2P router, provide you with those peers. You then connect to them with the I2P router until you find one who you can reach and build exploratory tunnels through. Reseed servers can tell that you bootstrapped from them, but nothing else about your traffic on the I2P network.
Yes, this is how a fully distributed peer-to-peer network works. Every node participates in routing packets for others, so your IP address must be known to establish connections. While the fact that your computer runs I2P is public, nobody can see your activities in it. You can't say if a user behind this IP address is sharing files, hosting a website, doing research or just running a node to contribute bandwidth to the project.
What I2P Does Not Do
The I2P network does not officially "Exit" traffic. It has outproxies to the Internet run by volunteers, which are centralized services. I2P is primarily a hidden service network and outproxying is not an official function, nor is it advised. The privacy benefits you get from participating in the the I2P network come from remaining in the network and not accessing the internet. I2P recommends that you use Tor Browser or a trusted VPN when you want to browse the Internet privately.
There are a great many other applications and projects working on anonymous communication and I2P has been inspired by much of their efforts. This is not a comprehensive list of anonymity resources - both freehaven's Anonymity Bibliography and GNUnet's related projects serve that purpose well. That said, a few systems stand out for further comparison. The following have individual comparison pages:
The following are discussed on the other networks page:
- Morphmix / Tarzan
- Mixminion / Mixmaster
- MUTE / AntsP2P