Tor / Onion Routing

Tor and Onion Routing are both anonymizing proxy networks, allowing people to tunnel out through their low latency mix network. The two primary differences between Tor / Onion-Routing and I2P are again related to differences in the threat model and the out-proxy design (though Tor supports hidden services as well). In addition, Tor takes the directory-based approach - providing a centralized point to manage the overall 'view' of the network, as well as gather and report statistics, as opposed to I2P's distributed network database and peer selection.

The I2P/Tor outproxy functionality does have a few substantial weaknesses against certain attackers - once the communication leaves the mixnet, global passive adversaries can more easily mount traffic analysis. In addition, the outproxies have access to the cleartext of the data transferred in both directions, and outproxies are prone to abuse, along with all of the other security issues we've come to know and love with normal Internet traffic.

However, many people don't need to worry about those situations, as they are outside their threat model. It is, also, outside I2P's (formal) functional scope (if people want to build outproxy functionality on top of an anonymous communication layer, they can). In fact, some I2P users currently take advantage of Tor to outproxy.

Comparison of Tor and I2P Terminology

While Tor and I2P are similar in many ways, much of the terminology is different.

Tor I2P Cell Message Client Router or Client Circuit Tunnel Directory NetDb Directory Server Floodfill Router Entry Guards Fast Peers Entry Node Inproxy Exit Node Outproxy Hidden Service Hidden Service, Eepsite or Destination Hidden Service Descriptor LeaseSet Introduction point Inbound Gateway Node Router Onion Proxy I2PTunnel Client (more or less) Onion Service Hidden Service, Eepsite or Destination Relay Router Rendezvous Point somewhat like Inbound Gateway + Outbound Endpoint Router Descriptor RouterInfo Server Router

Benefits of Tor over I2P

Much bigger user base; much more visibility in the academic and hacker communities; benefits from formal studies of anonymity, resistance, and performance; has a non-anonymous, visible, university-based leader

Has already solved some scaling issues I2P has yet to address

Has significant funding

Has more developers, including several that are funded

More resistant to state-level blocking due to TLS transport layer and bridges (I2P has proposals for "full restricted routes" but these are not yet implemented)

Big enough that it has had to adapt to blocking and DOS attempts

Designed and optimized for exit traffic, with a large number of exit nodes

Better documentation, has formal papers and specifications, better website, many more translations

More efficient with memory usage

Tor client nodes have very low bandwidth overhead

Centralized control reduces the complexity at each node and can efficiently address Sybil attacks

A core of high capacity nodes provides higher throughput and lower latency

C, not Java (ewww)

Benefits of I2P over Tor

Designed and optimized for hidden services, which are much faster than in Tor

Fully distributed and self organizing

Peers are selected by continuously profiling and ranking performance, rather than trusting claimed capacity

Floodfill peers ("directory servers") are varying and untrusted, rather than hardcoded

Small enough that it hasn't been blocked or DOSed much, or at all

Peer-to-peer friendly

Packet switched instead of circuit switched implicit transparent load balancing of messages across multiple peers, rather than a single path resilience vs. failures by running multiple tunnels in parallel, plus rotating tunnels scale each client's connections at O(1) instead of O(N) (Alice has e.g. 2 inbound tunnels that are used by all of the peers Alice is talking with, rather than a circuit for each)

Unidirectional tunnels instead of bidirectional circuits, doubling the number of nodes a peer has to compromise to get the same information. Counter-arguments and further discussion here.

Protection against detecting client activity, even when an attacker is participating in the tunnel, as tunnels are used for more than simply passing end to end messages (e.g. netDb, tunnel management, tunnel testing)

Tunnels in I2P are short lived, decreasing the number of samples that an attacker can use to mount an active attack with, unlike circuits in Tor, which are typically long lived.

I2P APIs are designed specifically for anonymity and security, while SOCKS is designed for functionality.

Essentially all peers participate in routing for others

The bandwidth overhead of being a full peer is low, while in Tor, while client nodes don't require much bandwidth, they don't fully participate in the mixnet.

Integrated automatic update mechanism

Both TCP and UDP transports

Java, not C (ewww)

Other potential benefits of I2P but not yet implemented

...and may never be implemented, so don't count on them!