The I2P Client Protocol (I2CP) exposes a strong separation of concerns between the router and any client that wishes to communicate over the network. It enables secure and asynchronous messaging by sending and receiving messages over a single TCP socket. With I2CP, a client application tells the router who they are (their "destination"), what anonymity, reliability, and latency tradeoffs to make, and where to send messages. In turn the router uses I2CP to tell the client when any messages have arrived, and to request authorization for some tunnels to be used.

The protocol itself is implemented in Java, to provide the Client SDK. This SDK is exposed in the i2p.jar package, which implements the client-side of I2CP. Clients should never need to access the router.jar package, which contains the router itself and the router-side of I2CP. There is also a C library implementation. A non-Java client would also have to implement the streaming library for TCP-style connections.

Applications can take advantage of the base I2CP plus the streaming and datagram libraries by using the Simple Anonymous Messaging or BOB protocols, which do not require clients to deal with any sort of cryptography. Also, clients may access the network by one of several proxies - HTTP, CONNECT, and SOCKS 4/4a/5. Alternatively, Java clients may access those libraries in ministreaming.jar and streaming.jar. So there are several options for both Java and non-Java applications.

Client-side end-to-end encryption (encrypting the data over the I2CP connection) was disabled in I2P release 0.6, leaving in place the ElGamal/AES end-to-end encryption which is implemented in the router. The only cryptography that client libraries must still implement is DSA public/private key signing for LeaseSets and Session Configurations, and management of those keys.

In a standard I2P installation, port 7654 is used by external java clients to communicate with the local router via I2CP. By default, the router binds to address 127.0.0.1. To bind to 0.0.0.0, set the router advanced configuration option i2cp.tcp.bindAllInterfaces=true and restart. Clients in the same JVM as the router pass messages directly to the router through an internal JVM interface.

The router also supports external connections over SSL. While SSL is not the default, it is strongly recommended for any traffic that may be exposed to the open Internet. The authorization user/password (if any), the Private Key and Signing Private Key for the Destination are all transmitted in-the-clear unless SSL is enabled.

I2CP Protocol Specification

Now on the I2CP Specification page.

I2CP Initialization

When a client connects to the router, it first sends a single protocol version byte (0x2A). Then it sends a GetDate Message and waits for the SetDate Message response. Next, it sends a CreateSession Message containing the session configuration. It next awaits a RequestLeaseSet Message from the router, indicating that inbound tunnels have been built, and responds with a CreateLeaseSetMessage containing the signed LeaseSet. The client may now initiate or receive connections from other I2P destinations.

I2CP Options

Router-side Options

The following options are traditionally passed to the router via a SessionConfig contained in a CreateSession Message or a ReconfigureSession Message.

Note: Large quantity, length, or variance settings may cause significant performance or reliability problems.

Note: As of release 0.7.7, option names and values must use UTF-8 encoding. This is primarily useful for nicknames. Prior to that release, options with multi-byte characters were corrupted. Since options are encoded in a Mapping, all option names and values are limited to 255 bytes (not characters) maximum.

Client-side Options

The following options are interpreted on the client side, and will be interpreted if passed to the I2PSession via the I2PClient.createSession() call. The streaming lib should also pass these options through to I2CP. Other implementations may have different defaults.

Note: All arguments, including numbers, are strings. True/false values are case-insensitive strings. Anything other than case-insensitive "true" is interpreted as false. All option names are case-sensitive.

I2CP Payload Data Format and Multiplexing

The end-to-end messages handled by I2CP (i.e. the data sent by the client in a SendMessageMessage and received by the client in a MessagePayloadMessage) are gzipped with a standard 10-byte gzip header beginning with 0x1F 0x8B 0x08 as specified by RFC 1952. As of release 0.7.1, I2P uses ignored portions of the gzip header to include protocol, from-port, and to-port information, thus supporting streaming and datagrams on the same destination, and allowing query/response using datagrams to work reliably in the presence of multiple channels.

The gzip function cannot be completely turned off, however setting i2cp.gzip=false turns the gzip effort setting to 0, which may save a little CPU. Implementations may select different gzip efforts on a per-socket or per-message basis, depending on an assessment of the compressibility of the contents. Due to the compressibility of destination padding implemented in API 0.9.57 (proposal 161), compression of the streaming SYN packets in each direction, and of repliable datagrams, is recommended even if the payload is not compressible. Implementations may wish to write a trivial gzip/gunzip function for a gzip effort of 0, which will provide large efficiency gains over a gzip library for this case.

Note: I2P protocol numbers 224-254 are reserved for experimental protocols. I2P protocol number 255 is reserved for future expansion.

Data integrity is verified with the standard gzip CRC-32 as specified by RFC 1952.

Important Differences from Standard IP

I2CP ports are for I2P sockets and datagrams. They are unrelated to your local sockets or ports. Because I2P did not support ports and protocol numbers prior to release 0.7.1, ports and protocol numbers are somewhat different from that in standard IP, for backward compatibility:

• Port 0 is valid and has special meaning.
• Ports 1-1023 are not special or privileged.
• Servers listen on port 0 by default, which means "all ports".
• Clients send to port 0 by default, which means "any port".
• Clients send from port 0 by default, which means "unspecified".
• Servers may have a service listening on port 0 and other services listening on higher ports. If so, the port 0 service is the default, and will be connected to if the incoming socket or datagram port does not match another service.
• Most I2P destinations only have one service running on them, so you may use the defaults, and ignore I2CP port configuration.
• Protocol 0 is valid and means "any protocol". However, this is not recommended, and probably will not work. Streaming requires that the protocol number is set to 6.
• Streaming sockets are tracked by an internal connection ID. Therefore, there is no requirement that the 5-tuple of dest:port:dest:port:protocol be unique. For example, there may be multiple sockets with the same ports between two destinations. Clients do not need to pick a "free port" for an outbound connection.

Future Work

• The current authorization mechanism could be modified to use hashed passwords.
• The Signing Private Keys is included in the Create Lease Set message, it is not required. Revocation is unimplemented. It should be replaced with random data or removed.
• Some improvements may be able to use messages previously defined but not implemented. For reference, here is the I2CP Protocol Specification Version 0.9 (PDF) dated August 28, 2003. That document also references the Common Data Structures Specification Version 0.9.

See Also