The Invisible Internet Client Protocol (I2CP) allows applications simplified access to the I2P network without requiring them to deal with the issues involved with the Invisible Internet Network Protocol (I2NP). Specifically, it defines the wire level protocol as well as semantics for the messages passed between clients and the router for communicating with the router over bidirectional TCP/IP sockets. I2CP does not specify how the client libraries are created, what APIs they expose to applications, or even what language they're written in.

## Requirements

I2CP requires that the client can access the router over bidirectional TCP sockets. In nearly all cases, these connections will be over to the local machine.

## Threat Model

I2CP does not provide data confidentiality or handle lossy connections beyond what TCP has built in. For this reason, I2CP really should only be run between clients and routers on the same machine or over trusted networks. Secured I2CP transports may be added in the future. I2CP however does not expose any private keys across the wire - proof of authorization to collect messages for a Destination is provided by public key signatures.

## Messages

These classes are the currently defined messages in the Invisible Internet Client Protocol (I2CP), though common data structures are located in the I2P Common Data Structure Specification. For simplicity, all I2CP messages begin with the same structure, though that structure is not listed below. Specifically, all I2CP messages transmitted begin with a 4 byte Integer specifying the entire size of the current message's body (the body being what's specified below), followed by a 1 byte Integer specifying the type of message (the id field below), after which the rest of the message is formatted according to the type of message, as specified below.

If there is a fatal error causing either the client or the router to desire to cancel sending a message part way through, it should drop its connection. Administrative sessions are not stateful (aka each administrative message from client to router must provide authentication), but normal client sessions are stateful and survive disconnects. Client sessions expire when either the client sends a DestroySessionMessage or the router times out the session according to its own configurable timer. If a client sends any message other than a CreateSessionMessage when there is no valid session, the router must reply with a SessionStatusMessage specifying that the session is not valid.

Note: the contents and bitbuckets for specific DataStructures are detailed in the I2P Data Structures Spec.