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.
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.
|Option||As Of Release||Recommended Arguments||Allowable Range||Default||Description|
|clientMessageTimeout||8*1000 - 120*1000||60*1000||The timeout (ms) for all sent messages. Unused. See the protocol specification for per-message settings.|
|crypto.lowTagThreshold||0.9.2||1-128||30||Minimum number of ElGamal/AES Session Tags before we send more. Recommended: approximately tagsToSend * 2/3|
|crypto.tagsToSend||0.9.2||1-128||40||Number of ElGamal/AES Session Tags to send at a time. For clients with relatively low bandwidth per-client-pair (IRC, some UDP apps), this may be set lower.|
|explicitPeers||null||Comma-separated list of Base 64 Hashes of peers to build tunnels through; for debugging only|
|i2cp.dontPublishLeaseSet||true, false||false||Should generally be set to true for clients and false for servers|
|i2cp.fastReceive||0.9.4||true, false||false||If true, the router just sends the MessagePayload instead of sending a MessageStatus and awaiting a ReceiveMessageBegin.|
|i2cp.messageReliability||BestEffort, None||BestEffort||Guaranteed is disabled; None implemented in 0.8.1; the streaming lib default is None as of 0.8.1, the client side default is None as of 0.9.4|
|i2cp.password||0.8.2||string||For authorization, if required by the router. If the client is running in the same JVM as a router, this option is not required. Warning - username and password are sent in the clear to the router, unless using SSL (i2cp.SSL=true). Authorization is only recommended when using SSL.|
|inbound.allowZeroHop||true, false||true||If incoming zero hop tunnel is allowed|
|outbound.allowZeroHop||true, false||true||If outgoing zero hop tunnel is allowed|
|inbound.backupQuantity||number from 0 to 3||No limit||0||Number of redundant fail-over for tunnels in|
|outbound.backupQuantity||number from 0 to 3||No limit||0||Number of redundant fail-over for tunnels out|
|inbound.IPRestriction||number from 0 to 4||0 to 4||2||Number of IP bytes to match to determine if two routers should not be in the same tunnel. 0 to disable.|
|outbound.IPRestriction||number from 0 to 4||0 to 4||2||Number of IP bytes to match to determine if two routers should not be in the same tunnel. 0 to disable.|
|inbound.length||number from 0 to 3||0 to 7||3||Length of tunnels in|
|outbound.length||number from 0 to 3||0 to 7||3||Length of tunnels out|
|inbound.lengthVariance||number from -1 to 2||-7 to 7||0||Random amount to add or subtract to the length of tunnels in. A positive number x means add a random amount from 0 to x inclusive. A negative number -x means add a random amount from -x to x inclusive. The router will limit the total length of the tunnel to 0 to 7 inclusive. The default variance was 1 prior to release 0.7.6.|
|outbound.lengthVariance||number from -1 to 2||-7 to 7||0||Random amount to add or subtract to the length of tunnels out. A positive number x means add a random amount from 0 to x inclusive. A negative number -x means add a random amount from -x to x inclusive. The router will limit the total length of the tunnel to 0 to 7 inclusive. The default variance was 1 prior to release 0.7.6.|
|inbound.nickname||string||Name of tunnel - generally used in routerconsole, which will use the first few characters of the Base64 hash of the destination by default.|
|outbound.nickname||string||Name of tunnel - generally ignored unless inbound.nickname is unset.|
|outbound.priority||0.9.4||number from -25 to 25||-25 to 25||0||Priority adjustment for outbound messages. Higher is higher priority.|
|inbound.quantity||number from 1 to 3||1 to 16||2||Number of tunnels in. Limit was increased from 6 to 16 in release 0.9; however, numbers higher than 6 are incompatible with older releases.|
|outbound.quantity||number from 1 to 3||No limit||2||Number of tunnels out|
|inbound.randomKey||0.9.17||Base 64 encoding of 32 random bytes||Used for consistent peer ordering across restarts.|
|outbound.randomKey||0.9.17||Base 64 encoding of 32 random bytes|
|inbound.*||Any other options prefixed with "inbound." are stored in the "unknown options" properties of the inbound tunnel pool's settings.|
|outbound.*||Any other options prefixed with "outbound." are stored in the "unknown options" properties of the outbound tunnel pool's settings.|
|shouldBundleReplyInfo||0.9.2||true, false||true||Set to false to disable ever bundling a reply LeaseSet.
For clients that do not publish their LeaseSet, this option must be true
for any reply to be possible. "true" is also recommended for multihomed servers
with long connection times.
Setting to "false" may save significant outbound bandwidth, especially if the client is configured with a large number of inbound tunnels (Leases). If replies are still required, this may shift the bandwidth burden to the far-end client and the floodfill. There are several cases where "false" may be appropriate:
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.
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.
|Option||As Of Release||Recommended Arguments||Allowable Range||Default||Description|
|i2cp.closeIdleTime||0.7.1||1800000||300000 minimum||(ms) Idle time required (default 30 minutes)|
|i2cp.closeOnIdle||0.7.1||true, false||false||Close I2P session when idle|
|i2cp.encryptLeaseSet||0.7.1||true, false||false||Encrypt the lease|
|i2cp.fastReceive||0.9.4||true, false||true||If true, the router just sends the MessagePayload instead of sending a MessageStatus and awaiting a ReceiveMessageBegin.|
|i2cp.gzip||0.6.5||true, false||true||Gzip outbound data|
|i2cp.leaseSetKey||0.7.1||For encrypted leasesets. Base 64 SessionKey (44 characters)|
|i2cp.leaseSetPrivateKey||0.9.18||Base 64 private key for encryption. Optionally preceded by the key type and ':'. Only "ELGAMAL_2048:" is supported, which is the default. I2CP will generate the public key from the private key. Use for persistent leaseset keys across restarts.|
|i2cp.leaseSetSigningPrivateKey||0.9.18||Base 64 private key for signatures. Optionally preceded by the key type and ':'. DSA_SHA1 is the default. Key type must match the signature type in the destination. I2CP will generate the public key from the private key. Use for persistent leaseset keys across restarts.|
|i2cp.messageReliability||BestEffort, None||None||Guaranteed is disabled; None implemented in 0.8.1; None is the default as of 0.9.4|
|i2cp.reduceIdleTime||0.7.1||1200000||300000 minimum||(ms) Idle time required (default 20 minutes, minimum 5 minutes)|
|i2cp.reduceOnIdle||0.7.1||true, false||false||Reduce tunnel quantity when idle|
|i2cp.reduceQuantity||0.7.1||1||1 to 5||1||Tunnel quantity when reduced (applies to both inbound and outbound)|
|i2cp.SSL||0.8.3||true, false||false||Connect to the router using SSL. If the client is running in the same JVM as a router, this option is ignored, and the client connects to that router internally.|
|i2cp.tcp.host||127.0.0.1||Router hostname. If the client is running in the same JVM as a router, this option is ignored, and the client connects to that router internally.|
|i2cp.tcp.port||1-65535||7654||Router I2CP port. If the client is running in the same JVM as a router, this option is ignored, and the client connects to that router internally.|
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.
|0-2||Gzip header 0x1F 0x8B 0x08|
|4-5||I2P Source port (Gzip mtime)|
|6-7||I2P Destination port (Gzip mtime)|
|9||I2P Protocol (6 = Streaming, 17 = Datagram, 18 = Raw Datagrams) (Gzip OS)|
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.
- 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.