i2p.www/www.i2p2/pages/i2cp.html

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Updated September 2010, current as of router version 0.8

<p>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, yet never exposing any private keys and authenticating itself
to the router only through signatures.  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.
</p>

<p>
The protocol itself has only been implemented in Java, to provide the
<a href="package-client.html">Client SDK</a>.  
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.
</p>

<p>
While implementing the client side of I2CP in a non-Java language is certainly feasible,
a non-Java client would also have to implement the
<a href="streaming.html">streaming library</a> for TCP-style connections.
Together, implementing I2CP and the streaming library would be a sizable task.
</p>

<p>
Applications can take advantage of the base I2CP plus the 
<a href="ministreaming">streaming</a> and <a href="datagrams">datagram</a> libraries
by using the <a href="sam">Simple Anonymous Messaging</a> or <a href="bob.html">BOB</a> 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.
</p>

<p>I2CP end-to-end encryption was disabled in I2P release 0.6,
leaving in place the end-to-end garlic encryption.
However, client libraries must still implement public/private key signing
for LeaseSets, and key management.
</p>

<p>In a standard I2P installation, port 7654 is used by 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 <tt>i2cp.tcp.bindAllInterfaces=true</tt> and restart.
</p>

<h2>I2CP Protocol Specification</h2>
<p>
Now on the
<a href="i2cp_spec.html">I2CP Specification page</a>.
</p>


<h2>I2CP Initialization</h2>
<p>
When a client connects to the router, it first sends a single protocol version byte (0x2A).
Then it sends a GetDateMessage and waits for the SetDateMessage response.
Next, it sends a CreateSessionMessage containing the session configuration.
It next awaits a RequestLeaseSetMessage 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.

<h2 id="options">I2CP Options</h2>
<p>
The following options are traditionally passed to the router via
a SessionConfig contained in a CreateSessionMessage or a ReconfigureSessionMessage.
<p>
<table border=1>
<tr><th>Option <th>Recommended Arguments <th>Allowable Range<th>Default<th>Description
<tr><td>inbound.quantity <td>number from 1 to 3 <td>1 to 6<td>2<td>Number of tunnels in
<tr><td>outbound.quantity <td>number from 1 to 3 <td>No limit<td>2<td>Number of tunnels out
<tr><td>inbound.length <td>number from 0 to 3 <td>0 to 7<td>2<td>Length of  tunnels in
<tr><td>outbound.length <td>number from 0 to 3 <td>0 to 7<td>2<td>Length of tunnels out
<tr><td>inbound.lengthVariance <td>number from -1 to 2 <td>-7 to 7<td>0<td>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.
<tr><td>outbound.lengthVariance <td>number from -1 to 2 <td>-7 to 7<td>0<td>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.
<tr><td>inbound.backupQuantity <td>number from 0 to 3 <td>No limit<td>0<td>Number of redundant fail-over for tunnels in
<tr><td>outbound.backupQuantity <td>number from 0 to 3 <td>No limit<td>0<td>Number of redundant fail-over for tunnels out
<tr><td>inbound.nickname <td>string<td>&nbsp;<td>&nbsp;<td>Name of tunnel - generally used in routerconsole, which will
     use the first few characters of the Base64 hash of the destination by default.
<tr><td>outbound.nickname <td>string<td>&nbsp;<td>&nbsp;<td>Name of tunnel - generally ignored unless inbound.nickname is unset.
<tr><td>inbound.allowZeroHop <td>true, false<td>&nbsp;<td>true<td>If incoming zero hop tunnel is allowed
<tr><td>outbound.allowZeroHop <td>true, false<td>&nbsp;<td>true<td>If outgoing zero hop tunnel is allowed
<tr><td>inbound.IPRestriction <td>number from 0 to 4 <td>0 to 4<td>2<td>Number of IP bytes to match to determine if
     two routers should not be in the same tunnel. 0 to disable.
<tr><td>outbound.IPRestriction <td>number from 0 to 4 <td>0 to 4<td>2<td>Number of IP bytes to match to determine if
     two routers should not be in the same tunnel. 0 to disable.
<tr><td>i2cp.dontPublishLeaseSet <td>true, false<td>&nbsp;<td>false<td>Should generally be set to true for clients
     and false for servers
<tr><td>i2cp.messageReliability <td>&nbsp;<td>BestEffort, Guaranteed<td>BestEffort<td>Guaranteed is disabled
<tr><td>explicitPeers<td>&nbsp;<td>&nbsp;<td>null<td>Comma-separated list of Base 64 Hashes of peers to build tunnels through; for debugging only
</table>
<p>
Note: Large quantity, length, or variance settings may cause significant performance or reliability problems.
<p>
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.

<p>
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.
<p>
<table border=1>
<tr><th>Option <th>As Of Release<th>Recommended Arguments <th>Allowable Range<th>Default<th>Description
<tr><td>i2cp.tcp.host <td>&nbsp;<td>&nbsp;<td>&nbsp;<td>localhost<td>Router hostname
<tr><td>i2cp.tcp.port <td>&nbsp;<td>&nbsp;<td>1-65535<td>7654<td>Router I2CP port
<tr><td>i2cp.gzip<td>0.6.5<td>true, false <td>&nbsp;<td>true<td>Gzip outbound data
<tr><td>i2cp.reduceOnIdle<td>0.7.1<td>true, false <td>&nbsp;<td>false<td>Reduce tunnel quantity when idle
<tr><td>i2cp.closeOnIdle<td>0.7.1<td>true, false <td>&nbsp;<td>false<td>Close I2P session when idle
<tr><td>i2cp.reduceIdleTime<td>0.7.1<td>1200000<td>300000 minimum<td>&nbsp;<td>(ms) Idle time required (default 20 minutes, minimum 5 minutes)
<tr><td>i2cp.closeIdleTime<td>0.7.1<td>1800000<td>300000 minimum<td>&nbsp;<td>(ms) Idle time required (default 30 minutes)
<tr><td>i2cp.reduceQuantity<td>0.7.1<td>1<td>1 to 5<td>1<td>Tunnel quantity when reduced (applies to both inbound and outbound)
<tr><td>i2cp.encryptLeaseSet<td>0.7.1<td>true, false <td>&nbsp;<td>false<td>Encrypt the lease
<tr><td>i2cp.leaseSetKey<td>0.7.1<td>&nbsp;<td>&nbsp;<td>&nbsp;<td>Base64 SessionKey (44 characters)
</table>
<p>
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.

<h2 id="format">I2CP Payload Data Format and Multiplexing</h2>
<p>
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) is gzipped with a standard 10-byte gzip
header beginning with 0x1F 0x8B 0x08 as
specified by <a href="http://www.ietf.org/rfc/rfc1952.txt">RFC 1952</a>.
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.
<p>
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.
<p>
<table border=1>
<tr><th>Bytes<th>Content
<tr><td>0-2<td>Gzip header 0x1F 0x8B 0x08
<tr><td>3<td>Gzip flags
<tr><td>4-5<td>I2P Source port (Gzip mtime)
<tr><td>6-7<td>I2P Destination port (Gzip mtime)
<tr><td>8<td>Gzip xflags
<tr><td>9<td>I2P Protocol (6 = Streaming, 17 = Datagram) (Gzip OS)
</table>

<p>
Data integrity is verified with the standard gzip CRC-32 as
specified by <a href="http://www.ietf.org/rfc/rfc1952.txt">RFC 1952</a>.
</p>


<h2 id="future">Future Work</h2>
<ul><li>
There is an in-JVM version of I2CP that uses a Java pipe instead of real sockets.
However, it still does message serialization and deserialization.
It should be refactored to pass the Message objects directly.
</li><li>
The protocol is highly inefficient, with three I2CP messages for every outgoing message,
and four I2CP messages for every incoming message.
Perhaps the acknowledgements and queueing could be eliminated, especially for the in-JVM version.
This would eliminate a lot of data structures and memory for holding
unacknowledged messages.
</li><li>
Implement i2cp.messageReliablity=none to disable the two Message Status Message responses
to a Send Message Message.
Make it the default? At least via the streaming lib.
</li><li>
The API does not support Dest Lookups or Get Bandwidth Limits Messages in a standard session,
only in a simple session.
This should be fixed.
</li><li>
The API does not support parallel Dest Lookups or Get Bandwidth Limits Messages in a session.
This should be fixed.
</li><li>
Implement I2CP and the streaming library in another programming language.
</li><li>
Is the initial Get Date / Set Date handshake required?
</li><li>
There is currently no authorization mechanism.
This would be required on the socket version only, not in the in-JVM version.
</li><li>
Private Keys are included in the Create Lease Set message,
are they really required? Revocation is unimplemented.
</li><li>
Some improvements may be able to use messages previously defined but not implemented.
For reference, here is the
<a href="/_static/pdf/I2CP_spec.pdf">I2CP Protocol Specification Version 0.9</a>
(PDF) dated August 28, 2003.
That document also references the
<a href="/_static/pdf/datastructures.pdf">Common Data Structures Specification Version 0.9</a>.
</li></ul>



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