how_intro.html 11.10 KiB
{% extends "_layout.html" %}
{% block title %}Introduction to How I2P Works{% endblock %}
{% block content %}<i>Note: the "how" documents have not been fully updated to include several changes
including the new
<a href="tunnel-alt.html">tunnel
routing and encryption</a> algorithms, addressing <a href="todo#tunnelId">several</a>
<a href="todo#tunnelLength">issues</a> (with the groundwork for addressing
<a href="todo#ordering">others</a>), and other changes.</i>
<p>I2P is an effort to build, deploy, and maintain a network to support secure and anonymous
communication. People using I2P are in control of the tradeoffs between anonymity, reliability,
bandwidth usage, and latency. There is no central point in the network on which pressure can be
exerted to compromise the integrity, security, or anonymity of the system. The network supports
dynamic reconfiguration in response to various attacks, and has been designed to make use of
additional resources as they become available. Of course, all aspects of the network are open
and freely available.</p>
<p>Unlike many other anonymizing networks, I2P doesn't try to provide anonymity by hiding the
originator of some communication and not the recipient, or the other way around. I2P is designed
to allow peers using I2P to communicate with each other anonymously — both sender and recipient
are unidentifiable to each other as well as to third parties. For example, today there are both
in-I2P web sites (allowing anonymous publishing / hosting) as well as HTTP proxies to the normal
web (allowing anonymous web browsing). Having the ability to run servers within I2P is essential,
as it is quite likely that any outbound proxies to the normal Internet will be monitored, disabled,
or even taken over to attempt more malicious attacks.</p>
<p>The network itself is message oriented - it is essentially a secure and anonymous IP layer,
where messages are addressed to cryptographic keys (Destinations) and can be significantly larger
than IP packets. Some example uses of the network include "eepsites" (webservers hosting normal web
applications within I2P), a <a href="http://bitconjurer.org/BitTorrent/">BitTorrent</a> port ("I2PSnark"),
or a distributed data store. With the help of mihi's <a href="i2ptunnel">I2PTunnel</a> application,
we are able to stream traditional TCP/IP applications over I2P, such as SSH, IRC, a squid proxy, and
even streaming audio. Most people will not use I2P directly, or even need to know they're using it.
Instead their view will be of one of the I2P enabled applications, or perhaps as a little controller
app to turn on and off various proxies to enable the anonymizing functionality.</p>
<p>An essential part of designing, developing, and testing an anonymizing network is to define the
<a href="how_threatmodel">threat model</a>, since there is no such thing as "true" anonymity, just
increasingly expensive costs to identify someone. Briefly, I2P's intent is to allow people to communicate
in arbitrarily hostile environments by providing good anonymity, mixed in with sufficient cover
traffic provided by the activity of people who require less anonymity. This way, some users can avoid
detection by a very powerful adversary, while others will try to evade a weaker entity,
<i>all on the same network</i>, where each one's messages are essentially indistinguishable from the
others.</p>
<h2>Why?</h2>
<p>There are a multitude of reasons why we need a system to support
anonymous communication, and everyone has their own personal rationale. There are many
<a href="how_networkcomparisons">other efforts</a> working on finding ways to provide varying degrees of
anonymity to people through the Internet, but we could not find any that met our needs or threat
model.</p>
<h2>How?</h2>
<p>The network at a glance is made up of a set of nodes ("routers") with a number of unidirectional
inbound and outbound virtual paths ("tunnels", as outlined on the <a href="how_tunnelrouting">tunnel routing</a> page).
Each router is identified by a cryptographic RouterIdentity which is typically long lived. These routers
communicate with each other through existing transport mechanisms (TCP, UDP, etc), passing various
messages. Client applications have their own cryptographic identifier ("Destination") which enables it
to send and receive messages. These clients can connect to any router and authorize the temporary
allocation ("lease") of some tunnels that will be used for sending and receiving messages through the
network. I2P has its own internal <a href="how_networkdatabase">network database</a> (using a modification of
the Kademlia algorithm) for scalable distributing routing and contact information securely.</p>
<p><center><div class="box"><img src="_static/images/net.png" alt="Network topology example" title="Network topology example" /></div></center></p><br>
<p>In the above, Alice, Bob, Charlie, and Dave are all running routers with a single Destination on their
local router. They each have a pair of 2-hop inbound tunnels per destination (labeled 1,2,3,4,5 and 6),
and a small subset of each of those router's outbound tunnel pool is shown with 2-hop outbound tunnels.For simplicity, Charlie's inbound tunnels and Dave's outbound tunnels are not shown, nor are the rest of
each router's outbound tunnel pool (typically stocked with 5-10 tunnels at a time). When Alice and Bob
talk to each other, Alice sends a message out one of her (pink) outbound tunnels targeting one of Bob's
(green) inbound tunnels (tunnel 3 or 4). She knows to send to those tunnels on the correct router by querying the
network database, which is constantly updated as new leases are authorized and old ones expire.</p>
<p>If Bob wants to reply to Alice, he simply goes through the same process - send a message out one of his
outbound tunnels targeting one of Alice's inbound tunnels (tunnel 1 or 2). To make things easier, most
messages sent between Alice and Bob are <a href="how_garlicrouting">garlic</a> wrapped, bundling the
sender's own current lease information so that the recipient can reply immediately without having to look
in the network database for the current data.</p>
<p>To deal with a wide range of attacks, I2P is fully distributed with no centralized resources - and
hence there are no directory servers keeping statistics regarding the performance and reliability of
routers within the network. As such, each router must keep and maintain profiles of various routers
and is responsible for selecting appropriate peers to meet the anonymity, performance, and reliability
needs of the users, as described in the <a href="how_peerselection">peer selection</a> page.</p>
<p>The network itself makes use of a significant number of <a href="how_cryptography">cryptographic techniques and algorithms</a> -
a full laundry list includes 2048bit ElGamal encryption, 256bit AES in CBC mode with PKCS#5 padding,
1024bit DSA signatures, SHA256 hashes, 2048bit Diffie-Hellman negotiated connections with station to
station authentication, and <a href="how_elgamalaes">ElGamal / AES+SessionTag</a>.</p>
<p>Content sent over I2P is encrypted through three layers garlic encryption (used to verify the delivery of the message to
the recipient), tunnel encryption (all messages passing through a tunnel is encrypted by the tunnel
gateway to the tunnel endpoint), and inter router transport layer encryption (e.g. the TCP transport
uses AES256 with ephemeral keys):</p>
<p>End-to-end (I2CP) encryption (client application to server application) was disabled in I2P release 0.6;
end-to-end (garlic) encryption (I2P client router to I2P server router) from Alice's router "a" to Bob's router "h" remains.
Notice the different use of terms! All data from a to h is end-to-end encrypted, but the I2CP connection
between the I2P router and the applications is not end-to-end encrypted!
A and h are the routers of Alice and Bob, while Alice and Bob in following chart are the applications running atop of I2P.</p>
<center><div class="box"><img src="_static/images/endToEndEncryption.png" alt="End to end layered encryption" title="End to end layered encryption." /></div></center><br>
<p>The specific use of these algorithms are outlined <a href="how_cryptography">elsewhere</a>.</p>
<p>The two main mechanisms for allowing people who need strong anonymity to use the network are
explicitly delayed garlic routed messages and more comprehensive tunnels to include support for pooling
and mixing messages. These are currently planned for release 3.0, but garlic routed messages with no
delays and FIFO tunnels are currently in place. Additionally, the 2.0 release will allow people to set
up and operate behind restricted routes (perhaps with trusted peers), as well as the deployment of more
flexible and anonymous transports.</p>
<p>Some questions have been raised with regards to the scalability of I2P, and reasonably so. There
will certainly be more analysis over time, but peer lookup and integration should be bounded by
<code>O(log(N))</code> due to the <a href="how_networkdatabase">network database</a>'s algorithm, while end to end
messages should be <code>O(1)</code> (scale free), since messages go out K hops through the outbound
tunnel and another K hops through the inbound tunnel, with K no longer than 3.
The size of the network (N) bears no impact.</p>
<h2>When?</h2>
<p>I2P initially began in Feb 2003 as a proposed modification to
<a href="http://freenetproject.org">Freenet</a> to allow it to use alternate transports, such as
<a href="http://java.sun.com/products/jms/index.jsp">JMS</a>, then grew into its own as an
'anonCommFramework' in April 2003, turning into I2P in July, with code being cut in earnest in August '03,
reaching the 0.2 release in September, 0.3 in March '04, and 0.4 in September '04.
Release 0.5 followed in early '05 and 0.6 in mid-'05.
I2P is currently moving forward according to
the <a href="roadmap">roadmap</a>.</p>
<h2>Who?</h2>
<p>We have a small <a href="team">team</a> spread around several continents, working to advance different
aspects of the project. We are very open to other developers who want to get involved and anyone else
who would like to contribute in other ways, such as critiques, peer review, testing, writing I2P enabled
applications, or documentation. The entire system is open source - the router and most of the SDK are
outright public domain with some BSD and Cryptix licensed code, while some applications like I2PTunnel
and I2PSnark are GPL. Almost everything is written in Java (1.5+), though some third party applications
are being written in Python. The code works on <a href="http://java.com/en/">Sun Java SE</a>, on the current <a href="http://www.kaffe.org/">Kaffe</a>, and
we are hoping to get it working on <a href="http://gcc.gnu.org/java/">GCJ</a> sooner rather than later.</p>
<h2>Where?</h2>
<p>Anyone interested should
join us on the IRC channel #i2p (hosted concurrently on irc.freenode.net, irc.postman.i2p, irc.freshcoffee.i2p, irc.welterde.i2p and irc.einirc.de).
There are currently no scheduled development meetings, however
<a href="meetings">archives are available</a>.</p>
<p>The current source is available in <a href="monotone.html">monotone</a>.</p>
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