i2p.www/www.i2p2/pages/techintro.html

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{% block title %}Introducing I2P{% endblock %}
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  <center>
    <b class="title"> 
    <h1>I2P: <font size="3">A scalable framework for anonymous communication</font></h1>
    </b> 
    <br/>
    <div class="toc"> 
      <table border="0">
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          <td valign="top" align="left" class="invisible"> <pre>
<font size="2"><b>IN THIS DOCUMENT</b></font><br />
* <a href="#intro">Introduction</a>
* <a href="#op">Operation</a>
  * <a href="#op.overview">Overview</a>
  * <a href="#op.tunnels">Tunnels</a>
  * <a href="#op.netdb">Network Database</a>
  * <a href="#op.transport">Transport protocols</a>
  * <a href="#op.crypto">Cryptography</a>
</pre></td>
          <td valign="top" align="left" class="invisible"> <pre>
<br/>
* <a href="#future">Future</a>
  * <a href="#future.restricted">Restricted routes</a>
  * <a href="#future.variablelatency">Variable latency</a>
  * <a href="#future.open">Open questions</a>
</pre></td>
          <td valign="top" align="left" class="invisible"> <pre>
<br/>
* <a href="#similar">Similar systems</a>
  * <a href="#similar.tor">Tor</a>
  * <a href="#similar.freenet">Freenet</a>
* <a href="#app">Appendix A: Application layer</a>
</pre></td>
        </tr>
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          <td height="10" colspan="3" align="left" valign="middle" class="invisible"> 
            <div class="underline"></div></td>
        </tr>
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          <td valign="top" align="left" class="invisible"><pre>
<font size="2"><b>FOR MORE INFORMATION</b></font><br>
  * <a href="package-client.html">Client Javadoc</a>
  * <a href="datagrams.html">Datagrams</a>
  * <a href="package-datagram.html">Datagram Javadoc</a>
  * <a href="_static/pdf/datastructures.pdf">Data Structures</a>
  * <a href="i2cp.html">I2CP</a>
  * <a href="i2np.html">I2NP</a>
  * <a href="i2ptunnel.html">I2PTunnel</a>
  * <a href="ministreaming.html">Ministreaming Lib</a>
  * <a href="how_networkdatabase.html">NetDb</a>
</pre></td>
          <td valign="top" align="left" class="invisible"><pre><br/>
  * <a href="naming.html">Naming and Addressbook</a>
  * <a href="package-naming.html">Naming Javadoc</a>
  * <a href="jbigi.html">Native BigInteger Lib</a>
  * <a href="ntcp.html">NTCP</a>
  * <a href="how_peerselection.html">Peer Selection</a>
  * <a href="performance.html">Performance</a>
  * <a href="protocols.html">Protocol Stack</a>
  * <a href="sam.html">SAM</a>
  * <a href="samv2.html">SAM V2</a>
  * <a href="samv3.html">SAM V3</a>
  * <a href="bob.html">BOB</a>
</pre></td>
          <td valign="top" align="left" class="invisible"><pre><br/>
  * <a href="socks.html">SOCKS</a>
  * <a href="udp.html">SSU</a>
  * <a href="streaming.html">Streaming Lib</a>
  * <a href="package-streaming.html">Streaming Javadoc</a>
  * <a href="http://syndie.i2p2.de/">Syndie</a>
  * <a href="package-tcp.html">Old TCP Javadoc</a>
  * <a href="tunnel-alt.html">Tunnel Implementation</a>
  * <a href="tunnel-alt-creation.html">Tunnel Creation</a>
  * <a href="tunnel.html">Old Tunnel Implementation</a>
</pre></td>
        </tr>
      </table>
    </div>
  </center>
  <br/>
  <h1 id="intro">Introduction</h1>
  <p>
    I2P is a scalable, self organizing, resilient packet switched anonymous 
    network layer, upon which any number of different anonymity or security conscious 
    applications can operate. Each of these applications may make their own anonymity, 
    latency, and throughput tradeoffs without worrying about the proper implementation 
    of a free route mixnet, allowing them to blend their activity with the larger 
    anonymity set of users already running on top of I2P.
  </p>
  <p>
    Applications available already provide the full range of typical Internet activities -
    <b>anonymous</b> web browsing, web hosting, chat, file sharing, e-mail,
    blogging and content syndication, newsgroups, as well as several other applications under development.
    <ul>
      <li>Web browsing: using any existing browser that supports using a proxy.</li>
      <li>Chat: IRC, Jabber, <a href="#app.i2pmessenger">I2P-messenger</a></li>
      <li>File sharing:
        <ul>
          <li><a href="#app.i2psnark">I2PSnark</a></li>
          <li><a href="#app.robert">Robert</a></li>
          <li><a href="#app.i2phex">I2Phex</a></li>
          <li><a href="#app.pybit">PyBit</a></li>
          <li><a href="#app.i2pbt">I2P-bt</a></li>
        </ul>
      </li>
      <li>E-mail: <a href="#app.i2pmail">susimail</a> and <a href="#app.i2pbote">I2P-Bote</a></li>
      <li>Using any newsgroup reader that supports using a proxy.</li>
    </ul>
  <p>
    Unlike web sites hosted within content distribution networks like <a href="#similar.freenet">Freenet</a> 
    or <a href="http://www.ovmj.org/GNUnet/">GNUnet</a>, the services hosted on 
    I2P are fully interactive - there are traditional web-style search engines, 
    bulletin boards, blogs you can comment on, database driven sites, and bridges 
    to query static systems like Freenet without needing to install it locally. 
  </p>
  <p>
    With all of these anonymity enabled applications, I2P takes on the role 
    of the message oriented middleware - applications say that they want to send 
    some data to a cryptographic identifier (a "destination") and I2P takes care 
    of making sure it gets there securely and anonymously. I2P also bundles a 
    simple <a href="#app.streaming">streaming</a> library to allow I2P's anonymous 
    best-effort messages to transfer as reliable, in-order streams, transparently 
    offering a TCP based congestion control algorithm tuned for the high bandwidth 
    delay product of the network. While there have been several simple SOCKS proxies 
    available to tie existing applications into the network, their value has been 
    limited as nearly every application routinely exposes what, in an anonymous 
    context, is sensitive information. The only safe way to go is to fully audit 
    an application to ensure proper operation, and to assist in that we provide 
    a series of APIs in various languages which can be used to make the most out 
    of the network.
  </p>
  <p> I2P is not a research project - academic, commercial, or governmental, but 
    is instead an engineering effort aimed at doing whatever is necessary to provide 
    a sufficient level of anonymity to those who need it. It has been in active 
    development since early 2003 with one full time developer and a dedicated 
    group of part time contributors from all over the world. All of the work done 
    on I2P is open source and freely available on the <a href="index.html">website</a>, 
    with the majority of the code released outright into the public domain, though 
    making use of a few cryptographic routines under BSD-style licenses. The people 
    working on I2P do not control what people release client applications under, 
    and there are several GPL'ed applications available (<a href="#app.i2ptunnel">I2PTunnel</a>, 
    <a href="#app.i2pmail">susimail</a>, <a href="#app.i2psnark">I2PSnark</a>, 
    <a href="#app.i2phex">I2Phex</a>).
    <a href="http://www.i2p.net/halloffame">Funding</a> for I2P comes entirely from donations,
    and does not receive any tax breaks in any jurisdiction at this time,
    as many of the developers are themselves anonymous.
  </p>
  <h1 id="op">Operation</h1>
  <h2 id="op.overview">Overview</h2>
  <p> To understand I2P's operation, it is essential to understand a few key concepts. 
    First, I2P makes a strict separation between the software participating in 
    the network (a "router") and the anonymous endpoints ("destinations") associated 
    with individual applications. The fact that someone is running I2P is not 
    usually a secret. What is hidden is information on what the user is doing, 
    if anything at all, as well as what router a particular destination is connected 
    to. End users will typically have several local destinations on their router 
    - for instance, one proxying in to IRC servers, another supporting the user's 
    anonymous webserver ("eepsite"), another for an I2Phex instance, another for 
    torrents, etc. </p>
  <p> Another critical concept to understand is the "tunnel" - a directed path 
    through an explicitly selected set of routers, making use of layered encryption 
    so that the messages sent in the tunnel's "gateway" appear entirely random 
    at each hop along the path until it reaches the tunnel's "endpoint". These 
    unidirectional tunnels can be seen as either "inbound" tunnels or "outbound" 
    tunnels, referring to whether they are bringing messages to the tunnel's creator 
    or away from them, respectively. The gateway of an inbound tunnel can receive 
    messages from any peer and will forward them down through the tunnel until 
    it reaches the (anonymous) endpoint (the creator). On the other hand, the 
    gateway of an outbound tunnel is the tunnel's creator, and messages sent through 
    that tunnel are encoded so that when they reach the outbound tunnel's endpoint, 
    that router has the instructions necessary to forward the message on to the 
    appropriate location. </p>
  <p> A third critical concept to understand is I2P's "network database" (or "netDb") 
    - a pair of algorithms used to share network metadata. The two types of metadata 
    carried are "routerInfo" and "leaseSets" - the routerInfo gives routers the 
    data necessary for contacting a particular router (their public keys, transport 
    addresses, etc), while the leaseSet gives routers the information necessary 
    for contacting a particular destination. Within each leaseSet, there are any 
    number of "leases", each of which specifies the gateway for one of that destination's 
    inbound tunnels as well as when that tunnel will expire. The leaseSet also 
    contains a pair of public keys which can be used for layered garlic encryption. 
  </p>
  <!--
<p>
I2P's operation can be understood by putting those three concepts together:
</p>

<p><img src="net.png"></p>
!-->
  <p> When Alice wants to send a message to Bob, she first does a lookup in the 
    netDb to find Bob's leaseSet, giving her his current inbound tunnel gateways. 
    She then picks one of her outbound tunnels and sends the message down it with 
    instructions for the outbound tunnel's endpoint to forward the message on 
    to one of Bob's inbound tunnel gateways. When the outbound tunnel endpoint 
    receives those instructions, it forwards the message as requested, and when 
    Bob's inbound tunnel gateway receives it, it is forwarded down the tunnel 
    to Bob's router. If Alice wants Bob to be able to reply to the message, she 
    needs to transmit her own destination explicitly as part of the message itself 
    (taken care of transparently in the <a href="#app.streaming">streaming</a> 
    library). Alice may also cut down on the response time by bundling her most 
    recent leaseSet with the message so that Bob doesn't need to do a netDb lookup 
    for it when he wants to reply, but this is optional. </p>
  <p> While the tunnels themselves have layered encryption to prevent unauthorized 
    disclosure to peers inside the network (as the transport layer itself does 
    to prevent unauthorized disclosure to peers outside the network), it is necessary 
    to add an additional end to end layer of encryption to hide the message from 
    the outbound tunnel endpoint and the inbound tunnel gateway. This "<a href="#op.garlic">garlic 
    encryption</a>" lets Alice's router wrap up multiple messages into a single 
    "garlic message", encrypted to a particular public key so that intermediary 
    peers cannot determine either how many messages are within the garlic, what 
    those messages say, or where those individual cloves are destined. For typical 
    end to end communication between Alice and Bob, the garlic will be encrypted 
    to the public key published in Bob's leaseSet, allowing the message to be 
    encrypted without giving out the public key to Bob's own router. </p>
  <p> Another important fact to keep in mind is that I2P is entirely message based 
    and that some messages may be lost along the way. Applications using I2P can 
    use the message oriented interfaces and take care of their own congestion 
    control and reliability needs, but most would be best served by reusing the 
    provided <a href="#app.streaming">streaming</a> library to view I2P as a streams 
    based network. </p>
  <h2 id="op.tunnels">Tunnels</h2>
  <p> Both inbound and outbound tunnels work along similar principles - the tunnel 
    gateway accumulates a number of tunnel messages, eventually preprocessing 
    them into something for tunnel delivery. Next, the gateway encrypts that preprocessed 
    data and forwards it to the first hop. That peer and subsequent tunnel participants 
    add on a layer of encryption after verifying that it isn't a duplicate before 
    forward it on to the next peer. Eventually, the message arrives at the endpoint 
    where the messages are split out again and forwarded on as requested. The 
    difference arises in what the tunnel's creator does - for inbound tunnels, 
    the creator is the endpoint and they simply decrypt all of the layers added, 
    while for outbound tunnels, the creator is the gateway and they pre-decrypt 
    all of the layers so that after all of the layers of per-hop encryption are 
    added, the message arrives in the clear at the tunnel endpoint. </p>
  <p> The choice of specific peers to pass on messages as well as their particular 
    ordering is important to understanding both I2P's anonymity and performance 
    characteristics. While the network database (below) has its own criteria for 
    picking what peers to query and store entries on, tunnels may use any peers 
    in the network in any order (and even any number of times) in a single tunnel. 
    If perfect latency and capacity data were globally known, selection and ordering 
    would be driven by the particular needs of the client in tandem with their 
    threat model. Unfortunately, latency and capacity data is not trivial to gather 
    anonymously, and depending upon untrusted peers to provide this information 
    has its own serious anonymity implications. </p>
  <p> From an anonymity perspective, the simplest technique would be to pick peers 
    randomly from the entire network, order them randomly, and use those peers 
    in that order for all eternity. From a performance perspective, the simplest 
    technique would be to pick the fastest peers with the necessary spare capacity, 
    spreading the load across different peers to handle transparent failover, 
    and to rebuild the tunnel whenever capacity information changes. While the 
    former is both brittle and inefficient, the later requires inaccessible information 
    and offers insufficient anonymity. I2P is instead working on offering a range 
    of peer selection strategies, coupled with anonymity aware measurement code 
    to organize the peers by their profiles. </p>
  <p> As a base, I2P is constantly profiling the peers with which it interacts 
    with by measuring their indirect behavior - for instance, when a peer responds 
    to a netDb lookup in 1.3 seconds, that round trip latency is recorded in the 
    profiles for all of the routers involved in the two tunnels (inbound and outbound) 
    through which the request and response passed, as well as the queried peer's 
    profile. Direct measurement, such as transport layer latency or congestion, 
    is not used as part of the profile, as it can be manipulated and associated 
    with the measuring router, exposing them to trivial attacks. While gathering 
    these profiles, a series of calculations are run on each to summarize its 
    performance - its latency, capacity to handle lots of activity, whether they 
    are currently overloaded, and how well integrated into the network they seem 
    to be. These calculations are then compared for active peers to organize the 
    routers into four tiers - fast and high capacity, high capacity, not failing, 
    and failing. The thresholds for those tiers are determined dynamically, and 
    while they currently use fairly simple algorithms, alternatives exist. </p>
  <p> Using this profile data, the simplest reasonable peer selection strategy 
    is to pick peers randomly from the top tier (fast and high capacity), and 
    this is currently deployed for client tunnels. Exploratory tunnels (used for 
    netDb and tunnel management) pick peers randomly from the not failing tier 
    (which includes routers in 'better' tiers as well), allowing the peer to sample 
    routers more widely, in effect optimizing the peer selection through randomized 
    hill climbing. These strategies alone do however leak information regarding 
    the peers in the router's top tier through predecessor and netDb harvesting 
    attacks. In turn, several alternatives exist which, while not balancing the 
    load as evenly, will address the attacks mounted by particular classes of 
    adversaries. </p>
  <p> By picking a random key and ordering the peers according to their XOR distance 
    from it, the information leaked is reduced in predecessor and harvesting attacks 
    according to the peers' failure rate and the tier's churn. Another simple 
    strategy for dealing with netDb harvesting attacks is to simply fix the inbound 
    tunnel gateway(s) yet randomize the peers further on in the tunnels. To deal 
    with predecessor attacks for adversaries which the client contacts, the outbound 
    tunnel endpoints would also remain fixed. The selection of which peer to fix 
    on the most exposed point would of course need to have a limit to the duration, 
    as all peers fail eventually, so it could either be reactively adjusted or 
    proactively avoided to mimic a measured mean time between failures of other 
    routers. These two strategies can in turn be combined, using a fixed exposed 
    peer and an XOR based ordering within the tunnels themselves. A more rigid 
    strategy would fix the exact peers and ordering of a potential tunnel, only 
    using individual peers if all of them agree to participate in the same way 
    each time. This varies from the XOR based ordering in that the predecessor 
    and successor of each peer is always the same, while the XOR only makes sure 
    their order doesn't change. </p>
  <p> As mentioned before, I2P currently (release 0.6.1.1) includes the tiered 
    random strategy above. Release 0.6.1.33 will contain the XOR-based ordering 
    strategy. Additional improvements may be included in the 0.6.2 release. A 
    more detailed discussion of the mechanics involved in tunnel operation, management, 
    and peer selection can be found in the <a href="tunnel-alt.html">tunnel spec</a>. 
  </p>
  <h2 id="op.netdb">Network Database</h2>
  <p> <i>Kademlia has been disabled - see <a href="how_networkdatabase.html">NetDb 
    Status</a></i> </p>
  <p> As mentioned earlier, I2P's netDb works to share the network's metadata. 
    Two algorithms are used to accomplish this - primarily, a small set of routers 
    are designated as "floodfill peers", while the rest of the routers participate 
    in the <a href="http://en.wikipedia.org/wiki/Kademlia">Kademlia </a> derived 
    distributed hash table for redundancy. To integrate the two algorithms, each 
    router always uses the Kademlia style store and fetch, but acts as if the 
    floodfill peers are 'closest' to the key in question. Additionally, when a 
    peer publishes a key into the netDb, after a brief delay they query another 
    random floodfill peer, asking them for the key, and if that peer does not 
    have it, they move on and republish the key again. Behind the scenes, when 
    one of the floodfill peers receives a new valid key, they republish it to 
    the other floodfill peers who then cache it locally. </p>
  <p> Each piece of data in the netDb is self authenticating - signed by the appropriate 
    party and verified by anyone who uses or stores it. In addition, the data 
    has liveliness information within it, allowing irrelevant entries to be dropped, 
    newer entries to replace older ones, and, for the paranoid, protection against 
    certain classes of attack. This is also why I2P bundles the necessary code 
    for maintaining the correct time, occasionally querying some SNTP servers 
    (the <a href="http://www.pool.ntp.org/">pool.ntp.org</a> round robin by default) 
    and detecting skew between routers at the transport layer. </p>
  <p> The routerInfo structure itself contains all of the information that one 
    router needs to know to securely send messages to another router. This includes 
    their identity (made up of a 2048bit ElGamal public key, a 1024bit DSA public 
    key, and a certificate), the transport addresses which they can be reached 
    on, such as an IP address and port, when the structure was published, and 
    a set of arbitrary uninterpreted text options. In addition, there is a signature 
    against all of that data as generated by the included DSA public key. The 
    key for this routerInfo structure in the netDb is the SHA256 hash of the router's 
    identity. The options published are often filled with information helpful 
    in debugging I2P's operation, but when I2P reaches the 1.0 release, the options 
    will be disabled and kept blank. </p>
  <p> The leaseSet structure is similar, in that it includes the I2P destination 
    (comprised of a 2048bit ElGamal public key, a 1024bit DSA public key, and 
    a certificate), a list of "leases", and a pair of public keys for garlic encrypting 
    messages to the destination. Each of the leases specify one of the destination's 
    inbound tunnel gateways by including the SHA256 of the gateway's identity, 
    a 4 byte tunnel id on that gateway, and when that tunnel will expire. The 
    key for the leaseSet in the netDb is the SHA256 of the destination itself. 
  </p>
  <p> As the router currently automatically bundles the leaseSet for the sender 
    inside a garlic message to the recipient, the leaseSet for destinations which 
    will not receive unsolicited messages do not need to be published in the netDb 
    at all. If the destination itself is sensitive, the leaseSet could instead 
    be transmitted through other means without ever going into the netDb. </p>
  <p> Bootstrapping the netDb itself is simple - once a router has at least one 
    routerInfo of a reachable peer, they query that router for references to other 
    routers in the network with the Kademlia healing algorithm. Each routerInfo 
    reference is stored in an individual file in the router's netDb subdirectory, 
    allowing people to easily share their references to bootstrap new users. </p>
  <p> Unlike traditional DHTs, the very act of conducting a search distributes 
    the data as well, since rather passing Kademlia's standard IP+port pairs, 
    references are given to the routers that the peer should query next (namely, 
    the SHA256 of those routers' identities). As such, iteratively searching for 
    a particular destination's leaseSet or router's routerInfo will also provide 
    you with the routerInfo of the peers along the way. In addition, due to the 
    time sensitivity of the data published, the information doesn't often need 
    to migrate between peers - since a tunnel is only valid for 10 minutes, the 
    leaseSet can be dropped after that time has passed. To take into account Sybil 
    attacks on the netDb, the Kademlia routing location used for any given key 
    varies over time. For instance, rather than storing a routerInfo on the peers 
    closest to SHA256(routerInfo.identity), they are stored on the peers closest 
    to SHA256(routerInfo.identity + YYYYMMDD), requiring an adversary to remount 
    the attack again daily so as to maintain their closeness to the current routing 
    key. As the very fact that a router is making a lookup for a given key may 
    expose sensitive data (and the fact that a router is <i>publishing</i> a given 
    key even more so), all netDb messages are transmitted through the router's 
    exploratory tunnels. </p>
  <p> The netDb plays a very specific role in the I2P network, and the algorithms 
    have been tuned towards our needs. This also means that it hasn't been tuned 
    to address the needs we have yet to run into. As the network grows, the primary 
    floodfill algorithm will need to be refined to exploit the capacity available, 
    or perhaps replaced with another technique for securely distributing the network 
    metadata. </p>
  <h2 id="op.transport">Transport protocols</h2>
  <p> Communication between routers needs to provide confidentiality and integrity 
    against external adversaries while authenticating that the router contacted 
    is the one who should receive a given message. The particulars of how routers 
    communicate with other routers aren't critical - three separate protocols 
    have been used at different points to provide those bare necessities. </p>
  <p> I2P started with a <a href="package-tcp.html">TCP-based protocol</a> which 
    has since been disabled. Then, to accommodate the need for high degree communication 
    (as a number of routers will end up speaking with many others), I2P moved 
    from a TCP based transport to a <a href="udp.html">UDP-based one</a> - "Secure 
    Semireliable UDP", or "SSU". </p>
  <p> As described in the <a href="udp.html">SSU spec</a>:</p>
  <blockquote> The goal of this protocol is to provide secure, authenticated, 
    semireliable, and unordered message delivery, exposing only a minimal amount 
    of data easily discernible to third parties. It should support high degree 
    communication as well as TCP-friendly congestion control, and may include 
    PMTU detection. It should be capable of efficiently moving bulk data at rates 
    sufficient for home users. In addition, it should support techniques for addressing 
    network obstacles, like most NATs or firewalls. </blockquote>
  <p> Following the introduction of SSU, after issues with congestion collapse 
    appeared, a new NIO-based TCP transport called <a href="ntcp.html">NTCP</a> 
    was implemented. It is enabled by default for outbound connections only. Those 
    who configure their NAT/firewall to allow inbound connections and specify 
    the external host and port (dyndns/etc is ok) on /config.jsp can receive inbound 
    connections. As NTCP is NIO based, so it doesn't suffer from the 1 thread 
    per connection issues of the old TCP transport. </p>
  <p> I2P supports multiple transports simultaneously. A particular transport 
    for an outbound connection is selected with "bids". Each transport bids for 
    the connection and the relative value of these bids assigns the priority. 
    Transports may reply with different bids, depending on whether there is already 
    an established connection to the peer. </p>
  <p> The current implementation ranks NTCP as the highest-priority transport 
    for outbound connections in most situations. SSU is enabled for both outbound 
    and inbound connections. Your firewall and your I2P router must be configured 
    to allow inbound NTCP connections. For further information see the <a href="ntcp.html">NTCP 
    page</a>. </p>
  <h2 id="op.crypto">Cryptography</h2>
  <p> A bare minimum set of cryptographic primitives are combined together to 
    provide I2P's layered defenses against a variety of adversaries. At the lowest 
    level, inter router communication is protected by the transport layer security 
    - SSU encrypts each packet with AES256/CBC with both an explicit IV and MAC 
    (HMAC-MD5-128) after agreeing upon an ephemeral session key through a 2048bit 
    Diffie-Hellman exchange, station-to-station authentication with the other 
    router's DSA key, plus each network message has their own hash for local integrity 
    checking. <a href="#op.tunnels">Tunnel</a> messages passed over the transports 
    have their own layered AES256/CBC encryption with an explicit IV and verified 
    at the tunnel endpoint with an additional SHA256 hash. Various other messages 
    are passed along inside "garlic messages", which are encrypted with ElGamal/AES+SessionTags 
    (explained below). </p>
  <h3 id="op.garlic">Garlic messages</h3>
  <p> Garlic messages are an extension of "onion" layered encryption, allowing 
    the contents of a single message to contain multiple "cloves" - fully formed 
    messages alongside their own instructions for delivery. Messages are wrapped 
    into a garlic message whenever the message would otherwise be passing in cleartext 
    through a peer who should not have access to the information - for instance, 
    when a router wants to ask another router to participate in a tunnel, they 
    wrap the request inside a garlic, encrypt that garlic to the receiving router's 
    2048bit ElGamal public key, and forward it through a tunnel. Another example 
    is when a client wants to send a message to a destination - the sender's router 
    will wrap up that data message (alongside some other messages) into a garlic, 
    encrypt that garlic to the 2048bit ElGamal public key published in the recipient's 
    leaseSet, and forward it through the appropriate tunnels. </p>
  <p> The "instructions" attached to each clove inside the encryption layer includes 
    the ability to request that the clove be forwarded locally, to a remote router, 
    or to a remote tunnel on a remote router. There are fields in those instructions 
    allowing a peer to request that the delivery be delayed until a certain time 
    or condition has been met, though they won't be honored until the <a href="#future.variablelatency">nontrivial 
    delays</a> are deployed. It is possible to explicitly route garlic messages 
    any number of hops without building tunnels, or even to reroute tunnel messages 
    by wrapping them in garlic messages and forwarding them a number of hops prior 
    to delivering them to the next hop in the tunnel, but those techniques are 
    not currently used in the existing implementation. </p>
  <h3 id="op.sessiontags">Session tags</h3>
  <p> As an unreliable, unordered, message based system, I2P uses a simple combination 
    of asymmetric and symmetric encryption algorithms to provide data confidentiality 
    and integrity to garlic messages. As a whole, the combination is referred 
    to as ElGamal/AES+SessionTags, but that is an excessively verbose way to describe 
    the simple use of 2048bit ElGamal, AES256, SHA256, and 32 byte nonces. </p>
  <p> The first time a router wants to encrypt a garlic message to another router, 
    they encrypt the keying material for an AES256 session key with ElGamal and 
    append the AES256/CBC encrypted payload after that encrypted ElGamal block. 
    In addition to the encrypted payload, the AES encrypted section contains the 
    payload length, the SHA256 hash of the unencrypted payload, as well as a number 
    of "session tags" - random 32 byte nonces. The next time the sender wants 
    to encrypt a garlic message to another router, rather than ElGamal encrypt 
    a new session key they simply pick one of the previously delivered session 
    tags and AES encrypt the payload like before, using the session key used with 
    that session tag, prepended with the session tag itself. When a router receives 
    a garlic encrypted message, they check the first 32 bytes to see if it matches 
    an available session tag - if it does, they simply AES decrypt the message, 
    but if it does not, they ElGamal decrypt the first block. </p>
  <p> Each session tag can be used only once so as to prevent internal adversaries 
    from unnecessarily correlating different messages as being between the same 
    routers. The sender of an ElGamal/AES+SessionTag encrypted message chooses 
    when and how many tags to deliver, prestocking the recipient with enough tags 
    to cover a volley of messages. Garlic messages may detect the successful tag 
    delivery by bundling a small additional message as a clove (a "delivery status 
    message") - when the garlic message arrives at the intended recipient and 
    is decrypted successfully, this small delivery status message is one of the 
    cloves exposed and has instructions for the recipient to send the clove back 
    to the original sender (through an inbound tunnel, of course). When the original 
    sender receives this delivery status message, they know that the session tags 
    bundled in the garlic message were successfully delivered. </p>
  <p> Session tags themselves have a very short lifetime, after which they are 
    discarded if not used. In addition, the quantity stored for each key is limited, 
    as are the number of keys themselves - if too many arrive, either new or old 
    messages may be dropped. The sender keeps track whether messages using session 
    tags are getting through, and if there isn't sufficient communication it may 
    drop the ones previously assumed to be properly delivered, reverting back 
    to the full expensive ElGamal encryption. </p>
  <p> One alternative is to transmit only a single session tag, and from that, 
    seed a deterministic PRNG for determining what tags to use or expect. By keeping 
    this PRNG roughly synchronized between the sender and recipient (the recipient 
    precomputes a window of the next e.g. 50 tags), the overhead of periodically 
    bundling a large number of tags is removed, allowing more options in the space/time 
    tradeoff, and perhaps reducing the number of ElGamal encryptions necessary. 
    However, it would depend upon the strength of the PRNG to provide the necessary 
    cover against internal adversaries, though perhaps by limiting the amount 
    of times each PRNG is used, any weaknesses can be minimized. At the moment, 
    there are no immediate plans to move towards these synchronized PRNGs. </p>
  <h1 id="future">Future</h1>
  <p> While I2P is currently functional and sufficient for many scenarios, there 
    are several areas which require further improvement to meet the needs of those 
    facing more powerful adversaries as well as substantial user experience optimization. 
  </p>
  <h2 id="future.restricted">Restricted route operation</h2>
  <p> I2P is an overlay network designed to be run on top of a functional packet 
    switched network, exploiting the end to end principle to offer anonymity and 
    security. While the Internet no longer fully embraces the end to end principle, 
    I2P does require a substantial portion of the network to be reachable - there 
    may be a number of peers along the edges running using restricted routes, 
    but I2P does not include an appropriate routing algorithm for the degenerate 
    case where most peers are unreachable. It would, however work on top of a 
    network employing such an algorithm. </p>
  <p> Restricted route operation, where there are limits to what peers are reachable 
    directly, has several different functional and anonymity implications, dependent 
    upon how the restricted routes are handled. At the most basic level, restricted 
    routes exist when a peer is behind a NAT or firewall which does not allow 
    inbound connections. This was largely addressed in I2P 0.6.0.6 by integrating 
    distributed hole punching into the transport layer, allowing people behind 
    most NATs and firewalls to receive unsolicited connections without any configuration. 
    However, this does not limit the exposure of the peer's IP address to routers 
    inside the network, as they can simply get introduced to the peer through 
    the published introducer. </p>
  <p> Beyond the functional handling of restricted routes, there are two levels 
    of restricted operation that can be used to limit the exposure of one's IP 
    address - using router-specific tunnels for communication, and offering 'client 
    routers'. For the former, routers can either build a new pool of tunnels or 
    reuse their exploratory pool, publishing the inbound gateways to some of them 
    as part of their routerInfo in place of their transport addresses. When a 
    peer wants to get in touch with them, they see those tunnel gateways in the 
    netDb and simply send the relevant message to them through one of the published 
    tunnels. If the peer behind the restricted route wants to reply, it may do 
    so either directly (if they are willing to expose their IP to the peer) or 
    indirectly through their outbound tunnels. When the routers that the peer 
    has direct connections to want to reach it (to forward tunnel messages, for 
    instance), they simply prioritize their direct connection over the published 
    tunnel gateway. The concept of 'client routers' simply extends the restricted 
    route by not publishing any router addresses. Such a router would not even 
    need to publish their routerInfo in the netDb, merely providing their self 
    signed routerInfo to the peers that it contacts (necessary to pass the router's 
    public keys). Both levels of restricted route operation are planned for I2P 
    2.0. </p>
  <p> There are tradeoffs for those behind restricted routes, as they would likely 
    participate in other people's tunnels less frequently, and the routers which 
    they are connected to would be able to infer traffic patterns that would not 
    otherwise be exposed. On the other hand, if the cost of that exposure is less 
    than the cost of an IP being made available, it may be worthwhile. This, of 
    course, assumes that the peers that the router behind a restricted route contacts 
    are not hostile - either the network is large enough that the probability 
    of using a hostile peer to get connected is small enough, or trusted (and 
    perhaps temporary) peers are used instead. </p>
  <h2 id="future.variablelatency">Variable latency</h2>
  <p> Even though the bulk of I2P's initial efforts have been on low latency communication, 
    it was designed with variable latency services in mind from the beginning. 
    At the most basic level, applications running on top of I2P can offer the 
    anonymity of medium and high latency communication while still blending their 
    traffic patterns in with low latency traffic. Internally though, I2P can offer 
    its own medium and high latency communication through the garlic encryption 
    - specifying that the message should be sent after a certain delay, at a certain 
    time, after a certain number of messages have passed, or another mix strategy. 
    With the layered encryption, only the router that the clove exposed the delay 
    request would know that the message requires high latency, allowing the traffic 
    to blend in further with the low latency traffic. Once the transmission precondition 
    is met, the router holding on to the clove (which itself would likely be a 
    garlic message) simply forwards it as requested - to a router, to a tunnel, 
    or, most likely, to a remote client destination. </p>
  <p> There are a substantial number of ways to exploit this capacity for high 
    latency comm in I2P, but for the moment, doing so has been scheduled for the 
    I2P 3.0 release. In the meantime, those requiring the anonymity that high 
    latency comm can offer should look towards the application layer to provide 
    it. </p>
  <h2 id="future.open">Open questions</h2>
  <pre>
How to get rid of the timing constraint?
Can we deal with the sessionTags more efficiently?
What, if any, batching/mixing strategies should be made available on the tunnels?
What other tunnel peer selection and ordering strategies should be available?
</pre>
  <h1 id="similar">Similar systems</h1>
  <p> I2P's architecture builds on the concepts of message oriented middleware, 
    the topology of DHTs, the anonymity and cryptography of free route mixnets, 
    and the adaptability of packet switched networking. The value comes not from 
    novel concepts of algorithms though, but from careful engineering combining 
    the research results of existing systems and papers. While there are a few 
    similar efforts worth reviewing, both for technical and functional comparisons, 
    two in particular are pulled out here - Tor and Freenet. </p>
  <p> See also the <a href="how_networkcomparisons.html">Network Comparisons Page</a>. 
  </p>

  <h2 id="similar.tor">Tor</h2>
  <p><i><a href="http://www.torproject.org/">website</a></i></p>
  <p> At first glance, Tor and I2P have many functional and anonymity related 
    similarities. While I2P's development began before we were aware of the early 
    stage efforts on Tor, many of the lessons of the original onion routing and 
    ZKS efforts were integrated into I2P's design. Rather than building an essentially 
    trusted, centralized system with directory servers, I2P has a self organizing 
    network database with each peer taking on the responsibility of profiling 
    other routers to determine how best to exploit available resources. Another 
    key difference is that while both I2P and Tor use layered and ordered paths 
    (tunnels and circuits/streams), I2P is fundamentally a packet switched network, 
    while Tor is fundamentally a circuit switched one, allowing I2P to transparently 
    route around congestion or other network failures, operate redundant pathways, 
    and load balance the data across available resources. While Tor offers the 
    useful outproxy functionality by offering integrated outproxy discovery and 
    selection, I2P leaves such application layer decisions up to applications 
    running on top of I2P - in fact, I2P has even externalized the TCP-like streaming 
    library itself to the application layer, allowing developers to experiment 
    with different strategies, exploiting their domain specific knowledge to offer 
    better performance. </p>
  <p> From an anonymity perspective, there is much similarity when the core networks 
    are compared. However, there are a few key differences. When dealing with 
    an internal adversary or most external adversaries, I2P's simplex tunnels 
    expose half as much traffic data than would be exposed with Tor's duplex circuits 
    by simply looking at the flows themselves - an HTTP request and response would 
    follow the same path in Tor, while in I2P the packets making up the request 
    would go out through one or more outbound tunnels and the packets making up 
    the response would come back through one or more different inbound tunnels. 
    While I2P's peer selection and ordering strategies should sufficiently address 
    predecessor attacks, I2P can trivially mimic Tor's non-redundant duplex tunnels 
    by simply building an inbound and outbound tunnel along the same routers.</p>
  <p> Another anonymity issue comes up in Tor's use of telescopic tunnel creation, 
    as simple packet counting and timing measurements as the cells in a circuit 
    pass through an adversary's node exposes statistical information regarding 
    where the adversary is within the circuit. I2P's unidirectional tunnel creation 
    with a single message so that this data is not exposed. Protecting the position 
    in a tunnel is important, as an adversary would otherwise be able to mounting 
    a series of powerful predecessor, intersection, and traffic confirmation attacks. 
  </p>
  <p> Tor's support for a second tier of "onion proxies" does offer a nontrivial 
    degree of anonymity while requiring a low cost of entry, while I2P will not 
    offer this topology until <a href="#future.restricted">2.0</a>. </p>
  <p> On the whole, Tor and I2P complement each other in their focus - Tor works 
    towards offering high speed anonymous Internet outproxying, while I2P works 
    towards offering a decentralized resilient network in itself. In theory, both 
    can be used to achieve both purposes, but given limited development resources, 
    they both have their strengths and weaknesses. The I2P developers have considered 
    the steps necessary to modify Tor to take advantage of I2P's design, but concerns 
    of Tor's viability under resource scarcity suggest that I2P's packet switching 
    architecture will be able to exploit scarce resources more effectively. </p>

  <h2 id="similar.freenet">Freenet</h2>
  <p><i><a href="http://www.freenetproject.org/">website</a></i></p>
  <p> Freenet played a large part in the initial stages of I2P's design - giving 
    proof to the viability of a vibrant pseudonymous community completely contained 
    within the network, demonstrating that the dangers inherent in outproxies 
    could be avoided. The first seed of I2P began as a replacement communication 
    layer for Freenet, attempting to factor out the complexities of a scalable, 
    anonymous and secure point to point communication from the complexities of 
    a censorship resistant distributed data store. Over time however, some of 
    the anonymity and scalability issues inherent in Freenet's algorithms made 
    it clear that I2P's focus should stay strictly on providing a generic anonymous 
    communication layer, rather than as a component of Freenet. Over the years, 
    the Freenet developers have come to see the weaknesses in the older design, 
    prompting them to suggest that they will require a "premix" layer to offer 
    substantial anonymity. In other words, Freenet needs to run on top of a mixnet 
    such as I2P or Tor, with "client nodes" requesting and publishing data through 
    the mixnet to the "server nodes" which then fetch and store the data according 
    to Freenet's heuristic distributed data storage algorithms. </p>
  <p> Freenet's functionality is very complementary to I2P's, as Freenet natively 
    provides many of the tools for operating medium and high latency systems, 
    while I2P natively provides the low latency mix network suitable for offering 
    adequate anonymity. The logic of separating the mixnet from the censorship 
    resistant distributed data store still seems self evident from an engineering, 
    anonymity, security, and resource allocation perspective, so hopefully the 
    Freenet team will pursue efforts in that direction, if not simply reusing 
    (or helping to improve, as necessary) existing mixnets like I2P or Tor. </p>
  <p> It is worth mentioning that there has recently been discussion and work 
    by the Freenet developers on a "globally scalable darknet" using restricted 
    routes between peers of various trust. While insufficient information has 
    been made publicly available regarding how such a system would operate for 
    a full review, from what has been said the anonymity and scalability claims 
    seem highly dubious. In particular, the appropriateness for use in hostile 
    regimes against state level adversaries has been tremendously overstated, 
    and any analysis on the implications of resource scarcity upon the scalability 
    of the network has seemingly been avoided. Further questions regarding susceptibility 
    to traffic analysis, trust, and other topics do exist, but a more in-depth 
    review of this "globally scalable darknet" will have to wait until the Freenet 
    team makes more information available. </p>

  <h1 id="app">Appendix A: Application layer</h1>

  <p>
    I2P itself doesn't really do much - it simply sends messages to remote destinations 
    and receives messages targeting local destinations - most of the interesting 
    work goes on at the layers above it. By itself, I2P could be seen as an anonymous 
    and secure IP layer, and the bundled <a href="#app.streaming">streaming library</a> 
    as an implementation of an anonymous and secure TCP layer on top of it. Beyond 
    that, <a href="#app.i2ptunnel">I2PTunnel</a> exposes a generic TCP proxying 
    system for either getting into or out of the I2P network, plus a variety of 
    network applications provide further functionality for end users.
  </p>

  <h2 id="app.streaming">Streaming library</h2>
  <p>
    The I2P streaming library can be viewed as a generic streaming interface (mirroring TCP sockets),
    and the implementation supports a <a href="http://en.wikipedia.org/wiki/Sliding_Window_Protocol">sliding window protocol</a>
    with several optimizations, to take into account the high delay over I2P.
    Individual streams may adjust the maximum packet size and 
    other options, though the default of 4KB compressed seems a reasonable tradeoff 
    between the bandwidth costs of retransmitting lost messages and the latency 
    of multiple messages.
  </p>
  <p>
    In addition, in consideration of the relatively high cost of subsequent 
    messages, the streaming library's protocol for scheduling and delivering messages 
    has been optimized to allow individual messages passed to contain as much 
    information as is available. For instance, a small HTTP transaction proxied 
    through the streaming library can be completed in a single round trip - the 
    first message bundles a SYN, FIN, and the small payload (an HTTP request typically 
    fits) and the reply bundles the SYN, FIN, ACK, and the small payload (many 
    HTTP responses fit). While an additional ACK must be transmitted to tell the 
    HTTP server that the SYN/FIN/ACK has been received, the local HTTP proxy can 
    deliver the full response to the browser immediately.
  </p>
  <p>
    On the whole, however, the streaming library bears much resemblance to an 
    abstraction of TCP, with its sliding windows, congestion control algorithms 
    (both slow start and congestion avoidance), and general packet behavior (ACK, 
    SYN, FIN, RST, etc).
  </p>

  <h2 id="app.naming">Naming library and addressbook</h2>
  <p><i> For more information see the <a href="naming.html">Naming and Addressbook</a> 
    page.</i></p>
  <p><i>Developed by: mihi, Ragnarok</i></p>
  <p> Naming within I2P has been an oft-debated topic since the very beginning 
    with advocates across the spectrum of possibilities. However, given I2P's 
    inherent demand for secure communication and decentralized operation, the 
    traditional DNS-style naming system is clearly out, as are "majority rules" 
    voting systems. Instead, I2P ships with a generic naming library and a base 
    implementation designed to work off a local name to destination mapping, as 
    well as an optional add-on application called the "addressbook". The addressbook 
    is a web-of-trust driven secure, distributed, and human readable naming system, 
    sacrificing only the call for all human readable names to be globally unique 
    by mandating only local uniqueness. While all messages in I2P are cryptographically 
    addressed by their destination, different people can have local addressbook 
    entries for "Alice" which refer to different destinations. People can still 
    discover new names by importing published addressbooks of peers specified 
    in their web of trust, by adding in the entries provided through a third party, 
    or (if some people organize a series of published addressbooks using a first 
    come first serve registration system) people can choose to treat these addressbooks 
    as name servers, emulating traditional DNS. </p>
  <p> I2P does not promote the use of DNS-like services though, as the damage 
    done by hijacking a site can be tremendous - and insecure destinations have 
    no value. DNSsec itself still falls back on registrars and certificate authorities, 
    while with I2P, requests sent to a destination cannot be intercepted or the 
    reply spoofed, as they are encrypted to the destination's public keys, and 
    a destination itself is just a pair of public keys and a certificate. DNS-style 
    systems on the other hand allow any of the name servers on the lookup path 
    to mount simple denial of service and spoofing attacks. Adding on a certificate 
    authenticating the responses as signed by some centralized certificate authority 
    would address many of the hostile nameserver issues but would leave open replay 
    attacks as well as hostile certificate authority attacks. </p>
  <p> Voting style naming is dangerous as well, especially given the effectiveness 
    of Sybil attacks in anonymous systems - the attacker can simply create an 
    arbitrarily high number of peers and "vote" with each to take over a given 
    name. Proof-of-work methods can be used to make identity non-free, but as 
    the network grows the load required to contact everyone to conduct online 
    voting is implausible, or if the full network is not queried, different sets 
    of answers may be reachable. </p>
  <p> As with the Internet however, I2P is keeping the design and operation of 
    a naming system out of the (IP-like) communication layer. The bundled naming 
    library includes a simple service provider interface which alternate naming 
    systems can plug into, allowing end users to drive what sort of naming tradeoffs 
    they prefer. </p>

  <h2 id="app.syndie">Syndie</h2>
  <p><i> The old Syndie bundled with I2P has been replaced by the new Syndie which 
    is distributed separately. For more information see the <a href="http://syndie.i2p2.de/">Syndie</a> 
    pages.</i></p>
  <p> Syndie is a safe, anonymous blogging / content publication / content aggregation 
    system. It lets you create information, share it with others, and read posts 
    from those you're interested in, all while taking into consideration your 
    needs for security and anonymity. Rather than building its own content distribution 
    network, Syndie is designed to run on top of existing networks, syndicating 
    content through eepsites, Tor hidden services, Freenet freesites, normal websites, 
    usenet newgroups, email lists, RSS feeds, etc. Data published with Syndie 
    is done so as to offer pseudonymous authentication to anyone reading or archiving 
    it. </p>

  <h2 id="app.i2ptunnel">I2PTunnel</h2>
  <p><i>Developed by: mihi</i></p>
  <p> I2PTunnel is probably I2P's most popular and versatile client application, 
    allowing generic proxying both into and out of the I2P network. I2PTunnel 
    can be viewed as four separate proxying applications - a "client" which receives 
    inbound TCP connections and forwards them to a given I2P destination, an "httpclient" 
    (aka "eepproxy") which acts like an HTTP proxy and forwards the requests to 
    the appropriate I2P destination (after querying the naming service if necessary), 
    a "server" which receives inbound I2P streaming connections on a destination 
    and forwards them to a given TCP host+port, and an "httpserver" which extends 
    the "server" by parsing the HTTP request and responses to allow safer operation. 
    There is an additional "socksclient" application, but its use is not encouraged 
    for reasons previously mentioned. </p>
  <p> I2P itself is not an outproxy network - the anonymity and security concerns 
    inherent in a mix net which forwards data into and out of the mix have kept 
    I2P's design focused on providing an anonymous network which capable of meeting 
    the user's needs without requiring external resources. However, the I2PTunnel 
    "httpclient" application offers a hook for outproxying - if the hostname requested 
    doesn't end in ".i2p", it picks a random destination from a user-provided 
    set of outproxies and forwards the request to them. These destinations are 
    simply I2PTunnel "server" instances run by volunteers who have explicitly 
    chosen to run outproxies - no one is an outproxy by default, and running an 
    outproxy doesn't automatically tell other people to proxy through you. While 
    outproxies do have inherent weaknesses, they offer a simple proof of concept 
    for using I2P and provide some functionality under a threat model which may 
    be sufficient for some users. </p>
  <p> I2PTunnel enables most of the applications in use. An "httpserver" pointing 
    at a webserver lets anyone run their own anonymous website (or "eepsite") 
    - a webserver is bundled with I2P for this purpose, but any webserver can 
    be used. Anyone may run a "client" pointing at one of the anonymously hosted 
    IRC servers, each of which are running a "server" pointing at their local 
    IRCd and communicating between IRCds over their own "client" tunnels. End 
    users also have "client" tunnels pointing at <a href="#app.i2pmail">I2Pmail's</a> 
    POP3 and SMTP destinations (which in turn are simply "server" instances pointing 
    at POP3 and SMTP servers), as well as "client" tunnels pointing at I2P's CVS 
    server, allowing anonymous development. At times people have even run "client" 
    proxies to access the "server" instances pointing at an NNTP server. </p>

  <h2 id="app.i2pbt">i2p-bt</h2>
  <p><i>Developed by: duck, et al</i></p>
  <p> i2p-bt is a port of the mainline python BitTorrent client to run both the 
    tracker and peer communication over I2P. Tracker requests are forwarded through 
    the eepproxy to eepsites specified in the torrent file while tracker responses 
    refer to peers by their destination explicitly, allowing i2p-bt to open up 
    a <a href="#app.streaming">streaming lib</a> connection to query them for 
    blocks. </p>
  <p> In addition to i2p-bt, a port of bytemonsoon has been made to I2P, making 
    a few modifications as necessary to strip any anonymity-compromising information 
    from the application and to take into consideration the fact that IPs cannot 
    be used for identifying peers. </p>

  <h2 id="app.i2psnark">I2PSnark</h2>
  <p><i>I2PSnark developed: jrandom, et al, ported from <a
href="http://www.klomp.org/mark/">mjw</a>'s <a
href="http://www.klomp.org/snark/">Snark</a> client</i></p>
  <p> Bundled with the I2P install, I2PSnark offers a simple anonymous BitTorrent 
    client with multitorrent capabilities, exposing all of the functionality through 
    a plain HTML web interface. </p>

  <h2 id="app.robert">Robert</h2>
  <p><i>Developed by: sponge</i></p>
  <p>Robert is a Bittorrent client written in Python.
    It is hosted on <a href="http://bob.i2p/Robert.html">http://bob.i2p/Robert.html</a></p> <!-- TODO: expand -->

  <h2 id="app.pybit">PyBit</h2>
  <p><i>Developed by: Blub</i></p>
  <p>PyBit is a Bittorrent client written in Python.
    It is hosted on <a href="http://pebcache.i2p/">http://pebcache.i2p/</a></p> <!-- TODO: expand -->

  <h2 id="app.i2phex">I2Phex</h2>
  <p><i>Developed by: sirup</i></p>
  <p> I2Phex is a fairly direct port of the Phex Gnutella filesharing client to 
    run entirely on top of I2P. While it has disabled some of Phex's functionality, 
    such as integration with Gnutella webcaches, the basic file sharing and chatting 
    system is fully functional. </p>
  <h2 id="app.i2pmail">I2Pmail/susimail</h2>
  <p><i>Developed by: postman, susi23, mastiejaner</i></p>
  <p> I2Pmail is more a service than an application - postman offers both internal 
    and external email with POP3 and SMTP service through I2PTunnel instances 
    accessing a series of components developed with mastiejaner, allowing people 
    to use their preferred mail clients to send and receive mail pseudonymously. 
    However, as most mail clients expose substantial identifying information, 
    I2P bundles susi23's web based susimail client which has been built specifically 
    with I2P's anonymity needs in mind. The I2Pmail/mail.i2p service offers transparent 
    virus filtering as well as denial of service prevention with hashcash augmented 
    quotas. In addition, each user has control of their batching strategy prior 
    to delivery through the mail.i2p outproxies, which are separate from the mail.i2p 
    SMTP and POP3 servers - both the outproxies and inproxies communicate with 
    the mail.i2p SMTP and POP3 servers through I2P itself, so compromising those 
    non-anonymous locations does not give access to the mail accounts or activity 
    patterns of the user. At the moment the developers work on a decentralized 
    mailsystem, called "v2mail". More information can be found on the eepsite 
    <a href="http://hq.postman.i2p/">hq.postman.i2p</a>.</p>

  <h2 id="app.i2pbote">I2P-Bote</h2>
  <p><i>Developed by: HungryHobo</i></p>
  <p>
    I2P-Bote is a distributed e-mail application. It does not use the traditional
    e-mail concept of sending an e-mail to a server and retrieving it from a server.
    Instead, it uses a Kademlia Distributed Hash Table to store mails.
    One user can push a mail into the DHT, while another can request the e-mail from the DHT.
  </p>

  <h2 id="app.i2pmessenger">I2P-messenger</h2>
  <p>
    I2P-messenger is an end-to-end encrypted serverless communication application.
    For communication between two users, they need to give each other their destination, to allow the other to connect.
  </p>
{% endblock %}