i2p.android.base/router/doc/tunnel-alt.html

<code>$Id: tunnel-alt.html,v 1.9 2005/07/27 14:04:07 jrandom Exp $</code>
<pre>
1) <a href="#tunnel.overview">Tunnel overview</a>
2) <a href="#tunnel.operation">Tunnel operation</a>
2.1) <a href="#tunnel.preprocessing">Message preprocessing</a>
2.2) <a href="#tunnel.gateway">Gateway processing</a>
2.3) <a href="#tunnel.participant">Participant processing</a>
2.4) <a href="#tunnel.endpoint">Endpoint processing</a>
2.5) <a href="#tunnel.padding">Padding</a>
2.6) <a href="#tunnel.fragmentation">Tunnel fragmentation</a>
2.7) <a href="#tunnel.alternatives">Alternatives</a>
2.7.1) <a href="#tunnel.reroute">Adjust tunnel processing midstream</a>
2.7.2) <a href="#tunnel.bidirectional">Use bidirectional tunnels</a>
2.7.3) <a href="#tunnel.backchannel">Backchannel communication</a>
2.7.4) <a href="#tunnel.variablesize">Variable size tunnel messages</a>
3) <a href="#tunnel.building">Tunnel building</a>
3.1) <a href="#tunnel.peerselection">Peer selection</a>
3.1.1) <a href="#tunnel.selection.exploratory">Exploratory tunnel peer selection</a>
3.1.2) <a href="#tunnel.selection.client">Client tunnel peer selection</a>
3.2) <a href="#tunnel.request">Request delivery</a>
3.3) <a href="#tunnel.pooling">Pooling</a>
3.4) <a href="#tunnel.building.alternatives">Alternatives</a>
3.4.1) <a href="#tunnel.building.telescoping">Telescopic building</a>
3.4.2) <a href="#tunnel.building.nonexploratory">Non-exploratory tunnels for management</a>
4) <a href="#tunnel.throttling">Tunnel throttling</a>
5) <a href="#tunnel.mixing">Mixing/batching</a>
</pre>

<h2>1) <a name="tunnel.overview">Tunnel overview</a></h2>

<p>Within I2P, messages are passed in one direction through a virtual
tunnel of peers, using whatever means are available to pass the 
message on to the next hop.  Messages arrive at the tunnel's 
gateway, get bundled up and/or fragmented into fixed sizes tunnel messages, 
and are forwarded on to the next hop in the tunnel, which processes and verifies
the validity of the message and sends it on to the next hop, and so on, until
it reaches the tunnel endpoint.  That endpoint takes the messages
bundled up by the gateway and forwards them as instructed - either
to another router, to another tunnel on another router, or locally.</p>

<p>Tunnels all work the same, but can be segmented into two different
groups - inbound tunnels and outbound tunnels.  The inbound tunnels
have an untrusted gateway which passes messages down towards the 
tunnel creator, which serves as the tunnel endpoint.  For outbound 
tunnels, the tunnel creator serves as the gateway, passing messages
out to the remote endpoint.</p>

<p>The tunnel's creator selects exactly which peers will participate
in the tunnel, and provides each with the necessary configuration
data.  They may have any number of hops, but may be constrained with various
proof-of-work requests to add on additional steps.  It is the intent to make
it hard for either participants or third parties to determine the length of 
a tunnel, or even for colluding participants to determine whether they are a
part of the same tunnel at all (barring the situation where colluding peers are
next to each other in the tunnel).</p>

<p>Beyond their length, there are additional configurable parameters
for each tunnel that can be used, such as a throttle on the frequency of 
messages delivered, how padding should be used, how long a tunnel should be 
in operation, whether to inject chaff messages, and what, if any, batching
strategies should be employed.</p>

<p>In practice, a series of tunnel pools are used for different
purposes - each local client destination has its own set of inbound
tunnels and outbound tunnels, configured to meet its anonymity and
performance needs.  In addition, the router itself maintains a series
of pools for participating in the network database and for managing
the tunnels themselves.</p>

<p>I2P is an inherently packet switched network, even with these 
tunnels, allowing it to take advantage of multiple tunnels running 
in parallel, increasing resilience and balancing load.  Outside of
the core I2P layer, there is an optional end to end streaming library 
available for client applications, exposing TCP-esque operation,
including message reordering, retransmission, congestion control, etc.</p>

<h2>2) <a name="tunnel.operation">Tunnel operation</a></h2>

<p>Tunnel operation has four distinct processes, taken on by various 
peers in the tunnel.  First, the tunnel gateway accumulates a number
of tunnel messages and preprocesses them into something for tunnel
delivery.  Next, that gateway encrypts that preprocessed data, then
forwards it to the first hop.  That peer, and subsequent tunnel 
participants, unwrap a layer of the encryption, verifying that it isn't
a duplicate, then forward it on to the next peer.  
Eventually, the message arrives at the endpoint where the messages
bundled by the gateway are split out again and forwarded on as 
requested.</p>

<p>Tunnel IDs are 4 byte numbers used at each hop - participants know what
tunnel ID to listen for messages with and what tunnel ID they should be forwarded
on as to the next hop, and each hop chooses the tunnel ID which they receive messages
on.  Tunnels themselves are short lived (10 minutes at the 
moment), and even if subsequent tunnels are built using the same sequence of 
peers, each hop's tunnel ID will change.</p>

<h3>2.1) <a name="tunnel.preprocessing">Message preprocessing</a></h3>

<p>When the gateway wants to deliver data through the tunnel, it first
gathers zero or more I2NP messages, selects how much padding will be used, 
fragments it across the necessary number of 1KB tunnel messages, and decides how
each I2NP message should be handled by the tunnel endpoint, encoding that
data into the raw tunnel payload:</p>
<ul>
<li>the first 4 bytes of the SHA256 of the remaining preprocessed data concatenated 
    with the IV, using the IV as will be seen on the tunnel endpoint (for
    outbound tunnels) or the IV as was seen on the tunnel gateway (for inbound
    tunnels) (see below for IV processing).</li>
<li>0 or more bytes containing random nonzero integers</li>
<li>1 byte containing 0x00</li>
<li>a series of zero or more { instructions, message } pairs</li>
</ul>

<p>The instructions are encoded with a single control byte, followed by any
necessary additional information.  The first bit in that control byte determines
how the remainder of the header is interpreted - if it is not set, the message 
is either not fragmented or this is the first fragment in the message.  If it is
set, this is a follow on fragment.</p>

<p>With the first bit being 0, the instructions are:</p>
<ul>
<li>1 byte control byte:<pre>
      bit 0: is follow on fragment?  (1 = true, 0 = false, must be 0)
   bits 1-2: delivery type
             (0x0 = LOCAL, 0x01 = TUNNEL, 0x02 = ROUTER)
      bit 3: delay included?  (1 = true, 0 = false)
      bit 4: fragmented?  (1 = true, 0 = false)
      bit 5: extended options?  (1 = true, 0 = false)
   bits 6-7: reserved</pre></li>
<li>if the delivery type was TUNNEL, a 4 byte tunnel ID</li>
<li>if the delivery type was TUNNEL or ROUTER, a 32 byte router hash</li>
<li>if the delay included flag is true, a 1 byte value:<pre>
      bit 0: type (0 = strict, 1 = randomized)
   bits 1-7: delay exponent (2^value minutes)</pre></li>
<li>if the fragmented flag is true, a 4 byte message ID</li>
<li>if the extended options flag is true:<pre>
   = a 1 byte option size (in bytes)
   = that many bytes</pre></li>
<li>2 byte size of the I2NP message or this fragment</li>
</ul>

<p>If the first bit being 1, the instructions are:</p>
<ul>
<li>1 byte control byte:<pre>
      bit 0: is follow on fragment?  (1 = true, 0 = false, must be 1)
   bits 1-6: fragment number
      bit 7: is last? (1 = true, 0 = false)</pre></li>
<li>4 byte message ID (same one defined in the first fragment)</li>
<li>2 byte size of this fragment</li>
</ul>

<p>The I2NP message is encoded in its standard form, and the 
preprocessed payload must be padded to a multiple of 16 bytes.</p>

<h3>2.2) <a name="tunnel.gateway">Gateway processing</a></h3>

<p>After the preprocessing of messages into a padded payload, the gateway builds
a random 16 byte IV value, iteratively encrypting it and the tunnel message as
necessary, and forwards the tuple {tunnelID, IV, encrypted tunnel message} to the next hop.</p>

<p>How encryption at the gateway is done depends on whether the tunnel is an
inbound or an outbound tunnel.  For inbound tunnels, they simply select a random
IV, postprocessing and updating it to generate the IV for the gateway and using 
that IV along side their own layer key to encrypt the preprocessed data.  For outbound 
tunnels they must iteratively decrypt the (unencrypted) IV and preprocessed 
data with the IV and layer keys for all hops in the tunnel.  The result of the outbound
tunnel encryption is that when each peer encrypts it, the endpoint will recover 
the initial preprocessed data.</p>

<h3>2.3) <a name="tunnel.participant">Participant processing</a></h3>

<p>When a peer receives a tunnel message, it checks that the message came from
the same previous hop as before (initialized when the first message comes through
the tunnel).  If the previous peer is a different router, or if the message has
already been seen, the message is dropped.  The participant then encrypts the 
received IV with AES256/ECB using their IV key to determine the current IV, uses 
that IV with the participant's layer key to encrypt the data, encrypts the 
current IV with AES256/ECB using their IV key again, then forwards the tuple 
{nextTunnelId, nextIV, encryptedData} to the next hop.  This double encryption
of the IV (both before and after use) help address a certain class of
confirmation attacks.</p>

<p>Duplicate message detection is handled by a decaying Bloom filter on message
IVs.  Each router maintains a single Bloom filter to contain the XOR of the IV and
the first block of the message received for all of the tunnels it is participating
in, modified to drop seen entries after 10-20 minutes (when the tunnels will have
expired).  The size of the bloom filter and the parameters used are sufficient to
more than saturate the router's network connection with a negligible chance of 
false positive.  The unique value fed into the Bloom filter is the XOR of the IV 
and the first block so as to prevent nonsequential colluding peers in the tunnel 
from tagging a message by resending it with the IV and first block switched.</p>

<h3>2.4) <a name="tunnel.endpoint">Endpoint processing</a></h3>

<p>After receiving and validating a tunnel message at the last hop in the tunnel,
how the endpoint recovers the data encoded by the gateway depends upon whether 
the tunnel is an inbound or an outbound tunnel.  For outbound tunnels, the 
endpoint encrypts the message with its layer key just like any other participant, 
exposing the preprocessed data.  For inbound tunnels, the endpoint is also the 
tunnel creator so they can merely iteratively decrypt the IV and message, using the 
layer and IV keys of each step in reverse order.</p>

<p>At this point, the tunnel endpoint has the preprocessed data sent by the gateway,
which it may then parse out into the included I2NP messages and forwards them as
requested in their delivery instructions.</p>

<h3>2.5) <a name="tunnel.padding">Padding</a></h3>

<p>Several tunnel padding strategies are possible, each with their own merits:</p>

<ul>
<li>No padding</li>
<li>Padding to a random size</li>
<li>Padding to a fixed size</li>
<li>Padding to the closest KB</li>
<li>Padding to the closest exponential size (2^n bytes)</li>
</ul>

<p>These padding strategies can be used on a variety of levels, addressing the
exposure of message size information to different adversaries.  After gathering
and reviewing some <a href="http://dev.i2p.net/~jrandom/messageSizes/">statistics</a>
from the 0.4 network, as well as exploring the anonymity tradeoffs, we're starting
with a fixed tunnel message size of 1024 bytes.  Within this however, the fragmented
messages themselves are not padded by the tunnel at all (though for end to end 
messages, they may be padded as part of the garlic wrapping).</p>

<h3>2.6) <a name="tunnel.fragmentation">Tunnel fragmentation</a></h3>

<p>To prevent adversaries from tagging the messages along the path by adjusting
the message size, all tunnel messages are a fixed 1024 bytes in size.  To accommodate 
larger I2NP messages as well as to support smaller ones more efficiently, the
gateway splits up the larger I2NP messages into fragments contained within each
tunnel message.  The endpoint will attempt to rebuild the I2NP message from the
fragments for a short period of time, but will discard them as necessary.</p>

<p>Routers have a lot of leeway as to how the fragments are arranged, whether 
they are stuffed inefficiently as discrete units, batched for a brief period to
fit more payload into the 1024 byte tunnel messages, or opportunistically padded
with other messages that the gateway wanted to send out.</p>

<h3>2.7) <a name="tunnel.alternatives">Alternatives</a></h3>

<h4>2.7.1) <a name="tunnel.reroute">Adjust tunnel processing midstream</a></h4>

<p>While the simple tunnel routing algorithm should be sufficient for most cases,
there are three alternatives that can be explored:</p>
<ul>
<li>Have a peer other than the endpoint temporarily act as the termination 
point for a tunnel by adjusting the encryption used at the gateway to give them
the plaintext of the preprocessed I2NP messages.  Each peer could check to see 
whether they had the plaintext, processing the message when received as if they
did.</li>
<li>Allow routers participating in a tunnel to remix the message before 
forwarding it on - bouncing it through one of that peer's own outbound tunnels,
bearing instructions for delivery to the next hop.</li>
<li>Implement code for the tunnel creator to redefine a peer's "next hop" in
the tunnel, allowing further dynamic redirection.</li>
</ul>

<h4>2.7.2) <a name="tunnel.bidirectional">Use bidirectional tunnels</a></h4>

<p>The current strategy of using two separate tunnels for inbound and outbound
communication is not the only technique available, and it does have anonymity
implications.  On the positive side, by using separate tunnels it lessens the
traffic data exposed for analysis to participants in a tunnel - for instance,
peers in an outbound tunnel from a web browser would only see the traffic of
an HTTP GET, while the peers in an inbound tunnel would see the payload 
delivered along the tunnel.  With bidirectional tunnels, all participants would
have access to the fact that e.g. 1KB was sent in one direction, then 100KB
in the other.  On the negative side, using unidirectional tunnels means that
there are two sets of peers which need to be profiled and accounted for, and
additional care must be taken to address the increased speed of predecessor
attacks.  The tunnel pooling and building process outlined below should
minimize the worries of the predecessor attack, though if it were desired,
it wouldn't be much trouble to build both the inbound and outbound tunnels
along the same peers.</p>

<h4>2.7.3) <a name="tunnel.backchannel">Backchannel communication</a></h4>

<p>At the moment, the IV values used are random values.  However, it is 
possible for that 16 byte value to be used to send control messages from the 
gateway to the endpoint, or on outbound tunnels, from the gateway to any of the
peers.  The inbound gateway could encode certain values in the IV once, which
the endpoint would be able to recover (since it knows the endpoint is also the
creator).  For outbound tunnels, the creator could deliver certain values to the 
participants during the tunnel creation (e.g. "if you see 0x0 as the IV, that
means X", "0x1 means Y", etc).  Since the gateway on the outbound tunnel is also
the creator, they can build a IV so that any of the peers will receive the 
correct value.  The tunnel creator could even give the inbound tunnel gateway
a series of IV values which that gateway could use to communicate with 
individual participants exactly one time (though this would have issues regarding
collusion detection)</p>

<p>This technique could later be used deliver message mid stream, or to allow the
inbound gateway to tell the endpoint that it is being DoS'ed or otherwise soon
to fail.  At the moment, there are no plans to exploit this backchannel.</p>

<h4>2.7.4) <a name="tunnel.variablesize">Variable size tunnel messages</a></h4>

<p>While the transport layer may have its own fixed or variable message size,
using its own fragmentation, the tunnel layer may instead use variable size
tunnel messages.  The difference is an issue of threat models - a fixed size 
at the transport layer helps reduce the information exposed to external 
adversaries (though overall flow analysis still works), but for internal 
adversaries (aka tunnel participants) the message size is exposed.  Fixed size
tunnel messages help reduce the information exposed to tunnel participants, but
does not hide the information exposed to tunnel endpoints and gateways.  Fixed
size end to end messages hide the information exposed to all peers in the 
network.</p>

<p>As always, its a question of who I2P is trying to protect against.  Variable 
sized tunnel messages are dangerous, as they allow participants to use the 
message size itself as a backchannel to other participants - e.g. if you see a 
1337 byte message, you're on the same tunnel as another colluding peer.  Even 
with a fixed set of allowable sizes (1024, 2048, 4096, etc), that backchannel 
still exists as peers could use the frequency of each size as the carrier (e.g.
two 1024 byte messages followed by an 8192).  Smaller messages do incur the 
overhead of the headers (IV, tunnel ID, hash portion, etc), but larger fixed size
messages either increase latency (due to batching) or dramatically increase 
overhead (due to padding).  Fragmentation helps ammortize the overhead, at the
cost of potential message loss due to lost fragments.</p>

<p>Timing attacks are also relevent when reviewing the effectiveness of fixed 
size messages, though they require a substantial view of network activity
patterns to be effective.  Excessive artificial delays in the tunnel will be 
detected by the tunnel's creator, due to periodic testing, causing that entire
tunnel to be scrapped and the profiles for peers within it to be adjusted.</p>

<h2>3) <a name="tunnel.building">Tunnel building</a></h2>

<p>When building a tunnel, the creator must send a request with the necessary
configuration data to each of the hops and wait for all of them to agree before
enabling the tunnel.  The requests are encrypted so that only the peers who need
to know a piece of information (such as the tunnel layer or IV key) has that
data.  In addition, only the tunnel creator will have access to the peer's
reply.  There are three important dimensions to keep in mind when producing
the tunnels: what peers are used (and where), how the requests are sent (and 
replies received), and how they are maintained.</p>

<h3>3.1) <a name="tunnel.peerselection">Peer selection</a></h3>

<p>Beyond the two types of tunnels - inbound and outbound - there are two styles
of peer selection used for different tunnels - exploratory and client.
Exploratory tunnels are used for both network database maintenance and tunnel
maintenance, while client tunnels are used for end to end client messages.  </p>

<h4>3.1.1) <a name="tunnel.selection.exploratory">Exploratory tunnel peer selection</a></h4>

<p>Exploratory tunnels are built out of a random selection of peers from a subset
of the network.  The particular subset varies on the local router and on what their
tunnel routing needs are.  In general, the exploratory tunnels are built out of
randomly selected peers who are in the peer's "not failing but active" profile
category.  The secondary purpose of the tunnels, beyond merely tunnel routing,
is to find underutilized high capacity peers so that they can be promoted for
use in client tunnels.</p>

<h4>3.1.2) <a name="tunnel.selection.client">Client tunnel peer selection</a></h4>

<p>Client tunnels are built with a more stringent set of requirements - the local
router will select peers out of its "fast and high capacity" profile category so
that performance and reliability will meet the needs of the client application.
However, there are several important details beyond that basic selection that 
should be adhered to, depending upon the client's anonymity needs.</p>
  
<p>For some clients who are worried about adversaries mounting a predecessor 
attack, the tunnel selection can keep the peers selected in a strict order -
if A, B, and C are in a tunnel, the hop after A is always B, and the hop after
B is always C.  A less strict ordering is also possible, assuring that while
the hop after A may be B, B may never be before A.  Other configuration options
include the ability for just the inbound tunnel gateways and outbound tunnel
endpoints to be fixed, or rotated on an MTBF rate.</p>

<p>In the initial implementation, only random ordering has been implemented, 
though more strict ordering will be developed and deployed over time, as well
as controls for the user to select which strategy to use for individual clients.</p>

<h3>3.2) <a name="tunnel.request">Request delivery</a></h3>

<p>A new tunnel request preparation, delivery, and response method has been
<a href="tunnel-alt-creation.html">devised</a>, which reduces the number of
predecessors exposed, cuts the number of messages transmitted, verifies proper
connectivity, and avoids the message counting attack of traditional telescopic
tunnel creation.  The old technique is listed below as an <a
href="#tunnel.building.exploratory">alternative</a>.</p>

<p>Peers may reject tunnel creation requests for a variety of reasons, though
a series of four increasingly severe rejections are known: probabalistic rejection
(due to approaching the router's capacity, or in response to a flood of requests), 
transient overload, bandwidth overload, and critical failure.  When received, 
those four are interpreted by the tunnel creator to help adjust their profile of
the router in question.</p>

<h3>3.3) <a name="tunnel.pooling">Pooling</a></h3>

<p>To allow efficient operation, the router maintains a series of tunnel pools,
each managing a group of tunnels used for a specific purpose with their own
configuration.  When a tunnel is needed for that purpose, the router selects one
out of the appropriate pool at random.  Overall, there are two exploratory tunnel
pools - one inbound and one outbound - each using the router's exploration 
defaults.  In addition, there is a pair of pools for each local destination -
one inbound and one outbound tunnel.  Those pools use the configuration specified
when the local destination connected to the router, or the router's defaults if
not specified.</p>

<p>Each pool has within its configuration a few key settings, defining how many
tunnels to keep active, how many backup tunnels to maintain in case of failure,
how frequently to test the tunnels, how long the tunnels should be, whether those
lengths should be randomized, how often replacement tunnels should be built, as 
well as any of the other settings allowed when configuring individual tunnels.</p>

<h3>3.4) <a name="tunnel.building.alternatives">Alternatives</a></h3>

<h4>3.4.1) <a name="tunnel.building.telescoping">Telescopic building</a></h4>

<p>One question that may arise regarding the use of the exploratory tunnels for
sending and receiving tunnel creation messages is how that impacts the tunnel's 
vulnerability to predecessor attacks.  While the endpoints and gateways of 
those tunnels will be randomly distributed across the network (perhaps even 
including the tunnel creator in that set), another alternative is to use the
tunnel pathways themselves to pass along the request and response, as is done
in <a href="http://tor.eff.org/">TOR</a>.  This, however, may lead to leaks 
during tunnel creation, allowing peers to discover how many hops there are later
on in the tunnel by monitoring the timing or <a
href="http://dev.i2p.net/pipermail/2005-October/001057.html">packet count</a> as
the tunnel is built.</p>

<h4>3.4.2) <a name="tunnel.building.nonexploratory">Non-exploratory tunnels for management</a></h4>

<p>A second alternative to the tunnel building process is to give the router 
an additional set of non-exploratory inbound and outbound pools, using those for
the tunnel request and response.  Assuming the router has a well integrated view
of the network, this should not be necessary, but if the router was partitioned
in some way, using non-exploratory pools for tunnel management would reduce the
leakage of information about what peers are in the router's partition.</p>

<h4>3.4.3) <a name="tunnel.building.exploratory">Exploratory request delivery</a></h4>

<p>A third alternative, used until I2P 0.6.2, garlic encrypts individual tunnel
request messages and delivers them to the hops individually, transmitting them
through exploratory tunnels with their reply coming back in a separate
exploratory tunnel.  This strategy has been dropped in favor of the one outlined
above.</p>

<h2>4) <a name="tunnel.throttling">Tunnel throttling</a></h2>

<p>Even though the tunnels within I2P bear a resemblance to a circuit switched
network, everything within I2P is strictly message based - tunnels are merely
accounting tricks to help organize the delivery of messages.  No assumptions are
made regarding reliability or ordering of messages, and retransmissions are left
to higher levels (e.g. I2P's client layer streaming library).  This allows I2P
to take advantage of throttling techniques available to both packet switched and
circuit switched networks.  For instance, each router may keep track of the 
moving average of how much data each tunnel is using, combine that with all of 
the averages used by other tunnels the router is participating in, and be able
to accept or reject additional tunnel participation requests based on its 
capacity and utilization.  On the other hand, each router can simply drop 
messages that are beyond its capacity, exploiting the research used on the 
normal internet.</p>

<h2>5) <a name="tunnel.mixing">Mixing/batching</a></h2>

<p>What strategies should be used at the gateway and at each hop for delaying,
reordering, rerouting, or padding messages?  To what extent should this be done
automatically, how much should be configured as a per tunnel or per hop setting,
and how should the tunnel's creator (and in turn, user) control this operation?
All of this is left as unknown, to be worked out for 
<a href="http://www.i2p.net/roadmap#3.0">I2P 3.0</a></p>