That page documents the current tunnel build implementation as of release 0.6.1.10. The older tunnel build method, used prior to release 0.6.1.10, is documented on the old tunnel page.
Beyond their length, there may be 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. None of these are currently implemented.
Several tunnel padding strategies are possible, each with their own merits:
- No padding
- Padding to a random size
- Padding to a fixed size
- Padding to the closest KB
- Padding to the closest exponential size (2^n bytes)
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 statistics 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).
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.
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.
While the simple tunnel routing algorithm should be sufficient for most cases, there are three alternatives that can be explored:
- 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.
- 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.
- Implement code for the tunnel creator to redefine a peer's "next hop" in the tunnel, allowing further dynamic redirection.
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.
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)
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.
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.
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 amortize the overhead, at the cost of potential message loss due to lost fragments.
Timing attacks are also relevant 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.Hashing it out in Public
Old tunnel build methodThe old tunnel build method, used prior to release 0.6.1.10, is documented on the old tunnel page. This was an "all at once" or "parallel" method, where messages were sent in parallel to each of the participants.
One question that arose 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 Tor. 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 packet count as the tunnel is built.
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.
A third alternative, used until I2P 0.6.1.10, 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.
More History and DiscussionBefore the introduction of the Variable Tunnel Build Message, there were at least two problems:
- The size of the messages (caused by an 8-hop maximum, when the typical tunnel length is 2 or 3 hops... and current research indicates that more than 3 hops does not enhance anonymity);
- The high build failure rate, especially for long (and exploratory) tunnels, since all hops must agree or the tunnel is discarded.
Welterde has proposed modifications to the parallel method to allow for reconfiguration. Sponge has proposed using 'tokens' of some sort.
Any students of tunnel building must study the historical record leading up to the current method, especially the various anonymity vulnerabilities that may exist in various methods. The mail archives from October 2005 at zzz.i2p are particularly helpful. As stated on the tunnel creation specification, the current strategy came about during a discussion on the I2P mailing list between Michael Rogers, Matthew Toseland (toad), and jrandom regarding the predecessor attack. See:Summary and Reasoning.
The build changes in 0.6.1.10, released February 2006, were the last incompatible change in i2p; i.e., all releases since 0.6.1.10 are backward compatible. Any tunnel build change would cause a similar 'flag day', unless we implemented code so that the build originator would only use the new method if all participants (and build/reply endpoints/gateways) supported it.
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 I2P 3.0