$Id$ Tor network discovery protocol 0. Scope This document proposes a way of doing more distributed network discovery while maintaining some amount of admission control. We don't recommend you implement this as-is; it needs more discussion. Terminology: - Client: The Tor component that chooses paths. - Server: A relay node that passes traffic along. 1. Goals. We want more decentralized discovery for network topology and status. In particular: 1a. We want to let clients learn about new servers from anywhere and build circuits through them if they wish. This means that Tor nodes need to be able to Extend to nodes they don't already know about. This is already implemented, but see the 'Extend policy' issue below. 1b. We want to provide a robust (available) and not-too-centralized mechanism for tracking network status (which nodes are up and working) and admission (which nodes are "recommended" for certain uses). 1c. [optional] We want to permit servers that can't route to all other servers, e.g. because they're behind NAT or otherwise firewalled.* 2. Assumptions. People get the code from us, and they trust us (or our gpg keys, or something down the trust chain that's equivalent). Even if the software allows humans to change the client configuration, most of them will use the default that's provided, so we should provide one that is the right balance of robust and safe. Assume that Sybil attackers can produce only a limited number of independent-looking nodes. Roger has only a limited amount of time for approving nodes, and doesn't want to be the time bottleneck anyway. We can trust servers to accurately report their characteristics (uptime, capacity, exit policies, etc), as long as we have some mechanism for notifying clients when we notice that they're lying. There exists a "main" core Internet in which most locations can access most locations. We'll focus on it first. 3. Some notes on how to achieve. We ship with S (e.g. 3) seed keys. We ship with N (e.g. 20) introducer locations and fingerprints. We ship with some set of signed timestamped certs for those introducers. Introducers serve signed network-status pages, listing their opinions of network status and which routers are good. They also serve descriptors in some way. These don't need to be signed by the introducers, since they're self-signed and timestamped by each server. A DHT is not so appropriate for distributing server descriptors as long as we expect each client to plan to collect all of them periodically. It would seem that each introducer might as well just keep its own big pile of descriptors, and they synchronize (pull) from each other periodically. Clients then get network-status pages from a threshold of introducers, fetch enough of the server descriptors to make them happy, and proceed as now. Anything wrong with this? Notice that this doesn't preclude other approaches to discovering different concurrent Tor networks. For example, a Tor network inside China could ship Tor with a different torrc and poof, they're using a different set of seed keys and a different set of introducers. Some smarter clients could be made to learn about both networks, and be told which nodes bridge the networks. 4. Unresolved: - What new features need to be added to server descriptors so they remain compact yet support new functionality? - How do we compactly describe seeds, introducers, and certs? Does Tor have built-in defaults still, that can be overridden? - How much cert functionality do we want in our PKI? Can we revoke introducers, or is that done by releasing a new version of the code? - By what mechanism will new servers contact the humans who run introducers, so they can be approved? - Is our network growing because of peoples' trust in Roger? Will it grow the same way, or as robustly, or more robustly, with no figurehead? - 'Extend policies' -- middleman doesn't really mean middleman, alas. ---------- (*) Regarding "Blossom: an unstructured overlay network for end-to-end connectivity." In this section we address possible solutions to the problem of how to allow Tor routers in different transport domains to communicate. First, we presume that for every interface between transport domains A and B, one Tor router T_A exists in transport domain A, one Tor router T_B exists in transport domain B, and (without loss of generality) T_A can open a persistent connection to T_B. Any Tor traffic between the two routers will occur over this connection, which effectively renders the routers equal partners in bridging between the two transport domains. We refer to the established link between two transport domains as a "bridge" (we use this term because there is no serious possibility of confusion with the notion of a layer 2 bridge). Next, suppose that the universe consists of transport domains connected by persistent connections in this manner. An individual router can open multiple connections to routers within the same foreign transport domain, and it can establish separate connections to routers within multiple foreign transport domains. As in regular Tor, each Blossom router pushes its descriptor to directory servers. These directory servers can be within the same transport domain, but they need not be. The trick is that if a directory server is in another transport domain, then that directory server must know through which Tor routers to send messages destined for the Tor router in question. Descriptors for Blossom routers held by the directory server must contain a special field for specifying a path through the overlay (i.e. an ordered list of router names/IDs) to a router in a foreign transport domain. (This field may be a set of paths rather than a single path.) A new router publishing to a directory server in a foreign transport should include a list of routers. This list should be either: a. ...a list of routers to which the router has persistent connections, or, if the new router does not have any persistent connections, b. ...a (not necessarily exhaustive) list of fellow routers that are in the same transport domain. The directory server will be able to use this information to derive a path to the new router, as follows. If the new router used approach (a), then the directory server will define the same path(s) in the descriptors for the router(s) specified in the list, with the corresponding specified router appended to each path. If the new router used approach (b), then the directory server will define the same path(s) in the descriptors for the routers specified in the list. The directory server will then insert the newly defined path into the descriptor from the router. If all directory servers are within the same transport domain, then the problem is solved: routers can exist within multiple transport domains, and as long as the network of transport domains is fully connected by bridges, any router will be able to access any other router in a foreign transport domain simply by extending along the path specified by the directory server. However, we want the system to be truly decentralized, which means not electing any particular transport domain to be the master domain in which entries are published. Generally speaking, directory servers share information with each other about routers. In order for a directory server to share information with a directory server in a foreign transport domain to which it cannot speak directly, it must use Tor, which means referring to the other directory server by using a router in the foreign transport domain. However, in order to use Tor, it must be able to reach that router, which means that a descriptor for that router must exist in its table, along with a means of reaching it. Therefore, in order for a mutual exchange of information between routers in transport domain A and those in transport domain B to be possible, when routers in transport domain A cannot establish direct connections with routers in transport domain B, then some router in transport domain B must have pushed its descriptor to a directory server in transport domain A, so that the directory server in transport domain A can use that router to reach the directory server in transport domain B. When confronted with the choice of multiple different paths to reach the same router, the Blossom nodes may use a route selection protocol similar in design to that used by BGP (may be a simple distance-vector route selection procedure that only takes into account path length, or may be more complex to avoid loops, cache results, etc.) in order to choose the best one.