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diff --git a/spec/path-spec/path-selection-constraints.md b/spec/path-spec/path-selection-constraints.md new file mode 100644 index 0000000..8cac4fc --- /dev/null +++ b/spec/path-spec/path-selection-constraints.md @@ -0,0 +1,146 @@ +<a id="path-spec.txt-2.2"></a> + +# Path selection and constraints + +We choose the path for each new circuit before we build it, +based on our current directory information. +(Clients and relays use the latest directory information they have; +directory authorities use their own opinions.) + +We choose the +exit node first, followed by the other nodes in the circuit, front to +back. (In other words, for a 3-hop circuit, we first pick hop 3, +then hop 1, then hop 2.) + +## Universal constraints + +All paths we generate obey the following +constraints: + +- We do not choose the same router twice for the same path. +- We do not choose any router in the same family as another in the same + path. (Two routers are in the same family if each one lists the other + in the "family" entries of its descriptor.) +- We do not choose more than one router in a given network range, + which defaults to /16 for IPv4 and /32 for IPv6. + (C Tor overrides this with `EnforceDistinctSubnets`; + Arti overrides this with `ipv[46]_subnet_family_prefix`.) +- The first node must be a Guard (see + discussion [below](./guard-nodes.md) and in the + [guard specification](../guard-spec)). +- XXXX Choosing the length + +## Special-purpose constraints + +Additionally, we may be building circuits with one or more requests in +mind. Each kind of request puts certain constraints on paths. + +Most circuits need to be "Fast". +For these, we only choose nodes with the `Fast` flag. +For non-"fast" circuits, nodes without the `Fast` flag are eligible. + +- TODO document which circuits (do not) need to be Fast. + +Similarly, some circuits need to be "Stable". +For these, we only choose nodes with the Stable flag. + +- All service-side introduction circuits and all rendezvous paths + should be Stable. +- All connection requests for connections that we think will need to + stay open a long time require Stable circuits. Currently, Tor decides + this by examining the request's target port, and comparing it to a + list of "long-lived" ports. (Default: 21, 22, 706, 1863, 5050, + 5190, 5222, 5223, 6667, 6697, 8300.) + +<a id="path-spec.txt-2.2.1"></a> + +## Weighting node selection + +For all circuits, we weight node selection according to router bandwidth. + +We also weight the bandwidth of Exit and Guard flagged nodes depending on +the fraction of total bandwidth that they make up and depending upon the +position they are being selected for. + +These weights are published in the consensus, and are computed as described +in +["Computing Bandwidth Weights"](../dir-spec/computing-consensus.md#computing-bandwidth-weights) +in the directory specification. +They are: + +```text + Wgg - Weight for Guard-flagged nodes in the guard position + Wgm - Weight for non-flagged nodes in the guard Position + Wgd - Weight for Guard+Exit-flagged nodes in the guard Position + + Wmg - Weight for Guard-flagged nodes in the middle Position + Wmm - Weight for non-flagged nodes in the middle Position + Wme - Weight for Exit-flagged nodes in the middle Position + Wmd - Weight for Guard+Exit flagged nodes in the middle Position + + Weg - Weight for Guard flagged nodes in the exit Position + Wem - Weight for non-flagged nodes in the exit Position + Wee - Weight for Exit-flagged nodes in the exit Position + Wed - Weight for Guard+Exit-flagged nodes in the exit Position + + Wgb - Weight for BEGIN_DIR-supporting Guard-flagged nodes + Wmb - Weight for BEGIN_DIR-supporting non-flagged nodes + Web - Weight for BEGIN_DIR-supporting Exit-flagged nodes + Wdb - Weight for BEGIN_DIR-supporting Guard+Exit-flagged nodes + + Wbg - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests + Wbm - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests + Wbe - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests + Wbd - Weight for Guard+Exit-flagged nodes for BEGIN_DIR requests +``` + +If any of those weights is malformed or not present in a consensus, +clients proceed with the regular path selection algorithm setting +the weights to the default value of 10000. + + +## Choosing an exit + +If we know what IP address we want to connect to, we can +trivially tell whether a given router will support it by simulating +its declared exit policy. + +(DNS resolve requests are only sent to relays whose +exit policy is not equivalent to "reject *:*".) + +Because we often connect to addresses of the form hostname:port, we do not +always know the target IP address when we select an exit node. In these +cases, we need to pick an exit node that "might support" connections to a +given address port with an unknown address. An exit node "might support" +such a connection if any clause that accepts any connections to that port +precedes all clauses (if any) that reject all connections to that port. + +Unless requested to do so by the user, we never choose an exit node +flagged as "BadExit" by more than half of the authorities who advertise +themselves as listing bad exits. + +<a id="path-spec.txt-2.2.2"></a> + +## User configuration + +Users can alter the default behavior for path selection with configuration +options. + +```text + - If "ExitNodes" is provided, then every request requires an exit node on + the ExitNodes list. (If a request is supported by no nodes on that list, + and StrictExitNodes is false, then Tor treats that request as if + ExitNodes were not provided.) + + - "EntryNodes" and "StrictEntryNodes" behave analogously. + + - If a user tries to connect to or resolve a hostname of the form + <target>.<servername>.exit, the request is rewritten to a request for + <target>, and the request is only supported by the exit whose nickname + or fingerprint is <servername>. + + - When set, "HSLayer2Nodes" and "HSLayer3Nodes" relax Tor's path + restrictions to allow nodes in the same /16 and node family to reappear + in the path. They also allow the guard node to be chosen as the RP, IP, + and HSDIR, and as the hop before those positions. +``` |