Optimize our path selection code
(This is a more complete ticket than #13739 (moved) and thus supersedes it.)
An onion service can be ask to open many rendezvous circuit by a client. With a vanilla Tor, the resource consumption there is roughly 1:2 (service:client) because the client needs to open two circuits, one to the RP and one to the IP and then sends one cell on that circuit to introduce. The service will do a bit less than that because its introduction circuit is already established so it simply needs to open a rendezvous circuit.
However, an attacker (DoS) can make a client skip the RP circuit creation, pin relays in a path to the introduction point and simply open an introduction circuit each time through that path (which in theory should be made very fast and also controlled by the attacker).
This means that the work ratio goes from 1:2 to 1:1 because the client only opens one circuit, send a single cell, close it and repeat.
But, in reality, it gets worst for the service because of the performance of our path selection code. It is heavy. For starter, we need to randomly select 3 nodes from the consensus for each rendezvous circuit. But to select those nodes, we go over all of them for each node to exclude some, include others, go through a series of tests such has
node_has_preferred_descriptor() that are quite heavy.
The CPU profile of an overloaded service shows that the majority of CPU is spent in selecting these nodes. Here is a perf report output for the top 5:
+ 11.10% tor tor [.] smartlist_remove + 10.96% tor tor [.] fmonty + 8.77% tor tor [.] tor_memeq + 6.56% tor tor [.] tor_addr_family + 5.40% tor tor [.] tor_addr_is_null
smartlist_remove() comes from:
- smartlist_remove - 11.10% smartlist_subtract router_choose_random_node choose_good_middle_server onion_extend_cpath onion_populate_cpath circuit_establish_circuit - circuit_launch_by_extend_info - 10.93% launch_rendezvous_point_circuit
Amazingly enough, we spend more time selecting path than doing crypto on a service that gets DDoS by introduction requests (#29607 (moved)).
There aren't many straight forward approach here. Optimizing the code path bits by bits could be an approach. Most likely, our first step is to have a benchmark for our current path selection code and try to improve on it incrementally.
There are also more hackish approach like pre-building a list of RP nodes at each consensus (or when torrc options are changed like
ExcludeNodes) and then randomly picking nodes from there would go faster because they wouldn't need to go through long iterations of tests, it would be already done.
One example is
router_choose_random_node() that goes over the entire nodes list just to pick nodes to exclude, then again to test each nodes
router_add_running_nodes_to_smartlist() and then potentially does a series of
smartlist_subtract() that iterate over the entire list again. This is in theory
O(n) + O(n) + O(n) actually theoretically being
O(n) but reality is far from the theory here in CPU consumption ;).