introduction point

an introduction point is a relay an onion service picks as a mailbox - clients hand their connection request there, and the service decides whether to meet them.

an onion service never accepts incoming connections directly. instead it builds circuits to a handful of relays and asks them to act as introduction points - buffers that hold connection requests until the service chooses to respond.

the flow works like this: the service publishes its introduction points inside its descriptor. you fetch that descriptor, pick an intro point, and send an encrypted introduce cell saying "i want to talk, here is a rendezvous point and a one-time secret". the intro point forwards it to the service over their pre-built circuit.

the service can then ignore the request, or connect out to the rendezvous point you named. crucially, the introduction point only ever sees encrypted request blobs flying between two circuits - it cannot decrypt them and it does not learn who either party is.

if a market seems unreachable even though its address checks out elsewhere, broken introduction circuits are one of the usual suspects alongside hsdir churn. it is also why services sometimes come back on their own: the underlying server was fine, only the mailboxes were down.