Try to specify what clients should do upon receiving INTRO_ACK

This comes out of a discussion at arti#913.

1 files changed, 9 insertions, 0 deletions

diff --git a/spec/rend-spec/introduction-protocol.md b/spec/rend-spec/introduction-protocol.md
index 94b7145..8b841fc 100644
--- a/spec/rend-spec/introduction-protocol.md
+++ b/spec/rend-spec/introduction-protocol.md
@@ -279,6 +279,15 @@ In reply, the introduction point sends an INTRODUCE_ACK cell back to
 the client, either informing it that its request has been delivered,
 or that its request will not succeed.
 
+If the INTRODUCE_ACK message indicates success,
+the client SHOULD close the circuit to the introduction point,
+and not use it for anything else.
+If the INTRODUCE_ACK message indicates failure,
+the client MAY try a different introduction point.
+It MAY reach the different introduction point
+either by extending its introduction circuit an additional hop,
+or by building a new introduction circuit.
+
 ```text
    [TODO: specify what tor should do when receiving a malformed cell. Drop it?
           Kill circuit? This goes for all possible cells.]