diff --git a/spam-prevention.md b/spam-prevention.md
index 42b5ab2..c703915 100644
--- a/spam-prevention.md
+++ b/spam-prevention.md
@@ -15,6 +15,7 @@ spamming attempts.
 * [Proposals](#proposals)
   * [Naive upfront payment](#naive-upfront-payment)
   * [Reverse upfront payment](#reverse-upfront-payment)
+  * [Bidirectional upfront payment](#bidirectional-upfront-payment)
   * [Web of trust HTLC hold fees](#web-of-trust-htlc-hold-fees)
 
 ## Description of the attack