diff --git a/docs/activitypub.7 b/docs/activitypub.7
index 17d7898..ba7211a 100644
--- a/docs/activitypub.7
+++ b/docs/activitypub.7
@@ -131,6 +131,15 @@ and
 Plain text and images in jpeg, gif, png, and webp formats are supported.
 Other formats are linked to origin.
 .El
+.Ss EXTENSIONS
+Honk also supports a
+.Vt Ping
+activity and will respond with a
+.Vt Pong
+activity.
+This is useful for debugging networking connectivity issues without
+visible side effects.
+See ping.txt for details.
 .Ss SECURITY
 Honk uses http signatures.
 .Ss WEBFINGER