diff --git a/21.md b/21.md
index eb6da25..c3a70ef 100644
--- a/21.md
+++ b/21.md
@@ -52,4 +52,9 @@ or
   "status": "ERROR",
   "reason": "Not found"
 }
-```
\ No newline at end of file
+```
+
+## Optional Long-Held HTTP Mode
+
+Services MAY support keeping the HTTP connection open until payment completion by accepting a `wait` parameter without a value, e.g. in the above example the URL would become `https://example.com/verify/894e7f7e...?wait`. The request should return the response when the payment is settled or expires whichever comes first. Clients specifying the `?wait` flag MUST be able to gracefully deal with earlier, non-final responses where `"settled": false` if the server doesn't support the long-held HTTP mode or closed connections due to server timeouts.
+