diff --git a/src/or/or.h b/src/or/or.h
index dcaea39d17..3510ab0421 100644
--- a/src/or/or.h
+++ b/src/or/or.h
@@ -3508,8 +3508,8 @@ typedef struct rend_intro_point_t {
   /** (Service side only) A digestmap recording the INTRODUCE2 cells
    * this intro point's circuit has received.  Each key is the digest
    * of the RSA-encrypted part of a received INTRODUCE2 cell; each
-   * value is a pointer to the time_t at which the cell was
-   * received. */
+   * value is a pointer to the time_t at which the cell was received.
+   * This digestmap is used to prevent replay attacks. */
   digestmap_t *accepted_intro_rsa_parts;
 
   /** (Service side only) The time at which this intro point was first
diff --git a/src/or/rendservice.c b/src/or/rendservice.c
index 2aa02f9e05..2c54f3059d 100644
--- a/src/or/rendservice.c
+++ b/src/or/rendservice.c
@@ -89,8 +89,12 @@ typedef struct rend_service_t {
                          * up-to-date. */
   time_t next_upload_time; /**< Scheduled next hidden service descriptor
                             * upload time. */
-  /** Map from digests of Diffie-Hellman values INTRODUCE2 to time_t of when
-   * they were received; used to prevent replays. */
+  /** Map from digests of Diffie-Hellman values INTRODUCE2 to time_t
+   * of when they were received.  Clients may send INTRODUCE1 cells
+   * for the same rendezvous point through two or more different
+   * introduction points; when they do, this digestmap keeps us from
+   * launching multiple simultaneous attempts to connect to the same
+   * rend point. */
   digestmap_t *accepted_intro_dh_parts;
   /** Time at which we last removed expired values from
    * accepted_intro_dh_parts. */