diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,13 @@
 # Revision history for net-spider-rpl
 
+## 0.2.1.0  -- 2019-07-19
+
+### DIO module
+
+* Export `TrickleInterval` type synonym. It should have been exported
+  in the first place.
+
+
 ## 0.2.0.0  -- 2019-07-15
 
 * [BREAKING CHANGE] Type class instances from net-spider-pangraph are
diff --git a/net-spider-rpl.cabal b/net-spider-rpl.cabal
--- a/net-spider-rpl.cabal
+++ b/net-spider-rpl.cabal
@@ -1,5 +1,5 @@
 name:                   net-spider-rpl
-version:                0.2.0.0
+version:                0.2.1.0
 author:                 Toshio Ito <debug.ito@gmail.com>
 maintainer:             Toshio Ito <debug.ito@gmail.com>
 license:                BSD3
diff --git a/src/NetSpider/RPL/DIO.hs b/src/NetSpider/RPL/DIO.hs
--- a/src/NetSpider/RPL/DIO.hs
+++ b/src/NetSpider/RPL/DIO.hs
@@ -14,6 +14,7 @@
     dioLinkState,
     MergedDIOLink(..),
     Rank,
+    TrickleInterval,
     NeighborType(..),
     neighborTypeToText,
     neighborTypeFromText,
@@ -55,6 +56,8 @@
 
 -- | The interval of Trickle timer as decribed as number of doublings
 -- of the minimum interval, i.e. log2(I / Imin).
+--
+-- @since 0.2.1.0
 type TrickleInterval = Word
 
 -- | Node attributes about DIO.