diff --git a/readme.md b/readme.md
index 8ba769c..fa0ee6f 100644
--- a/readme.md
+++ b/readme.md
@@ -16,8 +16,13 @@ The stellar components are:
 ![stellar-high-level-design](./docs/imgs/stellar-high-level-design.svg)
 
 ## Concepts
-Session is an abstract of Layer 4-7 connection, i.e., TCP Session, HTTP Session, DNS Session. A session can derive child sessions.
+Session is an abstract of Layer 4-7 connection, i.e., TCP Session, HTTP Session, DNS Session. 
+- A session can derive child sessions.
+- All session has three states, which are Opening, Active, Closing.
+- State changes generate session events.
+
 Session Extra Data is a plugin defined data structure that attached to a session, it's lifetime is same as the session.
+
 Event is something happened in a session, i.e., TCP OPENING, HTTP Request, Session Extra Data updates.
 
 ## Packet IO Library