BOLT #2: Normal Operation

[linzhm]

+-------+                            +-------+
|       |--(1)---- add_htlc   ------>|       |
|       |--(2)---- add_htlc   ------>|       |
|       |<-(3)---- add_htlc   -------|       |
|       |                            |       |
|       |--(4)----   commit   ------>|       |
|   A   |                            |   B   |
|       |<-(5)--- revoke_and_ack-----|       |
|       |<-(6)----   commit   -------|       |
|       |                            |       |
|       |--(7)--- revoke_and_ack---->|       |
+-------+                            +-------+

If the receiver A had given senderB (4)commit, and B drop the connection on purpse. Then sender B can have an commitment that can't be revoked. Even sender have the new commitment causing it to lose token, sender B can have a snapshot of current state. If they exchange commitment after that, and B has send more token to receiver A, B can then sign and publish the commit mentioned above to cheat.

[linzhm]

Sorry about the issue, I figured that an commit must be revoked until one make another commit.