Network security

Suppose we are using Lamport's hash, and Bob crashes before receiving Alice's reph pose an intruder, Trudy, can eavesdrop and detect that Bob crashed (maybe Trudy can cause Bob to crash). Then Trudy has a quantity (whatever Alice replied that Bob didre receive) which Trudy can use to impersonate Alice, if 'Trudy logs in before Alice attempeu log into Bob again. How can we modify Bob's behavior to prevent this threat? (Exactly do we overwrite Bob's database, and with what)?