From AMule Project FAQ
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In the Kademlia network, the contact type is a way to measure how much you can be confident on that contact.
Initially, contacts are given a Type 3 status when they are known.
Periodically the client will check if that contact is still alive.
Depending on how long that contact has been alive (on the current session) it will be promoted to more confident types:
- Type 4: The contact must be deleted (usually because it is not reachable).
- Type 3: This is the type all contacts are given initially, when they are known.
- Type 2: This are contacts which have been known for less than an hour but have proven to be still alive.
- Type 1: This are contacts which have been known for less than two hours but have proven to be still alive.
- Type 0: This are contacts which have been known for two hours or more and have proven to be still alive.
A contact's type is kept between sessions, so that your client knows those contacts which are most time connected to the Kademlia network.
To sum up, types can go from 0 to 4, being 0-contacts the best and 4-contacts the worst.