Some generalized negative test case which each protocol should have -
- send extra IE/AVP in the message.
- send a message with missing mandatory IE in the message.
- Behavior of Node when it receives an message with two mandatory IE/AVPs missing
- Shutdown and restart database (whatever ur DB) during test.
- Shutdown the database before starting of server and client node.
- Behavior of Node when it receives a message with correct header but extra large size payload.
- Requests from 'unknown peers'.
- Reconnecting after termination.
Now there are lot of test case which depends on the architecture of the product for which you need to get through with the internal document of the company and max number of cases would be interface specific please look into the respective 3GPP spec for that.