Finish wideo 04

ntnu/21v/ttk4145/summary/summary.md

@@ -107,4 +107,52 @@ To test how the systems responds for a unknown error is to insert a failed accep
   * Go with default if no messages have been received
 * The acceptancetest must be good.
 
- 
+
+### Fault model
+
+#### Example with storage functions.
+
+**Step 1: Failure modes**
+
+Find the failure modes: What could go wrong?
+
+* **Write**: May return "I failed". Does not know why it faield
+* **Read**: May return "I failed". Does not know why it failed.
+
+**Step 2: Detect, Simplify, Inject errors**
+
+* Write information on where/what/how the process is doing.
+* All errors --> Fail
+* Inject errors
+
+**Step 3: Handling with redundancy**
+
+* Have multiple copies of the the information
+  * Use only the newest
+
+#### Example with communication function
+
+**Step 1: Failure modes**
+
+* Message
+  * Lost
+  * Delayed
+  * Corrupted
+  * DUplicated
+  * Wrong recipient
+
+**Step 2: Detection, Merging of errormodes and error injection**
+
+* Adding information to message
+  * Checksum
+  * Session ID
+  * Sequence number
+* Adding "ack" on well recieved messages
+* All errors will be treaded as "Lost message"
+* Injection
+  * Occasionally throw away some messages
+
+**Step 3: Handling with redundancy**
+
+* Timeout 
+* Retransmit message