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+```
+2020-10-15:s5/net/mm12/noter #6be5
+```
+# Sidste gang
+## Konklusion af opgaver fra sidste gang
+
+Fejl i TCP afhænger meget af hvilke pakker der bliver tabt.
+
+## TCP wireless
+
+TCP performance is very affected by wireless errors.
+Solutions:
+
+ -  Can use some special ACK which uses bitfields for acking multiple packets.
+    Also called *SACK*, i think.
+
+ -  Can Also terminate TCP in the middle and switch over to another protocol.
+    Then move back to TCP at the other end of wireless.
+
+ -  Modify layer 2 to work better with TCP.
+     -  Can for example check if dupack and then drop it.
+
+FEJL PÅ SLIDES: slide 31 "down-stream" skal være "up-stream".
+
+# Fault tolerance
+
+Dette er noter fra mm12.
+
+How to handle faults in a network of services?
+
+1.  Define a list of possible errors, with likelihood.
+    Here errors can be prioritised based on likelihood and impact.
+
+    Also called *Fault analysis*.
+
+## Types of failures
+
+ -  Crash
+ -  Timing
+ -  Assertive
+ -  Arbitrary/Byzantine
+
+## Metrics
+
+**Availability** is the fraction of time the system is available.
+For very reliable systems this should be $1-10^5$.
+
+If there is **replication** the availability becomes $1-(1-A)^N$, where N is the
+number of servers.
+
+## Replication
+
+> If availability is to low, just throw more servers at it. /s
+
+Have multiple servers.
+How to split traffic between servers:
+
+ -  Just give a list of servers
+
+    Means clients must do failover themselves.
+    Sometimes this is not desirable.
+
+ -  Have servers on the same ip address.
+    So the client is unaware.
+
+    This makes a constraint on the network, which must to load balancing.
+    This could be done with a load balancer or with ARP.
+
+ -  Have name servers which update depending on server availability.
+
+TODO: Cluster framework.