Lb and failover overview (#241)

* Add overview section for load balancing/failover

overview/load.adoc

@@ -0,0 +1,34 @@
+include::../partials/attributes.adoc[]
+//Category: skupper-overview
+// Type: assembly
+[id="skupper-load"] 
+= Skupper load balancing and failover
+
+Skupper enables load balancing and failover across servers located across the service network.
+Specifically, Skupper balances *active TCP connections* across workloads deployed in distinct sites.
+If a workload at one site becomes unavailable, traffic is automatically rerouted to available sites. 
+For example, if you deploy the same backend code on two sites and expose the backend on the service network, concurrent requests from a third site to the backend service are processed by both sites.
+
+In a service network, the routing algorithm attempts to use the path with the lowest total *cost* from client to target server.
+
+== Understanding link cost
+
+Link cost is a configurable value when creating links between sites.
+In the earlier example, the link cost from the client to each server is the same by default (1).
+However, if one server is more powerful and has better latency, it processes TCP requests more quickly, resulting in more traffic being directed to that server.
+Skupper determines the perceived cost with the context of knowing the configured cost and the current number of open TCP connections and  directs traffic to the server with the lowest cost.
+
+* Local workloads have a link cost of 0.
+* If connecting to a workload requires traversing two or more links, the total of all link costs constitutes the link cost.
+
+NOTE: Traffic load is based on the number of concurrent TCP connections, so 'round robin' behavior should not be expected.
+
+
+== Using Skupper load balancing to achieve failover
+
+You can configure the network so that a specific location handles all traffic until failure. 
+After failure at that location, all traffic is handled by a different location. 
+To achieve this behavior, you set the cost from the client to the backup server high, for example 9999,  as described in {cost-link}.
+
+NOTE: Link cost is configured for all services between two sites.
+It is currently not possible to set different costs for distinct services.