diff --git a/content/notes/20-network-layer-control-plane.md b/content/notes/20-network-layer-control-plane.md
index c98a6cf27..64744ee2f 100644
--- a/content/notes/20-network-layer-control-plane.md
+++ b/content/notes/20-network-layer-control-plane.md
@@ -16,10 +16,67 @@ tags:
 
 ## SDN software defined networking
 a remote controller computes, and install fowarding tables in routers
-
+- much faster: less work for each router
+- separate control and data plane
 ![](https://i.imgur.com/H73GPsc.png)
 
 # routing algorithms
+- goal is to determine "good" paths from sending to recieving host through network of routers
+- path: sequence of routers
+- "good": least "cost", "fastest", "least congested"
+
+## graph abstraction
+- nodes: set of routers
+- edges: set of links
+
+$c_{a,b}$ cost the link directly connecting a and b. if there is no link the cost is ♾
+
+![slide](https://i.imgur.com/onryMTK.png)
+
+## algorithm classification
+global: all routers have complete toplogy, link cost into
+- link state algotihms
+
+decentralized: interative process of computation, exchange of into with neighbors
+- routers only know link costs to attacked neighbors
+- "distance vecotr algorithms
+
+static: routes change very slowly over time
+
+dynamic: routes change more quickly
+- periodic updates or in response to link cost changes
+
+## link state routing
+centralized: network topology, link costs are known to all nodes
+- each node gathers informatin on each link to its neighbors
+- build link state packets and flood to all other nodes
+
+computes least costs paths from one node to all other nodes
+- gives a fowarding table for that node 
+
+iterative: after k interations, know least cost path to k destinations
+
+## notations
+![](https://i.imgur.com/NfrM72Y.png)
+
+## Dijkstra's algorithm
+
+initialization
+```
+N` = {u}
+for all nodes v
+	if v adjacent to u
+		then D(v) = Cuv
+	else D(v) = infinity
+	
+loop until all nodes in N`
+	find W not in N` such that D(w) is a min
+	add w to N`
+	update D(v) for all v adjacent to w and not in N`:
+		D(v) = min(D(v), D(w)+Cwv)
+	//new least-path-cost to v is either old least-cost-path to v or known least-path-cost to w plus direct-cost from w to v
+
+```
 
 # routing in the internet