diff --git a/content/notes/19-network-layer-data-plane.md b/content/notes/19-network-layer-data-plane.md
index 1a718c415..9b326c5ce 100644
--- a/content/notes/19-network-layer-data-plane.md
+++ b/content/notes/19-network-layer-data-plane.md
@@ -22,14 +22,55 @@ tags:
 ![packet diagram](https://i.imgur.com/Y9ue6gE.png)
 
 # Forwarding
-move packets from  a routers input link to appropriate output link
-	
+forwarding
+- move packets from a routers input link to appropriate output link
+- local, per-router function
+- function in data plane
 
-# Routing
+routing
+- determine route taken by packets from source to destination
+	- network-wide logic
+	- function in control plane
+	- chooses shortest path through routers
 
+# network layer service model
+best effort
+- try its best to deliver packet
+- does not guarantee successful delivery
+- transport layer provides reliability
+- no big deal if packet is lost
+- does not guarantee timing or order of delivery
+	- each packet sent individually, possibly through different paths
+- does not guarantee bandwidth available
+	- network layer uses packet switching which unlike socket switching does not reserve resources
 
 # Routers
+![architecture overview](https://i.imgur.com/fNTeZnZ.png)
+
+destination-based forwarding: foward based only on destination IP address
+
+input port queueing: if datagrams arrive faster than fowarding rate into switch fabric
 
 # IP addressing
+- unreliable host-to-host communication protocol
+	- datagram formatting, IP addressing, packet switching
+
+![IP packet headers](https://i.imgur.com/2ehI0Kq.png)
+- ver: IP protocol versin number
+- head len: header length in bytes
+- type of service: diffserv(0:5) or ECN (6:7)
+- length: total datagram length (bytes)
+- time to live: remaining max hops (decremented at each router)
+- upper layer: which protocol use at transport layer
+- 16-bit id, flgs, fragment offset: fragmentation/reassembly
+- header checksum
+- source IP
+- destination IP
+- options: e.g., timestamp, record route taken
+
+overhead:
+- 20 bytes TCP
+- 20 bytes IP
+- = 40 bytes 
 
 # IPv6
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