diff --git a/content/notes/20-network-layer-control-plane.md b/content/notes/20-network-layer-control-plane.md
index 1133b4fdc..2c4c9f721 100644
--- a/content/notes/20-network-layer-control-plane.md
+++ b/content/notes/20-network-layer-control-plane.md
@@ -4,6 +4,9 @@ aliases:
 tags: 
 - cosc203
 - lecture
+sr-due: 2022-10-21
+sr-interval: 3
+sr-ease: 250
 ---
 
 # per-router control vs SDN
@@ -136,13 +139,31 @@ robustness
 	- each routers table is used by others: errors propagate through network
 
 
-## routing in internet
+# Routing in internet
 - autonomous system (AS): a group of networks and routers controled by a single administrative authority
 - Intra-AS routing:
 	- routing information protocol (RIP): distance vector routing
 	- open shortest path first (OSPF): link state routing
+- Inter-AS routing:
+	- border gateway protocol (BGP)L path-vector routing
 
+# ICMP
+- used by hosts and routers to communicate netowrk level information
+	- error reporting:
+	- echo requeset/reply
+- network layer "above" IP
+	- ICMP message carried in IP datagrams
+- ICMP message: type, code plus first 8 bytes of IP datagram causing error
 
-# routing in the internet
+![ICMP types and codes](https://i.imgur.com/cv0DRXL.png)
 
-# ICMP
\ No newline at end of file
+## traceroute
+- sends a set of UDP segments to destination
+- datagram in nth set arrives to nth router
+	- router discards datagram and sends source ICMP message (type 11 code 0)
+	- ICMP message possibly incldes name of router and IP
+- when ICMP message arrives at source: record RTTs
+
+stopping:
+- when a packet arrives that the destination, it returns ICMP "port unreachable message" (type 3 code 3)
+- source stops
\ No newline at end of file
diff --git a/content/notes/21-data-link-layer-LAN.md b/content/notes/21-data-link-layer-LAN.md
new file mode 100644
index 000000000..d17d2002c
--- /dev/null
+++ b/content/notes/21-data-link-layer-LAN.md
@@ -0,0 +1,44 @@
+---
+title: "21-data-link-layer-LAN"
+aliases: 
+tags: 
+- cosc203
+- lecture
+---
+
+# data link layer services
+- transfer datgram from one node to **physically adjacent** node over a link
+- terminology
+	- hosts and routers: node
+	- communication channels between adjacent nodes: links
+		- wired
+		- wireless
+		- LANs
+	- layer-2 packet: frame, encapsulates datagram
+		- ![](https://i.imgur.com/9RcK8WD.png)
+
+
+services
+- framing, link access:
+	- encapsulate datgram into frame, add head, trailer
+	- do not use IP address
+		- use MAC address in frame headers
+- reliable delivery of data between adjacent nodes
+	- flow control: pacing between adjacent sending and recieving nodes
+	- error detection and correction
+		- errors from: signal attenuation, noise
+		- receiver detects errors and signals retransmission, or drops frame
+		- reciever identifies and correct bit error(s) without retransmission
+
+implemented in network interface card: (NIC) or on a chip
+- ethernet, WiFi card or chip
+- implements link and physical layer
+
+each NIC has a unique ID - MAC (medium access control)
+- 48 bits: six groups of two hex digits
+- also called physical address, ethernet hardware address
+- manufacturer buys portion of MAC adress space (to assure uniqueness)
+
+# parity checking and CRC
+
+# MAC protocols
\ No newline at end of file
diff --git a/content/notes/cosc-203.md b/content/notes/cosc-203.md
index 3a31b78c5..1356b0c2a 100644
--- a/content/notes/cosc-203.md
+++ b/content/notes/cosc-203.md
@@ -49,7 +49,7 @@ tags:
 - [18-transport layer](notes/18-transport-layer)
 - [19-network layer data plane](19-network-layer-data-plane)
 - [20-network layer control plane](20-network-layer-control-plane)
-- [21-data link layer and LAN]
+- [21-data link layer and LAN](21-data-link-layer-LAN)
 - [22-LAN and physical layer]
 
 # Archive